3.4 Structure of a Rule
Now that you have seen some rules which are not-so-good but helpful in a way, let us see the structure of a Snort rule. All Snort rules have two logical parts: rule header and rule options. This is shown in Figure 3-1.
Figure 3-1. Basic structure of Snort rules.
The rule header contains information about what action a rule takes. It also contains criteria for matching a rule against data packets. The options part usually contains an alert message and information about which part of the packet should be used to generate the alert message. The options part contains additional criteria for matching a rule against data packets. A rule may detect one type or multiple types of intrusion activity. Intelligent rules should be able to apply to multiple intrusion signatures.
The general structure of a Snort rule header is shown in Figure 3-2.
Figure 3-2. Structure of Snort rule header.
The action part of the rule determines the type of action taken when criteria are met and a rule is exactly matched against a data packet. Typical actions are generating an alert or log message or invoking another rule. You will learn more about actions later in this chapter.
The protocol part is used to apply the rule on packets for a particular protocol only. This is the first criterion mentioned in the rule. Some examples of protocols used are IP, ICMP, UDP etc.
The address parts define source and destination addresses. Addresses may be a single host, multiple hosts or network addresses. You can also use these parts to exclude some addresses from a complete network. More about addresses will be discussed later. Note that there are two address fields in the rule. Source and destination addresses are determined based on direction field. As an example, if the direction field is “->”, the Address on the left side is source and the Address on the right side is destination.
In case of TCP or UDP protocol, the port parts determine the source and destination ports of a packet on which the rule is applied. In case of network layer protocols like IP and ICMP, port numbers have no significance.
The direction part of the rule actually determines which address and port number is used as source and which as destination.
For example, consider the following rule that generates an alert message whenever it detects an ICMP1 ping packet (ICMP ECHO REQUEST) with TTL equal to 100, as you have seen in Chapter 2.
alert icmp any any -> any any (msg: "Ping with TTL=100"; ttl: 100;)
The part of the rule before the starting parenthesis is called the rule header. The part of the rule that is enclosed by the parentheses is the options part. The header contains the following parts, in order:
A rule action. In this rule the action is “alert”, which means that an alert will be generated when conditions are met. Remember that packets are logged by default when an alert is generated. Depending on the action field, the rule options part may contain additional criteria for the rules.
Protocol. In this rule the protocol is ICMP, which means that the rule will be applied only on ICMP-type packets. In the Snort detection engine, if the protocol of a packet is not ICMP, the rest of the rule is not considered in order to save CPU time. The protocol part plays an important role when you want to apply Snort rules only to packets of a particular type.
Source address and source port. In this example both of them are set to “any”, which means that the rule will be applied on all packets coming from any source. Of course port numbers have no relevance to ICMP packets. Port numbers are relevant only when protocol is either TCP or UDP.
Direction. In this case the direction is set from left to right using the -> symbol. This shows that the address and port number on the left hand side of the symbol are source and those on the right hand side are destination. It also means that the rule will be applied on packets traveling from source to destination. You can also use a <- symbol to reverse the meaning of source and destination address of the packet. Note that a symbol <> can also be used to apply the rule on packets going in either direction.
Destination address and port address. In this example both are set to “any”, meaning the rule will be applied to all packets irrespective of their destination address. The direction in this rule does not play any role because the rule is applied to all ICMP packets moving in either direction, due to the use of the keyword “any” in both source and destination address parts.
The options part enclosed in parentheses shows that an alert message will be generated containing the text string “Ping with TTL=100” whenever the condition of TTL=100 is met. Note that TTL or Time To Live is a field in the IP packet header. Refer to RFC 791 at http://www.rfc-editor.org/rfc/rfc791.txt or Appendix C for information on IP packet headers.