Iptables Firewall Guide

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UFW
Introduction
For an introduction to firewalls, please see Firewall.

UFW - Uncomplicated Firewall
The default firewall configuration tool for Ubuntu is ufw. Developed to ease iptables firewall configuration, ufw provides a user friendly way to create an IPv4 or IPv6
host-based firewall. By default UFW is disabled.
Gufw is a GUI that is available as a frontend.

Basic Syntax and Examples
Default rules are fine for the average home user
When you turn UFW on, it uses a default set of rules (profile) that should be fine for the average home user. That's at least the goal of the Ubuntu developers. In short, all
'incoming' is being denied, with some exceptions to make things easier for home users.

Contents
1. Introduction
1. UFW - Uncomplicated Firewall
2. Basic Syntax and Examples
1. Default rules are fine for the average home user
2. Enable and Disable
3. Allow and Deny (specific rules)
4. Delete Existing Rule
5. Services
6. Status
7. Logging
3. Advanced Syntax
1. Allow Access
2. Deny Access
3. Working with numbered rules
4. Editing numbered rules
5. Advanced Example
4. Interpreting Log Entries
5. Other Resources

Enable and Disable
Enable UFW
To turn UFW on with the default set of rules:
sudo ufw enable

To check the status of UFW:
sudo ufw status verbose

The output should be like this:
youruser@yourcomputer:~$ sudo ufw status verbose
[sudo] password for youruser:
Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing)
New profiles: skip
youruser@yourcomputer:~$

Note that by default, deny is being applied to incoming. There are exceptions, which can be found in the output of this command:
sudo ufw show raw

You can also read the rules files in /etc/ufw (the files whose names end with .rules).

Disable UFW
To disable ufw use:
sudo ufw disable

Allow and Deny (specific rules)
Allow
sudo ufw allow /

example: To allow incoming tcp and udp packet on port 53
sudo ufw allow 53

example: To allow incoming tcp packets on port 53
sudo ufw allow 53/tcp

example: To allow incoming udp packets on port 53
sudo ufw allow 53/udp

Deny
sudo ufw deny /

example: To deny tcp and udp packets on port 53
sudo ufw deny 53

example: To deny incoming tcp packets on port 53
sudo ufw deny 53/tcp

example: To deny incoming udp packets on port 53
sudo ufw deny 53/udp

Delete Existing Rule
To delete a rule, simply prefix the original rule with delete. For example, if the original rule was:
ufw deny 80/tcp

Use this to delete it:
sudo ufw delete deny 80/tcp

Services
You can also allow or deny by service name since ufw reads from /etc/services To see get a list of services:
less /etc/services

Allow by Service Name
sudo ufw allow 

example: to allow ssh by name
sudo ufw allow ssh

Deny by Service Name
sudo ufw deny 

example: to deny ssh by name
sudo ufw deny ssh

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Status
Checking the status of ufw will tell you if ufw is enabled or disabled and also list the current ufw rules that are applied to your iptables.
To check the status of ufw:
sudo ufw status
Firewall loaded
To
-22:tcp
22:udp
22:tcp
22:udp
22:tcp
22:udp

Action
-----DENY
DENY
DENY
DENY
ALLOW
ALLOW

From
---192.168.0.1
192.168.0.1
192.168.0.7
192.168.0.7
192.168.0.0/24
192.168.0.0/24

if ufw was not enabled the output would be:
sudo ufw status
Status: inactive

Logging
To enable logging use:
sudo ufw logging on

To disable logging use:
sudo ufw logging off

Advanced Syntax
You can also use a fuller syntax, specifying the source and destination addresses, ports and protocols.

Allow Access
This section shows how to allow specific access.

Allow by Specific IP
sudo ufw allow from 

example:To allow packets from 207.46.232.182:
sudo ufw allow from 207.46.232.182

Allow by Subnet
You may use a net mask :
sudo ufw allow from 192.168.1.0/24

Allow by specific port and IP address
sudo ufw allow from  to  port 

example: allow IP address 192.168.0.4 access to port 22 for all protocols
sudo ufw allow from 192.168.0.4 to any port 22

Allow by specific port, IP address and protocol
sudo ufw allow from  to  port  proto 

example: allow IP address 192.168.0.4 access to port 22 using TCP
sudo ufw allow from 192.168.0.4 to any port 22 proto tcp

Enable PING
Note: Security by obscurity may be of very little actual benefit with modern cracker scripts. By default, UFW allows ping requests. You may find you wish to leave (icmp) ping requests enabled to diagnose networking problems.
In order to disable ping (icmp) requests, you need to edit /etc/ufw/before.rules and remove the following lines:
# ok icmp codes
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input

-p
-p
-p
-p
-p

icmp
icmp
icmp
icmp
icmp

--icmp-type
--icmp-type
--icmp-type
--icmp-type
--icmp-type

destination-unreachable -j ACCEPT
source-quench -j ACCEPT
time-exceeded -j ACCEPT
parameter-problem -j ACCEPT
echo-request -j ACCEPT

or change the "ACCEPT" to "DROP"
# ok icmp codes
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input
-A ufw-before-input

-p
-p
-p
-p
-p

icmp
icmp
icmp
icmp
icmp

--icmp-type
--icmp-type
--icmp-type
--icmp-type
--icmp-type

destination-unreachable -j DROP
source-quench -j DROP
time-exceeded -j DROP
parameter-problem -j DROP
echo-request -j DROP

Deny Access
Deny by specific IP
sudo ufw deny from 

example:To block packets from 207.46.232.182:
sudo ufw deny from 207.46.232.182

Deny by specific port and IP address
sudo ufw deny from  to  port 

example: deny ip address 192.168.0.1 access to port 22 for all protocols
sudo ufw deny from 192.168.0.1 to any port 22

Working with numbered rules
Listing rules with a reference number
You may use status numbered to show the order and id number of rules:
sudo ufw status numbered

Editing numbered rules
Delete numbered rule
You may then delete rules using the number. This will delete the first rule and rules will shift up to fill in the list.
sudo ufw delete 1

Insert numbered rule
sudo ufw insert 1 allow from 

Advanced Example
Scenario: You want to block access to port 22 from 192.168.0.1 and 192.168.0.7 but allow all other 192.168.0.x IPs to have access to port 22 using tcp

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sudo ufw deny from 192.168.0.1 to any port 22
sudo ufw deny from 192.168.0.7 to any port 22
sudo ufw allow from 192.168.0.0/24 to any port 22 proto tcp

This puts the specific rules first and the generic second. Once a rule is matched the others will not be evaluated (see manual below) so you must put the specific rules first. As rules change you may need to delete old rules to ensure that new rules are put in the proper order.
To check your rules orders you can check the status; for the scenario the output below is the desired output for the rules to work properly
sudo ufw status
Firewall loaded
To
-22:tcp
22:udp
22:tcp
22:udp
22:tcp

Action
-----DENY
DENY
DENY
DENY
ALLOW

From
---192.168.0.1
192.168.0.1
192.168.0.7
192.168.0.7
192.168.0.0/24

Scenario change: You want to block access to port 22 to 192.168.0.3 as well as 192.168.0.1 and 192.168.0.7.
sudo ufw delete allow from 192.168.0.0/24 to any port 22
sudo ufw status
Firewall loaded
To
-22:tcp
22:udp
22:tcp
22:udp

Action
-----DENY
DENY
DENY
DENY

From
---192.168.0.1
192.168.0.1
192.168.0.7
192.168.0.7

sudo ufw deny 192.168.0.3 to any port 22
sudo ufw allow 192.168.0.0/24 to any port 22 proto tcp
sudo ufw status
Firewall loaded
To
-22:tcp
22:udp
22:tcp
22:udp
22:tcp
22:udp
22:tcp

Action
-----DENY
DENY
DENY
DENY
DENY
DENY
ALLOW

From
---192.168.0.1
192.168.0.1
192.168.0.7
192.168.0.7
192.168.0.3
192.168.0.3
192.168.0.0/24

If you simply add the deny rule the allow would have been above it and been applied instead of the deny

Interpreting Log Entries
Based on the response to the post UFW log guide/tutorial ?.
The SPT and DPT values, along with SRC and DST values, will typically be the values you’ll focus on when analysing the firewall logs.

Pseudo Log Entry
Feb

4 23:33:37 hostname kernel: [ 3529.289825] [UFW BLOCK] IN=eth0 OUT= MAC=00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd SRC=444.333.222.111 DST=111.222.333.444 LEN=103 TOS=0x00 PREC=0x00 TTL=52 ID=0 DF PROTO=UDP SPT=53 DPT=36427 LEN=83

Date
It's good practice to watch the dates and times. If things are out of order or blocks of time are missing then an attacker probably messed with your logs.

Hostname
The server’s hostname

Uptime
The time in seconds since boot.

Logged Event
Short description of the logged event; e.g. [UFW BLOCK]

IN
If set, then the event was an incoming event.

OUT
If set, then the event was an outgoing event.

MAC
This provides a 14-byte combination of the Destination MAC, Source MAC, and EtherType fields, following the order found in the Ethernet II header. See Ethernet frame and EtherType for more information.

SRC
This indicates the source IP, who sent the packet initially. Some IPs are routable over the internet, some will only communicate over a LAN, and some will only route back to the source computer. See IP address for more information.

DST
This indicates the destination IP, who is meant to receive the packet. You can use whois.net or the cli whois to determine the owner of the IP address.

LEN
This indicates the length of the packet.

TOS
I believe this refers to the TOS field of the IPv4 header. See TCP Processing of the IPv4 Precedence Field for more information.

PREC
I believe this refers to the Precedence field of the IPv4 header.

TTL
This indicates the “Time to live” for the packet. Basically each packet will only bounce through the given number of routers before it dies and disappears. If it hasn’t found its destination before the TTL expires, then the packet will evaporate. This field keeps lost packets from clogging the
internet forever. See Time to live for more information.

ID
Not sure what this one is, but it's not really important for reading logs. It might be ufw’s internal ID system, it might be the operating system’s ID.

PROTO
This indicates the protocol of the packet - TCP or UDP. See TCP and UDP Ports Explained for more information.

SPT
This indicates the source. I believe this is the port, which the SRC IP sent the IP packet over. See List of TCP and UDP port numbers for more information.

DPT
This indicates the destination port. I believe this is the port, which the SRC IP sent its IP packet to, expecting a service to be running on this port.

WINDOW
This indicates the size of packet the sender is willing to receive.

RES

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This bit is reserved for future use & is always set to 0. Basically it’s irrelevant for log reading purposes.

SYN URGP
SYN indicates that this connection requires a three-way handshake, which is typical of TCP connections. URGP indicates whether the urgent pointer field is relevant. 0 means it's not. Doesn’t really matter for firewall log reading.

Other Resources
For instructions on using ufw first see the official server guide.
The most recent syntax and manual can be retrieved by getting the man page. Otherwise open a terminal window and type:
man ufw

Firewall - wiki homepage for firewall related documentation.
Iptables - interface to the netfilter subsystem in the Linux kernel.
Ubuntu:UncomplicatedFirewall - UFW Project wiki page.
Gufw - Graphic User Interface for UFW.
CategoryNetworking CategorySecurity
UFW (last edited 2017-03-31 14:52:46 by paulw2u @ host-62-252-189.68.not-set-yet.virginmedia.net[62.252.189.68]:paulw2u)

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