Writing Snort Rules
How To write Snort rules and keep your sanity
Current as of version 1.3.1.2
By Martin Roesch
Contents


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.

The first is that Snort rules must be completely contained on a single line, the Snort rule parser doesn't know how to handle rules on multiple lines.

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.

Here is an example rule:

alert tcp any any -> 192.168.1.0/24 111 (content:"|00 01 86 a5|"; msg: "mountd access";)
Figure 1 - Sample Snort Rule

The text up to the first parenthesis is the rule header and the section enclosed in parenthesis is the rule options.  The words before the colons in the rule options section are called option keywords.  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.  Let's begin by talking about the rule header section.


Rule Headers
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 action.  The rule action tells Snort what to do when it finds a packet that matches the rule criteria.  There are three available actions in Snort, alert, log, and pass.

Protocols:

The next field in a rule is the protocol.  There are three IP protocols that Snort currently analyzes for suspicious behavior, tcp, udp, and icmp.  In the future there may be more, such as ARP, IGRP, GRE, OSPF, RIP, IPX, etc.

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 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.

In Figure 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 2.

alert tcp !192.168.1.0/24 any -> 192.168.1.0/24 111 (content: "|00 01 86 a5|"; msg: "external mountd access";)
Figure 2 - Example IP Address Negation Rule

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".

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.
 
 

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 priveleged ports less than or equal to 1024 going to ports greater than or equal to 500
Figure 3 - Port Range Examples


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 4.
 
 

log tcp any any -> 192.168.1.0/24 !6000:6010
Figure 4 - Example of Port Negation


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 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 5.
 
 

log !192.168.1.0/24 any <> 192.168.1.0/24 23
Figure 5 - Snort rules using the Bidirectional Operator
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. As of this writing, there are fifteen rule option keywords  available for Snort:


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>";


Logto

The logto option 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>";


Minfrag

Minfrag sets a minimum size threshold for a fragmented packet.  It is generally used in conjunction with a single alert rule to set a boundry for the minimum fragment size that is acceptable on a network segment.  It makes a handy detector for attackers that like to break their fragments into tiny pieces before transmitting them to try to avoid detection mechaisms.  Generally speaking, there is virtually no commercial network equipment available that generates fragments smaller than 256-bytes, so people can take advantage of this fact by setting their minfrag value somewhere below that threshold.  See Figure 6 for an example of a rule that uses the minfrag option.

Format:

minfrag: "<number>";
alert tcp any any -> any any (minfrag: 256; msg: "Tiny fragments detected, possible hostile activity";)
Figure 6 - Minfrag Rule Option Example





TTL

This rule option is used to set a specific time-to-live value to test against.  The test it performs is only sucessful on an exact match.  This option keyword was intended for use in the detection of traceroute attempts.

Format:

ttl: "<number>";
ID

This option keyword is used to test for an exact match in the IP header fragment ID field.  Some hacking tools (and other programs) set this field specifically for various purposes, for example the value 31337 is very popular with some hackers.  This can be turned against them by putting a simple rule in place to test for this and some other "hacker numbers".

Format:

id: "<number>;


Dsize

The dsize option is used to test the packet payload size.  It may be set to any value, plus use the greater than/less than signs to indicate ranges and limits.  For example, if you know that a certain service has a buffer of a certain size, you can set this option to watch for attempted buffer overflows.  It has the added advantage of being a much faster way to test for a buffer overflow than a payload content check.

Format:

dsize: [>|<] <number>;
Note: The > and < operators are optional!


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 os 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 hexidecimal numbers and is a good shorthand method for describing complex binary data.  Figure 7 contains an example of mixed text and binary data in a Snort rule.
 
 

alert tcp any any -> 192.168.1.0/24 143 (content: "|90C8 C0FF FFFF|/bin/sh"; msg: "IMAP buffer overflow!";)
Figure 7 - Mixed Binary Bytecode and Text in a Content Rule Option

Format:

content: "<content string>";


Offset

The offset rule option is used as a modifier to rules using the content option keyword.  This keyword modifies the starting search position for the pattern match function from the beginning of the packet payload.  It is very useful for things like CGI scan detection rules where the content search string is never found in the first four bytes of the payload.  Care should be taken against setting the offset value too "tightly" and potentially missing an attack!  This rule option keyword cannot be used without also specifying a content rule option.

Format:

offset: <number>;


Depth

Depth is another content rule option modifier.  This sets the maximum search depth for the content pattern match function to search from the beginning of its search region.  It is useful for limiting the pattern match function from performing inefficient searches once the possible search region for a given set of content has been exceeded.  (Which is to say, if you're searching for "cgi-bin/phf" in a web-bound packet, you probably don't need to waste time searching the payload beyond the first 20 bytes!)  See Figure 8 for an example of a combined content, offset, and depth search rule.

Format:

depth: <number>;

 
alert tcp any any -> 192.168.1.0/24 80 (content: "cgi-bin/phf"; offset: 3; depth: 22; msg: "CGI-PHF attack";)
Figure 8 - Combined Content, Offset and Depth Rule






Flags

This rule tests the TCP flags for an exact match.  There are actually 8 flags variables available in Snort:

The reserved bits can be used to detect unusual behavior, such as IP stack fingerprinting attempts or other suspicious activity.  All of the flags are considered as a whole for this test, they must all be "up" for this rule option to be successful.  For instance, Figure 9 shows a SYN-FIN scan detection rule.

Format:

flags: <flag values>;

 
alert any any -> 192.168.1.0/24 any (flags: SF; msg: "Possible SYN FIN scan";)
Figure 9 - Sample TCP Flags Specification



Seq

This rule option refers to the TCP sequence number.  Essentially, it detects if the packet has a static sequence number set, and is therefore pretty much unused.  It was included for the sake of completeness.

Format:

seq: <number>;


Ack

The ack rule option keyword refers to the TCP header's acknowledge field.  This rule has one practical purpose so far: detecting  NMAP TCP pings.  A NMAP TCP ping sets this field to zero and sends a packet with the TCP ACK flag set to determine if a network host is active.  The rule to detect this activity is shown in Figure 10.

Format:

ack: <number>;

 
alert any any -> 192.168.1.0/24 any (flags: A; ack: 0; msg: "NMAP TCP ping";)
Figure 10 - TCP ACK Field Usage



Itype

This rule tests the value of the ICMP type field.  It is set using the numeric value of this field.  For a list of the available values, look in the decode.h file included with Snort or in any ICMP reference.  It should be noted that the values can be set out of range to detect invalid ICMP type values that are sometimes used in denial of service and flooding attacks.

Format:

itype: <number>;


Icode

The icode rule option keyword is pretty much identical to the itype rule, just set a numeric value in here and Snort will detect any traffic using that ICMP code value.  Out of range values can also be set to detect suspicious traffic.

Format:

icode: <number>;


Session

The session keyword is brand new as of version 1.3.1.1 and is used to extract the user data from TCP sessions.  It is extremely useful for seeing what users are typing in telnet, rlogin, ftp, or even web sessions.  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.  This function can slow Snort down considerably, so it shouldn't be used in heavy load situations, and is probably best suited for post-processing binary (tcpdump format) log files.  See Figure 11 for a good example of a telnet session logging rule.

Format:

session: [printable|all];

 
log tcp any any <> 192.168.1.0/24 23 (session: printable;)
Figure 11 - Logging Printable Telnet Session Data


Advanced Rule Concepts
 
Includes

Versions of Snort after 1.3.1.2 include new rules file parsing functionality developed by Christian Lademann, including two new rules file keywords.  The first of these keywords is include.  The include keyword allows other rule files to be included with the rules file that indicated on the Snort command line.

Format:

include: <include file path/name>
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 the Variables section for more information on defining and using variables in Snort rule files.

Variables

As of version 1.3.1.2, variables may be defined in Snort.  These are simple substitution variables set with the var keyword as in Figure 12.

Format:

var: <name> <value>

var MY_NET 192.168.1.0/24

alert tcp any any -> $MY_NET any (flags: S; msg: "SYN packet";)
 
Figure 12 - Example of Variable Definition and Usage

The 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 "-".

See figure 13 for an example of these rules modifiers in action.
 
 

var MY_NET $(MY_NET:-192.168.1.0/24)

log tcp any any -> $(MY_NET:?MY_NET is undefined!) 23
 
Figure 13 - Advanced Variable Usage Example

 


Building Good Rules
There are some general concepts to keep in mind when developing Snort rules to maximize efficiency and speed.  I will add to this section as my muse wills. :)

Content Rules are Case Sensitive

Don't forget that content rules are case sensitive and that many programs typically use uppercase letters to indicate commands.  FTP is a good example of this.  Consider the following two rules:

alert tcp any any -> 192.168.1.0/24 21 (content: "user root"; msg: "FTP root login";)
alert tcp any any -> 192.168.1.0/24 21 (content: "USER root"; msg: "FTP root login";)

The second of those two rules will catch most every automated root login attempt, but none that use lower case characters for "user".  Case insensitivity is a feature that will probably be added in future versions of Snort, but for now be aware that case counts!

Speeding Up Rules That Have Content Options

The order that rules are tested by the detection engine is completely independent of the order that they are written in a rule.  The last rule test that is done (when necessary) is always the content rule option.  Take advantage of this fact by using other faster rule options that can detect whether or not the content needs to be checked at all.  For instance, most of the time when data is sent from client to server after a TCP session is established, the PSH and ACK TCP flags are set on the packet containing the data.  This fact can be taken advantage of by rules that need to test payload content coming from the client to the sever with a simple TCP flag test that is far less computationally expensive than the pattern match algorithm.  Knowing this, a simple way to speed up rules that use content options is to also perform a flag test, as in Figure 14.  The basic idea is that if the PSH and ACK flags aren't set, there's no need to test the packet payload for the given rule.  If the flags are set, the additional computing power required to perform the test is negligible.
 
 

alert tcp any any -> 192.168.1.0/24 80 (content: "cgi-bin/phf"; flags: PA; msg: "CGI-PHF probe";)
Figure 14 - Using TCP Flag Tests to Hasten Content Rules

 



 

Version 1.0, All rights reserved, © Copyright 1999 Martin Roesch