Snort Manual

User Manual:

Open the PDF directly: View PDF .
Page Count: 178

Download
Open PDF In Browser	View PDF

SNORT R Users Manual
2.8.5
The Snort Project
October 19, 2009

Contents
1

Snort Overview

1.1

Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.2

Sniffer Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.3

Packet Logger Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4

Network Intrusion Detection System Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4.1

NIDS Mode Output Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4.2

Understanding Standard Alert Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4.3

High Performance Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4.4

Changing Alert Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Inline Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.1

Snort Inline Rule Application Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.2

Replacing Packets with Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.3

Installing Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.4

Running Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.5

Using the Honeynet Snort Inline Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5.6

Troubleshooting Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.6.1

Running Snort as a Daemon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.6.2

Running in Rule Stub Creation Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.6.3

Obfuscating IP Address Printouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.6.4

Specifying Multiple-Instance Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Reading Pcaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.7.1

Command line arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.7.2

Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tunneling Protocol Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.8.1

Multiple Encapsulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.8.2

Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

More Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.5

1.6

1.7

1.8

1.9

Configuring Snort

2.1

Includes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1.1

Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1.2

Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1.3

Config . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.1

Frag3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.2

Stream5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.3

sfPortscan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.4

RPC Decode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.5

Performance Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.6

HTTP Inspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.7

SMTP Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.8

FTP/Telnet Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.9

SSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.10 DCE/RPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.11 DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.12 SSL/TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.13 ARP Spoof Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.14 DCE/RPC 2 Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Decoder and Preprocessor Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.1

Configuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.2

Reverting to original behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Event Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.1

Rate Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.2

Event Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.3

Event Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.4

Event Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Performance Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5.1

Rule Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5.2

Preprocessor Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5.3

Packet Performance Monitoring (PPM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Output Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.1

alert syslog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.2

alert fast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.3

alert full . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.4

alert unixsock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.5

log tcpdump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.6.6

database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2

2.3

2.4

2.5

2.6

2.6.7

csv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

2.6.8

unified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

2.6.9

unified 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

2.6.10 alert prelude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
2.6.11 log null . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
2.6.12 alert aruba action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
2.6.13 Log Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
2.7

2.8

2.9

Host Attribute Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
2.7.1

Configuration Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

2.7.2

Attribute Table File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Dynamic Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
2.8.1

Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.8.2

Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Reloading a Snort Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
2.9.1

Enabling support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

2.9.2

Reloading a configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

2.9.3

Non-reloadable configuration options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

2.10 Multiple Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
2.10.1 Creating Multiple Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
2.10.2 Configuration Specific Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
2.10.3 How Configuration is applied? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3

Writing Snort Rules

113

3.1

The Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

3.2

Rules Headers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
3.2.1

Rule Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

3.2.2

Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

3.2.3

IP Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

3.2.4

Port Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

3.2.5

The Direction Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

3.2.6

Activate/Dynamic Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

3.3

Rule Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

3.4

General Rule Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.4.1

msg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

3.4.2

reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

3.4.3

gid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

3.4.4

sid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

3.4.5

rev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

3.4.6

classtype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

3.5

3.4.7

priority . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

3.4.8

metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.4.9

General Rule Quick Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Payload Detection Rule Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
3.5.1

content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

3.5.2

nocase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

3.5.3

rawbytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

3.5.4

depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

3.5.5

offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

3.5.6

distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

3.5.7

within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3.5.8

http client body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3.5.9

http cookie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3.5.10 http header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
3.5.11 http method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
3.5.12 http uri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
3.5.13 fast pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
3.5.14 uricontent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
3.5.15 urilen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
3.5.16 isdataat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
3.5.17 pcre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
3.5.18 byte test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
3.5.19 byte jump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
3.5.20 ftpbounce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
3.5.21 asn1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
3.5.22 cvs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
3.5.23 dce iface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
3.5.24 dce opnum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
3.5.25 dce stub data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
3.5.26 Payload Detection Quick Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
3.6

Non-Payload Detection Rule Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
3.6.1

fragoffset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

3.6.2

ttl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

3.6.3

tos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

3.6.4

id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

3.6.5

ipopts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

3.6.6

fragbits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

3.6.7

dsize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

3.6.8

flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

3.6.9

flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

3.6.10 flowbits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
3.6.11 seq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3.6.12 ack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3.6.13 window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
3.6.14 itype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.6.15 icode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.6.16 icmp id . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.6.17 icmp seq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.6.18 rpc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.6.19 ip proto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.6.20 sameip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.6.21 stream size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
3.6.22 Non-Payload Detection Quick Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
3.7

Post-Detection Rule Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
3.7.1

logto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

3.7.2

session . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

3.7.3

resp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

3.7.4

react . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

3.7.5

tag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

3.7.6

activates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

3.7.7

activated by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

3.7.8

count . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

3.7.9

replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

3.7.10 detection filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
3.7.11 Post-Detection Quick Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

3.8

Rule Thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

3.9

Writing Good Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Content Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

3.9.2

Catch the Vulnerability, Not the Exploit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

3.9.3

Catch the Oddities of the Protocol in the Rule . . . . . . . . . . . . . . . . . . . . . . . . . . 152

3.9.4

Optimizing Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

3.9.5

Testing Numerical Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

Making Snort Faster
4.1

3.9.1

157

MMAPed pcap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Dynamic Modules
5.1

158

Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
5.1.1

DynamicPluginMeta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6

5.2

5.3

5.1.2

DynamicPreprocessorData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.1.3

DynamicEngineData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

5.1.4

SFSnortPacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

5.1.5

Dynamic Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

Required Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
5.2.1

Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

5.2.2

Detection Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

5.2.3

Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
5.3.1

Preprocessor Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

5.3.2

Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Snort Development

174

6.1

Submitting Patches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

6.2

Snort Data Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

6.3

6.2.1

Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

6.2.2

Detection Plugins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

6.2.3

Output Plugins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

The Snort Team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

Chapter 1

Snort Overview
This manual is based on Writing Snort Rules by Martin Roesch and further work from Chris Green .
It was then maintained by Brian Caswell and now is maintained by the Snort Team. If you have a
better way to say something or find that something in the documentation is outdated, drop us a line and we will update
it. If you would like to submit patches for this document, you can find the latest version of the documentation in LATEX
format in the Snort CVS repository at /doc/snort_manual.tex. Small documentation updates are the easiest way to
help out the Snort Project.

1.1 Getting Started
Snort really isn’t very hard to use, but there are a lot of command line options to play with, and it’s not always obvious
which ones go together well. This file aims to make using Snort easier for new users.
Before we proceed, there are a few basic concepts you should understand about Snort. Snort can be configured to run
in three modes:
• Sniffer mode, which simply reads the packets off of the network and displays them for you in a continuous
stream on the console (screen).
• Packet Logger mode, which logs the packets to disk.
• Network Intrusion Detection System (NIDS) mode, the most complex and configurable configuration, which
allows Snort to analyze network traffic for matches against a user-defined rule set and performs several actions
based upon what it sees.
• Inline mode, which obtains packets from iptables instead of from libpcap and then causes iptables to drop or
pass packets based on Snort rules that use inline-specific rule types.

1.2 Sniffer Mode
First, let’s start with the basics. If you just want to print out the TCP/IP packet headers to the screen (i.e. sniffer mode),
try this:
./snort -v
This command will run Snort and just show the IP and TCP/UDP/ICMP headers, nothing else. If you want to see the
application data in transit, try the following:
./snort -vd
8

This instructs Snort to display the packet data as well as the headers. If you want an even more descriptive display,
showing the data link layer headers, do this:
./snort -vde
(As an aside, these switches may be divided up or smashed together in any combination. The last command could also
be typed out as:
./snort -d -v -e
and it would do the same thing.)

1.3 Packet Logger Mode
OK, all of these commands are pretty cool, but if you want to record the packets to the disk, you need to specify a
logging directory and Snort will automatically know to go into packet logger mode:
./snort -dev -l ./log
Of course, this assumes you have a directory named log in the current directory. If you don’t, Snort will exit with
an error message. When Snort runs in this mode, it collects every packet it sees and places it in a directory hierarchy
based upon the IP address of one of the hosts in the datagram.
If you just specify a plain -l switch, you may notice that Snort sometimes uses the address of the remote computer
as the directory in which it places packets and sometimes it uses the local host address. In order to log relative to the
home network, you need to tell Snort which network is the home network:
./snort -dev -l ./log -h 192.168.1.0/24
This rule tells Snort that you want to print out the data link and TCP/IP headers as well as application data into the
directory ./log, and you want to log the packets relative to the 192.168.1.0 class C network. All incoming packets
will be recorded into subdirectories of the log directory, with the directory names being based on the address of the
remote (non-192.168.1) host.

! NOTE
△
Note that if both the source and destination hosts are on the home network, they are logged to a directory
with a name based on the higher of the two port numbers or, in the case of a tie, the source address.
If you’re on a high speed network or you want to log the packets into a more compact form for later analysis, you
should consider logging in binary mode. Binary mode logs the packets in tcpdump format to a single binary file in the
logging directory:
./snort -l ./log -b
Note the command line changes here. We don’t need to specify a home network any longer because binary mode
logs everything into a single file, which eliminates the need to tell it how to format the output directory structure.
Additionally, you don’t need to run in verbose mode or specify the -d or -e switches because in binary mode the entire
packet is logged, not just sections of it. All you really need to do to place Snort into logger mode is to specify a logging
directory at the command line using the -l switch—the -b binary logging switch merely provides a modifier that tells
Snort to log the packets in something other than the default output format of plain ASCII text.
Once the packets have been logged to the binary file, you can read the packets back out of the file with any sniffer that
supports the tcpdump binary format (such as tcpdump or Ethereal). Snort can also read the packets back by using the
9

-r switch, which puts it into playback mode. Packets from any tcpdump formatted file can be processed through Snort
in any of its run modes. For example, if you wanted to run a binary log file through Snort in sniffer mode to dump the
packets to the screen, you can try something like this:
./snort -dv -r packet.log
You can manipulate the data in the file in a number of ways through Snort’s packet logging and intrusion detection
modes, as well as with the BPF interface that’s available from the command line. For example, if you only wanted to
see the ICMP packets from the log file, simply specify a BPF filter at the command line and Snort will only see the
ICMP packets in the file:
./snort -dvr packet.log icmp
For more info on how to use the BPF interface, read the Snort and tcpdump man pages.

1.4 Network Intrusion Detection System Mode
To enable Network Intrusion Detection System (NIDS) mode so that you don’t record every single packet sent down
the wire, try this:
./snort -dev -l ./log -h 192.168.1.0/24 -c snort.conf
where snort.conf is the name of your rules file. This will apply the rules configured in the snort.conf file to
each packet to decide if an action based upon the rule type in the file should be taken. If you don’t specify an output
directory for the program, it will default to /var/log/snort.
One thing to note about the last command line is that if Snort is going to be used in a long term way as an IDS, the
-v switch should be left off the command line for the sake of speed. The screen is a slow place to write data to, and
packets can be dropped while writing to the display.
It’s also not necessary to record the data link headers for most applications, so you can usually omit the -e switch, too.
./snort -d -h 192.168.1.0/24 -l ./log -c snort.conf
This will configure Snort to run in its most basic NIDS form, logging packets that trigger rules specified in the
snort.conf in plain ASCII to disk using a hierarchical directory structure (just like packet logger mode).

1.4.1 NIDS Mode Output Options
There are a number of ways to configure the output of Snort in NIDS mode. The default logging and alerting mechanisms are to log in decoded ASCII format and use full alerts. The full alert mechanism prints out the alert message in
addition to the full packet headers. There are several other alert output modes available at the command line, as well
as two logging facilities.
Alert modes are somewhat more complex. There are seven alert modes available at the command line: full, fast,
socket, syslog, console, cmg, and none. Six of these modes are accessed with the -A command line switch. These
options are:
Option
-A fast
-A full
-A
-A
-A
-A

unsock
none
console
cmg

Description
Fast alert mode. Writes the alert in a simple format with a timestamp, alert message, source and
destination IPs/ports.
Full alert mode. This is the default alert mode and will be used automatically if you do not specify
a mode.
Sends alerts to a UNIX socket that another program can listen on.
Turns off alerting.
Sends “fast-style” alerts to the console (screen).
Generates “cmg style” alerts.
10

Packets can be logged to their default decoded ASCII format or to a binary log file via the -b command line switch.
To disable packet logging altogether, use the -N command line switch.
For output modes available through the configuration file, see Section 2.6.

! NOTE
△

Command line logging options override any output options specified in the configuration file. This allows
debugging of configuration issues quickly via the command line.

To send alerts to syslog, use the -s switch. The default facilities for the syslog alerting mechanism are LOG AUTHPRIV
and LOG ALERT. If you want to configure other facilities for syslog output, use the output plugin directives in the
rules files. See Section 2.6.1 for more details on configuring syslog output.
For example, use the following command line to log to default (decoded ASCII) facility and send alerts to syslog:
./snort -c snort.conf -l ./log -h 192.168.1.0/24 -s
As another example, use the following command line to log to the default facility in /var/log/snort and send alerts to a
fast alert file:
./snort -c snort.conf -A fast -h 192.168.1.0/24

1.4.2 Understanding Standard Alert Output
When Snort generates an alert message, it will usually look like the following:
[**] [116:56:1] (snort_decoder): T/TCP Detected [**]
The first number is the Generator ID, this tells the user what component of Snort generated this alert. For a list of
GIDs, please read etc/generators in the Snort source. In this case, we know that this event came from the “decode”
(116) component of Snort.
The second number is the Snort ID (sometimes referred to as Signature ID). For a list of preprocessor SIDs, please see
etc/gen-msg.map. Rule-based SIDs are written directly into the rules with the sid option. In this case, 56 represents a
T/TCP event.
The third number is the revision ID. This number is primarily used when writing signatures, as each rendition of the
rule should increment this number with the rev option.

1.4.3 High Performance Configuration
If you want Snort to go fast (like keep up with a 1000 Mbps connection), you need to use unified logging and a unified
log reader such as barnyard. This allows Snort to log alerts in a binary form as fast as possible while another program
performs the slow actions, such as writing to a database.
If you want a text file that’s easily parsable, but still somewhat fast, try using binary logging with the “fast” output
mechanism.
This will log packets in tcpdump format and produce minimal alerts. For example:
./snort -b -A fast -c snort.conf

1.4.4 Changing Alert Order
The default way in which Snort applies its rules to packets may not be appropriate for all installations. The Pass rules
are applied first, then the Drop rules, then the Alert rules and finally, Log rules are applied.

! NOTE
△
Sometimes an errant pass rule could cause alerts to not show up, in which case you can change the default
ordering to allow Alert rules to be applied before Pass rules. For more information, please refer to the
--alert-before-pass option.
Several command line options are available to change the order in which rule actions are taken.
• --alert-before-pass option forces alert rules to take affect in favor of a pass rule.
• --treat-drop-as-alert causes drop, sdrop, and reject rules and any associated alerts to be logged as alerts,
rather then the normal action. This allows use of an inline policy with passive/IDS mode.
• --process-all-events option causes Snort to process every event associated with a packet, while taking the
actions based on the rules ordering. Without this option (default case), only the events for the first action based
on rules ordering are processed.

! NOTE
△
Pass rules are special cases here, in that the event processing is terminated when a pass rule is encountered,
regardless of the use of --process-all-events.

1.5 Inline Mode
Snort 2.3.0 RC1 integrated the intrusion prevention system (IPS) capability of Snort Inline into the official Snort
project. Snort Inline obtains packets from iptables instead of libpcap and then uses new rule types to help iptables
pass or drop packets based on Snort rules.
In order for Snort Inline to work properly, you must download and compile the iptables code to include “make
install-devel” (http://www.iptables.org). This will install the libipq library that allows Snort Inline to interface with iptables. Also, you must build and install LibNet, which is available from http://www.packetfactory.net.
There are three rule types you can use when running Snort with Snort Inline:
• drop - The drop rule type will tell iptables to drop the packet and log it via usual Snort means.
• reject - The reject rule type will tell iptables to drop the packet, log it via usual Snort means, and send a TCP
reset if the protocol is TCP or an icmp port unreachable if the protocol is UDP.
• sdrop - The sdrop rule type will tell iptables to drop the packet. Nothing is logged.

! NOTE
△
You can also replace sections of the packet payload when using Snort Inline. See Section 1.5.2 for more
information.
When using a reject rule, there are two options you can use to send TCP resets:
• You can use a RAW socket (the default behavior for Snort Inline), in which case you must have an interface
that has an IP address assigned to it. If there is not an interface with an IP address assigned with access to the
source of the packet, the packet will be logged and the reset packet will never make it onto the network.
12

• You can also now perform resets via a physical device when using iptables. We take the indev name from
ip queue and use this as the interface on which to send resets. We no longer need an IP loaded on the bridge,
and can remain pretty stealthy as the config layer2 resets in snort.conf takes a source MAC address which
we substitue for the MAC of the bridge. For example:
config layer2resets
tells Snort Inline to use layer2 resets and uses the MAC address of the bridge as the source MAC in the
packet, and:
config layer2resets: 00:06:76:DD:5F:E3
will tell Snort Inline to use layer2 resets and uses the source MAC of 00:06:76:DD:5F:E3 in the reset packet.
• The command-line option --disable-inline-initialization can be used to not initialize IPTables when in
inline mode. It should be used with command-line option -T to test for a valid configuration without requiring
opening inline devices and adversely affecting traffic flow.

1.5.1 Snort Inline Rule Application Order
The current rule application order is:
->activation->dynamic->pass->drop->sdrop->reject->alert->log
This will ensure that a drop rule has precedence over an alert or log rule.

1.5.2 Replacing Packets with Snort Inline
Additionally, Jed Haile’s content replace code allows you to modify packets before they leave the network. For
example:
alert tcp any any <> any 80 ( \
msg: "tcp replace"; content:"GET"; replace:"BET";)
alert udp any any <> any 53 ( \
msg: "udp replace"; content: "yahoo"; replace: "xxxxx";)
These rules will comb TCP port 80 traffic looking for GET, and UDP port 53 traffic looking for yahoo. Once they are
found, they are replaced with BET and xxxxx, respectively. The only catch is that the replace must be the same length
as the content.

1.5.3 Installing Snort Inline
To install Snort inline, use the following command:
./configure --enable-inline
make
make install

1.5.4 Running Snort Inline
First, you need to ensure that the ip queue module is loaded. Then, you need to send traffic to Snort Inline using the
QUEUE target. For example:
iptables -A OUTPUT -p tcp --dport 80 -j QUEUE
sends all TCP traffic leaving the firewall going to port 80 to the QUEUE target. This is what sends the packet from
kernel space to user space (Snort Inline). A quick way to get all outbound traffic going to the QUEUE is to use the
rc.firewall script created and maintained by the Honeynet Project (http://www.honeynet.org/papers/honeynet/tools/)
This script is well-documented and allows you to direct packets to Snort Inline by simply changing the QUEUE
variable to yes.
Finally, start Snort Inline:
snort -QDc ../etc/drop.conf -l /var/log/snort
You can use the following command line options:
• -Q - Gets packets from iptables.
• -D - Runs Snort Inline in daemon mode. The process ID is stored at /var/run/snort.pid
• -c - Reads the following configuration file.
• -l - Logs to the following directory.
Ideally, Snort Inline will be run using only its own drop.rules. If you want to use Snort for just alerting, a separate
process should be running with its own rule set.

1.5.5 Using the Honeynet Snort Inline Toolkit
The Honeynet Snort Inline Toolkit is a statically compiled Snort Inline binary put together by the Honeynet Project
for the Linux operating system. It comes with a set of drop.rules, the Snort Inline binary, a snort-inline rotation
shell script, and a good README. It can be found at:
http://www.honeynet.org/papers/honeynet/tools/

1.5.6 Troubleshooting Snort Inline
If you run Snort Inline and see something like this:
Initializing Output Plugins!
Reading from iptables
Log directory = /var/log/snort
Initializing Inline mode
InlineInit: : Failed to send netlink message: Connection refused
More than likely, the ip queue module is not loaded or ip queue support is not compiled into your kernel. Either
recompile your kernel to support ip queue, or load the module.
The ip queue module is loaded by executing:
insmod ip_queue
Also, if you want to ensure Snort Inline is getting packets, you can start it in the following manner:
snort -Qvc
This will display the header of every packet that Snort Inline sees.
14

1.6 Miscellaneous
1.6.1 Running Snort as a Daemon
If you want to run Snort as a daemon, you can the add -D switch to any combination described in the previous sections.
Please notice that if you want to be able to restart Snort by sending a SIGHUP signal to the daemon, you must specify
the full path to the Snort binary when you start it, for example:
/usr/local/bin/snort -d -h 192.168.1.0/24 \
-l /var/log/snortlogs -c /usr/local/etc/snort.conf -s -D
Relative paths are not supported due to security concerns.
Snort PID File
When Snort is run as a daemon , the daemon creates a PID file in the log directory. In Snort 2.6, the --pid-path
command line switch causes Snort to write the PID file in the directory specified.
Additionally, the --create-pidfile switch can be used to force creation of a PID file even when not running in
daemon mode.
The PID file will be locked so that other snort processes cannot start. Use the --nolock-pidfile switch to not lock
the PID file.

1.6.2 Running in Rule Stub Creation Mode
If you need to dump the shared object rules stub to a directory, you might need to use the –dump-dynamic-rules option.
These rule stub files are used in conjunction with the shared object rules. The path can be relative or absolute.
/usr/local/bin/snort -c /usr/local/etc/snort.conf \
--dump-dynamic-rules=/tmp
This path can also be configured in the snort.conf using the config option dump-dynamic-rules-path as follows:
config dump-dynamic-rules-path: /tmp/sorules
The path configured by command line has precedence over the one configured using dump-dynamic-rules-path.
/usr/local/bin/snort -c /usr/local/etc/snort.conf \
--dump-dynamic-rules
snort.conf:
config dump-dynamic-rules-path: /tmp/sorules
In the above mentioned scenario the dump path is set to /tmp/sorules.

1.6.3 Obfuscating IP Address Printouts
If you need to post packet logs to public mailing lists, you might want to use the -O switch. This switch obfuscates
your IP addresses in packet printouts. This is handy if you don’t want people on the mailing list to know the IP
addresses involved. You can also combine the -O switch with the -h switch to only obfuscate the IP addresses of hosts
on the home network. This is useful if you don’t care who sees the address of the attacking host. For example, you
could use the following command to read the packets from a log file and dump them to the screen, obfuscating only
the addresses from the 192.168.1.0/24 class C network:
./snort -d -v -r snort.log -O -h 192.168.1.0/24
15

1.6.4 Specifying Multiple-Instance Identifiers
In Snort v2.4, the -G command line option was added that specifies an instance identifier for the event logs. This option
can be used when running multiple instances of snort, either on different CPUs, or on the same CPU but a different
interface. Each Snort instance will use the value specified to generate unique event IDs. Users can specify either a
decimal value (-G 1) or hex value preceded by 0x (-G 0x11). This is also supported via a long option --logid.

1.7 Reading Pcaps
Instead of having Snort listen on an interface, you can give it a packet capture to read. Snort will read and analyze the
packets as if they came off the wire. This can be useful for testing and debugging Snort.

1.7.1 Command line arguments
Any of the below can be specified multiple times on the command line (-r included) and in addition to other Snort
command line options. Note, however, that specifying --pcap-reset and --pcap-show multiple times has the same
effect as specifying them once.
Option
-r
--pcap-single=
--pcap-file=
--pcap-list=""
--pcap-dir=
--pcap-filter=

--pcap-no-filter
--pcap-reset
--pcap-show

Description
Read a single pcap.
Same as -r. Added for completeness.
File that contains a list of pcaps to read. Can specifiy path to pcap or directory to
recurse to get pcaps.
A space separated list of pcaps to read.
A directory to recurse to look for pcaps. Sorted in ascii order.
Shell style filter to apply when getting pcaps from file or directory. This filter will apply to any --pcap-file or --pcap-dir arguments following. Use
--pcap-no-filter to delete filter for following --pcap-file or --pcap-dir
arguments or specifiy --pcap-filter again to forget previous filter and to apply
to following --pcap-file or --pcap-dir arguments.
Reset to use no filter when getting pcaps from file or directory.
If reading multiple pcaps, reset snort to post-configuration state before reading
next pcap. The default, i.e. without this option, is not to reset state.
Print a line saying what pcap is currently being read.

1.7.2 Examples
Read a single pcap
$ snort -r foo.pcap
$ snort --pcap-single=foo.pcap
Read pcaps from a file
$ cat foo.txt
foo1.pcap
foo2.pcap
/home/foo/pcaps
$ snort --pcap-file=foo.txt
This will read foo1.pcap, foo2.pcap and all files under /home/foo/pcaps. Note that Snort will not try to determine
whether the files under that directory are really pcap files or not.
16

Read pcaps from a command line list
$ snort --pcap-list="foo1.pcap foo2.pcap foo3.pcap"
This will read foo1.pcap, foo2.pcap and foo3.pcap.
Read pcaps under a directory
$ snort --pcap-dir="/home/foo/pcaps"
This will include all of the files under /home/foo/pcaps.
Using filters
$ cat foo.txt
foo1.pcap
foo2.pcap
/home/foo/pcaps
$ snort --pcap-filter="*.pcap" --pcap-file=foo.txt
$ snort --pcap-filter="*.pcap" --pcap-dir=/home/foo/pcaps
The above will only include files that match the shell pattern ”*.pcap”, in other words, any file ending in ”.pcap”.
$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \
> --pcap-filter="*.cap" --pcap-dir=/home/foo/pcaps
In the above, the first filter ”*.pcap” will only be applied to the pcaps in the file ”foo.txt” (and any directories that are
recursed in that file). The addition of the second filter ”*.cap” will cause the first filter to be forgotten and then applied
to the directory /home/foo/pcaps, so only files ending in ”.cap” will be included from that directory.
$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \
> --pcap-no-filter --pcap-dir=/home/foo/pcaps
In this example, the first filter will be applied to foo.txt, then no filter will be applied to the files found under
/home/foo/pcaps, so all files found under /home/foo/pcaps will be included.
$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \
> --pcap-no-filter --pcap-dir=/home/foo/pcaps \
> --pcap-filter="*.cap" --pcap-dir=/home/foo/pcaps2
In this example, the first filter will be applied to foo.txt, then no filter will be applied to the files found under
/home/foo/pcaps, so all files found under /home/foo/pcaps will be included, then the filter ”*.cap” will be applied
to files found under /home/foo/pcaps2.
Resetting state
$ snort --pcap-dir=/home/foo/pcaps --pcap-reset
The above example will read all of the files under /home/foo/pcaps, but after each pcap is read, Snort will be reset to
a post-configuration state, meaning all buffers will be flushed, statistics reset, etc. For each pcap, it will be like Snort
is seeing traffic for the first time.
17

Printing the pcap
$ snort --pcap-dir=/home/foo/pcaps --pcap-show
The above example will read all of the files under /home/foo/pcaps and will print a line indicating which pcap is
currently being read.

1.8 Tunneling Protocol Support
Snort supports decoding of GRE, IP in IP and PPTP. To enable support, an extra configuration option is necessary:
$ ./configure --enable-gre
To enable IPv6 support, one still needs to use the configuration option:
$ ./configure --enable-ipv6

1.8.1 Multiple Encapsulations
Snort will not decode more than one encapsulation. Scenarios such as
Eth IPv4 GRE IPv4 GRE IPv4 TCP Payload
or
Eth IPv4 IPv6 IPv4 TCP Payload
will not be handled and will generate a decoder alert.

1.8.2 Logging
Currently, only the encapsulated part of the packet is logged, e.g.
Eth IP1 GRE IP2 TCP Payload
gets logged as
Eth IP2 TCP Payload
and
Eth IP1 IP2 TCP Payload
gets logged as
Eth IP2 TCP Payload

! NOTE
△
Decoding of PPTP, which utilizes GRE and PPP, is not currently supported on architectures that require word
alignment such as SPARC.

1.9 More Information
Chapter 2 contains much information about many configuration options available in the configuration file. The Snort
manual page and the output of snort -? or snort --help contain information that can help you get Snort running
in several different modes.

! NOTE
△
In many shells, a backslash (\) is needed to escape the ?, so you may have to type snort -\? instead of
snort -? for a list of Snort command line options.
The Snort web page (http://www.snort.org) and the Snort Users mailing list:
http://marc.theaimsgroup.com/?l=snort-users
at snort-users@lists.sourceforge.net provide informative announcements as well as a venue for community
discussion and support. There’s a lot to Snort, so sit back with a beverage of your choosing and read the documentation
and mailing list archives.

Chapter 2

Configuring Snort
2.1 Includes
The include keyword allows other rules files to be included within the rules file indicated on the Snort command line.
It works much like an #include from the C programming language, reading the contents of the named file and adding
the contents in the place where the include statement appears in the file.

2.1.1 Format
include

! NOTE
△
Note that there is no semicolon at the end of this line.
Included files will substitute any predefined variable values into their own variable references. See Section 2.1.2 for
more information on defining and using variables in Snort rules files.

2.1.2 Variables
Three types of variables may be defined in Snort:
• var
• portvar
• ipvar

! NOTE
△
Note: ’ipvar’s are only enabled with IPv6 support. Without IPv6 support, use a regular ’var’.
These are simple substitution variables set with the var, ipvar, or portvar keywords as follows:
var RULES_PATH rules/
portvar MY_PORTS [22,80,1024:1050]
ipvar MY_NET [192.168.1.0/24,10.1.1.0/24]
alert tcp any any -> $MY_NET $MY_PORTS (flags:S; msg:"SYN packet";)
include $RULE_PATH/example.rule
20

IP Variables and IP Lists
IPs may be specified individually, in a list, as a CIDR block, or any combination of the three. If IPv6 support is
enabled, IP variables should be specified using ’ipvar’ instead of ’var’. Using ’var’ for an IP variable is still allowed
for backward compatibility, but it will be deprecated in a future release.
IPs, IP lists, and CIDR blocks may be negated with ’!’. Negation is handled differently compared with Snort versions
2.7.x and earlier. Previously, each element in a list was logically OR’ed together. IP lists now OR non-negated
elements and AND the result with the OR’ed negated elements.
The following example list will match the IP 1.1.1.1 and IP from 2.2.2.0 to 2.2.2.255, with the exception of IPs 2.2.2.2
and 2.2.2.3.
[1.1.1.1,2.2.2.0/24,![2.2.2.2,2.2.2.3]]
The order of the elements in the list does not matter. The element ’any’ can be used to match all IPs, although ’!any’
is not allowed. Also, negated IP ranges that are more general than non-negated IP ranges are not allowed.
See below for some valid examples if IP variables and IP lists.
ipvar EXAMPLE [1.1.1.1,2.2.2.0/24,![2.2.2.2,2.2.2.3]]
alert tcp $EXAMPLE any -> any any (msg:"Example"; sid:1;)
alert tcp [1.0.0.0/8,!1.1.1.0/24] any -> any any (msg:"Example";sid:2;)
The following examples demonstrate some invalid uses of IP variables and IP lists.
Use of !any:
ipvar EXAMPLE any
alert tcp !$EXAMPLE any -> any any (msg:"Example";sid:3;)
Different use of !any:
ipvar EXAMPLE !any
alert tcp $EXAMPLE any -> any any (msg:"Example";sid:3;)
Logical contradictions:
ipvar EXAMPLE [1.1.1.1,!1.1.1.1]
Nonsensical negations:
ipvar EXAMPLE [1.1.1.0/24,!1.1.0.0/16]
Port Variables and Port Lists
Portlists supports the declaration and lookup of ports and the representation of lists and ranges of ports. Variables,
ranges, or lists may all be negated with ’!’. Also, ’any’ will specify any ports, but ’!any’ is not allowed. Valid port
ranges are from 0 to 65535.
Lists of ports must be enclosed in brackets and port ranges may be specified with a ’:’, such as in:
[10:50,888:900]

Port variables should be specified using ’portvar’. The use of ’var’ to declare a port variable will be deprecated in a
future release. For backwards compatibility, a ’var’ can still be used to declare a port variable, provided the variable
name either ends with ’ PORT’ or begins with ’PORT ’.
The following examples demonstrate several valid usages of both port variables and port lists.
portvar EXAMPLE1 80
var EXAMPLE2_PORT [80:90]
var PORT_EXAMPLE2 [1]
portvar EXAMPLE3 any
portvar EXAMPLE4 [!70:90]
portvar EXAMPLE5 [80,91:95,100:200]
alert tcp any $EXAMPLE1 -> any $EXAMPLE2_PORT (msg:"Example"; sid:1;)
alert tcp any $PORT_EXAMPLE2 -> any any (msg:"Example"; sid:2;)
alert tcp any 90 -> any [100:1000,9999:20000] (msg:"Example"; sid:3;)
Several invalid examples of port variables and port lists are demonstrated below:
Use of !any:
portvar EXAMPLE5 !any
var EXAMPLE5 !any
Logical contradictions:
portvar EXAMPLE6 [80,!80]
Ports out of range:
portvar EXAMPLE7 [65536]
Incorrect declaration and use of a port variable:
var EXAMPLE8 80
alert tcp any $EXAMPLE8 -> any any (msg:"Example"; sid:4;)
Port variable used as an IP:
alert tcp $EXAMPLE1 any -> any any (msg:"Example"; sid:5;)
Variable Modifiers
Rule variable names can be modified in several ways. You can define meta-variables using the $ operator. These can
be used with the variable modifier operators ? and -, as described in the following table:

Variable Syntax
var
$(var) or $var
$(var:-default)
$(var:?message)

Description
Defines a meta-variable.
Replaces with the contents of variable var.
Replaces the contents of the variable var with “default” if var is undefined.
Replaces with the contents of variable var or prints out the error message and
exits.

Here is an example of advanced variable usage in action:
ipvar MY_NET 192.168.1.0/24
log tcp any any -> $(MY_NET:?MY_NET is undefined!) 23
Limitations
When embedding variables, types can not be mixed. For instance, port variables can be defined in terms of other port
variables, but old-style variables (with the ’var’ keyword) can not be embedded inside a ’portvar’.
Valid embedded variable:
portvar pvar1 80
portvar pvar2 [$pvar1,90]
Invalid embedded variable:
var pvar1 80
portvar pvar2 [$pvar1,90]
Likewise, variables can not be redefined if they were previously defined as a different type. They should be renamed
instead:
Invalid redefinition:
var pvar 80
portvar pvar 90

2.1.3 Config
Many configuration and command line options of Snort can be specified in the configuration file.
Format
config [: ]

Config Directive
config alert with interface name
config alertfile:
config asn1:

config autogenerate preprocessor
decoder rules

config bpf file:
config checksum drop:

config checksum mode:
config
config
config
config
config

chroot:
classification:
daemon
decode data link
default rule state:

config detection:
[lowmem] [no stream inserts]
[max queue events ]

Description
Appends interface name to alert (snort -I).
Sets the alerts output file.
Specifies the maximum number of nodes to track when doing
ASN1 decoding. See Section 3.5.21 for more information and
examples.
If Snort was configured to enable decoder and preprocessor
rules, this option will cause Snort to revert back to it’s original behavior of alerting if the decoder or preprocessor generates
an event.
Specifies BPF filters (snort -F).
Types of packets to drop if invalid checksums. Values: none,
noip, notcp, noicmp, noudp, ip, tcp, udp, icmp or all
(only applicable in inline mode and for packets checked per
checksum mode config option).
Types of packets to calculate checksums. Values: none, noip,
notcp, noicmp, noudp, ip, tcp, udp, icmp or all.
Chroots to specified dir (snort -t).
See Table 3.2 for a list of classifications.
Forks as a daemon (snort -D).
Decodes Layer2 headers (snort -e).
Global configuration directive to enable or disable the loading
of rules into the detection engine. Default (with or without directive) is enabled. Specify disabled to disable loading rules.
Makes changes to the detection engine. The following options
can be used:
• search-method
– ac Aho-Corasick Full (high memory, best performance)
– ac-std Aho-Corasick Standard (moderate memory,
high performance)
– ac-bnfa Aho-Corasick NFA (low memory, high
performance)
– acs Aho-Corasick Sparse (small memory, moderate
performance)
– ac-banded Aho-Corasick Banded (small memory,
moderate performance)
– ac-sparsebands Aho-Corasick Sparse-Banded
(small memory, high performance)
– lowmem Low Memory Keyword Trie (small memory, low performance)
• no stream inserts
• max queue events

config disable decode alerts
config disable inline init failopen

config disable ipopt alerts

Turns off the alerts generated by the decode phase of Snort.
Disables failopen thread that allows inline traffic to pass
while Snort is starting up.
Only useful if Snort was
configured with –enable-inline-init-failopen.
(snort
--disable-inline-init-failopen)
Disables IP option length validation alerts.

config disable tcpopt alerts
config
disable tcpopt experimental alerts
config disable tcpopt obsolete alerts
config disable tcpopt ttcp alerts
config disable ttcp alerts
config dump chars only
config dump payload
config dump payload verbose
config enable decode drops

Disables option length validation alerts.
Turns off alerts generated by experimental TCP options.

Turns off alerts generated by obsolete TCP options.
Turns off alerts generated by T/TCP options.
Turns off alerts generated by T/TCP options.
Turns on character dumps (snort -C).
Dumps application layer (snort -d).
Dumps raw packet starting at link layer (snort -X).
Enables the dropping of bad packets identified by decoder (only
applicable in inline mode).
config enable decode oversized alerts Enable alerting on packets that have headers containing length
fields for which the value is greater than the length of the packet.
config enable decode oversized drops Enable dropping packets that have headers containing length
fields for which the value is greater than the length of the packet.
enable decode oversized alerts must also be enabled for
this to be effective (only applicable in inline mode).
config enable ipopt drops
Enables the dropping of bad packets with bad/truncated IP options (only applicable in inline mode).
config enable mpls multicast
Enables support for MPLS multicast. This option is needed
when the network allows MPLS multicast traffic. When this
option is off and MPLS multicast traffic is detected, Snort will
generate an alert. By default, it is off.
config enable mpls overlapping ip
Enables support for overlapping IP addresses in an MPLS network. In a normal situation, where there are no overlapping
IP addresses, this configuration option should not be turned on.
However, there could be situations where two private networks
share the same IP space and different MPLS labels are used to
differentiate traffic from the two VPNs. In such a situation, this
configuration option should be turned on. By default, it is off.
config enable tcpopt drops
Enables the dropping of bad packets with bad/truncated TCP
option (only applicable in inline mode).
config
Enables the dropping of bad packets with experimental TCP openable tcpopt experimental drops
tion. (only applicable in inline mode).
config enable tcpopt obsolete drops
Enables the dropping of bad packets with obsolete TCP option.
(only applicable in inline mode).
enable tcpopt ttcp drops
Enables the dropping of bad packets with T/TCP option. (only
applicable in inline mode).
enable ttcp drops
Enables the dropping of bad packets with T/TCP option. (only
applicable in inline mode).
config event filter: memcap
Set global memcap in bytes for thresholding. Default is

1048576 bytes (1 megabyte).
config event queue: [max queue
Specifies conditions about Snort’s event queue. You can use the
] [log ] [order events
following options:
]
• max queue (max events supported)
• log (number of events to log)
• order events [priority|content length] (how to
order events within the queue)
See Section 2.4.4 for more information and examples.

config flexresp2 attempts:

config flexresp2 interface:

config flexresp2 memcap:

config flexresp2 rows:

config flowbits size:
config ignore ports:

config interface:
config ipv6 frag:
[bsd icmp frag alert on|off]
[, bad ipv6 frag alert on|off]
[, frag timeout ] [,
max frag sessions ]

Specify the number of TCP reset packets to send to the source
of the attack. Valid values are 0 to 20, however values less than
4 will default to 4. The default value without this option is 4.
(Snort must be compiled with –enable-flexresp2)
Specify the response interface to use. In Windows this can also
be the interface number. (Snort must be compiled with –enableflexresp2)
Specify the memcap for the hash table used to track the time
of responses. The times (hashed on a socket pair plus protocol)
are used to limit sending a response to the same half of a socket
pair every couple of seconds. Default is 1048576 bytes. (Snort
must be compiled with –enable-flexresp2)
Specify the number of rows for the hash table used to track the
time of responses. Default is 1024 rows. (Snort must be compiled with –enable-flexresp2)
Specifies the maximum number of flowbit tags that can be used
within a rule set.
Specifies ports to ignore (useful for ignoring noisy NFS traffic).
Specify the protocol (TCP, UDP, IP, or ICMP), followed by a
list of ports. Port ranges are supported.
Sets the network interface (snort -i).
The following options can be used:
• bsd icmp frag alert on|off (Specify whether or not
to alert. Default is on)
• bad ipv6 frag alert on|off (Specify whether or not
to alert. Default is on)
• frag timeout (Specify amount of time in
seconds to timeout first frag in hash table)
• max frag sessions (Specify the number
of fragments to track in the hash table)

config layer2resets:

config logdir:
config max attribute hosts:

config max mpls labelchain len:

config min ttl:
config mpls payload type:
ipv4|ipv6|ethernet
config
config
config
config

no promisc
nolog
nopcre
obfuscate

This option is only available when running in inline mode. See
Section 1.5.
Sets the logdir (snort -l).
Sets a limit on the maximum number of hosts to read from
the attribute table. Minimum value is 32 and the maximum is
524288 (512k). The default is 10000. If the number of hosts
in the attribute table exceeds this value, an error is logged and
the remainder of the hosts are ignored. This option is only supported with a Host Attribute Table (see section 2.7).
Sets a Snort-wide limit on the number of MPLS headers a
packet can have. Its default value is -1, which means that there
is no limit on label chain length.
Sets a Snort-wide minimum ttl to ignore all traffic.
Sets a Snort-wide MPLS payload type. In addition to ipv4, ipv6
and ethernet are also valid options. The default MPLS payload
type is ipv4
Disables promiscuous mode (snort -p).
Disables logging. Note: Alerts will still occur. (snort -N).
Disables pcre pattern matching.
Obfuscates IP Addresses (snort -O).

config order:

config pcre match limit:

config pcre match limit recursion:

config pkt count:
config policy version:

[]

config profile preprocs
config profile rules
config quiet
config read bin file:
config reference:

config reference net

config set gid:
set uid:
config show year
config snaplen:
config stateful
config tagged packet limit:

config threshold:

memcap

config timestats interval:

config umask:

Changes the order that rules are evaluated, eg: pass alert log
activation.
Restricts the amount of backtracking a given PCRE option. For
example, it will limit the number of nested repeats within a pattern. A value of -1 allows for unlimited PCRE, up to the PCRE
library compiled limit (around 10 million). A value of 0 results
in no PCRE evaluation. The snort default value is 1500.
Restricts the amount of stack used by a given PCRE option. A
value of -1 allows for unlimited PCRE, up to the PCRE library
compiled limit (around 10 million). A value of 0 results in no
PCRE evaluation. The snort default value is 1500. This option
is only useful if the value is less than the pcre match limit
Exits after N packets (snort -n).
Supply versioning information to configuration files. Base version should be a string in all configuration files including included ones. In addition, binding version must be in any file
configured with config binding. This option is used to avoid
race conditions when modifying and loading a configuration
within a short time span - before Snort has had a chance to load
a previous configuration.
Print statistics on preprocessor performance. See Section 2.5.2
for more details.
Print statistics on rule performance. See Section 2.5.1 for more
details.
Disables banner and status reports (snort -q).
Specifies a pcap file to use (instead of reading from network),
same effect as -r option.
Adds a new reference system to Snort, eg:
myref
http://myurl.com/?id=
For IP obfuscation, the obfuscated net will be used if the packet
contains an IP address in the reference net. Also used to determine how to set up the logging directory structure for the
session post detection rule option and ascii output plugin - an
attempt is made to name the log directories after the IP address
that is not in the reference net.
Changes GID to specified GID (snort -g).
Sets UID to (snort -u).
Shows year in timestamps (snort -y).
Set the snaplength of packet, same effect as -P or
--snaplen options.
Sets assurance mode for stream (stream is established).
When a metric other than packets is used in a tag option in
a rule, this option sets the maximum number of packets to be
tagged regardless of the amount defined by the other metric.
See Section 3.7.5 on using the tag option when writing rules
for more details. The default value when this option is not configured is 256 packets. Setting this option to a value of 0 will
disable the packet limit.
Set global memcap in bytes for thresholding. Default is
1048576 bytes (1 megabyte). (This is deprecated. Use config
event filter instead.)
Set the amount of time in seconds between logging time stats.
Default is 3600 (1 hour). Note this option is only available if
Snort was built to use time stats with --enable-timestats.
Sets umask when running (snort -m).

config utc
config verbose

Uses UTC instead of local time for timestamps (snort -U).
Uses verbose logging to STDOUT (snort -v).

2.2 Preprocessors
Preprocessors were introduced in version 1.5 of Snort. They allow the functionality of Snort to be extended by allowing
users and programmers to drop modular plugins into Snort fairly easily. Preprocessor code is run before the detection
engine is called, but after the packet has been decoded. The packet can be modified or analyzed in an out-of-band
manner using this mechanism.
Preprocessors are loaded and configured using the preprocessor keyword. The format of the preprocessor directive
in the Snort rules file is:
preprocessor :

2.2.1 Frag3
The frag3 preprocessor is a target-based IP defragmentation module for Snort. Frag3 is intended as a replacement for
the frag2 defragmentation module and was designed with the following goals:
1. Faster execution than frag2 with less complex data management.
2. Target-based host modeling anti-evasion techniques.
The frag2 preprocessor used splay trees extensively for managing the data structures associated with defragmenting
packets. Splay trees are excellent data structures to use when you have some assurance of locality of reference for the
data that you are handling but in high speed, heavily fragmented environments the nature of the splay trees worked
against the system and actually hindered performance. Frag3 uses the sfxhash data structure and linked lists for data
handling internally which allows it to have much more predictable and deterministic performance in any environment
which should aid us in managing heavily fragmented environments.
Target-based analysis is a relatively new concept in network-based intrusion detection. The idea of a target-based
system is to model the actual targets on the network instead of merely modeling the protocols and looking for attacks
within them. When IP stacks are written for different operating systems, they are usually implemented by people
who read the RFCs and then write their interpretation of what the RFC outlines into code. Unfortunately, there are
ambiguities in the way that the RFCs define some of the edge conditions that may occurr and when this happens
different people implement certain aspects of their IP stacks differently. For an IDS this is a big problem.
In an environment where the attacker can determine what style of IP defragmentation is being used on a particular target, the attacker can try to fragment packets such that the target will put them back together in a specific
manner while any passive systems trying to model the host traffic have to guess which way the target OS is going
to handle the overlaps and retransmits. As I like to say, if the attacker has more information about the targets on
a network than the IDS does, it is possible to evade the IDS. This is where the idea for “target-based IDS” came
from. For more detail on this issue and how it affects IDS, check out the famous Ptacek & Newsham paper at
http://www.snort.org/docs/idspaper/.
The basic idea behind target-based IDS is that we tell the IDS information about hosts on the network so that it can
avoid Ptacek & Newsham style evasion attacks based on information about how an individual target IP stack operates.
Vern Paxson and Umesh Shankar did a great paper on this very topic in 2003 that detailed mapping the hosts on a network and determining how their various IP stack implementations handled the types of problems seen in IP defragmentation and TCP stream reassembly. Check it out at http://www.icir.org/vern/papers/activemap-oak03.pdf.
We can also present the IDS with topology information to avoid TTL-based evasions and a variety of other issues, but
that’s a topic for another day. Once we have this information we can start to really change the game for these complex
modeling problems.
Frag3 was implemented to showcase and prototype a target-based module within Snort to test this idea.
28

Frag 3 Configuration
Frag3 configuration is somewhat more complex than frag2. There are at least two preprocessor directives required
to activate frag3, a global configuration directive and an engine instantiation. There can be an arbitrary number of
engines defined at startup with their own configuration, but only one global configuration.
Global Configuration
• Preprocessor name: frag3 global
• Available options: NOTE: Global configuration options are comma separated.
– max frags - Maximum simultaneous fragments to track. Default is 8192.
– memcap - Memory cap for self preservation. Default is 4MB.
– prealloc frags - Alternate memory management mode. Use preallocated fragment nodes
(faster in some situations).
Engine Configuration
• Preprocessor name: frag3 engine
• Available options: NOTE: Engine configuration options are space separated.
– timeout - Timeout for fragments. Fragments in the engine for longer than this period will
be automatically dropped. Default is 60 seconds.
– min ttl - Minimum acceptable TTL value for a fragment packet. Default is 1.
– detect anomalies - Detect fragment anomalies.
– bind to - IP List to bind this engine to. This engine will only run for packets with destination
addresses contained within the IP List. Default value is all.
– overlap limit - Limits the number of overlapping fragments per packet. The default is
”0” (unlimited), the minimum is ”0”, and the maximum is ”255”. This is an optional parameter. detect anomalies option must be configured for this option to take effect.
– min fragment length - Defines smallest fragment size (payload size) that should be considered valid. Fragments smaller than or equal to this limit are considered malicious and an event is raised, if
detect anomalies is also configured. The default is ”0” (unlimited), the minimum is ”0”, and the maximum
is ”255”. This is an optional parameter. detect anomalies option must be configured for this option to take
effect.
– policy - Select a target-based defragmentation mode. Available types are first, last, bsd, bsdright, linux. Default type is bsd.
The Paxson Active Mapping paper introduced the terminology frag3 is using to describe policy types. The
known mappings are as follows. Anyone who develops more mappings and would like to add to this list
please feel free to send us an email!

Platform
AIX 2
AIX 4.3 8.9.3
Cisco IOS
FreeBSD
HP JetDirect (printer)
HP-UX B.10.20
HP-UX 11.00
IRIX 4.0.5F
IRIX 6.2
IRIX 6.3
IRIX64 6.4
Linux 2.2.10
Linux 2.2.14-5.0
Linux 2.2.16-3
Linux 2.2.19-6.2.10smp
Linux 2.4.7-10
Linux 2.4.9-31SGI 1.0.2smp
Linux 2.4 (RedHat 7.1-7.3)
MacOS (version unknown)
NCD Thin Clients
OpenBSD (version unknown)
OpenBSD (version unknown)
OpenVMS 7.1
OS/2 (version unknown)
OSF1 V3.0
OSF1 V3.2
OSF1 V4.0,5.0,5.1
SunOS 4.1.4
SunOS 5.5.1,5.6,5.7,5.8
Tru64 Unix V5.0A,V5.1
Vax/VMS
Windows (95/98/NT4/W2K/XP)

Type
BSD
BSD
Last
BSD
BSD-right
BSD
First
BSD
BSD
BSD
BSD
linux
linux
linux
linux
linux
linux
linux
First
BSD
linux
linux
BSD
BSD
BSD
BSD
BSD
BSD
First
BSD
BSD
First

Format
Note in the advanced configuration below that there are three engines specified running with Linux, first and last
policies assigned. The first two engines are bound to specific IP address ranges and the last one applies to all other
traffic. Packets that don’t fall within the address requirements of the first two engines automatically fall through to the
third one.
Basic Configuration
preprocessor frag3_global
preprocessor frag3_engine
Advanced Configuration
preprocessor
preprocessor
preprocessor
preprocessor

frag3_global:
frag3_engine:
frag3_engine:
frag3_engine:

prealloc_nodes 8192
policy linux, bind_to 192.168.1.0/24
policy first, bind_to [10.1.47.0/24,172.16.8.0/24]
policy last, detect_anomalies

Frag 3 Alert Output
Frag3 is capable of detecting eight different types of anomalies. Its event output is packet-based so it will work with
all output modes of Snort. Read the documentation in the doc/signatures directory with filenames that begin with
“123-” for information on the different event types.

2.2.2 Stream5
The Stream5 preprocessor is a target-based TCP reassembly module for Snort. It is capable of tracking sessions for
both TCP and UDP. With Stream5, the rule ’flow’ and ’flowbits’ keywords are usable with TCP as well as UDP traffic.
Transport Protocols
TCP sessions are identified via the classic TCP ”connection”. UDP sessions are established as the result of a series of
UDP packets from two end points via the same set of ports. ICMP messages are tracked for the purposes of checking
for unreachable and service unavailable messages, which effectively terminate a TCP or UDP session.
Target-Based
Stream5, like Frag3, introduces target-based actions for handling of overlapping data and other TCP anomalies. The
methods for handling overlapping data, TCP Timestamps, Data on SYN, FIN and Reset sequence numbers, etc. and
the policies supported by Stream5 are the results of extensive research with many target operating systems.
Stream API
Stream5 fully supports the Stream API, other protocol normalizers/preprocessors to dynamically configure reassembly
behavior as required by the application layer protocol, identify sessions that may be ignored (large data transfers, etc),
and update the identifying information about the session (application protocol, direction, etc) that can later be used by
rules.
Anomaly Detection
TCP protocol anomalies, such as data on SYN packets, data received outside the TCP window, etc are configured via
the detect anomalies option to the TCP configuration. Some of these anomalies are detected on a per-target basis.
For example, a few operating systems allow data in TCP SYN packets, while others do not.
Stream5 Global Configuration
Global settings for the Stream5 preprocessor.
preprocessor stream5_global: \
[track_tcp ], [max_tcp ], \
[memcap ], \
[track_udp ], [max_udp ], \
[track_icmp ], [max_icmp ], \
[flush_on_alert], [show_rebuilt_packets], \
[prune_log_max ]

Option
track tcp
max tcp
memcap
track udp
max udp
track icmp
max icmp
flush on alert
show rebuilt packets
prune log max

Description
Track sessions for TCP. The default is ”yes”.
Maximum simultaneous TCP sessions tracked. The default is ”256000”, maximum is ”1052672”, minimum is ”1”.
Memcap for TCP packet storage. The default is ”8388608” (8MB), maximum is
”1073741824” (1GB), minimum is ”32768” (32KB).
Track sessions for UDP. The default is ”yes”.
Maximum simultaneous UDP sessions tracked. The default is ”128000”, maximum is ”1052672”, minimum is ”1”.
Track sessions for ICMP. The default is ”yes”.
Maximum simultaneous ICMP sessions tracked. The default is ”64000”, maximum is ”1052672”, minimum is ”1”.
Backwards compatibilty. Flush a TCP stream when an alert is generated on that
stream. The default is set to off.
Print/display packet after rebuilt (for debugging). The default is set to off.
Print a message when a session terminates that was consuming more than the
specified number of bytes. The default is ”1048576” (1MB), minimum is ”0”
(unlimited), maximum is not bounded, other than by the memcap.

Stream5 TCP Configuration
Provides a means on a per IP address target to configure TCP policy. This can have multiple occurances, per policy
that is bound to an IP address or network. One default policy must be specified, and that policy is not bound to an IP
address or network.
preprocessor stream5_tcp: \
[bind_to ], [timeout ], \
[policy ], [min_ttl ], \
[overlap_limit ], [max_window ], \
[require_3whs []], [detect_anomalies], \
[check_session_hijacking], [use_static_footprint_sizes], \
[dont_store_large_packets], [dont_reassemble_async], \
[max_queued_bytes ], [max_queued_segs ], \
[ports ], \
[ignore_any_rules]
Option
bind to
timeout

Description
IP address or network for this policy. The default is set to any.
Session timeout. The default is ”30”, the minimum is ”1”, and the maximum is ”86400” (approximately 1 day).

policy

min ttl
overlap limit
max window

require 3whs []

detect anomalies
check session hijacking

use static footprint sizes

dont store large packets

dont reassemble async
max queued bytes

The Operating System policy for the target OS. The policy id can be one
of the following:
Policy Name Operating Systems.
first
Favor first overlapped segment.
last
Favor first overlapped segment.
bsd
FresBSD 4.x and newer, NetBSD 2.x and
newer, OpenBSD 3.x and newer
linux
Linux 2.4 and newer
old-linux
Linux 2.2 and earlier
windows
Windows 2000, Windows XP, Windows
95/98/ME
win2003
Windows 2003 Server
vista
Windows Vista
solaris
Solaris 9.x and newer
hpux
HPUX 11 and newer
hpux10
HPUX 10
irix
IRIX 6 and newer
macos
MacOS 10.3 and newer
Minimum TTL. The default is ”1”, the minimum is ”1” and the maximum
is ”255”.
Limits the number of overlapping packets per session. The default is ”0”
(unlimited), the minimum is ”0”, and the maximum is ”255”.
Maximum TCP window allowed. The default is ”0” (unlimited), the
minimum is ”0”, and the maximum is ”1073725440” (65535 left shift
14). That is the highest possible TCP window per RFCs. This option is
intended to prevent a DoS against Stream5 by an attacker using an abnormally large window, so using a value near the maximum is discouraged.
Establish sessions only on completion of a SYN/SYN-ACK/ACK handshake. The default is set to off. The optional number of seconds specifies a startup timeout. This allows a grace period for existing sessions to
be considered established during that interval immediately after Snort is
started. The default is ”0” (don’t consider existing sessions established),
the minimum is ”0”, and the maximum is ”86400” (approximately 1
day).
Detect and alert on TCP protocol anomalies. The default is set to off.
Check for TCP session hijacking. This check validates the hardware
(MAC) address from both sides of the connect – as established on the
3-way handshake against subsequent packets received on the session. If
an ethernet layer is not part of the protocol stack received by Snort, there
are no checks performed. Alerts are generated (per ’detect anomalies’
option) for either the client or server when the MAC address for one side
or the other does not match. The default is set to off.
Use static values for determining when to build a reassembled packet to
allow for repeatable tests. This option should not be used production
environments. The default is set to off.
Performance improvement to not queue large packets in reassembly
buffer. The default is set to off. Using this option may result in missed
attacks.
Don’t queue packets for reassembly if traffic has not been seen in both
directions. The default is set to queue packets.
Limit the number of bytes queued for reassembly on a given TCP session
to bytes. Default is ”1048576” (1MB). A value of ”0” means unlimited,
with a non-zero minimum of ”1024”, and a maximum of ”1073741824”
(1GB). A message is written to console/syslog when this limit is enforced.

max queued segs

ports

ignore any rules

Limit the number of segments queued for reassembly on a given TCP
session. The default is ”2621”, derived based on an average size of 400
bytes. A value of ”0” means unlimited, with a non-zero minimum of
”2”, and a maximum of ”1073741824” (1GB). A message is written to
console/syslog when this limit is enforced.
Specify the client, server, or both and list of ports in which to perform
reassembly. This can appear more than once in a given config. The default settings are ports client 21 23 25 42 53 80 110 111 135
136 137 139 143 445 513 514 1433 1521 2401 3306. The minimum port allowed is ”1” and the maximum allowed is ”65535”.
Don’t process any -> any (ports) rules for TCP that attempt to match
payload if there are no port specific rules for the src or destination port.
Rules that have flow or flowbits will never be ignored. This is a performance improvement and may result in missed attacks. Using this does
not affect rules that look at protocol headers, only those with content,
PCRE, or byte test options. The default is ”off”. This option can be used
only in default policy.

! NOTE
△

If no options are specified for a given TCP policy, that is the default TCP policy. If only a bind to option is
used with no other options that TCP policy uses all of the default values.

Stream5 UDP Configuration
Configuration for UDP session tracking. Since there is no target based binding, there should be only one occurance of
the UDP configuration.
preprocessor stream5_udp: [timeout ], [ignore_any_rules]
Option
timeout
ignore any rules

Description
Session timeout. The default is ”30”, the minimum is ”1”, and the maximum is
”86400” (approximately 1 day).
Don’t process any -> any (ports) rules for UDP that attempt to match payload
if there are no port specific rules for the src or destination port. Rules that have
flow or flowbits will never be ignored. This is a performance improvement and
may result in missed attacks. Using this does not affect rules that look at protocol
headers, only those with content, PCRE, or byte test options. The default is ”off”.

! NOTE
△

With the ignore any rules option, a UDP rule will be ignored except when there is another port specific rule
that may be applied to the traffic. For example, if a UDP rule specifies destination port 53, the ’ignored’ any
-> any rule will be applied to traffic to/from port 53, but NOT to any other source or destination port. A list
of rule SIDs affected by this option are printed at Snort’s startup.

! NOTE
△

With the ignore any rules option, if a UDP rule that uses any -> any ports includes either flow or flowbits,
the ignore any rules option is effectively pointless. Because of the potential impact of disabling a flowbits
rule, the ignore any rules option will be disabled in this case.

Stream5 ICMP Configuration
Configuration for ICMP session tracking. Since there is no target based binding, there should be only one occurance
of the ICMP configuration.

! NOTE
△
ICMP is currently untested, in minimal code form and is NOT ready for use in production networks. It is not
turned on by default.
preprocessor stream5_icmp: [timeout ]
Option
timeout

Description
Session timeout. The default is ”30”, the minimum is ”1”, and the maximum is
”86400” (approximately 1 day).

Example Configurations
1. This example configuration is the default configuration in snort.conf and can be used for repeatable tests of
stream reassembly in readback mode.
preprocessor stream5_global: \
max_tcp 8192, track_tcp yes, track_udp yes, track_icmp no
preprocessor stream5_tcp: \
policy first, use_static_footprint_sizes
preprocessor stream5_udp: \
ignore_any_rules
2. This configuration maps two network segments to different OS policies, one for Windows and one for Linux,
with all other traffic going to the default policy of Solaris.
preprocessor
preprocessor
preprocessor
preprocessor

stream5_global: track_tcp yes
stream5_tcp: bind_to 192.168.1.0/24, policy windows
stream5_tcp: bind_to 10.1.1.0/24, policy linux
stream5_tcp: policy solaris

Alerts
Stream5 uses generator ID 129. It is capable of alerting on 8 (eight) anomalies, all of which relate to TCP anomalies.
There are no anomalies detected relating to UDP or ICMP.
The list of SIDs is as follows:
1. SYN on established session
2. Data on SYN packet
3. Data sent on stream not accepting data
4. TCP Timestamp is outside of PAWS window
5. Bad segment, overlap adjusted size less than/equal 0
6. Window size (after scaling) larger than policy allows
7. Limit on number of overlapping TCP packets reached
8. Data after Reset packet
35

2.2.3 sfPortscan
The sfPortscan module, developed by Sourcefire, is designed to detect the first phase in a network attack: Reconnaissance. In the Reconnaissance phase, an attacker determines what types of network protocols or services a host
supports. This is the traditional place where a portscan takes place. This phase assumes the attacking host has no prior
knowledge of what protocols or services are supported by the target; otherwise, this phase would not be necessary.
As the attacker has no beforehand knowledge of its intended target, most queries sent by the attacker will be negative
(meaning that the service ports are closed). In the nature of legitimate network communications, negative responses
from hosts are rare, and rarer still are multiple negative responses within a given amount of time. Our primary objective
in detecting portscans is to detect and track these negative responses.
One of the most common portscanning tools in use today is Nmap. Nmap encompasses many, if not all, of the current
portscanning techniques. sfPortscan was designed to be able to detect the different types of scans Nmap can produce.
sfPortscan will currently alert for the following types of Nmap scans:
• TCP Portscan
• UDP Portscan
• IP Portscan
These alerts are for one→one portscans, which are the traditional types of scans; one host scans multiple ports on
another host. Most of the port queries will be negative, since most hosts have relatively few services available.
sfPortscan also alerts for the following types of decoy portscans:
• TCP Decoy Portscan
• UDP Decoy Portscan
• IP Decoy Portscan
Decoy portscans are much like the Nmap portscans described above, only the attacker has a spoofed source address
inter-mixed with the real scanning address. This tactic helps hide the true identity of the attacker.
sfPortscan alerts for the following types of distributed portscans:
• TCP Distributed Portscan
• UDP Distributed Portscan
• IP Distributed Portscan
These are many→one portscans. Distributed portscans occur when multiple hosts query one host for open services.
This is used to evade an IDS and obfuscate command and control hosts.

! NOTE
△

Negative queries will be distributed among scanning hosts, so we track this type of scan through the scanned
host.

sfPortscan alerts for the following types of portsweeps:
• TCP Portsweep
• UDP Portsweep
• IP Portsweep
36

• ICMP Portsweep
These alerts are for one→many portsweeps. One host scans a single port on multiple hosts. This usually occurs when
a new exploit comes out and the attacker is looking for a specific service.

! NOTE
△

The characteristics of a portsweep scan may not result in many negative responses. For example, if an attacker
portsweeps a web farm for port 80, we will most likely not see many negative responses.

sfPortscan alerts on the following filtered portscans and portsweeps:
• TCP Filtered Portscan
• UDP Filtered Portscan
• IP Filtered Portscan
• TCP Filtered Decoy Portscan
• UDP Filtered Decoy Portscan
• IP Filtered Decoy Portscan
• TCP Filtered Portsweep
• UDP Filtered Portsweep
• IP Filtered Portsweep
• ICMP Filtered Portsweep
• TCP Filtered Distributed Portscan
• UDP Filtered Distributed Portscan
• IP Filtered Distributed Portscan
“Filtered” alerts indicate that there were no network errors (ICMP unreachables or TCP RSTs) or responses on closed
ports have been suppressed. It’s also a good indicator of whether the alert is just a very active legitimate host. Active
hosts, such as NATs, can trigger these alerts because they can send out many connection attempts within a very small
amount of time. A filtered alert may go off before responses from the remote hosts are received.
sfPortscan only generates one alert for each host pair in question during the time window (more on windows below).
On TCP scan alerts, sfPortscan will also display any open ports that were scanned. On TCP sweep alerts however,
sfPortscan will only track open ports after the alert has been triggered. Open port events are not individual alerts, but
tags based on the orginal scan alert.
sfPortscan Configuration
Use of the Stream5 preprocessor is required for sfPortscan. Stream gives portscan direction in the case of connectionless protocols like ICMP and UDP. You should enable the Stream preprocessor in your snort.conf, as described in
Section 2.2.2.
The parameters you can use to configure the portscan module are:
1. proto
Available options:
• TCP
37

• UDP
• IGMP
• ip proto
• all
2. scan type
Available options:
• portscan
• portsweep
• decoy portscan
• distributed portscan
• all
3. sense level
Available options:
• low - “Low” alerts are only generated on error packets sent from the target host, and because of the nature
of error responses, this setting should see very few false postives. However, this setting will never trigger
a Filtered Scan alert because of a lack of error responses. This setting is based on a static time window of
60 seconds, afterwhich this window is reset.
• medium - “Medium” alerts track connection counts, and so will generate filtered scan alerts. This setting
may false positive on active hosts (NATs, proxies, DNS caches, etc), so the user may need to deploy the
use of Ignore directives to properly tune this directive.
• high - “High” alerts continuously track hosts on a network using a time window to evaluate portscan
statistics for that host. A ”High” setting will catch some slow scans because of the continuous monitoring,
but is very sensitive to active hosts. This most definitely will require the user to tune sfPortscan.
4. watch ip
Defines which IPs, networks, and specific ports on those hosts to watch. The list is a comma separated list of
IP addresses, IP address using CIDR notation. Optionally, ports are specified after the IP address/CIDR using a
space and can be either a single port or a range denoted by a dash. IPs or networks not falling into this range are
ignored if this option is used.
5. ignore scanners
Ignores the source of scan alerts. The parameter is the same format as that of watch ip.
6. ignore scanned
Ignores the destination of scan alerts. The parameter is the same format as that of watch ip.
7. logfile
This option will output portscan events to the file specified. If file does not contain a leading slash, this file
will be placed in the Snort config dir.
8. include midstream
This option will include sessions picked up in midstream by Stream5. This can lead to false alerts, especially
under heavy load with dropped packets; which is why the option is off by default.
9. detect ack scans
This option will include sessions picked up in midstream by the stream module, which is necessary to detect
ACK scans. However, this can lead to false alerts, especially under heavy load with dropped packets; which is
why the option is off by default.

Format
preprocessor sfportscan: proto \
scan_type \
sense_level \
watch_ip \
ignore_scanners \
ignore_scanned \
logfile
Example
preprocessor flow: stats_interval 0 hash 2
preprocessor sfportscan:\
proto { all } \
scan_type { all } \
sense_level { low }
sfPortscan Alert Output
Unified Output In order to get all the portscan information logged with the alert, snort generates a pseudo-packet
and uses the payload portion to store the additional portscan information of priority count, connection count, IP count,
port count, IP range, and port range. The characteristics of the packet are:
Src/Dst MAC Addr == MACDAD
IP Protocol == 255
IP TTL == 0
Other than that, the packet looks like the IP portion of the packet that caused the portscan alert to be generated. This
includes any IP options, etc. The payload and payload size of the packet are equal to the length of the additional
portscan information that is logged. The size tends to be around 100 - 200 bytes.
Open port alerts differ from the other portscan alerts, because open port alerts utilize the tagged packet output system.
This means that if an output system that doesn’t print tagged packets is used, then the user won’t see open port alerts.
The open port information is stored in the IP payload and contains the port that is open.
The sfPortscan alert output was designed to work with unified packet logging, so it is possible to extend favorite Snort
GUIs to display portscan alerts and the additional information in the IP payload using the above packet characteristics.
Log File Output Log file output is displayed in the following format, and explained further below:
Time: 09/08-15:07:31.603880
event_id: 2
192.168.169.3 -> 192.168.169.5 (portscan) TCP Filtered Portscan
Priority Count: 0
Connection Count: 200
IP Count: 2
Scanner IP Range: 192.168.169.3:192.168.169.4
Port/Proto Count: 200
Port/Proto Range: 20:47557
If there are open ports on the target, one or more additional tagged packet(s) will be appended:
Time: 09/08-15:07:31.603881
event_ref: 2
39

192.168.169.3 -> 192.168.169.5 (portscan) Open Port
Open Port: 38458
1. Event id/Event ref
These fields are used to link an alert with the corresponding Open Port tagged packet
2. Priority Count
Priority Count keeps track of bad responses (resets, unreachables). The higher the priority count, the more
bad responses have been received.
3. Connection Count
Connection Count lists how many connections are active on the hosts (src or dst). This is accurate for
connection-based protocols, and is more of an estimate for others. Whether or not a portscan was filtered is
determined here. High connection count and low priority count would indicate filtered (no response received
from target).
4. IP Count
IP Count keeps track of the last IP to contact a host, and increments the count if the next IP is different. For
one-to-one scans, this is a low number. For active hosts this number will be high regardless, and one-to-one
scans may appear as a distributed scan.
5. Scanned/Scanner IP Range
This field changes depending on the type of alert. Portsweep (one-to-many) scans display the scanned IP range;
Portscans (one-to-one) display the scanner IP.
6. Port Count
Port Count keeps track of the last port contacted and increments this number when that changes. We use this
count (along with IP Count) to determine the difference between one-to-one portscans and one-to-one decoys.
Tuning sfPortscan
The most important aspect in detecting portscans is tuning the detection engine for your network(s). Here are some
tuning tips:
1. Use the watch ip, ignore scanners, and ignore scanned options.
It’s important to correctly set these options. The watch ip option is easy to understand. The analyst should set
this option to the list of Cidr blocks and IPs that they want to watch. If no watch ip is defined, sfPortscan will
watch all network traffic.
The ignore scanners and ignore scanned options come into play in weeding out legitimate hosts that are
very active on your network. Some of the most common examples are NAT IPs, DNS cache servers, syslog
servers, and nfs servers. sfPortscan may not generate false positives for these types of hosts, but be aware when
first tuning sfPortscan for these IPs. Depending on the type of alert that the host generates, the analyst will know
which to ignore it as. If the host is generating portsweep events, then add it to the ignore scanners option.
If the host is generating portscan alerts (and is the host that is being scanned), add it to the ignore scanned
option.
2. Filtered scan alerts are much more prone to false positives.
When determining false positives, the alert type is very important. Most of the false positives that sfPortscan
may generate are of the filtered scan alert type. So be much more suspicious of filtered portscans. Many times
this just indicates that a host was very active during the time period in question. If the host continually generates
these types of alerts, add it to the ignore scanners list or use a lower sensitivity level.
3. Make use of the Priority Count, Connection Count, IP Count, Port Count, IP Range, and Port Range to
determine false positives.

The portscan alert details are vital in determining the scope of a portscan and also the confidence of the portscan.
In the future, we hope to automate much of this analysis in assigning a scope level and confidence level, but
for now the user must manually do this. The easiest way to determine false positives is through simple ratio
estimations. The following is a list of ratios to estimate and the associated values that indicate a legimite scan
and not a false positive.
Connection Count / IP Count: This ratio indicates an estimated average of connections per IP. For portscans,
this ratio should be high, the higher the better. For portsweeps, this ratio should be low.
Port Count / IP Count: This ratio indicates an estimated average of ports connected to per IP. For portscans, this
ratio should be high and indicates that the scanned host’s ports were connected to by fewer IPs. For portsweeps,
this ratio should be low, indicating that the scanning host connected to few ports but on many hosts.
Connection Count / Port Count: This ratio indicates an estimated average of connections per port. For
portscans, this ratio should be low. This indicates that each connection was to a different port. For portsweeps,
this ratio should be high. This indicates that there were many connections to the same port.
The reason that Priority Count is not included, is because the priority count is included in the connection
count and the above comparisons take that into consideration. The Priority Count play an important role in
tuning because the higher the priority count the more likely it is a real portscan or portsweep (unless the host is
firewalled).
4. If all else fails, lower the sensitivity level.
If none of these other tuning techniques work or the analyst doesn’t have the time for tuning, lower the sensitivity
level. You get the best protection the higher the sensitivity level, but it’s also important that the portscan detection
engine generate alerts that the analyst will find informative. The low sensitivity level only generates alerts based
on error responses. These responses indicate a portscan and the alerts generated by the low sensitivity level are
highly accurate and require the least tuning. The low sensitivity level does not catch filtered scans; since these
are more prone to false positives.

2.2.4 RPC Decode
The rpc decode preprocessor normalizes RPC multiple fragmented records into a single un-fragmented record. It does
this by normalizing the packet into the packet buffer. If stream5 is enabled, it will only process client-side traffic. By
default, it runs against traffic on ports 111 and 32771.
Format
preprocessor rpc_decode: \
[ alert_fragments ] \
[no_alert_multiple_requests] \
[no_alert_large_fragments] \
[no_alert_incomplete]
Option
alert fragments
no alert multiple requests
no alert large fragments
no alert incomplete

Description
Alert on any fragmented RPC record.
Don’t alert when there are multiple records in one packet.
Don’t alert when the sum of fragmented records exceeds one packet.
Don’t alert when a single fragment record exceeds the size of one packet.

2.2.5 Performance Monitor
This preprocessor measures Snort’s real-time and theoretical maximum performance. Whenever this preprocessor is
turned on, it should have an output mode enabled, either “console” which prints statistics to the console window or
“file” with a file name, where statistics get printed to the specified file name. By default, Snort’s real-time statistics
are processed. This includes:
41

• Time Stamp
• Drop Rate
• Mbits/Sec (wire) [duplicated below for easy comparison with other rates]
• Alerts/Sec
• K-Pkts/Sec (wire) [duplicated below for easy comparison with other rates]
• Avg Bytes/Pkt (wire) [duplicated below for easy comparison with other rates]
• Pat-Matched [percent of data received that Snort processes in pattern matching]
• Syns/Sec
• SynAcks/Sec
• New Sessions Cached/Sec
• Sessions Del fr Cache/Sec
• Current Cached Sessions
• Max Cached Sessions
• Stream Flushes/Sec
• Stream Session Cache Faults
• Stream Session Cache Timeouts
• New Frag Trackers/Sec
• Frag-Completes/Sec
• Frag-Inserts/Sec
• Frag-Deletes/Sec
• Frag-Auto Deletes/Sec [memory DoS protection]
• Frag-Flushes/Sec
• Frag-Current [number of current Frag Trackers]
• Frag-Max [max number of Frag Trackers at any time]
• Frag-Timeouts
• Frag-Faults
• Number of CPUs [*** Only if compiled with LINUX SMP ***, the next three appear for each CPU]
• CPU usage (user)
• CPU usage (sys)
• CPU usage (Idle)
• Mbits/Sec (wire) [average mbits of total traffic]
• Mbits/Sec (ipfrag) [average mbits of IP fragmented traffic]
• Mbits/Sec (ipreass) [average mbits Snort injects after IP reassembly]
• Mbits/Sec (tcprebuilt) [average mbits Snort injects after TCP reassembly]
• Mbits/Sec (applayer) [average mbits seen by rules and protocol decoders]
42

• Avg Bytes/Pkt (wire)
• Avg Bytes/Pkt (ipfrag)
• Avg Bytes/Pkt (ipreass)
• Avg Bytes/Pkt (tcprebuilt)
• Avg Bytes/Pkt (applayer)
• K-Pkts/Sec (wire)
• K-Pkts/Sec (ipfrag)
• K-Pkts/Sec (ipreass)
• K-Pkts/Sec (tcprebuilt)
• K-Pkts/Sec (applayer)
• Total Packets Received
• Total Packets Dropped (not processed)
• Total Packets Blocked (inline)
• Percentage of Packets Dropped
• Total Filtered TCP Packets
• Total Filtered UDP Packets
• Midstream TCP Sessions/Sec
• Closed TCP Sessions/Sec
• Pruned TCP Sessions/Sec
• TimedOut TCP Sessions/Sec
• Dropped Async TCP Sessions/Sec
• TCP Sessions Initializing
• TCP Sessions Established
• TCP Sessions Closing
• Max TCP Sessions (interval)
• New Cached UDP Sessions/Sec
• Cached UDP Ssns Del/Sec
• Current Cached UDP Sessions
• Max Cached UDP Sessions
• Current Attribute Table Hosts (Target Based)
• Attribute Table Reloads (Target Based)
• Mbits/Sec (Snort)
• Mbits/Sec (sniffing)
• Mbits/Sec (combined)
• uSeconds/Pkt (Snort)
43

• uSeconds/Pkt (sniffing)
• uSeconds/Pkt (combined)
• KPkts/Sec (Snort)
• KPkts/Sec (sniffing)
• KPkts/Sec (combined)
The following options can be used with the performance monitor:
• flow - Prints out statistics about the type of traffic and protocol distributions that Snort is seeing. This option
can produce large amounts of output.
• events - Turns on event reporting. This prints out statistics as to the number of signatures that were matched
by the setwise pattern matcher (non-qualified events) and the number of those matches that were verified with
the signature flags (qualified events). This shows the user if there is a problem with the rule set that they are
running.
• max - Turns on the theoretical maximum performance that Snort calculates given the processor speed and current
performance. This is only valid for uniprocessor machines, since many operating systems don’t keep accurate
kernel statistics for multiple CPUs.
• console - Prints statistics at the console.
• file - Prints statistics in a comma-delimited format to the file that is specified. Not all statistics are output to
this file. You may also use snortfile which will output into your defined Snort log directory. Both of these
directives can be overridden on the command line with the -Z or --perfmon-file options.
• pktcnt - Adjusts the number of packets to process before checking for the time sample. This boosts performance, since checking the time sample reduces Snort’s performance. By default, this is 10000.
• time - Represents the number of seconds between intervals.
• accumulate or reset - Defines which type of drop statistics are kept by the operating system. By default,
reset is used.
• atexitonly - Dump stats for entire life of Snort.
• max file size - Defines the maximum size of the comma-delimited file. Before the file exceeds this size, it
will be rolled into a new date stamped file of the format YYYY-MM-DD, followed by YYYY-MM-DD.x, where
x will be incremented each time the comma delimiated file is rolled over. The minimum is 4096 bytes and the
maximum is 2147483648 bytes (2GB). The default is the same as the maximum.
Examples
preprocessor perfmonitor: \
time 30 events flow file stats.profile max console pktcnt 10000
preprocessor perfmonitor: \
time 300 file /var/tmp/snortstat pktcnt 10000

2.2.6 HTTP Inspect
HTTP Inspect is a generic HTTP decoder for user applications. Given a data buffer, HTTP Inspect will decode the
buffer, find HTTP fields, and normalize the fields. HTTP Inspect works on both client requests and server responses.
The current version of HTTP Inspect only handles stateless processing. This means that HTTP Inspect looks for HTTP
fields on a packet-by-packet basis, and will be fooled if packets are not reassembled. This works fine when there is
44

another module handling the reassembly, but there are limitations in analyzing the protocol. Future versions will have
a stateful processing mode which will hook into various reassembly modules.
HTTP Inspect has a very “rich” user configuration. Users can configure individual HTTP servers with a variety of
options, which should allow the user to emulate any type of web server. Within HTTP Inspect, there are two areas of
configuration: global and server.
Global Configuration
The global configuration deals with configuration options that determine the global functioning of HTTP Inspect. The
following example gives the generic global configuration format:
Format
preprocessor http_inspect: \
global \
iis_unicode_map \
codemap \
[detect_anomalous_servers] \
[proxy_alert]
You can only have a single global configuration, you’ll get an error if you try otherwise.
Configuration
1. iis unicode map [codemap ]
This is the global iis unicode map file. The iis unicode map is a required configuration parameter. The map
file can reside in the same directory as snort.conf or be specified via a fully-qualified path to the map file.
The iis unicode map file is a Unicode codepoint map which tells HTTP Inspect which codepage to use when
decoding Unicode characters. For US servers, the codemap is usually 1252.
A Microsoft US Unicode codepoint map is provided in the Snort source etc directory by default. It is called
unicode.map and should be used if no other codepoint map is available. A tool is supplied with Snort to generate
custom Unicode maps--ms unicode generator.c, which is available at http://www.snort.org/dl/contrib/.

! NOTE
△
Remember that this configuration is for the global IIS Unicode map, individual servers can reference their
own IIS Unicode map.
2. detect anomalous servers
This global configuration option enables generic HTTP server traffic inspection on non-HTTP configured ports,
and alerts if HTTP traffic is seen. Don’t turn this on if you don’t have a default server configuration that
encompasses all of the HTTP server ports that your users might access. In the future, we want to limit this to
specific networks so it’s more useful, but for right now, this inspects all network traffic.
3. proxy alert
This enables global alerting on HTTP server proxy usage. By configuring HTTP Inspect servers and enabling
allow proxy use, you will only receive proxy use alerts for web users that aren’t using the configured proxies
or are using a rogue proxy server.
Please note that if users aren’t required to configure web proxy use, then you may get a lot of proxy alerts. So,
please only use this feature with traditional proxy environments. Blind firewall proxies don’t count.

Example Global Configuration
preprocessor http_inspect: \
global iis_unicode_map unicode.map 1252
Server Configuration
There are two types of server configurations: default and by IP address.
Default This configuration supplies the default server configuration for any server that is not individually configured.
Most of your web servers will most likely end up using the default configuration.
Example Default Configuration
preprocessor http_inspect_server: \
server default profile all ports { 80 }
Configuration by IP Address This format is very similar to “default”, the only difference being that specific IPs
can be configured.
Example IP Configuration
preprocessor http_inspect_server: \
server 10.1.1.1 profile all ports { 80 }
Configuration by Multiple IP Addresses This format is very similar to “Configuration by IP Address”, the only
difference being that multiple IPs can be specified via a space separated list. There is a limit of 40 IP addresses or
CIDR notations per http inspect server line.
Example Multiple IP Configuration
preprocessor http_inspect_server: \
server { 10.1.1.1 10.2.2.0/24 } profile all ports { 80 }
Server Configuration Options
Important: Some configuration options have an argument of ‘yes’ or ‘no’. This argument specifies whether the user
wants the configuration option to generate an HTTP Inspect alert or not. The ‘yes/no’ argument does not specify
whether the configuration option itself is on or off, only the alerting functionality. In other words, whether set to ‘yes’
or ’no’, HTTP normalization will still occur, and rules based on HTTP traffic will still trigger.
1. profile
Users can configure HTTP Inspect by using pre-defined HTTP server profiles. Profiles allow the user to easily
configure the preprocessor for a certain type of server, but are not required for proper operation.
There are five profiles available: all, apache, iis, iis5 0, and iis4 0.
1-A. all
The all profile is meant to normalize the URI using most of the common tricks available. We alert on the
more serious forms of evasions. This is a great profile for detecting all types of attacks, regardless of the
HTTP server. profile all sets the configuration options described in Table 2.3.

Table 2.3: Options for the “all” Profile
Option
Setting
server flow depth
300
client flow depth
300
post depth
0
chunk encoding
alert on chunks larger than 500000 bytes
iis unicode map
codepoint map in the global configuration
ascii decoding
on, alert off
multiple slash
on, alert off
directory normalization on, alert off
apache whitespace
on, alert off
double decoding
on, alert on
%u decoding
on, alert on
bare byte decoding
on, alert on
iis unicode codepoints
on, alert on
iis backslash
on, alert off
iis delimiter
on, alert off
webroot
on, alert on
non strict URL parsing on
tab uri delimiter
is set
max header length
0, header length not checked
max headers
0, number of headers not checked

1-B.

apache
The apache profile is used for Apache web servers. This differs from the iis profile by only accepting
UTF-8 standard Unicode encoding and not accepting backslashes as legitimate slashes, like IIS does.
Apache also accepts tabs as whitespace. profile apache sets the configuration options described in
Table 2.4.
Table 2.4: Options for the apache Profile
Option
Setting
server flow depth
300
client flow depth
300
post depth
0
chunk encoding
alert on chunks larger than 500000 bytes
ascii decoding
on, alert off
multiple slash
on, alert off
directory normalization on, alert off
webroot
on, alert on
apache whitespace
on, alert on
utf 8 encoding
on, alert off
non strict url parsing
on
tab uri delimiter
is set
max header length
0, header length not checked
max headers
0, number of headers not checked

1-C. iis
The iis profile mimics IIS servers. So that means we use IIS Unicode codemaps for each server, %u
encoding, bare-byte encoding, double decoding, backslashes, etc. profile iis sets the configuration
options described in Table 2.5.
1-D.

iis4 0, iis5 0
In IIS 4.0 and IIS 5.0, there was a double decoding vulnerability. These two profiles are identical to iis,
47

Table 2.5: Options for the iis Profile
Option
Setting
server flow depth
300
client flow depth
300
post depth
0
chunk encoding
alert on chunks larger than 500000 bytes
iis unicode map
codepoint map in the global configuration
ascii decoding
on, alert off
multiple slash
on, alert off
directory normalization on, alert off
webroot
on, alert on
double decoding
on, alert on
%u decoding
on, alert on
bare byte decoding
on, alert on
iis unicode codepoints
on, alert on
iis backslash
on, alert off
iis delimiter
on, alert on
apache whitespace
on, alert on
non strict URL parsing on
max header length
0, header length not checked
max headers
0, number of headers not checked

except they will alert by default if a URL has a double encoding. Double decode is not supported in IIS
5.1 and beyond, so it’s disabled by default.
1-E.

default, no profile
The default options used by HTTP Inspect do not use a profile and are described in Table 2.6.
Table 2.6: Default HTTP Inspect Options
Option
Setting
port
80
server flow depth
300
client flow depth
300
post depth
0
chunk encoding
alert on chunks larger than 500000 bytes
ascii decoding
on, alert off
utf 8 encoding
on, alert off
multiple slash
on, alert off
directory normalization on, alert off
webroot
on, alert on
iis backslash
on, alert off
apache whitespace
on, alert off
iis delimiter
on, alert off
non strict URL parsing on
max header length
0, header length not checked
max headers
0, number of headers not checked
Profiles must be specified as the first server option and cannot be combined with any other options except:
•
•
•
•

ports
iis unicode map
allow proxy use
server flow depth

•
•
•
•
•
•
•
•
•

client flow depth
post depth
no alerts
inspect uri only
oversize dir length
normalize headers
normalize cookies
max header length
max headers

These options must be specified after the profile option.
Example
preprocessor http_inspect_server: \
server 1.1.1.1 profile all ports { 80 3128 }
2. ports { [< ... >]}
This is how the user configures which ports to decode on the HTTP server. However, HTTPS traffic is encrypted
and cannot be decoded with HTTP Inspect. To ignore HTTPS traffic, use the SSL preprocessor.
3. iis unicode map codemap
The IIS Unicode map is generated by the program ms unicode generator.c. This program is located on the
Snort.org web site at http://www.snort.org/dl/contrib/ directory. Executing this program generates a
Unicode map for the system that it was run on. So, to get the specific Unicode mappings for an IIS web server,
you run this program on that server and use that Unicode map in this configuration.
When using this option, the user needs to specify the file that contains the IIS Unicode map and also specify
the Unicode map to use. For US servers, this is usually 1252. But the ms unicode generator program tells you
which codemap to use for you server; it’s the ANSI code page. You can select the correct code page by looking
at the available code pages that the ms unicode generator outputs.
4. server flow depth
This specifies the amount of server response payload to inspect. This option significantly increases IDS performance because we are ignoring a large part of the network traffic (HTTP server response payloads). A small
percentage of Snort rules are targeted at this traffic and a small flow depth value may cause false negatives in
some of these rules. Most of these rules target either the HTTP header, or the content that is likely to be in the
first hundred or so bytes of non-header data. Headers are usually under 300 bytes long, but your mileage may
vary.
This value can be set from -1 to 1460. A value of -1 causes Snort to ignore all server side traffic for ports defined
in ports. Inversely, a value of 0 causes Snort to inspect all HTTP server payloads defined in ports (note that
this will likely slow down IDS performance). Values above 0 tell Snort the number of bytes to inspect in the
first packet of the server response.

! NOTE
△
server flow depth is the same as the old flow depth option, which will be deprecated in a future release.
5. client flow depth
This specifies the amount of raw client request payload to inspect. It is similar to server flow depth (above),
and has a default value of 300. It primarily eliminates Snort fro inspecting larger HTTP Cookies that appear at
the end of many client request Headers.
6. post depth
This specifies the amount of data to inspect in a client post message. The value can be set from 0 to 65495. The
default value is 0. This increases the perfomance by inspecting only specified bytes in the post message.
49

7. ascii
The ascii decode option tells us whether to decode encoded ASCII chars, a.k.a %2f = /, %2e = ., etc. It is
normal to see ASCII encoding usage in URLs, so it is recommended that you disable HTTP Inspect alerting for
this option.
8. utf 8
The utf-8 decode option tells HTTP Inspect to decode standard UTF-8 Unicode sequences that are in the URI.
This abides by the Unicode standard and only uses % encoding. Apache uses this standard, so for any Apache
servers, make sure you have this option turned on. As for alerting, you may be interested in knowing when you
have a UTF-8 encoded URI, but this will be prone to false positives as legitimate web clients use this type of
encoding. When utf 8 is enabled, ASCII decoding is also enabled to enforce correct functioning.
9. u encode
This option emulates the IIS %u encoding scheme. How the %u encoding scheme works is as follows: the
encoding scheme is started by a %u followed by 4 characters, like %uxxxx. The xxxx is a hex-encoded value
that correlates to an IIS Unicode codepoint. This value can most definitely be ASCII. An ASCII character is
encoded like %u002f = /, %u002e = ., etc. If no iis unicode map is specified before or after this option, the
default codemap is used.
You should alert on %u encodings, because we are not aware of any legitimate clients that use this encoding. So
it is most likely someone trying to be covert.
10. bare byte
Bare byte encoding is an IIS trick that uses non-ASCII characters as valid values when decoding UTF-8 values.
This is not in the HTTP standard, as all non-ASCII values have to be encoded with a %. Bare byte encoding
allows the user to emulate an IIS server and interpret non-standard encodings correctly.
The alert on this decoding should be enabled, because there are no legitimate clients that encode UTF-8 this
way since it is non-standard.
11. base36
This is an option to decode base36 encoded chars. This option is based on info from:
http://www.yk.rim.or.jp/˜shikap/patch/spp_http_decode.patch.
If %u encoding is enabled, this option will not work. You have to use the base36 option with the utf 8 option.
Don’t use the %u option, because base36 won’t work. When base36 is enabled, ASCII encoding is also enabled
to enforce correct behavior.
12. iis unicode
The iis unicode option turns on the Unicode codepoint mapping. If there is no iis unicode map option specified with the server config, iis unicode uses the default codemap. The iis unicode option handles the
mapping of non-ASCII codepoints that the IIS server accepts and decodes normal UTF-8 requests.
You should alert on the iis unicode option, because it is seen mainly in attacks and evasion attempts. When
iis unicode is enabled, ASCII and UTF-8 decoding are also enabled to enforce correct decoding. To alert on
UTF-8 decoding, you must enable also enable utf 8 yes.
13. double decode
The double decode option is once again IIS-specific and emulates IIS functionality. How this works is that IIS
does two passes through the request URI, doing decodes in each one. In the first pass, it seems that all types of
iis encoding is done: utf-8 unicode, ascii, bare byte, and %u. In the second pass, the following encodings are
done: ascii, bare byte, and %u. We leave out utf-8 because I think how this works is that the % encoded utf-8
is decoded to the Unicode byte in the first pass, and then UTF-8 is decoded in the second stage. Anyway, this
is really complex and adds tons of different encodings for one character. When double decode is enabled, so
ASCII is also enabled to enforce correct decoding.
14. non rfc char { []}
This option lets users receive an alert if certain non-RFC chars are used in a request URI. For instance, a user
may not want to see null bytes in the request URI and we can alert on that. Please use this option with care,
because you could configure it to say, alert on all ‘/’ or something like that. It’s flexible, so be careful.
50

15. multi slash
This option normalizes multiple slashes in a row, so something like: “foo/////////bar” get normalized to “foo/bar.”
If you want an alert when multiple slashes are seen, then configure with a yes; otherwise, use no.
16. iis backslash
Normalizes backslashes to slashes. This is again an IIS emulation. So a request URI of “/foo\bar” gets normalized to “/foo/bar.”
17. directory
This option normalizes directory traversals and self-referential directories.
The directory:
/foo/fake\_dir/../bar
gets normalized to:
/foo/bar
The directory:
/foo/./bar
gets normalized to:
/foo/bar
If you want to configure an alert, specify yes, otherwise, specify no. This alert may give false positives, since
some web sites refer to files using directory traversals.
18. apache whitespace
This option deals with the non-RFC standard of using tab for a space delimiter. Apache uses this, so if the
emulated web server is Apache, enable this option. Alerts on this option may be interesting, but may also be
false positive prone.
19. iis delimiter
This started out being IIS-specific, but Apache takes this non-standard delimiter was well. Since this is common,
we always take this as standard since the most popular web servers accept it. But you can still get an alert on
this option.
20. chunk length
This option is an anomaly detector for abnormally large chunk sizes. This picks up the Apache chunk encoding
exploits, and may also alert on HTTP tunneling that uses chunk encoding.
21. no pipeline req
This option turns HTTP pipeline decoding off, and is a performance enhancement if needed. By default, pipeline
requests are inspected for attacks, but when this option is enabled, pipeline requests are not decoded and analyzed per HTTP protocol field. It is only inspected with the generic pattern matching.
22. non strict
This option turns on non-strict URI parsing for the broken way in which Apache servers will decode a URI.
Only use this option on servers that will accept URIs like this: ”get /index.html alsjdfk alsj lj aj la jsj s\n”. The
non strict option assumes the URI is between the first and second space even if there is no valid HTTP identifier
after the second space.

23. allow proxy use
By specifying this keyword, the user is allowing proxy use on this server. This means that no alert will be
generated if the proxy alert global keyword has been used. If the proxy alert keyword is not enabled, then
this option does nothing. The allow proxy use keyword is just a way to suppress unauthorized proxy use for
an authorized server.
24. no alerts
This option turns off all alerts that are generated by the HTTP Inspect preprocessor module. This has no effect
on HTTP rules in the rule set. No argument is specified.
25. oversize dir length
This option takes a non-zero positive integer as an argument. The argument specifies the max char directory
length for URL directory. If a url directory is larger than this argument size, an alert is generated. A good
argument value is 300 characters. This should limit the alerts to IDS evasion type attacks, like whisker -i 4.
26. inspect uri only
This is a performance optimization. When enabled, only the URI portion of HTTP requests will be inspected
for attacks. As this field usually contains 90-95% of the web attacks, you’ll catch most of the attacks. So if
you need extra performance, enable this optimization. It’s important to note that if this option is used without
any uricontent rules, then no inspection will take place. This is obvious since the URI is only inspected with
uricontent rules, and if there are none available, then there is nothing to inspect.
For example, if we have the following rule set:
alert tcp any any -> any 80 ( msg:"content"; content: "foo"; )
and the we inspect the following URI:
get /foo.htm http/1.0\r\n\r\n
No alert will be generated when inspect uri only is enabled. The inspect uri only configuration turns off
all forms of detection except uricontent inspection.
27. max header length
This option takes an integer as an argument. The integer is the maximum length allowed for an HTTP client
request header field. Requests that exceed this length will cause a ”Long Header” alert. This alert is off by
default. To enable, specify an integer argument to max header length of 1 to 65535. Specifying a value of 0 is
treated as disabling the alert.
28. webroot
This option generates an alert when a directory traversal traverses past the web server root directory. This
generates much fewer false positives than the directory option, because it doesn’t alert on directory traversals
that stay within the web server directory structure. It only alerts when the directory traversals go past the web
server root directory, which is associated with certain web attacks.
29. tab uri delimiter
This option turns on the use of the tab character (0x09) as a delimiter for a URI. Apache accepts tab as a
delimiter; IIS does not. For IIS, a tab in the URI should be treated as any other character. Whether this option is
on or not, a tab is treated as whitespace if a space character (0x20) precedes it. No argument is specified.
30. normalize headers
This option turns on normalization for HTTP Header Fields, not including Cookies (using the same configuration
parameters as the URI normalization (ie, multi-slash, directory, etc.). It is useful for normalizing Referrer URIs
that may appear in the HTTP Header.
31. normalize cookies
This option turns on normalization for HTTP Cookie Fields (using the same configuration parameters as the
URI normalization (ie, multi-slash, directory, etc.). It is useful for normalizing data in HTTP Cookies that may
be encoded.
52

32. max headers
This option takes an integer as an argument. The integer is the maximum number of HTTP client request header
fields. Requests that contain more HTTP Headers than this value will cause a ”Max Header” alert. The alert is
off by default. To enable, specify an integer argumnet to max headers of 1 to 1024. Specifying a value of 0 is
treated as disabling the alert.
Examples
preprocessor http_inspect_server: \
server 10.1.1.1 \
ports { 80 3128 8080 } \
server_flow_depth 0 \
ascii no \
double_decode yes \
non_rfc_char { 0x00 } \
chunk_length 500000 \
non_strict \
no_alerts
preprocessor http_inspect_server: \
server default \
ports { 80 3128 } \
non_strict \
non_rfc_char { 0x00 } \
server_flow_depth 300 \
apache_whitespace yes \
directory no \
iis_backslash no \
u_encode yes \
ascii no \
chunk_length 500000 \
bare_byte yes \
double_decode yes \
iis_unicode yes \
iis_delimiter yes \
multi_slash no
preprocessor http_inspect_server: \
server default \
profile all \
ports { 80 8080 }

2.2.7 SMTP Preprocessor
The SMTP preprocessor is an SMTP decoder for user applications. Given a data buffer, SMTP will decode the buffer
and find SMTP commands and responses. It will also mark the command, data header data body sections, and TLS
data.
SMTP handles stateless and stateful processing. It saves state between individual packets. However maintaining
correct state is dependent on the reassembly of the client side of the stream (ie, a loss of coherent stream data results
in a loss of state).

Configuration
SMTP has the usual configuration items, such as port and inspection type. Also, SMTP command lines can be
normalized to remove extraneous spaces. TLS-encrypted traffic can be ignored, which improves performance. In
addition, regular mail data can be ignored for an additional performance boost. Since so few (none in the current snort
rule set) exploits are against mail data, this is relatively safe to do and can improve the performance of data inspection.
The configuration options are described below:
1. ports { [] ...

}

This specifies on what ports to check for SMTP data. Typically, this will include 25 and possibly 465, for
encrypted SMTP.
2. inspection type
Indicate whether to operate in stateful or stateless mode.
3. normalize
This turns on normalization. Normalization checks for more than one space character after a command. Space
characters are defined as space (ASCII 0x20) or tab (ASCII 0x09).
all checks all commands
none turns off normalization for all commands.
cmds just checks commands listed with the normalize cmds parameter.
4. ignore data
Ignore data section of mail (except for mail headers) when processing rules.
5. ignore tls data
Ignore TLS-encrypted data when processing rules.
6. max command line len
Alert if an SMTP command line is longer than this value. Absence of this option or a ”0” means never alert on
command line length. RFC 2821 recommends 512 as a maximum command line length.
7. max header line len
Alert if an SMTP DATA header line is longer than this value. Absence of this option or a ”0” means never alert
on data header line length. RFC 2821 recommends 1024 as a maximum data header line length.
8. max response line len
Alert if an SMTP response line is longer than this value. Absence of this option or a ”0” means never alert on
response line length. RFC 2821 recommends 512 as a maximum response line length.
9. alt max command line len { [] }
Overrides max command line len for specific commands.
10. no alerts
Turn off all alerts for this preprocessor.
11. invalid cmds { }
Alert if this command is sent from client side. Default is an empty list.
12. valid cmds { }
List of valid commands. We do not alert on commands in this list. Default is an empty list, but preprocessor has
this list hard-coded:
{ ATRN AUTH BDAT DATA DEBUG EHLO EMAL ESAM ESND ESOM ETRN EVFY EXPN } { HELO
HELP IDENT MAIL NOOP QUIT RCPT RSET SAML SOML SEND ONEX QUEU } { STARTTLS TICK
TIME TURN TURNME VERB VRFY X-EXPS X-LINK2STATE } { XADR XAUTH XCIR XEXCH50 XGEN
XLICENSE XQUE XSTA XTRN XUSR }
54

13. alert unknown cmds
Alert if we don’t recognize command. Default is off.
14. normalize cmds { }
Normalize this list of commands Default is { RCPT VRFY EXPN }.
15. xlink2state { enable | disable [drop] }
Enable/disable xlink2state alert. Drop if alerted. Default is enable.
16. print cmds
List all commands understood by the preprocessor. This not normally printed out with the configuration because
it can print so much data.
Example
preprocessor SMTP: \
ports { 25 } \
inspection_type stateful \
normalize cmds \
normalize_cmds { EXPN VRFY RCPT } \
ignore_data \
ignore_tls_data \
max_command_line_len 512 \
max_header_line_len
1024 \
max_response_line_len 512 \
no_alerts \
alt_max_command_line_len 300 { RCPT } \
invalid_cmds { } \
valid_cmds { } \
xlink2state { disable } \
print_cmds
Default
preprocessor SMTP: \
ports { 25 } \
inspection_type stateful \
normalize cmds \
normalize_cmds { EXPN VRFY RCPT } \
alt_max_command_line_len 260 { MAIL
alt_max_command_line_len 300 { RCPT
alt_max_command_line_len 500 { HELP
alt_max_command_line_len 255 { EXPN

} \
} \
HELO ETRN } \
VRFY }

Note
RCPT TO: and MAIL FROM: are SMTP commands. For the preprocessor configuration, they are referred to as RCPT
and MAIL, respectively. Within the code, the preprocessor actually maps RCPT and MAIL to the correct command
name.

2.2.8 FTP/Telnet Preprocessor
FTP/Telnet is an improvement to the Telnet decoder and provides stateful inspection capability for both FTP and
Telnet data streams. FTP/Telnet will decode the stream, identifying FTP commands and responses and Telnet escape
sequences and normalize the fields. FTP/Telnet works on both client requests and server responses.
55

FTP/Telnet has the capability to handle stateless processing, meaning it only looks for information on a packet-bypacket basis.
The default is to run FTP/Telent in stateful inspection mode, meaning it looks for information and handles reassembled
data correctly.
FTP/Telnet has a very “rich” user configuration, similar to that of HTTP Inspect (See 2.2.6). Users can configure
individual FTP servers and clients with a variety of options, which should allow the user to emulate any type of FTP
server or FTP Client. Within FTP/Telnet, there are four areas of configuration: Global, Telnet, FTP Client, and FTP
Server.

! NOTE
△
Some configuration options have an argument of yes or no. This argument specifies whether the user wants
the configuration option to generate a ftptelnet alert or not. The presence of the option indicates the option
itself is on, while the yes/no argument applies to the alerting functionality associated with that option.
Global Configuration
The global configuration deals with configuration options that determine the global functioning of FTP/Telnet. The
following example gives the generic global configuration format:
Format
preprocessor ftp_telnet: \
global \
inspection_type stateful \
encrypted_traffic yes \
check_encrypted
You can only have a single global configuration, you’ll get an error if you try otherwise. The FTP/Telnet global
configuration must appear before the other three areas of configuration.
Configuration
1. inspection type
This indicates whether to operate in stateful or stateless mode.
2. encrypted traffic
This option enables detection and alerting on encrypted Telnet and FTP command channels.

! NOTE
△
When inspection type is in stateless mode, checks for encrypted traffic will occur on every packet, whereas
in stateful mode, a particular session will be noted as encrypted and not inspected any further.
3. check encrypted
Instructs the the preprocessor to continue to check an encrypted session for a subsequent command to cease
encryption.
Example Global Configuration
preprocessor ftp_telnet: \
global inspection_type stateful encrypted_traffic no
56

Telnet Configuration
The telnet configuration deals with configuration options that determine the functioning of the Telnet portion of the
preprocessor. The following example gives the generic telnet configuration format:
Format
preprocessor ftp_telnet_protocol: \
telnet \
ports { 23 } \
normalize \
ayt_attack_thresh 6 \
detect_anomalies

There should only be a single telnet configuration, and subsequent instances will override previously set values.
Configuration
1. ports { [< ... >]}
This is how the user configures which ports to decode as telnet traffic. SSH tunnels cannot be decoded, so adding
port 22 will only yield false positives. Typically port 23 will be included.
2. normalize
This option tells the preprocessor to normalize the telnet traffic by eliminating the telnet escape sequences. It
functions similarly to its predecessor, the telnet decode preprocessor. Rules written with ’raw’ content options
will ignore the normailzed buffer that is created when this option is in use.
3. ayt attack thresh < number >
This option causes the preprocessor to alert when the number of consecutive telnet Are You There (AYT)
commands reaches the number specified. It is only applicable when the mode is stateful.
4. detect anomalies
In order to support certain options, Telnet supports subnegotiation. Per the Telnet RFC, subnegotiation begins
with SB (subnegotiation begin) and must end with an SE (subnegotiation end). However, certain implementations of Telnet servers will ignore the SB without a cooresponding SE. This is anomalous behavior which could
be an evasion case. Being that FTP uses the Telnet protocol on the control connection, it is also susceptible to
this behavior. The detect anomalies option enables alerting on Telnet SB without the corresponding SE.
Example Telnet Configuration
preprocessor ftp_telnet_protocol: \
telnet ports { 23 } normalize ayt_attack_thresh 6
FTP Server Configuration
There are two types of FTP server configurations: default and by IP address.
Default This configuration supplies the default server configuration for any FTP server that is not individually configured. Most of your FTP servers will most likely end up using the default configuration.

Example Default FTP Server Configuration
preprocessor ftp_telnet_protocol: \
ftp server default ports { 21 }
Refer to 60 for the list of options set in default ftp server configuration.
Configuration by IP Address This format is very similar to “default”, the only difference being that specific IPs
can be configured.
Example IP specific FTP Server Configuration
preprocessor _telnet_protocol: \
ftp server 10.1.1.1 ports { 21 } ftp_cmds { XPWD XCWD }
FTP Server Configuration Options
1. ports { [< ... >]}
This is how the user configures which ports to decode as FTP command channel traffic. Typically port 21 will
be included.
2. print cmds
During initialization, this option causes the preprocessor to print the configuration for each of the FTP commands
for this server.
3. ftp cmds {cmd[cmd]}
The preprocessor is configured to alert when it sees an FTP command that is not allowed by the server.
This option specifies a list of additional commands allowed by this server, outside of the default FTP command
set as specified in RFC 959. This may be used to allow the use of the ’X’ commands identified in RFC 775, as
well as any additional commands as needed.
For example:
ftp_cmds { XPWD XCWD XCUP XMKD XRMD }
4. def max param len
This specifies the default maximum allowed parameter length for an FTP command. It can be used as a basic
buffer overflow detection.
5. alt max param len {cmd[cmd]}
This specifies the maximum allowed parameter length for the specified FTP command(s). It can be used as a
more specific buffer overflow detection. For example the USER command – usernames may be no longer than
16 bytes, so the appropriate configuration would be:
alt_max_param_len 16 { USER }
6. chk str fmt {cmd[cmd]}
This option causes a check for string format attacks in the specified commands.
7. cmd validity cmd < fmt >
This option specifies the valid format for parameters of a given command.
fmt must be enclosed in <>’s and may contain the following:

Value
int
number
char
date

string
host port
long host port
extended host port
{}, |
{}, []

Description
Parameter must be an integer
Parameter must be an integer between 1 and 255
Parameter must be a single character, one of
Parameter follows format specified, where:
n
Number
C
Character
[]
optional format enclosed
|
OR
{}
choice of options
. + - literal
Parameter is a string (effectively unrestricted)
Parameter must be a host/port specified, per RFC 959
Parameter must be a long host port specified, per RFC
1639
Parameter must be an extended host port specified, per
RFC 2428
One of choices enclosed within, separated by |
One of the choices enclosed within {}, optional value
enclosed within []

Examples of the cmd validity option are shown below. These examples are the default checks, per RFC 959 and
others performed by the preprocessor.
cmd_validity
cmd_validity
cmd_validity
cmd_validity
cmd_validity

MODE
STRU
ALLO
TYPE
PORT

< int [ char R int ] >
< { char AE [ char NTC ] | char I | char L [ number ] } >
< host_port >

A cmd validity line can be used to override these defaults and/or add a check for other commands.
# This allows additional modes, including mode Z which allows for
# zip-style compression.
cmd_validity MODE < char ASBCZ >
# Allow for a date in the MDTM command.
cmd_validity MDTM < [ date nnnnnnnnnnnnnn[.n[n[n]]] ] string >
MDTM is an off case that is worth discussing. While not part of an established standard, certain FTP servers accept MDTM commands that set the modification time on a file. The most common among servers that do, accept
a format using YYYYMMDDHHmmss[.uuu]. Some others accept a format using YYYYMMDDHHmmss[+—]TZ format. The example above is for the first case (time format as specified in http://www.ietf.org/internetdrafts/draft-ietf-ftpext-mlst-16.txt)
To check validity for a server that uses the TZ format, use the following:
cmd_validity MDTM < [ date nnnnnnnnnnnnnn[{+|-}n[n]] ] string >
8. telnet cmds
This option turns on detection and alerting when telnet escape sequences are seen on the FTP command channel.
Injection of telnet escape sequences could be used as an evasion attempt on an FTP command channel.
9. ignore telnet erase cmds
This option allows Snort to ignore telnet escape sequences for erase character (TNC EAC) and erase line (TNC
EAL) when normalizing FTP command channel. Some FTP servers do not process those telnet escape sequences.
59

10. data chan
This option causes the rest of snort (rules, other preprocessors) to ignore FTP data channel connections. Using
this option means that NO INSPECTION other than TCP state will be performed on FTP data transfers. It
can be used to improve performance, especially with large file transfers from a trusted source. If your rule set
includes virus-type rules, it is recommended that this option not be used.
Use of the ”data chan” option is deprecated in favor of the ”ignore data chan” option. ”data chan” will be
removed in a future release.
11. ignore data chan
This option causes the rest of Snort (rules, other preprocessors) to ignore FTP data channel connections. Setting
this option to ”yes” means that NO INSPECTION other than TCP state will be performed on FTP data transfers.
It can be used to improve performance, especially with large file transfers from a trusted source. If your rule set
includes virus-type rules, it is recommended that this option not be used.
FTP Server Base Configuration Options
The base FTP server configuration is as follows. Options specified in the configuration file will modify this set of
options. FTP commands are added to the set of allowed commands. The other options will override those in the base
configuration.
def_max_param_len 100
ftp_cmds { USER PASS ACCT CWD CDUP SMNT
QUIT REIN TYPE STRU MODE RETR
STOR STOU APPE ALLO REST RNFR
RNTO ABOR DELE RMD MKD PWD LIST
NLST SITE SYST STAT HELP NOOP }
ftp_cmds { AUTH ADAT PROT PBSZ CONF ENC }
ftp_cmds { PORT PASV LPRT LPSV EPRT EPSV }
ftp_cmds { FEAT OPTS }
ftp_cmds { MDTM REST SIZE MLST MLSD }
alt_max_param_len 0 { CDUP QUIT REIN PASV STOU ABOR PWD SYST NOOP }
cmd_validity MODE < char SBC >
cmd_validity STRU < char FRPO [ string ] >
cmd_validity ALLO < int [ char R int ] >
cmd_validity TYPE < { char AE [ char NTC ] | char I | char L [ number ] } >
cmd_validity PORT < host_port >
cmd_validity LPRT < long_host_port >
cmd_validity EPRT < extd_host_port >
cmd_validity EPSV < [ { ’1’ | ’2’ | ’ALL’ } ] >
FTP Client Configuration
Similar to the FTP Server configuration, the FTP client configurations has two types: default, and by IP address.
Default This configuration supplies the default client configuration for any FTP client that is not individually configured. Most of your FTP clients will most likely end up using the default configuration.
Example Default FTP Client Configuration
preprocessor ftp_telnet_protocol: \
ftp client default bounce no max_resp_len 200

Configuration by IP Address This format is very similar to “default”, the only difference being that specific IPs
can be configured.
Example IP specific FTP Client Configuration
preprocessor ftp_telnet_protocol: \
ftp client 10.1.1.1 bounce yes max_resp_len 500
FTP Client Configuration Options
1. max resp len
This specifies the maximum allowed response length to an FTP command accepted by the client. It can be used
as a basic buffer overflow detection.
2. bounce
This option turns on detection and alerting of FTP bounce attacks. An FTP bounce attack occurs when the FTP
PORT command is issued and the specified host does not match the host of the client.
3. bounce to < CIDR,[port|portlow,porthi] >
When the bounce option is turned on, this allows the PORT command to use the IP address (in CIDR format) and
port (or inclusive port range) without generating an alert. It can be used to deal with proxied FTP connections
where the FTP data channel is different from the client.
A few examples:
• Allow bounces to 192.162.1.1 port 20020 – ie, the use of PORT 192,168,1,1,78,52.
bounce_to { 192.168.1.1,20020 }
• Allow bounces to 192.162.1.1 ports 20020 through 20040 – ie, the use of PORT 192,168,1,1,78,xx,
where xx is 52 through 72 inclusive.
bounce_to { 192.168.1.1,20020,20040 }
• Allow bounces to 192.162.1.1 port 20020 and 192.168.1.2 port 20030.
bounce_to { 192.168.1.1,20020 192.168.1.2,20030 }
4. telnet cmds
This option turns on detection and alerting when telnet escape sequences are seen on the FTP command channel.
Injection of telnet escape sequences could be used as an evasion attempt on an FTP command channel.
5. ignore telnet erase cmds
This option allows Snort to ignore telnet escape sequences for erase character (TNC EAC) and erase line (TNC
EAL) when normalizing FTP command channel. Some FTP clients do not process those telnet escape sequences.
Examples/Default Configuration from snort.conf
preprocessor ftp_telnet: \
global \
encrypted_traffic yes \
inspection_type stateful
preprocessor ftp_telnet_protocol:\
telnet \
normalize \
ayt_attack_thresh 200

#
#
#
#
#

This is consistent with the FTP rules as of 18 Sept 2004.
Set CWD to allow parameter length of 200
MODE has an additional mode of Z (compressed)
Check for string formats in USER & PASS commands
Check MDTM commands that set modification time on the file.

preprocessor ftp_telnet_protocol: \
ftp server default \
def_max_param_len 100 \
alt_max_param_len 200 { CWD } \
cmd_validity MODE < char ASBCZ > \
cmd_validity MDTM < [ date nnnnnnnnnnnnnn[.n[n[n]]] ] string > \
chk_str_fmt { USER PASS RNFR RNTO SITE MKD } \
telnet_cmds yes \
ignore_data_chan yes
preprocessor ftp_telnet_protocol: \
ftp client default \
max_resp_len 256 \
bounce yes \
telnet_cmds yes

2.2.9 SSH
The SSH preprocessor detects the following exploits: Challenge-Response Buffer Overflow, CRC 32, Secure CRT,
and the Protocol Mismatch exploit.
Both Challenge-Response Overflow and CRC 32 attacks occur after the key exchange, and are therefore encrypted.
Both attacks involve sending a large payload (20kb+) to the server immediately after the authentication challenge. To
detect the attacks, the SSH preprocessor counts the number of bytes transmitted to the server. If those bytes exceed a
predefined limit within a predefined number of packets, an alert is generated. Since the Challenge-Response Overflow
only effects SSHv2 and CRC 32 only effects SSHv1, the SSH version string exchange is used to distinguish the attacks.
The Secure CRT and protocol mismatch exploits are observable before the key exchange.
Configuration
By default, all alerts are disabled and the preprocessor checks traffic on port 22.
The available configuration options are described below.
1. server ports { [< ... >]}
This option specifies which ports the SSH preprocessor should inspect traffic to.
2. max encrypted packets < number >
The number of encrypted packets that Snort will inspect before ignoring a given SSH session. The SSH vulnerabilities that Snort can detect all happen at the very beginning of an SSH session. Once max encrypted packets
packets have been seen, Snort ignores the session to increase performance.
3. max client bytes < number >
The number of unanswered bytes allowed to be transferred before alerting on Challenge-Response Overflow or
CRC 32. This number must be hit before max encrypted packets packets are sent, or else Snort will ignore the
traffic.
4. max server version len < number >

The maximum number of bytes allowed in the SSH server version string before alerting on the Secure CRT
server version string overflow.
5. autodetect
Attempt to automatically detect SSH.
6. enable respoverflow
Enables checking for the Challenge-Response Overflow exploit.
7. enable ssh1crc32
Enables checking for the CRC 32 exploit.
8. enable srvoverflow
Enables checking for the Secure CRT exploit.
9. enable protomismatch
Enables checking for the Protocol Mismatch exploit.
10. enable badmsgdir
Enable alerts for traffic flowing the wrong direction. For instance, if the presumed server generates client traffic,
or if a client generates server traffic.
11. enable paysize
Enables alerts for invalid payload sizes.
12. enable recognition
Enable alerts for non-SSH traffic on SSH ports.
The SSH preprocessor should work by default. After max encrypted packets is reached, the preprocessor will stop
processing traffic for a given session. If Challenge-Respone Overflow or CRC 32 false positive, try increasing the
number of required client bytes with max client bytes.
Example Configuration from snort.conf
Looks for attacks on SSH server port 22. Alerts at 19600 unacknowledged bytes within 20 encrypted packets for the
Challenge-Response Overflow/CRC32 exploits.
preprocessor ssh: \
server_ports { 22 } \
max_client_bytes 19600 \
max_encrypted_packets 20 \
enable_respoverflow \
enable_ssh1crc32

2.2.10 DCE/RPC
The dcerpc preprocessor detects and decodes SMB and DCE/RPC traffic. It is primarily interested in DCE/RPC
requests, and only decodes SMB to get to the potential DCE/RPC requests carried by SMB.
Currently, the preprocessor only handles desegmentation (at SMB and TCP layers) and defragmentation of DCE/RPC.
Snort rules can be evaded by using both types of fragmentation. With the preprocessor enabled, the rules are given
reassembled DCE/RPC data to examine.
At the SMB layer, only segmentation using WriteAndX is currently reassembled. Other methods will be handled in
future versions of the preprocessor.

Autodetection of SMB is done by looking for ”\xFFSMB” at the start of the SMB data, as well as checking the NetBIOS
header (which is always present for SMB) for the type ”Session Message”.
Autodetection of DCE/RPC is not as reliable. Currently, two bytes are checked in the packet. Assuming that the data
is a DCE/RPC header, one byte is checked for DCE/RPC version 5 and another for a DCE/RPC PDU type of Request.
If both match, the preprocessor proceeds with the assumption that it is looking at DCE/RPC data. If subsequent checks
are nonsensical, it ends processing.
Configuration
The proprocessor has several optional configuration options. They are described below:
• autodetect
In addition to configured ports, try to autodetect DCE/RPC sessions. Note that DCE/RPC can run on practically
any port in addition to the more common ports. This option is not configured by default.
• ports smb { [ <...>] }
Ports that the preprocessor monitors for SMB traffic. Default are ports 139 and 445.
• ports dcerpc { [ <...>] }
Ports that the preprocessor monitors for DCE/RPC over TCP traffic. Default is port 135.
• disable smb frag
Do not do SMB desegmentation. Unless you are experiencing severe performance issues, this option should not
be configured as SMB segmentation provides for an easy evasion opportunity. This option is not configured by
default.
• disable dcerpc frag
Do not do DCE/RPC defragmentation. Unless you are experiencing severe performance issues, this option
should not be configured as DCE/RPC fragmentation provides for an easy evasion opportunity. This option is
not configured by default.
• max frag size
Maximum DCE/RPC fragment size to put in defragmentation buffer, in bytes. Default is 3000 bytes.
• memcap
Maximum amount of memory available to the DCE/RPC preprocessor for desegmentation and defragmentation,
in kilobytes. Default is 100000 kilobytes.
• alert memcap
Alert if memcap is exceeded. This option is not configured by default.
• reassemble increment
This option specifies how often the preprocessor should create a reassembled packet to send to the detection
engine with the data that’s been accrued in the segmentation and fragmentation reassembly buffers, before the
final desegmentation or defragmentation of the DCE/RPC request takes place. This will potentially catch an
attack earlier and is useful if in inline mode. Since the preprocessor looks at TCP reassembled packets (to avoid

TCP overlaps and segmentation evasions), the last packet of an attack using DCE/RPC segmented/fragmented
evasion techniques may have already gone through before the preprocessor looks at it, so looking at the data
early will likely catch the attack before all of the exploit data has gone through. Note, however, that in using
this option, Snort will potentially take a performance hit. Not recommended if Snort is running in passive
mode as it’s not really needed. The argument to the option specifies how often the preprocessor should create
a reassembled packet if there is data in the segmentation/fragmentation buffers. If not specified, this option is
disabled. A value of 0 will in effect disable this option as well.
Configuration Examples
In addition to defaults, autodetect SMB and DCE/RPC sessions on non-configured ports. Don’t do desegmentation on
SMB writes. Truncate DCE/RPC fragment if greater than 4000 bytes.
preprocessor dcerpc: \
autodetect \
disable_smb_frag \
max_frag_size 4000
In addition to defaults, don’t do DCE/RPC defragmentation. Set memory cap for desegmentation/defragmentation to
50,000 kilobytes. (Since no DCE/RPC defragmentation will be done the memory cap will only apply to desegmentation.)
preprocessor dcerpc: \
disable_dcerpc_frag \
memcap 50000
In addition to the defaults, detect on DCE/RPC (or TCP) ports 135 and 2103 (overrides default). Set memory cap for
desegmentation/defragmentation to 200,000 kilobytes. Create a reassembly packet every time through the preprocessor
if there is data in the desegmentation/defragmentation buffers.
preprocessor dcerpc: \
ports dcerpc { 135 2103 } \
memcap 200000 \
reassemble_increment 1
Default Configuration
If no options are given to the preprocessor, the default configuration will look like:
preprocessor dcerpc: \
ports smb { 139 445 } \
ports dcerpc { 135 } \
max_frag_size 3000 \
memcap 100000 \
reassemble_increment 0
Preprocessor Events
There is currently only one alert, which is triggered when the preprocessor has reached the memcap limit for memory
allocation. The alert is gid 130, sid 1.
Note
At the current time, there is not much to do with the dcerpc preprocessor other than turn it on and let it reassemble
fragmented DCE/RPC packets.
65

2.2.11 DNS
The DNS preprocessor decodes DNS Responses and can detect the following exploits: DNS Client RData Overflow,
Obsolete Record Types, and Experimental Record Types.
DNS looks at DNS Response traffic over UDP and TCP and it requires Stream preprocessor to be enabled for TCP
decoding.
Configuration
By default, all alerts are disabled and the preprocessor checks traffic on port 53.
The available configuration options are described below.
1. ports { [< ... >]}
This option specifies the source ports that the DNS preprocessor should inspect traffic.
2. enable obsolete types
Alert on Obsolete (per RFC 1035) Record Types
3. enable experimental types
Alert on Experimental (per RFC 1035) Record Types
4. enable rdata overflow
Check for DNS Client RData TXT Overflow
The DNS preprocessor does nothing if none of the 3 vulnerabilities it checks for are enabled. It will not operate on
TCP sessions picked up midstream, and it will cease operation on a session if it loses state because of missing data
(dropped packets).
Examples/Default Configuration from snort.conf
Looks for traffic on DNS server port 53. Check for the DNS Client RData overflow vulnerability. Do not alert on
obsolete or experimental RData record types.
preprocessor dns: \
ports { 53 } \
enable_rdata_overflow

2.2.12 SSL/TLS
Encrypted traffic should be ignored by Snort for both performance reasons and to reduce false positives. The SSL
Dynamic Preprocessor (SSLPP) decodes SSL and TLS traffic and optionally determines if and when Snort should
stop inspection of it.
Typically, SSL is used over port 443 as HTTPS. By enabling the SSLPP to inspect port 443 and enabling the noinspect encrypted option, only the SSL handshake of each connection will be inspected. Once the traffic is determined
to be encrypted, no further inspection of the data on the connection is made.
By default, SSLPP looks for a handshake followed by encrypted traffic traveling to both sides. If one side responds
with an indication that something has failed, such as the handshake, the session is not marked as encrypted. Verifying
that faultless encrypted traffic is sent from both endpoints ensures two things: the last client-side handshake packet
was not crafted to evade Snort, and that the traffic is legitimately encrypted.
In some cases, especially when packets may be missed, the only observed response from one endpoint will be TCP
ACKs. Therefore, if a user knows that server-side encrypted data can be trusted to mark the session as encrypted, the
user should use the ’trustservers’ option, documented below.
66

Configuration
1. ports { [< ... >]}
This option specifies which ports SSLPP will inspect traffic on.
By default, SSLPP watches the following ports:
• 443 HTTPS
• 465 SMTPS
• 563 NNTPS
• 636 LDAPS
• 989 FTPS
• 992 TelnetS
• 993 IMAPS
• 994 IRCS
• 995 POPS
2. noinspect encrypted
Disable inspection on traffic that is encrypted. Default is off.
3. trustservers
Disables the requirement that application (encrypted) data must be observed on both sides of the session before
a session is marked encrypted. Use this option for slightly better performance if you trust that your servers are
not compromised. This requires the noinspect encrypted option to be useful. Default is off.
Examples/Default Configuration from snort.conf
Enables the SSL preprocessor and tells it to disable inspection on encrypted traffic.
preprocessor ssl: noinspect_encrypted

2.2.13 ARP Spoof Preprocessor
The ARP spoof preprocessor decodes ARP packets and detects ARP attacks, unicast ARP requests, and inconsistent
Ethernet to IP mapping.
When no arguments are specified to arpspoof, the preprocessor inspects Ethernet addresses and the addresses in the
ARP packets. When inconsistency occurs, an alert with GID 112 and SID 2 or 3 is generated.
When ”-unicast” is specified as the argument of arpspoof, the preprocessor checks for unicast ARP requests. An
alert with GID 112 and SID 1 will be generated if a unicast ARP request is detected.
Specify a pair of IP and hardware address as the argument to arpspoof detect host. The host with the IP address
should be on the same layer 2 segment as Snort is. Specify one host IP MAC combo per line. The preprocessor will
use this list when detecting ARP cache overwrite attacks. Alert SID 4 is used in this case.
Format
preprocessor arpspoof[: -unicast]
preprocessor arpspoof_detect_host: ip mac

Option
ip
mac

Description
IP address.
The Ethernet address corresponding to the preceding IP.

Example Configuration
The first example configuration does neither unicast detection nor ARP mapping monitoring. The preprosessor merely
looks for Ethernet address inconsistencies.
preprocessor arpspoof
The next example configuration does not do unicast detection but monitors ARP mapping for hosts 192.168.40.1 and
192.168.40.2.
preprocessor arpspoof
preprocessor arpspoof_detect_host: 192.168.40.1 f0:0f:00:f0:0f:00
preprocessor arpspoof_detect_host: 192.168.40.2 f0:0f:00:f0:0f:01
The third example configuration has unicast detection enabled.
preprocessor arpspoof: -unicast
preprocessor arpspoof_detect_host: 192.168.40.1 f0:0f:00:f0:0f:00
preprocessor arpspoof_detect_host: 192.168.40.2 f0:0f:00:f0:0f:01

2.2.14 DCE/RPC 2 Preprocessor
The main purpose of the preprocessor is to perform SMB desegmentation and DCE/RPC defragmentation to avoid
rule evasion using these techniques. SMB desegmentation is performed for the following commands that can be
used to transport DCE/RPC requests and responses: Write, Write Block Raw, Write and Close, Write AndX,
Transaction, Transaction Secondary, Read, Read Block Raw and Read AndX. The following transports are supported for DCE/RPC: SMB, TCP, UDP and RPC over HTTP v.1 proxy and server. New rule options have been implemented to improve performance, reduce false positives and reduce the count and complexity of DCE/RPC based
rules.
Dependency Requirements
For proper functioning of the preprocessor:
• The dcerpc preprocessor (the initial iteration) must be disabled.
• Stream session tracking must be enabled, i.e. stream5. The preprocessor requires a session tracker to keep its
data.
• Stream reassembly must be performed for TCP sessions. If it is decided that a session is SMB or DCE/RPC, either through configured ports, servers or autodetecting, the dcerpc2 preprocessor will enable stream reassembly
for that session if necessary.
• IP defragmentation should be enabled, i.e. the frag3 preprocessor should be enabled and configured.

Target Based
There are enough important differences between Windows and Samba versions that a target based approach has been
implemented. Some important differences:
Named pipe instance tracking
A combination of valid login handle or UID, share handle or TID and file/named pipe handle or FID must be
used to write data to a named pipe. The binding between these is dependent on OS/software version.
Samba 3.0.22 and earlier
Any valid UID and TID, along with a valid FID can be used to make a request, however, if the TID
used in creating the FID is deleted (via a tree disconnect), the FID that was created using this TID
becomes invalid, i.e. no more requests can be written to that named pipe instance.
Samba greater than 3.0.22
Any valid TID, along with a valid FID can be used to make a request. However, only the UID used
in opening the named pipe can be used to make a request using the FID handle to the named pipe
instance. If the TID used to create the FID is deleted (via a tree disconnect), the FID that was created
using this TID becomes invalid, i.e. no more requests can be written to that named pipe instance. If
the UID used to create the named pipe instance is deleted (via a Logoff AndX), since it is necessary
in making a request to the named pipe, the FID becomes invalid.
Windows 2003
Windows XP
Windows Vista
These Windows versions require strict binding between the UID, TID and FID used to make a request
to a named pipe instance. Both the UID and TID used to open the named pipe instance must be
used when writing data to the same named pipe instance. Therefore, deleting either the UID or TID
invalidates the FID.
Windows 2000
Windows 2000 is interesting in that the first request to a named pipe must use the same binding as that
of the other Windows versions. However, requests after that follow the same binding as Samba 3.0.22
and earlier, i.e. no binding. It also follows Samba greater than 3.0.22 in that deleting the UID or TID
used to create the named pipe instance also invalidates it.
Accepted SMB commands
Samba in particular does not recognize certain commands under an IPC$ tree.
Samba (all versions)
Under an IPC$ tree, does not accept:
Open
Write And Close
Read
Read Block Raw
Write Block Raw
Windows (all versions)
Accepts all of the above commands under an IPC$ tree.
AndX command chaining

Windows is very strict in what command combinations it allows to be chained. Samba, on the other hand, is
very lax and allows some nonsensical combinations, e.g. multiple logins and tree connects (only one place to
return handles for these), login/logoff and tree connect/tree disconnect. Ultimately, we don’t want to keep track
of data that the server won’t accept. An evasion possibility would be accepting a fragment in a request that the
server won’t accept that gets sandwiched between an exploit.
Transaction tracking
The differences between a Transaction request and using one of the Write* commands to write data to a
named pipe are that (1) a Transaction performs the operations of a write and a read from the named pipe,
whereas in using the Write* commands, the client has to explicitly send one of the Read* requests to tell the
server to send the response and (2) a Transaction request is not written to the named pipe until all of the data is
received (via potential Transaction Secondary requests) whereas with the Write* commands, data is written
to the named pipe as it is received by the server. Multiple Transaction requests can be made simultaneously to
the same named pipe. These requests can also be segmented with Transaction Secondary commands. What
distinguishes them (when the same named pipe is being written to, i.e. having the same FID) are fields in the
SMB header representing a process id (PID) and multiplex id (MID). The PID represents the process this request
is a part of. An MID represents different sub-processes within a process (or under a PID). Segments for each
”thread” are stored separately and written to the named pipe when all segments are received. It is necessary to
track this so as not to munge these requests together (which would be a potential evasion opportunity).
Windows (all versions)
Uses a combination of PID and MID to define a ”thread”.
Samba (all versions)
Uses just the MID to define a ”thread”.
Multliple Bind requests
A Bind request is the first request that must be made in a connection-oriented DCE/RPC session in order to
specify the interface/interfaces that one wants to communicate with.
Windows (all versions)
For all of the Windows versions, only one Bind can ever be made on a session whether or not it
succeeds or fails. Any binding after that must use the Alter Context request. If another Bind is
made, all previous interface bindings are invalidated.
Samba 3.0.20 and earlier
Any amount of Bind requests can be made.
Samba later than 3.0.20
Another Bind request can be made if the first failed and no interfaces were successfully bound to. If
a Bind after a successful Bind is made, all previous interface bindings are invalidated.
DCE/RPC Fragmented requests - Context ID
Each fragment in a fragmented request carries the context id of the bound interface it wants to make the request
to.
Windows (all versions)
The context id that is ultimately used for the request is contained in the first fragment. The context id
field in any other fragment can contain any value.
Samba (all versions)
The context id that is ultimately used for the request is contained in the last fragment. The context id
field in any other fragment can contain any value.
DCE/RPC Fragmented requests - Operation number
70

Each fragment in a fragmented request carries an operation number (opnum) which is more or less a handle to
a function offered by the interface.
Samba (all versions)
Windows 2000
Windows 2003
Windows XP
The opnum that is ultimately used for the request is contained in the last fragment. The opnum field
in any other fragment can contain any value.
Windows Vista
The opnum that is ultimately used for the request is contained in the first fragment. The opnum field
in any other fragment can contain any value.
DCE/RPC Stub data byte order
The byte order of the stub data is determined differently for Windows and Samba.
Windows (all versions)
The byte order of the stub data is that which was used in the Bind request.
Samba (all versions)
The byte order of the stub data is that which is used in the request carrying the stub data.
Configuration
The dcerpc2 preprocessor has a global configuration and one or more server configurations. The global preprocessor
configuration name is dcerpc2 and the server preprocessor configuration name is dcerpc2 server.
Global Configuration
preprocessor dcerpc2
The global dcerpc2 configuration is required. Only one global dcerpc2 configuration can be specified.
Option syntax
Option
memcap
disable defrag
max frag len
events
reassemble threshold
memcap
max-frag-len
events
pseudo-event
event-list
event
re-thresh

=
=
=
=
=
=
=

Argument

NONE

Required
NO
NO
NO
NO
NO

Option explanations
memcap
71

Default
memcap 102400
OFF
OFF
events [smb, co, cl]
OFF

Specifies the maximum amount of run-time memory that can be allocated. Run-time memory includes any
memory allocated after configuration. Default is 100 MB.
disable defrag
Tells the preprocessor not to do DCE/RPC defragmentation. Default is to do defragmentation.
max frag len
Specifies the maximum fragment size that will be added to the defragmention module. If a fragment is
greater than this size, it is truncated before being added to the defragmentation module. Default is not set.
events
Specifies the classes of events to enable. (See Events section for an enumeration and explanation of events.)
memcap
Only one event. If the memcap is reached or exceeded, alert.
smb
Alert on events related to SMB processing.
co
Stands for connection-oriented DCE/RPC. Alert on events related to connection-oriented DCE/RPC
processing.
cl
Stands for connectionless DCE/RPC. Alert on events related to connectionless DCE/RPC processing. Defaults are smb, co and cl.
reassemble threshold
Specifies a minimum number of bytes in the DCE/RPC desegmentation and defragmentation buffers before
creating a reassembly packet to send to the detection engine. This option is useful in inline mode so as to
potentially catch an exploit early before full defragmentation is done. A value of 0 supplied as an argument
to this option will, in effect, disable this option. Default is disabled.
Option examples
memcap 30000
max_frag_len 16840
events none
events all
events smb
events co
events [co]
events [smb, co]
events [memcap, smb, co, cl]
reassemble_threshold 500

Configuration examples
preprocessor
preprocessor
preprocessor
preprocessor
preprocessor
preprocessor

dcerpc2
dcerpc2:
dcerpc2:
dcerpc2:
dcerpc2:
dcerpc2:

memcap 500000
max_frag_len 16840, memcap 300000, events smb
memcap 50000, events [memcap, smb, co, cl], max_frag_len 14440
disable_defrag, events [memcap, smb]
reassemble_threshold 500

Default global configuration
preprocessor dcerpc2: memcap 102400, events [smb, co, cl]

Server Configuration
72

preprocessor dcerpc2_server
The dcerpc2 server configuration is optional. A dcerpc2 server configuration must start with default or net
options. The default and net options are mutually exclusive. At most one default configuration can be specified. If
no default configuration is specified, default values will be used for the default configuration. Zero or more net
configurations can be specified. For any dcerpc2 server configuration, if non-required options are not specified, the
defaults will be used. When processing DCE/RPC traffic, the default configuration is used if no net configurations
match. If a net configuration matches, it will override the default configuration. A net configuration matches if the
packet’s server IP address matches an IP address or net specified in the net configuration. The net option supports
IPv6 addresses. Note that port and ip variables defined in snort.conf CANNOT be used.
Option syntax
Option
default
net
policy
detect

Argument
NONE

Required
YES
YES
NO
NO

autodetect

NONE

NO
NO

no autodetect http proxy ports
smb invalid shares
smb max chain
net
ip-list
ip
ip-addr
ip4-addr
ip6-addr
prefix
netmask
policy

=
=
=
=
=
=
=
=
=

detect
detect-list
detect-opt

=
=
=

transport

port-list
port-item
port-range
port
shares
share-list
share
word
var-word
max-chain

=
=
=
=
=
=
=
=
=
=

Default
NONE
NONE
policy WinXP
detect [smb [139,445], tcp 135,
udp 135, rpc-over-http-server
593]
autodetect [tcp 1025:, udp 1025:,
rpc-over-http-server 1025:]
DISABLED (The preprocessor autodetects
on all proxy ports by default)
NONE
smb max chain 3

Because the Snort main parser treats ’$’ as the start of a variable and tries to expand it, shares with ’$’ must be
enclosed quotes.
Option explanations
default
Specifies that this configuration is for the default server configuration.
73

net
Specifies that this configuration is an IP or net specific configuration. The configuration will only apply to
the IP addresses and nets supplied as an argument.
policy
Specifies the target-based policy to use when processing. Default is ”WinXP”.
detect
Specifies the DCE/RPC transport and server ports that should be detected on for the transport. Defaults
are ports 139 and 445 for SMB, 135 for TCP and UDP, 593 for RPC over HTTP server and 80 for RPC
over HTTP proxy.
autodetect
Specifies the DCE/RPC transport and server ports that the preprocessor should attempt to autodetect on
for the transport. The autodetect ports are only queried if no detect transport/ports match the packet. The
order in which the preprocessor will attempt to autodetect will be - TCP/UDP, RPC over HTTP server,
RPC over HTTP proxy and lastly SMB. Note that most dynamic DCE/RPC ports are above 1024 and ride
directly over TCP or UDP. It would be very uncommon to see SMB on anything other than ports 139 and
445. Defaults are 1025-65535 for TCP, UDP and RPC over HTTP server.
no autodetect http proxy ports
By default, the preprocessor will always attempt to autodetect for ports specified in the detect configuration
for rpc-over-http-proxy. This is because the proxy is likely a web server and the preprocessor should not
look at all web traffic. This option is useful if the RPC over HTTP proxy configured with the detect option
is only used to proxy DCE/RPC traffic. Default is to autodetect on RPC over HTTP proxy detect ports.
smb invalid shares
Specifies SMB shares that the preprocessor should alert on if an attempt is made to connect to them via a
Tree Connect or Tree Connect AndX. Default is empty.
smb max chain
Specifies the maximum amount of AndX command chaining that is allowed before an alert is generated.
Default maximum is 3 chained commands. A value of 0 disables this option.
Option examples
net 192.168.0.10
net 192.168.0.0/24
net [192.168.0.0/24]
net 192.168.0.0/255.255.255.0
net feab:45b3:ab92:8ac4:d322:007f:e5aa:7845
net feab:45b3:ab92:8ac4:d322:007f:e5aa:7845/128
net feab:45b3::/32
net [192.168.0.10, feab:45b3::/32]
net [192.168.0.0/24, feab:45b3:ab92:8ac4:d322:007f:e5aa:7845]
policy Win2000
policy Samba-3.0.22
detect none
detect smb
detect [smb]
detect smb 445
detect [smb 445]
detect smb [139,445]
detect [smb [139,445]]
detect [smb, tcp]
detect [smb 139, tcp [135,2103]]
detect [smb [139,445], tcp 135, udp 135, rpc-over-http-server [593,6002:6004]]

autodetect none
autodetect tcp
autodetect [tcp]
autodetect tcp 2025:
autodetect [tcp 2025:]
autodetect tcp [2025:3001,3003:]
autodetect [tcp [2025:3001,3003:]]
autodetect [tcp, udp]
autodetect [tcp 2025:, udp 2025:]
autodetect [tcp 2025:, udp, rpc-over-http-server [1025:6001,6005:]]
smb_invalid_shares private
smb_invalid_shares "private"
smb_invalid_shares "C$"
smb_invalid_shares [private, "C$"]
smb_invalid_shares ["private", "C$"]
smb_max_chain 1

Configuration examples
preprocessor dcerpc2_server: \
default
preprocessor dcerpc2_server: \
default, policy Win2000
preprocessor dcerpc2_server: \
default, policy Win2000, detect [smb, tcp], autodetect tcp 1025:, \
smb_invalid_shares ["C$", "D$", "ADMIN$"]
preprocessor dcerpc2_server: net 10.4.10.0/24, policy Win2000
preprocessor dcerpc2_server: \
net [10.4.10.0/24,feab:45b3::/126], policy WinVista, smb_max_chain 1
preprocessor dcerpc2_server: \
net [10.4.10.0/24,feab:45b3::/126], policy WinVista, \
detect [smb, tcp, rpc-over-http-proxy 8081],
autodetect [tcp, rpc-over-http-proxy [1025:6001,6005:]], \
smb_invalid_shares ["C$", "ADMIN$"], no_autodetect_http_proxy_ports
preprocessor dcerpc2_server: \
net [10.4.11.56,10.4.11.57], policy Samba, detect smb, autodetect none

Default server configuration
preprocessor dcerpc2_server: default, policy WinXP, \
detect [smb [139,445], tcp 135, udp 135, rpc-over-http-server 593], \
autodetect [tcp 1025:, udp 1025:, rpc-over-http-server 1025:], smb_max_chain 3

Complete dcerpc2 default configuration
preprocessor dcerpc2: \
memcap 102400, events [smb, co, cl]
preprocessor dcerpc2_server: \
default, policy WinXP, \
detect [smb [139,445], tcp 135, udp 135, rpc-over-http-server 593], \
autodetect [tcp 1025:, udp 1025:, rpc-over-http-server 1025:], smb_max_chain 3

Events
The preprocessor uses GID 133 to register events.
Memcap events

SID
1

Description
If the memory cap is reached and the preprocessor is configured to alert.

SMB events
SID
2

3
4
5

9
10
11
12
13
14

15
16

Description
An invalid NetBIOS Session Service type was specified in the header. Valid types are: Message,
Request (only from client), Positive Response (only from server), Negative Response
(only from server), Retarget Response (only from server) and Keep Alive.
An SMB message type was specified in the header. Either a request was made by the server or a
response was given by the client.
The SMB id does not equal \xffSMB. Note that since the preprocessor does not yet support
SMB2, id of \xfeSMB is turned away before an eventable point is reached.
The word count of the command header is invalid. SMB commands have pretty specific word
counts and if the preprocessor sees a command with a word count that doesn’t jive with that
command, the preprocessor will alert.
Some commands require a minimum number of bytes after the command header. If a command
requires this and the byte count is less than the minimum required byte count for that command,
the preprocessor will alert.
Some commands, especially the commands from the SMB Core implementation require a data
format field that specifies the kind of data that will be coming next. Some commands require a
specific format for the data. The preprocessor will alert if the format is not that which is expected
for that command.
Many SMB commands have a field containing an offset from the beginning of the SMB header to
where the data the command is carrying starts. If this offset puts us before data that has already
been processed or after the end of payload, the preprocessor will alert.
Some SMB commands, such as Transaction, have a field containing the total amount of data
to be transmitted. If this field is zero, the preprocessor will alert.
The preprocessor will alert if the NetBIOS Session Service length field contains a value less than
the size of an SMB header.
The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the
SMB command header to be decoded.
The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the
SMB command byte count specified in the command header.
The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the
SMB command data size specified in the command header.
The preprocessor will alert if the total data count specified in the SMB command header is less
than the data size specified in the SMB command header. (Total data count must always be
greater than or equal to current data size.)
The preprocessor will alert if the total amount of data sent in a transaction is greater than the total
data count specified in the SMB command header.
The preprocessor will alert if the byte count specified in the SMB command header is less than
the data size specified in the SMB command. (The byte count must always be greater than or
equal to the data size.)
Some of the Core Protocol commands (from the initial SMB implementation) require that the
byte count be some value greater than the data size exactly. The preprocessor will alert if the
byte count minus a predetermined amount based on the SMB command is not equal to the data
size.

20
21

For the Tree Connect command (and not the Tree Connect AndX command), the preprocessor
has to queue the requests up and wait for a server response to determine whether or not an IPC
share was successfully connected to (which is what the preprocessor is interested in). Unlike
the Tree Connect AndX response, there is no indication in the Tree Connect response as to
whether the share is IPC or not. There should be under normal circumstances no more than a few
pending tree connects at a time and the preprocessor will alert if this number is excessive.
After a client is done writing data using the Write* commands, it issues a Read* command to
the server to tell it to send a response to the data it has written. In this case the preprocessor
is concerned with the server response. The Read* request contains the file id associated with a
named pipe instance that the preprocessor will ultimately send the data to. The server response,
however, does not contain this file id, so it need to be queued with the request and dequeued with
the response. If multiple Read* requests are sent to the server, they are responded to in the order
they were sent. There should be under normal circumstances no more than a few pending Read*
requests at a time and the preprocessor will alert if this number is excessive.
The preprocessor will alert if the number of chained commands in a single request is greater than
or equal to the configured amount (default is 3).
With AndX command chaining it is possible to chain multiple Session Setup AndX commands
within the same request. There is, however, only one place in the SMB header to return a login
handle (or Uid). Windows does not allow this behavior, however Samba does. This is anomalous
behavior and the preprocessor will alert if it happens.
With AndX command chaining it is possible to chain multiple Tree Connect AndX commands
within the same request. There is, however, only one place in the SMB header to return a tree
handle (or Tid). Windows does not allow this behavior, however Samba does. This is anomalous
behavior and the preprocessor will alert if it happens.
When a Session Setup AndX request is sent to the server, the server responds (if the client
successfully authenticates) which a user id or login handle. This is used by the client in subsequent requests to indicate that it has authenticated. A Logoff AndX request is sent by the client
to indicate it wants to end the session and invalidate the login handle. With commands that are
chained after a Session Setup AndX request, the login handle returned by the server is used for
the subsequent chained commands. The combination of a Session Setup AndX command with
a chained Logoff AndX command, essentially logins in and logs off in the same request and is
anomalous behavior. The preprocessor will alert if it sees this.
A Tree Connect AndX command is used to connect to a share. The Tree Disconnect command is used to disconnect from that share. The combination of a Tree Connect AndX command with a chained Tree Disconnect command, essentially connects to a share and disconnects from the same share in the same request and is anomalous behavior. The preprocessor will
alert if it sees this.
An Open AndX or Nt Create AndX command is used to open/create a file or named pipe. (The
preprocessor is only interested in named pipes as this is where DCE/RPC requests are written to.)
The Close command is used to close that file or named pipe. The combination of a Open AndX
or Nt Create AndX command with a chained Close command, essentially opens and closes the
named pipe in the same request and is anomalous behavior. The preprocessor will alert if it sees
this.
The preprocessor will alert if it sees any of the invalid SMB shares configured. It looks for a
Tree Connect or Tree Connect AndX to the share.

Connection-oriented DCE/RPC events
SID
27
28

Description
The preprocessor will alert if the connection-oriented DCE/RPC major version contained in the
header is not equal to 5.
The preprocessor will alert if the connection-oriented DCE/RPC minor version contained in the
header is not equal to 0.
77

The preprocessor will alert if the connection-oriented DCE/RPC PDU type contained in the
header is not a valid PDU type.
The preprocessor will alert if the fragment length defined in the header is less than the size of the
header.
The preprocessor will alert if the remaining fragment length is less than the remaining packet
size.
The preprocessor will alert if in a Bind or Alter Context request, there are no context items
specified.
The preprocessor will alert if in a Bind or Alter Context request, there are no transfer syntaxes
to go with the requested interface.
The preprocessor will alert if a non-last fragment is less than the size of the negotiated maximum
fragment length. Most evasion techniques try to fragment the data as much as possible and
usually each fragment comes well below the negotiated transmit size.
The preprocessor will alert if a fragment is larger than the maximum negotiated fragment length.
The byte order of the request data is determined by the Bind in connection-oriented DCE/RPC
for Windows. It is anomalous behavior to attempt to change the byte order mid-session.
The call id for a set of fragments in a fragmented request should stay the same (it is incremented
for each complete request). The preprocessor will alert if it changes in a fragment mid-request.
The operation number specifies which function the request is calling on the bound interface. If a
request is fragmented, this number should stay the same for all fragments. The preprocessor will
alert if the opnum changes in a fragment mid-request.
The context id is a handle to a interface that was bound to. If a request if fragmented, this number
should stay the same for all fragments. The preprocessor will alert if the context id changes in a
fragment mid-request.

30
31
32
33
34

35
36
37
38

Connectionless DCE/RPC events
SID
40
41
42

Description
The preprocessor will alert if the connectionless DCE/RPC major version is not equal to 4.
The preprocessor will alert if the connectionless DCE/RPC pdu type is not a valid pdu type.
The preprocessor will alert if the packet data length is less than the size of the connectionless
header.
The preprocessor will alert if the sequence number uses in a request is the same or less than a
previously used sequence number on the session. In testing, wrapping the sequence number space
produces strange behavior from the server, so this should be considered anomalous behavior.

Rule Options
New rule options are supported by enabling the dcerpc2 preprocessor:
dce_iface
dce_opnum
dce_stub_data
New modifiers to existing byte test and byte jump rule options:
byte_test: dce
byte_jump: dce

dce iface
For DCE/RPC based rules it has been necessary to set flow-bits based on a client bind to a service to avoid
false positives. It is necessary for a client to bind to a service before being able to make a call to it. When a
client sends a bind request to the server, it can, however, specify one or more service interfaces to bind to. Each
interface is represented by a UUID. Each interface UUID is paired with a unique index (or context id) that future
requests can use to reference the service that the client is making a call to. The server will respond with the
interface UUIDs it accepts as valid and will allow the client to make requests to those services. When a client
makes a request, it will specify the context id so the server knows what service the client is making a request
to. Instead of using flow-bits, a rule can simply ask the preprocessor, using this rule option, whether or not the
client has bound to a specific interface UUID and whether or not this client request is making a request to it.
This can eliminate false positives where more than one service is bound to successfully since the preprocessor
can correlate the bind UUID to the context id used in the request. A DCE/RPC request can specify whether
numbers are represented as big endian or little endian. The representation of the interface UUID is different
depending on the endianness specified in the DCE/RPC previously requiring two rules - one for big endian and
one for little endian. The preprocessor eliminates the need for two rules by normalizing the UUID. An interface
contains a version. Some versions of an interface may not be vulnerable to a certain exploit. Also, a DCE/RPC
request can be broken up into 1 or more fragments. Flags (and a field in the connectionless header) are set in the
DCE/RPC header to indicate whether the fragment is the first, a middle or the last fragment. Many checks for
data in the DCE/RPC request are only relevant if the DCE/RPC request is a first fragment (or full request), since
subsequent fragments will contain data deeper into the DCE/RPC request. A rule which is looking for data,
say 5 bytes into the request (maybe it’s a length field), will be looking at the wrong data on a fragment other
than the first, since the beginning of subsequent fragments are already offset some length from the beginning of
the request. This can be a source of false positives in fragmented DCE/RPC traffic. By default it is reasonable
to only evaluate if the request is a first fragment (or full request). However, if the any frag option is used to
specify evaluating on all fragments.
Syntax
[ ’,’ ] [ ’,’ "any_frag" ]
uuid
hexlong
hexshort
hexbyte
operator
version

=
=
=
=
=
=

hexlong ’-’ hexshort ’-’ hexshort ’-’ 2hexbyte ’-’ 6hexbyte
4hexbyte
2hexbyte
2HEXDIGIT
’<’ | ’>’ | ’=’ | ’!’
0-65535

Examples
dce_iface:
dce_iface:
dce_iface:
dce_iface:

4b324fc8-1670-01d3-1278-5a47bf6ee188;
4b324fc8-1670-01d3-1278-5a47bf6ee188,<2;
4b324fc8-1670-01d3-1278-5a47bf6ee188,any_frag;
4b324fc8-1670-01d3-1278-5a47bf6ee188,=1,any_frag;

This option is used to specify an interface UUID. Optional arguments are an interface version and operator to
specify that the version be less than (’<’), greater than (’>’), equal to (’=’) or not equal to (’!’) the version
specified. Also, by default the rule will only be evaluated for a first fragment (or full request, i.e. not a fragment)
since most rules are written to start at the beginning of a request. The any frag argument says to evaluate for
middle and last fragments as well. This option requires tracking client Bind and Alter Context requests as
well as server Bind Ack and Alter Context responses for connection-oriented DCE/RPC in the preprocessor.
For each Bind and Alter Context request, the client specifies a list of interface UUIDs along with a handle
(or context id) for each interface UUID that will be used during the DCE/RPC session to reference the interface.
The server response indicates which interfaces it will allow the client to make requests to - it either accepts
or rejects the client’s wish to bind to a certain interface. This tracking is required so that when a request is
processed, the context id used in the request can be correlated with the interface UUID it is a handle for.
hexlong and hexshort will be specified and interpreted to be in big endian order (this is usually the default
way an interface UUID will be seen and represented). As an example, the following Messenger interface UUID
as taken off the wire from a little endian Bind request:

|f8 91 7b 5a 00 ff d0 11 a9 b2 00 c0 4f b6 e6 fc|
must be written as:
5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc
The same UUID taken off the wire from a big endian Bind request:
|5a 7b 91 f8 ff 00 11 d0 a9 b2 00 c0 4f b6 e6 fc|
must be written the same way:
5a7b91f8-ff00-11d0-a9b2-00c04fb6e6fc
This option matches if the specified interface UUID matches the interface UUID (as referred to by the context
id) of the DCE/RPC request and if supplied, the version operation is true. This option will not match if the
fragment is not a first fragment (or full request) unless the any frag option is supplied in which case only the
interface UUID and version need match. Note that a defragmented DCE/RPC request will be considered a full
request.
dce opnum
The opnum represents a specific function call to an interface. After is has been determined that a client has
bound to a specific interface and is making a request to it (see above - dce iface) usually we want to know
what function call it is making to that service. It is likely that an exploit lies in the particular DCE/RPC function
call.
Syntax

opnum-list
opnum-item
opnum-range
opnum

=
=
=
=

opnum-item | opnum-item ’,’ opnum-list
opnum | opnum-range
opnum ’-’ opnum
0-65535

Examples
dce_opnum:
dce_opnum:
dce_opnum:
dce_opnum:

15;
15-18;
15,18-20;
15,17,20-22;

This option is used to specify an opnum (or operation number), opnum range or list containing either or both
opnum and/or opnum-range. The opnum of a DCE/RPC request will be matched against the opnums specified
with this option. This option matches if any one of the opnums specified match the opnum of the DCE/RPC
request.
dce stub data
Since most netbios rules were doing protocol decoding only to get to the DCE/RPC stub data, i.e. the remote
procedure call or function call data, this option will alleviate this need and place the cursor at the beginning of
the DCE/RPC stub data. This reduces the number of rule option checks and the complexity of the rule.
This option takes no arguments.
Example
dce_stub_data;

This option is used to place the cursor (used to walk the packet payload in rules processing) at the beginning
of the DCE/RPC stub data, regardless of preceding rule options. There are no arguments to this option. This
option matches if there is DCE/RPC stub data.
byte test and byte jump
A DCE/RPC request can specify whether numbers are represented in big or little endian. These rule options will
take as a new argument dce and will work basically the same as the normal byte test/byte jump, but since
the DCE/RPC preprocessor will know the endianness of the request, it will be able to do the correct conversion.
byte test
Syntax
’,’ [ ’!’ ] ’,’ [ ’,’ [ ’,’ "relative" ]] \
’,’ "dce"
convert
operator
value
offset

=
=
=
=

1 | 2 | 4
’<’ | ’=’ | ’>’ | ’&’ | ’ˆ’
0-4294967295
-65535 to 65535

Examples
byte_test: 4,>,35000,0,relative,dce;
byte_test: 2,!=,2280,-10,relative,dce;

When using the dce argument to a byte test, the following normal byte test arguments will not be
allowed: big, little, string, hex, dec and oct.
byte jump
Syntax
’,’ [ ’,’ "relative" ] [ ’,’ "multiplier" ] \
[ ’,’ "align" ] [ ’,’ "post_offet" ] ’,’ "dce"
convert
offset
mult-value
adjustment-value

=
=
=
=

1 | 2 | 4
-65535 to 65535
0-65535
-65535 to 65535

Example
byte_jump:4,-4,relative,align,multiplier 2,post_offset -4,dce;

When using the dce argument to a byte jump, the following normal byte jump arguments will not be
allowed: big, little, string, hex, dec, oct and from beginning.
Example of rule complexity reduction
The following two rules using the new rule options replace 64 (set and isset flowbit) rules that are necessary if
the new rule options are not used:
alert tcp $EXTERNAL_NET any -> $HOME_NET [135,139,445,593,1024:] \
(msg:"dns R_Dnssrv funcs2 overflow attempt"; flow:established,to_server; \
dce_iface:50abc2a4-574d-40b3-9d66-ee4fd5fba076; dce_opnum:0-11; dce_stub_data; \
pcre:"/ˆ.{12}(\x00\x00\x00\x00|.{12})/sR"; byte_jump:4,-4,relative,align,dce; \
byte_test:4,>,256,4,relative,dce; reference:bugtraq,23470; reference:cve,2007-1748; \
classtype:attempted-admin; sid:1000068;)
alert udp $EXTERNAL_NET any -> $HOME_NET [135,1024:] \
(msg:"dns R_Dnssrv funcs2 overflow attempt"; flow:established,to_server; \
dce_iface:50abc2a4-574d-40b3-9d66-ee4fd5fba076; dce_opnum:0-11; dce_stub_data; \
pcre:"/ˆ.{12}(\x00\x00\x00\x00|.{12})/sR"; byte_jump:4,-4,relative,align,dce; \
byte_test:4,>,256,4,relative,dce; reference:bugtraq,23470; reference:cve,2007-1748; \
classtype:attempted-admin; sid:1000069;)

2.3 Decoder and Preprocessor Rules
Decoder and preprocessor rules allow one to enable and disable decoder and preprocessor events on a rule by rule
basis. They also allow one to specify the rule type or action of a decoder or preprocessor event on a rule by rule basis.
Decoder config options will still determine whether or not to generate decoder events. For example, if config
disable decode alerts is in snort.conf, decoder events will not be generated regardless of whether or not there
are corresponding rules for the event. Also note that if the decoder is configured to enable drops, e.g. config
enable decode drops, these options will take precedence over the event type of the rule. A packet will be dropped
if either a decoder config drop option is in snort.conf or the decoder or preprocessor rule type is drop. Of course,
the drop cases only apply if Snort is running inline. See doc/README.decode for config options that control decoder
events.

2.3.1 Configuring
The following options to configure will enable decoder and preprocessor rules:
$ ./configure --enable-decoder-preprocessor-rules
The decoder and preprocessor rules are located in the preproc rules/ directory in the top level source tree, and
have the names decoder.rules and preprocessor.rules respectively. These files are updated as new decoder and
preprocessor events are added to Snort.
To enable these rules in snort.conf, define the path to where the rules are located and uncomment the include lines
in snort.conf that reference the rules files.
var PREPROC_RULE_PATH /path/to/preproc_rules
...
include $PREPROC_RULE_PATH/preprocessor.rules
include $PREPROC_RULE_PATH/decoder.rules
To disable any rule, just comment it with a # or remove the rule completely from the file (commenting is recommended).
To change the rule type or action of a decoder/preprocessor rule, just replace alert with the desired rule type. Any
one of the following rule types can be used:
alert
log
pass
drop
sdrop
reject
For example one can change:
alert ( msg: "DECODE_NOT_IPV4_DGRAM"; sid: 1; gid: 116; rev: 1; \
metadata: rule-type decode ; classtype:protocol-command-decode;)
to
drop ( msg: "DECODE_NOT_IPV4_DGRAM"; sid: 1; gid: 116; rev: 1; \
metadata: rule-type decode ; classtype:protocol-command-decode;)
to drop (as well as alert on) packets where the Ethernet protocol is IPv4 but version field in IPv4 header has a value
other than 4.
See README.decode, README.gre and the various preprocessor READMEs for descriptions of the rules in decoder.rules
and preprocessor.rules.
82

2.3.2 Reverting to original behavior
If you have configured snort to use decoder and preprocessor rules, the following config option in snort.conf will
make Snort revert to the old behavior:
config autogenerate_preprocessor_decoder_rules
Note that if you want to revert to the old behavior, you also have to remove the decoder and preprocessor rules and
any reference to them from snort.conf, otherwise they will be loaded. This option applies to rules not specified and
the default behavior is to alert.

2.4 Event Processing
Snort provides a variety of mechanisms to tune event processing to suit your needs:
• Detection Filters
You can use detection filters to specifiy a threshold that must be exceeded before a rule generates an event. This
is covered in section 3.7.10.
• Rate Filters
You can use rate filters to change a rule action when the number or rate of events indicates a possible attack.
• Event Filters
You can use event filters to reduce the number of logged events for noisy rules. This can be tuned to significantly
reduce false alarms.
• Event Suppression
You can completely suppress the logging of unintersting events.

2.4.1 Rate Filtering
rate filter provides rate based attack prevention by allowing users to configure a new action to take for a specified
time when a given rate is exceeded. Multiple rate filters can be defined on the same rule, in which case they are
evaluated in the order they appear in the configuration file, and the first applicable action is taken.
Format
Rate filters are used as standalone configurations (outside of a rule) and have the following format:
rate_filter \
gen_id , sig_id , \
track , \
count , seconds , \
new_action alert|drop|pass|log|sdrop|reject, \
timeout \
[, apply_to ]
The options are described in the table below - all are required except apply to, which is optional.

Option
track by src | by dst |
by rule

count c
seconds s

apply to

Description
rate is tracked either by source IP address, destination IP address, or by
rule. This means the match statistics are maintained for each unique
source IP address, for each unique destination IP address, or they are
aggregated at rule level. For rules related to Stream5 sessions, source
and destination means client and server respectively. track by rule
and apply to may not be used together.
the maximum number of rule matches in s seconds before the rate filter
limit to is exceeded. c must be nonzero value.
the time period over which count is accrued. 0 seconds means count is
a total count instead of a specific rate. For example, rate filter may
be used to detect if the number of connections to a specific server exceed
a specific count. 0 seconds only applies to internal rules (gen id 135) and
other use will produce a fatal error by Snort.
new action replaces rule action for t seconds. drop, reject, and
sdrop can be used only when snort is used in inline mode. sdrop and
reject are conditionally compiled with GIDS.
revert to the original rule action after t seconds. If t is 0, then rule
action is never reverted back. An event filter may be used to manage
number of alerts after the rule action is enabled by rate filter.
restrict the configuration to only to source or destination IP address (indicated by track parameter) determined by . track by rule
and apply to may not be used together. Note that events are generated during the timeout period, even if the rate falls below the configured
limit.

Examples
Example 1 - allow a maximum of 100 connection attempts per second from any one IP address, and block further
connection attempts from that IP address for 10 seconds:
rate_filter \
gen_id 135, sig_id 1, \
track by_src, \
count 100, seconds 1, \
new_action drop, timeout 10
Example 2 - allow a maximum of 100 successful simultaneous connections from any one IP address, and block further
connections from that IP address for 10 seconds:
rate_filter \
gen_id 135, sig_id 2, \
track by_src, \
count 100, seconds 0, \
new_action drop, timeout 10

2.4.2 Event Filtering
Event filtering can be used to reduce the number of logged alerts for noisy rules by limiting the number of times a
particular event is logged during a specified time interval. This can be tuned to significantly reduce false alarms.
There are 3 types of event filters:
• limit
Alerts on the 1st m events during the time interval, then ignores events for the rest of the time interval.
84

• threshold
Alerts every m times we see this event during the time interval.
• both
Alerts once per time interval after seeing m occurrences of the event, then ignores any additional events during
the time interval.
Format
event_filter \
gen_id , sig_id , \
type , \
track , \
count , seconds
threshold \
gen_id , sig_id , \
type , \
track , \
count , seconds
threshold is an alias for event filter. Both formats are equivalent and support the options described below - all
are required. threshold is deprecated and will not be supported in future releases.
Option
gen id
sig id
type limit|threshold|both

track by src|by dst

count c
seconds s

Description
Specify the generator ID of an associated rule. gen id 0, sig id 0 can be used
to specify a ”global” threshold that applies to all rules.
Specify the signature ID of an associated rule. sig id 0 specifies a ”global” filter
because it applies to all sig ids for the given gen id.
type limit alerts on the 1st m events during the time interval, then ignores events
for the rest of the time interval. Type threshold alerts every m times we see
this event during the time interval. Type both alerts once per time interval after
seeing m occurrences of the event, then ignores any additional events during the
time interval.
rate is tracked either by source IP address, or destination IP address. This means
count is maintained for each unique source IP addresses, or for each unique destination IP addresses. Ports or anything else are not tracked.
number of rule matching in s seconds that will cause event filter limit to be
exceeded. c must be nonzero value.
time period over which count is accrued. s must be nonzero value.

! NOTE
△

Only one event filter may be defined for a given gen id, sig id. If more than one event filter is
applied to a specific gen id, sig id pair, Snort will terminate with an error while reading the configuration
information.

event filters with sig id 0 are considered ”global” because they apply to all rules with the given gen id. If
gen id is also 0, then the filter applies to all rules. (gen id 0, sig id != 0 is not allowed). Standard filtering tests
are applied first, if they do not block an event from being logged, the global filtering test is applied. Thresholds in a
rule (deprecated) will override a global event filter. Global event filters do not override what’s in a signature
or a more specific stand-alone event filter.

! NOTE
△

event filters can be used to suppress excessive rate filter alerts, however, the first new action event
of the timeout period is never suppressed. Such events indicate a change of state that are significant to the
user monitoring the network.

Examples
Limit logging to 1 event per 60 seconds:
event_filter \
gen_id 1, sig_id 1851, \
type limit, track by_src,
count 1, seconds 60

Limit logging to every 3rd event:
event_filter \
gen_id 1, sig_id 1852, \
type threshold, track by_src, \
count 3, seconds 60
Limit logging to just 1 event per 60 seconds, but only if we exceed 30 events in 60 seconds:
event_filter \
gen_id 1, sig_id 1853, \
type both, track by_src, \
count 30, seconds 60
Limit to logging 1 event per 60 seconds per IP triggering each rule (rule gen id is 1):
event_filter \
gen_id 1, sig_id 0, \
type limit, track by_src, \
count 1, seconds 60
Limit to logging 1 event per 60 seconds per IP, triggering each rule for each event generator:
event_filter \
gen_id 0, sig_id 0, \
type limit, track by_src, \
count 1, seconds 60
Events in Snort are generated in the usual way, event filters are handled as part of the output system. Read genmsg.map for details on gen ids.
Users can also configure a memcap for threshold with a “config:” option:
config event_filter: memcap
# this is deprecated:
config threshold: memcap

2.4.3 Event Suppression
Event suppression stops specified events from firing without removing the rule from the rule base. Suppression uses
an IP list to select specific networks and users for suppression. Suppression tests are performed prior to either standard
or global thresholding tests.
Suppression are standalone configurations that reference generators, SIDs, and IP addresses via an IP list . This allows
a rule to be completely suppressed, or suppressed when the causative traffic is going to or coming from a specific IP
or group of IP addresses.
You may apply multiple suppressions to a non-zero SID. You may also combine one event filter and several
suppressions to the same non-zero SID.
Format
The suppress configuration has two forms:
suppress \
gen_id , sig_id , \
suppress \
gen_id , sig_id , \
track , ip
Option
gen id
sig id
track by src|by dst
ip

Examples
Suppress this event completely:
suppress gen_id 1, sig_id 1852:
Suppress this event from this IP:
suppress gen_id 1, sig_id 1852, track by_src, ip 10.1.1.54
Suppress this event to this CIDR block:
suppress gen_id 1, sig_id 1852, track by_dst, ip 10.1.1.0/24

2.4.4 Event Logging
Snort supports logging multiple events per packet/stream that are prioritized with different insertion methods, such as
max content length or event ordering using the event queue.
The general configuration of the event queue is as follows:
config event_queue: [max_queue [size]] [log [size]] [order_events [TYPE]]
Event Queue Configuration Options There are three configuration options to the configuration parameter ’event queue’.
1. max queue
This determines the maximum size of the event queue. For example, if the event queue has a max size of 8, only
8 events will be stored for a single packet or stream.
The default value is 8.
2. log
This determines the number of events to log for a given packet or stream. You can’t log more than the max event
number that was specified.
The default value is 3.
3. order events
This argument determines the way that the incoming events are ordered. We currently have two different methods:
• priority - The highest priority (1 being the highest) events are ordered first.
• content length - Rules are ordered before decode or preprocessor alerts, and rules that have a longer
content are ordered before rules with shorter contents.
The method in which events are ordered does not affect rule types such as pass, alert, log, etc.
The default value is content length.
Event Queue Configuration Examples The default configuration:
config event_queue: max_queue 8 log 3 order_events content_length
Example of a reconfigured event queue:
config event_queue: max_queue 10 log 3 order_events content_length
Use the default event queue values, but change event order:
config event_queue: order_events priority
Use the default event queue values but change the number of logged events:
config event_queue: log 2

2.5 Performance Profiling
Snort can provide statistics on rule and preprocessor performance. Each require only a simple config option to
snort.conf and Snort will print statistics on the worst (or all) performers on exit. When a file name is provided in
profile rules or profile preprocs, the statistics will be saved in these files. If the append option is not present,
previous data in these files will be overwritten.
88

2.5.1 Rule Profiling
Format
config profile_rules: \
print [all | ], \
sort \
[,filename [append]]
• is the number of rules to print
• is one of:
checks
matches
nomatches
avg ticks
avg ticks per match
avg ticks per nomatch
total ticks
• is the output filename
• [append] dictates that the output will go to the same file each time (optional)
Examples
• Print all rules, sort by avg ticks (default configuration if option is turned on)
config profile rules
• Print all rules, sort by avg ticks, and append to file rules stats.txt
config profile rules filename rules stats.txt append
• Print the top 10 rules, based on highest average time
config profile rules:

print 10, sort avg ticks

• Print all rules, sorted by number of checks
config profile rules:

print all, sort checks

• Print top 100 rules, based on total time
config profile rules:

print 100, sort total ticks

• Print with default options, save results to performance.txt each time
config profile rules:

filename performance.txt append

• Print top 20 rules, save results to perf.txt with timestamp in filename
config profile rules:

print 20, filename perf.txt

Rule Profile Statistics (worst 4 rules)
==========================================================
Num
SID GID Rev
Checks
Matches
Alerts
===
=== === ===
======
=======
======
1
2389
1 12
1
1
1
2
2178
1 17
2
0
0
3
2179
1
8
2
0
0
4
1734
1 37
2
0
0

Ticks Avg/Check
===== =========
385698 385698.0
107822
53911.0
92458
46229.0
90054
45027.0

Avg/Match Avg/Nonmatch
========= ============
385698.0
0.0
0.0
53911.0
0.0
46229.0
0.0
45027.0

Figure 2.1: Rule Profiling Example Output
Output
Snort will print a table much like the following at exit.
Configuration line used to print the above table:
config profile rules:

print 4, sort total ticks

The columns represent:
• Number (rank)
• Sig ID
• Generator ID
• Checks (number of times rule was evaluated after fast pattern match within portgroup or any->any rules)
• Matches (number of times ALL rule options matched, will be high for rules that have no options)
• Alerts (number of alerts generated from this rule)
• CPU Ticks
• Avg Ticks per Check
• Avg Ticks per Match
• Avg Ticks per Nonmatch
Interpreting this info is the key. The Microsecs (or Ticks) column is important because that is the total time spent
evaluating a given rule. But, if that rule is causing alerts, it makes sense to leave it alone.
A high Avg/Check is a poor performing rule, that most likely contains PCRE. High Checks and low Avg/Check is
usually an any->any rule with few rule options and no content. Quick to check, the few options may or may not match.
We are looking at moving some of these into code, especially those with low SIDs.
By default, this information will be printed to the console when Snort exits. You can use the ”filename” option in
snort.conf to specify a file where this will be written. If ”append” is not specified, a new file will be created each time
Snort is run. The filenames will have timestamps appended to them. These files will be found in the logging directory.

2.5.2 Preprocessor Profiling
Format
config profile_preprocs: \
print [all | ], \
sort \
[, filename [append]]
• is the number of preprocessors to print
90

• is one of:
checks
avg ticks
total ticks
• is the output filename
• [append] dictates that the output will go to the same file each time (optional)
Examples
• Print all preprocessors, sort by avg ticks (default configuration if option is turned on)
config profile preprocs
• Print all preprocessors, sort by avg ticks, and append to file preprocs stats.txt
config profile preprocs, filename preprocs stats.txt append
• Print the top 10 preprocessors, based on highest average time
config profile preprocs:

print 10, sort avg ticks

• Print all preprocessors, sorted by number of checks
config profile preprocs:

print all, sort checks

Output
Snort will print a table much like the following at exit.
Configuration line used to print the above table:
config profile_rules: \
print 3, sort total_ticks
The columns represent:
• Number (rank) - The number is indented for each layer. Layer 1 preprocessors are listed under their respective
caller (and sorted similarly).
• Preprocessor Name
• Layer - When printing a specific number of preprocessors all subtasks info for a particular preprocessor is
printed for each layer 0 preprocessor stat.
• Checks (number of times preprocessor decided to look at a packet, ports matched, app layer header was correct,
etc)
• Exits (number of corresponding exits – just to verify code is instrumented correctly, should ALWAYS match
Checks, unless an exception was trapped)
• CPU Ticks
• Avg Ticks per Check
• Percent of caller - For non layer 0 preprocessors, i.e. subroutines within preprocessors, this identifies the percent
of the caller’s ticks that is spent for this subtask.
Because of task swapping, non-instrumented code, and other factors, the Pct of Caller field will not add up to 100%
of the caller’s time. It does give a reasonable indication of how much relative time is spent within each subtask.
By default, this information will be printed to the console when Snort exits. You can use the ”filename” option in
snort.conf to specify a file where this will be written. If ”append” is not specified, a new file will be created each time
Snort is run. The filenames will have timestamps appended to them. These files will be found in the logging directory.
91

Preprocessor Profile Statistics (all)
==========================================================
Num
Preprocessor Layer
Checks
Exits
===
============ =====
======
=====
1
ftptelnet_ftp
0
2697
2697
2
detect
0
930237
930237
1
rule eval
1
1347969
1347969
1
rule tree eval
2
1669390
1669390
1
pcre
3
488652
488652
2
asn1
3
1
1
3
uricontent
3
647122
647122
4
content
3
1043099
1043099
5
ftpbounce
3
23
23
6
byte_jump
3
9007
9007
7
byte_test
3
239015
239015
8
icmp_seq
3
2
2
9
fragbits
3
65259
65259
10
isdataat
3
5085
5085
11
flags
3
4147
4147
12
flowbits
3
2002630
2002630
13
ack
3
4042
4042
14
flow
3
1347822
1347822
15
icode
3
75538
75538
16
itype
3
27009
27009
17
icmp_id
3
41150
41150
18
ip_proto
3
142625
142625
19
ipopts
3
13690
13690
2
rtn eval
2
55836
55836
2
mpse
1
492836
492836
3
frag3
0
76925
76925
1
frag3insert
1
70885
70885
2
frag3rebuild
1
5419
5419
4
dcerpc
0
127332
127332
5
s5
0
809682
809682
1
s5tcp
1
765281
765281
1
s5TcpState
2
742464
742464
1
s5TcpFlush
3
51987
51987
1 s5TcpProcessRebuilt
4
47355
47355
2
s5TcpBuildPacket
4
47360
47360
2
s5TcpData
3
250035
250035
1
s5TcpPktInsert
4
88173
88173
2
s5TcpNewSess
2
60880
60880
6
eventq
0
2089428
2089428
7
httpinspect
0
296030
296030
8
smtp
0
137653
137653
9
decode
0
1057635
1057635
10
ftptelnet_telnet
0
175
175
11
sfportscan
0
881153
881153
12
backorifice
0
35369
35369
13
dns
0
16639
16639
total
total
0
1018323
1018323

Microsecs
=========
135720
31645670
26758596
26605086
18994719
8
2638614
3154396
19
3321
64401
0
10168
757
517
212231
261
79002
4280
1524
1618
5004
457
22763
4135697
1683797
434980
6280
2426830
14195602
14128577
13223585
92918
14548497
41711
141490
110136
81779
26690209
1862359
227982
1162456
175
518655
4875
1346
67046412

Avg/Check Pct of Caller Pct of Total
========= ============= ============
50.32
0.20
0.20
34.02
47.20
47.20
19.85
84.56
39.91
15.94
99.43
39.68
38.87
71.40
28.33
8.56
0.00
0.00
4.08
9.92
3.94
3.02
11.86
4.70
0.87
0.00
0.00
0.37
0.01
0.00
0.27
0.24
0.10
0.16
0.00
0.00
0.16
0.04
0.02
0.15
0.00
0.00
0.12
0.00
0.00
0.11
0.80
0.32
0.06
0.00
0.00
0.06
0.30
0.12
0.06
0.02
0.01
0.06
0.01
0.00
0.04
0.01
0.00
0.04
0.02
0.01
0.03
0.00
0.00
0.41
0.09
0.03
8.39
13.07
6.17
21.89
2.51
2.51
6.14
25.83
0.65
1.16
0.37
0.01
19.06
3.62
3.62
17.53
21.17
21.17
18.46
99.53
21.07
17.81
93.59
19.72
1.79
0.70
0.14
307.22
15657.23
21.70
0.88
44.89
0.06
0.57
1.07
0.21
1.25
77.84
0.16
1.34
0.58
0.12
12.77
39.81
39.81
6.29
2.78
2.78
1.66
0.34
0.34
1.10
1.73
1.73
1.00
0.00
0.00
0.59
0.77
0.77
0.14
0.01
0.01
0.08
0.00
0.00
65.84
0.00
0.00

Figure 2.2: Preprocessor Profiling Example Output

2.5.3 Packet Performance Monitoring (PPM)
PPM provides thresholding mechanisms that can be used to provide a basic level of latency control for snort. It does
not provide a hard and fast latency guarantee but should in effect provide a good average latency control. Both rules
and packets can be checked for latency. The action taken upon detection of excessive latency is configurable. The
following sections describe configuration, sample output, and some implementation details worth noting.
To use PPM, you must build with the –enable-ppm or the –enable-sourcefire option to configure.
PPM is configured as follows:
# Packet configuration:
config ppm: max-pkt-time , \
fastpath-expensive-packets, \
pkt-log, \
debug-pkts
# Rule configuration:
config ppm: max-rule-time , \
threshold count, \
suspend-expensive-rules, \
suspend-timeout , \
rule-log [log] [alert]
Packets and rules can be configured separately, as above, or together in just one config ppm statement. Packet and rule
monitoring is independent, so one or both or neither may be enabled.
Configuration
Packet Configuration Options
max-pkt-time
• enables packet latency thresholding using ’micros-secs’ as the limit.
• default is 0 (packet latency thresholding disabled)
• reasonable starting defaults: 100/250/1000 for 1G/100M/5M nets
fastpath-expensive-packets
• enables stopping further inspection of a packet if the max time is exceeded
• default is off
pkt-log
• enables logging packet event if packet exceeds max-pkt-time
• logging is to syslog or console depending upon snort configuration
• default is no logging
debug-pkts
• enables per packet timing stats to be printed after each packet
• default is off

Rule Configuration Options
max-rule-time
• enables rule latency thresholding using ’micros-secs’ as the limit.
• default is 0 (rule latency thresholding disabled)
• reasonable starting defaults: 100/250/1000 for 1G/100M/5M nets
threshold
• sets the number of consecutive rule time excesses before disabling a rule
• default is 5
suspend-expensive-rules
• enables suspending rule inspection if the max rule time is exceeded
• default is off
suspend-timeout
• rule suspension time in seconds
• default is 60 seconds
• set to zero to permanently disable expensive rules
rule-log [log] [alert]
• enables event logging output for rules
• default is no logging
• one or both of the options ’log’ and ’alert’ must be used with ’rule-log’
• the log option enables output to syslog or console depending upon snort configuration
Examples
Example 1: The following enables packet tracking:
config ppm: max-pkt-time 100
The following enables rule tracking:
config ppm: max-rule-time 50, threshold 5
If fastpath-expensive-packets or suspend-expensive-rules is not used, then no action is taken other than to increment
the count of the number of packets that should be fastpath’d or the rules that should be suspended. A summary of this
information is printed out when snort exits.
Example 2:
The following suspends rules and aborts packet inspection. These rules were used to generate the sample output that
follows.

config ppm: \
max-pkt-time 50, fastpath-expensive-packets, \
pkt-log, debug-pkt
config ppm: \
max-rule-time 50, threshold 5, suspend-expensive-rules, \
suspend-timeout 300, rule-log log alert
Sample Snort Output
Sample Snort Startup Output
Packet Performance Monitor Config:
ticks per usec : 1600 ticks
max packet time : 50 usecs
packet action
: fastpath-expensive-packets
packet logging : log
debug-pkts
: disabled
Rule Performance Monitor Config:
ticks per usec : 1600 ticks
max rule time
: 50 usecs
rule action
: suspend-expensive-rules
rule threshold : 5
suspend timeout : 300 secs
rule logging
: alert log
Sample Snort Run-time Output
...
PPM: Process-BeginPkt[61] caplen=60
PPM: Pkt[61] Used= 8.15385 usecs
PPM: Process-EndPkt[61]
PPM: Process-BeginPkt[62] caplen=342
PPM: Pkt[62] Used= 65.3659 usecs
PPM: Process-EndPkt[62]
PPM:
PPM:
PPM:
PPM:

Pkt-Event Pkt[63] used=56.0438 usecs, 0 rules, 1 nc-rules tested, packet fastpathed.
Process-BeginPkt[63] caplen=60
Pkt[63] Used= 8.394 usecs
Process-EndPkt[63]

PPM: Process-BeginPkt[64] caplen=60
PPM: Pkt[64] Used= 8.21764 usecs
PPM: Process-EndPkt[64]
...
Sample Snort Exit Output
Packet Performance Summary:
max packet time
: 50 usecs
packet events
: 1
avg pkt time
: 0.633125 usecs
Rule Performance Summary:
95

max rule time
rule events
avg nc-rule time

: 50 usecs
: 0
: 0.2675 usecs

Implementation Details
• Enforcement of packet and rule processing times is done after processing each rule. Latency control is not
enforced after each preprocessor.
• This implementation is software based and does not use an interrupt driven timing mechanism and is therefore
subject to the granularity of the software based timing tests. Due to the granularity of the timing measurements
any individual packet may exceed the user specified packet or rule processing time limit. Therefore this implementation cannot implement a precise latency guarantee with strict timing guarantees. Hence the reason this is
considered a best effort approach.
• Since this implementation depends on hardware based high performance frequency counters, latency thresholding is presently only available on Intel and PPC platforms.
• Time checks are made based on the total system time, not processor usage by Snort. This was a conscious design
decision because when a system is loaded, the latency for a packet is based on the total system time, not just the
processor time the Snort application receives. Therefore, it is recommended that you tune your thresholding to
operate optimally when your system is under load.

2.6 Output Modules
Output modules are new as of version 1.6. They allow Snort to be much more flexible in the formatting and presentation
of output to its users. The output modules are run when the alert or logging subsystems of Snort are called, after
the preprocessors and detection engine. The format of the directives in the rules file is very similar to that of the
preprocessors.
Multiple output plugins may be specified in the Snort configuration file. When multiple plugins of the same type (log,
alert) are specified, they are stacked and called in sequence when an event occurs. As with the standard logging and
alerting systems, output plugins send their data to /var/log/snort by default or to a user directed directory (using the -l
command line switch).
Output modules are loaded at runtime by specifying the output keyword in the rules file:
output :
output alert_syslog: log_auth log_alert

2.6.1 alert syslog
This module sends alerts to the syslog facility (much like the -s command line switch). This module also allows the
user to specify the logging facility and priority within the Snort rules file, giving users greater flexibility in logging
alerts.
Available Keywords
Facilities
• log auth
• log authpriv
• log daemon
96

• log local0
• log local1
• log local2
• log local3
• log local4
• log local5
• log local6
• log local7
• log user
Priorities
• log emerg
• log alert
• log crit
• log err
• log warning
• log notice
• log info
• log debug
Options
• log cons
• log ndelay
• log perror
• log pid
Format
alert_syslog: \

! NOTE
△

As WIN32 does not run syslog servers locally by default, a hostname and port can be passed as options. The
default host is 127.0.0.1. The default port is 514.
output alert_syslog: \
[host=],] \

Example
output alert_syslog: 10.1.1.1:514,

2.6.2 alert fast
This will print Snort alerts in a quick one-line format to a specified output file. It is a faster alerting method than full
alerts because it doesn’t need to print all of the packet headers to the output file and because it logs to only 1 file.
Format
output alert_fast: [ ["packet"] []]
::= [(’G’|’M’|K’)]
• filename: the name of the log file. The default name is ¡logdir¿/alert. You may specify ”stdout” for terminal
output. The name may include an absolute or relative path.
• packet: this option will cause multiline entries with full packet headers to be logged. By default, only brief
single-line entries are logged.
• limit: an optional limit on file size which defaults to 128 MB. The minimum is 1 KB. See 2.6.13 for more
information.
Example
output alert_fast: alert.fast

2.6.3 alert full
This will print Snort alert messages with full packet headers. The alerts will be written in the default logging directory
(/var/log/snort) or in the logging directory specified at the command line.
Inside the logging directory, a directory will be created per IP. These files will be decoded packet dumps of the packets
that triggered the alerts. The creation of these files slows Snort down considerably. This output method is discouraged
for all but the lightest traffic situations.
Format
output alert_full: [ []]
::= [(’G’|’M’|K’)]
• filename: the name of the log file. The default name is ¡logdir¿/alert. You may specify ”stdout” for terminal
output. The name may include an absolute or relative path.
• limit: an optional limit on file size which defaults to 128 MB. The minimum is 1 KB. See 2.6.13 for more
information.
Example
output alert_full: alert.full

2.6.4 alert unixsock
Sets up a UNIX domain socket and sends alert reports to it. External programs/processes can listen in on this socket
and receive Snort alert and packet data in real time. This is currently an experimental interface.
Format
alert_unixsock
Example
output alert_unixsock

2.6.5 log tcpdump
The log tcpdump module logs packets to a tcpdump-formatted file. This is useful for performing post-process analysis
on collected traffic with the vast number of tools that are available for examining tcpdump-formatted files.
Format
output log_tcpdump: [ []]
::= [(’G’|’M’|K’)]
• filename: the name of the log file. The default name is ¡logdir¿/snort.log. The name may include an absolute
or relative path. A UNIX timestamp is appended to the filename.
• limit: an optional limit on file size which defaults to 128 MB. When a sequence of packets is to be logged, the
aggregate size is used to test the rollover condition. See 2.6.13 for more information.
Example
output log_tcpdump: snort.log

2.6.6 database
This module from Jed Pickel sends Snort data to a variety of SQL databases. More information on installing and
configuring this module can be found on the [91]incident.org web page. The arguments to this plugin are the name of
the database to be logged to and a parameter list. Parameters are specified with the format parameter = argument. see
Figure 2.3 for example usage.
Format
database: , ,
The following parameters are available:
host - Host to connect to. If a non-zero-length string is specified, TCP/IP communication is used. Without a host
name, it will connect using a local UNIX domain socket.
port - Port number to connect to at the server host, or socket filename extension for UNIX-domain connections.
dbname - Database name
99

output database: \
log, mysql, dbname=snort user=snort host=localhost password=xyz
Figure 2.3: Database Output Plugin Configuration
user - Database username for authentication
password - Password used if the database demands password authentication
sensor name - Specify your own name for this Snort sensor. If you do not specify a name, one will be generated
automatically
encoding - Because the packet payload and option data is binary, there is no one simple and portable way to store it
in a database. Blobs are not used because they are not portable across databases. So i leave the encoding option
to you. You can choose from the following options. Each has its own advantages and disadvantages:
hex (default) - Represent binary data as a hex string.
Storage requirements - 2x the size of the binary
Searchability - very good
Human readability - not readable unless you are a true geek, requires post processing
base64 - Represent binary data as a base64 string.
Storage requirements - ∼1.3x the size of the binary
Searchability - impossible without post processing
Human readability - not readable requires post processing
ascii - Represent binary data as an ASCII string. This is the only option where you will actually lose data.
Non-ASCII Data is represented as a ‘.’. If you choose this option, then data for IP and TCP options will
still be represented as hex because it does not make any sense for that data to be ASCII.
Storage requirements - slightly larger than the binary because some characters are escaped (&,<,>)
Searchability - very good for searching for a text string impossible if you want to search for binary
human readability - very good
detail - How much detailed data do you want to store? The options are:
full (default) - Log all details of a packet that caused an alert (including IP/TCP options and the payload)
fast - Log only a minimum amount of data. You severely limit the potential of some analysis applications
if you choose this option, but this is still the best choice for some applications. The following fields are
logged: timestamp, signature, source ip, destination ip, source port, destination port, tcp
flags, and protocol)
Furthermore, there is a logging method and database type that must be defined. There are two logging types available,
log and alert. Setting the type to log attaches the database logging functionality to the log facility within the program.
If you set the type to log, the plugin will be called on the log output chain. Setting the type to alert attaches the plugin
to the alert output chain within the program.
There are five database types available in the current version of the plugin. These are mssql, mysql, postgresql,
oracle, and odbc. Set the type to match the database you are using.

! NOTE
△

The database output plugin does not have the ability to handle alerts that are generated by using the tag
keyword. See section 3.7.5 for more details.

100

2.6.7 csv
The csv output plugin allows alert data to be written in a format easily importable to a database. The output fields and
their order may be customized.
Format
output alert_csv: [ [ []]]
::= "default"|
::= (,)*
::= "dst"|"src"|"ttl" ...
::= [(’G’|’M’|K’)]
• filename: the name of the log file. The default name is ¡logdir¿/alert.csv. You may specify ”stdout” for terminal
output. The name may include an absolute or relative path.
• format: The list of formatting options is below. If the formatting option is ”default”, the output is in the order
of the formatting options listed.
– timestamp
– sig generator
– sig id
– sig rev
– msg
– proto
– src
– srcport
– dst
– dstport
– ethsrc
– ethdst
– ethlen
– tcpflags
– tcpseq
– tcpack
– tcplen
– tcpwindow
– ttl
– tos
– id
– dgmlen
– iplen
– icmptype
– icmpcode
– icmpid
– icmpseq
• limit: an optional limit on file size which defaults to 128 MB. The minimum is 1 KB. See 2.6.13 for more
information.
101

Example
output alert_csv: /var/log/alert.csv default
output alert_csv: /var/log/alert.csv timestamp, msg

2.6.8 unified
The unified output plugin is designed to be the fastest possible method of logging Snort events. The unified output
plugin logs events in binary format, allowing another programs to handle complex logging mechanisms that would
otherwise diminish the performance of Snort.
The name unified is a misnomer, as the unified output plugin creates two different files, an alert file, and a log file.
The alert file contains the high-level details of an event (eg: IPs, protocol, port, message id). The log file contains
the detailed packet information (a packet dump with the associated event ID). Both file types are written in a bimary
format described in spo unified.h.

! NOTE
△

Files have the file creation time (in Unix Epoch format) appended to each file when it is created.

Format
output alert_unified: [, ]
output log_unified: [, ]
Example
output alert_unified: snort.alert, limit 128
output log_unified: snort.log, limit 128

2.6.9 unified 2
The unified2 output plugin is a replacement for the unified output plugin. It has the same performance characteristics,
but a slightly different logging format. See section 2.6.8 on unified logging for more information.
Unified2 can work in one of three modes, packet logging, alert logging, or true unified logging. Packet logging
includes a capture of the entire packet and is specified with log unified2. Likewise, alert logging will only log
events and is specified with alert unified2. To include both logging styles in a single, unified file, simply specify
unified2.
When MPLS support is turned on, MPLS labels can be included in unified2 events. Use option mpls event types to
enable this. If option mpls event types is not used, then MPLS labels will be not be included in unified2 events.

! NOTE
△

By default, unified 2 files have the file creation time (in Unix Epoch format) appended to each file when it is
created.

Format
output alert_unified2: \
filename [, ] [, nostamp] [, mpls_event_types]

102

output log_unified2: \
filename [, ] [, nostamp]
output unified2: \
filename [, ] [, nostamp] [, mpls_event_types]
Example
output
output
output
output

alert_unified2: filename snort.alert, limit 128, nostamp
log_unified2: filename snort.log, limit 128, nostamp
unified2: filename merged.log, limit 128, nostamp
unified2: filename merged.log, limit 128, nostamp, mpls_event_types

2.6.10 alert prelude
! NOTE
△
support to use alert prelude is not built in by default. To use alert prelude, snort must be built with the
–enable-prelude argument passed to ./configure.

The alert prelude output plugin is used to log to a Prelude database. For more information on Prelude, see http://www.prelude-ids.or
Format
output alert_prelude: \
profile= \
[ info=] \
[ low=] \
[ medium=]
Example
output alert_prelude: profile=snort info=4 low=3 medium=2

2.6.11 log null
Sometimes it is useful to be able to create rules that will alert to certain types of traffic but will not cause packet log
entries. In Snort 1.8.2, the log null plugin was introduced. This is equivalent to using the -n command line option but
it is able to work within a ruletype.
Format
output log_null
Example
output log_null

# like using snort -n

ruletype info {
type alert
output alert_fast: info.alert
103

output log_null
}

2.6.12 alert aruba action
! NOTE
△
Support to use alert aruba action is not built in by default. To use alert aruba action, snort must be built with
the –enable-aruba argument passed to ./configure.
Communicates with an Aruba Networks wireless mobility controller to change the status of authenticated users. This
allows Snort to take action against users on the Aruba controller to control their network privilege levels.
For more information on Aruba Networks access control, see http://www.arubanetworks.com/.
Format
output alert_aruba_action: \

The following parameters are required:
controller address - Aruba mobility controller address.
secrettype - Secret type, one of ”sha1”, ”md5” or ”cleartext”.
secret - Authentication secret configured on the Aruba mobility controller with the ”aaa xml-api client” configuration command, represented as a sha1 or md5 hash, or a cleartext password.
action - Action to apply to the source IP address of the traffic generating an alert.
blacklist - Blacklist the station by disabling all radio communication.
setrole:rolename - Change the userś role to the specified rolename.
Example
output alert_aruba_action: \
10.3.9.6 cleartext foobar setrole:quarantine_role

2.6.13 Log Limits
This section pertains to logs produced by alert fast, alert full, alert csv, and log tcpdump. unified and
unified2 also may be given limits. Those limits are described in the respective sections.
When a configured limit is reached, the current log is closed and a new log is opened with a UNIX timestamp appended
to the configured log name.
Limits are configured as follows:
::= [(||)]
::= ’G’|’g’
::= ’M’|’m’
::= ’K’|’k’
Rollover will occur at most once per second so if limit is too small for logging rate, limit will be exceeded. Rollover
works correctly if snort is stopped/restarted.
104

2.7 Host Attribute Table
Starting with version 2.8.1, Snort has the capability to use information from an outside source to determine both the
protocol for use with Snort rules, and IP-Frag policy (see section 2.2.1) and TCP Stream reassembly policies (see
section 2.2.2). This information is stored in an attribute table, which is loaded at startup. The table is re-read during
run time upon receipt of signal number 30.
Snort associates a given packet with its attribute data from the table, if applicable.
For rule evaluation, service information is used instead of the ports when the protocol metadata in the rule matches the
service corresponding to the traffic. If the rule doesn’t have protocol metadata, or the traffic doesn’t have any matching
service information, the rule relies on the port information.

! NOTE
△

To use a host attribute table, Snort must be configured with the –enable-targetbased flag.

2.7.1 Configuration Format
attribute_table filename

2.7.2 Attribute Table File Format
The attribute table uses an XML format and consists of two sections, a mapping section, used to reduce the size of the
file for common data elements, and the host attribute section. The mapping section is optional.
An example of the file format is shown below.

1
Linux

2
ssh

192.168.1.234

1
100

Red Hat
99

2.6
98

linux

105

linux

22
100

tcp
100

2
100

OpenSSH
100

3.9p1
93

23
100

tcp
100

telnet
100

telnet
50

tcp
100

http
91

IE Http Browser
90

106

6.0
89

! NOTE
△

With Snort 2.8.1, for a given host entry, the stream and IP frag information are both used. Of the service
attributes, only the IP protocol (tcp, udp, etc), port, and protocol (http, ssh, etc) are used. The application
and version for a given service attribute, and any client attributes are ignored. They will be used in a future
release.

A DTD for verification of the Host Attribute Table XML file is provided with the snort packages.

2.8 Dynamic Modules
Dynamically loadable modules were introduced with Snort 2.6. They can be loaded via directives in snort.conf or
via command-line options.

! NOTE
△

To disable use of dynamic modules, Snort must be configured with the --disable-dynamicplugin flag.

2.8.1 Format

2.8.2 Directives
Syntax
dynamicpreprocessor [ file
|
directory ]

dynamicengine [ file | directory
]

Description
Tells snort to load the dynamic preprocessor shared library (if
file is used) or all dynamic preprocessor shared libraries (if directory is used).
Specify file, followed by the full or relative path to the shared library.
Or, specify directory, followed by the full or relative path to a directory of preprocessor
shared libraries. (Same effect as --dynamic-preprocessor-lib or
--dynamic-preprocessor-lib-dir options). See chapter 5 for more
information on dynamic preprocessor libraries.
Tells snort to load the dynamic engine shared library (if file is used) or
all dynamic engine shared libraries (if directory is used). Specify file,
followed by the full or relative path to the shared library. Or, specify
directory, followed by the full or relative path to a directory of preprocessor shared libraries. (Same effect as --dynamic-engine-lib or
--dynamic-preprocessor-lib-dir options). See chapter 5 for more
information on dynamic engine libraries.

107

dynamicdetection [ file
|
directory ]

Tells snort to load the dynamic detection rules shared library (if file
is used) or all dynamic detection rules shared libraries (if directory
is used). Specify file, followed by the full or relative path to the
shared library. Or, specify directory, followed by the full or relative
path to a directory of detection rules shared libraries. (Same effect as
--dynamic-detection-lib or --dynamic-detection-lib-dir options). See chapter 5 for more information on dynamic detection rules
libraries.

2.9 Reloading a Snort Configuration
Snort now supports reloading a configuration in lieu of restarting Snort in so as to provide seamless traffic inspection
during a configuration change. A separate thread will parse and create a swappable configuration object while the
main Snort packet processing thread continues inspecting traffic under the current configuration. When a swappable
configuration object is ready for use, the main Snort packet processing thread will swap in the new configuration to
use and will continue processing under the new configuration. Note that for some preprocessors, existing session data
will continue to use the configuration under which they were created in order to continue with proper state for that
session. All newly created sessions will, however, use the new configuration.

2.9.1 Enabling support
To enable support for reloading a configuration, add --enable-reload to configure when compiling.
There is also an ancillary option that determines how Snort should behave if any non-reloadable options are changed
(see section 2.9.3 below). This option is enabled by default and the behavior is for Snort to restart if any nonreloadable options are added/modified/removed. To disable this behavior and have Snort exit instead of restart, add
--disable-reload-error-restart in addition to --enable-reload to configure when compiling.

! NOTE
△

This functionality is not currently supported in Windows.

2.9.2 Reloading a configuration
First modify your snort.conf (the file passed to the -c option on the command line).
Then, to initiate a reload, send Snort a SIGHUP signal, e.g.
$ kill -SIGHUP

! NOTE
△

If reload support is not enabled, Snort will restart (as it always has) upon receipt of a SIGHUP.

! NOTE
△

An invalid configuration will still result in Snort fatal erroring, so you should test your new configuration
before issuing a reload, e.g. $ snort -c snort.conf -T

108

2.9.3 Non-reloadable configuration options
There are a number of option changes that are currently non-reloadable because they require changes to output, startup
memory allocations, etc. Modifying any of these options will cause Snort to restart (as a SIGHUP previously did) or
exit (if --disable-reload-error-restart was used to configure Snort).
Reloadable configuration options of note:
• Adding/modifying/removing text rules and variables are reloadable.
• Adding/modifying/removing preprocessor configurations are reloadable (except as noted below).
Non-reloadable configuration options of note:
• Adding/modifying/removing shared objects via dynamicdetection, dynamicengine and dynamicpreprocessor are
not reloadable, i.e. any new/modified/removed shared objects will require a restart.
• Any changes to output will require a restart.
Changes to the following options are not reloadable:
attribute_table
config alertfile
config asn1
config chroot
config daemon
config detection_filter
config flexresp2_attempts
config flexresp2_interface
config flexresp2_memcap
config flexresp2_rows
config flowbits_size
config interface
config logdir
config max_attribute_hosts
config nolog
config no_promisc
config pkt_count
config rate_filter
config read_bin_file
config set_gid
config set_uid
config snaplen
config threshold
dynamicdetection
dynamicengine
dynamicpreprocessor
output
In certain cases, only some of the parameters to a config option or preprocessor configuration are not reloadable.
Those parameters are listed below the relevant config option or preprocessor.
config ppm: max-rule-time
rule-log
config profile_rules
filename
print
109

sort
config profile_preprocs
filename
print
sort
preprocessor dcerpc2
memcap
preprocessor frag3_global
max_frags
memcap
prealloc_frags
prealloc_memcap
preprocessor perfmonitor
file
snortfile
preprocessor sfportscan
memcap
logfile
preprocessor stream5_global
memcap
max_tcp
max_udp
max_icmp
track_tcp
track_udp
track_icmp

2.10 Multiple Configurations
Snort now supports multiple configurations based on VLAN Id or IP subnet within a single instance of Snort. This will
allow administrators to specify multiple snort configuration files and bind each configuration to one or more VLANs
or subnets rather than running one Snort for each configuration required. Each unique snort configuration file will
create a new configuration instance within snort. VLANs/Subnets not bound to any specific configuration will use the
default configuration. Each configuration can have different preprocessor settings and detection rules.

2.10.1 Creating Multiple Configurations
Default configuration for snort is specified using the existing -c option. A default configuration binds multiple vlans
or networks to non-default configurations, using the following configuration line:
config binding: vlan
config binding: net
path to snort.conf - Refers to the absolute or relative path to the snort.conf for specific configuration.
vlanIdList - Refers to the comma seperated list of vlandIds and vlanId ranges. The format for ranges is two vlanId
separated by a ”-”. Spaces are allowed within ranges. Valid vlanId is any number in 0-4095 range. Negative
vland Ids and alphanumeric are not supported.
ipList - Refers to ip subnets. Subnets can be CIDR blocks for IPV6 or IPv4.

! NOTE
△
Vlan and Subnets can not be used in the same line. Configurations can be applied based on either Vlans or
Subnets not both.
110

! NOTE
△

Even though Vlan Ids 0 and 4095 are reserved, they are included as valid in terms of configuring Snort.

2.10.2 Configuration Specific Elements
Config Options
Generally config options defined within the default configuration are global by default i.e. their value applies to all
other configurations. The following config options are specific to each configuration.
policy_id
policy_mode
policy_version
The following config options are specific to each configuration. If not defined in a configuration, the default values of
the option (not the default configuration values) take effect.
config
config
config
config
config
config
config
config
config
config
config
config
config
config

checksum_drop
disable_decode_alerts
disable_decode_drops
disable_ipopt_alerts
disable_ipopt_drops
disable_tcpopt_alerts
disable_tcpopt_drops
disable_tcpopt_experimental_alerts
disable_tcpopt_experimental_drops
disable_tcpopt_obsolete_alerts
disable_tcpopt_obsolete_drops
disable_ttcp_alerts
disable_tcpopt_ttcp_alerts
disable_ttcp_drops

Rules
Rules are specific to configurations but only some parts of a rule can be customized for performance reasons. If a
rule is not specified in a configuration then the rule will never raise an event for the configuration. A rule shares all
parts of the rule options, including the general options, payload detection options, non-payload detection options, and
post-detection options. Parts of the rule header can be specified differently across configurations, limited to:
Source IP address and port
Destination IP address and port
Action
A higher revision of a rule in one configuration will override other revisions of the same rule in other configurations.
Variables
Variables defined using ”var”, ”portvar” and ”ipvar” are specific to configurations. If the rules in a configuration use
variables, those variables must be defined in that configuration.

111

Preprocessors
Preprocessors configurations can be defined within each vlan or subnet specific configuration. Options controlling
specific preprocessor memory usage, through specific limit on memory usage or number of instances, are processed
only in default policy. The options control total memory usage for a preprocessor across all policies. These options are
ignored in non-default policies without raising an error. A preprocessor must be configured in default configuration before it can be configured in non-default configuration. This is required as some mandatory preprocessor configuration
options are processed only in default configuration.
Events and Output
An unique policy id can be assigned by user, to each configuration using the following config line:
config policy_id:
id - Refers to a 16-bit unsigned value. This policy id will be used to identify alerts from a specific configuration in
the unified2 records.

! NOTE
△

If no policy id is specified, snort assigns 0 (zero) value to the configuration.

To enable vlanId logging in unified2 records the following option can be used.
output alert_unified2: vlan_event_types (alert logging only)
output unified2: filename , vlan_event_types (true unified logging)
filename - Refers to the absolute or relative filename.
vlan event types - When this option is set, snort will use unified2 event type 104 and 105 for IPv4 and IPv6
respectively.

! NOTE
△

Each event logged will have the vlanId from the packet if vlan headers are present otherwise 0 will be used.

2.10.3 How Configuration is applied?
Snort assigns every incoming packet to a unique configuration based on the following criteria. If VLANID is present,
then the innermost VLANID is used to find bound configuration. If the bound configuration is the default configuration, then destination IP address is searched to the most specific subnet that is bound to a non-default configuration.
The packet is assigned non-default configuration if found otherwise the check is repeated using source IP address. In
the end, default configuration is used if no other matching configuration is found.
For addressed based configuration binding, this can lead to conflicts between configurations if source address is bound
to one configuration and destination address is bound to another. In this case, snort will use the first configuration in
the order of definition, that can be applied to the packet.

112

Chapter 3

Writing Snort Rules
3.1 The Basics
Snort uses a simple, lightweight rules description language that is flexible and quite powerful. There are a number of
simple guidelines to remember when developing Snort rules that will help safeguard your sanity.
Most Snort rules are written in a single line. This was required in versions prior to 1.8. In current versions of Snort,
rules may span multiple lines by adding a backslash \ to the end of the line.
Snort rules are divided into two logical sections, the rule header and the rule options. The rule header contains
the rule’s action, protocol, source and destination IP addresses and netmasks, and the source and destination ports
information. The rule option section contains alert messages and information on which parts of the packet should be
inspected to determine if the rule action should be taken.
Figure 3.1 illustrates a sample Snort rule.

The text up to the first parenthesis is the rule header and the section enclosed in parenthesis contains the rule options.
The words before the colons in the rule options section are called option keywords.

! NOTE
△

Note that the rule options section is not specifically required by any rule, they are just used for the sake of
making tighter definitions of packets to collect or alert on (or drop, for that matter).

All of the elements in that make up a rule must be true for the indicated rule action to be taken. When taken together,
the elements can be considered to form a logical AND statement. At the same time, the various rules in a Snort rules
library file can be considered to form a large logical OR statement.

3.2 Rules Headers
3.2.1 Rule Actions
The rule header contains the information that defines the who, where, and what of a packet, as well as what to do in
the event that a packet with all the attributes indicated in the rule should show up. The first item in a rule is the rule
alert tcp any any -> 192.168.1.0/24 111 \
(content:"|00 01 86 a5|"; msg:"mountd access";)
Figure 3.1: Sample Snort Rule
113

action. The rule action tells Snort what to do when it finds a packet that matches the rule criteria. There are 5 available
default actions in Snort, alert, log, pass, activate, and dynamic. In addition, if you are running Snort in inline mode,
you have additional options which include drop, reject, and sdrop.
1. alert - generate an alert using the selected alert method, and then log the packet
2. log - log the packet
3. pass - ignore the packet
4. activate - alert and then turn on another dynamic rule
5. dynamic - remain idle until activated by an activate rule , then act as a log rule
6. drop - make iptables drop the packet and log the packet
7. reject - make iptables drop the packet, log it, and then send a TCP reset if the protocol is TCP or an ICMP port
unreachable message if the protocol is UDP.
8. sdrop - make iptables drop the packet but do not log it.
You can also define your own rule types and associate one or more output plugins with them. You can then use the
rule types as actions in Snort rules.
This example will create a type that will log to just tcpdump:
ruletype suspicious
{
type log
output log_tcpdump: suspicious.log
}
This example will create a rule type that will log to syslog and a MySQL database:
ruletype redalert
{
type alert
output alert_syslog: LOG_AUTH LOG_ALERT
output database: log, mysql, user=snort dbname=snort host=localhost
}

3.2.2 Protocols
The next field in a rule is the protocol. There are four protocols that Snort currently analyzes for suspicious behavior
– TCP, UDP, ICMP, and IP. In the future there may be more, such as ARP, IGRP, GRE, OSPF, RIP, IPX, etc.

3.2.3 IP Addresses
The next portion of the rule header deals with the IP address and port information for a given rule. The keyword any
may be used to define any address. Snort does not have a mechanism to provide host name lookup for the IP address
fields in the rules file. The addresses are formed by a straight numeric IP address and a CIDR[3] block. The CIDR
block indicates the netmask that should be applied to the rule’s address and any incoming packets that are tested against
the rule. A CIDR block mask of /24 indicates a Class C network, /16 a Class B network, and /32 indicates a specific
machine address. For example, the address/CIDR combination 192.168.1.0/24 would signify the block of addresses
from 192.168.1.1 to 192.168.1.255. Any rule that used this designation for, say, the destination address would match
on any address in that range. The CIDR designations give us a nice short-hand way to designate large address spaces
with just a few characters.
114

alert tcp !192.168.1.0/24 any -> 192.168.1.0/24 111 \
(content: "|00 01 86 a5|"; msg: "external mountd access";)
Figure 3.2: Example IP Address Negation Rule
alert tcp ![192.168.1.0/24,10.1.1.0/24] any -> \
[192.168.1.0/24,10.1.1.0/24] 111 (content: "|00 01 86 a5|"; \
msg: "external mountd access";)
Figure 3.3: IP Address Lists
In Figure 3.1, the source IP address was set to match for any computer talking, and the destination address was set to
match on the 192.168.1.0 Class C network.
There is an operator that can be applied to IP addresses, the negation operator. This operator tells Snort to match any
IP address except the one indicated by the listed IP address. The negation operator is indicated with a !. For example,
an easy modification to the initial example is to make it alert on any traffic that originates outside of the local net with
the negation operator as shown in Figure 3.2.

This rule’s IP addresses indicate any tcp packet with a source IP address not originating from the internal network and
a destination address on the internal network.
You may also specify lists of IP addresses. An IP list is specified by enclosing a comma separated list of IP addresses
and CIDR blocks within square brackets. For the time being, the IP list may not include spaces between the addresses.
See Figure 3.3 for an example of an IP list in action.

3.2.4 Port Numbers
Port numbers may be specified in a number of ways, including any ports, static port definitions, ranges, and by
negation. Any ports are a wildcard value, meaning literally any port. Static ports are indicated by a single port
number, such as 111 for portmapper, 23 for telnet, or 80 for http, etc. Port ranges are indicated with the range operator
:. The range operator may be applied in a number of ways to take on different meanings, such as in Figure 3.4.

Port negation is indicated by using the negation operator !. The negation operator may be applied against any of the
other rule types (except any, which would translate to none, how Zen...). For example, if for some twisted reason you
wanted to log everything except the X Windows ports, you could do something like the rule in Figure 3.5.

3.2.5 The Direction Operator
The direction operator -> indicates the orientation, or direction, of the traffic that the rule applies to. The IP address
and port numbers on the left side of the direction operator is considered to be the traffic coming from the source
log udp any any -> 192.168.1.0/24 1:1024 log udp
traffic coming from any port and destination ports ranging from 1 to 1024
log tcp any any -> 192.168.1.0/24 :6000
log tcp traffic from any port going to ports less than or equal to 6000
log tcp any :1024 -> 192.168.1.0/24 500:
log tcp traffic from privileged ports less than or equal to 1024 going to ports greater than or equal to 500
Figure 3.4: Port Range Examples
115

log tcp any any -> 192.168.1.0/24 !6000:6010
Figure 3.5: Example of Port Negation
log tcp !192.168.1.0/24 any <> 192.168.1.0/24 23
Figure 3.6: Snort rules using the Bidirectional Operator
host, and the address and port information on the right side of the operator is the destination host. There is also a
bidirectional operator, which is indicated with a <> symbol. This tells Snort to consider the address/port pairs in
either the source or destination orientation. This is handy for recording/analyzing both sides of a conversation, such as
telnet or POP3 sessions. An example of the bidirectional operator being used to record both sides of a telnet session is
shown in Figure 3.6.
Also, note that there is no <- operator. In Snort versions before 1.8.7, the direction operator did not have proper
error checking and many people used an invalid token. The reason the <- does not exist is so that rules always read
consistently.

3.2.6 Activate/Dynamic Rules
! NOTE
△

Activate and Dynamic rules are being phased out in favor of a combination of tagging (3.7.5) and flowbits
(3.6.10).

Activate/dynamic rule pairs give Snort a powerful capability. You can now have one rule activate another when it’s
action is performed for a set number of packets. This is very useful if you want to set Snort up to perform follow on
recording when a specific rule goes off. Activate rules act just like alert rules, except they have a *required* option
field: activates. Dynamic rules act just like log rules, but they have a different option field: activated by. Dynamic
rules have a second required field as well, count.
Activate rules are just like alerts but also tell Snort to add a rule when a specific network event occurs. Dynamic rules
are just like log rules except are dynamically enabled when the activate rule id goes off.
Put ’em together and they look like Figure 3.7.
These rules tell Snort to alert when it detects an IMAP buffer overflow and collect the next 50 packets headed for port
143 coming from outside $HOME NET headed to $HOME NET. If the buffer overflow happened and was successful,
there’s a very good possibility that useful data will be contained within the next 50 (or whatever) packets going to that
same service port on the network, so there’s value in collecting those packets for later analysis.

3.3 Rule Options
Rule options form the heart of Snort’s intrusion detection engine, combining ease of use with power and flexibility. All
Snort rule options are separated from each other using the semicolon (;) character. Rule option keywords are separated
from their arguments with a colon (:) character.
activate tcp !$HOME_NET any -> $HOME_NET 143 (flags: PA; \
content: "|E8C0FFFFFF|/bin"; activates: 1; \
msg: "IMAP buffer overflow!";)
dynamic tcp !$HOME_NET any -> $HOME_NET 143 (activated_by: 1; count: 50;)
Figure 3.7: Activate/Dynamic Rule Example

116

There are four major categories of rule options.
general These options provide information about the rule but do not have any affect during detection
payload These options all look for data inside the packet payload and can be inter-related
non-payload These options look for non-payload data
post-detection These options are rule specific triggers that happen after a rule has “fired.”

3.4 General Rule Options
3.4.1 msg
The msg rule option tells the logging and alerting engine the message to print along with a packet dump or to an alert.
It is a simple text string that utilizes the \ as an escape character to indicate a discrete character that might otherwise
confuse Snort’s rules parser (such as the semi-colon ; character).
Format
msg: "";

3.4.2 reference
The reference keyword allows rules to include references to external attack identification systems. The plugin currently
supports several specific systems as well as unique URLs. This plugin is to be used by output plugins to provide a link
to additional information about the alert produced.
Make sure to also take a look at http://www.snort.org/pub-bin/sigs-search.cgi/ for a system that is indexing
descriptions of alerts based on of the sid (See Section 3.4.4).

System
bugtraq
cve
nessus
arachnids
mcafee
url

Table 3.1: Supported Systems
URL Prefix
http://www.securityfocus.com/bid/
http://cve.mitre.org/cgi-bin/cvename.cgi?name=
http://cgi.nessus.org/plugins/dump.php3?id=
(currently down) http://www.whitehats.com/info/IDS
http://vil.nai.com/vil/dispVirus.asp?virus k=
http://

Format
reference: ,; [reference: ,;]
Examples
alert tcp any any -> any 7070 (msg:"IDS411/dos-realaudio"; \
flags:AP; content:"|fff4 fffd 06|"; reference:arachnids,IDS411;)
alert tcp any any -> any 21 (msg:"IDS287/ftp-wuftp260-venglin-linux"; \
flags:AP; content:"|31c031db 31c9b046 cd80 31c031db|"; \

117

reference:arachnids,IDS287; reference:bugtraq,1387; \
reference:cve,CAN-2000-1574;)

3.4.3 gid
The gid keyword (generator id) is used to identify what part of Snort generates the event when a particular rule
fires. For example gid 1 is associated with the rules subsystem and various gids over 100 are designated for specific
preprocessors and the decoder. See etc/generators in the source tree for the current generator ids in use. Note that the
gid keyword is optional and if it is not specified in a rule, it will default to 1 and the rule will be part of the general rule
subsystem. To avoid potential conflict with gids defined in Snort (that for some reason aren’t noted it etc/generators),
it is recommended that a value greater than 1,000,000 be used. For general rule writing, it is not recommended that
the gid keyword be used. This option should be used with the sid keyword. (See section 3.4.4)
The file etc/gen-msg.map contains contains more information on preprocessor and decoder gids.
Format
gid: ;
Example
This example is a rule with a generator id of 1000001.
alert tcp any any -> any 80 (content:"BOB"; gid:1000001; sid:1; rev:1;)

3.4.4 sid
The sid keyword is used to uniquely identify Snort rules. This information allows output plugins to identify rules
easily. This option should be used with the rev keyword. (See section 3.4.5)
• <100 Reserved for future use
• 100-1,000,000 Rules included with the Snort distribution
• >1,000,000 Used for local rules
The file sid-msg.map contains a mapping of alert messages to Snort rule IDs. This information is useful when postprocessing alert to map an ID to an alert message.
Format
sid: ;
Example
This example is a rule with the Snort Rule ID of 1000983.
alert tcp any any -> any 80 (content:"BOB"; sid:1000983; rev:1;)

118

3.4.5 rev
The rev keyword is used to uniquely identify revisions of Snort rules. Revisions, along with Snort rule id’s, allow
signatures and descriptions to be refined and replaced with updated information. This option should be used with the
sid keyword. (See section 3.4.4)
Format
rev: ;
Example
This example is a rule with the Snort Rule Revision of 1.
alert tcp any any -> any 80 (content:"BOB"; sid:1000983; rev:1;)

3.4.6 classtype
The classtype keyword is used to categorize a rule as detecting an attack that is part of a more general type of attack
class. Snort provides a default set of attack classes that are used by the default set of rules it provides. Defining
classifications for rules provides a way to better organize the event data Snort produces.
Format
classtype: ;
Example
alert tcp any any -> any 25 (msg:"SMTP expn root"; flags:A+; \
content:"expn root"; nocase; classtype:attempted-recon;)
Attack classifications defined by Snort reside in the classification.config file. The file uses the following syntax:
config classification:

These attack classifications are listed in Table 3.2. They are currently ordered with 3 default priorities. A priority of 1
(high) is the most severe and 3 (low) is the least severe.
Table 3.2: Snort Default Classifications
Classtype
attempted-admin
attempted-user
kickass-porn
policy-violation
shellcode-detect
successful-admin
successful-user
trojan-activity
unsuccessful-user
web-application-attack

Description
Attempted Administrator Privilege Gain
Attempted User Privilege Gain
SCORE! Get the lotion!
Potential Corporate Privacy Violation
Executable code was detected
Successful Administrator Privilege Gain
Successful User Privilege Gain
A Network Trojan was detected
Unsuccessful User Privilege Gain
Web Application Attack
119

Priority
high
high
high
high
high
high
high
high
high
high

attempted-dos
attempted-recon
bad-unknown
default-login-attempt
denial-of-service
misc-attack
non-standard-protocol
rpc-portmap-decode
successful-dos
successful-recon-largescale
successful-recon-limited
suspicious-filename-detect
suspicious-login
system-call-detect
unusual-client-port-connection
web-application-activity
icmp-event
misc-activity
network-scan
not-suspicious
protocol-command-decode
string-detect
unknown
tcp-connection

Attempted Denial of Service
Attempted Information Leak
Potentially Bad Traffic
Attempt to login by a default username and
password
Detection of a Denial of Service Attack
Misc Attack
Detection of a non-standard protocol or event
Decode of an RPC Query
Denial of Service
Large Scale Information Leak
Information Leak
A suspicious filename was detected
An attempted login using a suspicious username was detected
A system call was detected
A client was using an unusual port
Access to a potentially vulnerable web application
Generic ICMP event
Misc activity
Detection of a Network Scan
Not Suspicious Traffic
Generic Protocol Command Decode
A suspicious string was detected
Unknown Traffic
A TCP connection was detected

medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
medium
low
low
low
low
low
low
low
very low

Warnings
The classtype option can only use classifications that have been defined in snort.conf by using the config
classification option. Snort provides a default set of classifications in classification.config that are used
by the rules it provides.

3.4.7 priority
The priority tag assigns a severity level to rules. A classtype rule assigns a default priority (defined by the config
classification option) that may be overridden with a priority rule. Examples of each case are given below.
Format
priority: ;
Examples
alert TCP any any -> any 80 (msg: "WEB-MISC phf attempt"; flags:A+; \
content: "/cgi-bin/phf"; priority:10;)
alert tcp any any -> any 80 (msg:"EXPLOIT ntpdx overflow"; \
dsize: >128; classtype:attempted-admin; priority:10 );
120

3.4.8 metadata
The metadata tag allows a rule writer to embed additional information about the rule, typically in a key-value format.
Certain metadata keys and values have meaning to Snort and are listed in Table 3.3. Keys other than those listed in the
table are effectively ignored by Snort and can be free-form, with a key and a value. Multiple keys are separated by a
comma, while keys and values are separated by a space.

Key
engine
soid
service

Table 3.3: Snort Metadata Keys
Description
Indicate a Shared Library Rule
Shared Library Rule Generator and SID
Target-Based Service Identifier

Value Format
”shared”
gid|sid
”http”

! NOTE
△

The service Metadata Key is only meaningful when a Host Atttribute Table is provided. When the value
exactly matches the service ID as specified in the table, the rule is applied to that packet, otherwise, the rule
is not applied (even if the ports specified in the rule match). See Section 2.7 for details on the Host Attribute
Table.

.
Format
The examples below show an stub rule from a shared library rule. The first uses multiple metadata keywords, the
second a single metadata keyword, with keys separated by commas.
metadata: key1 value1;
metadata: key1 value1, key2 value2;
Examples
alert tcp any any -> any 80 (msg: "Shared Library Rule Example"; \
metadata:engine shared; metadata:soid 3|12345;)
alert tcp any any -> any 80 (msg: "Shared Library Rule Example"; \
metadata:engine shared, soid 3|12345;)
alert tcp any any -> any 80 (msg: "HTTP Service Rule Example"; \
metadata:service http;)

3.4.9 General Rule Quick Reference
Table 3.4: General rule option keywords
Keyword
msg
reference
gid

Description
The msg keyword tells the logging and alerting engine the message to print with
the packet dump or alert.
The reference keyword allows rules to include references to external attack identification systems.
The gid keyword (generator id) is used to identify what part of Snort generates the
event when a particular rule fires.
121

sid
rev
classtype
priority
metadata

The sid keyword is used to uniquely identify Snort rules.
The rev keyword is used to uniquely identify revisions of Snort rules.
The classtype keyword is used to categorize a rule as detecting an attack that is
part of a more general type of attack class.
The priority keyword assigns a severity level to rules.
The metadata keyword allows a rule writer to embed additional information about
the rule, typically in a key-value format.

3.5 Payload Detection Rule Options
3.5.1 content
The content keyword is one of the more important features of Snort. It allows the user to set rules that search for
specific content in the packet payload and trigger response based on that data. Whenever a content option pattern
match is performed, the Boyer-Moore pattern match function is called and the (rather computationally expensive) test
is performed against the packet contents. If data exactly matching the argument data string is contained anywhere
within the packet’s payload, the test is successful and the remainder of the rule option tests are performed. Be aware
that this test is case sensitive.
The option data for the content keyword is somewhat complex; it can contain mixed text and binary data. The binary
data is generally enclosed within the pipe (|) character and represented as bytecode. Bytecode represents binary data
as hexadecimal numbers and is a good shorthand method for describing complex binary data. The example below
shows use of mixed text and binary data in a Snort rule.
Note that multiple content rules can be specified in one rule. This allows rules to be tailored for less false positives.
If the rule is preceded by a !, the alert will be triggered on packets that do not contain this content. This is useful when
writing rules that want to alert on packets that do not match a certain pattern

! NOTE
△

Also note that the following characters must be escaped inside a content rule:
: ; \ "

Format
content: [!] "";
Examples
alert tcp any any -> any 139 (content:"|5c 00|P|00|I|00|P|00|E|00 5c|";)
alert tcp any any -> any 80 (content:!"GET";)

! NOTE
△

A ! modifier negates the results of the entire content search, modifiers included. For example, if using
content:!"A"; within:50; and there are only 5 bytes of payload and there is no ”A” in those 5 bytes, the
result will return a match. If there must be 50 bytes for a valid match, use isdataat as a pre-cursor to the
content.

122

Changing content behavior
The content keyword has a number of modifier keywords. The modifier keywords change how the previously specified content works. These modifier keywords are:
Table 3.5: Content Modifiers
Modifier
Section
nocase
3.5.2
rawbytes
3.5.3
depth
3.5.4
offset
3.5.5
distance
3.5.6
within
3.5.7
http client body
3.5.8
http cookie
3.5.9
http header
3.5.10
http method
3.5.11
http uri
3.5.12
fast pattern
3.5.13

3.5.2 nocase
The nocase keyword allows the rule writer to specify that the Snort should look for the specific pattern, ignoring case.
nocase modifies the previous ’content’ keyword in the rule.
Format
nocase;
Example
alert tcp any any -> any 21 (msg:"FTP ROOT"; content:"USER root"; nocase;)

3.5.3 rawbytes
The rawbytes keyword allows rules to look at the raw packet data, ignoring any decoding that was done by preprocessors. This acts as a modifier to the previous content 3.5.1 option.
format
rawbytes;
Example
This example tells the content pattern matcher to look at the raw traffic, instead of the decoded traffic provided by the
Telnet decoder.
alert tcp any any -> any 21 (msg: "Telnet NOP"; content: "|FF F1|"; rawbytes;)

123

3.5.4 depth
The depth keyword allows the rule writer to specify how far into a packet Snort should search for the specified pattern.
depth modifies the previous ‘content’ keyword in the rule.
A depth of 5 would tell Snort to only look for the specified pattern within the first 5 bytes of the payload.
As the depth keyword is a modifier to the previous ‘content’ keyword, there must be a content in the rule before ‘depth’
is specified.
Format
depth: ;

3.5.5 offset
The offset keyword allows the rule writer to specify where to start searching for a pattern within a packet. offset
modifies the previous ’content’ keyword in the rule.
An offset of 5 would tell Snort to start looking for the specified pattern after the first 5 bytes of the payload.
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’offset’ is
specified.
Format
offset: ;
Example
The following example shows use of a combined content, offset, and depth search rule.
alert tcp any any -> any 80 (content: "cgi-bin/phf"; offset:4; depth:20;)

3.5.6 distance
The distance keyword allows the rule writer to specify how far into a packet Snort should ignore before starting to
search for the specified pattern relative to the end of the previous pattern match.
This can be thought of as exactly the same thing as offset (See Section 3.5.5), except it is relative to the end of the last
pattern match instead of the beginning of the packet.
Format
distance: ;
Example
The rule below maps to a regular expression of /ABC.{1}DEF/.
alert tcp any any -> any any (content:"ABC"; content: "DEF"; distance:1;)

124

3.5.7 within
The within keyword is a content modifier that makes sure that at most N bytes are between pattern matches using the
content keyword ( See Section 3.5.1 ). It’s designed to be used in conjunction with the distance (Section 3.5.6) rule
option.
Format
within: ;
Examples
This rule constrains the search of EFG to not go past 10 bytes past the ABC match.
alert tcp any any -> any any (content:"ABC"; content: "EFG"; within:10;)

3.5.8 http client body
The http client body keyword is a content modifier that restricts the search to the NORMALIZED body of an HTTP
client request.
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’http client body’
is specified.
Format
http_client_body;
Examples
This rule constrains the search for the pattern ”EFG” to the NORMALIZED body of an HTTP client request.
alert tcp any any -> any 80 (content:"ABC"; content: "EFG"; http_client_body;)

! NOTE
△

The http client body modifier is not allowed to be used with the rawbytes modifier for the same content.

3.5.9 http cookie
The http cookie keyword is a content modifier that restricts the search to the extracted Cookie Header field of an HTTP
client request.
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’http cookie’
is specified.
The extracted Cookie Header field may be NORMALIZED, per the configuration of HttpInspect (see 2.2.6).
Format
http_cookie;

125

Examples
This rule constrains the search for the pattern ”EFG” to the extracted Cookie Header field of an HTTP client request.
alert tcp any any -> any 80 (content:"ABC"; content: "EFG"; http_cookie;)

! NOTE
△

The http cookie modifier is not allowed to be used with the rawbytes or fast pattern modifiers for the
same content.

3.5.10 http header
The http header keyword is a content modifier that restricts the search to the extracted Header fields of an HTTP client
request.
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’http header’
is specified.
The extracted Header fields may be NORMALIZED, per the configuration of HttpInspect (see 2.2.6).
Format
http_header;
Examples
This rule constrains the search for the pattern ”EFG” to the extracted Header fields of an HTTP client request.
alert tcp any any -> any 80 (content:"ABC"; content: "EFG"; http_header;)

! NOTE
△

The http header modifier is not allowed to be used with the rawbytes modifier for the same content.

3.5.11 http method
The http method keyword is a content modifier that restricts the search to the extracted Method from an HTTP client
request.
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’http method’
is specified.
Format
http_method;

126

Examples
This rule constrains the search for the pattern ”GET” to the extracted Method from an HTTP client request.
alert tcp any any -> any 80 (content:"ABC"; content: "GET"; http_method;)

! NOTE
△
The http method modifier is not allowed to be used with the rawbytes modifier for the same content.

3.5.12 http uri
The http uri keyword is a content modifier that restricts the search to the NORMALIZED request URI field . Using a
content rule option followed by a http uri modifier is the same as using a uricontent by itself (see: 3.5.14).
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’http uri’
is specified.
Format
http_uri;
Examples
This rule constrains the search for the pattern ”EFG” to the NORMALIZED URI.
alert tcp any any -> any 80 (content:"ABC"; content: "EFG"; http_uri;)

! NOTE
△
The http uri modifier is not allowed to be used with the rawbytes modifier for the same content.

3.5.13 fast pattern
The fast pattern keyword is a content modifier that sets the content within a rule to be used with the Fast Pattern
Matcher. It overrides the default of using the longest content within the rule.
fast pattern may be specified at most once for each of the buffer modifiers (excluding the http cookie modifier).
As this keyword is a modifier to the previous ’content’ keyword, there must be a content in the rule before ’fast pattern’
is specified.
Format
fast_pattern;
Examples
This rule causes the pattern ”EFG” to be used with the Fast Pattern Matcher, even though it is shorter than the earlier
pattern ”ABCD”.
alert tcp any any -> any 80 (content:"ABCD"; content: "EFG"; fast_pattern;)
127

! NOTE
△

The fast pattern modifier is not allowed to be used with the http cookie modifier for the same content,
nor with a content that is negated with a !.

3.5.14 uricontent
The uricontent keyword in the Snort rule language searches the NORMALIZED request URI field. This means that
if you are writing rules that include things that are normalized, such as %2f or directory traversals, these rules will not
alert. The reason is that the things you are looking for are normalized out of the URI buffer.
For example, the URI:
/scripts/..%c0%af../winnt/system32/cmd.exe?/c+ver
will get normalized into:
/winnt/system32/cmd.exe?/c+ver
Another example, the URI:
/cgi-bin/aaaaaaaaaaaaaaaaaaaaaaaaaa/..%252fp%68f?
will get normalized into:
/cgi-bin/phf?
When writing a uricontent rule, write the content that you want to find in the context that the URI will be normalized.
For example, if Snort normalizes directory traversals, do not include directory traversals.
You can write rules that look for the non-normalized content by using the content option. (See Section 3.5.1)
For a description of the parameters to this function, see the content rule options in Section 3.5.1.
This option works in conjunction with the HTTP Inspect preprocessor specified in Section 2.2.6.
Format
uricontent:[!];

! NOTE
△

uricontent cannot be modified by a rawbytes modifier.

3.5.15 urilen
The urilen keyword in the Snort rule language specifies the exact length, the minimum length, the maximum length,
or range of URI lengths to match.

128

Format
urilen: int<>int;
urilen: [<,>] ;
The following example will match URIs that are 5 bytes long:
urilen: 5
The following example will match URIs that are shorter than 5 bytes:
urilen: < 5
The following example will match URIs that are greater than 5 bytes and less than 10 bytes:
urilen: 5<>10
This option works in conjunction with the HTTP Inspect preprocessor specified in Section 2.2.6.

3.5.16 isdataat
Verify that the payload has data at a specified location, optionally looking for data relative to the end of the previous
content match.
Format
isdataat:[,relative];
Example
alert tcp any any -> any 111 (content:"PASS"; isdataat:50,relative; \
content:!"|0a|"; within:50;)
This rule looks for the string PASS exists in the packet, then verifies there is at least 50 bytes after the end of the string
PASS, then verifies that there is not a newline character within 50 bytes of the end of the PASS string.

3.5.17 pcre
The pcre keyword allows rules to be written using perl compatible regular expressions. For more detail on what can
be done via a pcre regular expression, check out the PCRE web site http://www.pcre.org
Format
pcre:[!]"(//|m)[ismxAEGRUBPHMCO]";
The post-re modifiers set compile time flags for the regular expression. See tables 3.6, 3.7, and 3.8 for descriptions of
each modifier.

! NOTE
△
The modifiers R and B should not be used together.
129

i
s
m

A
E

Table 3.6: Perl compatible modifiers for pcre
case insensitive
include newlines in the dot metacharacter
By default, the string is treated as one big line of characters. ˆ and $ match at
the beginning and ending of the string. When m is set, ˆ and $ match immediately
following or immediately before any newline in the buffer, as well as the very start
and very end of the buffer.
whitespace data characters in the pattern are ignored except when escaped or inside a character class

Table 3.7: PCRE compatible modifiers for pcre
the pattern must match only at the start of the buffer (same as ˆ )
Set $ to match only at the end of the subject string. Without E, $ also matches
immediately before the final character if it is a newline (but not before any other
newlines).
Inverts the ”greediness” of the quantifiers so that they are not greedy by default,
but become greedy if followed by ”?”.

Example
This example performs a case-insensitive search for the string BLAH in the payload.
alert ip any any -> any any (pcre:"/BLAH/i";)

! NOTE
△

Snort’s handling of multiple URIs with PCRE does not work as expected. PCRE when used without a
uricontent only evaluates the first URI. In order to use pcre to inspect all URIs, you must use either a
content or a uricontent.

3.5.18 byte test
Test a byte field against a specific value (with operator). Capable of testing binary values or converting representative
byte strings to their binary equivalent and testing them.
For a more detailed explanation, please read Section 3.9.5.
Format
byte_test: , [!], ,
[,relative] [,] [,, string];

130

R
U
P
H
M
C
B
O

Table 3.8: Snort specific modifiers for pcre
Match relative to the end of the last pattern match. (Similar to distance:0;)
Match the decoded URI buffers (Similar to uricontent and http uri)
Match normalized HTTP request body (Similar to http client body)
Match normalized HTTP request header (Similar to http header)
Match normalized HTTP request method (Similar to http method)
Match normalized HTTP request cookie (Similar to http cookie)
Do not use the decoded buffers (Similar to rawbytes)
Override the configured pcre match limit for this expression (See section 2.1.3)

Option
bytes to convert
operator

Description
Number of bytes to pick up from the packet
Operation to perform to test the value:
• < - less than
• > - greater than
• = - equal
• ! - not
• & - bitwise AND
• ˆ - bitwise OR

value
offset
relative
endian

Value to test the converted value against
Number of bytes into the payload to start processing
Use an offset relative to last pattern match
Endian type of the number being read:
• big - Process data as big endian (default)
• little - Process data as little endian

string
number type

Data is stored in string format in packet
Type of number being read:
• hex - Converted string data is represented in hexadecimal
• dec - Converted string data is represented in decimal
• oct - Converted string data is represented in octal

dce

Let the DCE/RPC 2 preprocessor determine the byte order of the value to be converted. See section 2.2.14 for a description and examples (2.2.14 for quick reference).

Any of the operators can also include ! to check if the operator is not true. If ! is specified without an operator, then
the operator is set to =.

! NOTE
△

Snort uses the C operators for each of these operators. If the & operator is used, then it would be the same as
using if (data & value) { do something();}

131

Examples
alert udp $EXTERNAL_NET any -> $HOME_NET any \
(msg:"AMD procedure 7 plog overflow "; \
content: "|00 04 93 F3|"; \
content: "|00 00 00 07|"; distance: 4; within: 4; \
byte_test: 4,>, 1000, 20, relative;)
alert tcp $EXTERNAL_NET any -> $HOME_NET any \
(msg:"AMD procedure 7 plog overflow "; \
content: "|00 04 93 F3|"; \
content: "|00 00 00 07|"; distance: 4; within: 4; \
byte_test: 4, >,1000, 20, relative;)
alert udp any any -> any 1234 \
(byte_test: 4, =, 1234, 0, string, dec; \
msg: "got 1234!";)
alert udp any any -> any 1235 \
(byte_test: 3, =, 123, 0, string, dec; \
msg: "got 123!";)
alert udp any any -> any 1236 \
(byte_test: 2, =, 12, 0, string, dec; \
msg: "got 12!";)
alert udp any any -> any 1237 \
(byte_test: 10, =, 1234567890, 0, string, dec; \
msg: "got 1234567890!";)
alert udp any any -> any 1238 \
(byte_test: 8, =, 0xdeadbeef, 0, string, hex; \
msg: "got DEADBEEF!";)

3.5.19 byte jump
The byte jump keyword allows rules to be written for length encoded protocols trivially. By having an option that
reads the length of a portion of data, then skips that far forward in the packet, rules can be written that skip over
specific portions of length-encoded protocols and perform detection in very specific locations.
The byte jump option does this by reading some number of bytes, convert them to their numeric representation, move
that many bytes forward and set a pointer for later detection. This pointer is known as the detect offset end pointer, or
doe ptr.
For a more detailed explanation, please read Section 3.9.5.
Format
byte_jump: , \
[,relative] [,multiplier ] [,big] [,little][,string]\
[,hex] [,dec] [,oct] [,align] [,from_beginning] [,post_offset ];

132

Option
bytes to convert
offset
relative
multiplier
big
little
string
hex
dec
oct
align
from beginning
post offset
dce

Description
Number of bytes to pick up from the packet
Number of bytes into the payload to start processing
Use an offset relative to last pattern match
Multiply the number of calculated bytes by and skip forward that number of bytes.
Process data as big endian (default)
Process data as little endian
Data is stored in string format in packet
Converted string data is represented in hexadecimal
Converted string data is represented in decimal
Converted string data is represented in octal
Round the number of converted bytes up to the next 32-bit boundary
Skip forward from the beginning of the packet payload instead of from the current
position in the packet.
Skip forward or backwards (positive of negative value) by number of
bytes after the other jump options have been applied.
Let the DCE/RPC 2 preprocessor determine the byte order of the value to be converted. See section 2.2.14 for a description and examples (2.2.14 for quick reference).

Example
alert udp any any -> any 32770:34000 (content: "|00 01 86 B8|"; \
content: "|00 00 00 01|"; distance: 4; within: 4; \
byte_jump: 4, 12, relative, align; \
byte_test: 4, >, 900, 20, relative; \
msg: "statd format string buffer overflow";)

3.5.20 ftpbounce
The ftpbounce keyword detects FTP bounce attacks.
Format
ftpbounce;
Example
alert tcp $EXTERNAL_NET any -> $HOME_NET 21 (msg:"FTP PORT bounce attempt"; \
flow:to_server,established; content:"PORT"; nocase; ftpbounce; pcre:"/ˆPORT/smi";\
classtype:misc-attack; sid:3441; rev:1;)

3.5.21 asn1
The ASN.1 detection plugin decodes a packet or a portion of a packet, and looks for various malicious encodings.
Multiple options can be used in an ’asn1’ option and the implied logic is boolean OR. So if any of the arguments
evaluate as true, the whole option evaluates as true.
The ASN.1 options provide programmatic detection capabilities as well as some more dynamic type detection. If an
option has an argument, the option and the argument are separated by a space or a comma. The preferred usage is to
use a space between option and argument.

133

Format
asn1: option[ argument][, option[ argument]] . . .
Option
bitstring overflow
double overflow

oversize length

absolute offset

relative offset

Description
Detects invalid bitstring encodings that are known to be remotely exploitable.
Detects a double ASCII encoding that is larger than a standard buffer. This is
known to be an exploitable function in Microsoft, but it is unknown at this time
which services may be exploitable.
Compares ASN.1 type lengths with the supplied argument. The syntax looks like,
“oversize length 500”. This means that if an ASN.1 type is greater than 500, then
this keyword is evaluated as true. This keyword must have one argument which
specifies the length to compare against.
This is the absolute offset from the beginning of the packet. For example,
if you wanted to decode snmp packets, you would say “absolute offset 0”.
absolute offset has one argument, the offset value. Offset may be positive
or negative.
This is the relative offset from the last content match or byte test/jump.
relative offset has one argument, the offset number. So if you wanted to
start decoding and ASN.1 sequence right after the content “foo”, you would specify ’content:"foo"; asn1: bitstring_overflow, relative_offset 0’.
Offset values may be positive or negative.

Examples
alert udp any any -> any 161 (msg:"Oversize SNMP Length"; \
asn1: oversize_length 10000, absolute_offset 0;)
alert tcp any any -> any 80 (msg:"ASN1 Relative Foo"; content:"foo"; \
asn1: bitstring_overflow, relative_offset 0;)

3.5.22 cvs
The CVS detection plugin aids in the detection of: Bugtraq-10384, CVE-2004-0396: ”Malformed Entry Modified and
Unchanged flag insertion”. Default CVS server ports are 2401 and 514 and are included in the default ports for stream
reassembly.

! NOTE
△
This plugin cannot do detection over encrypted sessions, e.g. SSH (usually port 22).
Format
cvs:

Source Exif Data:
File Type : PDF File Type Extension : pdf MIME Type : application/pdf PDF Version : 1.2 Linearized : No Page Count : 178 Page Mode : UseOutlines Producer : dvips + ESP Ghostscript 815.04 Create Date : 2009:10:19 17:18:48 Modify Date : 2009:10:19 17:18:48 Title : Subject : Creator : LaTeX with hyperref package Author :
EXIF Metadata provided by EXIF.tools

Snort Manual

Navigation menu

Versions of this User Manual:

Views

Navigation