Writing Snort Rules
How To write Snort rules and keep your sanity
Current as of version 1.7
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.

Includes

The include keyword allows other rule 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 putting them in place in the file in the place where the include appears.

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

Variables may be defined in Snort.  These are simple substitution variables set with the var keyword as in Figure 2.

Format:

var: <name> <value>

var MY_NET [192.168.1.0/24,10.1.1.0/24]

alert tcp any any -> $MY_NET any (flags: S; msg: "SYN packet";)
 
Figure 2 - 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 3 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 3 - Advanced Variable Usage Example
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 five available default actions in Snort, alert, log, pass, activate, and dynamic.

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
}

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 4.

alert tcp !192.168.1.0/24 any -> 192.168.1.0/24 111 (content: "|00 01 86 a5|"; msg: "external mountd access";)
Figure 4 - 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".

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 for an example of an IP list in action.

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";)

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 5.
 
 

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 5 - 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 6.
 
 

log tcp any any -> 192.168.1.0/24 !6000:6010
Figure 6 - 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 7.
 
 

log !192.168.1.0/24 any <> 192.168.1.0/24 23
Figure 7 - Snort rules using the Bidirectional Operator

Activate/Dynamic Rules

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".  When the "activate" rule goes off, it turns on the dynamic rule it is  linked to (indicated by the activates/activated_by option numbers) for  "count" number of packets (50 in this case).

Put 'em together and they look like  this:
 

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 8 - Activate/Dynamic rule example

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.
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 - prints a message in alerts and packet logs
  • logto - log the packet to a user specified filename instead of the standard output file
  • ttl - test the IP header's TTL field value
  • tos - test the IP header's TOS field value
  • id - test the IP header's fragment ID field for a specific value
  • ipoption - watch the IP option fields for specific codes
  • fragbits - test the fragmentation bits of the IP header
  • dsize - test the packet's payload size against a value
  • flags - test the TCP flags for certain values
  • seq - test the TCP sequence number field for a specific value
  • ack - test the TCP acknowledgement field for a specific value
  • itype - test the ICMP type field against a specific value
  • icode - test the ICMP code field against a specific value
  • icmp_id - test the ICMP ECHO ID field against a specific value
  • icmp_seq - test the ICMP ECHO sequence number against a specific value
  • content - search for a pattern in the  packet's payload
  • content-list - search for a set of patterns in the packet's payload
  • offset - modifier for the content option, sets the offset to begin attempting a pattern match
  • depth - modifier for the content option, sets the maximum search depth for a pattern match attempt
  • nocase - match the preceeding content string with case insensitivity
  • session - dumps the application layer information for a given session
  • rpc - watch RPC services for specific application/proceedure calls
  • resp - active response (knock down connections, etc)
  • react - active response (block web sites)
    		•

    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>";
    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>";
    TOS

    The "tos" keyword allows you to check the IP header TOS field for a specific value.  The test it performs is only sucessful on an exact match.

    Format:

    tos: "<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>";
    Ipoption

    If IP options are present in a packet, this option will search for a specific option in use, such as source routing. Valid arguments to this option are:

    The most frequently watched for IP options are strict and loose source routing which aren't used in any widespread internet applications. Only a single option may be specified per rule.

    Format:

    ipopts: <option>;
    Fragbits

    This rule inspects the fragment and reserved bits in the IP header.   There are three bits that can be checked, the Reserved Bit (RB), More Fragments (MF) bit, and the Dont Fragment (DF) bit.  These bits can be checked in a variety of combinations. Use the following values to indicate specific bits:

    You can also use modifiers to indicate logical match criteria for the specified bits: Format:
    fragbits: <bit values>;

     
    alert tcp !$HOME_NET any -> $HOME_NET any (fragbits: R+; msg: "Reserved IP bit set!";)
    Figure 9 - Example of fragbits detection usage
    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 10 - 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 access";)
    Figure 11 - Combined Content, Offset and Depth Rule
    Nocase

    The nocase option is used to deactivate case sensitivity in a "content" rule.  It is specified alone within a rule and any ASCII characters that are compared to the packet payload are treated as though they are either upper of lower case.

    Format:

    nocase;

     
    alert tcp any any -> 192.168.1.0/24 21 (content: "USER root"; nocase; msg: "FTP root user access attempt";)
    Figure 12 - Content rule with nocase modifier
    Flags

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

    There are also logical operators that can be used to specify matching criteria for the indicated flags: The reserved bits can be used to detect unusual behavior, such as IP stack fingerprinting attempts or other suspicious activity.  Figure 13 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 13 - 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 14.

    Format:

    ack: <number>;

     
    alert any any -> 192.168.1.0/24 any (flags: A; ack: 0; msg: "NMAP TCP ping";)
    Figure 14 - 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 15 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 15 - Logging Printable Telnet Session Data
    Icmp_id

    The icmp_id option examines an ICMP ECHO packet's ICMP ID number for a specific value. This is useful because some covert channel programs use static ICMP fields when they communicate. This particular plugin was developed to enable the stacheldraht detection rules written by Max Vision, but it is certainly useful for detection of a number of potential attacks.

    Format:

    icmp_id: <number>;
    Icmp_seq

    The icmp_id option examines an ICMP ECHO packet's ICMP sequence field for a specific value. This is useful because some covert channel programs use static ICMP fields when they communicate. This particular plugin was developed to enable the stacheldraht detection rules written by Max Vision, but it is certainly useful for detection of a number of potential attacks. (And yes, I know the info for this field is almost identical to the icmp_id description, it's practically the same damn thing!)

    Format:

    icmp_seq: <number>;
    Rpc

    This option looks at RPC requests and automatically decodes the application, procedure, and program version, indicating success when all three variables are matched. The format of the option call is "application, procedure, version". Wildcards are valid for both the procedure and version numbers and are indicated with a "*".

    Format:

    rpc: <number, [number|*], [number|*]>;

     
    alert tcp any any -> 192.168.1.0/24 111 (rpc: 100000,*,3; msg:"RPC getport (TCP)";)
    alert udp any any -> 192.168.1.0/24 111 (rpc: 100000,*,3; msg:"RPC getport (UDP)";)
    alert udp any any -> 192.168.1.0/24 111 (rpc: 100083,*,*; msg:"RPC ttdb";)
    alert udp any any -> 192.168.1.0/24 111 (rpc: 100232,10,*; msg:"RPC sadmin";)
    Figure 16 - Various RPC Call Alerts
    Resp

    The resp keyword implements flexible reponse (FlexResp) to traffic that matches a Snort rule.  The FlexResp code allows Snort to actively close offending connections.  The following arguments are valid for this module:

    These options can be combined to send multiple responses to the target host.  Multiple arguments are separated by a comma.

    Format:

    resp: <resp_modifier[, resp_modifier...]>;
    alert tcp any any -> 192.168.1.0/24 1524 (flags: S; resp: rst_all; msg: "Root shell backdoor attempt";)
    alert udp any any -> 192.168.1.0/24 31 (resp: icmp_port,icmp_host; msg: "Hacker's Paradise access attempt";)
    Figure 17 - FlexResp Usage Examples
    Content-list

    The content-list keyword allows multiple content strings to be specified in the place of a single content option. The patterns to be searched for must each be on a single line of content-list file as shown in Figure 1, but they are treated otherwise identically to content strings specified as an argument to a standard content directive. This option is the basis for the react keyword.

    React

    The react keyword based on flexible response (Flex Resp) implements flexible reaction 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 Flex Resp code allows Snort to actively close offending connections and/or send a visible notice to the browser (warn modifier available soon). The notice may include your own comment. The following arguments (basic modifiers) are valid for this option:
     

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

    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 through 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>
    preprocessor minfrag: 128
    Figure 20 - Preprocessor Directive Format Example

    Available Preprocessor Modules

    Minfrag

    The minfrag preprocessor examines fragmented packets for a specified size threshold.  When packets are fragmented, it is generally caused by routers between the source and destination.  Generally speaking, there is no piece of commercial network equipment that fragments packets in sizes smaller than 512 bytes, so we can use this fact to enable traffic to be monitored for tiny fragments that are generally indicative of someone trying to hide their traffic behind fragmentation.

    Format:

    minfrag: <threshold number>
    HTTP Decode

    HTTP Decode is used to process HTTP URI strings and convert their data to non-obfuscated ASCII strings.  This is done to defeat evasive web URL scanners and hostile attackers that could otherwise elude the content analysis strings used to examine HTTP traffic for suspicious activity.  The preprocessor module takes HTTP port numbers (separated by spaces) to be normalized as its arguments (typically 80 and 8080).

    Format:

    http_decode: <port list>
    preprocessor http_decode: 80 8080
    Figure 21 - HTTP Decode Directive Format Example
    Portscan Detector

    The Snort Portscan Preprocessor is developed by Patrick Mullen and (much) more information is available at his web page.

    What the Snort Portscan Preprocessor does:

    Log the start and end of portscans from a single source IP to the standard logging facility.

    If a log file is specified, logs the destination IPs and ports scanned as well as the type of scan.

    A portscan is defined as TCP connection attempts to more than P ports in T seconds or UDP packets sent to more than P ports in T seconds.  Ports can be spread across any number of destination IP addresses, and may all be the same port if spread across multiple IPs. This version does single->single and single->many portscans.  The next full release will do distributed portscans (multiple->single or multiple->multiple).  A portscan is also defined as a single "stealth scan" packet, such as NULL, FIN, SYNFIN, XMAS, etc.  This means that from scan-lib in the standard distribution of snort you should comment out the section for stealth scan packets.  The benefit is with the portscan module these alerts would only show once per scan, rather than once for each packet.  If you use the external logging feature you can look at the technique and type in the log file.

    The arguments to this module are:

    • network to monitor - The network/CIDR block to monitor for portscans
    • number of ports - number of ports accessed in the detection period
    • detection period - number of seconds to count that the port access threshold is considered for
    • logdir/filename - the directory/filename to place alerts in. Alerts are also written to the standard alert file
    Format:
    portscan: <network to monitor> <number of ports> <detection period> <logdir/filename>
    preprocessor portscan: 192.168.1.0/24 5 7 /var/log/portscan.log
    Figure 22 - Portscan Module Configuration Example
    Portscan Ignorehosts

    Another module from Patrick Mullen that modifies the portscan detection system's operation.  If you have servers which tend to trip off the portscan detector (such as NTP, NFS, and DNS servers), you can tell portscan to ignore TCP SYN and UDP portscans from certain hosts.   The arguments to this module are a list of IPs/CIDR blocks to be ignored.

    Format:

    portscan-ignorehosts: <host list>
    preprocessor portscan-ignorehosts: 192.168.1.5/32 192.168.3.0/24
    Figure 23 - Portscan Ignorehosts Module Configuration Example

     

    Defrag
    The defrag module (from Dragos Ruiu) allows Snort to perform full blown IP defragmentation, making it more difficult for hackers to simply circumvent the detection capabilities of the system.  It is very simple in its usage, merely requiring the addition of a preprocessor directive to the configuration file with no arguments.  This module generall supercedes the functionality of the minfrag module (i.e. you don't need to use minfrag if you're using defrag).

    Format:

    defrag
    preprocessor defrag
    Figure 24 - Defrag preprocessor configuration example

     

    Stream
    This module is still in BETA testing, use with caution!

    The stream plugin provides TCP stream reassembly functionality to Snort.  TCP streams on the configured ports with small segments will be reassembled into a stream of data that Snort can properly evaluate for suspicious activity.  This plugin takes a number of arguments:
     

    • timeout - the max time in seconds for which a stream will be kept alive if we haven't seen a packet for it
    • port - a server port to monitor.  we don't want to monitor all tcp streams (do we?)
    • maxbytes - maximum bytes in our reconstructed packets

      •  
    Format:
    stream: timeout <timeout>, ports <ports>, maxbytes <maxbytes>
    preprocessor stream: timeout 5, ports 21 23 80 8080, maxbytes 16384
    Figure 25 - TCP stream reassembler configuration example
    Spade: the Statistical Packet Anomaly Detection Engine

    In the interest of timeliness and sanity, I'd suggest checking out the README.Spade in the Snort distrbution as well as checking out http://www.silicondefense.com/spice/

    This module allows Snort to be able to perform statistical anomaly detection on your network, and it's essentially an entire new detection engine for Snort.  If you're interested in this kind of capability, you should definitely read the documentation in the Snort distribution as well as that on the SiliconDefense site.


    Output Modules
    Output Module Overview

    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
    Figure 26 - Output Module Configuration Example

    Available Output Modules

    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:

    • Options
      • LOG_CONS
      • LOG_NDELAY
      • LOG_PERROR
      • LOG_PID
    • 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
    Format:
    alert_syslog: <facility> <priority> <options>
    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

    Format:

    alert_fast: <output filename>
    output alert_fast: alert.fast
    Figure 27 - Fast alert configuration
    Alert_full

    Print Snort alert messages with full packet headers.  This alerting facility is generall pretty slow because it requires that the program do a whole lot of data parsing to format the data to be printed.  The alerts will be written in the default logging directory (/var/log/snort) or in the logging directory specified at the command line.

    Format:

    alert_full: <output filename>
    output alert_full: alert.full
    Figure 28 - Fast alert configuration
    Alert_smb

    This plugin sends WinPopup alert messages to the NETBIOS named machines indicated within the file specified as an argument to this output plugin.  It should be noted that use of this plugin is not encouraged as it executes an external executable binary (smbclient) at the same privilege level as Snort, commonly root.  The format of the workstation file is a list of the NETBIOS names of the hosts that wish to receive alerts, one per line in the file.

    Format:

    alert_smb: <alert workstation filename>
    output alert_smb: workstation.list
    Figure 29 - SMB alert configuration
    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
    output alert_unixsock
    Figure 30 - UnixSock alert configuration
    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 avialable for examining tcpdump formatted files.  This module only takes a single argument, the name of the output file.

    Format:

    log_tcpdump: <output filename>
    output log_tcpdump: snort.log
    Figure 31 - Tcpdump Output Module Configuration Example
    Xml

    The XML plug-in enables snort to log in SNML - simple network markup language aka (snort markup language) to a file or over a network.  The DTD is available in the contrib directory of the snort distribution and at: http://www.cert.org/DTD/snml-1.0.dtd. You can use this plug-in with on one or more snort sensors to log to a central database and create highly configurable intrusion detection infrastructures within your network. The plugin will also enable you to automatically report alerts to the CERT Coordination Center, your response team, or your
    managed IDS provider.

    This plugin was developed by Jed Pickel and Roman Danyliw at the CERT Coordination Center as part of the AIRCERT project.

    Be aware that the SNML DTD is in its early phases of development and is likely to be modified as it undergoes public scrutiny.

    See http://www.cert.org/kb/snortxml for the most up to date information and documentation about this plugin.

    The configuration line will be of the following format:

       output xml: [log | alert], [parameter list]

    Arguments:
     

    [log | alert ] - specify log or alert to connect the xml plugin to the log or alert facility.

    [parameter list] - The parameter list consists of key value pairs. The proper format is a list of key=value pairs each separated a space.
     
     

    file when this is the only parameter it will log to a file on the local machine.  Otherwise, if http or https is employed (see protocol), this is the script which is to be executed on the remote host.
    protocol The possible values for this field are 
    http  - send a POST over HTTP to a webserver (required: a [file] parameter)
    https - just like http but ssl encrypted and mutually authenticated.  (required: a [file], [cert], [key] parameter)
    tcp   - A simple tcp connection. You need to use some sort of listener (required: a [port] parameter) 
    iap   - An implementation of the Intrusion Alert Protocol (This does not work yet)
    host remote host where the logs are to be sent
    port The port number to connect to (default ports are)
    http  80
    https 443
    tcp   9000
    iap   9000
    cert the client X.509 certificate to use with https (PEM formatted)
    key the client private key to use with https (PEM formatted)
    ca the CA certificate used to validate the https server's certificate (PEM formatted)
    server the file containing a list of valid servers with which to communicate. It is used so that Snort canauthenticate the peer server. Each server is identified by a string formed by concatenating the subject of the server's X.509 certificate.  This string can be created by:
     
    % openssl x509 -subject -in <server certificate>.


    Typically only someone deploying the HTTPS will have to perform this task (since they have access to the server certificate). This entitity should publish this subject string for configuration inside each snort sensor.
    sanatize The argument is a a network/netmask combination for an IP range you wish to be sanitized. Any IP address within the range you specify will be represented as "xxx.xxx.xxx.xxx". Also, for sanitized alerts, no packet payload will be logged. You can use the sanitize parameter multiple times to represent multiple IP ranges.
    encoding Packet payload and option data is binary and there is not one standard way to represent it as ASCII text. You can choose the binary encoding option that is best suited for your environment.  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 loose 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)
    Format:
    xml: <output facility>
    output xml: log,   file=output
    output xml: log,   protocol=https host=air.cert.org file=alert.snort cert=mycert.crt key=mykey.pem ca=ca.crt server=srv_list.lst
    Figure 32 - XML output plugin setup examples
    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 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.  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
    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 loose 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 four database types available in the current version of the plugin are MySQL, PostgreSQL, Oracle, and unixODBC compliant databases.  Set the type to match the database you are using.

    Format:

    database: <log | alert>, <database type>, <parameter list>
    output database: log, mysql,  dbname=snort user=snort host=localhost password=xyz
    Figure 33 - Database output plugin configuration
    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 (unless you use the "nocase" option)

    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".

    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 23.  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 34 - Using TCP Flag Tests to Hasten Content Rules








    Version 1.2, All rights reserved, © Copyright 1999-2001 Martin Roesch