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PuTTY User Manual
PuTTY is a free (MIT-licensed) Windows Telnet and SSH client. This
manual documents PuTTY, and its companion utilities PSCP, PSFTP,
Plink, Pageant and PuTTYgen.
Note to Unix users: this manual currently primarily documents the
Windows versions of the PuTTY utilities. Some options are therefore
mentioned that are absent from the Unix version; the Unix version has
features not described here; and the pterm and command-line puttygen
utilities are not described at all. The only Unix-specific documentation
that currently exists is the man pages.
This manual is copyright 1997-2017 Simon Tatham. All rights reserved.
You may distribute this documentation under the MIT licence. See
appendix C for the licence text in full.
Chapter 1: Introduction to PuTTY
1.1 What are SSH, Telnet and Rlogin?
1.2 How do SSH, Telnet and Rlogin differ?
Chapter 2: Getting started with PuTTY
2.1 Starting a session
2.2 Verifying the host key (SSH only)
2.3 Logging in
2.4 After logging in
2.5 Logging out
Chapter 3: Using PuTTY
3.1 During your session
3.2 Creating a log file of your session
3.3 Altering your character set configuration
3.4 Using X11 forwarding in SSH
3.5 Using port forwarding in SSH
3.6 Making raw TCP connections
3.7 Connecting to a local serial line
3.8 The PuTTY command line
Chapter 4: Configuring PuTTY
4.1 The Session panel
4.2 The Logging panel

4.3 The Terminal panel
4.4 The Keyboard panel
4.5 The Bell panel
4.6 The Features panel
4.7 The Window panel
4.8 The Appearance panel
4.9 The Behaviour panel
4.10 The Translation panel
4.11 The Selection panel
4.12 The Colours panel
4.13 The Connection panel
4.14 The Data panel
4.15 The Proxy panel
4.16 The Telnet panel
4.17 The Rlogin panel
4.18 The SSH panel
4.19 The Kex panel
4.20 The Host Keys panel
4.21 The Cipher panel
4.22 The Auth panel
4.23 The GSSAPI panel
4.24 The TTY panel
4.25 The X11 panel
4.26 The Tunnels panel
4.27 The Bugs and More Bugs panels
4.28 The Serial panel
4.29 Storing configuration in a file
Chapter 5: Using PSCP to transfer files securely
5.1 Starting PSCP
5.2 PSCP Usage
Chapter 6: Using PSFTP to transfer files securely
6.1 Starting PSFTP
6.2 Running PSFTP
6.3 Using public key authentication with PSFTP
Chapter 7: Using the command-line connection tool Plink
7.1 Starting Plink
7.2 Using Plink
7.3 Using Plink in batch files and scripts
7.4 Using Plink with CVS

7.5 Using Plink with WinCVS
Chapter 8: Using public keys for SSH authentication
8.1 Public key authentication - an introduction
8.2 Using PuTTYgen, the PuTTY key generator
8.3 Getting ready for public key authentication
Chapter 9: Using Pageant for authentication
9.1 Getting started with Pageant
9.2 The Pageant main window
9.3 The Pageant command line
9.4 Using agent forwarding
9.5 Security considerations
Chapter 10: Common error messages
10.1 ‘The server's host key is not cached in the registry’
10.2 ‘WARNING - POTENTIAL SECURITY BREACH!’
10.3 ‘SSH protocol version 2 required by our configuration but
server only provides (old, insecure) SSH-1’
10.4 ‘The first cipher supported by the server is ... below the
configured warning threshold’
10.5 ‘Server sent disconnect message type 2 (protocol error):
"Too many authentication failures for root"’
10.6 ‘Out of memory’
10.7 ‘Internal error’, ‘Internal fault’, ‘Assertion failed’
10.8 ‘Unable to use this private key file’, ‘Couldn't load private
key’, ‘Key is of wrong type’
10.9 ‘Server refused our public key’ or ‘Key refused’
10.10 ‘Access denied’, ‘Authentication refused’
10.11 ‘No supported authentication methods available’
10.12 ‘Incorrect CRC received on packet’ or ‘Incorrect MAC
received on packet’
10.13 ‘Incoming packet was garbled on decryption’
10.14 ‘PuTTY X11 proxy: various errors’
10.15 ‘Network error: Software caused connection abort’
10.16 ‘Network error: Connection reset by peer’
10.17 ‘Network error: Connection refused’
10.18 ‘Network error: Connection timed out’
10.19 ‘Network error: Cannot assign requested address’
Appendix A: PuTTY FAQ
A.1 Introduction
A.2 Features supported in PuTTY

A.3 Ports to other operating systems
A.4 Embedding PuTTY in other programs
A.5 Details of PuTTY's operation
A.6 HOWTO questions
A.7 Troubleshooting
A.8 Security questions
A.9 Administrative questions
A.10 Miscellaneous questions
Appendix B: Feedback and bug reporting
B.1 General guidelines
B.2 Reporting bugs
B.3 Reporting security vulnerabilities
B.4 Requesting extra features
B.5 Requesting features that have already been requested
B.6 Support requests
B.7 Web server administration
B.8 Asking permission for things
B.9 Mirroring the PuTTY web site
B.10 Praise and compliments
B.11 E-mail address
Appendix C: PuTTY Licence
Appendix D: PuTTY hacking guide
D.1 Cross-OS portability
D.2 Multiple backends treated equally
D.3 Multiple sessions per process on some platforms
D.4 C, not C++
D.5 Security-conscious coding
D.6 Independence of specific compiler
D.7 Small code size
D.8 Single-threaded code
D.9 Keystrokes sent to the server wherever possible
D.10 640×480 friendliness in configuration panels
D.11 Automatically generated Makefiles
D.12 Coroutines in ssh.c
D.13 Single compilation of each source file
D.14 Do as we say, not as we do
Appendix E: PuTTY download keys and signatures
E.1 Public keys
E.2 Security details

E.3 Key rollover
Appendix F: SSH-2 names specified for PuTTY
F.1 Connection protocol channel request names
F.2 Key exchange method names
F.3 Encryption algorithm names

Chapter 1: Introduction to PuTTY
PuTTY is a free SSH, Telnet and Rlogin client for Windows systems.
1.1 What are SSH, Telnet and Rlogin?
1.2 How do SSH, Telnet and Rlogin differ?

1.1 What are SSH, Telnet and Rlogin?
If you already know what SSH, Telnet and Rlogin are, you can safely skip
on to the next section.
SSH, Telnet and Rlogin are three ways of doing the same thing: logging
in to a multi-user computer from another computer, over a network.
Multi-user operating systems, such as Unix and VMS, usually present a
command-line interface to the user, much like the ‘Command Prompt’ or
‘MS-DOS Prompt’ in Windows. The system prints a prompt, and you type
commands which the system will obey.
Using this type of interface, there is no need for you to be sitting at the
same machine you are typing commands to. The commands, and
responses, can be sent over a network, so you can sit at one computer
and give commands to another one, or even to more than one.
SSH, Telnet and Rlogin are network protocols that allow you to do this.
On the computer you sit at, you run a client, which makes a network
connection to the other computer (the server). The network connection
carries your keystrokes and commands from the client to the server, and
carries the server's responses back to you.
These protocols can also be used for other types of keyboard-based
interactive session. In particular, there are a lot of bulletin boards, talker
systems and MUDs (Multi-User Dungeons) which support access using
Telnet. There are even a few that support SSH.
You might want to use SSH, Telnet or Rlogin if:
you have an account on a Unix or VMS system which you want to be
able to access from somewhere else
your Internet Service Provider provides you with a login account on a
web server. (This might also be known as a shell account. A shell is
the program that runs on the server and interprets your commands
for you.)
you want to use a bulletin board system, talker or MUD which can be

accessed using Telnet.
You probably do not want to use SSH, Telnet or Rlogin if:
you only use Windows. Windows computers have their own ways of
networking between themselves, and unless you are doing
something fairly unusual, you will not need to use any of these
remote login protocols.

1.2 How do SSH, Telnet and Rlogin differ?
This list summarises some of the differences between SSH, Telnet and
Rlogin.
SSH (which stands for ‘secure shell’) is a recently designed, highsecurity protocol. It uses strong cryptography to protect your
connection against eavesdropping, hijacking and other attacks.
Telnet and Rlogin are both older protocols offering minimal security.
SSH and Rlogin both allow you to log in to the server without having
to type a password. (Rlogin's method of doing this is insecure, and
can allow an attacker to access your account on the server. SSH's
method is much more secure, and typically breaking the security
requires the attacker to have gained access to your actual client
machine.)
SSH allows you to connect to the server and automatically send a
command, so that the server will run that command and then
disconnect. So you can use it in automated processing.
The Internet is a hostile environment and security is everybody's
responsibility. If you are connecting across the open Internet, then we
recommend you use SSH. If the server you want to connect to doesn't
support SSH, it might be worth trying to persuade the administrator to
install it.
If your client and server are both behind the same (good) firewall, it is
more likely to be safe to use Telnet or Rlogin, but we still recommend you
use SSH.

Chapter 2: Getting started with PuTTY
This chapter gives a quick guide to the simplest types of interactive login
session using PuTTY.
2.1 Starting a session
2.2 Verifying the host key (SSH only)
2.3 Logging in
2.4 After logging in
2.5 Logging out

2.1 Starting a session
When you start PuTTY, you will see a dialog box. This dialog box allows
you to control everything PuTTY can do. See chapter 4 for details of all
the things you can control.
You don't usually need to change most of the configuration options. To
start the simplest kind of session, all you need to do is to enter a few
basic parameters.
In the ‘Host Name’ box, enter the Internet host name of the server you
want to connect to. You should have been told this by the provider of your
login account.
Now select a login protocol to use, from the ‘Connection type’ buttons.
For a login session, you should select Telnet, Rlogin or SSH. See section
1.2 for a description of the differences between the three protocols, and
advice on which one to use. The fourth protocol, Raw, is not used for
interactive login sessions; you would usually use this for debugging other
Internet services (see section 3.6). The fifth option, Serial, is used for
connecting to a local serial line, and works somewhat differently: see
section 3.7 for more information on this.
When you change the selected protocol, the number in the ‘Port’ box will
change. This is normal: it happens because the various login services are
usually provided on different network ports by the server machine. Most
servers will use the standard port numbers, so you will not need to
change the port setting. If your server provides login services on a nonstandard port, your system administrator should have told you which one.
(For example, many MUDs run Telnet service on a port other than 23.)
Once you have filled in the ‘Host Name’, ‘Protocol’, and possibly ‘Port’
settings, you are ready to connect. Press the ‘Open’ button at the bottom
of the dialog box, and PuTTY will begin trying to connect you to the
server.

2.2 Verifying the host key (SSH only)
If you are not using the SSH protocol, you can skip this section.
If you are using SSH to connect to a server for the first time, you will
probably see a message looking something like this:
The server's host key is not cached in the registry. You
have no guarantee that the server is the computer you
think it is.
The server's rsa2 key fingerprint is:
ssh-rsa 1024 7b:e5:6f:a7:f4:f9:81:62:5c:e3:1f:bf:8b:57:6c:5a
If you trust this host, hit Yes to add the key to
PuTTY's cache and carry on connecting.
If you want to carry on connecting just once, without
adding the key to the cache, hit No.
If you do not trust this host, hit Cancel to abandon the
connection.

This is a feature of the SSH protocol. It is designed to protect you against
a network attack known as spoofing: secretly redirecting your connection
to a different computer, so that you send your password to the wrong
machine. Using this technique, an attacker would be able to learn the
password that guards your login account, and could then log in as if they
were you and use the account for their own purposes.
To prevent this attack, each server has a unique identifying code, called a
host key. These keys are created in a way that prevents one server from
forging another server's key. So if you connect to a server and it sends
you a different host key from the one you were expecting, PuTTY can
warn you that the server may have been switched and that a spoofing
attack might be in progress.
PuTTY records the host key for each server you connect to, in the
Windows Registry. Every time you connect to a server, it checks that the
host key presented by the server is the same host key as it was the last
time you connected. If it is not, you will see a warning, and you will have
the chance to abandon your connection before you type any private
information (such as a password) into it.
However, when you connect to a server you have not connected to

before, PuTTY has no way of telling whether the host key is the right one
or not. So it gives the warning shown above, and asks you whether you
want to trust this host key or not.
Whether or not to trust the host key is your choice. If you are connecting
within a company network, you might feel that all the network users are
on the same side and spoofing attacks are unlikely, so you might choose
to trust the key without checking it. If you are connecting across a hostile
network (such as the Internet), you should check with your system
administrator, perhaps by telephone or in person. (Many servers have
more than one host key. If the system administrator sends you more than
one fingerprint, you should make sure the one PuTTY shows you is on
the list, but it doesn't matter which one it is.)
See section 4.20 for advanced options for managing host keys.

2.3 Logging in
After you have connected, and perhaps verified the server's host key, you
will be asked to log in, probably using a username and a password. Your
system administrator should have provided you with these. Enter the
username and the password, and the server should grant you access
and begin your session. If you have mistyped your password, most
servers will give you several chances to get it right.
If you are using SSH, be careful not to type your username wrongly,
because you will not have a chance to correct it after you press Return;
many SSH servers do not permit you to make two login attempts using
different usernames. If you type your username wrongly, you must close
PuTTY and start again.
If your password is refused but you are sure you have typed it correctly,
check that Caps Lock is not enabled. Many login servers, particularly
Unix computers, treat upper case and lower case as different when
checking your password; so if Caps Lock is on, your password will
probably be refused.

2.4 After logging in
After you log in to the server, what happens next is up to the server! Most
servers will print some sort of login message and then present a prompt,
at which you can type commands which the server will carry out. Some
servers will offer you on-line help; others might not. If you are in doubt
about what to do next, consult your system administrator.

2.5 Logging out
When you have finished your session, you should log out by typing the
server's own logout command. This might vary between servers; if in
doubt, try logout or exit, or consult a manual or your system
administrator. When the server processes your logout command, the
PuTTY window should close itself automatically.
You can close a PuTTY session using the Close button in the window
border, but this might confuse the server - a bit like hanging up a
telephone unexpectedly in the middle of a conversation. We recommend
you do not do this unless the server has stopped responding to you and
you cannot close the window any other way.

Chapter 3: Using PuTTY
This chapter provides a general introduction to some more advanced
features of PuTTY. For extreme detail and reference purposes, chapter 4
is likely to contain more information.
3.1 During your session
3.1.1 Copying and pasting text
3.1.2 Scrolling the screen back
3.1.3 The System menu
3.2 Creating a log file of your session
3.3 Altering your character set configuration
3.4 Using X11 forwarding in SSH
3.5 Using port forwarding in SSH
3.6 Making raw TCP connections
3.7 Connecting to a local serial line
3.8 The PuTTY command line
3.8.1 Starting a session from the command line
3.8.2 -cleanup
3.8.3 Standard command-line options

3.1 During your session
A lot of PuTTY's complexity and features are in the configuration panel.
Once you have worked your way through that and started a session,
things should be reasonably simple after that. Nevertheless, there are a
few more useful features available.
3.1.1 Copying and pasting text
3.1.2 Scrolling the screen back
3.1.3 The System menu
3.1.3.1 The PuTTY Event Log
3.1.3.2 Special commands
3.1.3.3 Starting new sessions
3.1.3.4 Changing your session settings
3.1.3.5 Copy All to Clipboard
3.1.3.6 Clearing and resetting the terminal
3.1.3.7 Full screen mode

3.1.1 Copying and pasting text
Often in a PuTTY session you will find text on your terminal screen which
you want to type in again. Like most other terminal emulators, PuTTY
allows you to copy and paste the text rather than having to type it again.
Also, copy and paste uses the Windows clipboard, so that you can paste
(for example) URLs into a web browser, or paste from a word processor
or spreadsheet into your terminal session.
PuTTY's copy and paste works entirely with the mouse. In order to copy
text to the clipboard, you just click the left mouse button in the terminal
window, and drag to select text. When you let go of the button, the text is
automatically copied to the clipboard. You do not need to press Ctrl-C or
Ctrl-Ins; in fact, if you do press Ctrl-C, PuTTY will send a Ctrl-C character
down your session to the server where it will probably cause a process to
be interrupted.
Pasting is done using the right button (or the middle mouse button, if you
have a three-button mouse and have set it up; see section 4.11.2).
(Pressing Shift-Ins, or selecting ‘Paste’ from the Ctrl+right-click context
menu, have the same effect.) When you click the right mouse button,
PuTTY will read whatever is in the Windows clipboard and paste it into
your session, exactly as if it had been typed at the keyboard. (Therefore,
be careful of pasting formatted text into an editor that does automatic
indenting; you may find that the spaces pasted from the clipboard plus
the spaces added by the editor add up to too many spaces and ruin the
formatting. There is nothing PuTTY can do about this.)
If you double-click the left mouse button, PuTTY will select a whole word.
If you double-click, hold down the second click, and drag the mouse,
PuTTY will select a sequence of whole words. (You can adjust precisely
what PuTTY considers to be part of a word; see section 4.11.5.) If you
triple-click, or triple-click and drag, then PuTTY will select a whole line or
sequence of lines.
If you want to select a rectangular region instead of selecting to the end
of each line, you can do this by holding down Alt when you make your
selection. You can also configure rectangular selection to be the default,

and then holding down Alt gives the normal behaviour instead: see
section 4.11.4 for details.
(In some Unix environments, Alt+drag is intercepted by the window
manager. Shift+Alt+drag should work for rectangular selection as well, so
you could try that instead.)
If you have a middle mouse button, then you can use it to adjust an
existing selection if you selected something slightly wrong. (If you have
configured the middle mouse button to paste, then the right mouse button
does this instead.) Click the button on the screen, and you can pick up
the nearest end of the selection and drag it to somewhere else.
It's possible for the server to ask to handle mouse clicks in the PuTTY
window itself. If this happens, the mouse pointer will turn into an arrow,
and using the mouse to copy and paste will only work if you hold down
Shift. See section 4.6.2 and section 4.11.3 for details of this feature and
how to configure it.

3.1.2 Scrolling the screen back
PuTTY keeps track of text that has scrolled up off the top of the terminal.
So if something appears on the screen that you want to read, but it
scrolls too fast and it's gone by the time you try to look for it, you can use
the scrollbar on the right side of the window to look back up the session
history and find it again.
As well as using the scrollbar, you can also page the scrollback up and
down by pressing Shift-PgUp and Shift-PgDn. You can scroll a line at a
time using Ctrl-PgUp and Ctrl-PgDn. These are still available if you
configure the scrollbar to be invisible.
By default the last 2000 lines scrolled off the top are preserved for you to
look at. You can increase (or decrease) this value using the configuration
box; see section 4.7.3.

3.1.3 The System menu
If you click the left mouse button on the icon in the top left corner of
PuTTY's terminal window, or click the right mouse button on the title bar,
you will see the standard Windows system menu containing items like
Minimise, Move, Size and Close.
PuTTY's system menu contains extra program features in addition to the
Windows standard options. These extra menu commands are described
below.
(These options are also available in a context menu brought up by
holding Ctrl and clicking with the right mouse button anywhere in the
PuTTY window.)
3.1.3.1 The PuTTY Event Log
3.1.3.2 Special commands
3.1.3.3 Starting new sessions
3.1.3.4 Changing your session settings
3.1.3.5 Copy All to Clipboard
3.1.3.6 Clearing and resetting the terminal
3.1.3.7 Full screen mode

3.1.3.1 The PuTTY Event Log
If you choose ‘Event Log’ from the system menu, a small window will pop
up in which PuTTY logs significant events during the connection. Most of
the events in the log will probably take place during session startup, but a
few can occur at any point in the session, and one or two occur right at
the end.
You can use the mouse to select one or more lines of the Event Log, and
hit the Copy button to copy them to the clipboard. If you are reporting a
bug, it's often useful to paste the contents of the Event Log into your bug
report.
(The Event Log is not the same as the facility to create a log file of your
session; that's described in section 3.2.)

3.1.3.2 Special commands
Depending on the protocol used for the current session, there may be a
submenu of ‘special commands’. These are protocol-specific tokens,
such as a ‘break’ signal, that can be sent down a connection in addition
to normal data. Their precise effect is usually up to the server. Currently
only Telnet, SSH, and serial connections have special commands.
The ‘break’ signal can also be invoked from the keyboard with Ctrl-Break.
The following special commands are available in Telnet:
Are You There
Break
Synch
Erase Character
PuTTY can also be configured to send this when the Backspace key
is pressed; see section 4.16.3.
Erase Line
Go Ahead
No Operation
Should have no effect.
Abort Process
Abort Output
Interrupt Process
PuTTY can also be configured to send this when Ctrl-C is typed; see
section 4.16.3.
Suspend Process
PuTTY can also be configured to send this when Ctrl-Z is typed; see
section 4.16.3.
End Of Record

End Of File
In an SSH connection, the following special commands are available:
IGNORE message
Should have no effect.
Repeat key exchange
Only available in SSH-2. Forces a repeat key exchange immediately
(and resets associated timers and counters). For more information
about repeat key exchanges, see section 4.19.2.
Cache new host key type
Only available in SSH-2. This submenu appears only if the server
has host keys of a type that PuTTY doesn't already have cached,
and so won't consider. Selecting a key here will allow PuTTY to use
that key now and in future: PuTTY will do a fresh key-exchange with
the selected key, and immediately add that key to its permanent
cache (relying on the host key used at the start of the connection to
cross-certify the new key). That key will be used for the rest of the
current session; it may not actually be used for future sessions,
depending on your preferences (see section 4.20.1).
Normally, PuTTY will carry on using a host key it already knows,
even if the server offers key formats that PuTTY would otherwise
prefer, to avoid host key prompts. As a result, if you've been using a
server for some years, you may still be using an older key than a
new user would use, due to server upgrades in the meantime. The
SSH protocol unfortunately does not have organised facilities for
host key migration and rollover, but this allows you to manually
upgrade.
Break
Only available in SSH-2, and only during a session. Optional
extension; may not be supported by server. PuTTY requests the
server's default break length.

Signals (SIGINT, SIGTERM etc)
Only available in SSH-2, and only during a session. Sends various
POSIX signals. Not honoured by all servers.
With a serial connection, the only available special command is ‘Break’.

3.1.3.3 Starting new sessions
PuTTY's system menu provides some shortcut ways to start new
sessions:
Selecting ‘New Session’ will start a completely new instance of
PuTTY, and bring up the configuration box as normal.
Selecting ‘Duplicate Session’ will start a session in a new window
with precisely the same options as your current one - connecting to
the same host using the same protocol, with all the same terminal
settings and everything.
In an inactive window, selecting ‘Restart Session’ will do the same as
‘Duplicate Session’, but in the current window.
The ‘Saved Sessions’ submenu gives you quick access to any sets
of stored session details you have previously saved. See section
4.1.2 for details of how to create saved sessions.

3.1.3.4 Changing your session settings
If you select ‘Change Settings’ from the system menu, PuTTY will display
a cut-down version of its initial configuration box. This allows you to
adjust most properties of your current session. You can change the
terminal size, the font, the actions of various keypresses, the colours, and
so on.
Some of the options that are available in the main configuration box are
not shown in the cut-down Change Settings box. These are usually
options which don't make sense to change in the middle of a session (for
example, you can't switch from SSH to Telnet in mid-session).
You can save the current settings to a saved session for future use from
this dialog box. See section 4.1.2 for more on saved sessions.

3.1.3.5 Copy All to Clipboard
This system menu option provides a convenient way to copy the whole
contents of the terminal screen (up to the last nonempty line) and
scrollback to the clipboard in one go.

3.1.3.6 Clearing and resetting the terminal
The ‘Clear Scrollback’ option on the system menu tells PuTTY to discard
all the lines of text that have been kept after they scrolled off the top of
the screen. This might be useful, for example, if you displayed sensitive
information and wanted to make sure nobody could look over your
shoulder and see it. (Note that this only prevents a casual user from
using the scrollbar to view the information; the text is not guaranteed not
to still be in PuTTY's memory.)
The ‘Reset Terminal’ option causes a full reset of the terminal emulation.
A VT-series terminal is a complex piece of software and can easily get
into a state where all the text printed becomes unreadable. (This can
happen, for example, if you accidentally output a binary file to your
terminal.) If this happens, selecting Reset Terminal should sort it out.

3.1.3.7 Full screen mode
If you find the title bar on a maximised window to be ugly or distracting,
you can select Full Screen mode to maximise PuTTY ‘even more’. When
you select this, PuTTY will expand to fill the whole screen and its
borders, title bar and scrollbar will disappear. (You can configure the
scrollbar not to disappear in full-screen mode if you want to keep it; see
section 4.7.3.)
When you are in full-screen mode, you can still access the system menu
if you click the left mouse button in the extreme top left corner of the
screen.

3.2 Creating a log file of your session
For some purposes you may find you want to log everything that appears
on your screen. You can do this using the ‘Logging’ panel in the
configuration box.
To begin a session log, select ‘Change Settings’ from the system menu
and go to the Logging panel. Enter a log file name, and select a logging
mode. (You can log all session output including the terminal control
sequences, or you can just log the printable text. It depends what you
want the log for.) Click ‘Apply’ and your log will be started. Later on, you
can go back to the Logging panel and select ‘Logging turned off
completely’ to stop logging; then PuTTY will close the log file and you can
safely read it.
See section 4.2 for more details and options.

3.3 Altering your character set configuration
If you find that special characters (accented characters, for example, or
line-drawing characters) are not being displayed correctly in your PuTTY
session, it may be that PuTTY is interpreting the characters sent by the
server according to the wrong character set. There are a lot of different
character sets available, and no good way for PuTTY to know which to
use, so it's entirely possible for this to happen.
If you click ‘Change Settings’ and look at the ‘Translation’ panel, you
should see a large number of character sets which you can select, and
other related options. Now all you need is to find out which of them you
want! (See section 4.10 for more information.)

3.4 Using X11 forwarding in SSH
The SSH protocol has the ability to securely forward X Window System
graphical applications over your encrypted SSH connection, so that you
can run an application on the SSH server machine and have it put its
windows up on your local machine without sending any X network traffic
in the clear.
In order to use this feature, you will need an X display server for your
Windows machine, such as Cygwin/X, X-Win32, or Exceed. This will
probably install itself as display number 0 on your local machine; if it
doesn't, the manual for the X server should tell you what it does do.
You should then tick the ‘Enable X11 forwarding’ box in the X11 panel
(see section 4.25) before starting your SSH session. The ‘X display
location’ box is blank by default, which means that PuTTY will try to use a
sensible default such as :0, which is the usual display location where
your X server will be installed. If that needs changing, then change it.
Now you should be able to log in to the SSH server as normal. To check
that X forwarding has been successfully negotiated during connection
startup, you can check the PuTTY Event Log (see section 3.1.3.1). It
should say something like this:
2001-12-05 17:22:01 Requesting X11 forwarding
2001-12-05 17:22:02 X11 forwarding enabled

If the remote system is Unix or Unix-like, you should also be able to see
that the DISPLAY environment variable has been set to point at display 10
or above on the SSH server machine itself:
fred@unixbox:~$ echo $DISPLAY
unixbox:10.0

If this works, you should then be able to run X applications in the remote
session and have them display their windows on your PC.
For more options relating to X11 forwarding, see section 4.25.

3.5 Using port forwarding in SSH
The SSH protocol has the ability to forward arbitrary network (TCP)
connections over your encrypted SSH connection, to avoid the network
traffic being sent in clear. For example, you could use this to connect
from your home computer to a POP-3 server on a remote machine
without your POP-3 password being visible to network sniffers.
In order to use port forwarding to connect from your local machine to a
port on a remote server, you need to:
Choose a port number on your local machine where PuTTY should
listen for incoming connections. There are likely to be plenty of
unused port numbers above 3000. (You can also use a local
loopback address here; see below for more details.)
Now, before you start your SSH connection, go to the Tunnels panel
(see section 4.26). Make sure the ‘Local’ radio button is set. Enter
the local port number into the ‘Source port’ box. Enter the destination
host name and port number into the ‘Destination’ box, separated by
a colon (for example, popserver.example.com:110 to connect to a
POP-3 server).
Now click the ‘Add’ button. The details of your port forwarding should
appear in the list box.
Now start your session and log in. (Port forwarding will not be enabled
until after you have logged in; otherwise it would be easy to perform
completely anonymous network attacks, and gain access to anyone's
virtual private network.) To check that PuTTY has set up the port
forwarding correctly, you can look at the PuTTY Event Log (see section
3.1.3.1). It should say something like this:
2001-12-05 17:22:10 Local port 3110 forwarding to
popserver.example.com:110

Now if you connect to the source port number on your local PC, you
should find that it answers you exactly as if it were the service running on
the destination machine. So in this example, you could then configure an
e-mail client to use localhost:3110 as a POP-3 server instead of
popserver.example.com:110. (Of course, the forwarding will stop

happening when your PuTTY session closes down.)
You can also forward ports in the other direction: arrange for a particular
port number on the server machine to be forwarded back to your PC as a
connection to a service on your PC or near it. To do this, just select the
‘Remote’ radio button instead of the ‘Local’ one. The ‘Source port’ box will
now specify a port number on the server (note that most servers will not
allow you to use port numbers under 1024 for this purpose).
An alternative way to forward local connections to remote hosts is to use
dynamic SOCKS proxying. In this mode, PuTTY acts as a SOCKS
server, which SOCKS-aware programs can connect to and open
forwarded connections to the destination of their choice, so this can be
an alternative to long lists of static forwardings. To use this mode, you will
need to select the ‘Dynamic’ radio button instead of ‘Local’, and then you
should not enter anything into the ‘Destination’ box (it will be ignored).
PuTTY will then listen for SOCKS connections on the port you have
specified. Most web browsers can be configured to connect to this
SOCKS proxy service; also, you can forward other PuTTY connections
through it by setting up the Proxy control panel (see section 4.15 for
details).
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are controls
in the Tunnels panel to change this:
The ‘Local ports accept connections from other hosts’ option allows
you to set up local-to-remote port forwardings (including dynamic
port forwardings) in such a way that machines other than your client
PC can connect to the forwarded port.
The ‘Remote ports do the same’ option does the same thing for
remote-to-local port forwardings (so that machines other than the
SSH server machine can connect to the forwarded port.) Note that
this feature is only available in the SSH-2 protocol, and not all SSH-2
servers honour it (in OpenSSH, for example, it's usually disabled by
default).
You can also specify an IP address to listen on. Typically a Windows

machine can be asked to listen on any single IP address in the 127.*.*.*
range, and all of these are loopback addresses available only to the local
machine. So if you forward (for example) 127.0.0.5:79 to a remote
machine's finger port, then you should be able to run commands such as
finger fred@127.0.0.5. This can be useful if the program connecting to
the forwarded port doesn't allow you to change the port number it uses.
This feature is available for local-to-remote forwarded ports; SSH-1 is
unable to support it for remote-to-local ports, while SSH-2 can support it
in theory but servers will not necessarily cooperate.
(Note that if you're using Windows XP Service Pack 2, you may need to
obtain a fix from Microsoft in order to use addresses like 127.0.0.5 - see
question A.7.17.)
For more options relating to port forwarding, see section 4.26.
If the connection you are forwarding over SSH is itself a second SSH
connection made by another copy of PuTTY, you might find the ‘logical
host name’ configuration option useful to warn PuTTY of which host key it
should be expecting. See section 4.13.5 for details of this.

3.6 Making raw TCP connections
A lot of Internet protocols are composed of commands and responses in
plain text. For example, SMTP (the protocol used to transfer e-mail),
NNTP (the protocol used to transfer Usenet news), and HTTP (the
protocol used to serve Web pages) all consist of commands in readable
plain text.
Sometimes it can be useful to connect directly to one of these services
and speak the protocol ‘by hand’, by typing protocol commands and
watching the responses. On Unix machines, you can do this using the
system's telnet command to connect to the right port number. For
example, telnet mailserver.example.com 25 might enable you to talk
directly to the SMTP service running on a mail server.
Although the Unix telnet program provides this functionality, the protocol
being used is not really Telnet. Really there is no actual protocol at all;
the bytes sent down the connection are exactly the ones you type, and
the bytes shown on the screen are exactly the ones sent by the server.
Unix telnet will attempt to detect or guess whether the service it is
talking to is a real Telnet service or not; PuTTY prefers to be told for
certain.
In order to make a debugging connection to a service of this type, you
simply select the fourth protocol name, ‘Raw’, from the ‘Protocol’ buttons
in the ‘Session’ configuration panel. (See section 4.1.1.) You can then
enter a host name and a port number, and make the connection.

3.7 Connecting to a local serial line
PuTTY can connect directly to a local serial line as an alternative to
making a network connection. In this mode, text typed into the PuTTY
window will be sent straight out of your computer's serial port, and data
received through that port will be displayed in the PuTTY window. You
might use this mode, for example, if your serial port is connected to
another computer which has a serial connection.
To make a connection of this type, simply select ‘Serial’ from the
‘Connection type’ radio buttons on the ‘Session’ configuration panel (see
section 4.1.1). The ‘Host Name’ and ‘Port’ boxes will transform into ‘Serial
line’ and ‘Speed’, allowing you to specify which serial line to use (if your
computer has more than one) and what speed (baud rate) to use when
transferring data. For further configuration options (data bits, stop bits,
parity, flow control), you can use the ‘Serial’ configuration panel (see
section 4.28).
After you start up PuTTY in serial mode, you might find that you have to
make the first move, by sending some data out of the serial line in order
to notify the device at the other end that someone is there for it to talk to.
This probably depends on the device. If you start up a PuTTY serial
session and nothing appears in the window, try pressing Return a few
times and see if that helps.
A serial line provides no well defined means for one end of the
connection to notify the other that the connection is finished. Therefore,
PuTTY in serial mode will remain connected until you close the window
using the close button.

3.8 The PuTTY command line
PuTTY can be made to do various things without user intervention by
supplying command-line arguments (e.g., from a command prompt
window, or a Windows shortcut).
3.8.1 Starting a session from the command line
3.8.2 -cleanup
3.8.3 Standard command-line options
3.8.3.1 -load: load a saved session
3.8.3.2 Selecting a protocol: -ssh, -telnet, -rlogin, -raw serial

3.8.3.3 -v: increase verbosity
3.8.3.4 -l: specify a login name
3.8.3.5 -L, -R and -D: set up port forwardings
3.8.3.6 -m: read a remote command or script from a file
3.8.3.7 -P: specify a port number
3.8.3.8 -pw: specify a password
3.8.3.9 -agent and -noagent: control use of Pageant for
authentication
3.8.3.10 -A and -a: control agent forwarding
3.8.3.11 -X and -x: control X11 forwarding
3.8.3.12 -t and -T: control pseudo-terminal allocation
3.8.3.13 -N: suppress starting a shell or command
3.8.3.14 -nc: make a remote network connection in place of a
remote shell or command
3.8.3.15 -C: enable compression
3.8.3.16 -1 and -2: specify an SSH protocol version
3.8.3.17 -4 and -6: specify an Internet protocol version
3.8.3.18 -i: specify an SSH private key
3.8.3.19 -loghost: specify a logical host name
3.8.3.20 -hostkey: manually specify an expected host key
3.8.3.21 -pgpfp: display PGP key fingerprints
3.8.3.22 -sercfg: specify serial port configuration
3.8.3.23 -sessionlog, -sshlog, -sshrawlog: specify session
logging
3.8.3.24 -proxycmd: specify a local proxy command
3.8.3.25 -restrict-acl: restrict the Windows process ACL

3.8.1 Starting a session from the command line
These options allow you to bypass the configuration window and launch
straight into a session.
To start a connection to a server called host:
putty.exe [-ssh | -telnet | -rlogin | -raw] [user@]host

If this syntax is used, settings are taken from the Default Settings (see
section 4.1.2); user overrides these settings if supplied. Also, you can
specify a protocol, which will override the default protocol (see section
3.8.3.2).
For telnet sessions, the following alternative syntax is supported (this
makes PuTTY suitable for use as a URL handler for telnet URLs in web
browsers):
putty.exe telnet://host[:port]/

To start a connection to a serial port, e.g. COM1:
putty.exe -serial com1

In order to start an existing saved session called sessionname, use the load option (described in section 3.8.3.1).
putty.exe -load "session name"

3.8.2 -cleanup
If invoked with the -cleanup option, rather than running as normal, PuTTY
will remove its registry entries and random seed file from the local
machine (after confirming with the user). It will also attempt to remove
information about recently launched sessions stored in the ‘jump list’ on
Windows 7 and up.
Note that on multi-user systems, -cleanup only removes registry entries
and files associated with the currently logged-in user.

3.8.3 Standard command-line options
PuTTY and its associated tools support a range of command-line options,
most of which are consistent across all the tools. This section lists the
available options in all tools. Options which are specific to a particular
tool are covered in the chapter about that tool.
3.8.3.1 -load: load a saved session
3.8.3.2 Selecting a protocol: -ssh, -telnet, -rlogin, -raw -serial
3.8.3.3 -v: increase verbosity
3.8.3.4 -l: specify a login name
3.8.3.5 -L, -R and -D: set up port forwardings
3.8.3.6 -m: read a remote command or script from a file
3.8.3.7 -P: specify a port number
3.8.3.8 -pw: specify a password
3.8.3.9 -agent and -noagent: control use of Pageant for
authentication
3.8.3.10 -A and -a: control agent forwarding
3.8.3.11 -X and -x: control X11 forwarding
3.8.3.12 -t and -T: control pseudo-terminal allocation
3.8.3.13 -N: suppress starting a shell or command
3.8.3.14 -nc: make a remote network connection in place of a remote
shell or command
3.8.3.15 -C: enable compression
3.8.3.16 -1 and -2: specify an SSH protocol version
3.8.3.17 -4 and -6: specify an Internet protocol version
3.8.3.18 -i: specify an SSH private key
3.8.3.19 -loghost: specify a logical host name
3.8.3.20 -hostkey: manually specify an expected host key
3.8.3.21 -pgpfp: display PGP key fingerprints
3.8.3.22 -sercfg: specify serial port configuration
3.8.3.23 -sessionlog, -sshlog, -sshrawlog: specify session logging
3.8.3.24 -proxycmd: specify a local proxy command
3.8.3.25 -restrict-acl: restrict the Windows process ACL

3.8.3.1 -load: load a saved session
The -load option causes PuTTY to load configuration details out of a
saved session. If these details include a host name, then this option is all
you need to make PuTTY start a session.
You need double quotes around the session name if it contains spaces.
If you want to create a Windows shortcut to start a PuTTY saved session,
this is the option you should use: your shortcut should call something like
d:\path\to\putty.exe -load "my session"

(Note that PuTTY itself supports an alternative form of this option, for
backwards compatibility. If you execute putty @sessionname it will have
the same effect as putty -load "sessionname". With the @ form, no
double quotes are required, and the @ sign must be the very first thing on
the command line. This form of the option is deprecated.)

3.8.3.2 Selecting a protocol: -ssh, -telnet, rlogin, -raw -serial
To choose which protocol you want to connect with, you can use one of
these options:
-ssh selects the SSH protocol.
-telnet selects the Telnet protocol.
-rlogin selects the Rlogin protocol.
-raw selects the raw protocol.
-serial selects a serial connection.

These options are not available in the file transfer tools PSCP and
PSFTP (which only work with the SSH protocol).
These options are equivalent to the protocol selection buttons in the
Session panel of the PuTTY configuration box (see section 4.1.1).

3.8.3.3 -v: increase verbosity
Most of the PuTTY tools can be made to tell you more about what they
are doing by supplying the -v option. If you are having trouble when
making a connection, or you're simply curious, you can turn this switch
on and hope to find out more about what is happening.

3.8.3.4 -l: specify a login name
You can specify the user name to log in as on the remote server using
the -l option. For example, plink login.example.com -l fred.
These options are equivalent to the username selection box in the
Connection panel of the PuTTY configuration box (see section 4.14.1).

3.8.3.5 -L, -R and -D: set up port forwardings
As well as setting up port forwardings in the PuTTY configuration (see
section 4.26), you can also set up forwardings on the command line. The
command-line options work just like the ones in Unix ssh programs.
To forward a local port (say 5110) to a remote destination (say
popserver.example.com port 110), you can write something like one of
these:
putty -L 5110:popserver.example.com:110 -load mysession
plink mysession -L 5110:popserver.example.com:110

To forward a remote port to a local destination, just use the -R option
instead of -L:
putty -R 5023:mytelnetserver.myhouse.org:23 -load mysession
plink mysession -R 5023:mytelnetserver.myhouse.org:23

To specify an IP address for the listening end of the tunnel, prepend it to
the argument:
plink -L 127.0.0.5:23:localhost:23 myhost

To set up SOCKS-based dynamic port forwarding on a local port, use the
-D option. For this one you only have to pass the port number:
putty -D 4096 -load mysession

For general information on port forwarding, see section 3.5.
These options are not available in the file transfer tools PSCP and
PSFTP.

3.8.3.6 -m: read a remote command or script
from a file
The -m option performs a similar function to the ‘Remote command’ box
in the SSH panel of the PuTTY configuration box (see section 4.18.1).
However, the -m option expects to be given a local file name, and it will
read a command from that file.
With some servers (particularly Unix systems), you can even put multiple
lines in this file and execute more than one command in sequence, or a
whole shell script; but this is arguably an abuse, and cannot be expected
to work on all servers. In particular, it is known not to work with certain
‘embedded’ servers, such as Cisco routers.
This option is not available in the file transfer tools PSCP and PSFTP.

3.8.3.7 -P: specify a port number
The -P option is used to specify the port number to connect to. If you
have a Telnet server running on port 9696 of a machine instead of port
23, for example:
putty -telnet -P 9696 host.name
plink -telnet -P 9696 host.name

(Note that this option is more useful in Plink than in PuTTY, because in
PuTTY you can write putty -telnet host.name 9696 in any case.)
This option is equivalent to the port number control in the Session panel
of the PuTTY configuration box (see section 4.1.1).

3.8.3.8 -pw: specify a password
A simple way to automate a remote login is to supply your password on
the command line. This is not recommended for reasons of security. If
you possibly can, we recommend you set up public-key authentication
instead. See chapter 8 for details.
Note that the -pw option only works when you are using the SSH protocol.
Due to fundamental limitations of Telnet and Rlogin, these protocols do
not support automated password authentication.

3.8.3.9 -agent and -noagent: control use of
Pageant for authentication
The -agent option turns on SSH authentication using Pageant, and noagent turns it off. These options are only meaningful if you are using
SSH.
See chapter 9 for general information on Pageant.
These options are equivalent to the agent authentication checkbox in the
Auth panel of the PuTTY configuration box (see section 4.22.3).

3.8.3.10 -A and -a: control agent forwarding
The -A option turns on SSH agent forwarding, and -a turns it off. These
options are only meaningful if you are using SSH.
See chapter 9 for general information on Pageant, and section 9.4 for
information on agent forwarding. Note that there is a security risk
involved with enabling this option; see section 9.5 for details.
These options are equivalent to the agent forwarding checkbox in the
Auth panel of the PuTTY configuration box (see section 4.22.6).
These options are not available in the file transfer tools PSCP and
PSFTP.

3.8.3.11 -X and -x: control X11 forwarding
The -X option turns on X11 forwarding in SSH, and -x turns it off. These
options are only meaningful if you are using SSH.
For information on X11 forwarding, see section 3.4.
These options are equivalent to the X11 forwarding checkbox in the X11
panel of the PuTTY configuration box (see section 4.25).
These options are not available in the file transfer tools PSCP and
PSFTP.

3.8.3.12 -t and -T: control pseudo-terminal
allocation
The -t option ensures PuTTY attempts to allocate a pseudo-terminal at
the server, and -T stops it from allocating one. These options are only
meaningful if you are using SSH.
These options are equivalent to the ‘Don't allocate a pseudo-terminal’
checkbox in the SSH panel of the PuTTY configuration box (see section
4.24.1).
These options are not available in the file transfer tools PSCP and
PSFTP.

3.8.3.13 -N: suppress starting a shell or
command
The -N option prevents PuTTY from attempting to start a shell or
command on the remote server. You might want to use this option if you
are only using the SSH connection for port forwarding, and your user
account on the server does not have the ability to run a shell.
This feature is only available in SSH protocol version 2 (since the version
1 protocol assumes you will always want to run a shell).
This option is equivalent to the ‘Don't start a shell or command at all’
checkbox in the SSH panel of the PuTTY configuration box (see section
4.18.2).
This option is not available in the file transfer tools PSCP and PSFTP.

3.8.3.14 -nc: make a remote network connection
in place of a remote shell or command
The -nc option prevents Plink (or PuTTY) from attempting to start a shell
or command on the remote server. Instead, it will instruct the remote
server to open a network connection to a host name and port number
specified by you, and treat that network connection as if it were the main
session.
You specify a host and port as an argument to the -nc option, with a
colon separating the host name from the port number, like this:
plink host1.example.com -nc host2.example.com:1234

You might want to use this feature if you needed to make an SSH
connection to a target host which you can only reach by going through a
proxy host, and rather than using port forwarding you prefer to use the
local proxy feature (see section 4.15.1 for more about local proxies). In
this situation you might select ‘Local’ proxy type, set your local proxy
command to be ‘plink %proxyhost -nc %host:%port’, enter the target host
name on the Session panel, and enter the directly reachable proxy host
name on the Proxy panel.
This feature is only available in SSH protocol version 2 (since the version
1 protocol assumes you will always want to run a shell). It is not available
in the file transfer tools PSCP and PSFTP. It is available in PuTTY itself,
although it is unlikely to be very useful in any tool other than Plink. Also, nc uses the same server functionality as port forwarding, so it will not
work if your server administrator has disabled port forwarding.
(The option is named -nc after the Unix program nc, short for ‘netcat’. The
command ‘plink host1 -nc host2:port’ is very similar in functionality to
‘plink host1 nc host2 port’, which invokes nc on the server and tells it to
connect to the specified destination. However, Plink's built-in -nc option
does not depend on the nc program being installed on the server.)

3.8.3.15 -C: enable compression
The -C option enables compression of the data sent across the network.
This option is only meaningful if you are using SSH.
This option is equivalent to the ‘Enable compression’ checkbox in the
SSH panel of the PuTTY configuration box (see section 4.18.3).

3.8.3.16 -1 and -2: specify an SSH protocol
version
The -1 and -2 options force PuTTY to use version 1 or version 2 of the
SSH protocol. These options are only meaningful if you are using SSH.
These options are equivalent to selecting the SSH protocol version in the
SSH panel of the PuTTY configuration box (see section 4.18.4).

3.8.3.17 -4 and -6: specify an Internet protocol
version
The -4 and -6 options force PuTTY to use the older Internet protocol IPv4
or the newer IPv6 for most outgoing connections.
These options are equivalent to selecting your preferred Internet protocol
version as ‘IPv4’ or ‘IPv6’ in the Connection panel of the PuTTY
configuration box (see section 4.13.4).

3.8.3.18 -i: specify an SSH private key
The -i option allows you to specify the name of a private key file in *.PPK
format which PuTTY will use to authenticate with the server. This option
is only meaningful if you are using SSH.
If you are using Pageant, you can also specify a public key file (in RFC
4716 or OpenSSH format) to identify a specific key file to use. (This won't
work if you're not running Pageant, of course.)
For general information on public-key authentication, see chapter 8.
This option is equivalent to the ‘Private key file for authentication’ box in
the Auth panel of the PuTTY configuration box (see section 4.22.8).

3.8.3.19 -loghost: specify a logical host name
This option overrides PuTTY's normal SSH host key caching policy by
telling it the name of the host you expect your connection to end up at (in
cases where this differs from the location PuTTY thinks it's connecting
to). It can be a plain host name, or a host name followed by a colon and a
port number. See section 4.13.5 for more detail on this.

3.8.3.20 -hostkey: manually specify an expected
host key
This option overrides PuTTY's normal SSH host key caching policy by
telling it exactly what host key to expect, which can be useful if the
normal automatic host key store in the Registry is unavailable. The
argument to this option should be either a host key fingerprint, or an
SSH-2 public key blob. See section 4.20.2 for more information.
You can specify this option more than once if you want to configure more
than one key to be accepted.

3.8.3.21 -pgpfp: display PGP key fingerprints
This option causes the PuTTY tools not to run as normal, but instead to
display the fingerprints of the PuTTY PGP Master Keys, in order to aid
with verifying new versions. See appendix E for more information.

3.8.3.22 -sercfg: specify serial port configuration
This option specifies the configuration parameters for the serial port
(baud rate, stop bits etc). Its argument is interpreted as a commaseparated list of configuration options, which can be as follows:
Any single digit from 5 to 9 sets the number of data bits.
‘1’, ‘1.5’ or ‘2’ sets the number of stop bits.
Any other numeric string is interpreted as a baud rate.
A single lower-case letter specifies the parity: ‘n’ for none, ‘o’ for odd,
‘e’ for even, ‘m’ for mark and ‘s’ for space.
A single upper-case letter specifies the flow control: ‘N’ for none, ‘X’
for XON/XOFF, ‘R’ for RTS/CTS and ‘D’ for DSR/DTR.
For example, ‘-sercfg 19200,8,n,1,N’ denotes a baud rate of 19200, 8
data bits, no parity, 1 stop bit and no flow control.

3.8.3.23 -sessionlog, -sshlog, -sshrawlog: specify
session logging
These options cause the PuTTY network tools to write out a log file. Each
of them expects a file name as an argument, e.g. ‘-sshlog putty.log’
causes an SSH packet log to be written to a file called ‘putty.log’. The
three different options select different logging modes, all available from
the GUI too:
-sessionlog selects ‘All session output’ logging mode.
-sshlog selects ‘SSH packets’ logging mode.
-sshrawlog selects ‘SSH packets and raw data’ logging

mode.

For more information on logging configuration, see section 4.2.

3.8.3.24 -proxycmd: specify a local proxy
command
This option enables PuTTY's mode for running a command on the local
machine and using it as a proxy for the network connection. It expects a
shell command string as an argument.
See section 4.15.1 for more information on this, and on other proxy
settings. In particular, note that since the special sequences described
there are understood in the argument string, literal backslashes must be
doubled (if you want \ in your command, you must put \\ on the
command line).

3.8.3.25 -restrict-acl: restrict the Windows
process ACL
This option (on Windows only) causes PuTTY (or another PuTTY tool) to
try to lock down the operating system's access control on its own
process. If this succeeds, it should present an extra obstacle to malware
that has managed to run under the same user id as the PuTTY process,
by preventing it from attaching to PuTTY using the same interfaces
debuggers use and either reading sensitive information out of its memory
or hijacking its network session.
This option is not enabled by default, because this form of interaction
between Windows programs has many legitimate uses, including
accessibility software such as screen readers. Also, it cannot provide full
security against this class of attack in any case, because PuTTY can only
lock down its own ACL after it has started up, and malware could still get
in if it attacks the process between startup and lockdown. So it trades
away noticeable convenience, and delivers less real security than you
might want. However, if you do want to make that tradeoff anyway, the
option is available.
A PuTTY process started with -restrict-acl will pass that on to any
processes started with Duplicate Session, New Session etc. (However, if
you're invoking PuTTY tools explicitly, for instance as a proxy command,
you'll need to arrange to pass them the -restrict-acl option yourself, if
that's what you want.)

Chapter 4: Configuring PuTTY
This chapter describes all the configuration options in PuTTY.
PuTTY is configured using the control panel that comes up before you
start a session. Some options can also be changed in the middle of a
session, by selecting ‘Change Settings’ from the window menu.
4.1 The Session panel
4.1.1 The host name section
4.1.2 Loading and storing saved sessions
4.1.3 ‘Close Window on Exit’
4.2 The Logging panel
4.2.1 ‘Log file name’
4.2.2 ‘What to do if the log file already exists’
4.2.3 ‘Flush log file frequently’
4.2.4 Options specific to SSH packet logging
4.3 The Terminal panel
4.3.1 ‘Auto wrap mode initially on’
4.3.2 ‘DEC Origin Mode initially on’
4.3.3 ‘Implicit CR in every LF’
4.3.4 ‘Implicit LF in every CR’
4.3.5 ‘Use background colour to erase screen’
4.3.6 ‘Enable blinking text’
4.3.7 ‘Answerback to ^E’
4.3.8 ‘Local echo’
4.3.9 ‘Local line editing’
4.3.10 Remote-controlled printing
4.4 The Keyboard panel
4.4.1 Changing the action of the Backspace key
4.4.2 Changing the action of the Home and End keys
4.4.3 Changing the action of the function keys and keypad
4.4.4 Controlling Application Cursor Keys mode
4.4.5 Controlling Application Keypad mode
4.4.6 Using NetHack keypad mode
4.4.7 Enabling a DEC-like Compose key
4.4.8 ‘Control-Alt is different from AltGr’
4.5 The Bell panel

4.5.1 ‘Set the style of bell’
4.5.2 ‘Taskbar/caption indication on bell’
4.5.3 ‘Control the bell overload behaviour’
4.6 The Features panel
4.6.1 Disabling application keypad and cursor keys
4.6.2 Disabling xterm-style mouse reporting
4.6.3 Disabling remote terminal resizing
4.6.4 Disabling switching to the alternate screen
4.6.5 Disabling remote window title changing
4.6.6 Response to remote window title querying
4.6.7 Disabling remote scrollback clearing
4.6.8 Disabling destructive backspace
4.6.9 Disabling remote character set configuration
4.6.10 Disabling Arabic text shaping
4.6.11 Disabling bidirectional text display
4.7 The Window panel
4.7.1 Setting the size of the PuTTY window
4.7.2 What to do when the window is resized
4.7.3 Controlling scrollback
4.7.4 ‘Push erased text into scrollback’
4.8 The Appearance panel
4.8.1 Controlling the appearance of the cursor
4.8.2 Controlling the font used in the terminal window
4.8.3 ‘Hide mouse pointer when typing in window’
4.8.4 Controlling the window border
4.9 The Behaviour panel
4.9.1 Controlling the window title
4.9.2 ‘Warn before closing window’
4.9.3 ‘Window closes on ALT-F4’
4.9.4 ‘System menu appears on ALT-Space’
4.9.5 ‘System menu appears on Alt alone’
4.9.6 ‘Ensure window is always on top’
4.9.7 ‘Full screen on Alt-Enter’
4.10 The Translation panel
4.10.1 Controlling character set translation
4.10.2 ‘Treat CJK ambiguous characters as wide’
4.10.3 ‘Caps Lock acts as Cyrillic switch’
4.10.4 Controlling display of line-drawing characters
4.10.5 Controlling copy and paste of line drawing characters

4.11 The Selection panel
4.11.1 Pasting in Rich Text Format
4.11.2 Changing the actions of the mouse buttons
4.11.3 ‘Shift overrides application's use of mouse’
4.11.4 Default selection mode
4.11.5 Configuring word-by-word selection
4.12 The Colours panel
4.12.1 ‘Allow terminal to specify ANSI colours’
4.12.2 ‘Allow terminal to use xterm 256-colour mode’
4.12.3 ‘Indicate bolded text by changing...’
4.12.4 ‘Attempt to use logical palettes’
4.12.5 ‘Use system colours’
4.12.6 Adjusting the colours in the terminal window
4.13 The Connection panel
4.13.1 Using keepalives to prevent disconnection
4.13.2 ‘Disable Nagle's algorithm’
4.13.3 ‘Enable TCP keepalives’
4.13.4 ‘Internet protocol’
4.13.5 ‘Logical name of remote host’
4.14 The Data panel
4.14.1 ‘Auto-login username’
4.14.2 Use of system username
4.14.3 ‘Terminal-type string’
4.14.4 ‘Terminal speeds’
4.14.5 Setting environment variables on the server
4.15 The Proxy panel
4.15.1 Setting the proxy type
4.15.2 Excluding parts of the network from proxying
4.15.3 Name resolution when using a proxy
4.15.4 Username and password
4.15.5 Specifying the Telnet or Local proxy command
4.15.6 Controlling proxy logging
4.16 The Telnet panel
4.16.1 ‘Handling of OLD_ENVIRON ambiguity’
4.16.2 Passive and active Telnet negotiation modes
4.16.3 ‘Keyboard sends Telnet special commands’
4.16.4 ‘Return key sends Telnet New Line instead of ^M’
4.17 The Rlogin panel
4.17.1 ‘Local username’

4.18 The SSH panel
4.18.1 Executing a specific command on the server
4.18.2 ‘Don't start a shell or command at all’
4.18.3 ‘Enable compression’
4.18.4 ‘SSH protocol version’
4.18.5 Sharing an SSH connection between PuTTY tools
4.19 The Kex panel
4.19.1 Key exchange algorithm selection
4.19.2 Repeat key exchange
4.20 The Host Keys panel
4.20.1 Host key type selection
4.20.2 Manually configuring host keys
4.21 The Cipher panel
4.22 The Auth panel
4.22.1 ‘Display pre-authentication banner’
4.22.2 ‘Bypass authentication entirely’
4.22.3 ‘Attempt authentication using Pageant’
4.22.4 ‘Attempt TIS or CryptoCard authentication’
4.22.5 ‘Attempt keyboard-interactive authentication’
4.22.6 ‘Allow agent forwarding’
4.22.7 ‘Allow attempted changes of username in SSH-2’
4.22.8 ‘Private key file for authentication’
4.23 The GSSAPI panel
4.23.1 ‘Allow GSSAPI credential delegation’
4.23.2 Preference order for GSSAPI libraries
4.24 The TTY panel
4.24.1 ‘Don't allocate a pseudo-terminal’
4.24.2 Sending terminal modes
4.25 The X11 panel
4.25.1 Remote X11 authentication
4.25.2 X authority file for local display
4.26 The Tunnels panel
4.26.1 Controlling the visibility of forwarded ports
4.26.2 Selecting Internet protocol version for forwarded ports
4.27 The Bugs and More Bugs panels
4.27.1 ‘Chokes on SSH-1 ignore messages’
4.27.2 ‘Refuses all SSH-1 password camouflage’
4.27.3 ‘Chokes on SSH-1 RSA authentication’
4.27.4 ‘Chokes on SSH-2 ignore messages’

4.27.5 ‘Chokes on PuTTY's SSH-2 ‘winadj’ requests’
4.27.6 ‘Miscomputes SSH-2 HMAC keys’
4.27.7 ‘Miscomputes SSH-2 encryption keys’
4.27.8 ‘Requires padding on SSH-2 RSA signatures’
4.27.9 ‘Misuses the session ID in SSH-2 PK auth’
4.27.10 ‘Handles SSH-2 key re-exchange badly’
4.27.11 ‘Ignores SSH-2 maximum packet size’
4.27.12 ‘Replies to requests on closed channels’
4.27.13 ‘Only supports pre-RFC4419 SSH-2 DH GEX’
4.28 The Serial panel
4.28.1 Selecting a serial line to connect to
4.28.2 Selecting the speed of your serial line
4.28.3 Selecting the number of data bits
4.28.4 Selecting the number of stop bits
4.28.5 Selecting the serial parity checking scheme
4.28.6 Selecting the serial flow control scheme
4.29 Storing configuration in a file

4.1 The Session panel
The Session configuration panel contains the basic options you need to
specify in order to open a session at all, and also allows you to save your
settings to be reloaded later.
4.1.1 The host name section
4.1.2 Loading and storing saved sessions
4.1.3 ‘Close Window on Exit’

4.1.1 The host name section
The top box on the Session panel, labelled ‘Specify your connection by
host name’, contains the details that need to be filled in before PuTTY
can open a session at all.
The ‘Host Name’ box is where you type the name, or the IP address,
of the server you want to connect to.
The ‘Connection type’ radio buttons let you choose what type of
connection you want to make: a raw connection, a Telnet connection,
an Rlogin connection, an SSH connection, or a connection to a local
serial line. (See section 1.2 for a summary of the differences
between SSH, Telnet and rlogin; see section 3.6 for an explanation
of ‘raw’ connections; see section 3.7 for information about using a
serial line.)
The ‘Port’ box lets you specify which port number on the server to
connect to. If you select Telnet, Rlogin, or SSH, this box will be filled
in automatically to the usual value, and you will only need to change
it if you have an unusual server. If you select Raw mode, you will
almost certainly need to fill in the ‘Port’ box yourself.
If you select ‘Serial’ from the ‘Connection type’ radio buttons, the ‘Host
Name’ and ‘Port’ boxes are replaced by ‘Serial line’ and ‘Speed’; see
section 4.28 for more details of these.

4.1.2 Loading and storing saved sessions
The next part of the Session configuration panel allows you to save your
preferred PuTTY options so they will appear automatically the next time
you start PuTTY. It also allows you to create saved sessions, which
contain a full set of configuration options plus a host name and protocol.
A saved session contains all the information PuTTY needs to start exactly
the session you want.
To save your default settings: first set up the settings the way you
want them saved. Then come back to the Session panel. Select the
‘Default Settings’ entry in the saved sessions list, with a single click.
Then press the ‘Save’ button.
If there is a specific host you want to store the details of how to connect
to, you should create a saved session, which will be separate from the
Default Settings.
To save a session: first go through the rest of the configuration box
setting up all the options you want. Then come back to the Session
panel. Enter a name for the saved session in the ‘Saved Sessions’
input box. (The server name is often a good choice for a saved
session name.) Then press the ‘Save’ button. Your saved session
name should now appear in the list box.
You can also save settings in mid-session, from the ‘Change
Settings’ dialog. Settings changed since the start of the session will
be saved with their current values; as well as settings changed
through the dialog, this includes changes in window size, window
title changes sent by the server, and so on.
To reload a saved session: single-click to select the session name in
the list box, and then press the ‘Load’ button. Your saved settings
should all appear in the configuration panel.
To modify a saved session: first load it as described above. Then
make the changes you want. Come back to the Session panel, and
press the ‘Save’ button. The new settings will be saved over the top
of the old ones.

To save the new settings under a different name, you can enter the
new name in the ‘Saved Sessions’ box, or single-click to select a
session name in the list box to overwrite that session. To save
‘Default Settings’, you must single-click the name before saving.
To start a saved session immediately: double-click on the session
name in the list box.
To delete a saved session: single-click to select the session name in
the list box, and then press the ‘Delete’ button.
Each saved session is independent of the Default Settings configuration.
If you change your preferences and update Default Settings, you must
also update every saved session separately.
Saved sessions are stored in the Registry, at the location
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\Sessions

If you need to store them in a file, you could try the method described in
section 4.29.

4.1.3 ‘Close Window on Exit’
Finally in the Session panel, there is an option labelled ‘Close Window on
Exit’. This controls whether the PuTTY terminal window disappears as
soon as the session inside it terminates. If you are likely to want to copy
and paste text out of the session after it has terminated, or restart the
session, you should arrange for this option to be off.
‘Close Window On Exit’ has three settings. ‘Always’ means always close
the window on exit; ‘Never’ means never close on exit (always leave the
window open, but inactive). The third setting, and the default one, is ‘Only
on clean exit’. In this mode, a session which terminates normally will
cause its window to close, but one which is aborted unexpectedly by
network trouble or a confusing message from the server will leave the
window up.

4.2 The Logging panel
The Logging configuration panel allows you to save log files of your
PuTTY sessions, for debugging, analysis or future reference.
The main option is a radio-button set that specifies whether PuTTY will
log anything at all. The options are:
‘None’. This is the default option; in this mode PuTTY will not create
a log file at all.
‘Printable output’. In this mode, a log file will be created and written
to, but only printable text will be saved into it. The various terminal
control codes that are typically sent down an interactive session
alongside the printable text will be omitted. This might be a useful
mode if you want to read a log file in a text editor and hope to be
able to make sense of it.
‘All session output’. In this mode, everything sent by the server into
your terminal session is logged. If you view the log file in a text
editor, therefore, you may well find it full of strange control
characters. This is a particularly useful mode if you are experiencing
problems with PuTTY's terminal handling: you can record everything
that went to the terminal, so that someone else can replay the
session later in slow motion and watch to see what went wrong.
‘SSH packets’. In this mode (which is only used by SSH
connections), the SSH message packets sent over the encrypted
connection are written to the log file (as well as Event Log entries).
You might need this to debug a network-level problem, or more likely
to send to the PuTTY authors as part of a bug report. BE WARNED
that if you log in using a password, the password can appear in the
log file; see section 4.2.4 for options that may help to remove
sensitive material from the log file before you send it to anyone else.
‘SSH packets and raw data’. In this mode, as well as the decrypted
packets (as in the previous mode), the raw (encrypted, compressed,
etc) packets are also logged. This could be useful to diagnose
corruption in transit. (The same caveats as the previous mode apply,
of course.)
Note that the non-SSH logging options (‘Printable output’ and ‘All session

output’) only work with PuTTY proper; in programs without terminal
emulation (such as Plink), they will have no effect, even if enabled via
saved settings.
4.2.1 ‘Log file name’
4.2.2 ‘What to do if the log file already exists’
4.2.3 ‘Flush log file frequently’
4.2.4 Options specific to SSH packet logging
4.2.4.1 ‘Omit known password fields’
4.2.4.2 ‘Omit session data’

4.2.1 ‘Log file name’
In this edit box you enter the name of the file you want to log the session
to. The ‘Browse’ button will let you look around your file system to find the
right place to put the file; or if you already know exactly where you want it
to go, you can just type a pathname into the edit box.
There are a few special features in this box. If you use the & character in
the file name box, PuTTY will insert details of the current session in the
name of the file it actually opens. The precise replacements it will do are:
will be replaced by the current year, as four digits.
will be replaced by the current month, as two digits.
will be replaced by the current day of the month, as two digits.
will be replaced by the current time, as six digits (HHMMSS) with
no punctuation.
&H will be replaced by the host name you are connecting to.
&P will be replaced by the port number you are connecting to on the
target host.
&Y
&M
&D
&T

For example, if you enter the host name c:\puttylogs\log-&h-&y&m&d&t.dat, you will end up with files looking like
log-server1.example.com-20010528-110859.dat
log-unixbox.somewhere.org-20010611-221001.dat

4.2.2 ‘What to do if the log file already exists’
This control allows you to specify what PuTTY should do if it tries to start
writing to a log file and it finds the file already exists. You might want to
automatically destroy the existing log file and start a new one with the
same name. Alternatively, you might want to open the existing log file and
add data to the end of it. Finally (the default option), you might not want
to have any automatic behaviour, but to ask the user every time the
problem comes up.

4.2.3 ‘Flush log file frequently’
This option allows you to control how frequently logged data is flushed to
disc. By default, PuTTY will flush data as soon as it is displayed, so that if
you view the log file while a session is still open, it will be up to date; and
if the client system crashes, there's a greater chance that the data will be
preserved.
However, this can incur a performance penalty. If PuTTY is running
slowly with logging enabled, you could try unchecking this option. Be
warned that the log file may not always be up to date as a result
(although it will of course be flushed when it is closed, for instance at the
end of a session).

4.2.4 Options specific to SSH packet logging
These options only apply if SSH packet data is being logged.
The following options allow particularly sensitive portions of unencrypted
packets to be automatically left out of the log file. They are only intended
to deter casual nosiness; an attacker could glean a lot of useful
information from even these obfuscated logs (e.g., length of password).
4.2.4.1 ‘Omit known password fields’
4.2.4.2 ‘Omit session data’

4.2.4.1 ‘Omit known password fields’
When checked, decrypted password fields are removed from the log of
transmitted packets. (This includes any user responses to challengeresponse authentication methods such as ‘keyboard-interactive’.) This
does not include X11 authentication data if using X11 forwarding.
Note that this will only omit data that PuTTY knows to be a password.
However, if you start another login session within your PuTTY session,
for instance, any password used will appear in the clear in the packet log.
The next option may be of use to protect against this.
This option is enabled by default.

4.2.4.2 ‘Omit session data’
When checked, all decrypted ‘session data’ is omitted; this is defined as
data in terminal sessions and in forwarded channels (TCP, X11, and
authentication agent). This will usually substantially reduce the size of the
resulting log file.
This option is disabled by default.

4.3 The Terminal panel
The Terminal configuration panel allows you to control the behaviour of
PuTTY's terminal emulation.
4.3.1 ‘Auto wrap mode initially on’
4.3.2 ‘DEC Origin Mode initially on’
4.3.3 ‘Implicit CR in every LF’
4.3.4 ‘Implicit LF in every CR’
4.3.5 ‘Use background colour to erase screen’
4.3.6 ‘Enable blinking text’
4.3.7 ‘Answerback to ^E’
4.3.8 ‘Local echo’
4.3.9 ‘Local line editing’
4.3.10 Remote-controlled printing

4.3.1 ‘Auto wrap mode initially on’
Auto wrap mode controls what happens when text printed in a PuTTY
window reaches the right-hand edge of the window.
With auto wrap mode on, if a long line of text reaches the right-hand
edge, it will wrap over on to the next line so you can still see all the text.
With auto wrap mode off, the cursor will stay at the right-hand edge of the
screen, and all the characters in the line will be printed on top of each
other.
If you are running a full-screen application and you occasionally find the
screen scrolling up when it looks as if it shouldn't, you could try turning
this option off.
Auto wrap mode can be turned on and off by control sequences sent by
the server. This configuration option controls the default state, which will
be restored when you reset the terminal (see section 3.1.3.6). However, if
you modify this option in mid-session using ‘Change Settings’, it will take
effect immediately.

4.3.2 ‘DEC Origin Mode initially on’
DEC Origin Mode is a minor option which controls how PuTTY interprets
cursor-position control sequences sent by the server.
The server can send a control sequence that restricts the scrolling region
of the display. For example, in an editor, the server might reserve a line at
the top of the screen and a line at the bottom, and might send a control
sequence that causes scrolling operations to affect only the remaining
lines.
With DEC Origin Mode on, cursor coordinates are counted from the top
of the scrolling region. With it turned off, cursor coordinates are counted
from the top of the whole screen regardless of the scrolling region.
It is unlikely you would need to change this option, but if you find a fullscreen application is displaying pieces of text in what looks like the wrong
part of the screen, you could try turning DEC Origin Mode on to see
whether that helps.
DEC Origin Mode can be turned on and off by control sequences sent by
the server. This configuration option controls the default state, which will
be restored when you reset the terminal (see section 3.1.3.6). However, if
you modify this option in mid-session using ‘Change Settings’, it will take
effect immediately.

4.3.3 ‘Implicit CR in every LF’
Most servers send two control characters, CR and LF, to start a new line
of the screen. The CR character makes the cursor return to the left-hand
side of the screen. The LF character makes the cursor move one line
down (and might make the screen scroll).
Some servers only send LF, and expect the terminal to move the cursor
over to the left automatically. If you come across a server that does this,
you will see a stepped effect on the screen, like this:
First line of text
Second line
Third line

If this happens to you, try enabling the ‘Implicit CR in every LF’ option,
and things might go back to normal:
First line of text
Second line
Third line

4.3.4 ‘Implicit LF in every CR’
Most servers send two control characters, CR and LF, to start a new line
of the screen. The CR character makes the cursor return to the left-hand
side of the screen. The LF character makes the cursor move one line
down (and might make the screen scroll).
Some servers only send CR, and so the newly written line is overwritten
by the following line. This option causes a line feed so that all lines are
displayed.

4.3.5 ‘Use background colour to erase screen’
Not all terminals agree on what colour to turn the screen when the server
sends a ‘clear screen’ sequence. Some terminals believe the screen
should always be cleared to the default background colour. Others
believe the screen should be cleared to whatever the server has selected
as a background colour.
There exist applications that expect both kinds of behaviour. Therefore,
PuTTY can be configured to do either.
With this option disabled, screen clearing is always done in the default
background colour. With this option enabled, it is done in the current
background colour.
Background-colour erase can be turned on and off by control sequences
sent by the server. This configuration option controls the default state,
which will be restored when you reset the terminal (see section 3.1.3.6).
However, if you modify this option in mid-session using ‘Change
Settings’, it will take effect immediately.

4.3.6 ‘Enable blinking text’
The server can ask PuTTY to display text that blinks on and off. This is
very distracting, so PuTTY allows you to turn blinking text off completely.
When blinking text is disabled and the server attempts to make some text
blink, PuTTY will instead display the text with a bolded background
colour.
Blinking text can be turned on and off by control sequences sent by the
server. This configuration option controls the default state, which will be
restored when you reset the terminal (see section 3.1.3.6). However, if
you modify this option in mid-session using ‘Change Settings’, it will take
effect immediately.

4.3.7 ‘Answerback to ^E’
This option controls what PuTTY will send back to the server if the server
sends it the ^E enquiry character. Normally it just sends the string
‘PuTTY’.
If you accidentally write the contents of a binary file to your terminal, you
will probably find that it contains more than one ^E character, and as a
result your next command line will probably read ‘PuTTYPuTTYPuTTY...’
as if you had typed the answerback string multiple times at the keyboard.
If you set the answerback string to be empty, this problem should go
away, but doing so might cause other problems.
Note that this is not the feature of PuTTY which the server will typically
use to determine your terminal type. That feature is the ‘Terminal-type
string’ in the Connection panel; see section 4.14.3 for details.
You can include control characters in the answerback string using ^C
notation. (Use ^~ to get a literal ^.)

4.3.8 ‘Local echo’
With local echo disabled, characters you type into the PuTTY window are
not echoed in the window by PuTTY. They are simply sent to the server.
(The server might choose to echo them back to you; this can't be
controlled from the PuTTY control panel.)
Some types of session need local echo, and many do not. In its default
mode, PuTTY will automatically attempt to deduce whether or not local
echo is appropriate for the session you are working in. If you find it has
made the wrong decision, you can use this configuration option to
override its choice: you can force local echo to be turned on, or force it to
be turned off, instead of relying on the automatic detection.

4.3.9 ‘Local line editing’
Normally, every character you type into the PuTTY window is sent
immediately to the server the moment you type it.
If you enable local line editing, this changes. PuTTY will let you edit a
whole line at a time locally, and the line will only be sent to the server
when you press Return. If you make a mistake, you can use the
Backspace key to correct it before you press Return, and the server will
never see the mistake.
Since it is hard to edit a line locally without being able to see it, local line
editing is mostly used in conjunction with local echo (section 4.3.8). This
makes it ideal for use in raw mode or when connecting to MUDs or
talkers. (Although some more advanced MUDs do occasionally turn local
line editing on and turn local echo off, in order to accept a password from
the user.)
Some types of session need local line editing, and many do not. In its
default mode, PuTTY will automatically attempt to deduce whether or not
local line editing is appropriate for the session you are working in. If you
find it has made the wrong decision, you can use this configuration option
to override its choice: you can force local line editing to be turned on, or
force it to be turned off, instead of relying on the automatic detection.

4.3.10 Remote-controlled printing
A lot of VT100-compatible terminals support printing under control of the
remote server (sometimes called ‘passthrough printing’). PuTTY supports
this feature as well, but it is turned off by default.
To enable remote-controlled printing, choose a printer from the ‘Printer to
send ANSI printer output to’ drop-down list box. This should allow you to
select from all the printers you have installed drivers for on your
computer. Alternatively, you can type the network name of a networked
printer (for example, \\printserver\printer1) even if you haven't already
installed a driver for it on your own machine.
When the remote server attempts to print some data, PuTTY will send
that data to the printer raw - without translating it, attempting to format it,
or doing anything else to it. It is up to you to ensure your remote server
knows what type of printer it is talking to.
Since PuTTY sends data to the printer raw, it cannot offer options such
as portrait versus landscape, print quality, or paper tray selection. All
these things would be done by your PC printer driver (which PuTTY
bypasses); if you need them done, you will have to find a way to
configure your remote server to do them.
To disable remote printing again, choose ‘None (printing disabled)’ from
the printer selection list. This is the default state.

4.4 The Keyboard panel
The Keyboard configuration panel allows you to control the behaviour of
the keyboard in PuTTY. The correct state for many of these settings
depends on what the server to which PuTTY is connecting expects. With
a Unix server, this is likely to depend on the termcap or terminfo entry it
uses, which in turn is likely to be controlled by the ‘Terminal-type string’
setting in the Connection panel; see section 4.14.3 for details. If none of
the settings here seems to help, you may find question A.7.13 to be
useful.
4.4.1 Changing the action of the Backspace key
4.4.2 Changing the action of the Home and End keys
4.4.3 Changing the action of the function keys and keypad
4.4.4 Controlling Application Cursor Keys mode
4.4.5 Controlling Application Keypad mode
4.4.6 Using NetHack keypad mode
4.4.7 Enabling a DEC-like Compose key
4.4.8 ‘Control-Alt is different from AltGr’

4.4.1 Changing the action of the Backspace key
Some terminals believe that the Backspace key should send the same
thing to the server as Control-H (ASCII code 8). Other terminals believe
that the Backspace key should send ASCII code 127 (usually known as
Control-?) so that it can be distinguished from Control-H. This option
allows you to choose which code PuTTY generates when you press
Backspace.
If you are connecting over SSH, PuTTY by default tells the server the
value of this option (see section 4.24.2), so you may find that the
Backspace key does the right thing either way. Similarly, if you are
connecting to a Unix system, you will probably find that the Unix stty
command lets you configure which the server expects to see, so again
you might not need to change which one PuTTY generates. On other
systems, the server's expectation might be fixed and you might have no
choice but to configure PuTTY.
If you do have the choice, we recommend configuring PuTTY to generate
Control-? and configuring the server to expect it, because that allows
applications such as emacs to use Control-H for help.
(Typing Shift-Backspace will cause PuTTY to send whichever code isn't
configured here as the default.)

4.4.2 Changing the action of the Home and End
keys
The Unix terminal emulator rxvt disagrees with the rest of the world
about what character sequences should be sent to the server by the
Home and End keys.
xterm,

and other terminals, send ESC [1~ for the Home key, and ESC [4~
for the End key. rxvt sends ESC [H for the Home key and ESC [Ow for the
End key.
If you find an application on which the Home and End keys aren't
working, you could try switching this option to see if it helps.

4.4.3 Changing the action of the function keys
and keypad
This option affects the function keys (F1 to F12) and the top row of the
numeric keypad.
In the default mode, labelled ESC [n~, the function keys generate
sequences like ESC [11~, ESC [12~ and so on. This matches the
general behaviour of Digital's terminals.
In Linux mode, F6 to F12 behave just like the default mode, but F1 to
F5 generate ESC [[A through to ESC [[E. This mimics the Linux
virtual console.
In Xterm R6 mode, F5 to F12 behave like the default mode, but F1 to
F4 generate ESC OP through to ESC OS, which are the sequences
produced by the top row of the keypad on Digital's terminals.
In VT400 mode, all the function keys behave like the default mode,
but the actual top row of the numeric keypad generates ESC OP
through to ESC OS.
In VT100+ mode, the function keys generate ESC OP through to ESC
O[

In SCO mode, the function keys F1 to F12 generate ESC [M through
to ESC [X. Together with shift, they generate ESC [Y through to ESC
[j. With control they generate ESC [k through to ESC [v, and with
shift and control together they generate ESC [w through to ESC [{.
If you don't know what any of this means, you probably don't need to
fiddle with it.

4.4.4 Controlling Application Cursor Keys mode
Application Cursor Keys mode is a way for the server to change the
control sequences sent by the arrow keys. In normal mode, the arrow
keys send ESC [A through to ESC [D. In application mode, they send ESC
OA through to ESC OD.
Application Cursor Keys mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the initial
state.
You can also disable application cursor keys mode completely, using the
‘Features’ configuration panel; see section 4.6.1.

4.4.5 Controlling Application Keypad mode
Application Keypad mode is a way for the server to change the behaviour
of the numeric keypad.
In normal mode, the keypad behaves like a normal Windows keypad:
with NumLock on, the number keys generate numbers, and with
NumLock off they act like the arrow keys and Home, End etc.
In application mode, all the keypad keys send special control sequences,
including Num Lock. Num Lock stops behaving like Num Lock and
becomes another function key.
Depending on which version of Windows you run, you may find the Num
Lock light still flashes on and off every time you press Num Lock, even
when application mode is active and Num Lock is acting like a function
key. This is unavoidable.
Application keypad mode can be turned on and off by the server,
depending on the application. PuTTY allows you to configure the initial
state.
You can also disable application keypad mode completely, using the
‘Features’ configuration panel; see section 4.6.1.

4.4.6 Using NetHack keypad mode
PuTTY has a special mode for playing NetHack. You can enable it by
selecting ‘NetHack’ in the ‘Initial state of numeric keypad’ control.
In this mode, the numeric keypad keys 1-9 generate the NetHack
movement commands (hjklyubn). The 5 key generates the . command
(do nothing).
In addition, pressing Shift or Ctrl with the keypad keys generate the Shiftor Ctrl-keys you would expect (e.g. keypad-7 generates ‘y’, so Shiftkeypad-7 generates ‘Y’ and Ctrl-keypad-7 generates Ctrl-Y); these
commands tell NetHack to keep moving you in the same direction until
you encounter something interesting.
For some reason, this feature only works properly when Num Lock is on.
We don't know why.

4.4.7 Enabling a DEC-like Compose key
DEC terminals have a Compose key, which provides an easy-toremember way of typing accented characters. You press Compose and
then type two more characters. The two characters are ‘combined’ to
produce an accented character. The choices of character are designed to
be easy to remember; for example, composing ‘e’ and ‘`’ produces the ‘è’
character.
If your keyboard has a Windows Application key, it acts as a Compose
key in PuTTY. Alternatively, if you enable the ‘AltGr acts as Compose key’
option, the AltGr key will become a Compose key.

4.4.8 ‘Control-Alt is different from AltGr’
Some old keyboards do not have an AltGr key, which can make it difficult
to type some characters. PuTTY can be configured to treat the key
combination Ctrl + Left Alt the same way as the AltGr key.
By default, this checkbox is checked, and the key combination Ctrl + Left
Alt does something completely different. PuTTY's usual handling of the
left Alt key is to prefix the Escape (Control-[) character to whatever
character sequence the rest of the keypress would generate. For
example, Alt-A generates Escape followed by a. So Alt-Ctrl-A would
generate Escape, followed by Control-A.
If you uncheck this box, Ctrl-Alt will become a synonym for AltGr, so you
can use it to type extra graphic characters if your keyboard has any.
(However, Ctrl-Alt will never act as a Compose key, regardless of the
setting of ‘AltGr acts as Compose key’ described in section 4.4.7.)

4.5 The Bell panel
The Bell panel controls the terminal bell feature: the server's ability to
cause PuTTY to beep at you.
In the default configuration, when the server sends the character with
ASCII code 7 (Control-G), PuTTY will play the Windows Default Beep
sound. This is not always what you want the terminal bell feature to do;
the Bell panel allows you to configure alternative actions.
4.5.1 ‘Set the style of bell’
4.5.2 ‘Taskbar/caption indication on bell’
4.5.3 ‘Control the bell overload behaviour’

4.5.1 ‘Set the style of bell’
This control allows you to select various different actions to occur on a
terminal bell:
Selecting ‘None’ disables the bell completely. In this mode, the
server can send as many Control-G characters as it likes and
nothing at all will happen.
‘Make default system alert sound’ is the default setting. It causes the
Windows ‘Default Beep’ sound to be played. To change what this
sound is, or to test it if nothing seems to be happening, use the
Sound configurer in the Windows Control Panel.
‘Visual bell’ is a silent alternative to a beeping computer. In this
mode, when the server sends a Control-G, the whole PuTTY window
will flash white for a fraction of a second.
‘Beep using the PC speaker’ is self-explanatory.
‘Play a custom sound file’ allows you to specify a particular sound file
to be used by PuTTY alone, or even by a particular individual PuTTY
session. This allows you to distinguish your PuTTY beeps from any
other beeps on the system. If you select this option, you will also
need to enter the name of your sound file in the edit control ‘Custom
sound file to play as a bell’.

4.5.2 ‘Taskbar/caption indication on bell’
This feature controls what happens to the PuTTY window's entry in the
Windows Taskbar if a bell occurs while the window does not have the
input focus.
In the default state (‘Disabled’) nothing unusual happens.
If you select ‘Steady’, then when a bell occurs and the window is not in
focus, the window's Taskbar entry and its title bar will change colour to let
you know that PuTTY session is asking for your attention. The change of
colour will persist until you select the window, so you can leave several
PuTTY windows minimised in your terminal, go away from your
keyboard, and be sure not to have missed any important beeps when you
get back.
‘Flashing’ is even more eye-catching: the Taskbar entry will continuously
flash on and off until you select the window.

4.5.3 ‘Control the bell overload behaviour’
A common user error in a terminal session is to accidentally run the Unix
command cat (or equivalent) on an inappropriate file type, such as an
executable, image file, or ZIP file. This produces a huge stream of nontext characters sent to the terminal, which typically includes a lot of bell
characters. As a result of this the terminal often doesn't stop beeping for
ten minutes, and everybody else in the office gets annoyed.
To try to avoid this behaviour, or any other cause of excessive beeping,
PuTTY includes a bell overload management feature. In the default
configuration, receiving more than five bell characters in a two-second
period will cause the overload feature to activate. Once the overload
feature is active, further bells will have no effect at all, so the rest of your
binary file will be sent to the screen in silence. After a period of five
seconds during which no further bells are received, the overload feature
will turn itself off again and bells will be re-enabled.
If you want this feature completely disabled, you can turn it off using the
checkbox ‘Bell is temporarily disabled when over-used’.
Alternatively, if you like the bell overload feature but don't agree with the
settings, you can configure the details: how many bells constitute an
overload, how short a time period they have to arrive in to do so, and how
much silent time is required before the overload feature will deactivate
itself.
Bell overload mode is always deactivated by any keypress in the
terminal. This means it can respond to large unexpected streams of data,
but does not interfere with ordinary command-line activities that generate
beeps (such as filename completion).

4.6 The Features panel
PuTTY's terminal emulation is very highly featured, and can do a lot of
things under remote server control. Some of these features can cause
problems due to buggy or strangely configured server applications.
The Features configuration panel allows you to disable some of PuTTY's
more advanced terminal features, in case they cause trouble.
4.6.1 Disabling application keypad and cursor keys
4.6.2 Disabling xterm-style mouse reporting
4.6.3 Disabling remote terminal resizing
4.6.4 Disabling switching to the alternate screen
4.6.5 Disabling remote window title changing
4.6.6 Response to remote window title querying
4.6.7 Disabling remote scrollback clearing
4.6.8 Disabling destructive backspace
4.6.9 Disabling remote character set configuration
4.6.10 Disabling Arabic text shaping
4.6.11 Disabling bidirectional text display

4.6.1 Disabling application keypad and cursor
keys
Application keypad mode (see section 4.4.5) and application cursor keys
mode (see section 4.4.4) alter the behaviour of the keypad and cursor
keys. Some applications enable these modes but then do not deal
correctly with the modified keys. You can force these modes to be
permanently disabled no matter what the server tries to do.

4.6.2 Disabling xterm-style mouse reporting
PuTTY allows the server to send control codes that let it take over the
mouse and use it for purposes other than copy and paste. Applications
which use this feature include the text-mode web browser links, the
Usenet newsreader trn version 4, and the file manager mc (Midnight
Commander).
If you find this feature inconvenient, you can disable it using the ‘Disable
xterm-style mouse reporting’ control. With this box ticked, the mouse will
always do copy and paste in the normal way.
Note that even if the application takes over the mouse, you can still
manage PuTTY's copy and paste by holding down the Shift key while you
select and paste, unless you have deliberately turned this feature off (see
section 4.11.3).

4.6.3 Disabling remote terminal resizing
PuTTY has the ability to change the terminal's size and position in
response to commands from the server. If you find PuTTY is doing this
unexpectedly or inconveniently, you can tell PuTTY not to respond to
those server commands.

4.6.4 Disabling switching to the alternate screen
Many terminals, including PuTTY, support an ‘alternate screen’. This is
the same size as the ordinary terminal screen, but separate. Typically a
screen-based program such as a text editor might switch the terminal to
the alternate screen before starting up. Then at the end of the run, it
switches back to the primary screen, and you see the screen contents
just as they were before starting the editor.
Some people prefer this not to happen. If you want your editor to run in
the same screen as the rest of your terminal activity, you can disable the
alternate screen feature completely.

4.6.5 Disabling remote window title changing
PuTTY has the ability to change the window title in response to
commands from the server. If you find PuTTY is doing this unexpectedly
or inconveniently, you can tell PuTTY not to respond to those server
commands.

4.6.6 Response to remote window title querying
PuTTY can optionally provide the xterm service of allowing server
applications to find out the local window title. This feature is disabled by
default, but you can turn it on if you really want it.
NOTE that this feature is a potential security hazard. If a malicious
application can write data to your terminal (for example, if you merely cat
a file owned by someone else on the server machine), it can change your
window title (unless you have disabled this as mentioned in section 4.6.5)
and then use this service to have the new window title sent back to the
server as if typed at the keyboard. This allows an attacker to fake
keypresses and potentially cause your server-side applications to do
things you didn't want. Therefore this feature is disabled by default, and
we recommend you do not set it to ‘Window title’ unless you really know
what you are doing.
There are three settings for this option:
‘None’
PuTTY makes no response whatsoever to the relevant escape
sequence. This may upset server-side software that is expecting
some sort of response.
‘Empty string’
PuTTY makes a well-formed response, but leaves it blank. Thus,
server-side software that expects a response is kept happy, but an
attacker cannot influence the response string. This is probably the
setting you want if you have no better ideas.
‘Window title’
PuTTY responds with the actual window title. This is dangerous for
the reasons described above.

4.6.7 Disabling remote scrollback clearing
PuTTY has the ability to clear the terminal's scrollback buffer in response
to a command from the server. If you find PuTTY is doing this
unexpectedly or inconveniently, you can tell PuTTY not to respond to that
server command.

4.6.8 Disabling destructive backspace
Normally, when PuTTY receives character 127 (^?) from the server, it will
perform a ‘destructive backspace’: move the cursor one space left and
delete the character under it. This can apparently cause problems in
some applications, so PuTTY provides the ability to configure character
127 to perform a normal backspace (without deleting a character)
instead.

4.6.9 Disabling remote character set
configuration
PuTTY has the ability to change its character set configuration in
response to commands from the server. Some programs send these
commands unexpectedly or inconveniently. In particular, BitchX (an IRC
client) seems to have a habit of reconfiguring the character set to
something other than the user intended.
If you find that accented characters are not showing up the way you
expect them to, particularly if you're running BitchX, you could try
disabling the remote character set configuration commands.

4.6.10 Disabling Arabic text shaping
PuTTY supports shaping of Arabic text, which means that if your server
sends text written in the basic Unicode Arabic alphabet then it will convert
it to the correct display forms before printing it on the screen.
If you are using full-screen software which was not expecting this to
happen (especially if you are not an Arabic speaker and you
unexpectedly find yourself dealing with Arabic text files in applications
which are not Arabic-aware), you might find that the display becomes
corrupted. By ticking this box, you can disable Arabic text shaping so that
PuTTY displays precisely the characters it is told to display.
You may also find you need to disable bidirectional text display; see
section 4.6.11.

4.6.11 Disabling bidirectional text display
PuTTY supports bidirectional text display, which means that if your server
sends text written in a language which is usually displayed from right to
left (such as Arabic or Hebrew) then PuTTY will automatically flip it round
so that it is displayed in the right direction on the screen.
If you are using full-screen software which was not expecting this to
happen (especially if you are not an Arabic speaker and you
unexpectedly find yourself dealing with Arabic text files in applications
which are not Arabic-aware), you might find that the display becomes
corrupted. By ticking this box, you can disable bidirectional text display,
so that PuTTY displays text from left to right in all situations.
You may also find you need to disable Arabic text shaping; see section
4.6.10.

4.7 The Window panel
The Window configuration panel allows you to control aspects of the
PuTTY window.
4.7.1 Setting the size of the PuTTY window
4.7.2 What to do when the window is resized
4.7.3 Controlling scrollback
4.7.4 ‘Push erased text into scrollback’

4.7.1 Setting the size of the PuTTY window
The ‘Columns’ and ‘Rows’ boxes let you set the PuTTY window to a
precise size. Of course you can also drag the window to a new size while
a session is running.

4.7.2 What to do when the window is resized
These options allow you to control what happens when the user tries to
resize the PuTTY window using its window furniture.
There are four options here:
‘Change the number of rows and columns’: the font size will not
change. (This is the default.)
‘Change the size of the font’: the number of rows and columns in the
terminal will stay the same, and the font size will change.
‘Change font size when maximised’: when the window is resized, the
number of rows and columns will change, except when the window is
maximised (or restored), when the font size will change. (In this
mode, holding down the Alt key while resizing will also cause the font
size to change.)
‘Forbid resizing completely’: the terminal will refuse to be resized at
all.

4.7.3 Controlling scrollback
These options let you configure the way PuTTY keeps text after it scrolls
off the top of the screen (see section 3.1.2).
The ‘Lines of scrollback’ box lets you configure how many lines of text
PuTTY keeps. The ‘Display scrollbar’ options allow you to hide the
scrollbar (although you can still view the scrollback using the keyboard as
described in section 3.1.2). You can separately configure whether the
scrollbar is shown in full-screen mode and in normal modes.
If you are viewing part of the scrollback when the server sends more text
to PuTTY, the screen will revert to showing the current terminal contents.
You can disable this behaviour by turning off ‘Reset scrollback on display
activity’. You can also make the screen revert when you press a key, by
turning on ‘Reset scrollback on keypress’.

4.7.4 ‘Push erased text into scrollback’
When this option is enabled, the contents of the terminal screen will be
pushed into the scrollback when a server-side application clears the
screen, so that your scrollback will contain a better record of what was on
your screen in the past.
If the application switches to the alternate screen (see section 4.6.4 for
more about this), then the contents of the primary screen will be visible in
the scrollback until the application switches back again.
This option is enabled by default.

4.8 The Appearance panel
The Appearance configuration panel allows you to control aspects of the
appearance of PuTTY's window.
4.8.1 Controlling the appearance of the cursor
4.8.2 Controlling the font used in the terminal window
4.8.3 ‘Hide mouse pointer when typing in window’
4.8.4 Controlling the window border

4.8.1 Controlling the appearance of the cursor
The ‘Cursor appearance’ option lets you configure the cursor to be a
block, an underline, or a vertical line. A block cursor becomes an empty
box when the window loses focus; an underline or a vertical line becomes
dotted.
The ‘Cursor blinks’ option makes the cursor blink on and off. This works
in any of the cursor modes.

4.8.2 Controlling the font used in the terminal
window
This option allows you to choose what font, in what size, the PuTTY
terminal window uses to display the text in the session.
By default, you will be offered a choice from all the fixed-width fonts
installed on the system, since VT100-style terminal handling expects a
fixed-width font. If you tick the box marked ‘Allow selection of variablepitch fonts’, however, PuTTY will offer variable-width fonts as well: if you
select one of these, the font will be coerced into fixed-size character
cells, which will probably not look very good (but can work OK with some
fonts).

4.8.3 ‘Hide mouse pointer when typing in
window’
If you enable this option, the mouse pointer will disappear if the PuTTY
window is selected and you press a key. This way, it will not obscure any
of the text in the window while you work in your session. As soon as you
move the mouse, the pointer will reappear.
This option is disabled by default, so the mouse pointer remains visible at
all times.

4.8.4 Controlling the window border
PuTTY allows you to configure the appearance of the window border to
some extent.
The checkbox marked ‘Sunken-edge border’ changes the appearance of
the window border to something more like a DOS box: the inside edge of
the border is highlighted as if it sank down to meet the surface inside the
window. This makes the border a little bit thicker as well. It's hard to
describe well. Try it and see if you like it.
You can also configure a completely blank gap between the text in the
window and the border, using the ‘Gap between text and window edge’
control. By default this is set at one pixel. You can reduce it to zero, or
increase it further.

4.9 The Behaviour panel
The Behaviour configuration panel allows you to control aspects of the
behaviour of PuTTY's window.
4.9.1 Controlling the window title
4.9.2 ‘Warn before closing window’
4.9.3 ‘Window closes on ALT-F4’
4.9.4 ‘System menu appears on ALT-Space’
4.9.5 ‘System menu appears on Alt alone’
4.9.6 ‘Ensure window is always on top’
4.9.7 ‘Full screen on Alt-Enter’

4.9.1 Controlling the window title
The ‘Window title’ edit box allows you to set the title of the PuTTY
window. By default the window title will contain the host name followed by
‘PuTTY’, for example server1.example.com - PuTTY. If you want a
different window title, this is where to set it.
PuTTY allows the server to send xterm control sequences which modify
the title of the window in mid-session (unless this is disabled - see
section 4.6.5); the title string set here is therefore only the initial window
title.
As well as the window title, there is also an xterm sequence to modify the
title of the window's icon. This makes sense in a windowing system
where the window becomes an icon when minimised, such as Windows
3.1 or most X Window System setups; but in the Windows 95-like user
interface it isn't as applicable.
By default, PuTTY only uses the server-supplied window title, and
ignores the icon title entirely. If for some reason you want to see both
titles, check the box marked ‘Separate window and icon titles’. If you do
this, PuTTY's window title and Taskbar caption will change into the
server-supplied icon title if you minimise the PuTTY window, and change
back to the server-supplied window title if you restore it. (If the server has
not bothered to supply a window or icon title, none of this will happen.)

4.9.2 ‘Warn before closing window’
If you press the Close button in a PuTTY window that contains a running
session, PuTTY will put up a warning window asking if you really meant
to close the window. A window whose session has already terminated
can always be closed without a warning.
If you want to be able to close a window quickly, you can disable the
‘Warn before closing window’ option.

4.9.3 ‘Window closes on ALT-F4’
By default, pressing ALT-F4 causes the window to close (or a warning
box to appear; see section 4.9.2). If you disable the ‘Window closes on
ALT-F4’ option, then pressing ALT-F4 will simply send a key sequence to
the server.

4.9.4 ‘System menu appears on ALT-Space’
If this option is enabled, then pressing ALT-Space will bring up the PuTTY
window's menu, like clicking on the top left corner. If it is disabled, then
pressing ALT-Space will just send ESC SPACE to the server.
Some accessibility programs for Windows may need this option enabling
to be able to control PuTTY's window successfully. For instance, Dragon
NaturallySpeaking requires it both to open the system menu via voice,
and to close, minimise, maximise and restore the window.

4.9.5 ‘System menu appears on Alt alone’
If this option is enabled, then pressing and releasing ALT will bring up the
PuTTY window's menu, like clicking on the top left corner. If it is disabled,
then pressing and releasing ALT will have no effect.

4.9.6 ‘Ensure window is always on top’
If this option is enabled, the PuTTY window will stay on top of all other
windows.

4.9.7 ‘Full screen on Alt-Enter’
If this option is enabled, then pressing Alt-Enter will cause the PuTTY
window to become full-screen. Pressing Alt-Enter again will restore the
previous window size.
The full-screen feature is also available from the System menu, even
when it is configured not to be available on the Alt-Enter key. See section
3.1.3.7.

4.10 The Translation panel
The Translation configuration panel allows you to control the translation
between the character set understood by the server and the character set
understood by PuTTY.
4.10.1 Controlling character set translation
4.10.2 ‘Treat CJK ambiguous characters as wide’
4.10.3 ‘Caps Lock acts as Cyrillic switch’
4.10.4 Controlling display of line-drawing characters
4.10.5 Controlling copy and paste of line drawing characters

4.10.1 Controlling character set translation
During an interactive session, PuTTY receives a stream of 8-bit bytes
from the server, and in order to display them on the screen it needs to
know what character set to interpret them in. Similarly, PuTTY needs to
know how to translate your keystrokes into the encoding the server
expects. Unfortunately, there is no satisfactory mechanism for PuTTY
and the server to communicate this information, so it must usually be
manually configured.
There are a lot of character sets to choose from. The ‘Remote character
set’ option lets you select one.
By default PuTTY will use the UTF-8 encoding of Unicode, which can
represent pretty much any character; data coming from the server is
interpreted as UTF-8, and keystrokes are sent UTF-8 encoded. This is
what most modern distributions of Linux will expect by default. However,
if this is wrong for your server, you can select a different character set
using this control.
A few other notable character sets are:
The ISO-8859 series are all standard character sets that include
various accented characters appropriate for different sets of
languages.
The Win125x series are defined by Microsoft, for similar purposes. In
particular Win1252 is almost equivalent to ISO-8859-1, but contains
a few extra characters such as matched quotes and the Euro
symbol.
If you want the old IBM PC character set with block graphics and
line-drawing characters, you can select ‘CP437’.
If you need support for a numeric code page which is not listed in the
drop-down list, such as code page 866, then you can try entering its
name manually (CP866 for example) in the list box. If the underlying
version of Windows has the appropriate translation table installed, PuTTY
will use it.

4.10.2 ‘Treat CJK ambiguous characters as wide’
There are some Unicode characters whose width is not well-defined. In
most contexts, such characters should be treated as single-width for the
purposes of wrapping and so on; however, in some CJK contexts, they
are better treated as double-width for historical reasons, and some
server-side applications may expect them to be displayed as such.
Setting this option will cause PuTTY to take the double-width
interpretation.
If you use legacy CJK applications, and you find your lines are wrapping
in the wrong places, or you are having other display problems, you might
want to play with this setting.
This option only has any effect in UTF-8 mode (see section 4.10.1).

4.10.3 ‘Caps Lock acts as Cyrillic switch’
This feature allows you to switch between a US/UK keyboard layout and
a Cyrillic keyboard layout by using the Caps Lock key, if you need to type
(for example) Russian and English side by side in the same document.
Currently this feature is not expected to work properly if your native
keyboard layout is not US or UK.

4.10.4 Controlling display of line-drawing
characters
VT100-series terminals allow the server to send control sequences that
shift temporarily into a separate character set for drawing simple lines
and boxes. However, there are a variety of ways in which PuTTY can
attempt to find appropriate characters, and the right one to use depends
on the locally configured font. In general you should probably try lots of
options until you find one that your particular font supports.
‘Use Unicode line drawing code points’ tries to use the box
characters that are present in Unicode. For good Unicode-supporting
fonts this is probably the most reliable and functional option.
‘Poor man's line drawing’ assumes that the font cannot generate the
line and box characters at all, so it will use the +, - and | characters
to draw approximations to boxes. You should use this option if none
of the other options works.
‘Font has XWindows encoding’ is for use with fonts that have a
special encoding, where the lowest 32 character positions (below the
ASCII printable range) contain the line-drawing characters. This is
unlikely to be the case with any standard Windows font; it will
probably only apply to custom-built fonts or fonts that have been
automatically converted from the X Window System.
‘Use font in both ANSI and OEM modes’ tries to use the same font in
two different character sets, to obtain a wider range of characters.
This doesn't always work; some fonts claim to be a different size
depending on which character set you try to use.
‘Use font in OEM mode only’ is more reliable than that, but can miss
out other characters from the main character set.

4.10.5 Controlling copy and paste of line
drawing characters
By default, when you copy and paste a piece of the PuTTY screen that
contains VT100 line and box drawing characters, PuTTY will paste them
in the form they appear on the screen: either Unicode line drawing code
points, or the ‘poor man's’ line-drawing characters +, - and |. The
checkbox ‘Copy and paste VT100 line drawing chars as lqqqk’ disables
this feature, so line-drawing characters will be pasted as the ASCII
characters that were printed to produce them. This will typically mean
they come out mostly as q and x, with a scattering of jklmntuvw at the
corners. This might be useful if you were trying to recreate the same box
layout in another program, for example.
Note that this option only applies to line-drawing characters which were
printed by using the VT100 mechanism. Line-drawing characters that
were received as Unicode code points will paste as Unicode always.

4.11 The Selection panel
The Selection panel allows you to control the way copy and paste work in
the PuTTY window.
4.11.1 Pasting in Rich Text Format
4.11.2 Changing the actions of the mouse buttons
4.11.3 ‘Shift overrides application's use of mouse’
4.11.4 Default selection mode
4.11.5 Configuring word-by-word selection

4.11.1 Pasting in Rich Text Format
If you enable ‘Paste to clipboard in RTF as well as plain text’, PuTTY will
write formatting information to the clipboard as well as the actual text you
copy. The effect of this is that if you paste into (say) a word processor, the
text will appear in the word processor in the same font, colour, and style
(e.g. bold, underline) PuTTY was using to display it.
This option can easily be inconvenient, so by default it is disabled.

4.11.2 Changing the actions of the mouse
buttons
PuTTY's copy and paste mechanism is by default modelled on the Unix
xterm application. The X Window System uses a three-button mouse, and
the convention is that the left button selects, the right button extends an
existing selection, and the middle button pastes.
Windows often only has two mouse buttons, so in PuTTY's default
configuration (‘Compromise’), the right button pastes, and the middle
button (if you have one) extends a selection.
If you have a three-button mouse and you are already used to the xterm
arrangement, you can select it using the ‘Action of mouse buttons’
control.
Alternatively, with the ‘Windows’ option selected, the middle button
extends, and the right button brings up a context menu (on which one of
the options is ‘Paste’). (This context menu is always available by holding
down Ctrl and right-clicking, regardless of the setting of this option.)

4.11.3 ‘Shift overrides application's use of
mouse’
PuTTY allows the server to send control codes that let it take over the
mouse and use it for purposes other than copy and paste. Applications
which use this feature include the text-mode web browser links, the
Usenet newsreader trn version 4, and the file manager mc (Midnight
Commander).
When running one of these applications, pressing the mouse buttons no
longer performs copy and paste. If you do need to copy and paste, you
can still do so if you hold down Shift while you do your mouse clicks.
However, it is possible in theory for applications to even detect and make
use of Shift + mouse clicks. We don't know of any applications that do
this, but in case someone ever writes one, unchecking the ‘Shift
overrides application's use of mouse’ checkbox will cause Shift + mouse
clicks to go to the server as well (so that mouse-driven copy and paste
will be completely disabled).
If you want to prevent the application from taking over the mouse at all,
you can do this using the Features control panel; see section 4.6.2.

4.11.4 Default selection mode
As described in section 3.1.1, PuTTY has two modes of selecting text to
be copied to the clipboard. In the default mode (‘Normal’), dragging the
mouse from point A to point B selects to the end of the line containing A,
all the lines in between, and from the very beginning of the line containing
B. In the other mode (‘Rectangular block’), dragging the mouse between
two points defines a rectangle, and everything within that rectangle is
copied.
Normally, you have to hold down Alt while dragging the mouse to select a
rectangular block. Using the ‘Default selection mode’ control, you can set
rectangular selection as the default, and then you have to hold down Alt
to get the normal behaviour.

4.11.5 Configuring word-by-word selection
PuTTY will select a word at a time in the terminal window if you doubleclick to begin the drag. This panel allows you to control precisely what is
considered to be a word.
Each character is given a class, which is a small number (typically 0, 1 or
2). PuTTY considers a single word to be any number of adjacent
characters in the same class. So by modifying the assignment of
characters to classes, you can modify the word-by-word selection
behaviour.
In the default configuration, the character classes are:
Class 0 contains white space and control characters.
Class 1 contains most punctuation.
Class 2 contains letters, numbers and a few pieces of punctuation
(the double quote, minus sign, period, forward slash and
underscore).
So, for example, if you assign the @ symbol into character class 2, you
will be able to select an e-mail address with just a double click.
In order to adjust these assignments, you start by selecting a group of
characters in the list box. Then enter a class number in the edit box
below, and press the ‘Set’ button.
This mechanism currently only covers ASCII characters, because it isn't
feasible to expand the list to cover the whole of Unicode.
Character class definitions can be modified by control sequences sent by
the server. This configuration option controls the default state, which will
be restored when you reset the terminal (see section 3.1.3.6). However, if
you modify this option in mid-session using ‘Change Settings’, it will take
effect immediately.

4.12 The Colours panel
The Colours panel allows you to control PuTTY's use of colour.
4.12.1 ‘Allow terminal to specify ANSI colours’
4.12.2 ‘Allow terminal to use xterm 256-colour mode’
4.12.3 ‘Indicate bolded text by changing...’
4.12.4 ‘Attempt to use logical palettes’
4.12.5 ‘Use system colours’
4.12.6 Adjusting the colours in the terminal window

4.12.1 ‘Allow terminal to specify ANSI colours’
This option is enabled by default. If it is disabled, PuTTY will ignore any
control sequences sent by the server to request coloured text.
If you have a particularly garish application, you might want to turn this
option off and make PuTTY only use the default foreground and
background colours.

4.12.2 ‘Allow terminal to use xterm 256-colour
mode’
This option is enabled by default. If it is disabled, PuTTY will ignore any
control sequences sent by the server which use the extended 256-colour
mode supported by recent versions of xterm.
If you have an application which is supposed to use 256-colour mode and
it isn't working, you may find you need to tell your server that your
terminal supports 256 colours. On Unix, you do this by ensuring that the
setting of TERM describes a 256-colour-capable terminal. You can check
this using a command such as infocmp:
$ infocmp | grep colors
colors#256, cols#80, it#8, lines#24, pairs#256,

If you do not see ‘colors#256’ in the output, you may need to change your
terminal setting. On modern Linux machines, you could try ‘xterm256color’.

4.12.3 ‘Indicate bolded text by changing...’
When the server sends a control sequence indicating that some text
should be displayed in bold, PuTTY can handle this in several ways. It
can either change the font for a bold version, or use the same font in a
brighter colour, or it can do both (brighten the colour and embolden the
font). This control lets you choose which.
By default bold is indicated by colour, so non-bold text is displayed in light
grey and bold text is displayed in bright white (and similarly in other
colours). If you change the setting to ‘The font’ box, bold and non-bold
text will be displayed in the same colour, and instead the font will change
to indicate the difference. If you select ‘Both’, the font and the colour will
both change.
Some applications rely on ‘bold black’ being distinguishable from a black
background; if you choose ‘The font’, their text may become invisible.

4.12.4 ‘Attempt to use logical palettes’
Logical palettes are a mechanism by which a Windows application
running on an 8-bit colour display can select precisely the colours it
wants instead of going with the Windows standard defaults.
If you are not getting the colours you ask for on an 8-bit display, you can
try enabling this option. However, be warned that it's never worked very
well.

4.12.5 ‘Use system colours’
Enabling this option will cause PuTTY to ignore the configured colours for
‘Default Background/Foreground’ and ‘Cursor Colour/Text’ (see section
4.12.6), instead going with the system-wide defaults.
Note that non-bold and bold text will be the same colour if this option is
enabled. You might want to change to indicating bold text by font
changes (see section 4.12.3).

4.12.6 Adjusting the colours in the terminal
window
The main colour control allows you to specify exactly what colours things
should be displayed in. To modify one of the PuTTY colours, use the list
box to select which colour you want to modify. The RGB values for that
colour will appear on the right-hand side of the list box. Now, if you press
the ‘Modify’ button, you will be presented with a colour selector, in which
you can choose a new colour to go in place of the old one. (You may also
edit the RGB values directly in the edit boxes, if you wish; each value is
an integer from 0 to 255.)
PuTTY allows you to set the cursor colour, the default foreground and
background, and the precise shades of all the ANSI configurable colours
(black, red, green, yellow, blue, magenta, cyan, and white). You can also
modify the precise shades used for the bold versions of these colours;
these are used to display bold text if you have chosen to indicate that by
colour (see section 4.12.3), and can also be used if the server asks
specifically to use them. (Note that ‘Default Bold Background’ is not the
background colour used for bold text; it is only used if the server
specifically asks for a bold background.)

4.13 The Connection panel
The Connection panel allows you to configure options that apply to more
than one type of connection.
4.13.1 Using keepalives to prevent disconnection
4.13.2 ‘Disable Nagle's algorithm’
4.13.3 ‘Enable TCP keepalives’
4.13.4 ‘Internet protocol’
4.13.5 ‘Logical name of remote host’

4.13.1 Using keepalives to prevent
disconnection
If you find your sessions are closing unexpectedly (most often with
‘Connection reset by peer’) after they have been idle for a while, you
might want to try using this option.
Some network routers and firewalls need to keep track of all connections
through them. Usually, these firewalls will assume a connection is dead if
no data is transferred in either direction after a certain time interval. This
can cause PuTTY sessions to be unexpectedly closed by the firewall if no
traffic is seen in the session for some time.
The keepalive option (‘Seconds between keepalives’) allows you to
configure PuTTY to send data through the session at regular intervals, in
a way that does not disrupt the actual terminal session. If you find your
firewall is cutting idle connections off, you can try entering a non-zero
value in this field. The value is measured in seconds; so, for example, if
your firewall cuts connections off after ten minutes then you might want to
enter 300 seconds (5 minutes) in the box.
Note that keepalives are not always helpful. They help if you have a
firewall which drops your connection after an idle period; but if the
network between you and the server suffers from breaks in connectivity
then keepalives can actually make things worse. If a session is idle, and
connectivity is temporarily lost between the endpoints, but the
connectivity is restored before either side tries to send anything, then
there will be no problem - neither endpoint will notice that anything was
wrong. However, if one side does send something during the break, it will
repeatedly try to re-send, and eventually give up and abandon the
connection. Then when connectivity is restored, the other side will find
that the first side doesn't believe there is an open connection any more.
Keepalives can make this sort of problem worse, because they increase
the probability that PuTTY will attempt to send data during a break in
connectivity. (Other types of periodic network activity can cause this
behaviour; in particular, SSH-2 re-keys can have this effect. See section
4.19.2.)

Therefore, you might find that keepalives help connection loss, or you
might find they make it worse, depending on what kind of network
problems you have between you and the server.
Keepalives are only supported in Telnet and SSH; the Rlogin and Raw
protocols offer no way of implementing them. (For an alternative, see
section 4.13.3.)
Note that if you are using SSH-1 and the server has a bug that makes it
unable to deal with SSH-1 ignore messages (see section 4.27.1),
enabling keepalives will have no effect.

4.13.2 ‘Disable Nagle's algorithm’
Nagle's algorithm is a detail of TCP/IP implementations that tries to
minimise the number of small data packets sent down a network
connection. With Nagle's algorithm enabled, PuTTY's bandwidth usage
will be slightly more efficient; with it disabled, you may find you get a
faster response to your keystrokes when connecting to some types of
server.
The Nagle algorithm is disabled by default for interactive connections.

4.13.3 ‘Enable TCP keepalives’
NOTE: TCP keepalives should not be confused with the application-level
keepalives described in section 4.13.1. If in doubt, you probably want
application-level keepalives; TCP keepalives are provided for
completeness.
The idea of TCP keepalives is similar to application-level keepalives, and
the same caveats apply. The main differences are:
TCP keepalives are available on all connection types, including Raw
and Rlogin.
The interval between TCP keepalives is usually much longer,
typically two hours; this is set by the operating system, and cannot
be configured within PuTTY.
If the operating system does not receive a response to a keepalive, it
may send out more in quick succession and terminate the
connection if no response is received.
TCP keepalives may be more useful for ensuring that half-open
connections are terminated than for keeping a connection alive.
TCP keepalives are disabled by default.

4.13.4 ‘Internet protocol’
This option allows the user to select between the old and new Internet
protocols and addressing schemes (IPv4 and IPv6). The selected
protocol will be used for most outgoing network connections (including
connections to proxies); however, tunnels have their own configuration,
for which see section 4.26.2.
The default setting is ‘Auto’, which means PuTTY will do something
sensible and try to guess which protocol you wanted. (If you specify a
literal Internet address, it will use whichever protocol that address
implies. If you provide a hostname, it will see what kinds of address exist
for that hostname; it will use IPv6 if there is an IPv6 address available,
and fall back to IPv4 if not.)
If you need to force PuTTY to use a particular protocol, you can explicitly
set this to ‘IPv4’ or ‘IPv6’.

4.13.5 ‘Logical name of remote host’
This allows you to tell PuTTY that the host it will really end up connecting
to is different from where it thinks it is making a network connection.
You might use this, for instance, if you had set up an SSH port forwarding
in one PuTTY session so that connections to some arbitrary port (say,
localhost port 10022) were forwarded to a second machine's SSH port
(say, foovax port 22), and then started a second PuTTY connecting to the
forwarded port.
In normal usage, the second PuTTY will access the host key cache under
the host name and port it actually connected to (i.e. localhost port 10022
in this example). Using the logical host name option, however, you can
configure the second PuTTY to cache the host key under the name of the
host you know that it's really going to end up talking to (here foovax).
This can be useful if you expect to connect to the same actual server
through many different channels (perhaps because your port forwarding
arrangements keep changing): by consistently setting the logical host
name, you can arrange that PuTTY will not keep asking you to reconfirm
its host key. Conversely, if you expect to use the same local port number
for port forwardings to lots of different servers, you probably didn't want
any particular server's host key cached under that local port number. (For
this latter case, you could instead explicitly configure host keys in the
relevant sessions; see section 4.20.2.)
If you just enter a host name for this option, PuTTY will cache the SSH
host key under the default SSH port for that host, irrespective of the port
you really connected to (since the typical scenario is like the above
example: you connect to a silly real port number and your connection
ends up forwarded to the normal port-22 SSH server of some other
machine). To override this, you can append a port number to the logical
host name, separated by a colon. E.g. entering ‘foovax:2200’ as the
logical host name will cause the host key to be cached as if you had
connected to port 2200 of foovax.
If you provide a host name using this option, it is also displayed in other

locations which contain the remote host name, such as the default
window title and the default SSH password prompt. This reflects the fact
that this is the host you're really connecting to, which is more important
than the mere means you happen to be using to contact that host. (This
applies even if you're using a protocol other than SSH.)

4.14 The Data panel
The Data panel allows you to configure various pieces of data which can
be sent to the server to affect your connection at the far end.
Each option on this panel applies to more than one protocol. Options
which apply to only one protocol appear on that protocol's configuration
panels.
4.14.1 ‘Auto-login username’
4.14.2 Use of system username
4.14.3 ‘Terminal-type string’
4.14.4 ‘Terminal speeds’
4.14.5 Setting environment variables on the server

4.14.1 ‘Auto-login username’
All three of the SSH, Telnet and Rlogin protocols allow you to specify
what user name you want to log in as, without having to type it explicitly
every time. (Some Telnet servers don't support this.)
In this box you can type that user name.

4.14.2 Use of system username
When the previous box (section 4.14.1) is left blank, by default, PuTTY
will prompt for a username at the time you make a connection.
In some environments, such as the networks of large organisations
implementing single sign-on, a more sensible default may be to use the
name of the user logged in to the local operating system (if any); this is
particularly likely to be useful with GSSAPI authentication (see section
4.23). This control allows you to change the default behaviour.
The current system username is displayed in the dialog as a
convenience. It is not saved in the configuration; if a saved session is
later used by a different user, that user's name will be used.

4.14.3 ‘Terminal-type string’
Most servers you might connect to with PuTTY are designed to be
connected to from lots of different types of terminal. In order to send the
right control sequences to each one, the server will need to know what
type of terminal it is dealing with. Therefore, each of the SSH, Telnet and
Rlogin protocols allow a text string to be sent down the connection
describing the terminal. On a Unix server, this selects an entry from the
termcap or terminfo database that tells applications what control
sequences to send to the terminal, and what character sequences to
expect the keyboard to generate.
PuTTY attempts to emulate the Unix xterm program, and by default it
reflects this by sending xterm as a terminal-type string. If you find this is
not doing what you want - perhaps the remote system reports ‘Unknown
terminal type’ - you could try setting this to something different, such as
vt220.
If you're not sure whether a problem is due to the terminal type setting or
not, you probably need to consult the manual for your application or your
server.

4.14.4 ‘Terminal speeds’
The Telnet, Rlogin, and SSH protocols allow the client to specify terminal
speeds to the server.
This parameter does not affect the actual speed of the connection, which
is always ‘as fast as possible’; it is just a hint that is sometimes used by
server software to modify its behaviour. For instance, if a slow speed is
indicated, the server may switch to a less bandwidth-hungry display
mode.
The value is usually meaningless in a network environment, but PuTTY
lets you configure it, in case you find the server is reacting badly to the
default value.
The format is a pair of numbers separated by a comma, for instance,
38400,38400. The first number represents the output speed (from the
server) in bits per second, and the second is the input speed (to the
server). (Only the first is used in the Rlogin protocol.)
This option has no effect on Raw connections.

4.14.5 Setting environment variables on the
server
The Telnet protocol provides a means for the client to pass environment
variables to the server. Many Telnet servers have stopped supporting this
feature due to security flaws, but PuTTY still supports it for the benefit of
any servers which have found other ways around the security problems
than just disabling the whole mechanism.
Version 2 of the SSH protocol also provides a similar mechanism, which
is easier to implement without security flaws. Newer SSH-2 servers are
more likely to support it than older ones.
This configuration data is not used in the SSH-1, rlogin or raw protocols.
To add an environment variable to the list transmitted down the
connection, you enter the variable name in the ‘Variable’ box, enter its
value in the ‘Value’ box, and press the ‘Add’ button. To remove one from
the list, select it in the list box and press ‘Remove’.

4.15 The Proxy panel
The Proxy panel allows you to configure PuTTY to use various types of
proxy in order to make its network connections. The settings in this panel
affect the primary network connection forming your PuTTY session, and
also any extra connections made as a result of SSH port forwarding (see
section 3.5).
Note that unlike some software (such as web browsers), PuTTY does not
attempt to automatically determine whether to use a proxy and (if so)
which one to use for a given destination. If you need to use a proxy, it
must always be explicitly configured.
4.15.1 Setting the proxy type
4.15.2 Excluding parts of the network from proxying
4.15.3 Name resolution when using a proxy
4.15.4 Username and password
4.15.5 Specifying the Telnet or Local proxy command
4.15.6 Controlling proxy logging

4.15.1 Setting the proxy type
The ‘Proxy type’ radio buttons allow you to configure what type of proxy
you want PuTTY to use for its network connections. The default setting is
‘None’; in this mode no proxy is used for any connection.
Selecting ‘HTTP’ allows you to proxy your connections through a
web server supporting the HTTP CONNECT command, as documented
in RFC 2817.
Selecting ‘SOCKS 4’ or ‘SOCKS 5’ allows you to proxy your
connections through a SOCKS server.
Many firewalls implement a less formal type of proxy in which a user
can make a Telnet connection directly to the firewall machine and
enter a command such as connect myhost.com 22 to connect through
to an external host. Selecting ‘Telnet’ allows you to tell PuTTY to use
this type of proxy.
Selecting ‘Local’ allows you to specify an arbitrary command on the
local machine to act as a proxy. When the session is started, instead
of creating a TCP connection, PuTTY runs the command (specified
in section 4.15.5), and uses its standard input and output streams.
This could be used, for instance, to talk to some kind of network
proxy that PuTTY does not natively support; or you could tunnel a
connection over something other than TCP/IP entirely.
If you want your local proxy command to make a secondary SSH
connection to a proxy host and then tunnel the primary connection
over that, you might well want the -nc command-line option in Plink.
See section 3.8.3.14 for more information.
You can also enable this mode on the command line; see section
3.8.3.24.

4.15.2 Excluding parts of the network from
proxying
Typically you will only need to use a proxy to connect to non-local parts of
your network; for example, your proxy might be required for connections
outside your company's internal network. In the ‘Exclude Hosts/IPs’ box
you can enter ranges of IP addresses, or ranges of DNS names, for
which PuTTY will avoid using the proxy and make a direct connection
instead.
The ‘Exclude Hosts/IPs’ box may contain more than one exclusion range,
separated by commas. Each range can be an IP address or a DNS
name, with a * character allowing wildcards. For example:
*.example.com

This excludes any host with a name ending in .example.com from
proxying.
192.168.88.*

This excludes any host with an IP address starting with 192.168.88 from
proxying.
192.168.88.*,*.example.com

This excludes both of the above ranges at once.
Connections to the local host (the host name localhost, and any
loopback IP address) are never proxied, even if the proxy exclude list
does not explicitly contain them. It is very unlikely that this behaviour
would ever cause problems, but if it does you can change it by enabling
‘Consider proxying local host connections’.
Note that if you are doing DNS at the proxy (see section 4.15.3), you
should make sure that your proxy exclusion settings do not depend on
knowing the IP address of a host. If the name is passed on to the proxy
without PuTTY looking it up, it will never know the IP address and cannot
check it against your list.

4.15.3 Name resolution when using a proxy
If you are using a proxy to access a private network, it can make a
difference whether DNS name resolution is performed by PuTTY itself
(on the client machine) or performed by the proxy.
The ‘Do DNS name lookup at proxy end’ configuration option allows you
to control this. If you set it to ‘No’, PuTTY will always do its own DNS, and
will always pass an IP address to the proxy. If you set it to ‘Yes’, PuTTY
will always pass host names straight to the proxy without trying to look
them up first.
If you set this option to ‘Auto’ (the default), PuTTY will do something it
considers appropriate for each type of proxy. Telnet, HTTP, and SOCKS5
proxies will have host names passed straight to them; SOCKS4 proxies
will not.
Note that if you are doing DNS at the proxy, you should make sure that
your proxy exclusion settings (see section 4.15.2) do not depend on
knowing the IP address of a host. If the name is passed on to the proxy
without PuTTY looking it up, it will never know the IP address and cannot
check it against your list.
The original SOCKS 4 protocol does not support proxy-side DNS. There
is a protocol extension (SOCKS 4A) which does support it, but not all
SOCKS 4 servers provide this extension. If you enable proxy DNS and
your SOCKS 4 server cannot deal with it, this might be why.

4.15.4 Username and password
If your proxy requires authentication, you can enter a username and a
password in the ‘Username’ and ‘Password’ boxes.
Note that if you save your session, the proxy password will be saved in
plain text, so anyone who can access your PuTTY configuration data will
be able to discover it.
Authentication is not fully supported for all forms of proxy:
Username and password authentication is supported for HTTP
proxies and SOCKS 5 proxies.
With SOCKS 5, authentication is via CHAP if the proxy supports
it (this is not supported in PuTTYtel); otherwise the password is
sent to the proxy in plain text.
With HTTP proxying, the only currently supported authentication
method is ‘basic’, where the password is sent to the proxy in
plain text.
SOCKS 4 can use the ‘Username’ field, but does not support
passwords.
You can specify a way to include a username and password in the
Telnet/Local proxy command (see section 4.15.5).

4.15.5 Specifying the Telnet or Local proxy
command
If you are using the Telnet proxy type, the usual command required by
the firewall's Telnet server is connect, followed by a host name and a port
number. If your proxy needs a different command, you can enter an
alternative here.
If you are using the Local proxy type, the local command to run is
specified here.
In this string, you can use \n to represent a new-line, \r to represent a
carriage return, \t to represent a tab character, and \x followed by two
hex digits to represent any other character. \\ is used to encode the \
character itself.
Also, the special strings %host and %port will be replaced by the host
name and port number you want to connect to. The strings %user and
%pass will be replaced by the proxy username and password you specify.
The strings %proxyhost and %proxyport will be replaced by the host
details specified on the Proxy panel, if any (this is most likely to be useful
for the Local proxy type). To get a literal % sign, enter %%.
If a Telnet proxy server prompts for a username and password before
commands can be sent, you can use a command such as:
%user\n%pass\nconnect %host %port\n

This will send your username and password as the first two lines to the
proxy, followed by a command to connect to the desired host and port.
Note that if you do not include the %user or %pass tokens in the Telnet
command, then the ‘Username’ and ‘Password’ configuration fields will
be ignored.

4.15.6 Controlling proxy logging
Often the proxy interaction has its own diagnostic output; this is
particularly the case for local proxy commands.
The setting ‘Print proxy diagnostics in the terminal window’ lets you
control how much of the proxy's diagnostics are printed to the main
terminal window, along with output from your main session.
By default (‘No’), proxy diagnostics are only sent to the Event Log; with
‘Yes’ they are also printed to the terminal, where they may get mixed up
with your main session. ‘Only until session starts’ is a compromise; proxy
messages will go to the terminal window until the main session is
deemed to have started (in a protocol-dependent way), which is when
they're most likely to be interesting; any further proxy-related messages
during the session will only go to the Event Log.

4.16 The Telnet panel
The Telnet panel allows you to configure options that only apply to Telnet
sessions.
4.16.1 ‘Handling of OLD_ENVIRON ambiguity’
4.16.2 Passive and active Telnet negotiation modes
4.16.3 ‘Keyboard sends Telnet special commands’
4.16.4 ‘Return key sends Telnet New Line instead of ^M’

4.16.1 ‘Handling of OLD_ENVIRON ambiguity’
The original Telnet mechanism for passing environment variables was
badly specified. At the time the standard (RFC 1408) was written, BSD
telnet implementations were already supporting the feature, and the
intention of the standard was to describe the behaviour the BSD
implementations were already using.
Sadly there was a typing error in the standard when it was issued, and
two vital function codes were specified the wrong way round. BSD
implementations did not change, and the standard was not corrected.
Therefore, it's possible you might find either BSD or RFC-compliant
implementations out there. This switch allows you to choose which one
PuTTY claims to be.
The problem was solved by issuing a second standard, defining a new
Telnet mechanism called NEW_ENVIRON, which behaved exactly like the
original OLD_ENVIRON but was not encumbered by existing
implementations. Most Telnet servers now support this, and it's
unambiguous. This feature should only be needed if you have trouble
passing environment variables to quite an old server.

4.16.2 Passive and active Telnet negotiation
modes
In a Telnet connection, there are two types of data passed between the
client and the server: actual text, and negotiations about which Telnet
extra features to use.
PuTTY can use two different strategies for negotiation:
In active mode, PuTTY starts to send negotiations as soon as the
connection is opened.
In passive mode, PuTTY will wait to negotiate until it sees a
negotiation from the server.
The obvious disadvantage of passive mode is that if the server is also
operating in a passive mode, then negotiation will never begin at all. For
this reason PuTTY defaults to active mode.
However, sometimes passive mode is required in order to successfully
get through certain types of firewall and Telnet proxy server. If you have
confusing trouble with a firewall, you could try enabling passive mode to
see if it helps.

4.16.3 ‘Keyboard sends Telnet special
commands’
If this box is checked, several key sequences will have their normal
actions modified:
the Backspace key on the keyboard will send the Telnet special
backspace code;
Control-C will send the Telnet special Interrupt Process code;
Control-Z will send the Telnet special Suspend Process code.
You probably shouldn't enable this unless you know what you're doing.

4.16.4 ‘Return key sends Telnet New Line
instead of ^M’
Unlike most other remote login protocols, the Telnet protocol has a
special ‘new line’ code that is not the same as the usual line endings of
Control-M or Control-J. By default, PuTTY sends the Telnet New Line
code when you press Return, instead of sending Control-M as it does in
most other protocols.
Most Unix-style Telnet servers don't mind whether they receive Telnet
New Line or Control-M; some servers do expect New Line, and some
servers prefer to see ^M. If you are seeing surprising behaviour when
you press Return in a Telnet session, you might try turning this option off
to see if it helps.

4.17 The Rlogin panel
The Rlogin panel allows you to configure options that only apply to Rlogin
sessions.
4.17.1 ‘Local username’

4.17.1 ‘Local username’
Rlogin allows an automated (password-free) form of login by means of a
file called .rhosts on the server. You put a line in your .rhosts file saying
something like jbloggs@pc1.example.com, and then when you make an
Rlogin connection the client transmits the username of the user running
the Rlogin client. The server checks the username and hostname against
.rhosts, and if they match it does not ask for a password.
This only works because Unix systems contain a safeguard to stop a
user from pretending to be another user in an Rlogin connection. Rlogin
connections have to come from port numbers below 1024, and Unix
systems prohibit this to unprivileged processes; so when the server sees
a connection from a low-numbered port, it assumes the client end of the
connection is held by a privileged (and therefore trusted) process, so it
believes the claim of who the user is.
Windows does not have this restriction: any user can initiate an outgoing
connection from a low-numbered port. Hence, the Rlogin .rhosts
mechanism is completely useless for securely distinguishing several
different users on a Windows machine. If you have a .rhosts entry
pointing at a Windows PC, you should assume that anyone using that PC
can spoof your username in an Rlogin connection and access your
account on the server.
The ‘Local username’ control allows you to specify what user name
PuTTY should claim you have, in case it doesn't match your Windows
user name (or in case you didn't bother to set up a Windows user name).

4.18 The SSH panel
The SSH panel allows you to configure options that only apply to SSH
sessions.
4.18.1 Executing a specific command on the server
4.18.2 ‘Don't start a shell or command at all’
4.18.3 ‘Enable compression’
4.18.4 ‘SSH protocol version’
4.18.5 Sharing an SSH connection between PuTTY tools

4.18.1 Executing a specific command on the
server
In SSH, you don't have to run a general shell session on the server.
Instead, you can choose to run a single specific command (such as a
mail user agent, for example). If you want to do this, enter the command
in the ‘Remote command’ box.
Note that most servers will close the session after executing the
command.

4.18.2 ‘Don't start a shell or command at all’
If you tick this box, PuTTY will not attempt to run a shell or command
after connecting to the remote server. You might want to use this option if
you are only using the SSH connection for port forwarding, and your user
account on the server does not have the ability to run a shell.
This feature is only available in SSH protocol version 2 (since the version
1 protocol assumes you will always want to run a shell).
This feature can also be enabled using the -N command-line option; see
section 3.8.3.13.
If you use this feature in Plink, you will not be able to terminate the Plink
process by any graceful means; the only way to kill it will be by pressing
Control-C or sending a kill signal from another program.

4.18.3 ‘Enable compression’
This enables data compression in the SSH connection: data sent by the
server is compressed before sending, and decompressed at the client
end. Likewise, data sent by PuTTY to the server is compressed first and
the server decompresses it at the other end. This can help make the
most of a low-bandwidth connection.

4.18.4 ‘SSH protocol version’
This allows you to select whether to use SSH protocol version 2 or the
older version 1.
You should normally leave this at the default of ‘2’. As well as having
fewer features, the older SSH-1 protocol is no longer developed, has
many known cryptographic weaknesses, and is generally not considered
to be secure. PuTTY's protocol 1 implementation is provided mainly for
compatibility, and is no longer being enhanced.
If a server offers both versions, prefer ‘2’. If you have some server or
piece of equipment that only talks SSH-1, select ‘1’ here, and do not treat
the resulting connection as secure.
PuTTY will not automatically fall back to the other version of the protocol
if the server turns out not to match your selection here; instead, it will put
up an error message and abort the connection. This prevents an active
attacker downgrading an intended SSH-2 connection to SSH-1.

4.18.5 Sharing an SSH connection between
PuTTY tools
The controls in this box allow you to configure PuTTY to reuse an
existing SSH connection, where possible.
The SSH-2 protocol permits you to run multiple data channels over the
same SSH connection, so that you can log in just once (and do the
expensive encryption setup just once) and then have more than one
terminal window open.
Each instance of PuTTY can still run at most one terminal session, but
using the controls in this box, you can configure PuTTY to check if
another instance of itself has already connected to the target host, and if
so, share that instance's SSH connection instead of starting a separate
new one.
To enable this feature, just tick the box ‘Share SSH connections if
possible’. Then, whenever you start up a PuTTY session connecting to a
particular host, it will try to reuse an existing SSH connection if one is
available. For example, selecting ‘Duplicate Session’ from the system
menu will launch another session on the same host, and if sharing is
enabled then it will reuse the existing SSH connection.
When this mode is in use, the first PuTTY that connected to a given
server becomes the ‘upstream’, which means that it is the one managing
the real SSH connection. All subsequent PuTTYs which reuse the
connection are referred to as ‘downstreams’: they do not connect to the
real server at all, but instead connect to the upstream PuTTY via local
inter-process communication methods.
For this system to be activated, both the upstream and downstream
instances of PuTTY must have the sharing option enabled.
The upstream PuTTY can therefore not terminate until all its
downstreams have closed. This is similar to the effect you get with port
forwarding or X11 forwarding, in which a PuTTY whose terminal session
has already finished will still remain open so as to keep serving

forwarded connections.
In case you need to configure this system in more detail, there are two
additional checkboxes which allow you to specify whether a particular
PuTTY can act as an upstream or a downstream or both. (These boxes
only take effect if the main ‘Share SSH connections if possible’ box is
also ticked.) By default both of these boxes are ticked, so that multiple
PuTTYs started from the same configuration will designate one of
themselves as the upstream and share a single connection; but if for
some reason you need a particular PuTTY configuration not to be an
upstream (e.g. because you definitely need it to close promptly) or not to
be a downstream (e.g. because it needs to do its own authentication
using a special private key) then you can untick one or the other of these
boxes.
I have referred to ‘PuTTY’ throughout the above discussion, but all the
other PuTTY tools which make SSH connections can use this
mechanism too. For example, if PSCP or PSFTP loads a configuration
with sharing enabled, then it can act as a downstream and use an
existing SSH connection set up by an instance of GUI PuTTY. The one
special case is that PSCP and PSFTP will never act as upstreams.
It is possible to test programmatically for the existence of a live upstream
using Plink. See section 7.2.3.3.

4.19 The Kex panel
The Kex panel (short for ‘key exchange’) allows you to configure options
related to SSH-2 key exchange.
Key exchange occurs at the start of an SSH connection (and occasionally
thereafter); it establishes a shared secret that is used as the basis for all
of SSH's security features. It is therefore very important for the security of
the connection that the key exchange is secure.
Key exchange is a cryptographically intensive process; if either the client
or the server is a relatively slow machine, the slower methods may take
several tens of seconds to complete.
If connection startup is too slow, or the connection hangs periodically, you
may want to try changing these settings.
If you don't understand what any of this means, it's safe to leave these
settings alone.
This entire panel is only relevant to SSH protocol version 2; none of
these settings affect SSH-1 at all.
4.19.1 Key exchange algorithm selection
4.19.2 Repeat key exchange

4.19.1 Key exchange algorithm selection
PuTTY supports a variety of SSH-2 key exchange methods, and allows
you to choose which one you prefer to use; configuration is similar to
cipher selection (see section 4.21).
PuTTY currently supports the following key exchange methods:
‘ECDH’: elliptic curve Diffie-Hellman key exchange.
‘Group 14’: Diffie-Hellman key exchange with a well-known 2048-bit
group.
‘Group 1’: Diffie-Hellman key exchange with a well-known 1024-bit
group. We no longer recommend using this method, and it's not used
by default in new installations; however, it may be the only method
supported by very old server software.
‘Group exchange’: with this method, instead of using a fixed group,
PuTTY requests that the server suggest a group to use for key
exchange; the server can avoid groups known to be weak, and
possibly invent new ones over time, without any changes required to
PuTTY's configuration. We recommend use of this method instead of
the well-known groups, if possible.
‘RSA key exchange’: this requires much less computational effort on
the part of the client, and somewhat less on the part of the server,
than Diffie-Hellman key exchange.
If the first algorithm PuTTY finds is below the ‘warn below here’ line, you
will see a warning box when you make the connection, similar to that for
cipher selection (see section 4.21).

4.19.2 Repeat key exchange
If the session key negotiated at connection startup is used too much or
for too long, it may become feasible to mount attacks against the SSH
connection. Therefore, the SSH-2 protocol specifies that a new key
exchange should take place every so often; this can be initiated by either
the client or the server.
While this renegotiation is taking place, no data can pass through the
SSH connection, so it may appear to ‘freeze’. (The occurrence of repeat
key exchange is noted in the Event Log; see section 3.1.3.1.) Usually the
same algorithm is used as at the start of the connection, with a similar
overhead.
These options control how often PuTTY will initiate a repeat key
exchange (‘rekey’). You can also force a key exchange at any time from
the Special Commands menu (see section 3.1.3.2).
‘Max minutes before rekey’ specifies the amount of time that is
allowed to elapse before a rekey is initiated. If this is set to zero,
PuTTY will not rekey due to elapsed time. The SSH-2 protocol
specification recommends a timeout of at most 60 minutes.
You might have a need to disable time-based rekeys completely for the
same reasons that keepalives aren't always helpful. If you anticipate
suffering a network dropout of several hours in the middle of an SSH
connection, but were not actually planning to send data down that
connection during those hours, then an attempted rekey in the middle of
the dropout will probably cause the connection to be abandoned,
whereas if rekeys are disabled then the connection should in principle
survive (in the absence of interfering firewalls). See section 4.13.1 for
more discussion of these issues; for these purposes, rekeys have much
the same properties as keepalives. (Except that rekeys have
cryptographic value in themselves, so you should bear that in mind when
deciding whether to turn them off.) Note, however, the the SSH server
can still initiate rekeys.
‘Max data before rekey’ specifies the amount of data (in bytes) that is

permitted to flow in either direction before a rekey is initiated. If this
is set to zero, PuTTY will not rekey due to transferred data. The
SSH-2 protocol specification recommends a limit of at most 1
gigabyte.
As well as specifying a value in bytes, the following shorthand can
be used:
‘1k’ specifies 1 kilobyte (1024 bytes).
‘1M’ specifies 1 megabyte (1024 kilobytes).
‘1G’ specifies 1 gigabyte (1024 megabytes).
Disabling data-based rekeys entirely is a bad idea. The integrity, and to a
lesser extent, confidentiality of the SSH-2 protocol depend in part on
rekeys occurring before a 32-bit packet sequence number wraps around.
Unlike time-based rekeys, data-based rekeys won't occur when the SSH
connection is idle, so they shouldn't cause the same problems. The SSH1 protocol, incidentally, has even weaker integrity protection than SSH-2
without rekeys.

4.20 The Host Keys panel
The Host Keys panel allows you to configure options related to SSH-2
host key management.
Host keys are used to prove the server's identity, and assure you that the
server is not being spoofed (either by a man-in-the-middle attack or by
completely replacing it on the network). See section 2.2 for a basic
introduction to host keys.
This entire panel is only relevant to SSH protocol version 2; none of
these settings affect SSH-1 at all.
4.20.1 Host key type selection
4.20.2 Manually configuring host keys

4.20.1 Host key type selection
PuTTY supports a variety of SSH-2 host key types, and allows you to
choose which one you prefer to use to identify the server. Configuration is
similar to cipher selection (see section 4.21).
PuTTY currently supports the following host key types:
‘Ed25519’: Edwards-curve DSA using a twisted Edwards curve with
modulus 2^255-19.
‘ECDSA’: elliptic curve DSA using one of the NIST-standardised
elliptic curves.
‘DSA’: straightforward DSA using modular exponentiation.
‘RSA’: the ordinary RSA algorithm.
If PuTTY already has one or more host keys stored for the server, it will
prefer to use one of those, even if the server has a key type that is higher
in the preference order. You can add such a key to PuTTY's cache from
within an existing session using the ‘Special Commands’ menu; see
section 3.1.3.2.
Otherwise, PuTTY will choose a key type based purely on the preference
order you specify in the configuration.
If the first key type PuTTY finds is below the ‘warn below here’ line, you
will see a warning box when you make the connection, similar to that for
cipher selection (see section 4.21).

4.20.2 Manually configuring host keys
In some situations, if PuTTY's automated host key management is not
doing what you need, you might need to manually configure PuTTY to
accept a specific host key, or one of a specific set of host keys.
One reason why you might want to do this is because the host name
PuTTY is connecting to is using round-robin DNS to return one of
multiple actual servers, and they all have different host keys. In that
situation, you might need to configure PuTTY to accept any of a list of
host keys for the possible servers, while still rejecting any key not in that
list.
Another reason is if PuTTY's automated host key management is
completely unavailable, e.g. because PuTTY (or Plink or PSFTP, etc) is
running in a Windows environment without access to the Registry. In that
situation, you will probably want to use the -hostkey command-line option
to configure the expected host key(s); see section 3.8.3.20.
For situations where PuTTY's automated host key management simply
picks the wrong host name to store a key under, you may want to
consider setting a ‘logical host name’ instead; see section 4.13.5.
To configure manual host keys via the GUI, enter some text describing
the host key into the edit box in the ‘Manually configure host keys for this
connection’ container, and press the ‘Add’ button. The text will appear in
the ‘Host keys or fingerprints to accept’ list box. You can remove keys
again with the ‘Remove’ button.
The text describing a host key can be in one of the following formats:
An MD5-based host key fingerprint of the form displayed in PuTTY's
Event Log and host key dialog boxes, i.e. sixteen 2-digit hex
numbers separated by colons.
A base64-encoded blob describing an SSH-2 public key in
OpenSSH's one-line public key format. How you acquire a public key
in this format is server-dependent; on an OpenSSH server it can
typically be found in a location like /etc/ssh/ssh_host_rsa_key.pub.

If this box contains at least one host key or fingerprint when PuTTY
makes an SSH connection, then PuTTY's automated host key
management is completely bypassed: the connection will be permitted if
and only if the host key presented by the server is one of the keys listed
in this box, and the host key store in the Registry will be neither read nor
written, unless you explicitly do so.
If the box is empty (as it usually is), then PuTTY's automated host key
management will work as normal.

4.21 The Cipher panel
PuTTY supports a variety of different encryption algorithms, and allows
you to choose which one you prefer to use. You can do this by dragging
the algorithms up and down in the list box (or moving them using the Up
and Down buttons) to specify a preference order. When you make an
SSH connection, PuTTY will search down the list from the top until it finds
an algorithm supported by the server, and then use that.
PuTTY currently supports the following algorithms:
ChaCha20-Poly1305, a combined cipher and MAC (SSH-2 only)
AES (Rijndael) - 256, 192, or 128-bit SDCTR or CBC (SSH-2 only)
Arcfour (RC4) - 256 or 128-bit stream cipher (SSH-2 only)
Blowfish - 256-bit SDCTR (SSH-2 only) or 128-bit CBC
Triple-DES - 168-bit SDCTR (SSH-2 only) or CBC
Single-DES - 56-bit CBC (see below for SSH-2)
If the algorithm PuTTY finds is below the ‘warn below here’ line, you will
see a warning box when you make the connection:
The first cipher supported by the server
is single-DES, which is below the configured
warning threshold.
Do you want to continue with this connection?

This warns you that the first available encryption is not a very secure one.
Typically you would put the ‘warn below here’ line between the
encryptions you consider secure and the ones you consider substandard.
By default, PuTTY supplies a preference order intended to reflect a
reasonable preference in terms of security and speed.
In SSH-2, the encryption algorithm is negotiated independently for each
direction of the connection, although PuTTY does not support separate
configuration of the preference orders. As a result you may get two
warnings similar to the one above, possibly with different encryptions.
Single-DES is not recommended in the SSH-2 protocol standards, but
one or two server implementations do support it. PuTTY can use single-

DES to interoperate with these servers if you enable the ‘Enable legacy
use of single-DES in SSH-2’ option; by default this is disabled and PuTTY
will stick to recommended ciphers.

4.22 The Auth panel
The Auth panel allows you to configure authentication options for SSH
sessions.
4.22.1 ‘Display pre-authentication banner’
4.22.2 ‘Bypass authentication entirely’
4.22.3 ‘Attempt authentication using Pageant’
4.22.4 ‘Attempt TIS or CryptoCard authentication’
4.22.5 ‘Attempt keyboard-interactive authentication’
4.22.6 ‘Allow agent forwarding’
4.22.7 ‘Allow attempted changes of username in SSH-2’
4.22.8 ‘Private key file for authentication’

4.22.1 ‘Display pre-authentication banner’
SSH-2 servers can provide a message for clients to display to the
prospective user before the user logs in; this is sometimes known as a
pre-authentication ‘banner’. Typically this is used to provide information
about the server and legal notices.
By default, PuTTY displays this message before prompting for a
password or similar credentials (although, unfortunately, not before
prompting for a login name, due to the nature of the protocol design). By
unchecking this option, display of the banner can be suppressed entirely.

4.22.2 ‘Bypass authentication entirely’
In SSH-2, it is in principle possible to establish a connection without using
SSH's mechanisms to identify or prove who you are to the server. An
SSH server could prefer to handle authentication in the data channel, for
instance, or simply require no user authentication whatsoever.
By default, PuTTY assumes the server requires authentication (we've
never heard of one that doesn't), and thus must start this process with a
username. If you find you are getting username prompts that you cannot
answer, you could try enabling this option. However, most SSH servers
will reject this.
This is not the option you want if you have a username and just want
PuTTY to remember it; for that see section 4.14.1. It's also probably not
what if you're trying to set up passwordless login to a mainstream SSH
server; depending on the server, you probably wanted public-key
authentication (chapter 8) or perhaps GSSAPI authentication (section
4.23). (These are still forms of authentication, even if you don't have to
interact with them.)
This option only affects SSH-2 connections. SSH-1 connections always
require an authentication step.

4.22.3 ‘Attempt authentication using Pageant’
If this option is enabled, then PuTTY will look for Pageant (the SSH
private-key storage agent) and attempt to authenticate with any suitable
public keys Pageant currently holds.
This behaviour is almost always desirable, and is therefore enabled by
default. In rare cases you might need to turn it off in order to force
authentication by some non-public-key method such as passwords.
This option can also be controlled using the -noagent command-line
option. See section 3.8.3.9.
See chapter 9 for more information about Pageant in general.

4.22.4 ‘Attempt TIS or CryptoCard
authentication’
TIS and CryptoCard authentication are (despite their names) generic
forms of simple challenge/response authentication available in SSH
protocol version 1 only. You might use them if you were using S/Key onetime passwords, for example, or if you had a physical security token that
generated responses to authentication challenges. They can even be
used to prompt for simple passwords.
With this switch enabled, PuTTY will attempt these forms of
authentication if the server is willing to try them. You will be presented
with a challenge string (which may be different every time) and must
supply the correct response in order to log in. If your server supports this,
you should talk to your system administrator about precisely what form
these challenges and responses take.

4.22.5 ‘Attempt keyboard-interactive
authentication’
The SSH-2 equivalent of TIS authentication is called ‘keyboardinteractive’. It is a flexible authentication method using an arbitrary
sequence of requests and responses; so it is not only useful for
challenge/response mechanisms such as S/Key, but it can also be used
for (for example) asking the user for a new password when the old one
has expired.
PuTTY leaves this option enabled by default, but supplies a switch to turn
it off in case you should have trouble with it.

4.22.6 ‘Allow agent forwarding’
This option allows the SSH server to open forwarded connections back to
your local copy of Pageant. If you are not running Pageant, this option
will do nothing.
See chapter 9 for general information on Pageant, and section 9.4 for
information on agent forwarding. Note that there is a security risk
involved with enabling this option; see section 9.5 for details.

4.22.7 ‘Allow attempted changes of username in
SSH-2’
In the SSH-1 protocol, it is impossible to change username after failing to
authenticate. So if you mis-type your username at the PuTTY ‘login as:’
prompt, you will not be able to change it except by restarting PuTTY.
The SSH-2 protocol does allow changes of username, in principle, but
does not make it mandatory for SSH-2 servers to accept them. In
particular, OpenSSH does not accept a change of username; once you
have sent one username, it will reject attempts to try to authenticate as
another user. (Depending on the version of OpenSSH, it may quietly
return failure for all login attempts, or it may send an error message.)
For this reason, PuTTY will by default not prompt you for your username
more than once, in case the server complains. If you know your server
can cope with it, you can enable the ‘Allow attempted changes of
username’ option to modify PuTTY's behaviour.

4.22.8 ‘Private key file for authentication’
This box is where you enter the name of your private key file if you are
using public key authentication. See chapter 8 for information about
public key authentication in SSH.
This key must be in PuTTY's native format (*.PPK). If you have a private
key in another format that you want to use with PuTTY, see section
8.2.12.
You can use the authentication agent Pageant so that you do not need to
explicitly configure a key here; see chapter 9.
If a private key file is specified here with Pageant running, PuTTY will first
try asking Pageant to authenticate with that key, and ignore any other
keys Pageant may have. If that fails, PuTTY will ask for a passphrase as
normal. You can also specify a public key file in this case (in RFC 4716 or
OpenSSH format), as that's sufficient to identify the key to Pageant, but
of course if Pageant isn't present PuTTY can't fall back to using this file
itself.

4.23 The GSSAPI panel
The ‘GSSAPI’ subpanel of the ‘Auth’ panel controls the use of GSSAPI
authentication. This is a mechanism which delegates the authentication
exchange to a library elsewhere on the client machine, which in principle
can authenticate in many different ways but in practice is usually used
with the Kerberos single sign-on protocol to implement passwordless
login.
GSSAPI is only available in the SSH-2 protocol.
The topmost control on the GSSAPI subpanel is the checkbox labelled
‘Attempt GSSAPI authentication’. If this is disabled, GSSAPI will not be
attempted at all and the rest of this panel is unused. If it is enabled,
GSSAPI authentication will be attempted, and (typically) if your client
machine has valid Kerberos credentials loaded, then PuTTY should be
able to authenticate automatically to servers that support Kerberos
logins.
4.23.1 ‘Allow GSSAPI credential delegation’
4.23.2 Preference order for GSSAPI libraries

4.23.1 ‘Allow GSSAPI credential delegation’
GSSAPI credential delegation is a mechanism for passing on your
Kerberos (or other) identity to the session on the SSH server. If you
enable this option, then not only will PuTTY be able to log in
automatically to a server that accepts your Kerberos credentials, but also
you will be able to connect out from that server to other Kerberossupporting services and use the same credentials just as automatically.
(This option is the Kerberos analogue of SSH agent forwarding; see
section 9.4 for some information on that.)
Note that, like SSH agent forwarding, there is a security implication in the
use of this option: the administrator of the server you connect to, or
anyone else who has cracked the administrator account on that server,
could fake your identity when connecting to further Kerberos-supporting
services. However, Kerberos sites are typically run by a central authority,
so the administrator of one server is likely to already have access to the
other services too; so this would typically be less of a risk than SSH
agent forwarding.

4.23.2 Preference order for GSSAPI libraries
GSSAPI is a mechanism which allows more than one authentication
method to be accessed through the same interface. Therefore, more than
one authentication library may exist on your system which can be
accessed using GSSAPI.
PuTTY contains native support for a few well-known such libraries, and
will look for all of them on your system and use whichever it finds. If more
than one exists on your system and you need to use a specific one, you
can adjust the order in which it will search using this preference list
control.
One of the options in the preference list is to use a user-specified
GSSAPI library. If the library you want to use is not mentioned by name in
PuTTY's list of options, you can enter its full pathname in the ‘Usersupplied GSSAPI library path’ field, and move the ‘User-supplied
GSSAPI library’ option in the preference list to make sure it is selected
before anything else.
On Windows, such libraries are files with a .dll extension, and must
have been built in the same way as the PuTTY executable you're
running; if you have a 32-bit DLL, you must run a 32-bit version of
PuTTY, and the same with 64-bit (see question A.6.10). On Unix, shared
libraries generally have a .so extension.

4.24 The TTY panel
The TTY panel lets you configure the remote pseudo-terminal.
4.24.1 ‘Don't allocate a pseudo-terminal’
4.24.2 Sending terminal modes

4.24.1 ‘Don't allocate a pseudo-terminal’
When connecting to a Unix system, most interactive shell sessions are
run in a pseudo-terminal, which allows the Unix system to pretend it's
talking to a real physical terminal device but allows the SSH server to
catch all the data coming from that fake device and send it back to the
client.
Occasionally you might find you have a need to run a session not in a
pseudo-terminal. In PuTTY, this is generally only useful for very specialist
purposes; although in Plink (see chapter 7) it is the usual way of working.

4.24.2 Sending terminal modes
The SSH protocol allows the client to send ‘terminal modes’ for the
remote pseudo-terminal. These usually control the server's expectation of
the local terminal's behaviour.
If your server does not have sensible defaults for these modes, you may
find that changing them here helps, although the server is at liberty to
ignore your changes. If you don't understand any of this, it's safe to leave
these settings alone.
(None of these settings will have any effect if no pseudo-terminal is
requested or allocated.)
You can change what happens for a particular mode by selecting it in the
list, choosing one of the options and specifying the exact value if
necessary, and hitting ‘Set’. The effect of the options is as follows:
If the ‘Auto’ option is selected, the PuTTY tools will decide whether to
specify that mode to the server, and if so, will send a sensible value.
PuTTY proper will send modes that it has an opinion on (currently
only the code for the Backspace key, ERASE, and whether the
character set is UTF-8, IUTF8). Plink on Unix will propagate
appropriate modes from the local terminal, if any.
If ‘Nothing’ is selected, no value for the mode will be specified to the
server under any circumstances.
If a value is specified, it will be sent to the server under all
circumstances. The precise syntax of the value box depends on the
mode.
By default, all of the available modes are listed as ‘Auto’, which should do
the right thing in most circumstances.
The precise effect of each setting, if any, is up to the server. Their names
come from POSIX and other Unix systems, and they are most likely to
have a useful effect on such systems. (These are the same settings that
can usually be changed using the stty command once logged in to such

servers.)
Some notable modes are described below; for fuller explanations, see
your server documentation.
is the character that when typed by the user will delete one
space to the left. When set to ‘Auto’ (the default setting), this follows
the setting of the local Backspace key in PuTTY (see section 4.4.1).
ERASE

This and other special characters are specified using ^C notation for
Ctrl-C, and so on. Use ^<27> or ^<0x1B> to specify a character
numerically, and ^~ to get a literal ^. Other non-control characters are
denoted by themselves. Leaving the box entirely blank indicates that
no character should be assigned to the specified function, although
this may not be supported by all servers.
is a special character that usually forcefully ends the current
process on the server (SIGQUIT). On many servers its default setting
is Ctrl-backslash (^\), which is easy to accidentally invoke on many
keyboards. If this is getting in your way, you may want to change it to
another character or turn it off entirely.
Boolean modes such as ECHO and ICANON can be specified in PuTTY
in a variety of ways, such as true/false, yes/no, and 0/1. (Explicitly
specifying a value of no is different from not sending the mode at all.)
The boolean mode IUTF8 signals to the server whether the terminal
character set is UTF-8 or not, for purposes such as basic line editing;
if this is set incorrectly, the backspace key may erase the wrong
amount of text, for instance. However, simply setting this is not
usually sufficient for the server to use UTF-8; POSIX servers will
generally also require the locale to be set (by some serverdependent means), although many newer installations default to
UTF-8. Also, since this mode was added to the SSH protocol much
later than the others, many servers (particularly older servers) do not
honour this mode sent over SSH; indeed, a few poorly-written
servers object to its mere presence, so you may find you need to set
it to not be sent at all. When set to ‘Auto’, this follows the local
configured character set (see section 4.10.1).
Terminal speeds are configured elsewhere; see section 4.14.4.
QUIT

4.25 The X11 panel
The X11 panel allows you to configure forwarding of X11 over an SSH
connection.
If your server lets you run X Window System graphical applications, X11
forwarding allows you to securely give those applications access to a
local X display on your PC.
To enable X11 forwarding, check the ‘Enable X11 forwarding’ box. If your
X display is somewhere unusual, you will need to enter its location in the
‘X display location’ box; if this is left blank, PuTTY will try to find a
sensible default in the environment, or use the primary local display (:0) if
that fails.
See section 3.4 for more information about X11 forwarding.
4.25.1 Remote X11 authentication
4.25.2 X authority file for local display

4.25.1 Remote X11 authentication
If you are using X11 forwarding, the virtual X server created on the SSH
server machine will be protected by authorisation data. This data is
invented, and checked, by PuTTY.
The usual authorisation method used for this is called MIT-MAGIC-COOKIE1. This is a simple password-style protocol: the X client sends some
cookie data to the server, and the server checks that it matches the real
cookie. The cookie data is sent over an unencrypted X11 connection; so
if you allow a client on a third machine to access the virtual X server, then
the cookie will be sent in the clear.
PuTTY offers the alternative protocol XDM-AUTHORIZATION-1. This is a
cryptographically authenticated protocol: the data sent by the X client is
different every time, and it depends on the IP address and port of the
client's end of the connection and is also stamped with the current time.
So an eavesdropper who captures an XDM-AUTHORIZATION-1 string cannot
immediately re-use it for their own X connection.
PuTTY's support for XDM-AUTHORIZATION-1 is a somewhat experimental
feature, and may encounter several problems:
Some X clients probably do not even support XDM-AUTHORIZATION-1,
so they will not know what to do with the data PuTTY has provided.
This authentication mechanism will only work in SSH-2. In SSH-1,
the SSH server does not tell the client the source address of a
forwarded connection in a machine-readable format, so it's
impossible to verify the XDM-AUTHORIZATION-1 data.
You may find this feature causes problems with some SSH servers,
which will not clean up XDM-AUTHORIZATION-1 data after a session, so
that if you then connect to the same server using a client which only
does MIT-MAGIC-COOKIE-1 and are allocated the same remote display
number, you might find that out-of-date authentication data is still
present on your server and your X connections fail.
PuTTY's default is MIT-MAGIC-COOKIE-1. If you change it, you should be
sure you know what you're doing.

4.25.2 X authority file for local display
If you are using X11 forwarding, the local X server to which your
forwarded connections are eventually directed may itself require
authorisation.
Some Windows X servers do not require this: they do authorisation by
simpler means, such as accepting any connection from the local machine
but not from anywhere else. However, if your X server does require
authorisation, then PuTTY needs to know what authorisation is required.
One way in which this data might be made available is for the X server to
store it somewhere in a file which has the same format as the Unix
.Xauthority file. If this is how your Windows X server works, then you
can tell PuTTY where to find this file by configuring this option. By
default, PuTTY will not attempt to find any authorisation for your local
display.

4.26 The Tunnels panel
The Tunnels panel allows you to configure tunnelling of arbitrary
connection types through an SSH connection.
Port forwarding allows you to tunnel other types of network connection
down an SSH session. See section 3.5 for a general discussion of port
forwarding and how it works.
The port forwarding section in the Tunnels panel shows a list of all the
port forwardings that PuTTY will try to set up when it connects to the
server. By default no port forwardings are set up, so this list is empty.
To add a port forwarding:
Set one of the ‘Local’ or ‘Remote’ radio buttons, depending on
whether you want to forward a local port to a remote destination
(‘Local’) or forward a remote port to a local destination (‘Remote’).
Alternatively, select ‘Dynamic’ if you want PuTTY to provide a local
SOCKS 4/4A/5 proxy on a local port (note that this proxy only
supports TCP connections; the SSH protocol does not support
forwarding UDP).
Enter a source port number into the ‘Source port’ box. For local
forwardings, PuTTY will listen on this port of your PC. For remote
forwardings, your SSH server will listen on this port of the remote
machine. Note that most servers will not allow you to listen on port
numbers less than 1024.
If you have selected ‘Local’ or ‘Remote’ (this step is not needed with
‘Dynamic’), enter a hostname and port number separated by a colon,
in the ‘Destination’ box. Connections received on the source port will
be directed to this destination. For example, to connect to a POP-3
server, you might enter popserver.example.com:110. (If you need to
enter a literal IPv6 address, enclose it in square brackets, for
instance ‘[::1]:2200’.)
Click the ‘Add’ button. Your forwarding details should appear in the
list box.
To remove a port forwarding, simply select its details in the list box, and

click the ‘Remove’ button.
In the ‘Source port’ box, you can also optionally enter an IP address to
listen on, by specifying (for instance) 127.0.0.5:79. See section 3.5 for
more information on how this works and its restrictions.
In place of port numbers, you can enter service names, if they are known
to the local system. For instance, in the ‘Destination’ box, you could enter
popserver.example.com:pop3.
You can modify the currently active set of port forwardings in mid-session
using ‘Change Settings’ (see section 3.1.3.4). If you delete a local or
dynamic port forwarding in mid-session, PuTTY will stop listening for
connections on that port, so it can be re-used by another program. If you
delete a remote port forwarding, note that:
The SSH-1 protocol contains no mechanism for asking the server to
stop listening on a remote port.
The SSH-2 protocol does contain such a mechanism, but not all
SSH servers support it. (In particular, OpenSSH does not support it
in any version earlier than 3.9.)
If you ask to delete a remote port forwarding and PuTTY cannot make
the server actually stop listening on the port, it will instead just start
refusing incoming connections on that port. Therefore, although the port
cannot be reused by another program, you can at least be reasonably
sure that server-side programs can no longer access the service at your
end of the port forwarding.
If you delete a forwarding, any existing connections established using
that forwarding remain open. Similarly, changes to global settings such
as ‘Local ports accept connections from other hosts’ only take effect on
new forwardings.
If the connection you are forwarding over SSH is itself a second SSH
connection made by another copy of PuTTY, you might find the ‘logical
host name’ configuration option useful to warn PuTTY of which host key it
should be expecting. See section 4.13.5 for details of this.
4.26.1 Controlling the visibility of forwarded ports

4.26.2 Selecting Internet protocol version for forwarded ports

4.26.1 Controlling the visibility of forwarded
ports
The source port for a forwarded connection usually does not accept
connections from any machine except the SSH client or server machine
itself (for local and remote forwardings respectively). There are controls
in the Tunnels panel to change this:
The ‘Local ports accept connections from other hosts’ option allows
you to set up local-to-remote port forwardings in such a way that
machines other than your client PC can connect to the forwarded
port. (This also applies to dynamic SOCKS forwarding.)
The ‘Remote ports do the same’ option does the same thing for
remote-to-local port forwardings (so that machines other than the
SSH server machine can connect to the forwarded port.) Note that
this feature is only available in the SSH-2 protocol, and not all SSH-2
servers support it (OpenSSH 3.0 does not, for example).

4.26.2 Selecting Internet protocol version for
forwarded ports
This switch allows you to select a specific Internet protocol (IPv4 or IPv6)
for the local end of a forwarded port. By default, it is set on ‘Auto’, which
means that:
for a local-to-remote port forwarding, PuTTY will listen for incoming
connections in both IPv4 and (if available) IPv6
for a remote-to-local port forwarding, PuTTY will choose a sensible
protocol for the outgoing connection.
This overrides the general Internet protocol version preference on the
Connection panel (see section 4.13.4).
Note that some operating systems may listen for incoming connections in
IPv4 even if you specifically asked for IPv6, because their IPv4 and IPv6
protocol stacks are linked together. Apparently Linux does this, and
Windows does not. So if you're running PuTTY on Windows and you tick
‘IPv6’ for a local or dynamic port forwarding, it will only be usable by
connecting to it using IPv6; whereas if you do the same on Linux, you
can also use it with IPv4. However, ticking ‘Auto’ should always give you
a port which you can connect to using either protocol.

4.27 The Bugs and More Bugs panels
Not all SSH servers work properly. Various existing servers have bugs in
them, which can make it impossible for a client to talk to them unless it
knows about the bug and works around it.
Since most servers announce their software version number at the
beginning of the SSH connection, PuTTY will attempt to detect which
bugs it can expect to see in the server and automatically enable
workarounds. However, sometimes it will make mistakes; if the server
has been deliberately configured to conceal its version number, or if the
server is a version which PuTTY's bug database does not know about,
then PuTTY will not know what bugs to expect.
The Bugs and More Bugs panels (there are two because we have so
many bug compatibility modes) allow you to manually configure the bugs
PuTTY expects to see in the server. Each bug can be configured in three
states:
‘Off’: PuTTY will assume the server does not have the bug.
‘On’: PuTTY will assume the server does have the bug.
‘Auto’: PuTTY will use the server's version number announcement to
try to guess whether or not the server has the bug.
4.27.1 ‘Chokes on SSH-1 ignore messages’
4.27.2 ‘Refuses all SSH-1 password camouflage’
4.27.3 ‘Chokes on SSH-1 RSA authentication’
4.27.4 ‘Chokes on SSH-2 ignore messages’
4.27.5 ‘Chokes on PuTTY's SSH-2 ‘winadj’ requests’
4.27.6 ‘Miscomputes SSH-2 HMAC keys’
4.27.7 ‘Miscomputes SSH-2 encryption keys’
4.27.8 ‘Requires padding on SSH-2 RSA signatures’
4.27.9 ‘Misuses the session ID in SSH-2 PK auth’
4.27.10 ‘Handles SSH-2 key re-exchange badly’
4.27.11 ‘Ignores SSH-2 maximum packet size’
4.27.12 ‘Replies to requests on closed channels’
4.27.13 ‘Only supports pre-RFC4419 SSH-2 DH GEX’

4.27.1 ‘Chokes on SSH-1 ignore messages’
An ignore message (SSH_MSG_IGNORE) is a message in the SSH
protocol which can be sent from the client to the server, or from the
server to the client, at any time. Either side is required to ignore the
message whenever it receives it. PuTTY uses ignore messages to hide
the password packet in SSH-1, so that a listener cannot tell the length of
the user's password; it also uses ignore messages for connection
keepalives (see section 4.13.1).
If this bug is detected, PuTTY will stop using ignore messages. This
means that keepalives will stop working, and PuTTY will have to fall back
to a secondary defence against SSH-1 password-length eavesdropping.
See section 4.27.2. If this bug is enabled when talking to a correct server,
the session will succeed, but keepalives will not work and the session
might be more vulnerable to eavesdroppers than it could be.

4.27.2 ‘Refuses all SSH-1 password camouflage’
When talking to an SSH-1 server which cannot deal with ignore
messages (see section 4.27.1), PuTTY will attempt to disguise the length
of the user's password by sending additional padding within the
password packet. This is technically a violation of the SSH-1
specification, and so PuTTY will only do it when it cannot use standardscompliant ignore messages as camouflage. In this sense, for a server to
refuse to accept a padded password packet is not really a bug, but it
does make life inconvenient if the server can also not handle ignore
messages.
If this ‘bug’ is detected, PuTTY will assume that neither ignore messages
nor padding are acceptable, and that it thus has no choice but to send
the user's password with no form of camouflage, so that an
eavesdropping user will be easily able to find out the exact length of the
password. If this bug is enabled when talking to a correct server, the
session will succeed, but will be more vulnerable to eavesdroppers than it
could be.
This is an SSH-1-specific bug. SSH-2 is secure against this type of
attack.

4.27.3 ‘Chokes on SSH-1 RSA authentication’
Some SSH-1 servers cannot deal with RSA authentication messages at
all. If Pageant is running and contains any SSH-1 keys, PuTTY will
normally automatically try RSA authentication before falling back to
passwords, so these servers will crash when they see the RSA attempt.
If this bug is detected, PuTTY will go straight to password authentication.
If this bug is enabled when talking to a correct server, the session will
succeed, but of course RSA authentication will be impossible.
This is an SSH-1-specific bug.

4.27.4 ‘Chokes on SSH-2 ignore messages’
An ignore message (SSH_MSG_IGNORE) is a message in the SSH
protocol which can be sent from the client to the server, or from the
server to the client, at any time. Either side is required to ignore the
message whenever it receives it. PuTTY uses ignore messages in SSH-2
to confuse the encrypted data stream and make it harder to cryptanalyse.
It also uses ignore messages for connection keepalives (see section
4.13.1).
If it believes the server to have this bug, PuTTY will stop using ignore
messages. If this bug is enabled when talking to a correct server, the
session will succeed, but keepalives will not work and the session might
be less cryptographically secure than it could be.

4.27.5 ‘Chokes on PuTTY's SSH-2 ‘winadj’
requests’
PuTTY sometimes sends a special request to SSH servers in the middle
of channel data, with the name winadj@putty.projects.tartarus.org (see
section F.1). The purpose of this request is to measure the round-trip time
to the server, which PuTTY uses to tune its flow control. The server does
not actually have to understand the message; it is expected to send back
a SSH_MSG_CHANNEL_FAILURE message indicating that it didn't understand it.
(All PuTTY needs for its timing calculations is some kind of response.)
It has been known for some SSH servers to get confused by this
message in one way or another – because it has a long name, or
because they can't cope with unrecognised request names even to the
extent of sending back the correct failure response, or because they
handle it sensibly but fill up the server's log file with pointless spam, or
whatever. PuTTY therefore supports this bug-compatibility flag: if it
believes the server has this bug, it will never send its
‘winadj@putty.projects.tartarus.org’ request, and will make do without
its timing data.

4.27.6 ‘Miscomputes SSH-2 HMAC keys’
Versions 2.3.0 and below of the SSH server software from ssh.com
compute the keys for their HMAC message authentication codes
incorrectly. A typical symptom of this problem is that PuTTY dies
unexpectedly at the beginning of the session, saying ‘Incorrect MAC
received on packet’.
If this bug is detected, PuTTY will compute its HMAC keys in the same
way as the buggy server, so that communication will still be possible. If
this bug is enabled when talking to a correct server, communication will
fail.
This is an SSH-2-specific bug.

4.27.7 ‘Miscomputes SSH-2 encryption keys’
Versions below 2.0.11 of the SSH server software from ssh.com compute
the keys for the session encryption incorrectly. This problem can cause
various error messages, such as ‘Incoming packet was garbled on
decryption’, or possibly even ‘Out of memory’.
If this bug is detected, PuTTY will compute its encryption keys in the
same way as the buggy server, so that communication will still be
possible. If this bug is enabled when talking to a correct server,
communication will fail.
This is an SSH-2-specific bug.

4.27.8 ‘Requires padding on SSH-2 RSA
signatures’
Versions below 3.3 of OpenSSH require SSH-2 RSA signatures to be
padded with zero bytes to the same length as the RSA key modulus. The
SSH-2 specification says that an unpadded signature MUST be
accepted, so this is a bug. A typical symptom of this problem is that
PuTTY mysteriously fails RSA authentication once in every few hundred
attempts, and falls back to passwords.
If this bug is detected, PuTTY will pad its signatures in the way OpenSSH
expects. If this bug is enabled when talking to a correct server, it is likely
that no damage will be done, since correct servers usually still accept
padded signatures because they're used to talking to OpenSSH.
This is an SSH-2-specific bug.

4.27.9 ‘Misuses the session ID in SSH-2 PK auth’
Versions below 2.3 of OpenSSH require SSH-2 public-key authentication
to be done slightly differently: the data to be signed by the client contains
the session ID formatted in a different way. If public-key authentication
mysteriously does not work but the Event Log (see section 3.1.3.1) thinks
it has successfully sent a signature, it might be worth enabling the
workaround for this bug to see if it helps.
If this bug is detected, PuTTY will sign data in the way OpenSSH
expects. If this bug is enabled when talking to a correct server, SSH-2
public-key authentication will fail.
This is an SSH-2-specific bug.

4.27.10 ‘Handles SSH-2 key re-exchange badly’
Some SSH servers cannot cope with repeat key exchange at all, and will
ignore attempts by the client to start one. Since PuTTY pauses the
session while performing a repeat key exchange, the effect of this would
be to cause the session to hang after an hour (unless you have your
rekey timeout set differently; see section 4.19.2 for more about rekeys).
Other, very old, SSH servers handle repeat key exchange even more
badly, and disconnect upon receiving a repeat key exchange request.
If this bug is detected, PuTTY will never initiate a repeat key exchange. If
this bug is enabled when talking to a correct server, the session should
still function, but may be less secure than you would expect.
This is an SSH-2-specific bug.

4.27.11 ‘Ignores SSH-2 maximum packet size’
When an SSH-2 channel is set up, each end announces the maximum
size of data packet that it is willing to receive for that channel. Some
servers ignore PuTTY's announcement and send packets larger than
PuTTY is willing to accept, causing it to report ‘Incoming packet was
garbled on decryption’.
If this bug is detected, PuTTY never allows the channel's flow-control
window to grow large enough to allow the server to send an over-sized
packet. If this bug is enabled when talking to a correct server, the session
will work correctly, but download performance will be less than it could
be.

4.27.12 ‘Replies to requests on closed channels’
The SSH protocol as published in RFC 4254 has an ambiguity which
arises if one side of a connection tries to close a channel, while the other
side simultaneously sends a request within the channel and asks for a
reply. RFC 4254 leaves it unclear whether the closing side should reply to
the channel request after having announced its intention to close the
channel.
Discussion on the ietf-ssh mailing list in April 2014 formed a clear
consensus that the right answer is no. However, because of the
ambiguity in the specification, some SSH servers have implemented the
other policy; for example, OpenSSH used to until it was fixed.
Because PuTTY sends channel requests with the ‘want reply’ flag
throughout channels' lifetime (see section 4.27.5), it's possible that when
connecting to such a server it might receive a reply to a request after it
thinks the channel has entirely closed, and terminate with an error along
the lines of ‘Received SSH2_MSG_CHANNEL_FAILURE for nonexistent channel
256’.

4.27.13 ‘Only supports pre-RFC4419 SSH-2 DH
GEX’
The SSH key exchange method that uses Diffie-Hellman group exchange
was redesigned after its original release, to use a slightly more
sophisticated setup message. Almost all SSH implementations switched
over to the new version. (PuTTY was one of the last.) A few old servers
still only support the old one.
If this bug is detected, and the client and server negotiate Diffie-Hellman
group exchange, then PuTTY will send the old message now known as
SSH2_MSG_KEX_DH_GEX_REQUEST_OLD in place of the new
SSH2_MSG_KEX_DH_GEX_REQUEST.
This is an SSH-2-specific bug.

4.28 The Serial panel
The Serial panel allows you to configure options that only apply when
PuTTY is connecting to a local serial line.
4.28.1 Selecting a serial line to connect to
4.28.2 Selecting the speed of your serial line
4.28.3 Selecting the number of data bits
4.28.4 Selecting the number of stop bits
4.28.5 Selecting the serial parity checking scheme
4.28.6 Selecting the serial flow control scheme

4.28.1 Selecting a serial line to connect to
The ‘Serial line to connect to’ box allows you to choose which serial line
you want PuTTY to talk to, if your computer has more than one serial
port.
On Windows, the first serial line is called COM1, and if there is a second it
is called COM2, and so on.
This configuration setting is also visible on the Session panel, where it
replaces the ‘Host Name’ box (see section 4.1.1) if the connection type is
set to ‘Serial’.

4.28.2 Selecting the speed of your serial line
The ‘Speed’ box allows you to choose the speed (or ‘baud rate’) at which
to talk to the serial line. Typical values might be 9600, 19200, 38400 or
57600. Which one you need will depend on the device at the other end of
the serial cable; consult the manual for that device if you are in doubt.
This configuration setting is also visible on the Session panel, where it
replaces the ‘Port’ box (see section 4.1.1) if the connection type is set to
‘Serial’.

4.28.3 Selecting the number of data bits
The ‘Data bits’ box allows you to choose how many data bits are
transmitted in each byte sent or received through the serial line. Typical
values are 7 or 8.

4.28.4 Selecting the number of stop bits
The ‘Stop bits’ box allows you to choose how many stop bits are used in
the serial line protocol. Typical values are 1, 1.5 or 2.

4.28.5 Selecting the serial parity checking
scheme
The ‘Parity’ box allows you to choose what type of parity checking is used
on the serial line. The settings are:
‘None’: no parity bit is sent at all.
‘Odd’: an extra parity bit is sent alongside each byte, and arranged
so that the total number of 1 bits is odd.
‘Even’: an extra parity bit is sent alongside each byte, and arranged
so that the total number of 1 bits is even.
‘Mark’: an extra parity bit is sent alongside each byte, and always set
to 1.
‘Space’: an extra parity bit is sent alongside each byte, and always
set to 0.

4.28.6 Selecting the serial flow control scheme
The ‘Flow control’ box allows you to choose what type of flow control
checking is used on the serial line. The settings are:
‘None’: no flow control is done. Data may be lost if either side
attempts to send faster than the serial line permits.
‘XON/XOFF’: flow control is done by sending XON and XOFF
characters within the data stream.
‘RTS/CTS’: flow control is done using the RTS and CTS wires on the
serial line.
‘DSR/DTR’: flow control is done using the DSR and DTR wires on
the serial line.

4.29 Storing configuration in a file
PuTTY does not currently support storing its configuration in a file instead
of the Registry. However, you can work around this with a couple of batch
files.
You will need a file called (say) PUTTY.BAT which imports the contents of a
file into the Registry, then runs PuTTY, exports the contents of the
Registry back into the file, and deletes the Registry entries. This can all
be done using the Regedit command line options, so it's all automatic.
Here is what you need in PUTTY.BAT:
@ECHO OFF
regedit /s putty.reg
regedit /s puttyrnd.reg
start /w putty.exe
regedit /ea new.reg HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
copy new.reg putty.reg
del new.reg
regedit /s puttydel.reg

This batch file needs two auxiliary files: PUTTYRND.REG which sets up an
initial safe location for the PUTTY.RND random seed file, and PUTTYDEL.REG
which destroys everything in the Registry once it's been successfully
saved back to the file.
Here is PUTTYDEL.REG:
REGEDIT4
[-HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]

Here is an example PUTTYRND.REG file:
REGEDIT4
[HKEY_CURRENT_USER\Software\SimonTatham\PuTTY]
"RandSeedFile"="a:\\putty.rnd"

You should replace a:\putty.rnd with the location where you want to
store your random number data. If the aim is to carry around PuTTY and
its settings on one USB stick, you probably want to store it on the USB

stick.

Chapter 5: Using PSCP to transfer files securely
PSCP, the PuTTY Secure Copy client, is a tool for transferring files
securely between computers using an SSH connection.
If you have an SSH-2 server, you might prefer PSFTP (see chapter 6) for
interactive use. PSFTP does not in general work with SSH-1 servers,
however.
5.1 Starting PSCP
5.2 PSCP Usage
5.2.1 The basics
5.2.2 Options
5.2.3 Return value
5.2.4 Using public key authentication with PSCP

5.1 Starting PSCP
PSCP is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring up a
console window. With Windows 95, 98, and ME, this is called an ‘MSDOS Prompt’ and with Windows NT, 2000, and XP, it is called a
‘Command Prompt’. It should be available from the Programs section of
your Start Menu.
To start PSCP it will need either to be on your PATH or in your current
directory. To add the directory containing PSCP to your PATH environment
variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%

This will only work for the lifetime of that particular console window. To
set your PATH more permanently on Windows NT, 2000, and XP, use the
Environment tab of the System Control Panel. On Windows 95, 98, and
ME, you will need to edit your AUTOEXEC.BAT to include a set command
like the one above.

5.2 PSCP Usage
Once you've got a console window to type into, you can just type pscp on
its own to bring up a usage message. This tells you the version of PSCP
you're using, and gives you a brief summary of how to use PSCP:
Z:\owendadmin>pscp
PuTTY Secure Copy client
Release 0.70
Usage: pscp [options] [user@]host:source target
pscp [options] source [source...] [user@]host:target
pscp [options] -ls [user@]host:filespec
Options:
-V
print version information and exit
-pgpfp
print PGP key fingerprints and exit
-p
preserve file attributes
-q
quiet, don't show statistics
-r
copy directories recursively
-v
show verbose messages
-load sessname Load settings from saved session
-P port
connect to specified port
-l user
connect with specified username
-pw passw login with specified password
-1 -2
force use of particular SSH protocol version
-4 -6
force use of IPv4 or IPv6
-C
enable compression
-i key
private key file for user authentication
-noagent disable use of Pageant
-agent
enable use of Pageant
-hostkey aa:bb:cc:...
manually specify a host key (may be repeated)
-batch
disable all interactive prompts
-proxycmd command
use 'command' as local proxy
-unsafe
allow server-side wildcards (DANGEROUS)
-sftp
force use of SFTP protocol
-scp
force use of SCP protocol
-sshlog file
-sshrawlog file
log protocol details to a file

(PSCP's interface is much like the Unix scp command, if you're familiar
with that.)
5.2.1 The basics

5.2.1.1 user
5.2.1.2 host
5.2.1.3 source
5.2.1.4 target
5.2.2 Options
5.2.2.1 -ls list remote files
5.2.2.2 -p preserve file attributes
5.2.2.3 -q quiet, don't show statistics
5.2.2.4 -r copies directories recursively
5.2.2.5 -batch avoid interactive prompts
5.2.2.6 -sftp, -scp force use of particular protocol
5.2.3 Return value
5.2.4 Using public key authentication with PSCP

5.2.1 The basics
To receive (a) file(s) from a remote server:
pscp [options] [user@]host:source target

So to copy the file /etc/hosts from the server example.com as user fred to
the file c:\temp\example-hosts.txt, you would type:
pscp fred@example.com:/etc/hosts c:\temp\example-hosts.txt

To send (a) file(s) to a remote server:
pscp [options] source [source...] [user@]host:target

So to copy the local file c:\documents\foo.txt to the server example.com
as user fred to the file /tmp/foo you would type:
pscp c:\documents\foo.txt fred@example.com:/tmp/foo

You can use wildcards to transfer multiple files in either direction, like this:
pscp c:\documents\*.doc fred@example.com:docfiles
pscp fred@example.com:source/*.c c:\source

However, in the second case (using a wildcard for multiple remote files)
you may see a warning saying something like ‘warning: remote host tried
to write to a file called ‘terminal.c’ when we requested a file called ‘*.c’.
If this is a wildcard, consider upgrading to SSH-2 or using the ‘-unsafe’
option. Renaming of this file has been disallowed’.
This is due to a fundamental insecurity in the old-style SCP protocol: the
client sends the wildcard string (*.c) to the server, and the server sends
back a sequence of file names that match the wildcard pattern. However,
there is nothing to stop the server sending back a different pattern and
writing over one of your other files: if you request *.c, the server might
send back the file name AUTOEXEC.BAT and install a virus for you. Since
the wildcard matching rules are decided by the server, the client cannot
reliably verify that the filenames sent back match the pattern.

PSCP will attempt to use the newer SFTP protocol (part of SSH-2) where
possible, which does not suffer from this security flaw. If you are talking to
an SSH-2 server which supports SFTP, you will never see this warning.
(You can force use of the SFTP protocol, if available, with -sftp - see
section 5.2.2.6.)
If you really need to use a server-side wildcard with an SSH-1 server, you
can use the -unsafe command line option with PSCP:
pscp -unsafe fred@example.com:source/*.c c:\source

This will suppress the warning message and the file transfer will happen.
However, you should be aware that by using this option you are giving
the server the ability to write to any file in the target directory, so you
should only use this option if you trust the server administrator not to be
malicious (and not to let the server machine be cracked by malicious
people). Alternatively, do any such download in a newly created empty
directory. (Even in ‘unsafe’ mode, PSCP will still protect you against the
server trying to get out of that directory using pathnames including ‘..’.)
5.2.1.1 user
5.2.1.2 host
5.2.1.3 source
5.2.1.4 target

5.2.1.1 user
The login name on the remote server. If this is omitted, and host is a
PuTTY saved session, PSCP will use any username specified by that
saved session. Otherwise, PSCP will attempt to use the local Windows
username.

5.2.1.2 host
The name of the remote server, or the name of an existing PuTTY saved
session. In the latter case, the session's settings for hostname, port
number, cipher type and username will be used.

5.2.1.3 source
One or more source files. Wildcards are allowed. The syntax of wildcards
depends on the system to which they apply, so if you are copying from a
Windows system to a UNIX system, you should use Windows wildcard
syntax (e.g. *.*), but if you are copying from a UNIX system to a
Windows system, you would use the wildcard syntax allowed by your
UNIX shell (e.g. *).
If the source is a remote server and you do not specify a full pathname
(in UNIX, a pathname beginning with a / (slash) character), what you
specify as a source will be interpreted relative to your home directory on
the remote server.

5.2.1.4 target
The filename or directory to put the file(s). When copying from a remote
server to a local host, you may wish simply to place the file(s) in the
current directory. To do this, you should specify a target of .. For
example:
pscp fred@example.com:/home/tom/.emacs .

...would copy /home/tom/.emacs on the remote server to the current
directory.
As with the source parameter, if the target is on a remote server and is
not a full path name, it is interpreted relative to your home directory on
the remote server.

5.2.2 Options
PSCP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.8.3 for a description of these options. (The ones not
supported by PSCP are clearly marked.)
PSCP also supports some of its own options. The following sections
describe PSCP's specific command-line options.
5.2.2.1 -ls list remote files
5.2.2.2 -p preserve file attributes
5.2.2.3 -q quiet, don't show statistics
5.2.2.4 -r copies directories recursively
5.2.2.5 -batch avoid interactive prompts
5.2.2.6 -sftp, -scp force use of particular protocol

5.2.2.1 -ls list remote files
If the -ls option is given, no files are transferred; instead, remote files are
listed. Only a hostname specification and optional remote file
specification need be given. For example:
pscp -ls fred@example.com:dir1

The SCP protocol does not contain within itself a means of listing files. If
SCP is in use, this option therefore assumes that the server responds
appropriately to the command ls -la; this may not work with all servers.
If SFTP is in use, this option should work with all servers.

5.2.2.2 -p preserve file attributes
By default, files copied with PSCP are timestamped with the date and
time they were copied. The -p option preserves the original timestamp on
copied files.

5.2.2.3 -q quiet, don't show statistics
By default, PSCP displays a meter displaying the progress of the current
transfer:
mibs.tar

|

168 kB |

84.0 kB/s | ETA: 00:00:13 |

13%

The fields in this display are (from left to right), filename, size (in
kilobytes) of file transferred so far, estimate of how fast the file is being
transferred (in kilobytes per second), estimated time that the transfer will
be complete, and percentage of the file so far transferred. The -q option
to PSCP suppresses the printing of these statistics.

5.2.2.4 -r copies directories recursively
By default, PSCP will only copy files. Any directories you specify to copy
will be skipped, as will their contents. The -r option tells PSCP to
descend into any directories you specify, and to copy them and their
contents. This allows you to use PSCP to transfer whole directory
structures between machines.

5.2.2.5 -batch avoid interactive prompts
If you use the -batch option, PSCP will never give an interactive prompt
while establishing the connection. If the server's host key is invalid, for
example (see section 2.2), then the connection will simply be abandoned
instead of asking you what to do next.
This may help PSCP's behaviour when it is used in automated scripts:
using -batch, if something goes wrong at connection time, the batch job
will fail rather than hang.

5.2.2.6 -sftp, -scp force use of particular
protocol
As mentioned in section 5.2.1, there are two different file transfer
protocols in use with SSH. Despite its name, PSCP (like many other
ostensible scp clients) can use either of these protocols.
The older SCP protocol does not have a written specification and leaves
a lot of detail to the server platform. Wildcards are expanded on the
server. The simple design means that any wildcard specification
supported by the server platform (such as brace expansion) can be used,
but also leads to interoperability issues such as with filename quoting (for
instance, where filenames contain spaces), and also the security issue
described in section 5.2.1.
The newer SFTP protocol, which is usually associated with SSH-2
servers, is specified in a more platform independent way, and leaves
issues such as wildcard syntax up to the client. (PuTTY's SFTP wildcard
syntax is described in section 6.2.2.) This makes it more consistent
across platforms, more suitable for scripting and automation, and avoids
security issues with wildcard matching.
Normally PSCP will attempt to use the SFTP protocol, and only fall back
to the SCP protocol if SFTP is not available on the server.
The -scp option forces PSCP to use the SCP protocol or quit.
The -sftp option forces PSCP to use the SFTP protocol or quit. When
this option is specified, PSCP looks harder for an SFTP server, which
may allow use of SFTP with SSH-1 depending on server setup.

5.2.3 Return value
PSCP returns an ERRORLEVEL of zero (success) only if the files were
correctly transferred. You can test for this in a batch file, using code such
as this:
pscp file*.* user@hostname:
if errorlevel 1 echo There was an error

5.2.4 Using public key authentication with PSCP
Like PuTTY, PSCP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSCP can use PuTTY saved sessions in place of hostnames (see
section 5.2.1.2). So you would do this:
Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.22.8). You will
probably also want to specify a username to log in as (see section
4.14.1).
In PSCP, you can now use the name of the session instead of a
hostname: type pscp sessionname:file localfile, where
sessionname is replaced by the name of your saved session.
Secondly, you can supply the name of a private key file on the command
line, with the -i option. See section 3.8.3.18 for more information.
Thirdly, PSCP will attempt to authenticate using Pageant if Pageant is
running (see chapter 9). So you would do this:
Ensure Pageant is running, and has your private key stored in it.
Specify a user and host name to PSCP as normal. PSCP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see chapter 8.

Chapter 6: Using PSFTP to transfer files
securely
PSFTP, the PuTTY SFTP client, is a tool for transferring files securely
between computers using an SSH connection.
PSFTP differs from PSCP in the following ways:
PSCP should work on virtually every SSH server. PSFTP uses the
new SFTP protocol, which is a feature of SSH-2 only. (PSCP will
also use this protocol if it can, but there is an SSH-1 equivalent it can
fall back to if it cannot.)
PSFTP allows you to run an interactive file transfer session, much
like the Windows ftp program. You can list the contents of
directories, browse around the file system, issue multiple get and put
commands, and eventually log out. By contrast, PSCP is designed to
do a single file transfer operation and immediately terminate.
6.1 Starting PSFTP
6.1.1 -b: specify a file containing batch commands
6.1.2 -bc: display batch commands as they are run
6.1.3 -be: continue batch processing on errors
6.1.4 -batch: avoid interactive prompts
6.2 Running PSFTP
6.2.1 General quoting rules for PSFTP commands
6.2.2 Wildcards in PSFTP
6.2.3 The open command: start a session
6.2.4 The quit command: end your session
6.2.5 The close command: close your connection
6.2.6 The help command: get quick online help
6.2.7 The cd and pwd commands: changing the remote working
directory
6.2.8 The lcd and lpwd commands: changing the local working
directory
6.2.9 The get command: fetch a file from the server
6.2.10 The put command: send a file to the server
6.2.11 The mget and mput commands: fetch or send multiple files

6.2.12 The reget and reput commands: resuming file transfers
6.2.13 The dir command: list remote files
6.2.14 The chmod command: change permissions on remote files
6.2.15 The del command: delete remote files
6.2.16 The mkdir command: create remote directories
6.2.17 The rmdir command: remove remote directories
6.2.18 The mv command: move and rename remote files
6.2.19 The ! command: run a local Windows command
6.3 Using public key authentication with PSFTP

6.1 Starting PSFTP
The usual way to start PSFTP is from a command prompt, much like
PSCP. To do this, it will need either to be on your PATH or in your current
directory. To add the directory containing PSFTP to your PATH
environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%

Unlike PSCP, however, PSFTP has no complex command-line syntax;
you just specify a host name and perhaps a user name:
psftp server.example.com

or perhaps
psftp fred@server.example.com

Alternatively, if you just type psftp on its own (or double-click the PSFTP
icon in the Windows GUI), you will see the PSFTP prompt, and a
message telling you PSFTP has not connected to any server:
C:\>psftp
psftp: no hostname specified; use "open host.name" to connect
psftp>

At this point you can type open server.example.com or open
fred@server.example.com to start a session.
PSFTP accepts all the general command line options supported by the
PuTTY tools, except the ones which make no sense in a file transfer
utility. See section 3.8.3 for a description of these options. (The ones not
supported by PSFTP are clearly marked.)
PSFTP also supports some of its own options. The following sections
describe PSFTP's specific command-line options.
6.1.1 -b: specify a file containing batch commands
6.1.2 -bc: display batch commands as they are run
6.1.3 -be: continue batch processing on errors

6.1.4 -batch: avoid interactive prompts

6.1.1 -b: specify a file containing batch
commands
In normal operation, PSFTP is an interactive program which displays a
command line and accepts commands from the keyboard.
If you need to do automated tasks with PSFTP, you would probably
prefer to specify a set of commands in advance and have them executed
automatically. The -b option allows you to do this. You use it with a file
name containing batch commands. For example, you might create a file
called myscript.scr containing lines like this:
cd /home/ftp/users/jeff
del jam-old.tar.gz
ren jam.tar.gz jam-old.tar.gz
put jam.tar.gz
chmod a+r jam.tar.gz

and then you could run the script by typing
psftp user@hostname -b myscript.scr

When you run a batch script in this way, PSFTP will abort the script if any
command fails to complete successfully. To change this behaviour, you
can add the -be option (section 6.1.3).
PSFTP will terminate after it finishes executing the batch script.

6.1.2 -bc: display batch commands as they are
run
The -bc option alters what PSFTP displays while processing a batch
script specified with -b. With the -bc option, PSFTP will display prompts
and commands just as if the commands had been typed at the keyboard.
So instead of seeing this:
C:\>psftp fred@hostname -b batchfile
Sent username "fred"
Remote working directory is /home/fred
Listing directory /home/fred/lib
drwxrwsr-x
4 fred
fred
1024
drwxr-sr-x
25 fred
fred
2048
drwxrwsr-x
3 fred
fred
1024
lrwxrwxrwx
1 fred
fred
24
drwxrwsr-x
2 fred
fred
1024

Sep
Dec
Apr
Apr
Mar

6 10:42 .
14 09:36 ..
17 2000 jed
17 2000 timber
13 2000 trn

Sep
Dec
Apr
Apr
Mar

6 10:42 .
14 09:36 ..
17 2000 jed
17 2000 timber
13 2000 trn

you might see this:
C:\>psftp fred@hostname -bc -b batchfile
Sent username "fred"
Remote working directory is /home/fred
psftp> dir lib
Listing directory /home/fred/lib
drwxrwsr-x
4 fred
fred
1024
drwxr-sr-x
25 fred
fred
2048
drwxrwsr-x
3 fred
fred
1024
lrwxrwxrwx
1 fred
fred
24
drwxrwsr-x
2 fred
fred
1024
psftp> quit

6.1.3 -be: continue batch processing on errors
When running a batch file, this additional option causes PSFTP to
continue processing even if a command fails to complete successfully.
You might want this to happen if you wanted to delete a file and didn't
care if it was already not present, for example.

6.1.4 -batch: avoid interactive prompts
If you use the -batch option, PSFTP will never give an interactive prompt
while establishing the connection. If the server's host key is invalid, for
example (see section 2.2), then the connection will simply be abandoned
instead of asking you what to do next.
This may help PSFTP's behaviour when it is used in automated scripts:
using -batch, if something goes wrong at connection time, the batch job
will fail rather than hang.

6.2 Running PSFTP
Once you have started your PSFTP session, you will see a psftp>
prompt. You can now type commands to perform file-transfer functions.
This section lists all the available commands.
Any line starting with a # will be treated as a comment and ignored.
6.2.1 General quoting rules for PSFTP commands
6.2.2 Wildcards in PSFTP
6.2.3 The open command: start a session
6.2.4 The quit command: end your session
6.2.5 The close command: close your connection
6.2.6 The help command: get quick online help
6.2.7 The cd and pwd commands: changing the remote working
directory
6.2.8 The lcd and lpwd commands: changing the local working
directory
6.2.9 The get command: fetch a file from the server
6.2.10 The put command: send a file to the server
6.2.11 The mget and mput commands: fetch or send multiple files
6.2.12 The reget and reput commands: resuming file transfers
6.2.13 The dir command: list remote files
6.2.14 The chmod command: change permissions on remote files
6.2.15 The del command: delete remote files
6.2.16 The mkdir command: create remote directories
6.2.17 The rmdir command: remove remote directories
6.2.18 The mv command: move and rename remote files
6.2.19 The ! command: run a local Windows command

6.2.1 General quoting rules for PSFTP
commands
Most PSFTP commands are considered by the PSFTP command
interpreter as a sequence of words, separated by spaces. For example,
the command ren oldfilename newfilename splits up into three words:
ren (the command name), oldfilename (the name of the file to be
renamed), and newfilename (the new name to give the file).
Sometimes you will need to specify file names that contain spaces. In
order to do this, you can surround the file name with double quotes. This
works equally well for local file names and remote file names:
psftp> get "spacey file name.txt" "save it under this name.txt"

The double quotes themselves will not appear as part of the file names;
they are removed by PSFTP and their only effect is to stop the spaces
inside them from acting as word separators.
If you need to use a double quote (on some types of remote system,
such as Unix, you are allowed to use double quotes in file names), you
can do this by doubling it. This works both inside and outside double
quotes. For example, this command
psftp> ren ""this"" "a file with ""quotes"" in it"

will take a file whose current name is "this" (with a double quote
character at the beginning and the end) and rename it to a file whose
name is a file with "quotes" in it.
(The one exception to the PSFTP quoting rules is the ! command, which
passes its command line straight to Windows without splitting it up into
words at all. See section 6.2.19.)

6.2.2 Wildcards in PSFTP
Several commands in PSFTP support ‘wildcards’ to select multiple files.
For local file specifications (such as the first argument to put), wildcard
rules for the local operating system are used. For instance, PSFTP
running on Windows might require the use of *.* where PSFTP on Unix
would need *.
For remote file specifications (such as the first argument to get), PSFTP
uses a standard wildcard syntax (similar to POSIX wildcards):
matches any sequence of characters (including a zero-length
sequence).
? matches exactly one character.
[abc] matches exactly one character which can be a, b, or c.
*

[a-z]

matches any character in the range a to z.

[^abc]

matches a single character that is not a, b, or c.

Special cases: [-a] matches a literal hyphen (-) or a; [^-a] matches
all other characters. [a^] matches a literal caret (^) or a.
(backslash) before any of the above characters (or itself) removes
that character's special meaning.
\

A leading period (.) on a filename is not treated specially, unlike in some
Unix contexts; get * will fetch all files, whether or not they start with a
leading period.

6.2.3 The open command: start a session
If you started PSFTP by double-clicking in the GUI, or just by typing psftp
at the command line, you will need to open a connection to an SFTP
server before you can issue any other commands (except help and quit).
To create a connection, type open host.name, or if you need to specify a
user name as well you can type open user@host.name. You can optionally
specify a port as well: open user@host.name 22.
Once you have issued this command, you will not be able to issue it
again, even if the command fails (for example, if you mistype the host
name or the connection times out). So if the connection is not opened
successfully, PSFTP will terminate immediately.

6.2.4 The quit command: end your session
When you have finished your session, type the command quit to close
the connection, terminate PSFTP and return to the command line (or just
close the PSFTP console window if you started it from the GUI).
You can also use the bye and exit commands, which have exactly the
same effect.

6.2.5 The close command: close your
connection
If you just want to close the network connection but keep PSFTP running,
you can use the close command. You can then use the open command to
open a new connection.

6.2.6 The help command: get quick online help
If you type help, PSFTP will give a short list of the available commands.
If you type help with a command name - for example, help get - then
PSFTP will give a short piece of help on that particular command.

6.2.7 The cd and pwd commands: changing the
remote working directory
PSFTP maintains a notion of your ‘working directory’ on the server. This
is the default directory that other commands will operate on. For
example, if you type get filename.dat then PSFTP will look for
filename.dat in your remote working directory on the server.
To change your remote working directory, use the cd command. If you
don't provide an argument, cd will return you to your home directory on
the server (more precisely, the remote directory you were in at the start of
the connection).
To display your current remote working directory, type pwd.

6.2.8 The lcd and lpwd commands: changing the
local working directory
As well as having a working directory on the remote server, PSFTP also
has a working directory on your local machine (just like any other
Windows process). This is the default local directory that other
commands will operate on. For example, if you type get filename.dat
then PSFTP will save the resulting file as filename.dat in your local
working directory.
To change your local working directory, use the lcd command. To display
your current local working directory, type lpwd.

6.2.9 The get command: fetch a file from the
server
To download a file from the server and store it on your local PC, you use
the get command.
In its simplest form, you just use this with a file name:
get myfile.dat

If you want to store the file locally under a different name, specify the
local file name after the remote one:
get myfile.dat newname.dat

This will fetch the file on the server called myfile.dat, but will save it to
your local machine under the name newname.dat.
To fetch an entire directory recursively, you can use the -r option:
get -r mydir
get -r mydir newname

(If you want to fetch a file whose name starts with a hyphen, you may
have to use the -- special argument, which stops get from interpreting
anything as a switch after it. For example, ‘get -- -silly-name-’.)

6.2.10 The put command: send a file to the
server
To upload a file to the server from your local PC, you use the put
command.
In its simplest form, you just use this with a file name:
put myfile.dat

If you want to store the file remotely under a different name, specify the
remote file name after the local one:
put myfile.dat newname.dat

This will send the local file called myfile.dat, but will store it on the server
under the name newname.dat.
To send an entire directory recursively, you can use the -r option:
put -r mydir
put -r mydir newname

(If you want to send a file whose name starts with a hyphen, you may
have to use the -- special argument, which stops put from interpreting
anything as a switch after it. For example, ‘put -- -silly-name-’.)

6.2.11 The mget and mput commands: fetch or
send multiple files
works almost exactly like get, except that it allows you to specify
more than one file to fetch at once. You can do this in two ways:
mget

by giving two or more explicit file names (‘mget
file2.txt’)
by using a wildcard (‘mget *.txt’).

file1.txt

Every argument to mget is treated as the name of a file to fetch (unlike
get, which will interpret at most one argument like that, and a second
argument will be treated as an alternative name under which to store the
retrieved file), or a wildcard expression matching more than one file.
The -r and -- options from get are also available with mget.
mput

is similar to put, with the same differences.

6.2.12 The reget and reput commands: resuming
file transfers
If a file transfer fails half way through, and you end up with half the file
stored on your disk, you can resume the file transfer using the reget and
reput commands. These work exactly like the get and put commands,
but they check for the presence of the half-written destination file and
start transferring from where the last attempt left off.
The syntax of reget and reput is exactly the same as the syntax of get
and put:
reget myfile.dat
reget myfile.dat newname.dat
reget -r mydir

These commands are intended mainly for resuming interrupted transfers.
They assume that the remote file or directory structure has not changed
in any way; if there have been changes, you may end up with corrupted
files. In particular, the -r option will not pick up changes to files or
directories already transferred in full.

6.2.13 The dir command: list remote files
To list the files in your remote working directory, just type dir.
You can also list the contents of a different directory by typing dir
followed by the directory name:
dir /home/fred
dir sources

And you can list a subset of the contents of a directory by providing a
wildcard:
dir /home/fred/*.txt
dir sources/*.c

The ls command works exactly the same way as dir.

6.2.14 The chmod command: change permissions
on remote files
PSFTP allows you to modify the file permissions on files and directories
on the server. You do this using the chmod command, which works very
much like the Unix chmod command.
The basic syntax is chmod modes file, where modes represents a
modification to the file permissions, and file is the filename to modify.
You can specify multiple files or wildcards. For example:
chmod go-rwx,u+w privatefile
chmod a+r public*
chmod 640 groupfile1 groupfile2

The modes parameter can be a set of octal digits in the Unix style. (If you
don't know what this means, you probably don't want to be using it!)
Alternatively, it can be a list of permission modifications, separated by
commas. Each modification consists of:
The people affected by the modification. This can be u (the owning
user), g (members of the owning group), or o (everybody else ‘others’), or some combination of those. It can also be a (‘all’) to
affect everybody at once.
A + or - sign, indicating whether permissions are to be added or
removed.
The actual permissions being added or removed. These can be r
(permission to read the file), w (permission to write to the file), and x
(permission to execute the file, or in the case of a directory,
permission to access files within the directory).
So the above examples would do:
The first example: go-rwx removes read, write and execute
permissions for members of the owning group and everybody else
(so the only permissions left are the ones for the file owner). u+w
adds write permission for the file owner.
The second example: a+r adds read permission for everybody to all

files and directories starting with ‘public’.
In addition to all this, there are a few extra special cases for Unix
systems. On non-Unix systems these are unlikely to be useful:
You can specify u+s and u-s to add or remove the Unix set-user-ID
bit. This is typically only useful for special purposes; refer to your
Unix documentation if you're not sure about it.
You can specify g+s and g-s to add or remove the Unix set-group-ID
bit. On a file, this works similarly to the set-user-ID bit (see your Unix
documentation again); on a directory it ensures that files created in
the directory are accessible by members of the group that owns the
directory.
You can specify +t and -t to add or remove the Unix ‘sticky bit’.
When applied to a directory, this means that the owner of a file in
that directory can delete the file (whereas normally only the owner of
the directory would be allowed to).

6.2.15 The del command: delete remote files
To delete a file on the server, type del and then the filename or
filenames:
del oldfile.dat
del file1.txt file2.txt
del *.o

Files will be deleted without further prompting, even if multiple files are
specified.
will only delete files. You cannot use it to delete directories; use rmdir
for that.
del

The rm command works exactly the same way as del.

6.2.16 The mkdir command: create remote
directories
To create a directory on the server, type mkdir and then the directory
name:
mkdir newstuff

You can specify multiple directories to create at once:
mkdir dir1 dir2 dir3

6.2.17 The rmdir command: remove remote
directories
To remove a directory on the server, type rmdir and then the directory
name or names:
rmdir oldstuff
rmdir *.old ancient

Directories will be deleted without further prompting, even if multiple
directories are specified.
Most SFTP servers will probably refuse to remove a directory if the
directory has anything in it, so you will need to delete the contents first.

6.2.18 The mv command: move and rename
remote files
To rename a single file on the server, type mv, then the current file name,
and then the new file name:
mv oldfile newname

You can also move the file into a different directory and change the
name:
mv oldfile dir/newname

To move one or more files into an existing subdirectory, specify the files
(using wildcards if desired), and then the destination directory:
mv file dir
mv file1 dir1/file2 dir2
mv *.c *.h ..

The rename and ren commands work exactly the same way as mv.

6.2.19 The ! command: run a local Windows
command
You can run local Windows commands using the ! command. This is the
only PSFTP command that is not subject to the command quoting rules
given in section 6.2.1. If any command line begins with the ! character,
then the rest of the line will be passed straight to Windows without further
translation.
For example, if you want to move an existing copy of a file out of the way
before downloading an updated version, you might type:
psftp> !ren myfile.dat myfile.bak
psftp> get myfile.dat

using the Windows ren command to rename files on your local PC.

6.3 Using public key authentication with PSFTP
Like PuTTY, PSFTP can authenticate using a public key instead of a
password. There are three ways you can do this.
Firstly, PSFTP can use PuTTY saved sessions in place of hostnames. So
you might do this:
Run PuTTY, and create a PuTTY saved session (see section 4.1.2)
which specifies your private key file (see section 4.22.8). You will
probably also want to specify a username to log in as (see section
4.14.1).
In PSFTP, you can now use the name of the session instead of a
hostname: type psftp sessionname, where sessionname is replaced
by the name of your saved session.
Secondly, you can supply the name of a private key file on the command
line, with the -i option. See section 3.8.3.18 for more information.
Thirdly, PSFTP will attempt to authenticate using Pageant if Pageant is
running (see chapter 9). So you would do this:
Ensure Pageant is running, and has your private key stored in it.
Specify a user and host name to PSFTP as normal. PSFTP will
automatically detect Pageant and try to use the keys within it.
For more general information on public-key authentication, see chapter 8.

Chapter 7: Using the command-line connection
tool Plink
Plink is a command-line connection tool similar to UNIX ssh. It is mostly
used for automated operations, such as making CVS access a repository
on a remote server.
Plink is probably not what you want if you want to run an interactive
session in a console window.
7.1 Starting Plink
7.2 Using Plink
7.2.1 Using Plink for interactive logins
7.2.2 Using Plink for automated connections
7.2.3 Plink command line options
7.3 Using Plink in batch files and scripts
7.4 Using Plink with CVS
7.5 Using Plink with WinCVS

7.1 Starting Plink
Plink is a command line application. This means that you cannot just
double-click on its icon to run it and instead you have to bring up a
console window. In Windows 95, 98, and ME, this is called an ‘MS-DOS
Prompt’, and in Windows NT, 2000, and XP, it is called a ‘Command
Prompt’. It should be available from the Programs section of your Start
Menu.
In order to use Plink, the file plink.exe will need either to be on your PATH
or in your current directory. To add the directory containing Plink to your
PATH environment variable, type into the console window:
set PATH=C:\path\to\putty\directory;%PATH%

This will only work for the lifetime of that particular console window. To
set your PATH more permanently on Windows NT, 2000, and XP, use the
Environment tab of the System Control Panel. On Windows 95, 98, and
ME, you will need to edit your AUTOEXEC.BAT to include a set command
like the one above.

7.2 Using Plink
This section describes the basics of how to use Plink for interactive logins
and for automated processes.
Once you've got a console window to type into, you can just type plink
on its own to bring up a usage message. This tells you the version of
Plink you're using, and gives you a brief summary of how to use Plink:
Z:\sysosd>plink
Plink: command-line connection utility
Release 0.70
Usage: plink [options] [user@]host [command]
("host" can also be a PuTTY saved session name)
Options:
-V
print version information and exit
-pgpfp
print PGP key fingerprints and exit
-v
show verbose messages
-load sessname Load settings from saved session
-ssh -telnet -rlogin -raw -serial
force use of a particular protocol
-P port
connect to specified port
-l user
connect with specified username
-batch
disable all interactive prompts
-proxycmd command
use 'command' as local proxy
-sercfg configuration-string (e.g. 19200,8,n,1,X)
Specify the serial configuration (serial only)
The following options only apply to SSH connections:
-pw passw login with specified password
-D [listen-IP:]listen-port
Dynamic SOCKS-based port forwarding
-L [listen-IP:]listen-port:host:port
Forward local port to remote address
-R [listen-IP:]listen-port:host:port
Forward remote port to local address
-X -x
enable / disable X11 forwarding
-A -a
enable / disable agent forwarding
-t -T
enable / disable pty allocation
-1 -2
force use of particular protocol version
-4 -6
force use of IPv4 or IPv6
-C
enable compression
-i key
private key file for user authentication
-noagent disable use of Pageant
-agent
enable use of Pageant
-hostkey aa:bb:cc:...

manually specify a host key (may be repeated)
-m file
read remote command(s) from file
-s
remote command is an SSH subsystem (SSH-2 only)
-N
don't start a shell/command (SSH-2 only)
-nc host:port
open tunnel in place of session (SSH-2 only)
-sshlog file
-sshrawlog file
log protocol details to a file
-shareexists
test whether a connection-sharing upstream exists

Once this works, you are ready to use Plink.
7.2.1 Using Plink for interactive logins
7.2.2 Using Plink for automated connections
7.2.3 Plink command line options
7.2.3.1 -batch: disable all interactive prompts
7.2.3.2 -s: remote command is SSH subsystem
7.2.3.3 -shareexists: test for connection-sharing upstream

7.2.1 Using Plink for interactive logins
To make a simple interactive connection to a remote server, just type
plink and then the host name:
Z:\sysosd>plink login.example.com
Debian GNU/Linux 2.2 flunky.example.com
flunky login:

You should then be able to log in as normal and run a session. The
output sent by the server will be written straight to your command prompt
window, which will most likely not interpret terminal control codes in the
way the server expects it to. So if you run any full-screen applications, for
example, you can expect to see strange characters appearing in your
window. Interactive connections like this are not the main point of Plink.
In order to connect with a different protocol, you can give the command
line options -ssh, -telnet, -rlogin or -raw. To make an SSH connection,
for example:
Z:\sysosd>plink -ssh login.example.com
login as:

If you have already set up a PuTTY saved session, then instead of
supplying a host name, you can give the saved session name. This
allows you to use public-key authentication, specify a user name, and
use most of the other features of PuTTY:
Z:\sysosd>plink my-ssh-session
Sent username "fred"
Authenticating with public key "fred@winbox"
Last login: Thu Dec 6 19:25:33 2001 from :0.0
fred@flunky:~$

(You can also use the -load command-line option to load a saved
session; see section 3.8.3.1. If you use -load, the saved session exists,
and it specifies a hostname, you cannot also specify a host or user@host
argument - it will be treated as part of the remote command.)

7.2.2 Using Plink for automated connections
More typically Plink is used with the SSH protocol, to enable you to talk
directly to a program running on the server. To do this you have to ensure
Plink is using the SSH protocol. You can do this in several ways:
Use the -ssh option as described in section 7.2.1.
Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the protocol as SSH.
Set the Windows environment variable PLINK_PROTOCOL to the word
ssh.
Usually Plink is not invoked directly by a user, but run automatically by
another process. Therefore you typically do not want Plink to prompt you
for a user name or a password.
Next, you are likely to need to avoid the various interactive prompts Plink
can produce. You might be prompted to verify the host key of the server
you're connecting to, to enter a user name, or to enter a password.
To avoid being prompted for the server host key when using Plink for an
automated connection, you should first make a manual connection (using
either of PuTTY or Plink) to the same server, verify the host key (see
section 2.2 for more information), and select Yes to add the host key to
the Registry. After that, Plink commands connecting to that server should
not give a host key prompt unless the host key changes.
To avoid being prompted for a user name, you can:
Use the -l option to specify a user name on the command line. For
example, plink login.example.com -l fred.
Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies the username to log in as (see
section 4.14.1).
To avoid being prompted for a password, you should almost certainly set
up public-key authentication. (See chapter 8 for a general introduction to
public-key authentication.) Again, you can do this in two ways:

Set up a PuTTY saved session that describes the server you are
connecting to, and that also specifies a private key file (see section
4.22.8). For this to work without prompting, your private key will need
to have no passphrase.
Store the private key in Pageant. See chapter 9 for further
information.
Once you have done all this, you should be able to run a remote
command on the SSH server machine and have it execute automatically
with no prompting:
Z:\sysosd>plink login.example.com -l fred echo hello, world
hello, world
Z:\sysosd>

Or, if you have set up a saved session with all the connection details:
Z:\sysosd>plink mysession echo hello, world
hello, world
Z:\sysosd>

Then you can set up other programs to run this Plink command and talk
to it as if it were a process on the server machine.

7.2.3 Plink command line options
Plink accepts all the general command line options supported by the
PuTTY tools. See section 3.8.3 for a description of these options.
Plink also supports some of its own options. The following sections
describe Plink's specific command-line options.
7.2.3.1 -batch: disable all interactive prompts
7.2.3.2 -s: remote command is SSH subsystem
7.2.3.3 -shareexists: test for connection-sharing upstream

7.2.3.1 -batch: disable all interactive prompts
If you use the -batch option, Plink will never give an interactive prompt
while establishing the connection. If the server's host key is invalid, for
example (see section 2.2), then the connection will simply be abandoned
instead of asking you what to do next.
This may help Plink's behaviour when it is used in automated scripts:
using -batch, if something goes wrong at connection time, the batch job
will fail rather than hang.

7.2.3.2 -s: remote command is SSH subsystem
If you specify the -s option, Plink passes the specified command as the
name of an SSH ‘subsystem’ rather than an ordinary command line.
(This option is only meaningful with the SSH-2 protocol.)

7.2.3.3 -shareexists: test for connection-sharing
upstream
This option does not make a new connection; instead it allows testing for
the presence of an existing connection that can be shared. (See section
4.18.5 for more information about SSH connection sharing.)
A Plink invocation of the form:
plink -shareexists 

will test whether there is currently a viable ‘upstream’ for the session in
question, which can be specified using any syntax you'd normally use
with Plink to make an actual connection (a host/port number, a bare
saved session name, -load, etc). It returns a zero exit status if a usable
‘upstream’ exists, nonzero otherwise.
(This option is only meaningful with the SSH-2 protocol.)

7.3 Using Plink in batch files and scripts
Once you have set up Plink to be able to log in to a remote server without
any interactive prompting (see section 7.2.2), you can use it for lots of
scripting and batch purposes. For example, to start a backup on a remote
machine, you might use a command like:
plink root@myserver /etc/backups/do-backup.sh

Or perhaps you want to fetch all system log lines relating to a particular
web area:
plink mysession grep /~fred/ /var/log/httpd/access.log > fredlog

Any non-interactive command you could usefully run on the server
command line, you can run in a batch file using Plink in this way.

7.4 Using Plink with CVS
To use Plink with CVS, you need to set the environment variable CVS_RSH
to point to Plink:
set CVS_RSH=\path\to\plink.exe

You also need to arrange to be able to connect to a remote host without
any interactive prompts, as described in section 7.2.2.
You should then be able to run CVS as follows:
cvs -d :ext:user@sessionname:/path/to/repository co module

If you specified a username in your saved session, you don't even need
to specify the ‘user’ part of this, and you can just say:
cvs -d :ext:sessionname:/path/to/repository co module

7.5 Using Plink with WinCVS
Plink can also be used with WinCVS. Firstly, arrange for Plink to be able
to connect to a remote host non-interactively, as described in section
7.2.2.
Then, in WinCVS, bring up the ‘Preferences’ dialogue box from the
Admin menu, and switch to the ‘Ports’ tab. Tick the box there labelled
‘Check for an alternate rsh name’ and in the text entry field to the right
enter the full path to plink.exe. Select ‘OK’ on the ‘Preferences’ dialogue
box.
Next, select ‘Command Line’ from the WinCVS ‘Admin’ menu, and type a
CVS command as in section 7.4, for example:
cvs -d :ext:user@hostname:/path/to/repository co module

or (if you're using a saved session):
cvs -d :ext:user@sessionname:/path/to/repository co module

Select the folder you want to check out to with the ‘Change Folder’
button, and click ‘OK’ to check out your module. Once you've got
modules checked out, WinCVS will happily invoke plink from the GUI for
CVS operations.

Chapter 8: Using public keys for SSH
authentication
8.1 Public key authentication - an introduction
8.2 Using PuTTYgen, the PuTTY key generator
8.2.1 Generating a new key
8.2.2 Selecting the type of key
8.2.3 Selecting the size (strength) of the key
8.2.4 The ‘Generate’ button
8.2.5 The ‘Key fingerprint’ box
8.2.6 Setting a comment for your key
8.2.7 Setting a passphrase for your key
8.2.8 Saving your private key to a disk file
8.2.9 Saving your public key to a disk file
8.2.10 ‘Public key for pasting into authorized_keys file’
8.2.11 Reloading a private key
8.2.12 Dealing with private keys in other formats
8.3 Getting ready for public key authentication

8.1 Public key authentication - an introduction
Public key authentication is an alternative means of identifying yourself to
a login server, instead of typing a password. It is more secure and more
flexible, but more difficult to set up.
In conventional password authentication, you prove you are who you
claim to be by proving that you know the correct password. The only way
to prove you know the password is to tell the server what you think the
password is. This means that if the server has been hacked, or spoofed
(see section 2.2), an attacker can learn your password.
Public key authentication solves this problem. You generate a key pair,
consisting of a public key (which everybody is allowed to know) and a
private key (which you keep secret and do not give to anybody). The
private key is able to generate signatures. A signature created using your
private key cannot be forged by anybody who does not have that key; but
anybody who has your public key can verify that a particular signature is
genuine.
So you generate a key pair on your own computer, and you copy the
public key to the server. Then, when the server asks you to prove who
you are, PuTTY can generate a signature using your private key. The
server can verify that signature (since it has your public key) and allow
you to log in. Now if the server is hacked or spoofed, the attacker does
not gain your private key or password; they only gain one signature. And
signatures cannot be re-used, so they have gained nothing.
There is a problem with this: if your private key is stored unprotected on
your own computer, then anybody who gains access to that will be able
to generate signatures as if they were you. So they will be able to log in
to your server under your account. For this reason, your private key is
usually encrypted when it is stored on your local machine, using a
passphrase of your choice. In order to generate a signature, PuTTY must
decrypt the key, so you have to type your passphrase.
This can make public-key authentication less convenient than password
authentication: every time you log in to the server, instead of typing a

short password, you have to type a longer passphrase. One solution to
this is to use an authentication agent, a separate program which holds
decrypted private keys and generates signatures on request. PuTTY's
authentication agent is called Pageant. When you begin a Windows
session, you start Pageant and load your private key into it (typing your
passphrase once). For the rest of your session, you can start PuTTY any
number of times and Pageant will automatically generate signatures
without you having to do anything. When you close your Windows
session, Pageant shuts down, without ever having stored your decrypted
private key on disk. Many people feel this is a good compromise between
security and convenience. See chapter 9 for further details.
There is more than one public-key algorithm available. The most
common are RSA and ECDSA, but others exist, notably DSA (otherwise
known as DSS), the USA's federal Digital Signature Standard. The key
types supported by PuTTY are described in section 8.2.2.

8.2 Using PuTTYgen, the PuTTY key generator
PuTTYgen is a key generator. It generates pairs of public and private
keys to be used with PuTTY, PSCP, and Plink, as well as the PuTTY
authentication agent, Pageant (see chapter 9). PuTTYgen generates
RSA, DSA, ECDSA, and Ed25519 keys.
When you run PuTTYgen you will see a window where you have two
choices: ‘Generate’, to generate a new public/private key pair, or ‘Load’ to
load in an existing private key.
8.2.1 Generating a new key
8.2.2 Selecting the type of key
8.2.3 Selecting the size (strength) of the key
8.2.4 The ‘Generate’ button
8.2.5 The ‘Key fingerprint’ box
8.2.6 Setting a comment for your key
8.2.7 Setting a passphrase for your key
8.2.8 Saving your private key to a disk file
8.2.9 Saving your public key to a disk file
8.2.10 ‘Public key for pasting into authorized_keys file’
8.2.11 Reloading a private key
8.2.12 Dealing with private keys in other formats

8.2.1 Generating a new key
This is a general outline of the procedure for generating a new key pair.
The following sections describe the process in more detail.
First, you need to select which type of key you want to generate, and
also select the strength of the key. This is described in more detail in
section 8.2.2 and section 8.2.3.
Then press the ‘Generate’ button, to actually generate the key.
Section 8.2.4 describes this step.
Once you have generated the key, select a comment field (section
8.2.6) and a passphrase (section 8.2.7).
Now you're ready to save the private key to disk; press the ‘Save
private key’ button. (See section 8.2.8).
Your key pair is now ready for use. You may also want to copy the public
key to your server, either by copying it out of the ‘Public key for pasting
into authorized_keys file’ box (see section 8.2.10), or by using the ‘Save
public key’ button (section 8.2.9). However, you don't need to do this
immediately; if you want, you can load the private key back into
PuTTYgen later (see section 8.2.11) and the public key will be available
for copying and pasting again.
Section 8.3 describes the typical process of configuring PuTTY to attempt
public-key authentication, and configuring your SSH server to accept it.

8.2.2 Selecting the type of key
Before generating a key pair using PuTTYgen, you need to select which
type of key you need. PuTTYgen currently supports these types of key:
An RSA key for use with the SSH-1 protocol.
An RSA key for use with the SSH-2 protocol.
A DSA key for use with the SSH-2 protocol.
An ECDSA (elliptic curve DSA) key for use with the SSH-2 protocol.
An Ed25519 key (another elliptic curve algorithm) for use with the
SSH-2 protocol.
The SSH-1 protocol only supports RSA keys; if you will be connecting
using the SSH-1 protocol, you must select the first key type or your key
will be completely useless.
The SSH-2 protocol supports more than one key type. The types
supported by PuTTY are RSA, DSA, ECDSA, and Ed25519.

8.2.3 Selecting the size (strength) of the key
The ‘Number of bits’ input box allows you to choose the strength of the
key PuTTYgen will generate.
For RSA, 2048 bits should currently be sufficient for most purposes.
For ECDSA, only 256, 384, and 521 bits are supported. (ECDSA
offers equivalent security to RSA with smaller key sizes.)
For Ed25519, the only valid size is 256 bits.

8.2.4 The ‘Generate’ button
Once you have chosen the type of key you want, and the strength of the
key, press the ‘Generate’ button and PuTTYgen will begin the process of
actually generating the key.
First, a progress bar will appear and PuTTYgen will ask you to move the
mouse around to generate randomness. Wave the mouse in circles over
the blank area in the PuTTYgen window, and the progress bar will
gradually fill up as PuTTYgen collects enough randomness. You don't
need to wave the mouse in particularly imaginative patterns (although it
can't hurt); PuTTYgen will collect enough randomness just from the fine
detail of exactly how far the mouse has moved each time Windows
samples its position.
When the progress bar reaches the end, PuTTYgen will begin creating
the key. The progress bar will reset to the start, and gradually move up
again to track the progress of the key generation. It will not move evenly,
and may occasionally slow down to a stop; this is unfortunately
unavoidable, because key generation is a random process and it is
impossible to reliably predict how long it will take.
When the key generation is complete, a new set of controls will appear in
the window to indicate this.

8.2.5 The ‘Key fingerprint’ box
The ‘Key fingerprint’ box shows you a fingerprint value for the generated
key. This is derived cryptographically from the public key value, so it
doesn't need to be kept secret; it is supposed to be more manageable for
human beings than the public key itself.
The fingerprint value is intended to be cryptographically secure, in the
sense that it is computationally infeasible for someone to invent a second
key with the same fingerprint, or to find a key with a particular fingerprint.
So some utilities, such as the Pageant key list box (see section 9.2.1)
and the Unix ssh-add utility, will list key fingerprints rather than the whole
public key.

8.2.6 Setting a comment for your key
If you have more than one key and use them for different purposes, you
don't need to memorise the key fingerprints in order to tell them apart.
PuTTYgen allows you to enter a comment for your key, which will be
displayed whenever PuTTY or Pageant asks you for the passphrase.
The default comment format, if you don't specify one, contains the key
type and the date of generation, such as rsa-key-20011212. Another
commonly used approach is to use your name and the name of the
computer the key will be used on, such as simon@simons-pc.
To alter the key comment, just type your comment text into the ‘Key
comment’ box before saving the private key. If you want to change the
comment later, you can load the private key back into PuTTYgen, change
the comment, and save it again.

8.2.7 Setting a passphrase for your key
The ‘Key passphrase’ and ‘Confirm passphrase’ boxes allow you to
choose a passphrase for your key. The passphrase will be used to
encrypt the key on disk, so you will not be able to use the key without first
entering the passphrase.
When you save the key, PuTTYgen will check that the ‘Key passphrase’
and ‘Confirm passphrase’ boxes both contain exactly the same
passphrase, and will refuse to save the key otherwise.
If you leave the passphrase fields blank, the key will be saved
unencrypted. You should not do this without good reason; if you do, your
private key file on disk will be all an attacker needs to gain access to any
machine configured to accept that key. If you want to be able to log in
without having to type a passphrase every time, you should consider
using Pageant (chapter 9) so that your decrypted key is only held in
memory rather than on disk.
Under special circumstances you may genuinely need to use a key with
no passphrase; for example, if you need to run an automated batch script
that needs to make an SSH connection, you can't be there to type the
passphrase. In this case we recommend you generate a special key for
each specific batch script (or whatever) that needs one, and on the
server side you should arrange that each key is restricted so that it can
only be used for that specific purpose. The documentation for your SSH
server should explain how to do this (it will probably vary between
servers).
Choosing a good passphrase is difficult. Just as you shouldn't use a
dictionary word as a password because it's easy for an attacker to run
through a whole dictionary, you should not use a song lyric, quotation or
other well-known sentence as a passphrase. DiceWare
(www.diceware.com) recommends using at least five words each
generated randomly by rolling five dice, which gives over 2^64 possible
passphrases and is probably not a bad scheme. If you want your
passphrase to make grammatical sense, this cuts down the possibilities a
lot and you should use a longer one as a result.

Do not forget your passphrase. There is no way to recover it.

8.2.8 Saving your private key to a disk file
Once you have generated a key, set a comment field and set a
passphrase, you are ready to save your private key to disk.
Press the ‘Save private key’ button. PuTTYgen will put up a dialog box
asking you where to save the file. Select a directory, type in a file name,
and press ‘Save’.
This file is in PuTTY's native format (*.PPK); it is the one you will need to
tell PuTTY to use for authentication (see section 4.22.8) or tell Pageant to
load (see section 9.2.2).

8.2.9 Saving your public key to a disk file
RFC 4716 specifies a standard format for storing SSH-2 public keys on
disk. Some SSH servers (such as ssh.com's) require a public key in this
format in order to accept authentication with the corresponding private
key. (Others, such as OpenSSH, use a different format; see section
8.2.10.)
To save your public key in the SSH-2 standard format, press the ‘Save
public key’ button in PuTTYgen. PuTTYgen will put up a dialog box
asking you where to save the file. Select a directory, type in a file name,
and press ‘Save’.
You will then probably want to copy the public key file to your SSH server
machine. See section 8.3 for general instructions on configuring publickey authentication once you have generated a key.
If you use this option with an SSH-1 key, the file PuTTYgen saves will
contain exactly the same text that appears in the ‘Public key for pasting’
box. This is the only existing standard for SSH-1 public keys.

8.2.10 ‘Public key for pasting into
authorized_keys file’
All SSH-1 servers require your public key to be given to it in a one-line
format before it will accept authentication with your private key. The
OpenSSH server also requires this for SSH-2.
The ‘Public key for pasting into authorized_keys file’ gives the public-key
data in the correct one-line format. Typically you will want to select the
entire contents of the box using the mouse, press Ctrl+C to copy it to the
clipboard, and then paste the data into a PuTTY session which is already
connected to the server.
See section 8.3 for general instructions on configuring public-key
authentication once you have generated a key.

8.2.11 Reloading a private key
PuTTYgen allows you to load an existing private key file into memory. If
you do this, you can then change the passphrase and comment before
saving it again; you can also make extra copies of the public key.
To load an existing key, press the ‘Load’ button. PuTTYgen will put up a
dialog box where you can browse around the file system and find your
key file. Once you select the file, PuTTYgen will ask you for a passphrase
(if necessary) and will then display the key details in the same way as if it
had just generated the key.
If you use the Load command to load a foreign key format, it will work,
but you will see a message box warning you that the key you have
loaded is not a PuTTY native key. See section 8.2.12 for information
about importing foreign key formats.

8.2.12 Dealing with private keys in other formats
Most SSH-1 clients use a standard format for storing private keys on
disk. PuTTY uses this format as well; so if you have generated an SSH-1
private key using OpenSSH or ssh.com's client, you can use it with
PuTTY, and vice versa.
However, SSH-2 private keys have no standard format. OpenSSH and
ssh.com have different formats, and PuTTY's is different again. So a key
generated with one client cannot immediately be used with another.
Using the ‘Import’ command from the ‘Conversions’ menu, PuTTYgen
can load SSH-2 private keys in OpenSSH's format and ssh.com's format.
Once you have loaded one of these key types, you can then save it back
out as a PuTTY-format key (*.PPK) so that you can use it with the PuTTY
suite. The passphrase will be unchanged by this process (unless you
deliberately change it). You may want to change the key comment before
you save the key, since OpenSSH's SSH-2 key format contains no space
for a comment and ssh.com's default comment format is long and
verbose.
PuTTYgen can also export private keys in OpenSSH format and in
ssh.com format. To do so, select one of the ‘Export’ options from the
‘Conversions’ menu. Exporting a key works exactly like saving it (see
section 8.2.8) - you need to have typed your passphrase in beforehand,
and you will be warned if you are about to save a key without a
passphrase.
For OpenSSH there are two options. Modern OpenSSH actually has two
formats it uses for storing private keys. ‘Export OpenSSH key’ will
automatically choose the oldest format supported for the key type, for
maximum backward compatibility with older versions of OpenSSH; for
newer key types like Ed25519, it will use the newer format as that is the
only legal option. If you have some specific reason for wanting to use
OpenSSH's newer format even for RSA, DSA, or ECDSA keys, you can
choose ‘Export OpenSSH key (force new file format)’.
Note that since only SSH-2 keys come in different formats, the export

options are not available if you have generated an SSH-1 key.

8.3 Getting ready for public key authentication
Connect to your SSH server using PuTTY with the SSH protocol. When
the connection succeeds you will be prompted for your user name and
password to login. Once logged in, you must configure the server to
accept your public key for authentication:
If your server is using the SSH-1 protocol, you should change into
the .ssh directory and open the file authorized_keys with your
favourite editor. (You may have to create this file if this is the first key
you have put in it). Then switch to the PuTTYgen window, select all
of the text in the ‘Public key for pasting into authorized_keys file’ box
(see section 8.2.10), and copy it to the clipboard (Ctrl+C). Then,
switch back to the PuTTY window and insert the data into the open
file, making sure it ends up all on one line. Save the file.
If your server is OpenSSH and is using the SSH-2 protocol, you
should follow the same instructions, except that in earlier versions of
OpenSSH 2 the file might be called authorized_keys2. (In modern
versions the same authorized_keys file is used for both SSH-1 and
SSH-2 keys.)
If your server is ssh.com's product and is using SSH-2, you need to
save a public key file from PuTTYgen (see section 8.2.9), and copy
that into the .ssh2 directory on the server. Then you should go into
that .ssh2 directory, and edit (or create) a file called authorization.
In this file you should put a line like Key mykey.pub, with mykey.pub
replaced by the name of your key file.
For other SSH server software, you should refer to the manual for
that server.
You may also need to ensure that your home directory, your .ssh
directory, and any other files involved (such as authorized_keys,
authorized_keys2 or authorization) are not group-writable or worldwritable. You can typically do this by using a command such as
chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keys

Your server should now be configured to accept authentication using your
private key. Now you need to configure PuTTY to attempt authentication

using your private key. You can do this in any of three ways:
Select the private key in PuTTY's configuration. See section 4.22.8
for details.
Specify the key file on the command line with the -i option. See
section 3.8.3.18 for details.
Load the private key into Pageant (see chapter 9). In this case
PuTTY will automatically try to use it for authentication if it can.

Chapter 9: Using Pageant for authentication
Pageant is an SSH authentication agent. It holds your private keys in
memory, already decoded, so that you can use them often without
needing to type a passphrase.
9.1 Getting started with Pageant
9.2 The Pageant main window
9.2.1 The key list box
9.2.2 The ‘Add Key’ button
9.2.3 The ‘Remove Key’ button
9.3 The Pageant command line
9.3.1 Making Pageant automatically load keys on startup
9.3.2 Making Pageant run another program
9.4 Using agent forwarding
9.5 Security considerations

9.1 Getting started with Pageant
Before you run Pageant, you need to have a private key in *.PPK format.
See chapter 8 to find out how to generate and use one.
When you run Pageant, it will put an icon of a computer wearing a hat
into the System tray. It will then sit and do nothing, until you load a private
key into it.
If you click the Pageant icon with the right mouse button, you will see a
menu. Select ‘View Keys’ from this menu. The Pageant main window will
appear. (You can also bring this window up by double-clicking on the
Pageant icon.)
The Pageant window contains a list box. This shows the private keys
Pageant is holding. When you start Pageant, it has no keys, so the list
box will be empty. After you add one or more keys, they will show up in
the list box.
To add a key to Pageant, press the ‘Add Key’ button. Pageant will bring
up a file dialog, labelled ‘Select Private Key File’. Find your private key
file in this dialog, and press ‘Open’.
Pageant will now load the private key. If the key is protected by a
passphrase, Pageant will ask you to type the passphrase. When the key
has been loaded, it will appear in the list in the Pageant window.
Now start PuTTY and open an SSH session to a site that accepts your
key. PuTTY will notice that Pageant is running, retrieve the key
automatically from Pageant, and use it to authenticate. You can now
open as many PuTTY sessions as you like without having to type your
passphrase again.
(PuTTY can be configured not to try to use Pageant, but it will try by
default. See section 4.22.3 and section 3.8.3.9 for more information.)
When you want to shut down Pageant, click the right button on the
Pageant icon in the System tray, and select ‘Exit’ from the menu. Closing
the Pageant main window does not shut down Pageant.

9.2 The Pageant main window
The Pageant main window appears when you left-click on the Pageant
system tray icon, or alternatively right-click and select ‘View Keys’ from
the menu. You can use it to keep track of what keys are currently loaded
into Pageant, and to add new ones or remove the existing keys.
9.2.1 The key list box
9.2.2 The ‘Add Key’ button
9.2.3 The ‘Remove Key’ button

9.2.1 The key list box
The large list box in the Pageant main window lists the private keys that
are currently loaded into Pageant. The list might look something like this:
ssh-rsa 2048 22:d6:69:c9:22:51:ac:cb:b9:15:67:47:f7:65:6d:d7 k1
ssh-dss 2048 e4:6c:69:f3:4f:fc:cf:fc:96:c0:88:34:a7:1e:59:d7 k2

For each key, the list box will tell you:
The type of the key. Currently, this can be ssh1 (an RSA key for use
with the SSH-1 protocol), ssh-rsa (an RSA key for use with the SSH2 protocol), ssh-dss (a DSA key for use with the SSH-2 protocol),
ecdsa-sha2-* (an ECDSA key for use with the SSH-2 protocol), or
ssh-ed25519 (an Ed25519 key for use with the SSH-2 protocol).
The size (in bits) of the key.
The fingerprint for the public key. This should be the same fingerprint
given by PuTTYgen, and (hopefully) also the same fingerprint shown
by remote utilities such as ssh-keygen when applied to your
authorized_keys file.
The comment attached to the key.

9.2.2 The ‘Add Key’ button
To add a key to Pageant by reading it out of a local disk file, press the
‘Add Key’ button in the Pageant main window, or alternatively right-click
on the Pageant icon in the system tray and select ‘Add Key’ from there.
Pageant will bring up a file dialog, labelled ‘Select Private Key File’. Find
your private key file in this dialog, and press ‘Open’. If you want to add
more than one key at once, you can select multiple files using Shift-click
(to select several adjacent files) or Ctrl-click (to select non-adjacent files).
Pageant will now load the private key(s). If a key is protected by a
passphrase, Pageant will ask you to type the passphrase.
(This is not the only way to add a private key to Pageant. You can also
add one from a remote system by using agent forwarding; see section
9.4 for details.)

9.2.3 The ‘Remove Key’ button
If you need to remove a key from Pageant, select that key in the list box,
and press the ‘Remove Key’ button. Pageant will remove the key from its
memory.
You can apply this to keys you added using the ‘Add Key’ button, or to
keys you added remotely using agent forwarding (see section 9.4); it
makes no difference.

9.3 The Pageant command line
Pageant can be made to do things automatically when it starts up, by
specifying instructions on its command line. If you're starting Pageant
from the Windows GUI, you can arrange this by editing the properties of
the Windows shortcut that it was started from.
If Pageant is already running, invoking it again with the options below
causes actions to be performed with the existing instance, not a new one.
9.3.1 Making Pageant automatically load keys on startup
9.3.2 Making Pageant run another program

9.3.1 Making Pageant automatically load keys on
startup
Pageant can automatically load one or more private keys when it starts
up, if you provide them on the Pageant command line. Your command
line might then look like:
C:\PuTTY\pageant.exe d:\main.ppk d:\secondary.ppk

If the keys are stored encrypted, Pageant will request the passphrases
on startup.
If Pageant is already running, this syntax loads keys into the existing
Pageant.

9.3.2 Making Pageant run another program
You can arrange for Pageant to start another program once it has
initialised itself and loaded any keys specified on its command line. This
program (perhaps a PuTTY, or a WinCVS making use of Plink, or
whatever) will then be able to use the keys Pageant has loaded.
You do this by specifying the -c option followed by the command, like
this:
C:\PuTTY\pageant.exe d:\main.ppk -c C:\PuTTY\putty.exe

9.4 Using agent forwarding
Agent forwarding is a mechanism that allows applications on your SSH
server machine to talk to the agent on your client machine.
Note that at present, agent forwarding in SSH-2 is only available when
your SSH server is OpenSSH. The ssh.com server uses a different agent
protocol, which PuTTY does not yet support.
To enable agent forwarding, first start Pageant. Then set up a PuTTY
SSH session in which ‘Allow agent forwarding’ is enabled (see section
4.22.6). Open the session as normal. (Alternatively, you can use the -A
command line option; see section 3.8.3.10 for details.)
If this has worked, your applications on the server should now have
access to a Unix domain socket which the SSH server will forward back
to PuTTY, and PuTTY will forward on to the agent. To check that this has
actually happened, you can try this command on Unix server machines:
unixbox:~$ echo $SSH_AUTH_SOCK
/tmp/ssh-XXNP18Jz/agent.28794
unixbox:~$

If the result line comes up blank, agent forwarding has not been enabled
at all.
Now if you run ssh on the server and use it to connect through to another
server that accepts one of the keys in Pageant, you should be able to log
in without a password:
unixbox:~$ ssh -v otherunixbox
[...]
debug: next auth method to try is publickey
debug: userauth_pubkey_agent: trying agent key my-putty-key
debug: ssh-userauth2 successful: method publickey
[...]

If you enable agent forwarding on that SSH connection as well (see the
manual for your server-side SSH client to find out how to do this), your
authentication keys will still be available on the next machine you connect
to - two SSH connections away from where they're actually stored.

In addition, if you have a private key on one of the SSH servers, you can
send it all the way back to Pageant using the local ssh-add command:
unixbox:~$ ssh-add ~/.ssh/id_rsa
Need passphrase for /home/fred/.ssh/id_rsa
Enter passphrase for /home/fred/.ssh/id_rsa:
Identity added: /home/fred/.ssh/id_rsa (/home/simon/.ssh/id_rsa)
unixbox:~$

and then it's available to every machine that has agent forwarding
available (not just the ones downstream of the place you added it).

9.5 Security considerations
Using Pageant for public-key authentication gives you the convenience of
being able to open multiple SSH sessions without having to type a
passphrase every time, but also gives you the security benefit of never
storing a decrypted private key on disk. Many people feel this is a good
compromise between security and convenience.
It is a compromise, however. Holding your decrypted private keys in
Pageant is better than storing them in easy-to-find disk files, but still less
secure than not storing them anywhere at all. This is for two reasons:
Windows unfortunately provides no way to protect pieces of memory
from being written to the system swap file. So if Pageant is holding
your private keys for a long period of time, it's possible that
decrypted private key data may be written to the system swap file,
and an attacker who gained access to your hard disk later on might
be able to recover that data. (However, if you stored an unencrypted
key in a disk file they would certainly be able to recover it.)
Although, like most modern operating systems, Windows prevents
programs from accidentally accessing one another's memory space,
it does allow programs to access one another's memory space
deliberately, for special purposes such as debugging. This means
that if you allow a virus, trojan, or other malicious program on to your
Windows system while Pageant is running, it could access the
memory of the Pageant process, extract your decrypted
authentication keys, and send them back to its master.
Similarly, use of agent forwarding is a security improvement on other
methods of one-touch authentication, but not perfect. Holding your keys
in Pageant on your Windows box has a security advantage over holding
them on the remote server machine itself (either in an agent or just
unencrypted on disk), because if the server machine ever sees your
unencrypted private key then the sysadmin or anyone who cracks the
machine can steal the keys and pretend to be you for as long as they
want.
However, the sysadmin of the server machine can always pretend to be

you on that machine. So if you forward your agent to a server machine,
then the sysadmin of that machine can access the forwarded agent
connection and request signatures from any of your private keys, and can
therefore log in to other machines as you. They can only do this to a
limited extent - when the agent forwarding disappears they lose the ability
- but using Pageant doesn't actually prevent the sysadmin (or hackers)
on the server from doing this.
Therefore, if you don't trust the sysadmin of a server machine, you
should never use agent forwarding to that machine. (Of course you also
shouldn't store private keys on that machine, type passphrases into it, or
log into other machines from it in any way at all; Pageant is hardly unique
in this respect.)

Chapter 10: Common error messages
This chapter lists a number of common error messages which PuTTY
and its associated tools can produce, and explains what they mean in
more detail.
We do not attempt to list all error messages here: there are many which
should never occur, and some which should be self-explanatory. If you
get an error message which is not listed in this chapter and which you
don't understand, report it to us as a bug (see appendix B) and we will
add documentation for it.
10.1 ‘The server's host key is not cached in the registry’
10.2 ‘WARNING - POTENTIAL SECURITY BREACH!’
10.3 ‘SSH protocol version 2 required by our configuration but server
only provides (old, insecure) SSH-1’
10.4 ‘The first cipher supported by the server is ... below the
configured warning threshold’
10.5 ‘Server sent disconnect message type 2 (protocol error): "Too
many authentication failures for root"’
10.6 ‘Out of memory’
10.7 ‘Internal error’, ‘Internal fault’, ‘Assertion failed’
10.8 ‘Unable to use this private key file’, ‘Couldn't load private key’,
‘Key is of wrong type’
10.9 ‘Server refused our public key’ or ‘Key refused’
10.10 ‘Access denied’, ‘Authentication refused’
10.11 ‘No supported authentication methods available’
10.12 ‘Incorrect CRC received on packet’ or ‘Incorrect MAC received
on packet’
10.13 ‘Incoming packet was garbled on decryption’
10.14 ‘PuTTY X11 proxy: various errors’
10.15 ‘Network error: Software caused connection abort’
10.16 ‘Network error: Connection reset by peer’
10.17 ‘Network error: Connection refused’
10.18 ‘Network error: Connection timed out’
10.19 ‘Network error: Cannot assign requested address’

10.1 ‘The server's host key is not cached in the
registry’
This error message occurs when PuTTY connects to a new SSH server.
Every server identifies itself by means of a host key; once PuTTY knows
the host key for a server, it will be able to detect if a malicious attacker
redirects your connection to another machine.
If you see this message, it means that PuTTY has not seen this host key
before, and has no way of knowing whether it is correct or not. You
should attempt to verify the host key by other means, such as asking the
machine's administrator.
If you see this message and you know that your installation of PuTTY has
connected to the same server before, it may have been recently
upgraded to SSH protocol version 2. SSH protocols 1 and 2 use separate
host keys, so when you first use SSH-2 with a server you have only used
SSH-1 with before, you will see this message again. You should verify
the correctness of the key as before.
See section 2.2 for more information on host keys.

10.2 ‘WARNING - POTENTIAL SECURITY
BREACH!’
This message, followed by ‘The server's host key does not match the one
PuTTY has cached in the registry’, means that PuTTY has connected to
the SSH server before, knows what its host key should be, but has found
a different one.
This may mean that a malicious attacker has replaced your server with a
different one, or has redirected your network connection to their own
machine. On the other hand, it may simply mean that the administrator of
your server has accidentally changed the key while upgrading the SSH
software; this shouldn't happen but it is unfortunately possible.
You should contact your server's administrator and see whether they
expect the host key to have changed. If so, verify the new host key in the
same way as you would if it was new.
See section 2.2 for more information on host keys.

10.3 ‘SSH protocol version 2 required by our
configuration but server only provides (old,
insecure) SSH-1’
By default, PuTTY only supports connecting to SSH servers that
implement SSH protocol version 2. If you see this message, the server
you're trying to connect to only supports the older SSH-1 protocol.
If the server genuinely only supports SSH-1, then you need to either
change the ‘SSH protocol version’ setting (see section 4.18.4), or use the
-1 command-line option; in any case, you should not treat the resulting
connection as secure.
You might start seeing this message with new versions of PuTTY (from
0.68 onwards) where you didn't before, because it used to be possible to
configure PuTTY to automatically fall back from SSH-2 to SSH-1. This is
no longer supported, to prevent the possibility of a downgrade attack.

10.4 ‘The first cipher supported by the server is
... below the configured warning threshold’
This occurs when the SSH server does not offer any ciphers which you
have configured PuTTY to consider strong enough. By default, PuTTY
puts up this warning only for single-DES and Arcfour encryption.
See section 4.21 for more information on this message.

10.5 ‘Server sent disconnect message type 2
(protocol error): "Too many authentication
failures for root"’
This message is produced by an OpenSSH (or Sun SSH) server if it
receives more failed authentication attempts than it is willing to tolerate.
This can easily happen if you are using Pageant and have a large
number of keys loaded into it, since these servers count each offer of a
public key as an authentication attempt. This can be worked around by
specifying the key that's required for the authentication in the PuTTY
configuration (see section 4.22.8); PuTTY will ignore any other keys
Pageant may have, but will ask Pageant to do the authentication, so that
you don't have to type your passphrase.
On the server, this can be worked around by disabling public-key
authentication or (for Sun SSH only) by increasing MaxAuthTries in
sshd_config.

10.6 ‘Out of memory’
This occurs when PuTTY tries to allocate more memory than the system
can give it. This may happen for genuine reasons: if the computer really
has run out of memory, or if you have configured an extremely large
number of lines of scrollback in your terminal. PuTTY is not able to
recover from running out of memory; it will terminate immediately after
giving this error.
However, this error can also occur when memory is not running out at all,
because PuTTY receives data in the wrong format. In SSH-2 and also in
SFTP, the server sends the length of each message before the message
itself; so PuTTY will receive the length, try to allocate space for the
message, and then receive the rest of the message. If the length PuTTY
receives is garbage, it will try to allocate a ridiculous amount of memory,
and will terminate with an ‘Out of memory’ error.
This can happen in SSH-2, if PuTTY and the server have not enabled
encryption in the same way (see question A.7.3 in the FAQ).
This can also happen in PSCP or PSFTP, if your login scripts on the
server generate output: the client program will be expecting an SFTP
message starting with a length, and if it receives some text from your
login scripts instead it will try to interpret them as a message length. See
question A.7.4 for details of this.

10.7 ‘Internal error’, ‘Internal fault’, ‘Assertion
failed’
Any error beginning with the word ‘Internal’ should never occur. If it does,
there is a bug in PuTTY by definition; please see appendix B and report it
to us.
Similarly, any error message starting with ‘Assertion failed’ is a bug in
PuTTY. Please report it to us, and include the exact text from the error
message box.

10.8 ‘Unable to use this private key file’,
‘Couldn't load private key’, ‘Key is of wrong
type’
Various forms of this error are printed in the PuTTY window, or written to
the PuTTY Event Log (see section 3.1.3.1) when trying public-key
authentication, or given by Pageant when trying to load a private key.
If you see one of these messages, it often indicates that you've tried to
load a key of an inappropriate type into PuTTY, Plink, PSCP, PSFTP, or
Pageant.
You may have specified a key that's inappropriate for the connection
you're making. The SSH-1 and SSH-2 protocols require different private
key formats, and a SSH-1 key can't be used for a SSH-2 connection (or
vice versa).
Alternatively, you may have tried to load an SSH-2 key in a ‘foreign’
format (OpenSSH or ssh.com) directly into one of the PuTTY tools, in
which case you need to import it into PuTTY's native format (*.PPK) using
PuTTYgen - see section 8.2.12.

10.9 ‘Server refused our public key’ or ‘Key
refused’
Various forms of this error are printed in the PuTTY window, or written to
the PuTTY Event Log (see section 3.1.3.1) when trying public-key
authentication.
If you see one of these messages, it means that PuTTY has sent a public
key to the server and offered to authenticate with it, and the server has
refused to accept authentication. This usually means that the server is
not configured to accept this key to authenticate this user.
This is almost certainly not a problem with PuTTY. If you see this type of
message, the first thing you should do is check your server configuration
carefully. Common errors include having the wrong permissions or
ownership set on the public key or the user's home directory on the
server. Also, read the PuTTY Event Log; the server may have sent
diagnostic messages explaining exactly what problem it had with your
setup.
Section 8.3 has some hints on server-side public key setup.

10.10 ‘Access denied’, ‘Authentication refused’
Various forms of this error are printed in the PuTTY window, or written to
the PuTTY Event Log (see section 3.1.3.1) during authentication.
If you see one of these messages, it means that the server has refused
all the forms of authentication PuTTY has tried and it has no further
ideas.
It may be worth checking the Event Log for diagnostic messages from the
server giving more detail.
This error can be caused by buggy SSH-1 servers that fail to cope with
the various strategies we use for camouflaging passwords in transit.
Upgrade your server, or use the workarounds described in section 4.27.1
and possibly section 4.27.2.

10.11 ‘No supported authentication methods
available’
This error indicates that PuTTY has run out of ways to authenticate you
to an SSH server. This may be because PuTTY has TIS or keyboardinteractive authentication disabled, in which case section 4.22.4 and
section 4.22.5.

10.12 ‘Incorrect CRC received on packet’ or
‘Incorrect MAC received on packet’
This error occurs when PuTTY decrypts an SSH packet and its
checksum is not correct. This probably means something has gone
wrong in the encryption or decryption process. It's difficult to tell from this
error message whether the problem is in the client, in the server, or in
between.
In particular, if the network is corrupting data at the TCP level, it may only
be obvious with cryptographic protocols such as SSH, which explicitly
check the integrity of the transferred data and complain loudly if the
checks fail. Corruption of protocols without integrity protection (such as
HTTP) will manifest in more subtle failures (such as misdisplayed text or
images in a web browser) which may not be noticed.
Occasionally this has been caused by server bugs. An example is the
bug described at section 4.27.6, although you're very unlikely to
encounter that one these days.
In this context MAC stands for Message Authentication Code. It's a
cryptographic term, and it has nothing at all to do with Ethernet MAC
(Media Access Control) addresses, or with the Apple computer.

10.13 ‘Incoming packet was garbled on
decryption’
This error occurs when PuTTY decrypts an SSH packet and the
decrypted data makes no sense. This probably means something has
gone wrong in the encryption or decryption process. It's difficult to tell
from this error message whether the problem is in the client, in the
server, or in between.
If you get this error, one thing you could try would be to fiddle with the
setting of ‘Miscomputes SSH-2 encryption keys’ (see section 4.27.7) or
‘Ignores SSH-2 maximum packet size’ (see section 4.27.11) on the Bugs
panel.

10.14 ‘PuTTY X11 proxy: various errors’
This family of errors are reported when PuTTY is doing X forwarding.
They are sent back to the X application running on the SSH server, which
will usually report the error to the user.
When PuTTY enables X forwarding (see section 3.4) it creates a virtual X
display running on the SSH server. This display requires authentication to
connect to it (this is how PuTTY prevents other users on your server
machine from connecting through the PuTTY proxy to your real X
display). PuTTY also sends the server the details it needs to enable
clients to connect, and the server should put this mechanism in place
automatically, so your X applications should just work.
A common reason why people see one of these messages is because
they used SSH to log in as one user (let's say ‘fred’), and then used the
Unix su command to become another user (typically ‘root’). The original
user, ‘fred’, has access to the X authentication data provided by the SSH
server, and can run X applications which are forwarded over the SSH
connection. However, the second user (‘root’) does not automatically
have the authentication data passed on to it, so attempting to run an X
application as that user often fails with this error.
If this happens, it is not a problem with PuTTY. You need to arrange for
your X authentication data to be passed from the user you logged in as to
the user you used su to become. How you do this depends on your
particular system; in fact many modern versions of su do it automatically.

10.15 ‘Network error: Software caused
connection abort’
This is a generic error produced by the Windows network code when it
kills an established connection for some reason. For example, it might
happen if you pull the network cable out of the back of an Ethernetconnected computer, or if Windows has any other similar reason to
believe the entire network has become unreachable.
Windows also generates this error if it has given up on the machine at the
other end of the connection ever responding to it. If the network between
your client and server goes down and your client then tries to send some
data, Windows will make several attempts to send the data and will then
give up and kill the connection. In particular, this can occur even if you
didn't type anything, if you are using SSH-2 and PuTTY attempts a key
re-exchange. (See section 4.19.2 for more about key re-exchange.)
(It can also occur if you are using keepalives in your connection. Other
people have reported that keepalives fix this error for them. See section
4.13.1 for a discussion of the pros and cons of keepalives.)
We are not aware of any reason why this error might occur that would
represent a bug in PuTTY. The problem is between you, your Windows
system, your network and the remote system.

10.16 ‘Network error: Connection reset by peer’
This error occurs when the machines at each end of a network
connection lose track of the state of the connection between them. For
example, you might see it if your SSH server crashes, and manages to
reboot fully before you next attempt to send data to it.
However, the most common reason to see this message is if you are
connecting through a firewall or a NAT router which has timed the
connection out. See question A.7.8 in the FAQ for more details. You may
be able to improve the situation by using keepalives; see section 4.13.1
for details on this.
Note that Windows can produce this error in some circumstances without
seeing a connection reset from the server, for instance if the connection
to the network is lost.

10.17 ‘Network error: Connection refused’
This error means that the network connection PuTTY tried to make to
your server was rejected by the server. Usually this happens because the
server does not provide the service which PuTTY is trying to access.
Check that you are connecting with the correct protocol (SSH, Telnet or
Rlogin), and check that the port number is correct. If that fails, consult the
administrator of your server.

10.18 ‘Network error: Connection timed out’
This error means that the network connection PuTTY tried to make to
your server received no response at all from the server. Usually this
happens because the server machine is completely isolated from the
network, or because it is turned off.
Check that you have correctly entered the host name or IP address of
your server machine. If that fails, consult the administrator of your server.
Unix also generates this error when it tries to send data down a
connection and contact with the server has been completely lost during a
connection. (There is a delay of minutes before Unix gives up on
receiving a reply from the server.) This can occur if you type things into
PuTTY while the network is down, but it can also occur if PuTTY decides
of its own accord to send data: due to a repeat key exchange in SSH-2
(see section 4.19.2) or due to keepalives (section 4.13.1).

10.19 ‘Network error: Cannot assign requested
address’
This means that the operating system rejected the parameters of the
network connection PuTTY tried to make, usually without actually trying
to connect to anything, because they were simply invalid.
A common way to provoke this error is to accidentally try to connect to
port 0, which is not a valid port number.

Appendix A: PuTTY FAQ
This FAQ is published on the PuTTY web site, and also provided as an
appendix in the manual.
A.1 Introduction
A.1.1 What is PuTTY?
A.2 Features supported in PuTTY
A.2.1 Does PuTTY support SSH-2?
A.2.2 Does PuTTY support reading OpenSSH or ssh.com SSH-2
private key files?
A.2.3 Does PuTTY support SSH-1?
A.2.4 Does PuTTY support local echo?
A.2.5 Does PuTTY support storing settings, so I don't have to
change them every time?
A.2.6 Does PuTTY support storing its settings in a disk file?
A.2.7 Does PuTTY support full-screen mode, like a DOS box?
A.2.8 Does PuTTY have the ability to remember my password
so I don't have to type it every time?
A.2.9 Is there an option to turn off the annoying host key
prompts?
A.2.10 Will you write an SSH server for the PuTTY suite, to go
with the client?
A.2.11 Can PSCP or PSFTP transfer files in ASCII mode?
A.3 Ports to other operating systems
A.3.1 What ports of PuTTY exist?
A.3.2 Is there a port to Unix?
A.3.3 What's the point of the Unix port? Unix has OpenSSH.
A.3.4 Will there be a port to Windows CE or PocketPC?
A.3.5 Is there a port to Windows 3.1?
A.3.6 Will there be a port to the Mac?
A.3.7 Will there be a port to EPOC?
A.3.8 Will there be a port to the iPhone?
A.4 Embedding PuTTY in other programs
A.4.1 Is the SSH or Telnet code available as a DLL?
A.4.2 Is the SSH or Telnet code available as a Visual Basic
component?
A.4.3 How can I use PuTTY to make an SSH connection from

within another program?
A.5 Details of PuTTY's operation
A.5.1 What terminal type does PuTTY use?
A.5.2 Where does PuTTY store its data?
A.6 HOWTO questions
A.6.1 What login name / password should I use?
A.6.2 What commands can I type into my PuTTY terminal
window?
A.6.3 How can I make PuTTY start up maximised?
A.6.4 How can I create a Windows shortcut to start a particular
saved session directly?
A.6.5 How can I start an SSH session straight from the
command line?
A.6.6 How do I copy and paste between PuTTY and other
Windows applications?
A.6.7 How do I use all PuTTY's features (public keys, proxying,
cipher selection, etc.) in PSCP, PSFTP and Plink?
A.6.8 How do I use PSCP.EXE? When I double-click it gives me
a command prompt window which then closes instantly.
A.6.9 How do I use PSCP to copy a file whose name has
spaces in?
A.6.10 Should I run the 32-bit or the 64-bit version?
A.7 Troubleshooting
A.7.1 Why do I see ‘Fatal: Protocol error: Expected control
record’ in PSCP?
A.7.2 I clicked on a colour in the Colours panel, and the colour
didn't change in my terminal.
A.7.3 After trying to establish an SSH-2 connection, PuTTY says
‘Out of memory’ and dies.
A.7.4 When attempting a file transfer, either PSCP or PSFTP
says ‘Out of memory’ and dies.
A.7.5 PSFTP transfers files much slower than PSCP.
A.7.6 When I run full-colour applications, I see areas of black
space where colour ought to be, or vice versa.
A.7.7 When I change some terminal settings, nothing happens.
A.7.8 My PuTTY sessions unexpectedly close after they are idle
for a while.
A.7.9 PuTTY's network connections time out too quickly when
network connectivity is temporarily lost.

A.7.10 When I cat a binary file, I get ‘PuTTYPuTTYPuTTY’ on
my command line.
A.7.11 When I cat a binary file, my window title changes to a
nonsense string.
A.7.12 My keyboard stops working once PuTTY displays the
password prompt.
A.7.13 One or more function keys don't do what I expected in a
server-side application.
A.7.14 Why do I see ‘Couldn't load private key from ...’? Why
can PuTTYgen load my key but not PuTTY?
A.7.15 When I'm connected to a Red Hat Linux 8.0 system,
some characters don't display properly.
A.7.16 Since I upgraded to PuTTY 0.54, the scrollback has
stopped working when I run screen.
A.7.17 Since I upgraded Windows XP to Service Pack 2, I can't
use addresses like 127.0.0.2.
A.7.18 PSFTP commands seem to be missing a directory
separator (slash).
A.7.19 Do you want to hear about ‘Software caused connection
abort’?
A.7.20 My SSH-2 session locks up for a few seconds every so
often.
A.7.21 PuTTY fails to start up. Windows claims that ‘the
application configuration is incorrect’.
A.7.22 When I put 32-bit PuTTY in C:\WINDOWS\SYSTEM32 on my
64-bit Windows system, ‘Duplicate Session’ doesn't work.
A.8 Security questions
A.8.1 Is it safe for me to download PuTTY and use it on a public
PC?
A.8.2 What does PuTTY leave on a system? How can I clean up
after it?
A.8.3 How come PuTTY now supports DSA, when the website
used to say how insecure it was?
A.8.4 Couldn't Pageant use VirtualLock() to stop private keys
being written to disk?
A.9 Administrative questions
A.9.1 Would you like me to register you a nicer domain name?
A.9.2 Would you like free web hosting for the PuTTY web site?
A.9.3 Would you link to my web site from the PuTTY web site?

A.9.4 Why don't you move PuTTY to SourceForge?
A.9.5 Why can't I subscribe to the putty-bugs mailing list?
A.9.6 If putty-bugs isn't a general-subscription mailing list, what
is?
A.9.7 How can I donate to PuTTY development?
A.9.8 Can I have permission to put PuTTY on a cover disk /
distribute it with other software / etc?
A.9.9 Can you sign an agreement indemnifying us against
security problems in PuTTY?
A.9.10 Can you sign this form granting us permission to
use/distribute PuTTY?
A.9.11 Can you write us a formal notice of permission to use
PuTTY?
A.9.12 Can you sign anything for us?
A.9.13 If you won't sign anything, can you give us some sort of
assurance that you won't make PuTTY closed-source in future?
A.9.14 Can you provide us with export control information / FIPS
certification for PuTTY?
A.9.15 As one of our existing software vendors, can you just fill
in this questionnaire for us?
A.9.16 The sha1sums / sha256sums / etc files on your download
page don't match the binaries.
A.10 Miscellaneous questions
A.10.1 Is PuTTY a port of OpenSSH, or based on OpenSSH or
OpenSSL?
A.10.2 Where can I buy silly putty?
A.10.3 What does ‘PuTTY’ mean?
A.10.4 How do I pronounce ‘PuTTY’?

A.1 Introduction
A.1.1 What is PuTTY?

A.1.1 What is PuTTY?
PuTTY is a client program for the SSH, Telnet and Rlogin network
protocols.
These protocols are all used to run a remote session on a computer, over
a network. PuTTY implements the client end of that session: the end at
which the session is displayed, rather than the end at which it runs.
In really simple terms: you run PuTTY on a Windows machine, and tell it
to connect to (for example) a Unix machine. PuTTY opens a window.
Then, anything you type into that window is sent straight to the Unix
machine, and everything the Unix machine sends back is displayed in the
window. So you can work on the Unix machine as if you were sitting at its
console, while actually sitting somewhere else.

A.2 Features supported in PuTTY
In general, if you want to know if PuTTY supports a particular feature,
you should look for it on the PuTTY web site. In particular:
try the changes page, and see if you can find the feature on there. If
a feature is listed there, it's been implemented. If it's listed as a
change made since the latest version, it should be available in the
development snapshots, in which case testing will be very welcome.
try the Wishlist page, and see if you can find the feature there. If it's
on there, and not in the ‘Recently fixed’ section, it probably hasn't
been implemented.
A.2.1 Does PuTTY support SSH-2?
A.2.2 Does PuTTY support reading OpenSSH or ssh.com SSH-2
private key files?
A.2.3 Does PuTTY support SSH-1?
A.2.4 Does PuTTY support local echo?
A.2.5 Does PuTTY support storing settings, so I don't have to
change them every time?
A.2.6 Does PuTTY support storing its settings in a disk file?
A.2.7 Does PuTTY support full-screen mode, like a DOS box?
A.2.8 Does PuTTY have the ability to remember my password so I
don't have to type it every time?
A.2.9 Is there an option to turn off the annoying host key prompts?
A.2.10 Will you write an SSH server for the PuTTY suite, to go with
the client?
A.2.11 Can PSCP or PSFTP transfer files in ASCII mode?

A.2.1 Does PuTTY support SSH-2?
Yes. SSH-2 support has been available in PuTTY since version 0.50.
Public key authentication (both RSA and DSA) in SSH-2 is new in version
0.52.

A.2.2 Does PuTTY support reading OpenSSH or
ssh.com SSH-2 private key files?
PuTTY doesn't support this natively (see the wishlist entry for reasons
why not), but as of 0.53 PuTTYgen can convert both OpenSSH and
ssh.com private key files into PuTTY's format.

A.2.3 Does PuTTY support SSH-1?
Yes. SSH-1 support has always been available in PuTTY.
However, the SSH-1 protocol has many weaknesses and is no longer
considered secure; you should use SSH-2 instead if at all possible.
As of 0.68, PuTTY will no longer fall back to SSH-1 if the server doesn't
appear to support SSH-2; you must explicitly ask for SSH-1.

A.2.4 Does PuTTY support local echo?
Yes. Version 0.52 has proper support for local echo.
In version 0.51 and before, local echo could not be separated from local
line editing (where you type a line of text locally, and it is not sent to the
server until you press Return, so you have the chance to edit it and
correct mistakes before the server sees it). New in version 0.52, local
echo and local line editing are separate options, and by default PuTTY
will try to determine automatically whether to enable them or not, based
on which protocol you have selected and also based on hints from the
server. If you have a problem with PuTTY's default choice, you can force
each option to be enabled or disabled as you choose. The controls are in
the Terminal panel, in the section marked ‘Line discipline options’.

A.2.5 Does PuTTY support storing settings, so I
don't have to change them every time?
Yes, all of PuTTY's settings can be saved in named session profiles. You
can also change the default settings that are used for new sessions. See
section 4.1.2 in the documentation for how to do this.

A.2.6 Does PuTTY support storing its settings in
a disk file?
Not at present, although section 4.29 in the documentation gives a
method of achieving the same effect.

A.2.7 Does PuTTY support full-screen mode, like
a DOS box?
Yes; this is a new feature in version 0.52.

A.2.8 Does PuTTY have the ability to remember
my password so I don't have to type it every
time?
No, it doesn't.
Remembering your password is a bad plan for obvious security reasons:
anyone who gains access to your machine while you're away from your
desk can find out the remembered password, and use it, abuse it or
change it.
In addition, it's not even possible for PuTTY to automatically send your
password in a Telnet session, because Telnet doesn't give the client
software any indication of which part of the login process is the password
prompt. PuTTY would have to guess, by looking for words like ‘password’
in the session data; and if your login program is written in something
other than English, this won't work.
In SSH, remembering your password would be possible in theory, but
there doesn't seem to be much point since SSH supports public key
authentication, which is more flexible and more secure. See chapter 8 in
the documentation for a full discussion of public key authentication.

A.2.9 Is there an option to turn off the annoying
host key prompts?
No, there isn't. And there won't be. Even if you write it yourself and send
us the patch, we won't accept it.
Those annoying host key prompts are the whole point of SSH. Without
them, all the cryptographic technology SSH uses to secure your session
is doing nothing more than making an attacker's job slightly harder;
instead of sitting between you and the server with a packet sniffer, the
attacker must actually subvert a router and start modifying the packets
going back and forth. But that's not all that much harder than just sniffing;
and without host key checking, it will go completely undetected by client
or server.
Host key checking is your guarantee that the encryption you put on your
data at the client end is the same encryption taken off the data at the
server end; it's your guarantee that it hasn't been removed and replaced
somewhere on the way. Host key checking makes the attacker's job
astronomically hard, compared to packet sniffing, and even compared to
subverting a router. Instead of applying a little intelligence and keeping an
eye on Bugtraq, the attacker must now perform a brute-force attack
against at least one military-strength cipher. That insignificant host key
prompt really does make that much difference.
If you're having a specific problem with host key checking - perhaps you
want an automated batch job to make use of PSCP or Plink, and the
interactive host key prompt is hanging the batch process - then the right
way to fix it is to add the correct host key to the Registry in advance, or if
the Registry is not available, to use the -hostkey command-line option.
That way, you retain the important feature of host key checking: the right
key will be accepted and the wrong ones will not. Adding an option to turn
host key checking off completely is the wrong solution and we will not do
it.
If you have host keys available in the common known_hosts format, we
have a script called kh2reg.py to convert them to a Windows .REG file,
which can be installed ahead of time by double-clicking or using REGEDIT.

A.2.10 Will you write an SSH server for the
PuTTY suite, to go with the client?
No. The only reason we might want to would be if we could easily re-use
existing code and significantly cut down the effort. We don't believe this is
the case; there just isn't enough common ground between an SSH client
and server to make it worthwhile.
If someone else wants to use bits of PuTTY in the process of writing a
Windows SSH server, they'd be perfectly welcome to of course, but I
really can't see it being a lot less effort for us to do that than it would be
for us to write a server from the ground up. We don't have time, and we
don't have motivation. The code is available if anyone else wants to try it.

A.2.11 Can PSCP or PSFTP transfer files in ASCII
mode?
Unfortunately not.
Until recently, this was a limitation of the file transfer protocols: the SCP
and SFTP protocols had no notion of transferring a file in anything other
than binary mode. (This is still true of SCP.)
The current draft protocol spec of SFTP proposes a means of
implementing ASCII transfer. At some point PSCP/PSFTP may
implement this proposal.

A.3 Ports to other operating systems
The eventual goal is for PuTTY to be a multi-platform program, able to
run on at least Windows, Mac OS and Unix.
Porting will become easier once PuTTY has a generalised porting layer,
drawing a clear line between platform-dependent and platformindependent code. The general intention was for this porting layer to
evolve naturally as part of the process of doing the first port; a Unix port
has now been released and the plan seems to be working so far.
A.3.1 What ports of PuTTY exist?
A.3.2 Is there a port to Unix?
A.3.3 What's the point of the Unix port? Unix has OpenSSH.
A.3.4 Will there be a port to Windows CE or PocketPC?
A.3.5 Is there a port to Windows 3.1?
A.3.6 Will there be a port to the Mac?
A.3.7 Will there be a port to EPOC?
A.3.8 Will there be a port to the iPhone?

A.3.1 What ports of PuTTY exist?
Currently, release versions of PuTTY tools only run on Windows systems
and Unix.
As of 0.68, the supplied PuTTY executables run on versions of Windows
from XP onwards, up to and including Windows 10; and we know of no
reason why PuTTY should not continue to work on future versions of
Windows. We provide 32-bit and 64-bit Windows executables; see
question A.6.10 for discussion of the compatibility issues around that.
(We used to also provide executables for Windows for the Alpha
processor, but stopped after 0.58 due to lack of interest.)
In the development code, a partial port to Mac OS exists (see question
A.3.6).
Currently PuTTY does not run on Windows CE (see question A.3.4).
We do not have release-quality ports for any other systems at the present
time. If anyone told you we had an Android port, or an iOS port, or any
other port of PuTTY, they were mistaken. We don't.
There are some third-party ports to various platforms, mentioned on the
Links page of our website.

A.3.2 Is there a port to Unix?
As of 0.54, there are Unix ports of most of the traditional PuTTY tools,
and also one entirely new application.
If you look at the source release, you should find a unix subdirectory.
There are a couple of ways of building it, including the usual
configure/make; see the file README in the source distribution. This should
build you Unix ports of Plink, PuTTY itself, PuTTYgen, PSCP, PSFTP,
Pageant, and also pterm - an xterm-type program which supports the
same terminal emulation as PuTTY.
If you don't have Gtk, you should still be able to build the command-line
tools.

A.3.3 What's the point of the Unix port? Unix has
OpenSSH.
All sorts of little things. pterm is directly useful to anyone who prefers
PuTTY's terminal emulation to xterm's, which at least some people do.
Unix Plink has apparently found a niche among people who find the
complexity of OpenSSL makes OpenSSH hard to install (and who don't
mind Plink not having as many features). Some users want to generate a
large number of SSH keys on Unix and then copy them all into PuTTY,
and the Unix PuTTYgen should allow them to automate that conversion
process.
There were development advantages as well; porting PuTTY to Unix was
a valuable path-finding effort for other future ports, and also allowed us to
use the excellent Linux tool Valgrind to help with debugging, which has
already improved PuTTY's stability on all platforms.
However, if you're a Unix user and you can see no reason to switch from
OpenSSH to PuTTY/Plink, then you're probably right. We don't expect
our Unix port to be the right thing for everybody.

A.3.4 Will there be a port to Windows CE or
PocketPC?
We once did some work on such a port, but it only reached an early
stage, and certainly not a useful one. It's no longer being actively worked
on.

A.3.5 Is there a port to Windows 3.1?
PuTTY is a 32-bit application from the ground up, so it won't run on
Windows 3.1 as a native 16-bit program; and it would be very hard to port
it to do so, because of Windows 3.1's vile memory allocation
mechanisms.
However, it is possible in theory to compile the existing PuTTY source in
such a way that it will run under Win32s (an extension to Windows 3.1 to
let you run 32-bit programs). In order to do this you'll need the right kind
of C compiler - modern versions of Visual C at least have stopped being
backwards compatible to Win32s. Also, the last time we tried this it didn't
work very well.

A.3.6 Will there be a port to the Mac?
We hope so!
We attempted one around 2005, written as a native Cocoa application,
but it turned out to be very slow to redraw its window for some reason we
never got to the bottom of.
In 2015, after porting the GTK front end to work with GTK 3, we began
another attempt based on making small changes to the GTK code and
building it against the OS X Quartz version of GTK 3. This doesn't seem
to have the window redrawing problem any more, so it's already got
further than the last effort, but it is still substantially unfinished.
If any OS X and/or GTK programming experts are keen to have a
finished version of this, we urge them to help out with some of the
remaining problems!

A.3.7 Will there be a port to EPOC?
I hope so, but given that ports aren't really progressing very fast even on
systems the developers do already know how to program for, it might be
a long time before any of us get round to learning a new system and
doing the port for that.
However, some of the work has been done by other people; see the
Links page of our website for various third-party ports.

A.3.8 Will there be a port to the iPhone?
We have no plans to write such a port ourselves; none of us has an
iPhone, and developing and publishing applications for it looks awkward
and expensive.
However, there is a third-party SSH client for the iPhone and iPod Touch
called pTerm, which is apparently based on PuTTY. (This is nothing to do
with our similarly-named pterm, which is a standalone terminal emulator
for Unix systems; see question A.3.2.)

A.4 Embedding PuTTY in other programs
A.4.1 Is the SSH or Telnet code available as a DLL?
A.4.2 Is the SSH or Telnet code available as a Visual Basic
component?
A.4.3 How can I use PuTTY to make an SSH connection from within
another program?

A.4.1 Is the SSH or Telnet code available as a
DLL?
No, it isn't. It would take a reasonable amount of rewriting for this to be
possible, and since the PuTTY project itself doesn't believe in DLLs (they
make installation more error-prone) none of us has taken the time to do
it.
Most of the code cleanup work would be a good thing to happen in
general, so if anyone feels like helping, we wouldn't say no.
See also the wishlist entry.

A.4.2 Is the SSH or Telnet code available as a
Visual Basic component?
No, it isn't. None of the PuTTY team uses Visual Basic, and none of us
has any particular need to make SSH connections from a Visual Basic
application. In addition, all the preliminary work to turn it into a DLL would
be necessary first; and furthermore, we don't even know how to write VB
components.
If someone offers to do some of this work for us, we might consider it, but
unless that happens I can't see VB integration being anywhere other than
the very bottom of our priority list.

A.4.3 How can I use PuTTY to make an SSH
connection from within another program?
Probably your best bet is to use Plink, the command-line connection tool.
If you can start Plink as a second Windows process, and arrange for your
primary process to be able to send data to the Plink process, and receive
data from it, through pipes, then you should be able to make SSH
connections from your program.
This is what CVS for Windows does, for example.

A.5 Details of PuTTY's operation
A.5.1 What terminal type does PuTTY use?
A.5.2 Where does PuTTY store its data?

A.5.1 What terminal type does PuTTY use?
For most purposes, PuTTY can be considered to be an xterm terminal.
PuTTY also supports some terminal control sequences not supported by
the real xterm: notably the Linux console sequences that reconfigure the
colour palette, and the title bar control sequences used by DECterm (which
are different from the xterm ones; PuTTY supports both).
By default, PuTTY announces its terminal type to the server as xterm. If
you have a problem with this, you can reconfigure it to say something
else; vt220 might help if you have trouble.

A.5.2 Where does PuTTY store its data?
On Windows, PuTTY stores most of its data (saved sessions, SSH host
keys) in the Registry. The precise location is
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY

and within that area, saved sessions are stored under Sessions while
host keys are stored under SshHostKeys.
PuTTY also requires a random number seed file, to improve the
unpredictability of randomly chosen data needed as part of the SSH
cryptography. This is stored by default in a file called PUTTY.RND; this is
stored by default in the ‘Application Data’ directory, or failing that, one of
a number of fallback locations. If you want to change the location of the
random number seed file, you can put your chosen pathname in the
Registry, at
HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile

You can ask PuTTY to delete all this data; see question A.8.2.
On Unix, PuTTY stores all of this data in a directory ~/.putty by default.

A.6 HOWTO questions
A.6.1 What login name / password should I use?
A.6.2 What commands can I type into my PuTTY terminal window?
A.6.3 How can I make PuTTY start up maximised?
A.6.4 How can I create a Windows shortcut to start a particular
saved session directly?
A.6.5 How can I start an SSH session straight from the command
line?
A.6.6 How do I copy and paste between PuTTY and other Windows
applications?
A.6.7 How do I use all PuTTY's features (public keys, proxying,
cipher selection, etc.) in PSCP, PSFTP and Plink?
A.6.8 How do I use PSCP.EXE? When I double-click it gives me a
command prompt window which then closes instantly.
A.6.9 How do I use PSCP to copy a file whose name has spaces in?
A.6.10 Should I run the 32-bit or the 64-bit version?

A.6.1 What login name / password should I use?
This is not a question you should be asking us.
PuTTY is a communications tool, for making connections to other
computers. We maintain the tool; we don't administer any computers that
you're likely to be able to use, in the same way that the people who make
web browsers aren't responsible for most of the content you can view in
them. We cannot help with questions of this sort.
If you know the name of the computer you want to connect to, but don't
know what login name or password to use, you should talk to whoever
administers that computer. If you don't know who that is, see the next
question for some possible ways to find out.

A.6.2 What commands can I type into my PuTTY
terminal window?
Again, this is not a question you should be asking us. You need to read
the manuals, or ask the administrator, of the computer you have
connected to.
PuTTY does not process the commands you type into it. It's only a
communications tool. It makes a connection to another computer; it
passes the commands you type to that other computer; and it passes the
other computer's responses back to you. Therefore, the precise range of
commands you can use will not depend on PuTTY, but on what kind of
computer you have connected to and what software is running on it. The
PuTTY team cannot help you with that.
(Think of PuTTY as being a bit like a telephone. If you phone somebody
up and you don't know what language to speak to make them understand
you, it isn't the telephone company's job to find that out for you. We just
provide the means for you to get in touch; making yourself understood is
somebody else's problem.)
If you are unsure of where to start looking for the administrator of your
server, a good place to start might be to remember how you found out the
host name in the PuTTY configuration. If you were given that host name
by e-mail, for example, you could try asking the person who sent you that
e-mail. If your company's IT department provided you with ready-made
PuTTY saved sessions, then that IT department can probably also tell
you something about what commands you can type during those
sessions. But the PuTTY maintainer team does not administer any server
you are likely to be connecting to, and cannot help you with questions of
this type.

A.6.3 How can I make PuTTY start up
maximised?
Create a Windows shortcut to start PuTTY from, and set it as ‘Run
Maximized’.

A.6.4 How can I create a Windows shortcut to
start a particular saved session directly?
To run a PuTTY session saved under the name ‘mysession’, create a
Windows shortcut that invokes PuTTY with a command line like
\path\name\to\putty.exe -load "mysession"

(Note: prior to 0.53, the syntax was @session. This is now deprecated and
may be removed at some point.)

A.6.5 How can I start an SSH session straight
from the command line?
Use the command line putty -ssh host.name. Alternatively, create a
saved session that specifies the SSH protocol, and start the saved
session as shown in question A.6.4.

A.6.6 How do I copy and paste between PuTTY
and other Windows applications?
Copy and paste works similarly to the X Window System. You use the left
mouse button to select text in the PuTTY window. The act of selection
automatically copies the text to the clipboard: there is no need to press
Ctrl-Ins or Ctrl-C or anything else. In fact, pressing Ctrl-C will send a CtrlC character to the other end of your connection (just like it does the rest
of the time), which may have unpleasant effects. The only thing you need
to do, to copy text to the clipboard, is to select it.
To paste the clipboard contents into a PuTTY window, by default you click
the right mouse button. If you have a three-button mouse and are used to
X applications, you can configure pasting to be done by the middle button
instead, but this is not the default because most Windows users don't
have a middle button at all.
You can also paste by pressing Shift-Ins.

A.6.7 How do I use all PuTTY's features (public
keys, proxying, cipher selection, etc.) in PSCP,
PSFTP and Plink?
Most major features (e.g., public keys, port forwarding) are available
through command line options. See the documentation.
Not all features are accessible from the command line yet, although we'd
like to fix this. In the meantime, you can use most of PuTTY's features if
you create a PuTTY saved session, and then use the name of the saved
session on the command line in place of a hostname. This works for
PSCP, PSFTP and Plink (but don't expect port forwarding in the file
transfer applications!).

A.6.8 How do I use PSCP.EXE? When I doubleclick it gives me a command prompt window
which then closes instantly.
PSCP is a command-line application, not a GUI application. If you run it
without arguments, it will simply print a help message and terminate.
To use PSCP properly, run it from a Command Prompt window. See
chapter 5 in the documentation for more details.

A.6.9 How do I use PSCP to copy a file whose
name has spaces in?
If PSCP is using the traditional SCP protocol, this is confusing. If you're
specifying a file at the local end, you just use one set of quotes as you
would normally do:
pscp "local filename with spaces" user@host:
pscp user@host:myfile "local filename with spaces"

But if the filename you're specifying is on the remote side, you have to
use backslashes and two sets of quotes:
pscp user@host:"\"remote filename with spaces\"" local_filename
pscp local_filename user@host:"\"remote filename with spaces\""

Worse still, in a remote-to-local copy you have to specify the local file
name explicitly, otherwise PSCP will complain that they don't match
(unless you specified the -unsafe option). The following command will
give an error message:
c:\>pscp user@host:"\"oo er\"" .
warning: remote host tried to write to a file called 'oo er'
when we requested a file called '"oo er"'.

Instead, you need to specify the local file name in full:
c:\>pscp user@host:"\"oo er\"" "oo er"

If PSCP is using the newer SFTP protocol, none of this is a problem, and
all filenames with spaces in are specified using a single pair of quotes in
the obvious way:
pscp "local file" user@host:
pscp user@host:"remote file" .

A.6.10 Should I run the 32-bit or the 64-bit
version?
If you're not sure, the 32-bit version is generally the safe option. It will run
perfectly well on all processors and on all versions of Windows that
PuTTY supports. PuTTY doesn't require to run as a 64-bit application to
work well, and having a 32-bit PuTTY on a 64-bit system isn't likely to
cause you any trouble.
The 64-bit version (first released in 0.68) will only run if you have a 64-bit
processor and a 64-bit edition of Windows (both of these things are likely
to be true of any recent Windows PC). It will run somewhat faster (in
particular, the cryptography will be faster, especially during link setup),
but it will consume slightly more memory.
If you need to use an external DLL for GSSAPI authentication, that DLL
may only be available in a 32-bit or 64-bit form, and that will dictate the
version of PuTTY you need to use. (You will probably know if you're
doing this; see section 4.23.2 in the documentation.)

A.7 Troubleshooting
A.7.1 Why do I see ‘Fatal: Protocol error: Expected control record’ in
PSCP?
A.7.2 I clicked on a colour in the Colours panel, and the colour didn't
change in my terminal.
A.7.3 After trying to establish an SSH-2 connection, PuTTY says
‘Out of memory’ and dies.
A.7.4 When attempting a file transfer, either PSCP or PSFTP says
‘Out of memory’ and dies.
A.7.5 PSFTP transfers files much slower than PSCP.
A.7.6 When I run full-colour applications, I see areas of black space
where colour ought to be, or vice versa.
A.7.7 When I change some terminal settings, nothing happens.
A.7.8 My PuTTY sessions unexpectedly close after they are idle for
a while.
A.7.9 PuTTY's network connections time out too quickly when
network connectivity is temporarily lost.
A.7.10 When I cat a binary file, I get ‘PuTTYPuTTYPuTTY’ on my
command line.
A.7.11 When I cat a binary file, my window title changes to a
nonsense string.
A.7.12 My keyboard stops working once PuTTY displays the
password prompt.
A.7.13 One or more function keys don't do what I expected in a
server-side application.
A.7.14 Why do I see ‘Couldn't load private key from ...’? Why can
PuTTYgen load my key but not PuTTY?
A.7.15 When I'm connected to a Red Hat Linux 8.0 system, some
characters don't display properly.
A.7.16 Since I upgraded to PuTTY 0.54, the scrollback has stopped
working when I run screen.
A.7.17 Since I upgraded Windows XP to Service Pack 2, I can't use
addresses like 127.0.0.2.
A.7.18 PSFTP commands seem to be missing a directory separator
(slash).
A.7.19 Do you want to hear about ‘Software caused connection
abort’?

A.7.20 My SSH-2 session locks up for a few seconds every so often.
A.7.21 PuTTY fails to start up. Windows claims that ‘the application
configuration is incorrect’.
A.7.22 When I put 32-bit PuTTY in C:\WINDOWS\SYSTEM32 on my 64-bit
Windows system, ‘Duplicate Session’ doesn't work.

A.7.1 Why do I see ‘Fatal: Protocol error:
Expected control record’ in PSCP?
This happens because PSCP was expecting to see data from the server
that was part of the PSCP protocol exchange, and instead it saw data
that it couldn't make any sense of at all.
This almost always happens because the startup scripts in your account
on the server machine are generating output. This is impossible for
PSCP, or any other SCP client, to work around. You should never use
startup files (.bashrc, .cshrc and so on) which generate output in noninteractive sessions.
This is not actually a PuTTY problem. If PSCP fails in this way, then all
other SCP clients are likely to fail in exactly the same way. The problem
is at the server end.

A.7.2 I clicked on a colour in the Colours panel,
and the colour didn't change in my terminal.
That isn't how you're supposed to use the Colours panel.
During the course of a session, PuTTY potentially uses all the colours
listed in the Colours panel. It's not a question of using only one of them
and you choosing which one; PuTTY will use them all. The purpose of
the Colours panel is to let you adjust the appearance of all the colours.
So to change the colour of the cursor, for example, you would select
‘Cursor Colour’, press the ‘Modify’ button, and select a new colour from
the dialog box that appeared. Similarly, if you want your session to
appear in green, you should select ‘Default Foreground’ and press
‘Modify’. Clicking on ‘ANSI Green’ won't turn your session green; it will
only allow you to adjust the shade of green used when PuTTY is
instructed by the server to display green text.

A.7.3 After trying to establish an SSH-2
connection, PuTTY says ‘Out of memory’ and
dies.
If this happens just while the connection is starting up, this often indicates
that for some reason the client and server have failed to establish a
session encryption key. Somehow, they have performed calculations that
should have given each of them the same key, but have ended up with
different keys; so data encrypted by one and decrypted by the other looks
like random garbage.
This causes an ‘out of memory’ error because the first encrypted data
PuTTY expects to see is the length of an SSH message. Normally this
will be something well under 100 bytes. If the decryption has failed,
PuTTY will see a completely random length in the region of two
gigabytes, and will try to allocate enough memory to store this nonexistent message. This will immediately lead to it thinking it doesn't have
enough memory, and panicking.
If this happens to you, it is quite likely to still be a PuTTY bug and you
should report it (although it might be a bug in your SSH server instead);
but it doesn't necessarily mean you've actually run out of memory.

A.7.4 When attempting a file transfer, either
PSCP or PSFTP says ‘Out of memory’ and dies.
This is almost always caused by your login scripts on the server
generating output. PSCP or PSFTP will receive that output when they
were expecting to see the start of a file transfer protocol, and they will
attempt to interpret the output as file-transfer protocol. This will usually
lead to an ‘out of memory’ error for much the same reasons as given in
question A.7.3.
This is a setup problem in your account on your server, not a
PSCP/PSFTP bug. Your login scripts should never generate output
during non-interactive sessions; secure file transfer is not the only form of
remote access that will break if they do.
On Unix, a simple fix is to ensure that all the parts of your login script that
might generate output are in .profile (if you use a Bourne shell
derivative) or .login (if you use a C shell). Putting them in more general
files such as .bashrc or .cshrc is liable to lead to problems.

A.7.5 PSFTP transfers files much slower than
PSCP.
The throughput of PSFTP 0.54 should be much better than 0.53b and
prior; we've added code to the SFTP backend to queue several blocks of
data rather than waiting for an acknowledgement for each. (The SCP
backend did not suffer from this performance issue because SCP is a
much simpler protocol.)

A.7.6 When I run full-colour applications, I see
areas of black space where colour ought to be,
or vice versa.
You almost certainly need to change the ‘Use background colour to erase
screen’ setting in the Terminal panel. If there is too much black space (the
commoner situation), you should enable it, while if there is too much
colour, you should disable it. (See section 4.3.5.)
In old versions of PuTTY, this was disabled by default, and would not
take effect until you reset the terminal (see question A.7.7). Since 0.54, it
is enabled by default, and changes take effect immediately.

A.7.7 When I change some terminal settings,
nothing happens.
Some of the terminal options (notably Auto Wrap and background-colour
screen erase) actually represent the default setting, rather than the
currently active setting. The server can send sequences that modify
these options in mid-session, but when the terminal is reset (by server
action, or by you choosing ‘Reset Terminal’ from the System menu) the
defaults are restored.
In versions 0.53b and prior, if you change one of these options in the
middle of a session, you will find that the change does not immediately
take effect. It will only take effect once you reset the terminal.
In version 0.54, the behaviour has changed - changes to these settings
take effect immediately.

A.7.8 My PuTTY sessions unexpectedly close
after they are idle for a while.
Some types of firewall, and almost any router doing Network Address
Translation (NAT, also known as IP masquerading), will forget about a
connection through them if the connection does nothing for too long. This
will cause the connection to be rudely cut off when contact is resumed.
You can try to combat this by telling PuTTY to send keepalives: packets
of data which have no effect on the actual session, but which reassure
the router or firewall that the network connection is still active and worth
remembering about.
Keepalives don't solve everything, unfortunately; although they cause
greater robustness against this sort of router, they can also cause a loss
of robustness against network dropouts. See section 4.13.1 in the
documentation for more discussion of this.

A.7.9 PuTTY's network connections time out too
quickly when network connectivity is
temporarily lost.
This is a Windows problem, not a PuTTY problem. The timeout value
can't be set on per application or per session basis. To increase the TCP
timeout globally, you need to tinker with the Registry.
On Windows 95, 98 or ME, the registry key you need to create or change
is
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
MSTCP\MaxDataRetries

(it must be of type DWORD in Win95, or String in Win98/ME). (See MS
Knowledge Base article 158474 for more information.)
On Windows NT, 2000, or XP, the registry key to create or change is
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
Parameters\TcpMaxDataRetransmissions

and it must be of type DWORD. (See MS Knowledge Base articles
120642 and 314053 for more information.)
Set the key's value to something like 10. This will cause Windows to try
harder to keep connections alive instead of abandoning them.

A.7.10 When I cat a binary file, I get
‘PuTTYPuTTYPuTTY’ on my command line.
Don't do that, then.
This is designed behaviour; when PuTTY receives the character ControlE from the remote server, it interprets it as a request to identify itself, and
so it sends back the string ‘PuTTY’ as if that string had been entered at the
keyboard. Control-E should only be sent by programs that are prepared
to deal with the response. Writing a binary file to your terminal is likely to
output many Control-E characters, and cause this behaviour. Don't do it.
It's a bad plan.
To mitigate the effects, you could configure the answerback string to be
empty (see section 4.3.7); but writing binary files to your terminal is likely
to cause various other unpleasant behaviour, so this is only a small
remedy.

A.7.11 When I cat a binary file, my window title
changes to a nonsense string.
Don't do that, then.
It is designed behaviour that PuTTY should have the ability to adjust the
window title on instructions from the server. Normally the control
sequence that does this should only be sent deliberately, by programs
that know what they are doing and intend to put meaningful text in the
window title. Writing a binary file to your terminal runs the risk of sending
the same control sequence by accident, and cause unexpected changes
in the window title. Don't do it.

A.7.12 My keyboard stops working once PuTTY
displays the password prompt.
No, it doesn't. PuTTY just doesn't display the password you type, so that
someone looking at your screen can't see what it is.
Unlike the Windows login prompts, PuTTY doesn't display the password
as a row of asterisks either. This is so that someone looking at your
screen can't even tell how long your password is, which might be
valuable information.

A.7.13 One or more function keys don't do what I
expected in a server-side application.
If you've already tried all the relevant options in the PuTTY Keyboard
panel, you may need to mail the PuTTY maintainers and ask.
It is not usually helpful just to tell us which application, which server
operating system, and which key isn't working; in order to replicate the
problem we would need to have a copy of every operating system, and
every application, that anyone has ever complained about.
PuTTY responds to function key presses by sending a sequence of
control characters to the server. If a function key isn't doing what you
expect, it's likely that the character sequence your application is
expecting to receive is not the same as the one PuTTY is sending.
Therefore what we really need to know is what sequence the application
is expecting.
The simplest way to investigate this is to find some other terminal
environment, in which that function key does work; and then investigate
what sequence the function key is sending in that situation. One
reasonably easy way to do this on a Unix system is to type the command
cat, and then press the function key. This is likely to produce output of
the form ^[[11~. You can also do this in PuTTY, to find out what
sequence the function key is producing in that. Then you can mail the
PuTTY maintainers and tell us ‘I wanted the F1 key to send ^[[11~, but
instead it's sending ^[OP, can this be done?’, or something similar.
You should still read the Feedback page on the PuTTY website (also
provided as appendix B in the manual), and follow the guidelines
contained in that.

A.7.14 Why do I see ‘Couldn't load private key
from ...’? Why can PuTTYgen load my key but
not PuTTY?
It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
but you're trying to use it in an SSH-1 connection. SSH-1 and SSH-2
keys have different formats, and (at least in 0.52) PuTTY's reporting of a
key in the wrong format isn't optimal.
To connect using SSH-2 to a server that supports both versions, you
need to change the configuration from the default (see question A.2.1).

A.7.15 When I'm connected to a Red Hat Linux
8.0 system, some characters don't display
properly.
A common complaint is that hyphens in man pages show up as a-acute.
With release 8.0, Red Hat appear to have made UTF-8 the default
character set. There appears to be no way for terminal emulators such as
PuTTY to know this (as far as we know, the appropriate escape
sequence to switch into UTF-8 mode isn't sent).
A fix is to configure sessions to RH8 systems to use UTF-8 translation see section 4.10.1 in the documentation. (Note that if you use ‘Change
Settings’, changes may not take place immediately - see question A.7.7.)
If you really want to change the character set used by the server, the right
place is /etc/sysconfig/i18n, but this shouldn't be necessary.

A.7.16 Since I upgraded to PuTTY 0.54, the
scrollback has stopped working when I run
screen.
PuTTY's terminal emulator has always had the policy that when the
‘alternate screen’ is in use, nothing is added to the scrollback. This is
because the usual sorts of programs which use the alternate screen are
things like text editors, which tend to scroll back and forth in the same
document a lot; so (a) they would fill up the scrollback with a large
amount of unhelpfully disordered text, and (b) they contain their own
method for the user to scroll back to the bit they were interested in. We
have generally found this policy to do the Right Thing in almost all
situations.
Unfortunately, screen is one exception: it uses the alternate screen, but
it's still usually helpful to have PuTTY's scrollback continue working. The
simplest solution is to go to the Features control panel and tick ‘Disable
switching to alternate terminal screen’. (See section 4.6.4 for more
details.) Alternatively, you can tell screen itself not to use the alternate
screen: the screen FAQ suggests adding the line ‘termcapinfo xterm
ti@:te@’ to your .screenrc file.
The reason why this only started to be a problem in 0.54 is because
screen typically uses an unusual control sequence to switch to the
alternate screen, and previous versions of PuTTY did not support this
sequence.

A.7.17 Since I upgraded Windows XP to Service
Pack 2, I can't use addresses like 127.0.0.2.
Some people who ask PuTTY to listen on localhost addresses other than
127.0.0.1 to forward services such as SMB and Windows Terminal
Services have found that doing so no longer works since they upgraded
to WinXP SP2.
This is apparently an issue with SP2 that is acknowledged by Microsoft in
MS Knowledge Base article 884020. The article links to a fix you can
download.
(However, we've been told that SP2 also fixes the bug that means you
need to use non-127.0.0.1 addresses to forward Terminal Services in the
first place.)

A.7.18 PSFTP commands seem to be missing a
directory separator (slash).
Some people have reported the following incorrect behaviour with
PSFTP:
psftp> pwd
Remote directory is /dir1/dir2
psftp> get filename.ext
/dir1/dir2filename.ext: no such file or directory

This is not a bug in PSFTP. There is a known bug in some versions of
portable OpenSSH (bug 697) that causes these symptoms; it appears to
have been introduced around 3.7.x. It manifests only on certain platforms
(AIX is what has been reported to us).
There is a patch for OpenSSH attached to that bug; it's also fixed in
recent versions of portable OpenSSH (from around 3.8).

A.7.19 Do you want to hear about ‘Software
caused connection abort’?
In the documentation for PuTTY 0.53 and 0.53b, we mentioned that we'd
like to hear about any occurrences of this error. Since the release of
PuTTY 0.54, however, we've been convinced that this error doesn't
indicate that PuTTY's doing anything wrong, and we don't need to hear
about further occurrences. See section 10.15 for our current
documentation of this error.

A.7.20 My SSH-2 session locks up for a few
seconds every so often.
Recent versions of PuTTY automatically initiate repeat key exchange
once per hour, to improve session security. If your client or server
machine is slow, you may experience this as a delay of anything up to
thirty seconds or so.
These delays are inconvenient, but they are there for your protection. If
they really cause you a problem, you can choose to turn off periodic
rekeying using the ‘Kex’ configuration panel (see section 4.19), but be
aware that you will be sacrificing security for this. (Falling back to SSH-1
would also remove the delays, but would lose a lot more security still. We
do not recommend it.)

A.7.21 PuTTY fails to start up. Windows claims
that ‘the application configuration is incorrect’.
This is caused by a bug in certain versions of Windows XP which is
triggered by PuTTY 0.58. This was fixed in 0.59. The ‘xp-wont-run’ entry
in PuTTY's wishlist has more details.

A.7.22 When I put 32-bit PuTTY in
C:\WINDOWS\SYSTEM32 on my 64-bit Windows
system, ‘Duplicate Session’ doesn't work.
The short answer is not to put the PuTTY executables in that location.
On 64-bit systems, C:\WINDOWS\SYSTEM32 is intended to contain only 64-bit
binaries; Windows' 32-bit binaries live in C:\WINDOWS\SYSWOW64. When a
32-bit PuTTY executable runs on a 64-bit system, it cannot by default see
the ‘real’ C:\WINDOWS\SYSTEM32 at all, because the File System Redirector
arranges that the running program sees the appropriate kind of binaries
in SYSTEM32. Thus, operations in the PuTTY suite that involve it accessing
its own executables, such as ‘New Session’ and ‘Duplicate Session’, will
not work.

A.8 Security questions
A.8.1 Is it safe for me to download PuTTY and use it on a public PC?
A.8.2 What does PuTTY leave on a system? How can I clean up
after it?
A.8.3 How come PuTTY now supports DSA, when the website used
to say how insecure it was?
A.8.4 Couldn't Pageant use VirtualLock() to stop private keys being
written to disk?

A.8.1 Is it safe for me to download PuTTY and
use it on a public PC?
It depends on whether you trust that PC. If you don't trust the public PC,
don't use PuTTY on it, and don't use any other software you plan to type
passwords into either. It might be watching your keystrokes, or it might
tamper with the PuTTY binary you download. There is no program safe
enough that you can run it on an actively malicious PC and get away with
typing passwords into it.
If you do trust the PC, then it's probably OK to use PuTTY on it (but if you
don't trust the network, then the PuTTY download might be tampered
with, so it would be better to carry PuTTY with you on a USB stick).

A.8.2 What does PuTTY leave on a system? How
can I clean up after it?
PuTTY will leave some Registry entries, and a random seed file, on the
PC (see question A.5.2). Windows 7 and up also remember some
information about recently launched sessions for the ‘jump list’ feature.
If you are using PuTTY on a public PC, or somebody else's PC, you
might want to clean this information up when you leave. You can do that
automatically, by running the command putty -cleanup. See section
3.8.2 in the documentation for more detail. (Note that this only removes
settings for the currently logged-in user on multi-user systems.)
If PuTTY was installed from the installer package, it will also appear in
‘Add/Remove Programs’. Current versions of the installer do not offer to
remove the above-mentioned items, so if you want them removed you
should run putty -cleanup before uninstalling.

A.8.3 How come PuTTY now supports DSA,
when the website used to say how insecure it
was?
DSA has a major weakness if badly implemented: it relies on a random
number generator to far too great an extent. If the random number
generator produces a number an attacker can predict, the DSA private
key is exposed - meaning that the attacker can log in as you on all
systems that accept that key.
The PuTTY policy changed because the developers were informed of
ways to implement DSA which do not suffer nearly as badly from this
weakness, and indeed which don't need to rely on random numbers at
all. For this reason we now believe PuTTY's DSA implementation is
probably OK.
The recently added elliptic-curve signature methods are also DSA-style
algorithms, so they have this same weakness in principle. Our ECDSA
implementation uses the same defence as DSA, while our Ed25519
implementation uses the similar system (but different in details) that the
Ed25519 spec mandates.

A.8.4 Couldn't Pageant use VirtualLock() to stop
private keys being written to disk?
Unfortunately not. The VirtualLock() function in the Windows API
doesn't do a proper job: it may prevent small pieces of a process's
memory from being paged to disk while the process is running, but it
doesn't stop the process's memory as a whole from being swapped
completely out to disk when the process is long-term inactive. And
Pageant spends most of its time inactive.

A.9 Administrative questions
A.9.1 Would you like me to register you a nicer domain name?
A.9.2 Would you like free web hosting for the PuTTY web site?
A.9.3 Would you link to my web site from the PuTTY web site?
A.9.4 Why don't you move PuTTY to SourceForge?
A.9.5 Why can't I subscribe to the putty-bugs mailing list?
A.9.6 If putty-bugs isn't a general-subscription mailing list, what is?
A.9.7 How can I donate to PuTTY development?
A.9.8 Can I have permission to put PuTTY on a cover disk /
distribute it with other software / etc?
A.9.9 Can you sign an agreement indemnifying us against security
problems in PuTTY?
A.9.10 Can you sign this form granting us permission to
use/distribute PuTTY?
A.9.11 Can you write us a formal notice of permission to use
PuTTY?
A.9.12 Can you sign anything for us?
A.9.13 If you won't sign anything, can you give us some sort of
assurance that you won't make PuTTY closed-source in future?
A.9.14 Can you provide us with export control information / FIPS
certification for PuTTY?
A.9.15 As one of our existing software vendors, can you just fill in
this questionnaire for us?
A.9.16 The sha1sums / sha256sums / etc files on your download page
don't match the binaries.

A.9.1 Would you like me to register you a nicer
domain name?
No, thank you. Even if you can find one (most of them seem to have
been registered already, by people who didn't ask whether we actually
wanted it before they applied), we're happy with the PuTTY web site
being exactly where it is. It's not hard to find (just type ‘putty’ into
google.com and we're the first link returned), and we don't believe the
administrative hassle of moving the site would be worth the benefit.
In addition, if we did want a custom domain name, we would want to run
it ourselves, so we knew for certain that it would continue to point where
we wanted it, and wouldn't suddenly change or do strange things. Having
it registered for us by a third party who we don't even know is not the
best way to achieve this.

A.9.2 Would you like free web hosting for the
PuTTY web site?
We already have some, thanks.

A.9.3 Would you link to my web site from the
PuTTY web site?
Only if the content of your web page is of definite direct interest to PuTTY
users. If your content is unrelated, or only tangentially related, to PuTTY,
then the link would simply be advertising for you.
One very nice effect of the Google ranking mechanism is that by and
large, the most popular web sites get the highest rankings. This means
that when an ordinary person does a search, the top item in the search is
very likely to be a high-quality site or the site they actually wanted, rather
than the site which paid the most money for its ranking.
The PuTTY web site is held in high esteem by Google, for precisely this
reason: lots of people have linked to it simply because they like PuTTY,
without us ever having to ask anyone to link to us. We feel that it would
be an abuse of this esteem to use it to boost the ranking of random
advertisers' web sites. If you want your web site to have a high Google
ranking, we'd prefer that you achieve this the way we did - by being good
enough at what you do that people will link to you simply because they
like you.
In particular, we aren't interested in trading links for money (see above),
and we certainly aren't interested in trading links for other links (since we
have no advertising on our web site, our Google ranking is not even
directly worth anything to us). If we don't want to link to you for free, then
we probably won't want to link to you at all.
If you have software based on PuTTY, or specifically designed to
interoperate with PuTTY, or in some other way of genuine interest to
PuTTY users, then we will probably be happy to add a link to you on our
Links page. And if you're running a particularly valuable mirror of the
PuTTY web site, we might be interested in linking to you from our Mirrors
page.

A.9.4 Why don't you move PuTTY to
SourceForge?
Partly, because we don't want to move the web site location (see
question A.9.1).
Also, security reasons. PuTTY is a security product, and as such it is
particularly important to guard the code and the web site against
unauthorised modifications which might introduce subtle security flaws.
Therefore, we prefer that the Git repository, web site and FTP site remain
where they are, under the direct control of system administrators we
know and trust personally, rather than being run by a large organisation
full of people we've never met and which is known to have had breakins
in the past.
No offence to SourceForge; I think they do a wonderful job. But they're
not ideal for everyone, and in particular they're not ideal for us.

A.9.5 Why can't I subscribe to the putty-bugs
mailing list?
Because you're not a member of the PuTTY core development team. The
putty-bugs mailing list is not a general newsgroup-like discussion forum;
it's a contact address for the core developers, and an internal mailing list
for us to discuss things among ourselves. If we opened it up for
everybody to subscribe to, it would turn into something more like a
newsgroup and we would be completely overwhelmed by the volume of
traffic. It's hard enough to keep up with the list as it is.

A.9.6 If putty-bugs isn't a general-subscription
mailing list, what is?
There isn't one, that we know of.
If someone else wants to set up a mailing list or other forum for PuTTY
users to help each other with common problems, that would be fine with
us, though the PuTTY team would almost certainly not have the time to
read it. It's probably better to use one of the established newsgroups for
this purpose (see section B.1.2).

A.9.7 How can I donate to PuTTY development?
Please, please don't feel you have to. PuTTY is completely free software,
and not shareware. We think it's very important that everybody who
wants to use PuTTY should be able to, whether they have any money or
not; so the last thing we would want is for a PuTTY user to feel guilty
because they haven't paid us any money. If you want to keep your
money, please do keep it. We wouldn't dream of asking for any.
Having said all that, if you still really want to give us money, we won't
argue :-) The easiest way for us to accept donations is if you send money
to  using PayPal (www.paypal.com). If you don't like
PayPal, talk to us; we can probably arrange some alternative means.
Small donations (tens of dollars or tens of euros) will probably be spent
on beer or curry, which helps motivate our volunteer team to continue
doing this for the world. Larger donations will be spent on something that
actually helps development, if we can find anything (perhaps new
hardware, or a copy of Windows XP), but if we can't find anything then
we'll just distribute the money among the developers. If you want to be
sure your donation is going towards something worthwhile, ask us first. If
you don't like these terms, feel perfectly free not to donate. We don't
mind.

A.9.8 Can I have permission to put PuTTY on a
cover disk / distribute it with other software /
etc?
Yes. For most things, you need not bother asking us explicitly for
permission; our licence already grants you permission.
See section B.8 for more details.

A.9.9 Can you sign an agreement indemnifying
us against security problems in PuTTY?
No!
A vendor of physical security products (e.g. locks) might plausibly be
willing to accept financial liability for a product that failed to perform as
advertised and resulted in damage (e.g. valuables being stolen). The
reason they can afford to do this is because they sell a lot of units, and
only a small proportion of them will fail; so they can meet their financial
liability out of the income from all the rest of their sales, and still have
enough left over to make a profit. Financial liability is intrinsically linked to
selling your product for money.
There are two reasons why PuTTY is not analogous to a physical lock in
this context. One is that software products don't exhibit random variation:
if PuTTY has a security hole (which does happen, although we do our
utmost to prevent it and to respond quickly when it does), every copy of
PuTTY will have the same hole, so it's likely to affect all the users at the
same time. So even if our users were all paying us to use PuTTY, we
wouldn't be able to simultaneously pay every affected user compensation
in excess of the amount they had paid us in the first place. It just wouldn't
work.
The second, much more important, reason is that PuTTY users don't pay
us. The PuTTY team does not have an income; it's a volunteer effort
composed of people spending their spare time to try to write useful
software. We aren't even a company or any kind of legally recognised
organisation. We're just a bunch of people who happen to do some stuff
in our spare time.
Therefore, to ask us to assume financial liability is to ask us to assume a
risk of having to pay it out of our own personal pockets: out of the same
budget from which we buy food and clothes and pay our rent. That's
more than we're willing to give. We're already giving a lot of our spare
time to developing software for free; if we had to pay our own money to
do it as well, we'd start to wonder why we were bothering.

Free software fundamentally does not work on the basis of financial
guarantees. Your guarantee of the software functioning correctly is simply
that you have the source code and can check it before you use it. If you
want to be sure there aren't any security holes, do a security audit of the
PuTTY code, or hire a security engineer if you don't have the necessary
skills yourself: instead of trying to ensure you can get compensation in
the event of a disaster, try to ensure there isn't a disaster in the first
place.
If you really want financial security, see if you can find a security engineer
who will take financial responsibility for the correctness of their review.
(This might be less likely to suffer from the everything-failing-at-once
problem mentioned above, because such an engineer would probably be
reviewing a lot of different products which would tend to fail
independently.) Failing that, see if you can persuade an insurance
company to insure you against security incidents, and if the insurer
demands it as a condition then get our code reviewed by a security
engineer they're happy with.

A.9.10 Can you sign this form granting us
permission to use/distribute PuTTY?
If your form contains any clause along the lines of ‘the undersigned
represents and warrants’, we're not going to sign it. This is particularly
true if it asks us to warrant that PuTTY is secure; see question A.9.9 for
more discussion of this. But it doesn't really matter what we're supposed
to be warranting: even if it's something we already believe is true, such
as that we don't infringe any third-party copyright, we will not sign a
document accepting any legal or financial liability. This is simply because
the PuTTY development project has no income out of which to satisfy
that liability, or pay legal costs, should it become necessary. We cannot
afford to be sued. We are assuring you that we have done our best; if
that isn't good enough for you, tough.
The existing PuTTY licence document already gives you permission to
use or distribute PuTTY in pretty much any way which does not involve
pretending you wrote it or suing us if it goes wrong. We think that really
ought to be enough for anybody.
See also question A.9.12 for another reason why we don't want to do this
sort of thing.

A.9.11 Can you write us a formal notice of
permission to use PuTTY?
We could, in principle, but it isn't clear what use it would be. If you think
there's a serious chance of one of the PuTTY copyright holders suing you
(which we don't!), you would presumably want a signed notice from all of
them; and we couldn't provide that even if we wanted to, because many
of the copyright holders are people who contributed some code in the
past and with whom we subsequently lost contact. Therefore the best we
would be able to do even in theory would be to have the core
development team sign the document, which wouldn't guarantee you that
some other copyright holder might not sue.
See also question A.9.12 for another reason why we don't want to do this
sort of thing.

A.9.12 Can you sign anything for us?
Not unless there's an incredibly good reason.
We are generally unwilling to set a precedent that involves us having to
enter into individual agreements with PuTTY users. We estimate that we
have literally millions of users, and we absolutely would not have time to
go round signing specific agreements with every one of them. So if you
want us to sign something specific for you, you might usefully stop to
consider whether there's anything special that distinguishes you from
999,999 other users, and therefore any reason we should be willing to
sign something for you without it setting such a precedent.
If your company policy requires you to have an individual agreement with
the supplier of any software you use, then your company policy is simply
not well suited to using popular free software, and we urge you to
consider this as a flaw in your policy.

A.9.13 If you won't sign anything, can you give
us some sort of assurance that you won't make
PuTTY closed-source in future?
Yes and no.
If what you want is an assurance that some current version of PuTTY
which you've already downloaded will remain free, then you already have
that assurance: it's called the PuTTY Licence. It grants you permission to
use, distribute and copy the software to which it applies; once we've
granted that permission (which we have), we can't just revoke it.
On the other hand, if you want an assurance that future versions of
PuTTY won't be closed-source, that's more difficult. We could in principle
sign a document stating that we would never release a closed-source
PuTTY, but that wouldn't assure you that we would keep releasing opensource PuTTYs: we would still have the option of ceasing to develop
PuTTY at all, which would surely be even worse for you than making it
closed-source! (And we almost certainly wouldn't want to sign a
document guaranteeing that we would actually continue to do
development work on PuTTY; we certainly wouldn't sign it for free.
Documents like that are called contracts of employment, and are
generally not signed except in return for a sizeable salary.)
If we were to stop developing PuTTY, or to decide to make all future
releases closed-source, then you would still be free to copy the last open
release in accordance with the current licence, and in particular you could
start your own fork of the project from that release. If this happened, I
confidently predict that somebody would do that, and that some kind of a
free PuTTY would continue to be developed. There's already precedent
for that sort of thing happening in free software. We can't guarantee that
somebody other than you would do it, of course; you might have to do it
yourself. But we can assure you that there would be nothing preventing
anyone from continuing free development if we stopped.
(Finally, we can also confidently predict that if we made PuTTY closedsource and someone made an open-source fork, most people would

switch to the latter. Therefore, it would be pretty stupid of us to try it.)

A.9.14 Can you provide us with export control
information / FIPS certification for PuTTY?
Some people have asked us for an Export Control Classification Number
(ECCN) for PuTTY. We don't know whether we have one, and as a team
of free software developers based in the UK we don't have the time,
money, or effort to deal with US bureaucracy to investigate any further.
We believe that PuTTY falls under 5D002 on the US Commerce Control
List, but that shouldn't be taken as definitive. If you need to know more
you should seek professional legal advice. The same applies to any other
country's legal requirements and restrictions.
Similarly, some people have asked us for FIPS certification of the PuTTY
tools. Unless someone else is prepared to do the necessary work and
pay any costs, we can't provide this.

A.9.15 As one of our existing software vendors,
can you just fill in this questionnaire for us?
We periodically receive requests like this, from organisations which have
apparently sent out a form letter to everyone listed in their big
spreadsheet of ‘software vendors’ requiring them all to answer some long
list of questions about supported OS versions, paid support
arrangements, compliance with assorted local regulations we haven't
heard of, contact phone numbers, and other such administrivia. Many of
the questions are obviously meaningless when applied to PuTTY (we
don't provide any paid support in the first place!), most of the rest could
have been answered with only a very quick look at our website, and
some we are actively unwilling to answer (we are private individuals, why
would we want to give out our home phone numbers to large
corporations?).
We don't make a habit of responding in full to these questionnaires,
because we are not a software vendor.
A software vendor is a company to which you are paying lots of money in
return for some software. They know who you are, and they know you're
paying them money; so they have an incentive to fill in your forms and
questionnaires, to research any local regulations you cite if they don't
already know about them, and generally to provide every scrap of
information you might possibly need in the most convenient manner for
you, because they want to keep being paid.
But we are a team of free software developers, and that means your
relationship with us is nothing like that at all. If you once downloaded our
software from our website, that's great and we hope you found it useful,
but it doesn't mean we have the least idea who you are, or any incentive
to do lots of unpaid work to support our ‘relationship’ with you.
It's not that we are unwilling to provide information. We put as much of it
as we can on our website for your convenience, and if you actually need
to know some fact about PuTTY which you haven't been able to find on
the website (and which is not obviously inapplicable to free software in
the first place) then please do ask us, and we'll try to answer as best we

can. But we put up the website and this FAQ precisely so that we don't
have to keep answering the same questions over and over again, so we
aren't prepared to fill in completely generic form-letter questionnaires for
people who haven't done their best to find the answers here first.
If you work for an organisation which you think might be at risk of making
this mistake, we urge you to reorganise your list of software suppliers so
that it clearly distinguishes paid vendors who know about you from free
software developers who don't have any idea who you are. Then, only
send out these mass mailings to the former.

A.9.16 The sha1sums / sha256sums / etc files on
your download page don't match the binaries.
People report this every so often, and usually the reason turns out to be
that they've matched up the wrong checksums file with the wrong
binaries.
The PuTTY download page contains more than one version of the
software. There's a latest release version; there are the development
snapshots; and when we're in the run-up to making a release, there are
also pre-release builds of the upcoming new version. Each one has its
own collection of binaries, and its own collection of checksums files to go
with them.
So if you've downloaded the release version of the actual program, you
need the release version of the checksums too, otherwise you will see a
mismatch. Similarly, the development snapshot binaries go with the
development snapshot checksums, and so on. (We've colour-coded the
download page in an effort to reduce this confusion a bit.)
If you have double-checked that, and you still think there's a real
mismatch, then please send us a report carefully quoting everything
relevant:
the exact URL you got your binary from
the checksum of the binary after you downloaded
the exact URL you got your checksums file from
the checksum that file says the binary should have.

A.10 Miscellaneous questions
A.10.1 Is PuTTY a port of OpenSSH, or based on OpenSSH or
OpenSSL?
A.10.2 Where can I buy silly putty?
A.10.3 What does ‘PuTTY’ mean?
A.10.4 How do I pronounce ‘PuTTY’?

A.10.1 Is PuTTY a port of OpenSSH, or based on
OpenSSH or OpenSSL?
No, it isn't. PuTTY is almost completely composed of code written from
scratch for PuTTY. The only code we share with OpenSSH is the detector
for SSH-1 CRC compensation attacks, written by CORE SDI S.A; we
share no code at all with OpenSSL.

A.10.2 Where can I buy silly putty?
You're looking at the wrong web site; the only PuTTY we know about
here is the name of a computer program.
If you want the kind of putty you can buy as an executive toy, the PuTTY
team can personally recommend Thinking Putty, which you can buy from
Crazy Aaron's Putty World, at www.puttyworld.com.

A.10.3 What does ‘PuTTY’ mean?
It's the name of a popular SSH and Telnet client. Any other meaning is in
the eye of the beholder. It's been rumoured that ‘PuTTY’ is the antonym
of ‘getty’, or that it's the stuff that makes your Windows useful, or that it's
a kind of plutonium Teletype. We couldn't possibly comment on such
allegations.

A.10.4 How do I pronounce ‘PuTTY’?
Exactly like the English word ‘putty’, which we pronounce /ˈpʌti/.

Appendix B: Feedback and bug reporting
This is a guide to providing feedback to the PuTTY development team. It
is provided as both a web page on the PuTTY site, and an appendix in
the PuTTY manual.
Section B.1 gives some general guidelines for sending any kind of e-mail
to the development team. Following sections give more specific
guidelines for particular types of e-mail, such as bug reports and feature
requests.
B.1 General guidelines
B.1.1 Sending large attachments
B.1.2 Other places to ask for help
B.2 Reporting bugs
B.3 Reporting security vulnerabilities
B.4 Requesting extra features
B.5 Requesting features that have already been requested
B.6 Support requests
B.7 Web server administration
B.8 Asking permission for things
B.9 Mirroring the PuTTY web site
B.10 Praise and compliments
B.11 E-mail address

B.1 General guidelines
The PuTTY development team gets a lot of mail. If you can possibly
solve your own problem by reading the manual, reading the FAQ, reading
the web site, asking a fellow user, perhaps posting to a newsgroup (see
section B.1.2), or some other means, then it would make our lives much
easier.
We get so much e-mail that we literally do not have time to answer it all.
We regret this, but there's nothing we can do about it. So if you can
possibly avoid sending mail to the PuTTY team, we recommend you do
so. In particular, support requests (section B.6) are probably better sent
to newsgroups, or passed to a local expert if possible.
The PuTTY contact email address is a private mailing list containing four
or five core developers. Don't be put off by it being a mailing list: if you
need to send confidential data as part of a bug report, you can trust the
people on the list to respect that confidence. Also, the archives aren't
publicly available, so you shouldn't be letting yourself in for any spam by
sending us mail.
Please use a meaningful subject line on your message. We get a lot of
mail, and it's hard to find the message we're looking for if they all have
subject lines like ‘PuTTY bug’.
B.1.1 Sending large attachments
B.1.2 Other places to ask for help

B.1.1 Sending large attachments
Since the PuTTY contact address is a mailing list, e-mails larger than
40Kb will be held for inspection by the list administrator, and will not be
allowed through unless they really appear to be worth their large size.
If you are considering sending any kind of large data file to the PuTTY
team, it's almost always a bad idea, or at the very least it would be better
to ask us first whether we actually need the file. Alternatively, you could
put the file on a web site and just send us the URL; that way, we don't
have to download it unless we decide we actually need it, and only one of
us needs to download it instead of it being automatically copied to all the
developers.
(If the file contains confidential information, then you could encrypt it with
our Secure Contact Key; see section E.1 for details.)
Some people like to send mail in MS Word format. Please don't send us
bug reports, or any other mail, as a Word document. Word documents
are roughly fifty times larger than writing the same report in plain text. In
addition, most of the PuTTY team read their e-mail on Unix machines, so
copying the file to a Windows box to run Word is very inconvenient. Not
only that, but several of us don't even have a copy of Word!
Some people like to send us screen shots when demonstrating a
problem. Please don't do this without checking with us first - we almost
never actually need the information in the screen shot. Sending a screen
shot of an error box is almost certainly unnecessary when you could just
tell us in plain text what the error was. (On some versions of Windows,
pressing Ctrl-C when the error box is displayed will copy the text of the
message to the clipboard.) Sending a full-screen shot is occasionally
useful, but it's probably still wise to check whether we need it before
sending it.
If you must mail a screen shot, don't send it as a .BMP file. BMPs have no
compression and they are much larger than other image formats such as
PNG, TIFF and GIF. Convert the file to a properly compressed image
format before sending it.

Please don't mail us executables, at all. Our mail server blocks all
incoming e-mail containing executables, as a defence against the vast
numbers of e-mail viruses we receive every day. If you mail us an
executable, it will just bounce.
If you have made a tiny modification to the PuTTY code, please send us
a patch to the source code if possible, rather than sending us a huge
.ZIP file containing the complete sources plus your modification. If you've
only changed 10 lines, we'd prefer to receive a mail that's 30 lines long
than one containing multiple megabytes of data we already have.

B.1.2 Other places to ask for help
There are two Usenet newsgroups that are particularly relevant to the
PuTTY tools:
comp.security.ssh,
comp.terminals, for

for questions specific to using the SSH protocol;
issues relating to terminal emulation (for
instance, keyboard problems).
Please use the newsgroup most appropriate to your query, and
remember that these are general newsgroups, not specifically about
PuTTY.
If you don't have direct access to Usenet, you can access these
newsgroups through Google Groups (groups.google.com).

B.2 Reporting bugs
If you think you have found a bug in PuTTY, your first steps should be:
Check the Wishlist page on the PuTTY website, and see if we
already know about the problem. If we do, it is almost certainly not
necessary to mail us about it, unless you think you have extra
information that might be helpful to us in fixing it. (Of course, if we
actually need specific extra information about a particular bug, the
Wishlist page will say so.)
Check the Change Log on the PuTTY website, and see if we have
already fixed the bug in the development snapshots.
Check the FAQ on the PuTTY website (also provided as appendix A
in the manual), and see if it answers your question. The FAQ lists the
most common things which people think are bugs, but which aren't
bugs.
Download the latest development snapshot and see if the problem
still happens with that. This really is worth doing. As a general rule
we aren't very interested in bugs that appear in the release version
but not in the development version, because that usually means they
are bugs we have already fixed. On the other hand, if you can find a
bug in the development version that doesn't appear in the release,
that's likely to be a new bug we've introduced since the release and
we're definitely interested in it.
If none of those options solved your problem, and you still need to report
a bug to us, it is useful if you include some general information:
Tell us what version of PuTTY you are running. To find this out, use
the ‘About PuTTY’ option from the System menu. Please do not just
tell us ‘I'm running the latest version’; e-mail can be delayed and it
may not be obvious which version was the latest at the time you sent
the message.
PuTTY is a multi-platform application; tell us what version of what
OS you are running PuTTY on. (If you're running on Unix, or
Windows for Alpha, tell us, or we'll assume you're running on
Windows for Intel as this is overwhelmingly the case.)
Tell us what protocol you are connecting with: SSH, Telnet, Rlogin or

Raw mode.
Tell us what kind of server you are connecting to; what OS, and if
possible what SSH server (if you're using SSH). You can get some of
this information from the PuTTY Event Log (see section 3.1.3.1 in
the manual).
Send us the contents of the PuTTY Event Log, unless you have a
specific reason not to (for example, if it contains confidential
information that you think we should be able to solve your problem
without needing to know).
Try to give us as much information as you can to help us see the
problem for ourselves. If possible, give us a step-by-step sequence
of precise instructions for reproducing the fault.
Don't just tell us that PuTTY ‘does the wrong thing’; tell us exactly
and precisely what it did, and also tell us exactly and precisely what
you think it should have done instead. Some people tell us PuTTY
does the wrong thing, and it turns out that it was doing the right thing
and their expectations were wrong. Help to avoid this problem by
telling us exactly what you think it should have done, and exactly
what it did do.
If you think you can, you're welcome to try to fix the problem
yourself. A patch to the code which fixes a bug is an excellent
addition to a bug report. However, a patch is never a substitute for a
good bug report; if your patch is wrong or inappropriate, and you
haven't supplied us with full information about the actual bug, then
we won't be able to find a better solution.
https://www.chiark.greenend.org.uk/~sgtatham/bugs.html is an
article on how to report bugs effectively in general. If your bug report
is particularly unclear, we may ask you to go away, read this article,
and then report the bug again.
It is reasonable to report bugs in PuTTY's documentation, if you think the
documentation is unclear or unhelpful. But we do need to be given exact
details of what you think the documentation has failed to tell you, or how
you think it could be made clearer. If your problem is simply that you don't
understand the documentation, we suggest posting to a newsgroup (see
section B.1.2) and seeing if someone will explain what you need to know.
Then, if you think the documentation could usefully have told you that,
send us a bug report and explain how you think we should change it.

B.3 Reporting security vulnerabilities
If you've found a security vulnerability in PuTTY, you might well want to
notify us using an encrypted communications channel, to avoid disclosing
information about the vulnerability before a fixed release is available.
For this purpose, we provide a GPG key suitable for encryption: the
Secure Contact Key. See section E.1 for details of this.
(Of course, vulnerabilities are also bugs, so please do include as much
information as possible about them, the same way you would with any
other bug report.)

B.4 Requesting extra features
If you want to request a new feature in PuTTY, the very first things you
should do are:
Check the Wishlist page on the PuTTY website, and see if your
feature is already on the list. If it is, it probably won't achieve very
much to repeat the request. (But see section B.5 if you want to
persuade us to give your particular feature higher priority.)
Check the Wishlist and Change Log on the PuTTY website, and see
if we have already added your feature in the development
snapshots. If it isn't clear, download the latest development snapshot
and see if the feature is present. If it is, then it will also be in the next
release and there is no need to mail us at all.
If you can't find your feature in either the development snapshots or the
Wishlist, then you probably do need to submit a feature request. Since
the PuTTY authors are very busy, it helps if you try to do some of the
work for us:
Do as much of the design as you can. Think about ‘corner cases’;
think about how your feature interacts with other existing features.
Think about the user interface; if you can't come up with a simple
and intuitive interface to your feature, you shouldn't be surprised if
we can't either. Always imagine whether it's possible for there to be
more than one, or less than one, of something you'd assumed there
would be one of. (For example, if you were to want PuTTY to put an
icon in the System tray rather than the Taskbar, you should think
about what happens if there's more than one PuTTY active; how
would the user tell which was which?)
If you can program, it may be worth offering to write the feature
yourself and send us a patch. However, it is likely to be helpful if you
confer with us first; there may be design issues you haven't thought
of, or we may be about to make big changes to the code which your
patch would clash with, or something. If you check with the
maintainers first, there is a better chance of your code actually being
usable. Also, read the design principles listed in appendix D: if you
do not conform to them, we will probably not be able to accept your

patch.

B.5 Requesting features that have already been
requested
If a feature is already listed on the Wishlist, then it usually means we
would like to add it to PuTTY at some point. However, this may not be in
the near future. If there's a feature on the Wishlist which you would like to
see in the near future, there are several things you can do to try to
increase its priority level:
Mail us and vote for it. (Be sure to mention that you've seen it on the
Wishlist, or we might think you haven't even read the Wishlist). This
probably won't have very much effect; if a huge number of people
vote for something then it may make a difference, but one or two
extra votes for a particular feature are unlikely to change our priority
list immediately. Offering a new and compelling justification might
help. Also, don't expect a reply.
Offer us money if we do the work sooner rather than later. This
sometimes works, but not always. The PuTTY team all have full-time
jobs and we're doing all of this work in our free time; we may
sometimes be willing to give up some more of our free time in
exchange for some money, but if you try to bribe us for a big feature
it's entirely possible that we simply won't have the time to spare whether you pay us or not. (Also, we don't accept bribes to add bad
features to the Wishlist, because our desire to provide high-quality
software to the users comes first.)
Offer to help us write the code. This is probably the only way to get a
feature implemented quickly, if it's a big one that we don't have time
to do ourselves.

B.6 Support requests
If you're trying to make PuTTY do something for you and it isn't working,
but you're not sure whether it's a bug or not, then please consider looking
for help somewhere else. This is one of the most common types of mail
the PuTTY team receives, and we simply don't have time to answer all
the questions. Questions of this type include:
If you want to do something with PuTTY but have no idea where to
start, and reading the manual hasn't helped, try posting to a
newsgroup (see section B.1.2) and see if someone can explain it to
you.
If you have tried to do something with PuTTY but it hasn't worked,
and you aren't sure whether it's a bug in PuTTY or a bug in your
SSH server or simply that you're not doing it right, then try posting to
a newsgroup (see section B.1.2) and see if someone can solve your
problem. Or try doing the same thing with a different SSH client and
see if it works with that. Please do not report it as a PuTTY bug
unless you are really sure it is a bug in PuTTY.
If someone else installed PuTTY for you, or you're using PuTTY on
someone else's computer, try asking them for help first. They're
more likely to understand how they installed it and what they
expected you to use it for than we are.
If you have successfully made a connection to your server and now
need to know what to type at the server's command prompt, or other
details of how to use the server-end software, talk to your server's
system administrator. This is not the PuTTY team's problem. PuTTY
is only a communications tool, like a telephone; if you can't speak
the same language as the person at the other end of the phone, it
isn't the telephone company's job to teach it to you.
If you absolutely cannot get a support question answered any other way,
you can try mailing it to us, but we can't guarantee to have time to
answer it.

B.7 Web server administration
If the PuTTY web site is down (Connection Timed Out), please don't
bother mailing us to tell us about it. Most of us read our e-mail on the
same machines that host the web site, so if those machines are down
then we will notice before we read our e-mail. So there's no point telling
us our servers are down.
Of course, if the web site has some other error (Connection Refused, 404
Not Found, 403 Forbidden, or something else) then we might not have
noticed and it might still be worth telling us about it.
If you want to report a problem with our web site, check that you're
looking at our real web site and not a mirror. The real web site is at
https://www.chiark.greenend.org.uk/~sgtatham/putty/; if that's not
where you're reading this, then don't report the problem to us until you've
checked that it's really a problem with the main site. If it's only a problem
with the mirror, you should try to contact the administrator of that mirror
site first, and only contact us if that doesn't solve the problem (in case we
need to remove the mirror from our list).

B.8 Asking permission for things
PuTTY is distributed under the MIT Licence (see appendix C for details).
This means you can do almost anything you like with our software, our
source code, and our documentation. The only things you aren't allowed
to do are to remove our copyright notices or the licence text itself, or to
hold us legally responsible if something goes wrong.
So if you want permission to include PuTTY on a magazine cover disk, or
as part of a collection of useful software on a CD or a web site, then
permission is already granted. You don't have to mail us and ask. Just go
ahead and do it. We don't mind.
(If you want to distribute PuTTY alongside your own application for use
with that application, or if you want to distribute PuTTY within your own
organisation, then we recommend, but do not insist, that you offer your
own first-line technical support, to answer questions about the interaction
of PuTTY with your environment. If your users mail us directly, we won't
be able to tell them anything useful about your specific setup.)
If you want to use parts of the PuTTY source code in another program,
then it might be worth mailing us to talk about technical details, but if all
you want is to ask permission then you don't need to bother. You already
have permission.
If you just want to link to our web site, just go ahead. (It's not clear that
we could stop you doing this, even if we wanted to!)

B.9 Mirroring the PuTTY web site
If you want to set up a mirror of the PuTTY website, go ahead and set
one up. Please don't bother asking us for permission before setting up a
mirror. You already have permission.
If the mirror is in a country where we don't already have plenty of mirrors,
we may be willing to add it to the list on our mirrors page. Read the
guidelines on that page, make sure your mirror works, and email us the
information listed at the bottom of the page.
Note that we do not promise to list your mirror: we get a lot of mirror
notifications and yours may not happen to find its way to the top of the
list.
Also note that we link to all our mirror sites using the rel="nofollow"
attribute. Running a PuTTY mirror is not intended to be a cheap way to
gain search rankings.
If you have technical questions about the process of mirroring, then you
might want to mail us before setting up the mirror (see also the guidelines
on the Mirrors page); but if you just want to ask for permission, you don't
need to. You already have permission.

B.10 Praise and compliments
One of the most rewarding things about maintaining free software is
getting e-mails that just say ‘thanks’. We are always happy to receive emails of this type.
Regrettably we don't have time to answer them all in person. If you mail
us a compliment and don't receive a reply, please don't think we've
ignored you. We did receive it and we were happy about it; we just didn't
have time to tell you so personally.
To everyone who's ever sent us praise and compliments, in the past and
the future: you're welcome!

B.11 E-mail address
The actual address to mail is .

Appendix C: PuTTY Licence
PuTTY is copyright 1997-2017 Simon Tatham.
Portions copyright Robert de Bath, Joris van Rantwijk, Delian Delchev,
Andreas Schultz, Jeroen Massar, Wez Furlong, Nicolas Barry, Justin
Bradford, Ben Harris, Malcolm Smith, Ahmad Khalifa, Markus Kuhn,
Colin Watson, Christopher Staite, and CORE SDI S.A.
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the ‘Software’),
to deal in the Software without restriction, including without limitation the
rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
sell copies of the Software, and to permit persons to whom the Software
is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED ‘AS IS’, WITHOUT WARRANTY OF
ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

Appendix D: PuTTY hacking guide
This appendix lists a selection of the design principles applying to the
PuTTY source code. If you are planning to send code contributions, you
should read this first.
D.1 Cross-OS portability
D.2 Multiple backends treated equally
D.3 Multiple sessions per process on some platforms
D.4 C, not C++
D.5 Security-conscious coding
D.6 Independence of specific compiler
D.7 Small code size
D.8 Single-threaded code
D.9 Keystrokes sent to the server wherever possible
D.10 640×480 friendliness in configuration panels
D.11 Automatically generated Makefiles
D.12 Coroutines in ssh.c
D.13 Single compilation of each source file
D.14 Do as we say, not as we do

D.1 Cross-OS portability
Despite Windows being its main area of fame, PuTTY is no longer a
Windows-only application suite. It has a working Unix port; a Mac port is
in progress; more ports may or may not happen at a later date.
Therefore, embedding Windows-specific code in core modules such as
ssh.c is not acceptable. We went to great lengths to remove all the
Windows-specific stuff from our core modules, and to shift it out into
Windows-specific modules. Adding large amounts of Windows-specific
stuff in parts of the code that should be portable is almost guaranteed to
make us reject a contribution.
The PuTTY source base is divided into platform-specific modules and
platform-generic modules. The Unix-specific modules are all in the unix
subdirectory; the Mac-specific modules are in the mac subdirectory; the
Windows-specific modules are in the windows subdirectory.
All the modules in the main source directory - notably all of the code for
the various back ends - are platform-generic. We want to keep them that
way.
This also means you should stick to what you are guaranteed by
ANSI/ISO C (that is, the original C89/C90 standard, not C99). Try not to
make assumptions about the precise size of basic types such as int and
long int; don't use pointer casts to do endianness-dependent
operations, and so on.
(There are one or two aspects of ANSI C portability which we don't care
about. In particular, we expect PuTTY to be compiled on 32-bit
architectures or bigger; so it's safe to assume that int is at least 32 bits
wide, not just the 16 you are guaranteed by ANSI C. Similarly, we
assume that the execution character encoding is a superset of the
printable characters of ASCII, though we don't assume the numeric
values of control characters, particularly '\n' and '\r'. Also, the X
forwarding code assumes that time_t has the Unix format and semantics,
i.e. an integer giving the number of seconds since 1970.)

D.2 Multiple backends treated equally
PuTTY is not an SSH client with some other stuff tacked on the side.
PuTTY is a generic, multiple-backend, remote VT-terminal client which
happens to support one backend which is larger, more popular and more
useful than the rest. Any extra feature which can possibly be general
across all backends should be so: localising features unnecessarily into
the SSH back end is a design error. (For example, we had several code
submissions for proxy support which worked by hacking ssh.c. Clearly
this is completely wrong: the network.h abstraction is the place to put it,
so that it will apply to all back ends equally, and indeed we eventually put
it there after another contributor sent a better patch.)
The rest of PuTTY should try to avoid knowing anything about specific
back ends if at all possible. To support a feature which is only available in
one network protocol, for example, the back end interface should be
extended in a general manner such that any back end which is able to
provide that feature can do so. If it so happens that only one back end
actually does, that's just the way it is, but it shouldn't be relied upon by
any code.

D.3 Multiple sessions per process on some
platforms
Some ports of PuTTY - notably the in-progress Mac port - are
constrained by the operating system to run as a single process potentially
managing multiple sessions.
Therefore, the platform-independent parts of PuTTY never use global
variables to store per-session data. The global variables that do exist are
tolerated because they are not specific to a particular login session: flags
defines properties that are expected to apply equally to all the sessions
run by a single PuTTY process, the random number state in sshrand.c
and the timer list in timing.c serve all sessions equally, and so on. But
most data is specific to a particular network session, and is therefore
stored in dynamically allocated data structures, and pointers to these
structures are passed around between functions.
Platform-specific code can reverse this decision if it likes. The Windows
code, for historical reasons, stores most of its data as global variables.
That's OK, because on Windows we know there is only one session per
PuTTY process, so it's safe to do that. But changes to the platformindependent code should avoid introducing global variables, unless they
are genuinely cross-session.

D.4 C, not C++
PuTTY is written entirely in C, not in C++.
We have made some effort to make it easy to compile our code using a
C++ compiler: notably, our snew, snewn and sresize macros explicitly cast
the return values of malloc and realloc to the target type. (This has type
checking advantages even in C: it means you never accidentally allocate
the wrong size piece of memory for the pointer type you're assigning it to.
C++ friendliness is really a side benefit.)
We want PuTTY to continue being pure C, at least in the platformindependent parts and the currently existing ports. Patches which switch
the Makefiles to compile it as C++ and start using classes will not be
accepted. Also, in particular, we disapprove of // comments, at least for
the moment. (Perhaps once C99 becomes genuinely widespread we
might be more lenient.)
The one exception: a port to a new platform may use languages other
than C if they are necessary to code on that platform. If your favourite
PDA has a GUI with a C++ API, then there's no way you can do a port of
PuTTY without using C++, so go ahead and use it. But keep the C++
restricted to that platform's subdirectory; if your changes force the Unix or
Windows ports to be compiled as C++, they will be unacceptable to us.

D.5 Security-conscious coding
PuTTY is a network application and a security application. Assume your
code will end up being fed deliberately malicious data by attackers, and
try to code in a way that makes it unlikely to be a security risk.
In particular, try not to use fixed-size buffers for variable-size data such
as strings received from the network (or even the user). We provide
functions such as dupcat and dupprintf, which dynamically allocate
buffers of the right size for the string they construct. Use these wherever
possible.

D.6 Independence of specific compiler
Windows PuTTY can currently be compiled with any of four Windows
compilers: MS Visual C, Borland's freely downloadable C compiler, the
Cygwin / mingw32 GNU tools, and lcc-win32.
This is a really useful property of PuTTY, because it means people who
want to contribute to the coding don't depend on having a specific
compiler; so they don't have to fork out money for MSVC if they don't
already have it, but on the other hand if they do have it they also don't
have to spend effort installing gcc alongside it. They can use whichever
compiler they happen to have available, or install whichever is cheapest
and easiest if they don't have one.
Therefore, we don't want PuTTY to start depending on which compiler
you're using. Using GNU extensions to the C language, for example,
would ruin this useful property (not that anyone's ever tried it!); and more
realistically, depending on an MS-specific library function supplied by the
MSVC C library (_snprintf, for example) is a mistake, because that
function won't be available under the other compilers. Any function
supplied in an official Windows DLL as part of the Windows API is fine,
and anything defined in the C library standard is also fine, because those
should be available irrespective of compilation environment. But things in
between, available as non-standard library and language extensions in
only one compiler, are disallowed.
(_snprintf in particular should be unnecessary, since we provide
dupprintf; see section D.5.)
Compiler independence should apply on all platforms, of course, not just
on Windows.

D.7 Small code size
PuTTY is tiny, compared to many other Windows applications. And it's
easy to install: it depends on no DLLs, no other applications, no service
packs or system upgrades. It's just one executable. You install that
executable wherever you want to, and run it.
We want to keep both these properties - the small size, and the ease of
installation - if at all possible. So code contributions that depend critically
on external DLLs, or that add a huge amount to the code size for a
feature which is only useful to a small minority of users, are likely to be
thrown out immediately.
We do vaguely intend to introduce a DLL plugin interface for PuTTY,
whereby seriously large extra features can be implemented in plugin
modules. The important thing, though, is that those DLLs will be optional;
if PuTTY can't find them on startup, it should run perfectly happily and
just won't provide those particular features. A full installation of PuTTY
might one day contain ten or twenty little DLL plugins, which would cut
down a little on the ease of installation - but if you really needed ease of
installation you could still just install the one PuTTY binary, or just the
DLLs you really needed, and it would still work fine.
Depending on external DLLs is something we'd like to avoid if at all
possible (though for some purposes, such as complex SSH
authentication mechanisms, it may be unavoidable). If it can't be avoided,
the important thing is to follow the same principle of graceful degradation:
if a DLL can't be found, then PuTTY should run happily and just not
supply the feature that depended on it.

D.8 Single-threaded code
PuTTY and its supporting tools, or at least the vast majority of them, run
in only one OS thread.
This means that if you're devising some piece of internal mechanism,
there's no need to use locks to make sure it doesn't get called by two
threads at once. The only way code can be called re-entrantly is by
recursion.
That said, most of Windows PuTTY's network handling is triggered off
Windows messages requested by WSAAsyncSelect(), so if you call
MessageBox() deep within some network event handling code you should
be aware that you might be re-entered if a network event comes in and is
passed on to our window procedure by the MessageBox() message loop.
Also, the front ends (in particular Windows Plink) can use multiple
threads if they like. However, Windows Plink keeps very tight control of its
auxiliary threads, and uses them pretty much exclusively as a form of
select(). Pretty much all the code outside windows/winplink.c is only
ever called from the one primary thread; the others just loop round
blocking on file handles and send messages to the main thread when
some real work needs doing. This is not considered a portability hazard
because that bit of windows/winplink.c will need rewriting on other
platforms in any case.
One important consequence of this: PuTTY has only one thread in which
to do everything. That ‘everything’ may include managing more than one
login session (section D.3), managing multiple data channels within an
SSH session, responding to GUI events even when nothing is happening
on the network, and responding to network requests from the server
(such as repeat key exchange) even when the program is dealing with
complex user interaction such as the re-configuration dialog box. This
means that almost none of the PuTTY code can safely block.

D.9 Keystrokes sent to the server wherever
possible
In almost all cases, PuTTY sends keystrokes to the server. Even weird
keystrokes that you think should be hot keys controlling PuTTY. Even AltF4 or Alt-Space, for example. If a keystroke has a well-defined escape
sequence that it could usefully be sending to the server, then it should do
so, or at the very least it should be configurably able to do so.
To unconditionally turn a key combination into a hot key to control PuTTY
is almost always a design error. If a hot key is really truly required, then
try to find a key combination for it which isn't already used in existing
PuTTYs (either it sends nothing to the server, or it sends the same thing
as some other combination). Even then, be prepared for the possibility
that one day that key combination might end up being needed to send
something to the server - so make sure that there's an alternative way to
invoke whatever PuTTY feature it controls.

D.10 640×480 friendliness in configuration
panels
There's a reason we have lots of tiny configuration panels instead of a
few huge ones, and that reason is that not everyone has a 1600×1200
desktop. 640×480 is still a viable resolution for running Windows (and
indeed it's still the default if you start up in safe mode), so it's still a
resolution we care about.
Accordingly, the PuTTY configuration box, and the PuTTYgen control
window, are deliberately kept just small enough to fit comfortably on a
640×480 display. If you're adding controls to either of these boxes and
you find yourself wanting to increase the size of the whole box, don't.
Split it into more panels instead.

D.11 Automatically generated Makefiles
PuTTY is intended to compile on multiple platforms, and with multiple
compilers. It would be horrifying to try to maintain a single Makefile which
handled all possible situations, and just as painful to try to directly
maintain a set of matching Makefiles for each different compilation
environment.
Therefore, we have moved the problem up by one level. In the PuTTY
source archive is a file called Recipe, which lists which source files
combine to produce which binaries; and there is also a script called
mkfiles.pl, which reads Recipe and writes out the real Makefiles. (The
script also reads all the source files and analyses their dependencies on
header files, so we get an extra benefit from doing it this way, which is
that we can supply correct dependency information even in environments
where it's difficult to set up an automated make depend phase.)
You should never edit any of the PuTTY Makefiles directly. They are not
stored in our source repository at all. They are automatically generated
by mkfiles.pl from the file Recipe.
If you need to add a new object file to a particular binary, the right thing to
do is to edit Recipe and re-run mkfiles.pl. This will cause the new object
file to be added in every tool that requires it, on every platform where it
matters, in every Makefile to which it is relevant, and to get all the
dependency data right.
If you send us a patch that modifies one of the Makefiles, you just waste
our time, because we will have to convert it into a change to Recipe. If
you send us a patch that modifies all of the Makefiles, you will have
wasted a lot of your time as well!
(There is a comment at the top of every Makefile in the PuTTY source
archive saying this, but many people don't seem to read it, so it's worth
repeating here.)

D.12 Coroutines in ssh.c
Large parts of the code in ssh.c are structured using a set of macros that
implement (something close to) Donald Knuth's ‘coroutines’ concept in C.
Essentially, the purpose of these macros are to arrange that a function
can call crReturn() to return to its caller, and the next time it is called
control will resume from just after that crReturn statement.
This means that any local (automatic) variables declared in such a
function will be corrupted every time you call crReturn. If you need a
variable to persist for longer than that, you must make it a field in one of
the persistent state structures: either the local state structures s or st in
each function, or the backend-wide structure ssh.
See https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html for
a more in-depth discussion of what these macros are for and how they
work.

D.13 Single compilation of each source file
The PuTTY build system for any given platform works on the following
very simple model:
Each source file is compiled precisely once, to produce a single
object file.
Each binary is created by linking together some combination of those
object files.
Therefore, if you need to introduce functionality to a particular module
which is only available in some of the tool binaries (for example, a
cryptographic proxy authentication mechanism which needs to be left out
of PuTTYtel to maintain its usability in crypto-hostile jurisdictions), the
wrong way to do it is by adding #ifdefs in (say) proxy.c. This would
require separate compilation of proxy.c for PuTTY and PuTTYtel, which
means that the entire Makefile-generation architecture (see section D.11)
would have to be significantly redesigned. Unless you are prepared to do
that redesign yourself, and guarantee that it will still port to any future
platforms we might decide to run on, you should not attempt this!
The right way to introduce a feature like this is to put the new code in a
separate source file, and (if necessary) introduce a second new source
file defining the same set of functions, but defining them as stubs which
don't provide the feature. Then the module whose behaviour needs to
vary (proxy.c in this example) can call the functions defined in these two
modules, and it will either provide the new feature or not provide it
according to which of your new modules it is linked with.
Of course, object files are never shared between platforms; so it is
allowable to use #ifdef to select between platforms. This happens in
puttyps.h (choosing which of the platform-specific include files to use),
and also in misc.c (the Windows-specific ‘Minefield’ memory diagnostic
system). It should be used sparingly, though, if at all.

D.14 Do as we say, not as we do
The current PuTTY code probably does not conform strictly to all of the
principles listed above. There may be the occasional SSH-specific piece
of code in what should be a backend-independent module, or the
occasional dependence on a non-standard X library function under Unix.
This should not be taken as a licence to go ahead and violate the rules.
Where we violate them ourselves, we're not happy about it, and we
would welcome patches that fix any existing problems. Please try to help
us make our code better, not worse!

Appendix E: PuTTY download keys and
signatures
We create GPG signatures for all the PuTTY files distributed from our
web site, so that users can be confident that the files have not been
tampered with. Here we identify our public keys, and explain our
signature policy so you can have an accurate idea of what each signature
guarantees. This description is provided as both a web page on the
PuTTY site, and an appendix in the PuTTY manual.
As of release 0.58, all of the PuTTY executables contain fingerprint
material (usually accessed via the -pgpfp command-line option), such
that if you have an executable you trust, you can use it to establish a trust
path, for instance to a newer version downloaded from the Internet.
(Note that none of the keys, signatures, etc mentioned here have
anything to do with keys used with SSH - they are purely for verifying the
origin of files distributed by the PuTTY team.)
E.1 Public keys
E.2 Security details
E.2.1 The Development Snapshots key
E.2.2 The Releases key
E.2.3 The Secure Contact Key
E.2.4 The Master Keys
E.3 Key rollover

E.1 Public keys
We maintain multiple keys, stored with different levels of security due to
being used in different ways. See section E.2 below for details.
The keys we provide are:
Snapshot Key
Used to sign routine development builds of PuTTY: nightly
snapshots, pre-releases, and sometimes also custom diagnostic
builds we send to particular users.
Release Key
Used to sign manually released versions of PuTTY.
Secure Contact Key
An encryption-capable key suitable for people to send confidential
messages to the PuTTY team, e.g. reports of vulnerabilities.
Master Key
Used to tie all the above keys into the GPG web of trust. The Master
Key signs all the other keys, and other GPG users have signed it in
turn.
The current issue of those keys are available for download from the
PuTTY website, and are also available on PGP keyservers using the key
IDs listed below.
Master Key
RSA, 4096-bit. Key ID: 4096R/04676F7C (long version:
4096R/AB585DC604676F7C). Fingerprint:
440D E3B5 B7A1 CA85 B3CC

1718 AB58 5DC6 0467 6F7C

Release Key
RSA, 2048-bit. Key ID: 2048R/B43434E4 (long version:
2048R/9DFE2648B43434E4). Fingerprint:
0054 DDAA 8ADA 15D2 768A

6DE7 9DFE 2648 B434 34E4

Secure Contact Key
RSA, 2048-bit. Main key ID: 2048R/8A0AF00B (long version:
2048R/C4FCAAD08A0AF00B). Encryption subkey ID: 2048R/50C2CF5C
(long version: 2048R/9EB39CC150C2CF5C). Fingerprint:
8A26 250E 763F E359 75F3

Snapshot Key

118F C4FC AAD0 8A0A F00B

RSA, 2048-bit. Key ID: 2048R/D15F7E8A (long version:
2048R/EEF20295D15F7E8A). Fingerprint:
0A3B 0048 FE49 9B67 A234

FEB6 EEF2 0295 D15F 7E8A

E.2 Security details
The various keys have various different security levels. This section
explains what those security levels are, and how far you can expect to
trust each key.
E.2.1 The Development Snapshots key
E.2.2 The Releases key
E.2.3 The Secure Contact Key
E.2.4 The Master Keys

E.2.1 The Development Snapshots key
The Development Snapshots private key is stored without a passphrase.
This is necessary, because the snapshots are generated every night
without human intervention, so nobody would be able to type a
passphrase.
The snapshots are built and signed on a team member's home
computers, before being uploaded to the web server from which you
download them.
Therefore, a signature from the Development Snapshots key DOES
protect you against:
People tampering with the PuTTY binaries between the PuTTY web
site and you.
The maintainers of our web server attempting to abuse their root
privilege to tamper with the binaries.
But it DOES NOT protect you against:
People tampering with the binaries before they are uploaded to our
download servers.
People tampering with the build machines so that the next set of
binaries they build will be malicious in some way.
People stealing the unencrypted private key from the build machine
it lives on.
Of course, we take all reasonable precautions to guard the build
machines. But when you see a signature, you should always be certain of
precisely what it guarantees and precisely what it does not.

E.2.2 The Releases key
The Releases key is more secure: because it is only used at release
time, to sign each release by hand, we can store it encrypted.
The Releases private key is kept encrypted on the developers' own local
machines. So an attacker wanting to steal it would have to also steal the
passphrase.

E.2.3 The Secure Contact Key
The Secure Contact Key is stored with a similar level of security to the
Release Key: it is stored with a passphrase, and no automated script has
access to it.

E.2.4 The Master Keys
The Master Key signs almost nothing. Its purpose is to bind the other
keys together and certify that they are all owned by the same people and
part of the same integrated setup. The only signatures produced by the
Master Key, ever, should be the signatures on the other keys.
The Master Key is especially long, and its private key and passphrase
are stored with special care.
We have collected some third-party signatures on the Master Key, in
order to increase the chances that you can find a suitable trust path to
them.
We have uploaded our various keys to public keyservers, so that even if
you don't know any of the people who have signed our keys, you can still
be reasonably confident that an attacker would find it hard to substitute
fake keys on all the public keyservers at once.

E.3 Key rollover
Our current keys were generated in September 2015, except for the
Secure Contact Key which was generated in February 2016 (we didn't
think of it until later).
Prior to that, we had a much older set of keys generated in 2000. For
each of the key types above (other than the Secure Contact Key), we
provided both an RSA key and a DSA key (because at the time we
generated them, RSA was not in practice available to everyone, due to
export restrictions).
The new Master Key is signed with both of the old ones, to show that it
really is owned by the same people and not substituted by an attacker.
Also, we have retrospectively signed the old Release Keys with the new
Master Key, in case you're trying to verify the signatures on a release
prior to the rollover and can find a chain of trust to those keys from any of
the people who have signed our new Master Key.
Future releases will be signed with the up-to-date keys shown above.
Releases prior to the rollover are signed with the old Release Keys.
For completeness, those old keys are given here:
Master Key (original RSA)
RSA, 1024-bit. Key ID: 1024R/1E34AC41 (long version:
1024R/9D5877BF1E34AC41). Fingerprint:
8F 15 97 DA 25 30 AB 0D

88 D1 92 54 11 CF 0C 4C

Master Key (original DSA)
DSA, 1024-bit. Key ID: 1024D/6A93B34E (long version:
1024D/4F5E6DF56A93B34E). Fingerprint:
313C 3E76 4B74 C2C5 F2AE

83A8 4F5E 6DF5 6A93 B34E

Release Key (original RSA)
RSA, 1024-bit. Key ID: 1024R/B41CAE29 (long version:
1024R/EF39CCC0B41CAE29). Fingerprint:
AE 65 D3 F7 85 D3 18 E0

3B 0C 9B 02 FF 3A 81 FE

Release Key (original DSA)
DSA, 1024-bit. Key ID: 1024D/08B0A90B (long version:
1024D/FECD6F3F08B0A90B). Fingerprint:

00B1 1009 38E6 9800 6518

F0AB FECD 6F3F 08B0 A90B

Snapshot Key (original RSA)
RSA, 1024-bit. Key ID: 1024R/32B903A9 (long version:
1024R/FAAED21532B903A9). Fingerprint:
86 8B 1F 79 9C F4 7F BD

8B 1B D7 8E C6 4E 4C 03

Snapshot Key (original DSA)
DSA, 1024-bit. Key ID: 1024D/7D3E4A00 (long version:
1024D/165E56F77D3E4A00). Fingerprint:
63DD 8EF8 32F5 D777 9FF0

2947 165E 56F7 7D3E 4A00

Appendix F: SSH-2 names specified for PuTTY
There are various parts of the SSH-2 protocol where things are specified
using a textual name. Names ending in @putty.projects.tartarus.org
are reserved for allocation by the PuTTY team. Allocated names are
documented here.
F.1 Connection protocol channel request names
F.2 Key exchange method names
F.3 Encryption algorithm names

F.1 Connection protocol channel request names
These names can be sent in a SSH_MSG_CHANNEL_REQUEST message.
simple@putty.projects.tartarus.org

This is sent by a client to announce that it will not have more than
one channel open at a time in the current connection (that one being
the one the request is sent on). The intention is that the server,
knowing this, can set the window on that one channel to something
very large, and leave flow control to TCP. There is no messagespecific data.
winadj@putty.projects.tartarus.org

PuTTY sends this request along with some
SSH_MSG_CHANNEL_WINDOW_ADJUST messages as part of its window-size
tuning. It can be sent on any type of channel. There is no messagespecific data. Servers MUST treat it as an unrecognised request and
respond with SSH_MSG_CHANNEL_FAILURE.
(Some SSH servers get confused by this message, so there is a
bug-compatibility mode for disabling it. See section 4.27.5.)

F.2 Key exchange method names
rsa-sha1-draft-00@putty.projects.tartarus.org
rsa-sha256-draft-00@putty.projects.tartarus.org
rsa1024-sha1-draft-01@putty.projects.tartarus.org
rsa1024-sha256-draft-01@putty.projects.tartarus.org
rsa2048-sha256-draft-01@putty.projects.tartarus.org
rsa1024-sha1-draft-02@putty.projects.tartarus.org
rsa2048-sha512-draft-02@putty.projects.tartarus.org
rsa1024-sha1-draft-03@putty.projects.tartarus.org
rsa2048-sha256-draft-03@putty.projects.tartarus.org
rsa1024-sha1-draft-04@putty.projects.tartarus.org
rsa2048-sha256-draft-04@putty.projects.tartarus.org

These appeared in various drafts of what eventually became
RFC 4432. They have been superseded by rsa1024-sha1 and
rsa2048-sha256.

F.3 Encryption algorithm names
arcfour128-draft-00@putty.projects.tartarus.org
arcfour256-draft-00@putty.projects.tartarus.org

These were used in drafts of what eventually became RFC 4345.
They have been superseded by arcfour128 and arcfour256.



---

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