Snort Manual

User Manual:

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Snort Overview
Configuring Snort
Writing Snort Rules
Making Snort Faster
- MMAPed pcap
Dynamic Modules
Snort Development

SNORT R

Users Manual

2.8.5

The Snort Project

October 19, 2009

1998-2003 Martin Roesch

2001-2003 Chris Green

2003-2009 Sourceﬁre, Inc.

Contents

1 Snort Overview 8

1.1 Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2 Sniffer Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Packet Logger Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Network Intrusion Detection System Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.1 NIDS Mode Output Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.2 Understanding Standard Alert Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4.3 High Performance Conﬁguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4.4 Changing Alert Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.5 Inline Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.5.1 Snort Inline Rule Application Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.5.2 Replacing Packets with Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.5.3 Installing Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.5.4 Running Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.5.5 Using the Honeynet Snort Inline Toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.5.6 Troubleshooting Snort Inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.6 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.6.1 Running Snort as a Daemon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.6.2 Running in Rule Stub Creation Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.6.3 Obfuscating IP Address Printouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.6.4 Specifying Multiple-Instance Identiﬁers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.7 Reading Pcaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.7.1 Command line arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.7.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.8 Tunneling Protocol Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.8.1 Multiple Encapsulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.8.2 Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.9 More Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2 Conﬁguring Snort 20

2.1 Includes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.1.1 Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.1.2 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.1.3 Conﬁg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2 Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2.1 Frag3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2.2 Stream5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.3 sfPortscan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2.4 RPC Decode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.2.5 Performance Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.2.6 HTTP Inspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.2.7 SMTP Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

2.2.8 FTP/Telnet Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.2.9 SSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

2.2.10 DCE/RPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

2.2.11 DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.2.12 SSL/TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

2.2.13 ARP Spoof Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

2.2.14 DCE/RPC 2 Preprocessor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

2.3 Decoder and Preprocessor Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

2.3.1 Conﬁguring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

2.3.2 Reverting to original behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

2.4 Event Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

2.4.1 Rate Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

2.4.2 Event Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

2.4.3 Event Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

2.4.4 Event Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

2.5 Performance Proﬁling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

2.5.1 Rule Proﬁling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

2.5.2 Preprocessor Proﬁling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

2.5.3 Packet Performance Monitoring (PPM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

2.6 Output Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

2.6.1 alert syslog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

2.6.2 alert fast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2.6.3 alert full . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2.6.4 alert unixsock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

2.6.5 log tcpdump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

2.6.6 database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

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

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

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

2.8 Dynamic Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.8.1 Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.8.2 Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

2.9 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

3.4.7 priority . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

3.4.8 metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

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

3.5 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

3.9.1 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

4 Making Snort Faster 157

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

5 Dynamic Modules 158

5.1 Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.1.1 DynamicPluginMeta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.1.2 DynamicPreprocessorData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.1.3 DynamicEngineData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

5.1.4 SFSnortPacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

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

5.2 Required Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

5.2.1 Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

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

5.2.3 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

5.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

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

5.3.2 Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

6 Snort Development 174

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

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

6.2.1 Preprocessors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

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

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

6.3 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 <cmg@snort.org>.

It was then maintained by Brian Caswell <bmc@snort.org>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 L

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

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

-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 mecha-

nisms 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 Description

-A fast

Fast alert mode. Writes the alert in a simple format with a timestamp, alert message, source and

destination IPs/ports.

-A full

Full alert mode. This is the default alert mode and will be used automatically if you do not specify

a mode.

-A unsock

Sends alerts to a UNIX socket that another program can listen on.

-A none

Turns off alerting.

-A console

Sends “fast-style” alerts to the console (screen).

-A cmg

Generates “cmg style” alerts.

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 inter-

face 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.

•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 <configuration file>

This will display the header of every packet that Snort Inline sees.

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-rulesoption.

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

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 Description

-r <file>

Read a single pcap.

--pcap-single=<file>

Same as -r. Added for completeness.

--pcap-file=<file>

File that contains a list of pcaps to read. Can speciﬁy path to pcap or directory to

recurse to get pcaps.

--pcap-list="<list>"

A space separated list of pcaps to read.

--pcap-dir=<dir>

A directory to recurse to look for pcaps. Sorted in ascii order.

--pcap-filter=<filter>

Shell style ﬁlter to apply when getting pcaps from ﬁle or directory. This ﬁl-

ter will apply to any

--pcap-file

--pcap-dir

arguments following. Use

--pcap-no-filter

to delete ﬁlter for following

--pcap-file

--pcap-dir

arguments or speciﬁy

--pcap-filter

again to forget previous ﬁlter and to apply

to following

--pcap-file

--pcap-dir

arguments.

--pcap-no-filter

Reset to use no ﬁlter when getting pcaps from ﬁle or directory.

--pcap-reset

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.

--pcap-show

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.

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.

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

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 -?

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

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 <include file path/name>

△

!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

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 Description

var

Deﬁnes a meta-variable.

$(var) or $var

Replaces with the contents of variable

var

$(var:-default)

Replaces the contents of the variable

var

with “default” if

var

is undeﬁned.

$(var:?message)

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 <directive> [: <value>]

Conﬁg Directive Description

config alert with interface name

Appends interface name to alert (

snort -I

config alertfile: <filename>

Sets the alerts output ﬁle.

config asn1: <max-nodes>

Speciﬁes the maximum number of nodes to track when doing

ASN1 decoding. See Section 3.5.21 for more information and

examples.

config autogenerate preprocessor

decoder rules

If Snort was conﬁgured to enable decoder and preprocessor

rules, this option will cause Snort to revert back to it’s origi-

nal behavior of alerting if the decoder or preprocessor generates

an event.

config bpf file: <filename>

Speciﬁes BPF ﬁlters (

snort -F

config checksum drop: <types>

Types of packets to drop if invalid checksums. Values:

none

noip

notcp

noicmp

noudp

tcp

udp

icmp

all

(only applicable in inline mode and for packets checked per

checksum mode

conﬁg option).

config checksum mode: <types>

Types of packets to calculate checksums. Values:

none

noip

notcp

noicmp

noudp

tcp

udp

icmp

all

config chroot: <dir>

Chroots to speciﬁed dir (

snort -t

config classification: <class>

See Table 3.2 for a list of classiﬁcations.

config daemon

Forks as a daemon (

snort -D

config decode data link

Decodes Layer2 headers (

snort -e

config default rule state: <state>

Global conﬁguration directive to enable or disable the loading

of rules into the detection engine. Default (with or without di-

rective) is enabled. Specify

disabled

to disable loading rules.

config detection: <search-method>

[lowmem] [no stream inserts]

[max queue events <num>]

Makes changes to the detection engine. The following options

can be used:

•

search-method

ac-std

ac-bnfa

acs

ac-banded

ac-sparsebands

lowmem

–

Aho-Corasick Full (high memory, best perfor-

mance)

–

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 mem-

ory, low performance)

•

no stream inserts

•

max queue events

integer

config disable decode alerts

Turns off the alerts generated by the decode phase of Snort.

config disable inline init failopen

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

)

config disable ipopt alerts

Disables IP option length validation alerts.

config disable tcpopt alerts

Disables option length validation alerts.

config

disable tcpopt experimental alerts

Turns off alerts generated by experimental TCP options.

config disable tcpopt obsolete alerts

Turns off alerts generated by obsolete TCP options.

config disable tcpopt ttcp alerts

Turns off alerts generated by T/TCP options.

config disable ttcp alerts

Turns off alerts generated by T/TCP options.

config dump chars only

Turns on character dumps (

snort -C

config dump payload

Dumps application layer (

snort -d

config dump payload verbose

Dumps raw packet starting at link layer (

snort -X

config enable decode drops

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 op-

tions (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 net-

work. 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

enable tcpopt experimental drops

Enables the dropping of bad packets with experimental TCP op-

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

<bytes>

Set global memcap in bytes for thresholding. Default is

1048576 bytes (1 megabyte).

config event queue: [max queue

<num>] [log <num>] [order events

<order>]

Speciﬁes conditions about Snort’s event queue. You can use the

following options:

•

max queue

integer

>(max events supported)

•

log

integer

>(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:

<num-resets>

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)

config flexresp2 interface:

<iface>

Specify the response interface to use. In Windows this can also

be the interface number. (Snort must be compiled with –enable-

ﬂexresp2)

config flexresp2 memcap: <bytes>

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)

config flexresp2 rows: <num-rows>

Specify the number of rows for the hash table used to track the

time of responses. Default is 1024 rows. (Snort must be com-

piled with –enable-ﬂexresp2)

config flowbits size: <num-bits>

Speciﬁes the maximum number of ﬂowbit tags that can be used

within a rule set.

config ignore ports: <proto>

<port-list>

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.

config interface: <iface>

Sets the network interface (

snort -i

config ipv6 frag:

[bsd icmp frag alert on|off]

[, bad ipv6 frag alert on|off]

[, frag timeout <secs>] [,

max frag sessions <max-track>]

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

integer

>(Specify amount of time in

seconds to timeout ﬁrst frag in hash table)

•

max frag sessions

integer

>(Specify the number

of fragments to track in the hash table)

config layer2resets: <mac-addr>

This option is only available when running in inline mode. See

Section 1.5.

config logdir: <dir>

Sets the logdir (

snort -l

config max attribute hosts: <hosts>

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 sup-

ported with a Host Attribute Table (see section 2.7).

config max mpls labelchain len:

<num-hdrs>

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.

config min ttl: <ttl>

Sets a Snort-wide minimum ttl to ignore all trafﬁc.

config mpls payload type:

ipv4|ipv6|ethernet

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

config no promisc

Disables promiscuous mode (

snort -p

config nolog

Disables logging. Note: Alerts will still occur. (

snort -N

config nopcre

Disables pcre pattern matching.

config obfuscate

Obfuscates IP Addresses (

snort -O

config order: <order>

Changes the order that rules are evaluated, eg: pass alert log

activation.

config pcre match limit:

integer

Restricts the amount of backtracking a given PCRE option. For

example, it will limit the number of nested repeats within a pat-

tern. 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.

config pcre match limit recursion:

integer

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

config pkt count: <N>

Exits after N packets (

snort -n

config policy version:

base-version-string

[

binding-version-string

]

Supply versioning information to conﬁguration ﬁles. Base ver-

sion should be a string in all conﬁguration ﬁles including in-

cluded 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.

config profile preprocs

Print statistics on preprocessor performance. See Section 2.5.2

for more details.

config profile rules

Print statistics on rule performance. See Section 2.5.1 for more

details.

config quiet

Disables banner and status reports (

snort -q

config read bin file: <pcap>

Speciﬁes a pcap ﬁle to use (instead of reading from network),

same effect as -r <tf>option.

config reference: <ref>

Adds a new reference system to Snort, eg: myref

http://myurl.com/?id=

config reference net <cidr>

For IP obfuscation, the obfuscated net will be used if the packet

contains an IP address in the reference net. Also used to de-

termine 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.

config set gid: <gid>

Changes GID to speciﬁed GID (

snort -g

set uid: <uid>

Sets UID to <id>(

snort -u

config show year

Shows year in timestamps (

snort -y

config snaplen: <bytes>

Set the snaplength of packet, same effect as

-P

snaplen

>or

--snaplen

snaplen

>options.

config stateful

Sets assurance mode for stream (stream is established).

config tagged packet limit:

<max-tag>

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 con-

ﬁgured is 256 packets. Setting this option to a value of 0 will

disable the packet limit.

config threshold: memcap <bytes>

Set global memcap in bytes for thresholding. Default is

1048576 bytes (1 megabyte). (This is deprecated. Use conﬁg

event ﬁlter instead.)

config timestats interval: <secs>

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

config umask: <umask>

Sets umask when running (

snort -m

config utc

Uses UTC instead of local time for timestamps (

snort -U

config verbose

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 <name>: <options>

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 partic-

ular 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 net-

work and determining how their various IP stack implementations handled the types of problems seen in IP defragmen-

tation 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.

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

number

>- Maximum simultaneous fragments to track. Default is 8192.

–

memcap

bytes

>- Memory cap for self preservation. Default is 4MB.

–

prealloc frags

number

>- 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

seconds

>- Timeout for fragments. Fragments in the engine for longer than this period will

be automatically dropped. Default is 60 seconds.

–

min ttl

value

>- Minimum acceptable TTL value for a fragment packet. Default is 1.

–

detect anomalies

- Detect fragment anomalies.

–

bind to

ip list

>- 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 <number>

- 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. de-

tect anomalies option must be conﬁgured for this option to take effect.

–

min fragment length <number>

- Deﬁnes smallest fragment size (payload size) that should be consid-

ered 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

type

>- Select a target-based defragmentation mode. Available types are ﬁrst, last, bsd, bsd-

right, 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 Type

AIX 2 BSD

AIX 4.3 8.9.3 BSD

Cisco IOS Last

FreeBSD BSD

HP JetDirect (printer) BSD-right

HP-UX B.10.20 BSD

HP-UX 11.00 First

IRIX 4.0.5F BSD

IRIX 6.2 BSD

IRIX 6.3 BSD

IRIX64 6.4 BSD

Linux 2.2.10 linux

Linux 2.2.14-5.0 linux

Linux 2.2.16-3 linux

Linux 2.2.19-6.2.10smp linux

Linux 2.4.7-10 linux

Linux 2.4.9-31SGI 1.0.2smp linux

Linux 2.4 (RedHat 7.1-7.3) linux

MacOS (version unknown) First

NCD Thin Clients BSD

OpenBSD (version unknown) linux

OpenVMS 7.1 BSD

OS/2 (version unknown) BSD

OSF1 V3.0 BSD

OSF1 V3.2 BSD

OSF1 V4.0,5.0,5.1 BSD

SunOS 4.1.4 BSD

SunOS 5.5.1,5.6,5.7,5.8 First

Tru64 Unix V5.0A,V5.1 BSD

Vax/VMS BSD

Windows (95/98/NT4/W2K/XP) 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 frag3_global: prealloc_nodes 8192

preprocessor frag3_engine: policy linux, bind_to 192.168.1.0/24

preprocessor frag3_engine: policy first, bind_to [10.1.47.0/24,172.16.8.0/24]

preprocessor frag3_engine: 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 <yes|no>], [max_tcp <number>], \

[memcap <number bytes>], \

[track_udp <yes|no>], [max_udp <number>], \

[track_icmp <yes|no>], [max_icmp <number>], \

[flush_on_alert], [show_rebuilt_packets], \

[prune_log_max <bytes>]

Option Description

track tcp <yes|no>

Track sessions for TCP. The default is ”yes”.

max tcp <num sessions>

Maximum simultaneous TCP sessions tracked. The default is ”256000”, maxi-

mum is ”1052672”, minimum is ”1”.

memcap <num bytes>

Memcap for TCP packet storage. The default is ”8388608” (8MB), maximum is

”1073741824” (1GB), minimum is ”32768” (32KB).

track udp <yes|no>

Track sessions for UDP. The default is ”yes”.

max udp <num sessions>

Maximum simultaneous UDP sessions tracked. The default is ”128000”, maxi-

mum is ”1052672”, minimum is ”1”.

track icmp <yes|no>

Track sessions for ICMP. The default is ”yes”.

max icmp <num sessions>

Maximum simultaneous ICMP sessions tracked. The default is ”64000”, maxi-

mum is ”1052672”, minimum is ”1”.

flush on alert

Backwards compatibilty. Flush a TCP stream when an alert is generated on that

stream. The default is set to off.

show rebuilt packets

Print/display packet after rebuilt (for debugging). The default is set to off.

prune log max <num bytes>

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 <ip_addr>], [timeout <number secs>], \

[policy <policy_id>], [min_ttl <number>], \

[overlap_limit <number>], [max_window <number>], \

[require_3whs [<number secs>]], [detect_anomalies], \

[check_session_hijacking], [use_static_footprint_sizes], \

[dont_store_large_packets], [dont_reassemble_async], \

[max_queued_bytes <bytes>], [max_queued_segs <number segs>], \

[ports <client|server|both> <all|number [number]*>], \

[ignore_any_rules]

Option Description

bind to <ip addr>

IP address or network for this policy. The default is set to any.

timeout <num seconds>

Session timeout. The default is ”30”, the minimum is ”1”, and the maxi-

mum is ”86400” (approximately 1 day).

policy <policy id>

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

min ttl <number>

Minimum TTL. The default is ”1”, the minimum is ”1” and the maximum

is ”255”.

overlap limit <number>

Limits the number of overlapping packets per session. The default is ”0”

(unlimited), the minimum is ”0”, and the maximum is ”255”.

max window <number>

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 abnor-

mally large window, so using a value near the maximum is discouraged.

require 3whs [<number

seconds>]

Establish sessions only on completion of a SYN/SYN-ACK/ACK hand-

shake. The default is set to off. The optional number of seconds speci-

ﬁes 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 anomalies

Detect and alert on TCP protocol anomalies. The default is set to off.

check session hijacking

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 footprint sizes

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.

dont store large packets

Performance improvement to not queue large packets in reassembly

buffer. The default is set to off. Using this option may result in missed

attacks.

dont reassemble async

Don’t queue packets for reassembly if trafﬁc has not been seen in both

directions. The default is set to queue packets.

max queued bytes <bytes>

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 en-

forced.

max queued segs <num>

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.

ports <client|server|both>

<all|number(s)>

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 de-

fault 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 mini-

mum port allowed is ”1” and the maximum allowed is ”65535”.

ignore any rules

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 perfor-

mance 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 <number secs>], [ignore_any_rules]

Option Description

timeout <num seconds>

Session timeout. The default is ”30”, the minimum is ”1”, and the maximum is

”86400” (approximately 1 day).

ignore any rules

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 <number secs>]

Option Description

timeout <num seconds>

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 stream5_global: track_tcp yes

preprocessor stream5_tcp: bind_to 192.168.1.0/24, policy windows

preprocessor stream5_tcp: bind_to 10.1.1.0/24, policy linux

preprocessor 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

2.2.3 sfPortscan

The sfPortscan module, developed by Sourceﬁre, is designed to detect the ﬁrst phase in a network attack: Recon-

naissance. 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 negativeresponses 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

•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 connection-

less 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 <protocol>

Available options:

•

TCP

•

UDP

•

IGMP

•

ip proto

•

all

2. scan type <scan type>

Available options:

•

portscan

•

portsweep

•

decoy portscan

•

distributed portscan

•

all

3. sense level <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 <ip1|ip2/cidr[ [port|port2-port3]]>

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 <ip1|ip2/cidr[ [port|port2-port3]]>

Ignores the source of scan alerts. The parameter is the same format as that of

watch ip

6. ignore scanned <ip1|ip2/cidr[ [port|port2-port3]]>

Ignores the destination of scan alerts. The parameter is the same format as that of

watch ip

7. logﬁle <ﬁle>

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 <protocols> \

scan_type <portscan|portsweep|decoy_portscan|distributed_portscan|all> \

sense_level <low|medium|high> \

watch_ip <IP or IP/CIDR> \

ignore_scanners <IP list> \

ignore_scanned <IP list> \

logfile <path and filename>

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

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: \

<ports> [ alert_fragments ] \

[no_alert_multiple_requests] \

[no_alert_large_fragments] \

[no_alert_incomplete]

Option Description

alert fragments

Alert on any fragmented RPC record.

no alert multiple requests

Don’t alert when there are multiple records in one packet.

no alert large fragments

Don’t alert when the sum of fragmented records exceeds one packet.

no alert incomplete

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:

•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]

•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)

•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

--perfmon-file

options.

•

pktcnt

- Adjusts the number of packets to process before checking for the time sample. This boosts perfor-

mance, since checking the time sample reduces Snort’s performance. By default, this is 10000.

•

time

- Represents the number of seconds between intervals.

•

accumulate

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

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 <map_filename> \

codemap <integer> \

[detect_anomalous_servers] \

[proxy_alert]

You can only have a single global conﬁguration, you’ll get an error if you try otherwise.

Conﬁguration

iis unicode map

map filename

[codemap

integer

]

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.

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.

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.

profile

all

apache

iis

iis5 0

iis4 0

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

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 }

ports

port

>[<

port

>< ... >]}

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.

iis unicode map

map filename

codemap

integer

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.

server flow depth

integer

This speciﬁes the amount of server response payload to inspect. This option signiﬁcantly increases IDS perfor-

mance 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

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.

client flow depth

integer

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.

post depth

integer

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.

ascii

yes

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.

utf 8

yes

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.

u encode

yes

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

yes

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

yes

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

yes

The

iis unicode

option turns on the Unicode codepoint mapping. If there is no iis unicode map option spec-

iﬁed 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

yes

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

byte

>[<

byte ...

>]}

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.

15.

multi slash

yes

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

16.

iis backslash

yes

Normalizes backslashes to slashes. This is again an IIS emulation. So a request URI of “/foo\bar” gets normal-

ized to “/foo/bar.”

17.

directory

yes

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

. This alert may give false positives, since

some web sites refer to ﬁles using directory traversals.

18.

apache whitespace

yes

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

yes

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

non-zero positive integer

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 ana-

lyzed 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

non-zero positive integer

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

positive integer up to 65535

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

yes

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.

32.

max headers

positive integer up to 1024

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:

ports

{

<port> [<port>] ...

}

This speciﬁes on what ports to check for SMTP data. Typically, this will include 25 and possibly 465, for

encrypted SMTP.

inspection type <stateful | stateless>

Indicate whether to operate in stateful or stateless mode.

normalize <all | none | cmds>

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.

ignore data

Ignore data section of mail (except for mail headers) when processing rules.

ignore tls data

Ignore TLS-encrypted data when processing rules.

max command line len <int>

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.

max header line len <int>

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.

max response line len <int>

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.

alt max command line len <int>

{

<cmd> [<cmd>]

}

Overrides

max command line len

for speciﬁc commands.

10.

no alerts

Turn off all alerts for this preprocessor.

11.

invalid cmds

{

<Space-delimited list of commands>

}

Alert if this command is sent from client side. Default is an empty list.

12.

valid cmds

{

<Space-delimited list of commands>

}

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 }

13.

alert unknown cmds

Alert if we don’t recognize command. Default is off.

14.

normalize cmds

{

<Space-delimited list of commands>

}

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 HELO ETRN } \

alt_max_command_line_len 255 { EXPN 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.

FTP/Telnet has the capability to handle stateless processing, meaning it only looks for information on a packet-by-

packet 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

. 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

inspection type

This indicates whether to operate in stateful or stateless mode.

encrypted traffic

yes|no

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.

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

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

ports

port

>[<

port

>< ... >]}

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.

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.

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.

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 implementa-

tions 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 con-

ﬁgured. 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

ports

port

>[<

port

>< ... >]}

This is how the user conﬁgures which ports to decode as FTP command channel trafﬁc. Typically port 21 will

be included.

print cmds

During initialization, this option causes the preprocessor to print the conﬁguration for each of the FTP commands

for this server.

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 }

def max param len

number

This speciﬁes the default maximum allowed parameter length for an FTP command. It can be used as a basic

buffer overﬂow detection.

alt max param len

number

>{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 }

chk str fmt

{cmd[cmd]}

This option causes a check for string format attacks in the speciﬁed commands.

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 Description

int Parameter must be an integer

number Parameter must be an integer between 1 and 255

char <chars>Parameter must be a single character, one of <chars>

date <datefmt>Parameter follows format speciﬁed, where:

n Number

C Character

[] optional format enclosed

|OR

{} choice of options

. + - literal

string Parameter is a string (effectively unrestricted)

host port Parameter must be a host/port speciﬁed, per RFC 959

long host port Parameter must be a long host port speciﬁed, per RFC

1639

extended host port 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 MODE <char SBC>

cmd_validity STRU <char FRP>

cmd_validity ALLO < int [ char R int ] >

cmd_validity TYPE < { char AE [ char NTC ] | char I | char L [ number ] } >

cmd_validity PORT < 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 ac-

cept 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/internet-

drafts/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 >

telnet cmds

yes

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.

ignore telnet erase cmds

yes|no

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 se-

quences.

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

yes

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 con-

ﬁgured. 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

max resp len

number

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.

bounce

yes|no

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.

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 }

telnet cmds

yes|no

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.

ignore telnet erase cmds

yes|no

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.

server ports

port

>[<

port

>< ... >]}

This option speciﬁes which ports the SSH preprocessor should inspect trafﬁc to.

max encrypted packets

number

The number of encrypted packets that Snort will inspect before ignoring a given SSH session. The SSH vulner-

abilities 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.

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.

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.

autodetect

Attempt to automatically detect SSH.

enable respoverflow

Enables checking for the Challenge-Response Overﬂow exploit.

enable ssh1crc32

Enables checking for the CRC 32 exploit.

enable srvoverflow

Enables checking for the Secure CRT exploit.

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

port

> <...>]

}

Ports that the preprocessor monitors for SMB trafﬁc. Default are ports 139 and 445.

•

ports dcerpc

port

> <...>]

}

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

number

Maximum DCE/RPC fragment size to put in defragmentation buffer, in bytes. Default is 3000 bytes.

•

memcap

number

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

number

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 desegmenta-

tion.)

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/defragmentationto 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.

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.

ports

port

>[<

port

>< ... >]}

This option speciﬁes the source ports that the DNS preprocessor should inspect trafﬁc.

enable obsolete types

Alert on Obsolete (per RFC 1035) Record Types

enable experimental types

Alert on Experimental (per RFC 1035) Record Types

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 noin-

spect 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.

Conﬁguration

ports

port

>[<

port

>< ... >]}

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

noinspect encrypted

Disable inspection on trafﬁc that is encrypted. Default is off.

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 Description

IP address.

mac

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 sup-

ported for DCE/RPC: SMB, TCP, UDP and RPC over HTTP v.1 proxy and server. New rule options have been im-

plemented 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, ei-

ther 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

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

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

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

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 Argument Required Default

memcap <memcap>

memcap 102400

disable defrag

NONE NO OFF

max frag len <max-frag-len>

NO OFF

events <events>

events [smb, co, cl]

reassemble threshold <re-thresh>

NO OFF

memcap = 1024-4194303 (kilobytes)

max-frag-len = 1514-65535

events = pseudo-event | event | ’[’ event-list ’]’

pseudo-event = "none" | "all"

event-list = event | event ’,’ event-list

event = "memcap" | "smb" | "co" | "cl"

re-thresh = 0-65535

Option explanations

memcap

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.

Stands for connection-oriented DCE/RPC. Alert on events related to connection-oriented DCE/RPC

processing.

Stands for connectionless DCE/RPC. Alert on events related to connectionless DCE/RPC pro-

cessing. Defaults are

smb

and

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 dcerpc2

preprocessor dcerpc2: memcap 500000

preprocessor dcerpc2: max_frag_len 16840, memcap 300000, events smb

preprocessor dcerpc2: memcap 50000, events [memcap, smb, co, cl], max_frag_len 14440

preprocessor dcerpc2: disable_defrag, events [memcap, smb]

preprocessor dcerpc2: reassemble_threshold 500

Default global conﬁguration

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

Server Conﬁguration

preprocessor dcerpc2_server

The

dcerpc2 server

conﬁguration is optional. A

dcerpc2 server

conﬁguration must start with

default

net

options. The

default

and

net

options are mutually exclusive. At most one default conﬁguration can be speciﬁed. If

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 Argument Required Default

default

NONE YES NONE

net <net>

YES NONE

policy <policy>

policy WinXP

detect <detect>

detect [smb [139,445], tcp 135,

udp 135, rpc-over-http-server

593]

autodetect <detect>

autodetect [tcp 1025:, udp 1025:,

rpc-over-http-server 1025:]

no autodetect http proxy ports

NONE NO DISABLED (The preprocessor autodetects

on all proxy ports by default)

smb invalid shares <shares>

NO NONE

smb max chain <max-chain>

smb max chain 3

net = ip | ’[’ ip-list ’]’

ip-list = ip | ip ’,’ ip-list

ip = ip-addr | ip-addr ’/’ prefix | ip4-addr ’/’ netmask

ip-addr = ip4-addr | ip6-addr

ip4-addr = a valid IPv4 address

ip6-addr = a valid IPv6 address (can be compressed)

prefix = a valid CIDR

netmask = a valid netmask

policy = "Win2000" | "Win2003" | "WinXP" | "WinVista" |

"Samba" | "Samba-3.0.22" | "Samba-3.0.20"

detect = "none" | detect-opt | ’[’ detect-list ’]’

detect-list = detect-opt | detect-opt ’,’ detect-list

detect-opt = transport | transport port-item |

transport ’[’ port-list ’]’

transport = "smb" | "tcp" | "udp" | "rpc-over-http-proxy" |

"rpc-over-http-server"

port-list = port-item | port-item ’,’ port-list

port-item = port | port-range

port-range = ’:’ port | port ’:’ | port ’:’ port

port = 0-65535

shares = share | ’[’ share-list ’]’

share-list = share | share ’,’ share-list

share = word | ’"’ word ’"’ | ’"’ var-word ’"’

word = graphical ascii characters except ’,’ ’"’ ’]’ ’[’ ’$’

var-word = graphical ascii characters except ’,’ ’"’ ’]’ ’[’

max-chain = 0-255

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.

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

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 Description

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

SMB events

SID Description

2 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

3 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.

4 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.

5 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.

6 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.

7 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.

8 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.

9 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.

10 The preprocessor will alert if the NetBIOS Session Service length ﬁeld contains a value less than

the size of an SMB header.

11 The preprocessor will alert if the remaining NetBIOS packet length is less than the size of the

SMB command header to be decoded.

12 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.

13 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.

14 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.)

15 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.

16 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.)

17 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.

18 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.

19 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.

20 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).

21 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.

22 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.

23 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 subse-

quent 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.

24 A

Tree Connect AndX

command is used to connect to a share. The

Tree Disconnect

com-

mand is used to disconnect from that share. The combination of a

Tree Connect AndX

com-

mand with a chained

Tree Disconnect

command, essentially connects to a share and discon-

nects from the same share in the same request and is anomalous behavior. The preprocessor will

alert if it sees this.

25 An

Open AndX

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

command is used to close that ﬁle or named pipe. The combination of a

Open AndX

Nt Create AndX

command with a chained

command, essentially opens and closes the

named pipe in the same request and is anomalous behavior. The preprocessor will alert if it sees

this.

26 The preprocessor will alert if it sees any of the invalid SMB shares conﬁgured. It looks for a

Tree Connect

Tree Connect AndX

to the share.

Connection-oriented DCE/RPC events

SID Description

27 The preprocessor will alert if the connection-oriented DCE/RPC major version contained in the

header is not equal to 5.

28 The preprocessor will alert if the connection-oriented DCE/RPC minor version contained in the

header is not equal to 0.

29 The preprocessor will alert if the connection-oriented DCE/RPC PDU type contained in the

header is not a valid PDU type.

30 The preprocessor will alert if the fragment length deﬁned in the header is less than the size of the

header.

31 The preprocessor will alert if the remaining fragment length is less than the remaining packet

size.

32 The preprocessor will alert if in a

Bind

Alter Context

request, there are no context items

speciﬁed.

33 The preprocessor will alert if in a

Bind

Alter Context

request, there are no transfer syntaxes

to go with the requested interface.

34 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.

35 The preprocessor will alert if a fragment is larger than the maximum negotiated fragment length.

36 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.

37 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.

38 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.

39 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.

Connectionless DCE/RPC events

SID Description

40 The preprocessor will alert if the connectionless DCE/RPC major version is not equal to 4.

41 The preprocessor will alert if the connectionless DCE/RPC pdu type is not a valid pdu type.

42 The preprocessor will alert if the packet data length is less than the size of the connectionless

header.

43 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

<uuid> [ ’,’ <operator> <version> ] [ ’,’ "any_frag" ]

uuid = hexlong ’-’ hexshort ’-’ hexshort ’-’ 2hexbyte ’-’ 6hexbyte

hexlong = 4hexbyte

hexshort = 2hexbyte

hexbyte = 2HEXDIGIT

operator = ’<’ | ’>’ | ’=’ | ’!’

version = 0-65535

Examples

dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188;

dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188,<2;

dce_iface: 4b324fc8-1670-01d3-1278-5a47bf6ee188,any_frag;

dce_iface: 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-list = opnum-item | opnum-item ’,’ opnum-list

opnum-item = opnum | opnum-range

opnum-range = opnum ’-’ opnum

opnum = 0-65535

Examples

dce_opnum: 15;

dce_opnum: 15-18;

dce_opnum: 15,18-20;

dce_opnum: 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

<convert> ’,’ [ ’!’ ] <operator> ’,’ <value> [ ’,’ <offset> [ ’,’ "relative" ]] \

’,’ "dce"

convert = 1 | 2 | 4

operator = ’<’ | ’=’ | ’>’ | ’&’ | ’ˆ’

value = 0-4294967295

offset = -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

<convert> ’,’ <offset> [ ’,’ "relative" ] [ ’,’ "multiplier" <mult-value> ] \

[ ’,’ "align" ] [ ’,’ "post_offet" <adjustment-value> ] ’,’ "dce"

convert = 1 | 2 | 4

offset = -65535 to 65535

mult-value = 0-65535

adjustment-value = -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

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 recom-

mended).

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;)

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

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 <gid>, sig_id <sid>, \

track <by_src|by_dst|by_rule>, \

count <c>, seconds <s>, \

timeout <seconds> \

[, apply_to <ip-list>]

The options are described in the table below - all are required except

apply to

, which is optional.

Option Description

track by src | by dst |

by rule

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.

count c

the maximum number of rule matches in

seconds before the rate ﬁlter

limit to is exceeded.

must be nonzero value.

seconds s

the time period over which

count

is accrued. 0 seconds means

count

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 alert | drop |

pass | log | sdrop | reject

new action

replaces rule action for

seconds.

drop

reject

, and

sdrop

can be used only when snort is used in inline mode.

sdrop

and

reject

are conditionally compiled with GIDS.

timeout t

revert to the original rule action after

seconds. If

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

apply to <ip-list>

restrict the conﬁguration to only to source or destination IP address (in-

dicated by track parameter) determined by

<ip-list>

track by rule

and

apply to

may not be used together. Note that events are gener-

ated 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 mevents during the time interval, then ignores events for the rest of the time interval.

•

threshold

Alerts every mtimes we see this event during the time interval.

•

both

Alerts once per time interval after seeing moccurrences of the event, then ignores any additional events during

the time interval.

Format

event_filter \

gen_id <gid>, sig_id <sid>, \

type <limit|threshold|both>, \

track <by_src|by_dst>, \

count <c>, seconds <s>

threshold \

gen_id <gid>, sig_id <sid>, \

type <limit|threshold|both>, \

track <by_src|by_dst>, \

count <c>, seconds <s>

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 Description

gen id <gid>

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.

sig id <sid>

Specify the signature ID of an associated rule.

sig id 0

speciﬁes a ”global” ﬁlter

because it applies to all

sig id

s for the given

gen id

type limit|threshold|both

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.

track by src|by dst

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 desti-

nation IP addresses. Ports or anything else are not tracked.

count c

number of rule matching in s seconds that will cause

event filter

limit to be

exceeded.

must be nonzero value.

seconds s

time period over which

count

is accrued.

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

applied to a speciﬁc

gen id, sig id

pair, Snort will terminate with an error while reading the conﬁguration

information.

event filter

s 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 filter

s 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 gen-

msg.map for details on gen ids.

Users can also conﬁgure a memcap for threshold with a “conﬁg:” option:

config event_filter: memcap <bytes>

# this is deprecated:

config threshold: memcap <bytes>

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 <gid>, sig_id <sid>, \

suppress \

gen_id <gid>, sig_id <sid>, \

track <by_src|by_dst>, ip <ip-list>

Option Description

gen id <gid>

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.

sig id <sid>

Specify the signature ID of an associated rule.

sig id 0

speciﬁes a ”global” ﬁlter

because it applies to all

sig id

s for the given

gen id

track by src|by dst

Suppress by source IP address or destination IP address. This is optional, but if

present,

must be provided as well.

ip <list>

Restrict the suppression to only source or destination IP addresses (indicated by

track

parameter) determined by ¡list¿. If track is provided, ip must be provided

as well.

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’.

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.

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.

order events

This argument determines the way that the incoming events are ordered. We currently have two different meth-

ods:

•

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

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.

2.5.1 Rule Proﬁling

Format

config profile_rules: \

print [all | <num>], \

sort <sort_option> \

[,filename <filename> [append]]

•

<num>

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 Ticks Avg/Check Avg/Match Avg/Nonmatch

=== === === === ====== ======= ====== ===== ========= ========= ============

1 2389 1 12 1 1 1 385698 385698.0 385698.0 0.0

2 2178 1 17 2 0 0 107822 53911.0 0.0 53911.0

3 2179 1 8 2 0 0 92458 46229.0 0.0 46229.0

4 1734 1 37 2 0 0 90054 45027.0 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 | <num>], \

sort <sort_option> \

[, filename <filename> [append]]

•

<num>

is the number of preprocessors to print

•

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.

Preprocessor Profile Statistics (all)

==========================================================

Num Preprocessor Layer Checks Exits Microsecs Avg/Check Pct of Caller Pct of Total

=== ============ ===== ====== ===== ========= ========= ============= ============

1 ftptelnet_ftp 0 2697 2697 135720 50.32 0.20 0.20

2 detect 0 930237 930237 31645670 34.02 47.20 47.20

1 rule eval 1 1347969 1347969 26758596 19.85 84.56 39.91

1 rule tree eval 2 1669390 1669390 26605086 15.94 99.43 39.68

1 pcre 3 488652 488652 18994719 38.87 71.40 28.33

2 asn1 3 1 1 8 8.56 0.00 0.00

3 uricontent 3 647122 647122 2638614 4.08 9.92 3.94

4 content 3 1043099 1043099 3154396 3.02 11.86 4.70

5 ftpbounce 3 23 23 19 0.87 0.00 0.00

6 byte_jump 3 9007 9007 3321 0.37 0.01 0.00

7 byte_test 3 239015 239015 64401 0.27 0.24 0.10

8 icmp_seq 3 2 2 0 0.16 0.00 0.00

9 fragbits 3 65259 65259 10168 0.16 0.04 0.02

10 isdataat 3 5085 5085 757 0.15 0.00 0.00

11 flags 3 4147 4147 517 0.12 0.00 0.00

12 flowbits 3 2002630 2002630 212231 0.11 0.80 0.32

13 ack 3 4042 4042 261 0.06 0.00 0.00

14 flow 3 1347822 1347822 79002 0.06 0.30 0.12

15 icode 3 75538 75538 4280 0.06 0.02 0.01

16 itype 3 27009 27009 1524 0.06 0.01 0.00

17 icmp_id 3 41150 41150 1618 0.04 0.01 0.00

18 ip_proto 3 142625 142625 5004 0.04 0.02 0.01

19 ipopts 3 13690 13690 457 0.03 0.00 0.00

2 rtn eval 2 55836 55836 22763 0.41 0.09 0.03

2 mpse 1 492836 492836 4135697 8.39 13.07 6.17

3 frag3 0 76925 76925 1683797 21.89 2.51 2.51

1 frag3insert 1 70885 70885 434980 6.14 25.83 0.65

2 frag3rebuild 1 5419 5419 6280 1.16 0.37 0.01

4 dcerpc 0 127332 127332 2426830 19.06 3.62 3.62

5 s5 0 809682 809682 14195602 17.53 21.17 21.17

1 s5tcp 1 765281 765281 14128577 18.46 99.53 21.07

1 s5TcpState 2 742464 742464 13223585 17.81 93.59 19.72

1 s5TcpFlush 3 51987 51987 92918 1.79 0.70 0.14

1 s5TcpProcessRebuilt 4 47355 47355 14548497 307.22 15657.23 21.70

2 s5TcpBuildPacket 4 47360 47360 41711 0.88 44.89 0.06

2 s5TcpData 3 250035 250035 141490 0.57 1.07 0.21

1 s5TcpPktInsert 4 88173 88173 110136 1.25 77.84 0.16

2 s5TcpNewSess 2 60880 60880 81779 1.34 0.58 0.12

6 eventq 0 2089428 2089428 26690209 12.77 39.81 39.81

7 httpinspect 0 296030 296030 1862359 6.29 2.78 2.78

8 smtp 0 137653 137653 227982 1.66 0.34 0.34

9 decode 0 1057635 1057635 1162456 1.10 1.73 1.73

10 ftptelnet_telnet 0 175 175 175 1.00 0.00 0.00

11 sfportscan 0 881153 881153 518655 0.59 0.77 0.77

12 backorifice 0 35369 35369 4875 0.14 0.01 0.01

13 dns 0 16639 16639 1346 0.08 0.00 0.00

total total 0 1018323 1018323 67046412 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 <micro-secs>, \

fastpath-expensive-packets, \

pkt-log, \

debug-pkts

# Rule configuration:

config ppm: max-rule-time <micro-secs>, \

threshold count, \

suspend-expensive-rules, \

suspend-timeout <seconds>, \

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 <micro-secs>

•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 <micro-secs>

•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 <count>

•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 <seconds>

•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: Pkt-Event Pkt[63] used=56.0438 usecs, 0 rules, 1 nc-rules tested, packet fastpathed.

PPM: Process-BeginPkt[63] caplen=60

PPM: Pkt[63] Used= 8.394 usecs

PPM: 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:

max rule time : 50 usecs

rule events : 0

avg nc-rule time : 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 imple-

mentation 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 threshold-

ing 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 <name>: <options>

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

•

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=<hostname[:<port>],] \

Example

output alert_syslog: 10.1.1.1:514, <facility> <priority> <options>

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: [<filename> ["packet"] [<limit>]]

<limit> ::= <number>[(’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: [<filename> [<limit>]]

<limit> ::= <number>[(’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: [<filename> [<limit>]]

<limit> ::= <number>[(’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: <log | alert>, <database type>, <parameter list>

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

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: [<filename> [<format> [<limit>]]]

<format> ::= "default"|<list>

<list> ::= <field>(,<field>)*

<field> ::= "dst"|"src"|"ttl" ...

<limit> ::= <number>[(’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

–

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: <base file name> [, <limit <file size limit in MB>]

output log_unified: <base file name> [, <limit <file size limit in MB>]

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

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 <base filename> [, <limit <size in MB>] [, nostamp] [, mpls_event_types]

102

output log_unified2: \

filename <base filename> [, <limit <size in MB>] [, nostamp]

output unified2: \

filename <base file name> [, <limit <size in MB>] [, nostamp] [, mpls_event_types]

Example

output alert_unified2: filename snort.alert, limit 128, nostamp

output log_unified2: filename snort.log, limit 128, nostamp

output unified2: filename merged.log, limit 128, nostamp

output 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.org

Format

output alert_prelude: \

profile=<name of prelude profile> \

[ info=<priority number for info priority alerts>] \

[ low=<priority number for low priority alerts>] \

[ medium=<priority number for medium priority alerts>]

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” conﬁgura-

tion 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 user´s 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:

<limit> ::= <number>[(<gb>|<mb>|<kb>)]

<gb> ::= ’G’|’g’

<mb> ::= ’M’|’m’

<kb> ::= ’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 <path to file>

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.

<SNORT_ATTRIBUTES>

<ATTRIBUTE_MAP>

<ENTRY>

<VALUE>Linux</VALUE>

</ENTRY>

<ENTRY>

</ENTRY>

</ATTRIBUTE_MAP>

<ATTRIBUTE_TABLE>

<HOST>

<OPERATING_SYSTEM>

<NAME>

<ATTRIBUTE_ID>1</ATTRIBUTE_ID>

</NAME>

<ATTRIBUTE_VALUE>Red Hat</ATTRIBUTE_VALUE>

</VENDOR>

<ATTRIBUTE_VALUE>2.6</ATTRIBUTE_VALUE>

</VERSION>

<FRAG_POLICY>linux</FRAG_POLICY>

105

<STREAM_POLICY>linux</STREAM_POLICY>

</OPERATING_SYSTEM>

<PORT>

<ATTRIBUTE_VALUE>22</ATTRIBUTE_VALUE>

</PORT>

<ATTRIBUTE_VALUE>tcp</ATTRIBUTE_VALUE>

</IPPROTO>

<ATTRIBUTE_ID>2</ATTRIBUTE_ID>

</PROTOCOL>

<ATTRIBUTE_ID>OpenSSH</ATTRIBUTE_ID>

<ATTRIBUTE_VALUE>3.9p1</ATTRIBUTE_VALUE>

</VERSION>

</APPLICATION>

</SERVICE>

<PORT>

<ATTRIBUTE_VALUE>23</ATTRIBUTE_VALUE>

</PORT>

<ATTRIBUTE_VALUE>tcp</ATTRIBUTE_VALUE>

</IPPROTO>

<ATTRIBUTE_VALUE>telnet</ATTRIBUTE_VALUE>

</PROTOCOL>

<ATTRIBUTE_VALUE>telnet</ATTRIBUTE_VALUE>

</APPLICATION>

</SERVICE>

</SERVICES>

<ATTRIBUTE_VALUE>tcp</ATTRIBUTE_VALUE>

</IPPROTO>

<ATTRIBUTE_ID>http</ATTRIBUTE_ID>

</PROTOCOL>

<ATTRIBUTE_ID>IE Http Browser</ATTRIBUTE_ID>

106

<ATTRIBUTE_VALUE>6.0</ATTRIBUTE_VALUE>

</VERSION>

</APPLICATION>

</CLIENT>

</CLIENTS>

</HOST>

</ATTRIBUTE_TABLE>

</SNORT_ATTRIBUTES>

△

!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

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 Description

dynamicpreprocessor

[

file

shared library path

directory

directory of

shared libraries

Tells snort to load the dynamic preprocessor shared library (if

ﬁle is used) or all dynamic preprocessor shared libraries (if di-

rectory is used). Specify

file

, followed by the full or rel-

ative path to the shared library. Or, specify

directory

, fol-

lowed by the full or relative path to a directory of preprocessor

shared libraries. (Same effect as

--dynamic-preprocessor-lib

--dynamic-preprocessor-lib-dir

options). See chapter 5 for more

information on dynamic preprocessor libraries.

dynamicengine

[

file

shared

library path

directory

directory of shared

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 pre-

processor shared libraries. (Same effect as

--dynamic-engine-lib

--dynamic-preprocessor-lib-dir

options). See chapter 5 for more

information on dynamic engine libraries.

107

dynamicdetection

[

file

shared library path

directory

directory of

shared libraries

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

--dynamic-detection-lib-dir

op-

tions). 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 non-

reloadable 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 <snort pid>

△

!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/removingshared 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 <int>

rule-log

config profile_rules

filename

109

sort

config profile_preprocs

filename

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: <path_to_snort.conf> vlan <vlanIdList>

config binding: <path_to_snort.conf> net <ipList>

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 checksum_drop

config disable_decode_alerts

config disable_decode_drops

config disable_ipopt_alerts

config disable_ipopt_drops

config disable_tcpopt_alerts

config disable_tcpopt_drops

config disable_tcpopt_experimental_alerts

config disable_tcpopt_experimental_drops

config disable_tcpopt_obsolete_alerts

config disable_tcpopt_obsolete_drops

config disable_ttcp_alerts

config disable_tcpopt_ttcp_alerts

config 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 be-

fore 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>

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 <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 conﬁgura-

tion, 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: "<message text>";

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).

Table 3.1: Supported Systems

System URL Preﬁx

bugtraq http://www.securityfocus.com/bid/

cve http://cve.mitre.org/cgi-bin/cvename.cgi?name=

nessus http://cgi.nessus.org/plugins/dump.php3?id=

arachnids (currently down) http://www.whitehats.com/info/IDS

mcafee http://vil.nai.com/vil/dispVirus.asp?virus k=

url http://

Format

reference: <id system>,<id>; [reference: <id system>,<id>;]

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: <generator id>;

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 post-

processing alert to map an ID to an alert message.

Format

sid: <snort rules id>;

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: <revision integer>;

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: <class name>;

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: <class name>,<class description>,<default priority>

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 Description Priority

attempted-admin Attempted Administrator Privilege Gain high

attempted-user Attempted User Privilege Gain high

kickass-porn SCORE! Get the lotion! high

policy-violation Potential Corporate Privacy Violation high

shellcode-detect Executable code was detected high

successful-admin Successful Administrator Privilege Gain high

successful-user Successful User Privilege Gain high

trojan-activity A Network Trojan was detected high

unsuccessful-user Unsuccessful User Privilege Gain high

web-application-attack Web Application Attack high

119

attempted-dos Attempted Denial of Service medium

attempted-recon Attempted Information Leak medium

bad-unknown Potentially Bad Trafﬁc medium

default-login-attempt Attempt to login by a default username and

password medium

denial-of-service Detection of a Denial of Service Attack medium

misc-attack Misc Attack medium

non-standard-protocol Detection of a non-standard protocol or event medium

rpc-portmap-decode Decode of an RPC Query medium

successful-dos Denial of Service medium

successful-recon-largescale Large Scale Information Leak medium

successful-recon-limited Information Leak medium

suspicious-ﬁlename-detect A suspicious ﬁlename was detected medium

suspicious-login An attempted login using a suspicious user-

name was detected medium

system-call-detect A system call was detected medium

unusual-client-port-connection A client was using an unusual port medium

web-application-activity Access to a potentially vulnerable web appli-

cation medium

icmp-event Generic ICMP event low

misc-activity Misc activity low

network-scan Detection of a Network Scan low

not-suspicious Not Suspicious Trafﬁc low

protocol-command-decode Generic Protocol Command Decode low

string-detect A suspicious string was detected low

unknown Unknown Trafﬁc low

tcp-connection A TCP connection was detected 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: <priority integer>;

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.

Table 3.3: Snort Metadata Keys

Key Description Value Format

engine

Indicate a Shared Library Rule ”shared”

soid

Shared Library Rule Generator and SID gid|sid

service

Target-Based Service Identiﬁer ”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 Description

msg

The msg keyword tells the logging and alerting engine the message to print with

the packet dump or alert.

reference

The reference keyword allows rules to include references to external attack iden-

tiﬁcation systems.

gid

The gid keyword (generator id) is used to identify what part of Snort generates the

event when a particular rule ﬁres.

121

sid

The sid keyword is used to uniquely identify Snort rules.

rev

The rev keyword is used to uniquely identify revisions of Snort rules.

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.

priority

The priority keyword assigns a severity level to rules.

metadata

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: [!] "<content string>";

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

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 speci-

ﬁed 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 preproces-

sors. 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: <number>;

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: <number>;

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: <byte count>;

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: <byte count>;

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

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:[!]<content string>;

△

!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: [<,>] <int>;

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:<int>[,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:[!]"(/<regex>/|m<delim><regex><delim>)[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

Table 3.6: Perl compatible modiﬁers for

pcre

i case insensitive

s include newlines in the dot metacharacter

m 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.

x whitespace data characters in the pattern are ignored except when escaped or in-

side a character class

Table 3.7: PCRE compatible modiﬁers for

pcre

A the pattern must match only at the start of the buffer (same as ˆ )

E 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).

G 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: <bytes to convert>, [!]<operator>, <value>, <offset> \

[,relative] [,<endian>] [,<number type>, string];

130

Table 3.8: Snort speciﬁc modiﬁers for

pcre

R Match relative to the end of the last pattern match. (Similar to distance:0;)

U Match the decoded URI buffers (Similar to

uricontent

and

http uri

)

P Match normalized HTTP request body (Similar to

http client body

)

H Match normalized HTTP request header (Similar to

http header

)

M Match normalized HTTP request method (Similar to

http method

)

C Match normalized HTTP request cookie (Similar to

http cookie

)

B Do not use the decoded buffers (Similar to rawbytes)

O Override the conﬁgured pcre match limit for this expression (See section 2.1.3)

Option Description

bytes to convert

Number of bytes to pick up from the packet

operator

Operation to perform to test the value:

•<- less than

•>- greater than

•= - equal

•! - not

•& - bitwise AND

•ˆ - bitwise OR

value

Value to test the converted value against

offset

Number of bytes into the payload to start processing

relative

Use an offset relative to last pattern match

endian

Endian type of the number being read:

•

big

- Process data as big endian (default)

•

little

- Process data as little endian

string

Data is stored in string format in packet

number type

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 con-

verted. See section 2.2.14 for a description and examples (2.2.14 for quick refer-

ence).

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: <bytes_to_convert>, <offset> \

[,relative] [,multiplier <multiplier value>] [,big] [,little][,string]\

[,hex] [,dec] [,oct] [,align] [,from_beginning] [,post_offset <adjustment value>];

132

Option Description

bytes to convert

Number of bytes to pick up from the packet

offset

Number of bytes into the payload to start processing

relative

Use an offset relative to last pattern match

multiplier

value

>Multiply the number of calculated bytes by <

value

>and skip forward that num-

ber of bytes.

big

Process data as big endian (default)

little

Process data as little endian

string

Data is stored in string format in packet

hex

Converted string data is represented in hexadecimal

dec

Converted string data is represented in decimal

oct

Converted string data is represented in octal

align

Round the number of converted bytes up to the next 32-bit boundary

from beginning

Skip forward from the beginning of the packet payload instead of from the current

position in the packet.

post offset

value

>Skip forward or backwards (positive of negative value)

value

>number of

bytes after the other jump options have been applied.

dce

Let the DCE/RPC 2 preprocessor determine the byte order of the value to be con-

verted. See section 2.2.14 for a description and examples (2.2.14 for quick refer-

ence).

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 Description

bitstring overflow

Detects invalid bitstring encodings that are known to be remotely exploitable.

double overflow

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.

oversize length

value

>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.

absolute offset

value

>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.

relative offset

value

>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 spec-

ify

’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:<option>;

Option Description

invalid-entry

Looks for an invalid Entry string, which is a way of causing a heap overﬂow

(see CVE-2004-0396) and bad pointer derefenece in versions of CVS 1.11.15 and

before.

Examples

alert tcp any any -> any 2401 (msg:"CVS Invalid-entry"; \

flow:to_server,established; cvs:invalid-entry;)

134

3.5.23 dce iface

See the DCE/RPC 2 Preprocessor section 2.2.14 for a description and examples of using this rule option.

3.5.24 dce opnum

See the DCE/RPC 2 Preprocessor section 2.2.14 for a description and examples of using this rule option.

3.5.25 dce stub data

See the DCE/RPC 2 Preprocessor section 2.2.14 for a description and examples of using this rule option.

3.5.26 Payload Detection Quick Reference

Table 3.9: Payload detection rule option keywords

Keyword Description

content

The content keyword allows the user to set rules that search for speciﬁc content in

the packet payload and trigger response based on that data.

rawbytes

The rawbytes keyword allows rules to look at the raw packet data, ignoring any

decoding that was done by preprocessors.

depth

The depth keyword allows the rule writer to specify how far into a packet Snort

should search for the speciﬁed pattern.

offset

The offset keyword allows the rule writer to specify where to start searching for a

pattern within a packet.

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.

within

The within keyword is a content modiﬁer that makes sure that at most N bytes are

between pattern matches using the content keyword.

uricontent

The uricontent keyword in the Snort rule language searches the normalized request

URI ﬁeld.

isdataat

The isdataat keyword veriﬁes that the payload has data at a speciﬁed location.

pcre

The pcre keyword allows rules to be written using perl compatible regular expres-

sions.

byte test

The byte test keyword tests a byte ﬁeld against a speciﬁc value (with operator).

byte jump

The byte jump keyword allows rules to read the length of a portion of data, then

skip that far forward in the packet.

ftpbounce

The ftpbounce keyword detects FTP bounce attacks.

asn1

The asn1 detection plugin decodes a packet or a portion of a packet, and looks for

various malicious encodings.

cvs

The cvs keyword detects invalid entry strings.

dce iface

See the DCE/RPC 2 Preprocessor section 2.2.14.

dce opnum

See the DCE/RPC 2 Preprocessor section 2.2.14.

dce stub data

See the DCE/RPC 2 Preprocessor section 2.2.14.

135

3.6 Non-Payload Detection Rule Options

3.6.1 fragoffset

The fragoffset keyword allows one to compare the IP fragment offset ﬁeld against a decimal value. To catch all the ﬁrst

fragments of an IP session, you could use the fragbits keyword and look for the More fragments option in conjunction

with a fragoffset of 0.

Format

fragoffset:[<|>]<number>;

Example

alert ip any any -> any any \

(msg: "First Fragment"; fragbits: M; fragoffset: 0;)

3.6.2 ttl

The ttl keyword is used to check the IP time-to-live value. This option keyword was intended for use in the detection

of traceroute attempts.

Format

ttl:[[<number>-]><=]<number>;

Example

This example checks for a time-to-live value that is less than 3.

ttl:<3;

This example checks for a time-to-live value that between 3 and 5.

ttl:3-5;

3.6.3 tos

The tos keyword is used to check the IP TOS ﬁeld for a speciﬁc value.

Format

tos:[!]<number>;

Example

This example looks for a tos value that is not 4

tos:!4;

136

3.6.4 id

The id keyword is used to check the IP ID ﬁeld for a speciﬁc value. Some tools (exploits, scanners and other odd

programs) set this ﬁeld speciﬁcally for various purposes, for example, the value 31337 is very popular with some

hackers.

Format

id:<number>;

Example

This example looks for the IP ID of 31337.

id:31337;

3.6.5 ipopts

The ipopts keyword is used to check if a speciﬁc IP option is present.

The following options may be checked:

rr - Record Route

eol - End of list

nop - No Op

ts - Time Stamp

sec - IP Security

esec - IP Extended Security

lsrr - Loose Source Routing

ssrr - Strict Source Routing

satid - Stream identiﬁer

any - any IP options are set

The most frequently watched for IP options are strict and loose source routing which aren’t used in any widespread

internet applications.

Format

ipopts:<rr|eol|nop|ts|sec|esec|lsrr|ssrr|satid|any>;

Example

This example looks for the IP Option of Loose Source Routing.

ipopts:lsrr;

137

Warning

Only a single ipopts keyword may be speciﬁed per rule.

3.6.6 fragbits

The

fragbits

keyword is used to check if fragmentation and reserved bits are set in the IP header.

The following bits may be checked:

M- More Fragments

D- Don’t Fragment

R- Reserved Bit

The following modiﬁers can be set to change the match criteria:

+match on the speciﬁed bits, plus any others

*match if any of the speciﬁed bits are set

!match if the speciﬁed bits are not set

Format

fragbits:[+*!]<[MDR]>;

Example

This example checks if the More Fragments bit and the Do not Fragment bit are set.

fragbits:MD+;

3.6.7 dsize

The dsize keyword is used to test the packet payload size. This may be used to check for abnormally sized packets. In

many cases, it is useful for detecting buffer overﬂows.

Format

dsize: [<>]<number>[<><number>];

Example

This example looks for a dsize that is between 300 and 400 bytes.

dsize:300<>400;

Warning

dsize will fail on stream rebuilt packets, regardless of the size of the payload.

138

3.6.8 ﬂags

The ﬂags keyword is used to check if speciﬁc TCP ﬂag bits are present.

The following bits may be checked:

F- FIN (LSB in TCP Flags byte)

S- SYN

R- RST

P- PSH

A- ACK

U- URG

1- Reserved bit 1 (MSB in TCP Flags byte)

2- Reserved bit 2

0- No TCP Flags Set

The following modiﬁers can be set to change the match criteria:

+- match on the speciﬁed bits, plus any others

*- match if any of the speciﬁed bits are set

!- match if the speciﬁed bits are not set

To handle writing rules for session initiation packets such as ECN where a SYN packet is sent with the previously

reserved bits 1 and 2 set, an option mask may be speciﬁed. A rule could check for a ﬂags value of S,12 if one wishes

to ﬁnd packets with just the syn bit, regardless of the values of the reserved bits.

Format

flags:[!|*|+]<FSRPAU120>[,<FSRPAU120>];

Example

This example checks if just the SYN and the FIN bits are set, ignoring reserved bit 1 and reserved bit 2.

alert tcp any any -> any any (flags:SF,12;)

3.6.9 ﬂow

The ﬂow keyword is used in conjunction with TCP stream reassembly (see Section 2.2.2). It allows rules to only apply

to certain directions of the trafﬁc ﬂow.

This allows rules to only apply to clients or servers. This allows packets related to $HOME NET clients viewing web

pages to be distinguished from servers running in the $HOME NET.

The established keyword will replace the

flags: A+

used in many places to show established TCP connections.

139

Options

Option Description

to client

Trigger on server responses from A to B

to server

Trigger on client requests from A to B

from client

Trigger on client requests from A to B

from server

Trigger on server responses from A to B

established

Trigger only on established TCP connections

stateless

Trigger regardless of the state of the stream processor (useful for packets that are

designed to cause machines to crash)

no stream

Do not trigger on rebuilt stream packets (useful for dsize and stream5)

only stream

Only trigger on rebuilt stream packets

Format

flow: [(established|stateless)]

[,(to_client|to_server|from_client|from_server)]

[,(no_stream|only_stream)];

Examples

alert tcp !$HOME_NET any -> $HOME_NET 21 (msg:"cd incoming detected"; \

flow:from_client; content:"CWD incoming"; nocase;)

alert tcp !$HOME_NET 0 -> $HOME_NET 0 (msg: "Port 0 TCP traffic"; \

flow:stateless;)

3.6.10 ﬂowbits

The

flowbits

keyword is used in conjunction with conversation tracking from the Stream preprocessor (see Section2.2.2).

It allows rules to track states across transport protocol sessions. The ﬂowbits option is most useful for TCP sessions,

as it allows rules to generically track the state of an application protocol.

There are seven keywords associated with ﬂowbits. Most of the options need a user-deﬁned name for the speciﬁc

state that is being checked. This string should be limited to any alphanumeric string including periods, dashes, and

underscores.

Option Description

set

Sets the speciﬁed state for the current ﬂow.

unset

Unsets the speciﬁed state for the current ﬂow.

toggle

Sets the speciﬁed state if the state is unset, otherwise unsets the state if the state is

set.

isset

Checks if the speciﬁed state is set.

isnotset

Checks if the speciﬁed state is not set.

noalert

Cause the rule to not generate an alert, regardless of the rest of the detection

options.

Format

Examples

alert tcp any 143 -> any any (msg:"IMAP login";

140

content:"OK LOGIN"; flowbits:set,logged_in;

flowbits:noalert;)

alert tcp any any -> any 143 (msg:"IMAP LIST"; content:"LIST";

flowbits:isset,logged_in;)

3.6.11 seq

The seq keyword is used to check for a speciﬁc TCP sequence number.

Format

seq:<number>;

Example

This example looks for a TCP sequence number of 0.

seq:0;

3.6.12 ack

The ack keyword is used to check for a speciﬁc TCP acknowledge number.

Format

ack: <number>;

Example

This example looks for a TCP acknowledge number of 0.

ack:0;

3.6.13 window

The window keyword is used to check for a speciﬁc TCP window size.

Format

window:[!]<number>;

Example

This example looks for a TCP window size of 55808.

window:55808;

141

3.6.14 itype

The itype keyword is used to check for a speciﬁc ICMP type value.

Format

itype:[<|>]<number>[<><number>];

Example

This example looks for an ICMP type greater than 30.

itype:>30;

3.6.15 icode

The icode keyword is used to check for a speciﬁc ICMP code value.

Format

icode: [<|>]<number>[<><number>];

Example

This example looks for an ICMP code greater than 30.

code:>30;

3.6.16 icmp id

The icmp id keyword is used to check for a speciﬁc ICMP ID value.

This is useful because some covert channel programs use static ICMP ﬁelds when they communicate. This particular

plugin was developed to detect the stacheldraht DDoS agent.

Format

icmp_id:<number>;

Example

This example looks for an ICMP ID of 0.

icmp_id:0;

3.6.17 icmp seq

The icmp seq keyword is used to check for a speciﬁc ICMP sequence value.

This is useful because some covert channel programs use static ICMP ﬁelds when they communicate. This particular

plugin was developed to detect the stacheldraht DDoS agent.

142

Format

icmp_seq:<number>;

Example

This example looks for an ICMP Sequence of 0.

icmp_seq:0;

3.6.18 rpc

The rpc keyword is used to check for a RPC application, version, and procedure numbers in SUNRPC CALL requests.

Wildcards are valid for both version and procedure numbers by using ’*’;

Format

rpc: <application number>, [<version number>|*], [<procedure number>|*]>;

Example

The following example looks for an RPC portmap GETPORT request.

alert tcp any any -> any 111 (rpc: 100000,*,3;);

Warning

Because of the fast pattern matching engine, the RPC keyword is slower than looking for the RPC values by using

normal content matching.

3.6.19 ip proto

The ip proto keyword allows checks against the IP protocol header. For a list of protocols that may be speciﬁed by

name, see /etc/protocols.

Format

ip_proto:[!|>|<] <name or number>;

Example

This example looks for IGMP trafﬁc.

alert ip any any -> any any (ip_proto:igmp;)

3.6.20 sameip

The sameip keyword allows rules to check if the source ip is the same as the destination IP.

143

Format

sameip;

Example

This example looks for any trafﬁc where the Source IP and the Destination IP is the same.

alert ip any any -> any any (sameip;)

3.6.21 stream size

The stream size keyword allows a rule to match trafﬁc according to the number of bytes observed, as determined by

the TCP sequence numbers.

△

!NOTE

The stream size option is only available when the Stream5 preprocessor is enabled.

Format

stream_size:<server|client|both|either>,<operator>,<number>

Where the operator is one of the following:

•<- less than

•>- greater than

•= - equal

•!= - not

•<= - less than or equal

•>= - greater than or equal

Example

For example, to look for a session that is less that 6 bytes from the client side, use:

alert tcp any any -> any any (stream_size:client,<,6;)

3.6.22 Non-Payload Detection Quick Reference

Table 3.10: Non-payload detection rule option keywords

Keyword Description

fragoffset

The fragoffset keyword allows one to compare the IP fragment offset ﬁeld against

a decimal value.

ttl

The ttl keyword is used to check the IP time-to-live value.

tos

The tos keyword is used to check the IP TOS ﬁeld for a speciﬁc value.

The id keyword is used to check the IP ID ﬁeld for a speciﬁc value.

144

ipopts

The ipopts keyword is used to check if a speciﬁc IP option is present.

fragbits

The fragbits keyword is used to check if fragmentation and reserved bits are set in

the IP header.

dsize

The dsize keyword is used to test the packet payload size.

flags

The ﬂags keyword is used to check if speciﬁc TCP ﬂag bits are present.

flow

The ﬂow keyword allows rules to only apply to certain directions of the trafﬁc

ﬂow.

flowbits

The ﬂowbits keyword allows rules to track states across transport protocol ses-

sions.

seq

The seq keyword is used to check for a speciﬁc TCP sequence number.

ack

The ack keyword is used to check for a speciﬁc TCP acknowledge number.

window

The window keyword is used to check for a speciﬁc TCP window size.

itype

The itype keyword is used to check for a speciﬁc ICMP type value.

icode

The icode keyword is used to check for a speciﬁc ICMP code value.

icmp id

The icmp id keyword is used to check for a speciﬁc ICMP ID value.

icmp seq

The icmp seq keyword is used to check for a speciﬁc ICMP sequence value.

rpc

The rpc keyword is used to check for a RPC application, version, and procedure

numbers in SUNRPC CALL requests.

ip proto

The ip proto keyword allows checks against the IP protocol header.

sameip

The sameip keyword allows rules to check if the source ip is the same as the

destination IP.

3.7 Post-Detection Rule Options

3.7.1 logto

The logto keyword tells Snort to log all packets that trigger this rule to a special output log ﬁle. This is especially

handy for combining data from things like NMAP activity, HTTP CGI scans, etc. It should be noted that this option

does not work when Snort is in binary logging mode.

Format

logto:"filename";

3.7.2 session

The session keyword is built to extract user data from TCP Sessions. There are many cases where seeing what users

are typing in telnet, rlogin, ftp, or even web sessions is very useful.

There are two available argument keywords for the session rule option, printable or all. The printable keyword only

prints out data that the user would normally see or be able to type.

The all keyword substitutes non-printable characters with their hexadecimal equivalents.

Format

session: [printable|all];

145

Example

The following example logs all printable strings in a telnet packet.

log tcp any any <> any 23 (session:printable;)

Warnings

Using the session keyword can slow Snort down considerably, so it should not be used in heavy load situations. The

session keyword is best suited for post-processing binary (pcap) log ﬁles.

3.7.3 resp

The resp keyword is used to attempt to close sessions when an alert is triggered. In Snort, this is called ﬂexible

response.

Flexible Response supports the following mechanisms for attempting to close sessions:

Option Description

rst snd

Send TCP-RST packets to the sending socket

rst rcv

Send TCP-RST packets to the receiving socket

rst all

Send TCP RST packets in both directions

icmp net

Send a ICMP NET UNREACH to the sender

icmp host

Send a ICMP HOST UNREACH to the sender

icmp port

Send a ICMP PORT UNREACH to the sender

icmp all

Send all above ICMP packets to the sender

These options can be combined to send multiple responses to the target host.

Format

resp: <resp_mechanism>[,<resp_mechanism>[,<resp_mechanism>]];

Warnings

This functionality is not built in by default. Use the – –enable-ﬂexresp ﬂag to conﬁgure when building Snort to enable

this functionality.

Be very careful when using Flexible Response. It is quite easy to get Snort into an inﬁnite loop by deﬁning a rule such

as:

alert tcp any any -> any any (resp:rst_all;)

It is easy to be fooled into interfering with normal network trafﬁc as well.

Example

The following example attempts to reset any TCP connection to port 1524.

alert tcp any any -> any 1524 (flags:S; resp:rst_all;)

146

3.7.4 react

This keyword implements an ability for users to react to trafﬁc that matches a Snort rule. The basic reaction is blocking

interesting sites users want to access: New York Times, slashdot, or something really important - napster and porn

sites. The React code allows Snort to actively close offending connections and send a visible notice to the browser.

The notice may include your own comment. The following arguments (basic modiﬁers) are valid for this option:

•block - close connection and send the visible notice

The basic argument may be combined with the following arguments (additional modiﬁers):

•msg - include the msg option text into the blocking visible notice

•proxy <port nr>- use the proxy port to send the visible notice

Multiple additional arguments are separated by a comma. The react keyword should be placed as the last one in the

option list.

Format

react: block[, <react_additional_modifier>];

Example

alert tcp any any <> 192.168.1.0/24 80 (content: "bad.htm"; \

msg: "Not for children!"; react: block, msg, proxy 8000;)

Warnings

React functionality is not built in by default; you must conﬁgure with –enable-react to build it. (Note that react may

now be enabled independently of ﬂexresp and ﬂexresp2.)

Be very careful when using react. Causing a network trafﬁc generation loop is very easy to do with this functionality.

3.7.5 tag

The tag keyword allow rules to log more than just the single packet that triggered the rule. Once a rule is triggered,

additional trafﬁc involving the source and/or destination host is tagged. Tagged trafﬁc is logged to allow analysis of

response codes and post-attack trafﬁc. tagged alerts will be sent to the same output plugins as the original alert, but it

is the responsibility of the output plugin to properly handle these special alerts. Currently, the database output plugin,

described in Section 2.6.6, does not properly handle tagged alerts.

Format

tag: <type>, <count>, <metric>, [direction];

type

•

session

- Log packets in the session that set off the rule

•

host

- Log packets from the host that caused the tag to activate (uses [direction] modiﬁer)

count

147

•

- Count is speciﬁed as a number of units. Units are speciﬁed in the <metric>ﬁeld.

metric

•

packets

- Tag the host/session for <count>packets

•

seconds

- Tag the host/session for <count>seconds

•

bytes

- Tag the host/session for <count>bytes

direction - only relevant if host type is used.

•

src

- Tag packets containing the source IP address of the packet that generated the initial event.

•

dst

- Tag packets containing the destination IP address of the packet that generated the initial event.

Note, any packets that generate an alert will not be tagged. For example, it may seem that the following rule will tag

the ﬁrst 600 seconds of any packet involving 10.1.1.1.

alert tcp any any <> 10.1.1.1 any (tag:host,600,seconds,src;)

However, since the rule will ﬁre on every packet involving 10.1.1.1, no packets will get tagged. The ﬂowbits option

would be useful here.

alert tcp any any <> 10.1.1.1 any (flowbits:isnotset,tagged;

flowbits:set,tagged; tag:host,600,seconds,src;)

Also note that if you have a tag option in a rule that uses a metric other than

packets

, a

tagged packet limit

will

be used to limit the number of tagged packets regardless of whether the

seconds

bytes

count has been reached.

The default

tagged packet limit

value is 256 and can be modiﬁed by using a conﬁg option in your snort.conf ﬁle

(see Section 2.1.3 on how to use the

tagged packet limit

conﬁg option). You can disable this packet limit for

a particular rule by adding a

packets

metric to your tag option and setting its count to 0 (This can be done on a

global scale by setting the

tagged packet limit

option in snort.conf to 0). Doing this will ensure that packets are

tagged for the full amount of

seconds

bytes

and will not be cut off by the

tagged packet limit

. (Note that the

tagged packet limit

was introduced to avoid DoS situations on high bandwidth sensors for tag rules with a high

seconds

bytes

counts.)

alert tcp 10.1.1.4 any -> 10.1.1.1 any \

(content:"TAGMYPACKETS"; tag:host,0,packets,600,seconds,src;)

Example

This example logs the ﬁrst 10 seconds or the

tagged packet limit

(whichever comes ﬁrst) of any telnet session.

alert tcp any any -> any 23 (flags:s,12; tag:session,10,seconds;)

3.7.6 activates

The

activates

keyword allows the rule writer to specify a rule to add when a speciﬁc network event occurs. See

Section 3.2.6 for more information.

Format

activates: 1;

148

3.7.7 activated by

The

activated by

keyword allows the rule writer to dynamically enable a rule when a speciﬁc activate rule is trig-

gered. See Section 3.2.6 for more information.

Format

activated_by: 1;

3.7.8 count

The

count

keyword must be used in combination with the

activated by

keyword. It allows the rule writer to specify

how many packets to leave the rule enabled for after it is activated. See Section 3.2.6 for more information.

Format

activated_by: 1; count: 50;

3.7.9 replace

The

replace

keyword is a feature available in inline mode which will cause Snort to replace the prior matching

content with the given string. Both the new string and the content it is to replace must have the same length. You can

have multiple replacements within a rule, one per content.

See section 1.5 for more on operating in inline mode.

replace: <string>;

3.7.10 detection ﬁlter

detection ﬁlter deﬁnes a rate which must be exceeded by a source or destination host before a rule can generate an

event. detection ﬁlter has the following format:

detection_filter: \

track <by_src|by_dst>, \

count <c>, seconds <s>;

Option Description

track

by src|by dst

Rate is tracked either by source IP address or destination IP address. This means

count is maintained for each unique source IP address or each unique destination

IP address.

count c

The maximum number of rule matches in s seconds allowed before the detection

ﬁlter limit to be exceeded. C must be nonzero.

seconds s

Time period over which count is accrued. The value must be nonzero.

Snort evaluates a

detection filter

as the last step of the detection phase, after evaluating all other rule options

(regardless of the position of the ﬁlter within the rule source). At most one

detection filter

is permitted per rule.

Example - this rule will ﬁre on every failed login attempt from 10.1.2.100 during one sampling period of 60 seconds,

after the ﬁrst 30 failed login attempts:

149

drop tcp 10.1.2.100 any > 10.1.1.100 22 ( \

msg:"SSH Brute Force Attempt";

flow:established,to_server; \

content:"SSH"; nocase; offset:0; depth:4; \

detection_filter: track by_src, count 30, seconds 60; \

sid:1000001; rev:1;)

Since potentially many events will be generated, a

detection filter

would normally be used in conjunction with

event filter

to reduce the number of logged events.

3.7.11 Post-Detection Quick Reference

Table 3.11: Post-detection rule option keywords

Keyword Description

logto

The logto keyword tells Snort to log all packets that trigger this rule to a special

output log ﬁle.

session

The session keyword is built to extract user data from TCP Sessions.

resp

The resp keyword is used attempt to close sessions when an alert is triggered.

react

This keyword implements an ability for users to react to trafﬁc that matches a

Snort rule by closing connection and sending a notice.

tag

The tag keyword allow rules to log more than just the single packet that triggered

the rule.

activates

This keyword allows the rule writer to specify a rule to add when a speciﬁc net-

work event occurs.

activated by

This keyword allows the rule writer to dynamically enable a rule when a speciﬁc

activate rule is triggered.

count

This keyword must be used in combination with the

activated by

keyword. It

allows the rule writer to specify how many packets to leave the rule enabled for

after it is activated.

replace

Replace the prior matching content with the given string of the same length. Avail-

able in inline mode only.

detection filter

Track by source or destination IP address and if the rule otherwise matches more

than the conﬁgured rate it will ﬁre.

3.8 Rule Thresholds

△

!NOTE

Rule thresholds are deprecated and will not be supported in a future release. Use

detection filter

(3.7.10) within rules, or

event filter

s (2.4.2) as standalone conﬁgurations instead.

threshold

can be included as part of a rule, or you can use standalone thresholds that reference the generator and

SID they are applied to. There is no functional difference between adding a threshold to a rule, or using a standalone

threshold applied to the same rule. There is a logical difference. Some rules may only make sense with a threshold.

These should incorporate the threshold into the rule. For instance, a rule for detecting a too many login password

attempts may require more than 5 attempts. This can be done using the ‘limit’ type of threshold. It makes sense that

the threshold feature is an integral part of this rule.

Format

150

threshold: \

type <limit|threshold|both>, \

track <by_src|by_dst>, \

count <c>, seconds <s>;

Option Description

type limit|threshold|both

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.

track by src|by dst

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 desti-

nation IP addresses. Ports or anything else are not tracked.

count c

number of rule matching in s seconds that will cause

event filter

limit to be

exceeded.

must be nonzero value.

seconds s

time period over which

count

is accrued.

must be nonzero value.

Examples

This rule logs the ﬁrst event of this SID every 60 seconds.

alert tcp $external_net any -> $http_servers $http_ports \

(msg:"web-misc robots.txt access"; flow:to_server, established; \

uricontent:"/robots.txt"; nocase; reference:nessus,10302; \

classtype:web-application-activity; threshold: type limit, track \

by_src, count 1 , seconds 60 ; sid:1000852; rev:1;)

This rule logs every 10th event on this SID during a 60 second interval. So if less than 10 events occur in 60 seconds,

nothing gets logged. Once an event is logged, a new time period starts for type=threshold.

alert tcp $external_net any -> $http_servers $http_ports \

(msg:"web-misc robots.txt access"; flow:to_server, established; \

uricontent:"/robots.txt"; nocase; reference:nessus,10302; \

classtype:web-application-activity; threshold: type threshold, \

track by_dst, count 10 , seconds 60 ; sid:1000852; rev:1;)

This rule logs at most one event every 60 seconds if at least 10 events on this SID are ﬁred.

alert tcp $external_net any -> $http_servers $http_ports \

(msg:"web-misc robots.txt access"; flow:to_server, established; \

uricontent:"/robots.txt"; nocase; reference:nessus,10302; \

classtype:web-application-activity; threshold: type both , track \

by_dst, count 10 , seconds 60 ; sid:1000852; rev:1;)

3.9 Writing Good Rules

There are some general concepts to keep in mind when developing Snort rules to maximize efﬁciency and speed.

151

3.9.1 Content Matching

The 2.0 detection engine changes the way Snort works slightly by having the ﬁrst phase be a setwise pattern match.

The longer a content option is, the more exact the match. Rules without content (or uricontent) slow the entire system

down.

While some detection options, such as pcre and byte test, perform detection in the payload section of the packet, they

do not use the setwise pattern matching engine. If at all possible, try and have at least one content option if at all

possible.

3.9.2 Catch the Vulnerability, Not the Exploit

Try to write rules that target the vulnerability, instead of a speciﬁc exploit.

For example, look for a the vulnerable command with an argument that is too large, instead of shellcode that binds a

shell.

By writing rules for the vulnerability, the rule is less vulnerable to evasion when an attacker changes the exploit

slightly.

3.9.3 Catch the Oddities of the Protocol in the Rule

Many services typically send the commands in upper case letters. FTP is a good example. In FTP, to send the

username, the client sends:

user username_here

A simple rule to look for FTP root login attempts could be:

alert tcp any any -> any any 21 (content:"user root";)

While it may seem trivial to write a rule that looks for the username root, a good rule will handle all of the odd things

that the protocol might handle when accepting the user command.

For example, each of the following are accepted by most FTP servers:

user root

user<tab>root

To handle all of the cases that the FTP server might handle, the rule needs more smarts than a simple string match.

A good rule that looks for root login on ftp would be:

alert tcp any any -> any 21 (flow:to_server,established; \

content:"root"; pcre:"/user\s+root/i";)

There are a few important things to note in this rule:

•The rule has a ﬂow option, verifying this is trafﬁc going to the server on an enstablished session.

•The rule has a content option, looking for root, which is the longest, most unique string in the attack. This option

is added to allow Snort’s setwise pattern match detection engine to give Snort a boost in speed.

•The rule has a pcre option, looking for user, followed at least one space character (which includes tab), followed

by root, ignoring case.

152

3.9.4 Optimizing Rules

The content matching portion of the detection engine has recursion to handle a few evasion cases. Rules that are not

properly written can cause Snort to waste time duplicating checks.

The way the recursion works now is if a pattern matches, and if any of the detection options after that pattern fail, then

look for the pattern again after where it was found the previous time. Repeat until the pattern is not found again or the

opt functions all succeed.

On ﬁrst read, that may not sound like a smart idea, but it is needed. For example, take the following rule:

alert ip any any -> any any (content:"a"; content:"b"; within:1;)

This rule would look for “a”, immediately followed by “b”. Without recursion, the payload “aab” would fail, even

though it is obvious that the payload “aab” has “a” immediately followed by “b”, because the ﬁrst ”a” is not immedi-

ately followed by “b”.

While recursion is important for detection, the recursion implementation is not very smart.

For example, the following rule options are not optimized:

content:"|13|"; dsize:1;

By looking at this rule snippit, it is obvious the rule looks for a packet with a single byte of 0x13. However, because

of recursion, a packet with 1024 bytes of 0x13 could cause 1023 too many pattern match attempts and 1023 too many

dsize checks. Why? The content 0x13 would be found in the ﬁrst byte, then the dsize option would fail, and because

of recursion, the content 0x13 would be found again starting after where the previous 0x13 was found, once it is found,

then check the dsize again, repeating until 0x13 is not found in the payload again.

Reordering the rule options so that discrete checks (such as dsize) are moved to the begining of the rule speed up

Snort.

The optimized rule snipping would be:

dsize:1; content:"|13|";

A packet of 1024 bytes of 0x13 would fail immediately, as the dsize check is the ﬁrst option checked and dsize is a

discrete check without recursion.

The following rule options are discrete and should generally be placed at the begining of any rule:

•

dsize

•

flags

•

flow

•

fragbits

•

icmp id

•

icmp seq

•

icode

•

ipopts

•

ip proto

•

itype

153

•

seq

•

session

•

tos

•

ttl

•

ack

•

window

•

resp

•

sameip

3.9.5 Testing Numerical Values

The rule options byte test and byte jump were written to support writing rules for protocols that have length encoded

data. RPC was the protocol that spawned the requirement for these two rule options, as RPC uses simple length based

encoding for passing data.

In order to understand why byte test and byte jump are useful, let’s go through an exploit attempt against the sadmind

service.

This is the payload of the exploit:

89 09 9c e2 00 00 00 00 00 00 00 02 00 01 87 88 ................

00 00 00 0a 00 00 00 01 00 00 00 01 00 00 00 20 ...............

40 28 3a 10 00 00 00 0a 4d 45 54 41 53 50 4c 4f @(:.....metasplo

49 54 00 00 00 00 00 00 00 00 00 00 00 00 00 00 it..............

00 00 00 00 00 00 00 00 40 28 3a 14 00 07 45 df ........@(:...e.

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 04 ................

7f 00 00 01 00 01 87 88 00 00 00 0a 00 00 00 04 ................

7f 00 00 01 00 01 87 88 00 00 00 0a 00 00 00 11 ................

00 00 00 1e 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 3b 4d 45 54 41 53 50 4c 4f .......;metasplo

49 54 00 00 00 00 00 00 00 00 00 00 00 00 00 00 it..............

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 06 73 79 73 74 65 6d 00 00 ........system..

00 00 00 15 2e 2e 2f 2e 2e 2f 2e 2e 2f 2e 2e 2f ....../../../../

2e 2e 2f 62 69 6e 2f 73 68 00 00 00 00 00 04 1e ../bin/sh.......

<snip>

Let’s break this up, describe each of the ﬁelds, and ﬁgure out how to write a rule to catch this exploit.

There are a few things to note with RPC:

•Numbers are written as uint32s, taking four bytes. The number 26 would show up as 0x0000001a.

•Strings are written as a uint32 specifying the length of the string, the string, and then null bytes to pad the length

of the string to end on a 4 byte boundary. The string “bob” would show up as 0x00000003626f6200.

89 09 9c e2 - the request id, a random uint32, unique to each request

00 00 00 00 - rpc type (call = 0, response = 1)

00 00 00 02 - rpc version (2)

00 01 87 88 - rpc program (0x00018788 = 100232 = sadmind)

154

00 00 00 0a - rpc program version (0x0000000a = 10)

00 00 00 01 - rpc procedure (0x00000001 = 1)

00 00 00 01 - credential flavor (1 = auth\_unix)

00 00 00 20 - length of auth\_unix data (0x20 = 32

## the next 32 bytes are the auth\_unix data

40 28 3a 10 - unix timestamp (0x40283a10 = 1076378128 = feb 10 01:55:28 2004 gmt)

00 00 00 0a - length of the client machine name (0x0a = 10)

4d 45 54 41 53 50 4c 4f 49 54 00 00 - metasploit

00 00 00 00 - uid of requesting user (0)

00 00 00 00 - gid of requesting user (0)

00 00 00 00 - extra group ids (0)

00 00 00 00 - verifier flavor (0 = auth\_null, aka none)

00 00 00 00 - length of verifier (0, aka none)

The rest of the packet is the request that gets passed to procedure 1 of sadmind.

However, we know the vulnerability is that sadmind trusts the uid coming from the client. sadmind runs any request

where the client’s uid is 0 as root. As such, we have decoded enough of the request to write our rule.

First, we need to make sure that our packet is an RPC call.

content:"|00 00 00 00|"; offset:4; depth:4;

Then, we need to make sure that our packet is a call to sadmind.

content:"|00 01 87 88|"; offset:12; depth:4;

Then, we need to make sure that our packet is a call to the procedure 1, the vulnerable procedure.

content:"|00 00 00 01|"; offset:16; depth:4;

Then, we need to make sure that our packet has auth unix credentials.

content:"|00 00 00 01|"; offset:20; depth:4;

We don’t care about the hostname, but we want to skip over it and check a number value after the hostname. This is

where byte test is useful. Starting at the length of the hostname, the data we have is:

00 00 00 0a 4d 45 54 41 53 50 4c 4f 49 54 00 00

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

00 00 00 00

We want to read 4 bytes, turn it into a number, and jump that many bytes forward, making sure to account for the

padding that RPC requires on strings. If we do that, we are now at:

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

00 00 00 00

which happens to be the exact location of the uid, the value we want to check.

In english, we want to read 4 bytes, 36 bytes from the beginning of the packet, and turn those 4 bytes into an integer

and jump that many bytes forward, aligning on the 4 byte boundary. To do that in a Snort rule, we use:

155

byte_jump:4,36,align;

then we want to look for the uid of 0.

content:"|00 00 00 00|"; within:4;

Now that we have all the detection capabilities for our rule, let’s put them all together.

content:"|00 00 00 00|"; offset:4; depth:4;

content:"|00 01 87 88|"; offset:12; depth:4;

content:"|00 00 00 01|"; offset:16; depth:4;

content:"|00 00 00 01|"; offset:20; depth:4;

byte_jump:4,36,align;

content:"|00 00 00 00|"; within:4;

The 3rd and fourth string match are right next to each other, so we should combine those patterns. We end up with:

content:"|00 00 00 00|"; offset:4; depth:4;

content:"|00 01 87 88|"; offset:12; depth:4;

content:"|00 00 00 01 00 00 00 01|"; offset:16; depth:8;

byte_jump:4,36,align;

content:"|00 00 00 00|"; within:4;

If the sadmind service was vulnerable to a buffer overﬂow when reading the client’s hostname, instead of reading the

length of the hostname and jumping that many bytes forward, we would check the length of the hostname to make

sure it is not too large.

To do that, we would read 4 bytes, starting 36 bytes into the packet, turn it into a number, and then make sure it is not

too large (let’s say bigger than 200 bytes). In Snort, we do:

byte_test:4,>,200,36;

Our full rule would be:

content:"|00 00 00 00|"; offset:4; depth:4;

content:"|00 01 87 88|"; offset:12; depth:4;

content:"|00 00 00 01 00 00 00 01|"; offset:16; depth:8;

byte_test:4,>,200,36;

156

Chapter 4

Making Snort Faster

4.1 MMAPed pcap

On Linux, a modiﬁed version of libpcap is available that implements a shared memory ring buffer. Phil Woods

(cpw@lanl.gov) is the current maintainer of the libpcap implementation of the shared memory ring buffer. The shared

memory ring buffer libpcap can be downloaded from his website at

http://public.lanl.gov/cpw/

Instead of the normal mechanism of copying the packets from kernel memory into userland memory, by using a shared

memory ring buffer, libpcap is able to queue packets into a shared buffer that Snort is able to read directly. This change

speeds up Snort by limiting the number of times the packet is copied before Snort gets to perform its detection upon

it.

Once Snort linked against the shared memory libpcap, enabling the ring buffer is done via setting the enviornment

variable PCAP FRAMES.PCAP FRAMES is the size of the ring buffer. According to Phil, the maximum size is

32768, as this appears to be the maximum number of iovecs the kernel can handle. By using PCAP FRAMES=max,

libpcap will automatically use the most frames possible. On Ethernet, this ends up being 1530 bytes per frame, for a

total of around 52 Mbytes of memory for the ring buffer alone.

157

Chapter 5

Dynamic Modules

Preprocessors, detection capabilities, and rules can now be developed as dynamically loadable module to snort. When

enabled via the –enable-dynamicplugin conﬁgure option, the dynamic API presents a means for loading dynamic

libraries and allowing the module to utilize certain functions within the main snort code.

The remainder of this chapter will highlight the data structures and API functions used in developing preprocessors,

detection engines, and rules as a dynamic plugin to snort.

Beware: the deﬁnitions herein may be out of date; check the appropriate header ﬁles for the current deﬁnitions.

5.1 Data Structures

A number of data structures are central to the API. The deﬁnition of each is deﬁned in the following sections.

5.1.1 DynamicPluginMeta

The DynamicPluginMeta structure deﬁnes the type of dynamic module (preprocessor, rules, or detection engine), the

version information, and path to the shared library. A shared library can implement all three types, but typically is

limited to a single functionality such as a preprocessor. It is deﬁned in

sf dynamic meta.h

as:

#define MAX_NAME_LEN 1024

#define TYPE_ENGINE 0x01

#define TYPE_DETECTION 0x02

#define TYPE_PREPROCESSOR 0x04

typedef struct _DynamicPluginMeta

{

int type;

int major;

int minor;

int build;

char uniqueName[MAX_NAME_LEN];

char *libraryPath;

} DynamicPluginMeta;

5.1.2 DynamicPreprocessorData

The DynamicPreprocessorData structure deﬁnes the interface the preprocessor uses to interact with snort itself. This

inclues functions to register the preprocessor’s conﬁguration parsing, restart, exit, and processing functions. It includes

158

function to log messages, errors, fatal errors, and debugging info. It also includes information for setting alerts,

handling Inline drops, access to the StreamAPI, and it provides access to the normalized http and alternate data

buffers. This data structure should be initialized when the preprocessor shared library is loaded. It is deﬁned in

sf dynamic preprocessor.h

. Check the header ﬁle for the current deﬁnition.

5.1.3 DynamicEngineData

The DynamicEngineData structure deﬁnes the interface a detection engine uses to interact with snort itself. This

includes functions for logging messages, errors, fatal errors, and debugging info as well as a means to register and

check ﬂowbits. It also includes a location to store rule-stubs for dynamic rules that are loaded, and it provides access

to the normalized http and alternate data buffers. It is deﬁned in

sf dynamic engine.h

as:

typedef struct _DynamicEngineData

{

int version;

u_int8_t *altBuffer;

UriInfo *uriBuffers[MAX_URIINFOS];

RegisterRule ruleRegister;

RegisterBit flowbitRegister;

CheckFlowbit flowbitCheck;

DetectAsn1 asn1Detect;

LogMsgFunc logMsg;

LogMsgFunc errMsg;

LogMsgFunc fatalMsg;

char *dataDumpDirectory;

GetPreprocRuleOptFuncs getPreprocOptFuncs;

SetRuleData setRuleData;

GetRuleData getRuleData;

DebugMsgFunc debugMsg;

#ifdef HAVE_WCHAR_H

DebugWideMsgFunc debugWideMsg;

#endif

char **debugMsgFile;

int *debugMsgLine;

PCRECompileFunc pcreCompile;

PCREStudyFunc pcreStudy;

PCREExecFunc pcreExec;

} DynamicEngineData;

5.1.4 SFSnortPacket

The SFSnortPacket structure mirrors the snort Packet structure and provides access to all of the data contained in a

given packet.

It and the data structures it incorporates are deﬁned in

sf snort packet.h

. Additional data structures may be deﬁned

to reference other protocol ﬁelds. Check the header ﬁle for the current deﬁnitions.

159

5.1.5 Dynamic Rules

A dynamic rule should use any of the following data structures. The following structures are deﬁned in

sf snort plugin api.h

Rule

The Rule structure deﬁnes the basic outline of a rule and contains the same set of information that is seen in a text

rule. That includes protocol, address and port information and rule information (classiﬁcation, generator and signature

IDs, revision, priority, classiﬁcation, and a list of references). It also includes a list of rule options and an optional

evaluation function.

#define RULE_MATCH 1

#define RULE_NOMATCH 0

typedef struct _Rule

{

IPInfo ip;

RuleInformation info;

RuleOption **options; /* NULL terminated array of RuleOption union */

ruleEvalFunc evalFunc;

char initialized; /* Rule Initialized, used internally */

u_int32_t numOptions; /* Rule option count, used internally */

char noAlert; /* Flag with no alert, used internally */

void *ruleData; /* Hash table for dynamic data pointers */

} Rule;

The rule evaluation function is deﬁned as

typedef int (*ruleEvalFunc)(void *);

where the parameter is a pointer to the SFSnortPacket structure.

RuleInformation

The RuleInformation structure deﬁnes the meta data for a rule and includes generator ID, signature ID, revision,

classiﬁcation, priority, message text, and a list of references.

typedef struct _RuleInformation

{

u_int32_t genID;

u_int32_t sigID;

u_int32_t revision;

char *classification; /* String format of classification name */

u_int32_t priority;

char *message;

RuleReference **references; /* NULL terminated array of references */

RuleMetaData **meta; /* NULL terminated array of references */

} RuleInformation;

160

RuleReference

The RuleReference structure deﬁnes a single rule reference, including the system name and rereference identiﬁer.

typedef struct _RuleReference

{

char *systemName;

char *refIdentifier;

} RuleReference;

IPInfo

The IPInfo structure deﬁnes the initial matching criteria for a rule and includes the protocol, src address and port, des-

tination address and port, and direction. Some of the standard strings and variables are predeﬁned - any, HOME NET,

HTTP SERVERS, HTTP PORTS, etc.

typedef struct _IPInfo

{

u_int8_t protocol;

char * src_addr;

char * src_port; /* 0 for non TCP/UDP */

char direction; /* non-zero is bi-directional */

char * dst_addr;

char * dst_port; /* 0 for non TCP/UDP */

} IPInfo;

#define ANY_NET "any"

#define HOME_NET "$HOME_NET"

#define EXTERNAL_NET "$EXTERNAL_NET"

#define ANY_PORT "any"

#define HTTP_SERVERS "$HTTP_SERVERS"

#define HTTP_PORTS "$HTTP_PORTS"

#define SMTP_SERVERS "$SMTP_SERVERS"

RuleOption

The RuleOption structure deﬁnes a single rule option as an option type and a reference to the data speciﬁc to that

option. Each option has a ﬂags ﬁeld that contains speciﬁc ﬂags for that option as well as a ”Not” ﬂag. The ”Not” ﬂag

is used to negate the results of evaluating that option.

typedef enum DynamicOptionType {

OPTION_TYPE_PREPROCESSOR,

OPTION_TYPE_CONTENT,

OPTION_TYPE_PCRE,

OPTION_TYPE_FLOWBIT,

OPTION_TYPE_FLOWFLAGS,

OPTION_TYPE_ASN1,

OPTION_TYPE_CURSOR,

OPTION_TYPE_HDR_CHECK,

OPTION_TYPE_BYTE_TEST,

OPTION_TYPE_BYTE_JUMP,

OPTION_TYPE_BYTE_EXTRACT,

OPTION_TYPE_SET_CURSOR,

OPTION_TYPE_LOOP,

OPTION_TYPE_MAX

161

};

typedef struct _RuleOption

{

int optionType;

union

{

void *ptr;

ContentInfo *content;

CursorInfo *cursor;

PCREInfo *pcre;

FlowBitsInfo *flowBit;

ByteData *byte;

ByteExtract *byteExtract;

FlowFlags *flowFlags;

Asn1Context *asn1;

HdrOptCheck *hdrData;

LoopInfo *loop;

PreprocessorOption *preprocOpt;

} option_u;

} RuleOption;

#define NOT_FLAG 0x10000000

Some options also contain information that is initialized at run time, such as the compiled PCRE information, Boyer-

Moore content information, the integer ID for a ﬂowbit, etc.

The option types and related structures are listed below.

•OptionType: Content & Structure: ContentInfo

The ContentInfo structure deﬁnes an option for a content search. It includes the pattern, depth and offset, and

ﬂags (one of which must specify the buffer – raw, URI or normalized – to search). Additional ﬂags include

nocase, relative, unicode, and a designation that this content is to be used for snorts fast pattern evaluation. The

most unique content, that which distinguishes this rule as a possible match to a packet, should be marked for

fast pattern evaluation. In the dynamic detection engine provided with Snort, if no ContentInfo structure in a

given rules uses that ﬂag, the one with the longest content length will be used.

typedef struct _ContentInfo

{

u_int8_t *pattern;

u_int32_t depth;

int32_t offset;

u_int32_t flags; /* must include a CONTENT_BUF_X */

void *boyer_ptr;

u_int8_t *patternByteForm;

u_int32_t patternByteFormLength;

u_int32_t incrementLength;

} ContentInfo;

#define CONTENT_NOCASE 0x01

#define CONTENT_RELATIVE 0x02

#define CONTENT_UNICODE2BYTE 0x04

#define CONTENT_UNICODE4BYTE 0x08

#define CONTENT_FAST_PATTERN 0x10

#define CONTENT_END_BUFFER 0x20

#define CONTENT_BUF_NORMALIZED 0x100

162

#define CONTENT_BUF_RAW 0x200

#define CONTENT_BUF_URI 0x400

•OptionType: PCRE & Structure: PCREInfo

The PCREInfo structure deﬁnes an option for a PCRE search. It includes the PCRE expression, pcre ﬂags such

as caseless, as deﬁned in PCRE.h, and ﬂags to specify the buffer.

pcre.h provides flags:

PCRE_CASELESS

PCRE_MULTILINE

PCRE_DOTALL

PCRE_EXTENDED

PCRE_ANCHORED

PCRE_DOLLAR_ENDONLY

PCRE_UNGREEDY

typedef struct _PCREInfo

{

char *expr;

void *compiled_expr;

void *compiled_extra;

u_int32_t compile_flags;

u_int32_t flags; /* must include a CONTENT_BUF_X */

} PCREInfo;

•OptionType: Flowbit & Structure: FlowBitsInfo

The FlowBitsInfo structure deﬁnes a ﬂowbits option. It includes the name of the ﬂowbit and the operation (set,

unset, toggle, isset, isnotset).

#define FLOWBIT_SET 0x01

#define FLOWBIT_UNSET 0x02

#define FLOWBIT_TOGGLE 0x04

#define FLOWBIT_ISSET 0x08

#define FLOWBIT_ISNOTSET 0x10

#define FLOWBIT_RESET 0x20

#define FLOWBIT_NOALERT 0x40

typedef struct _FlowBitsInfo

{

char *flowBitsName;

u_int8_t operation;

u_int32_t id;

u_int32_t flags;

} FlowBitsInfo;

•OptionType: Flow Flags & Structure: FlowFlags

The FlowFlags structure deﬁnes a ﬂow option. It includes the ﬂags, which specify the direction (from server,

to server), established session, etc.

#define FLOW_ESTABLISHED 0x10

#define FLOW_IGNORE_REASSEMBLED 0x1000

#define FLOW_ONLY_REASSMBLED 0x2000

163

#define FLOW_FR_SERVER 0x40

#define FLOW_TO_CLIENT 0x40 /* Just for redundancy */

#define FLOW_TO_SERVER 0x80

#define FLOW_FR_CLIENT 0x80 /* Just for redundancy */

typedef struct _FlowFlags

{

u_int32_t flags;

} FlowFlags;

•OptionType: ASN.1 & Structure: Asn1Context

The Asn1Context structure deﬁnes the information for an ASN1 option. It mirrors the ASN1 rule option and

also includes a ﬂags ﬁeld.

#define ASN1_ABS_OFFSET 1

#define ASN1_REL_OFFSET 2

typedef struct _Asn1Context

{

int bs_overflow;

int double_overflow;

int print;

int length;

unsigned int max_length;

int offset;

int offset_type;

u_int32_t flags;

} Asn1Context;

•OptionType: Cursor Check & Structure: CursorInfo

The CursorInfo structure deﬁnes an option for a cursor evaluation. The cursor is the current position within the

evaluation buffer, as related to content and PCRE searches, as well as byte tests and byte jumps. It includes an

offset and ﬂags that specify the buffer. This can be used to verify there is sufﬁcient data to continue evaluation,

similar to the isdataat rule option.

typedef struct _CursorInfo

{

int32_t offset;

u_int32_t flags; /* specify one of CONTENT_BUF_X */

} CursorInfo;

•OptionType: Protocol Header & Structure: HdrOptCheck

The HdrOptCheck structure deﬁnes an option to check a protocol header for a speciﬁc value. It incldues the

header ﬁeld, the operation (¡,¿,=,etc), a value, a mask to ignore that part of the header ﬁeld, and ﬂags.

#define IP_HDR_ID 0x0001 /* IP Header ID */

#define IP_HDR_PROTO 0x0002 /* IP Protocol */

#define IP_HDR_FRAGBITS 0x0003 /* Frag Flags set in IP Header */

#define IP_HDR_FRAGOFFSET 0x0004 /* Frag Offset set in IP Header */

#define IP_HDR_OPTIONS 0x0005 /* IP Options -- is option xx included */

#define IP_HDR_TTL 0x0006 /* IP Time to live */

#define IP_HDR_TOS 0x0007 /* IP Type of Service */

#define IP_HDR_OPTCHECK_MASK 0x000f

#define TCP_HDR_ACK 0x0010 /* TCP Ack Value */

164

#define TCP_HDR_SEQ 0x0020 /* TCP Seq Value */

#define TCP_HDR_FLAGS 0x0030 /* Flags set in TCP Header */

#define TCP_HDR_OPTIONS 0x0040 /* TCP Options -- is option xx included */

#define TCP_HDR_WIN 0x0050 /* TCP Window */

#define TCP_HDR_OPTCHECK_MASK 0x00f0

#define ICMP_HDR_CODE 0x1000 /* ICMP Header Code */

#define ICMP_HDR_TYPE 0x2000 /* ICMP Header Type */

#define ICMP_HDR_ID 0x3000 /* ICMP ID for ICMP_ECHO/ICMP_ECHO_REPLY */

#define ICMP_HDR_SEQ 0x4000 /* ICMP ID for ICMP_ECHO/ICMP_ECHO_REPLY */

#define ICMP_HDR_OPTCHECK_MASK 0xf000

typedef struct _HdrOptCheck

{

u_int16_t hdrField; /* Field to check */

u_int32_t op; /* Type of comparison */

u_int32_t value; /* Value to compare value against */

u_int32_t mask_value; /* bits of value to ignore */

u_int32_t flags;

} HdrOptCheck;

•OptionType: Byte Test & Structure: ByteData

The ByteData structure deﬁnes the information for both ByteTest and ByteJump operations. It includes the

number of bytes, an operation (for ByteTest, ¡,¿,=,etc), a value, an offset, multiplier, and ﬂags. The ﬂags must

specify the buffer.

#define CHECK_EQ 0

#define CHECK_NEQ 1

#define CHECK_LT 2

#define CHECK_GT 3

#define CHECK_LTE 4

#define CHECK_GTE 5

#define CHECK_AND 6

#define CHECK_XOR 7

#define CHECK_ALL 8

#define CHECK_ATLEASTONE 9

#define CHECK_NONE 10

typedef struct _ByteData

{

u_int32_t bytes; /* Number of bytes to extract */

u_int32_t op; /* Type of byte comparison, for checkValue */

u_int32_t value; /* Value to compare value against, for checkValue, or extracted value */

int32_t offset; /* Offset from cursor */

u_int32_t multiplier; /* Used for byte jump -- 32bits is MORE than enough */

u_int32_t flags; /* must include a CONTENT_BUF_X */

} ByteData;

•OptionType: Byte Jump & Structure: ByteData

See Byte Test above.

•OptionType: Set Cursor & Structure: CursorInfo

See Cursor Check above.

•OptionType: Loop & Structures: LoopInfo,ByteExtract,DynamicElement

The LoopInfo structure deﬁnes the information for a set of options that are to be evaluated repeatedly. The loop

option acts like a FOR loop and includes start, end, and increment values as well as the comparison operation for

165

termination. It includes a cursor adjust that happens through each iteration of the loop, a reference to a RuleInfo

structure that deﬁnes the RuleOptions are to be evaluated through each iteration. One of those options may be a

ByteExtract.

typedef struct _LoopInfo

{

DynamicElement *start; /* Starting value of FOR loop (i=start) */

DynamicElement *end; /* Ending value of FOR loop (i OP end) */

DynamicElement *increment; /* Increment value of FOR loop (i+= increment) */

u_int32_t op; /* Type of comparison for loop termination */

CursorInfo *cursorAdjust; /* How to move cursor each iteration of loop */

struct _Rule *subRule; /* Pointer to SubRule & options to evaluate within

* the loop */

u_int8_t initialized; /* Loop initialized properly (safeguard) */

u_int32_t flags; /* can be used to negate loop results, specifies

} LoopInfo;

The ByteExtract structure deﬁnes the information to use when extracting bytes for a DynamicElement used a in

Loop evaltion. It includes the number of bytes, an offset, multiplier, ﬂags specifying the buffer, and a reference

to the DynamicElement.

typedef struct _ByteExtract

{

u_int32_t bytes; /* Number of bytes to extract */

int32_t offset; /* Offset from cursor */

u_int32_t multiplier; /* Multiply value by this (similar to byte jump) */

u_int32_t flags; /* must include a CONTENT_BUF_X */

char *refId; /* To match up with a DynamicElement refId */

void *memoryLocation; /* Location to store the data extracted */

} ByteExtract;

The DynamicElement structure is used to deﬁne the values for a looping evaluation. It includes whether the

element is static (an integer) or dynamic (extracted from a buffer in the packet) and the value. For a dynamic

element, the value is ﬁlled by a related ByteExtract option that is part of the loop.

#define DYNAMIC_TYPE_INT_STATIC 1

#define DYNAMIC_TYPE_INT_REF 2

typedef struct _DynamicElement

{

char dynamicType; /* type of this field - static or reference */

char *refId; /* reference ID (NULL if static) */

union

{

void *voidPtr; /* Holder */

int32_t staticInt; /* Value of static */

int32_t *dynamicInt; /* Pointer to value of dynamic */

} data;

} DynamicElement;

5.2 Required Functions

Each dynamic module must deﬁne a set of functions and data objects to work within this framework.

166

5.2.1 Preprocessors

Each dynamic preprocessor library must deﬁne the following functions. These are deﬁned in the ﬁle

sf dynamic preproc lib.c

The metadata and setup function for the preprocessor should be deﬁned

sf preproc info.h

•int LibVersion(DynamicPluginMeta *)

This function returns the metadata for the shared library.

•int InitializePreprocessor(DynamicPreprocessorData *)

This function initializes the data structure for use by the preprocessor into a library global variable,

dpd

and

invokes the setup function.

5.2.2 Detection Engine

Each dynamic detection engine library must deﬁne the following functions.

•int LibVersion(DynamicPluginMeta *)

This function returns the metadata for the shared library.

•int InitializeEngineLib(DynamicEngineData *)

This function initializes the data structure for use by the engine.

The sample code provided with Snort predeﬁnes those functions and deﬁnes the following APIs to be used by a

dynamic rules library.

•int RegisterRules(Rule **)

This is the function to iterate through each rule in the list, initialize it to setup content searches, PCRE evalution

data, and register ﬂowbits.

•int DumpRules(char *,Rule **)

This is the function to iterate through each rule in the list and write a rule-stop to be used by snort to control the

action of the rule (alert, log, drop, etc).

•int ruleMatch(void *p, Rule *rule)

This is the function to evaluate a rule if the rule does not have its own Rule Evaluation Function. This uses the

individual functions outlined below for each of the rule options and handles repetitive content issues.

Each of the functions below returns RULE MATCH if the option matches based on the current criteria (cursor

position, etc).

–int contentMatch(void *p, ContentInfo* content, u int8 t **cursor)

This function evaluates a single content for a given packet, checking for the existence of that content as

delimited by ContentInfo and cursor. Cursor position is updated and returned in *cursor.

With a text rule, the with option corresponds to depth, and the distance option corresponds to offset.

–int checkFlow(void *p, FlowFlags *ﬂowﬂags)

This function evaluates the ﬂow for a given packet.

–int extractValue(void *p, ByteExtract *byteExtract, u int8 t *cursor)

This function extracts the bytes from a given packet, as speciﬁed by ByteExtract and delimited by cursor.

Value extracted is stored in ByteExtract memoryLocation paraneter.

–int processFlowbits(void *p, FlowBitsInfo *ﬂowbits)

This function evaluates the ﬂowbits for a given packet, as speciﬁed by FlowBitsInfo. It will interact with

ﬂowbits used by text-based rules.

167

–int setCursor(void *p, CursorInfo *cursorInfo, u int8 t **cursor)

This function adjusts the cursor as delimited by CursorInfo. New cursor position is returned in *cursor.

It handles bounds checking for the speciﬁed buffer and returns RULE NOMATCH if the cursor is moved

out of bounds.

It is also used by contentMatch, byteJump, and pcreMatch to adjust the cursor position after a successful

match.

–int checkCursor(void *p, CursorInfo *cursorInfo, u int8 t *cursor)

This function validates that the cursor is within bounds of the speciﬁed buffer.

–int checkValue(void *p, ByteData *byteData, u int32 t value, u int8 t *cursor)

This function compares the value to the value stored in ByteData.

–int byteTest(void *p, ByteData *byteData, u int8 t *cursor)

This is a wrapper for extractValue() followed by checkValue().

–int byteJump(void *p, ByteData *byteData, u int8 t **cursor)

This is a wrapper for extractValue() followed by setCursor().

–int pcreMatch(void *p, PCREInfo *pcre, u int8 t **cursor)

This function evaluates a single pcre for a given packet, checking for the existence of the expression as

delimited by PCREInfo and cursor. Cursor position is updated and returned in *cursor.

–int detectAsn1(void *p, Asn1Context *asn1, u int8 t *cursor)

This function evaluates an ASN.1 check for a given packet, as delimited by Asn1Context and cursor.

–int checkHdrOpt(void *p, HdrOptCheck *optData)

This function evaluates the given packet’s protocol headers, as speciﬁed by HdrOptCheck.

–int loopEval(void *p, LoopInfo *loop, u int8 t **cursor)

This function iterates through the SubRule of LoopInfo, as delimited by LoopInfo and cursor. Cursor

position is updated and returned in *cursor.

–int preprocOptionEval(void *p, PreprocessorOption *preprocOpt, u int8 t **cursor)

This function evaluates the preprocessor deﬁned option, as spepcifed by PreprocessorOption. Cursor po-

sition is updated and returned in *cursor.

–void setTempCursor(u int8 t **temp cursor, u int8 t **cursor)

This function is used to handled repetitive contents to save off a cursor position temporarily to be reset at

later point.

–void revertTempCursor(u int8 t **temp cursor, u int8 t **cursor)

This function is used to revert to a previously saved temporary cursor position.

△

!NOTE

If you decide to write you own rule evaluation function, patterns that occur more than once may result in false

negatives. Take extra care to handle this situation and search for the matched pattern again if subsequent rule

options fail to match. This should be done for both content and PCRE options.

5.2.3 Rules

Each dynamic rules library must deﬁne the following functions. Examples are deﬁned in the ﬁle

sfnort dynamic detection lib.c

The metadata and setup function for the preprocessor should be deﬁned in

sfsnort dynamic detection lib.h

•int LibVersion(DynamicPluginMeta *)

This function returns the metadata for the shared library.

•int EngineVersion(DynamicPluginMeta *)

This function deﬁnes the version requirements for the corresponding detection engine library.

168

•int DumpSkeletonRules()

This functions writes out the rule-stubs for rules that are loaded.

•int InitializeDetection()

This function registers each rule in the rules library. It should set up fast pattern-matcher content, register

ﬂowbits, etc.

The sample code provided with Snort predeﬁnes those functions and uses the following data within the dynamic rules

library.

•Rule *rules[]

A NULL terminated list of Rule structures that this library deﬁnes.

5.3 Examples

This section provides a simple example of a dynamic preprocessor and a dynamic rule.

5.3.1 Preprocessor Example

The following is an example of a simple preprocessor. This preprocessor always alerts on a Packet if the TCP port

matches the one conﬁgured.

This assumes the the ﬁles sf dynamic preproc lib.c and sf dynamic preproc lib.h are used.

This is the metadata for this preprocessor, deﬁned in sf preproc info.h.

#define MAJOR_VERSION 1

#define MINOR_VERSION 0

#define BUILD_VERSION 0

#define PREPROC_NAME "SF_Dynamic_Example_Preprocessor"

#define DYNAMIC_PREPROC_SETUP ExampleSetup

extern void ExampleSetup();

The remainder of the code is deﬁned in spp example.c and is compiled together with sf dynamic preproc lib.c into

lib sfdynamic preprocessor example.so.

Deﬁne the Setup function to register the initialization function.

#define GENERATOR_EXAMPLE 256

extern DynamicPreprocessorData _dpd;

void ExampleInit(unsigned char *);

void ExampleProcess(void *, void *);

void ExampleSetup()

{

_dpd.registerPreproc("dynamic_example", ExampleInit);

DEBUG_WRAP(_dpd.debugMsg(DEBUG_PLUGIN, "Preprocessor: Example is setup\n"););

}

The initialization function to parse the keywords from

snort.conf

169

u_int16_t portToCheck;

void ExampleInit(unsigned char *args)

{

char *arg;

char *argEnd;

unsigned long port;

_dpd.logMsg("Example dynamic preprocessor configuration\n");

arg = strtok(args, " \t\n\r");

if(!strcasecmp("port", arg))

{

arg = strtok(NULL, "\t\n\r");

if (!arg)

{

_dpd.fatalMsg("ExamplePreproc: Missing port\n");

}

port = strtoul(arg, &argEnd, 10);

if (port < 0 || port > 65535)

{

_dpd.fatalMsg("ExamplePreproc: Invalid port %d\n", port);

}

portToCheck = port;

_dpd.logMsg(" Port: %d\n", portToCheck);

}

else

{

_dpd.fatalMsg("ExamplePreproc: Invalid option %s\n", arg);

}

/* Register the preprocessor function, Transport layer, ID 10000 */

_dpd.addPreproc(ExampleProcess, PRIORITY_TRANSPORT, 10000);

DEBUG_WRAP(_dpd.debugMsg(DEBUG_PLUGIN, "Preprocessor: Example is initialized\n"););

}

The function to process the packet and log an alert if the either port matches.

#define SRC_PORT_MATCH 1

#define SRC_PORT_MATCH_STR "example_preprocessor: src port match"

#define DST_PORT_MATCH 2

#define DST_PORT_MATCH_STR "example_preprocessor: dest port match"

void ExampleProcess(void *pkt, void *context)

{

SFSnortPacket *p = (SFSnortPacket *)pkt;

if (!p->ip4_header || p->ip4_header->proto != IPPROTO_TCP || !p->tcp_header)

{

/* Not for me, return */

return;

}

if (p->src_port == portToCheck)

{

170

/* Source port matched, log alert */

_dpd.alertAdd(GENERATOR_EXAMPLE, SRC_PORT_MATCH,

1, 0, 3, SRC_PORT_MATCH_STR, 0);

return;

}

if (p->dst_port == portToCheck)

{

/* Destination port matched, log alert */

_dpd.alertAdd(GENERATOR_EXAMPLE, DST_PORT_MATCH,

1, 0, 3, DST_PORT_MATCH_STR, 0);

return;

}

5.3.2 Rules

The following is an example of a simple rule, take from the current rule set, SID 109. It is implemented to work with

the detection engine provided with snort.

The snort rule in normal format:

alert tcp $HOME_NET 12345:12346 -> $EXTERNAL_NET any \

(msg:"BACKDOOR netbus active"; flow:from_server,established; \

content:"NetBus"; reference:arachnids,401; classtype:misc-activity; \

sid:109; rev:5;)

This is the metadata for this rule library, deﬁned in detection lib meta.h.

/* Version for this rule library */

#define DETECTION_LIB_MAJOR_VERSION 1

#define DETECTION_LIB_MINOR_VERSION 0

#define DETECTION_LIB_BUILD_VERSION 1

#define DETECTION_LIB_NAME "Snort_Dynamic_Rule_Example"

/* Required version and name of the engine */

#define REQ_ENGINE_LIB_MAJOR_VERSION 1

#define REQ_ENGINE_LIB_MINOR_VERSION 0

#define REQ_ENGINE_LIB_NAME "SF_SNORT_DETECTION_ENGINE"

The deﬁnition of each data structure for this rule is in sid109.c.

Declaration of the data structures.

•Flow option

Deﬁne the FlowFlags structure and its corresponding RuleOption. Per the text version, ﬂow is from server,established.

static FlowFlags sid109flow =

{

FLOW_ESTABLISHED|FLOW_TO_CLIENT

};

static RuleOption sid109option1 =

{

171

OPTION_TYPE_FLOWFLAGS,

{

&sid109flow

}

};

•Content Option

Deﬁne the ContentInfo structure and its corresponding RuleOption. Per the text version, content is ”NetBus”,

no depth or offset, case sensitive, and non-relative. Search on the normalized buffer by default. NOTE: This

content will be used for the fast pattern matcher since it is the longest content option for this rule and no contents

have a ﬂag of CONTENT FAST PATTERN.

static ContentInfo sid109content =

{

"NetBus", /* pattern to search for */

0, /* depth */

0, /* offset */

CONTENT_BUF_NORMALIZED, /* flags */

NULL, /* holder for boyer/moore info */

NULL, /* holder for byte representation of "NetBus" */

0, /* holder for length of byte representation */

0 /* holder for increment length */

};

static RuleOption sid109option2 =

{

OPTION_TYPE_CONTENT,

{

&sid109content

}

};

•Rule and Meta Data

Deﬁne the references.

static RuleReference sid109ref_arachnids =

{

"arachnids", /* Type */

"401" /* value */

};

static RuleReference *sid109refs[] =

{

&sid109ref_arachnids,

NULL

};

The list of rule options. Rule options are evaluated in the order speciﬁed.

RuleOption *sid109options[] =

{

&sid109option1,

&sid109option2,

NULL

};

172

The rule itself, with the protocl header, meta data (sid, classiﬁcation, message, etc).

Rule sid109 =

{

/* protocol header, akin to => tcp any any -> any any */

{

IPPROTO_TCP, /* proto */

HOME_NET, /* source IP */

"12345:12346", /* source port(s) */

0, /* Direction */

EXTERNAL_NET, /* destination IP */

ANY_PORT, /* destination port */

/* metadata */

{

3, /* genid -- use 3 to distinguish a C rule */

109, /* sigid */

5, /* revision */

"misc-activity", /* classification */

0, /* priority */

"BACKDOOR netbus active", /* message */

sid109refs /* ptr to references */

sid109options, /* ptr to rule options */

NULL, /* Use internal eval func */

0, /* Holder, not yet initialized, used internally */

0, /* Holder, option count, used internally */

0, /* Holder, no alert, used internally for flowbits */

NULL /* Holder, rule data, used internally */

•The List of rules deﬁned by this rules library

The NULL terminated list of rules. The InitializeDetection iterates through each Rule in the list and initializes

the content, ﬂowbits, pcre, etc.

extern Rule sid109;

extern Rule sid637;

Rule *rules[] =

{

&sid109,

&sid637,

NULL

};

173

Chapter 6

Snort Development

Currently, this chapter is here as a place holder. It will someday contain references on how to create new detection

plugins and preprocessors. End users don’t really need to be reading this section. This is intended to help developers

get a basic understanding of whats going on quickly.

If you are going to be helping out with Snort development, please use the HEAD branch of cvs. We’ve had problems

in the past of people submitting patches only to the stable branch (since they are likely writing this stuff for their own

IDS purposes). Bugﬁxes are what goes into STABLE. Features go into HEAD.

6.1 Submitting Patches

Patches to Snort should be sent to the

snort-devel@lists.sourceforge.net

mailing list. Patches should done

with the command

diff -nu snort-orig snort-new

6.2 Snort Data Flow

First, trafﬁc is acquired from the network link via libpcap. Packets are passed through a series of decoder routines that

ﬁrst ﬁll out the packet structure for link level protocols then are further decoded for things like TCP and UDP ports.

Packets are then sent through the registered set of preprocessors. Each preprocessor checks to see if this packet is

something it should look at.

Packets are then sent through the detection engine. The detection engine checks each packet against the various

options listed in the Snort rules ﬁles. Each of the keyword options is a plugin. This allows this to be easily extensible.

6.2.1 Preprocessors

For example, a TCP analysis preprocessor could simply return if the packet does not have a TCP header. It can do this

by checking:

if (p->tcph==null)

return;

Similarly, there are a lot of packet ﬂags available that can be used to mark a packet as “reassembled” or logged. Check

out src/decode.h for the list of pkt * constants.

174

6.2.2 Detection Plugins

Basically, look at an existing output plugin and copy it to a new item and change a few things. Later, we’ll document

what these few things are.

6.2.3 Output Plugins

Generally, new output plugins should go into the barnyard project rather than the Snort project. We are currently

cleaning house on the available output options.

175

6.3 The Snort Team

Creator and Lead Architect Marty Roesch

Lead Snort Developers Steve Sturges

Todd Wease

Russ Combs

Ryan Jordan

Dilbagh Chahal

Bhagyashree Bantwal

Snort Rules Maintainer Brian Caswell

Snort Rules Team Nigel Houghton

Alex Kirk

Matt Watchinski

Win32 Maintainer Snort Team

RPM Maintainers JP Vossen

Daniel Wittenberg

Inline Developers Victor Julien

Rob McMillen

William Metcalf

Major Contributors Erek Adams

Andrew Baker

Scott Campbell

Roman D.

Michael Davis

Chris Green

Jed Haile

Jeremy Hewlett

Glenn Mansﬁeld Keeni

Adam Keeton

Chad Kreimendahl

Kevin Liu

Andrew Mullican

Jeff Nathan

Marc Norton

Judy Novak

Andreas Ostling

Chris Reid

Daniel Roelker

Dragos Ruiu

Fyodor Yarochkin

Phil Wood

176

Bibliography

[1] http://packetstorm.securify.com/mag/phrack/phrack49/p49-06

[2] http://www.nmap.org

[3] http://public.pacbell.net/dedicated/cidr.html

[4] http://www.whitehats.com

[5] http://www.incident.org/snortdb

[6] http://www.pcre.org

177

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