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 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 - block and log the packet

  7. reject - block 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 - block 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 tcpdump: database:

    ruletype redalert
          type alert 
          output alert_syslog: LOG_AUTH LOG_ALERT 
          output log_tcpdump: suspicious.log

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 config 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 would signify the block of addresses from to 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 [*], the source IP address was set to match for any computer talking, and the destination address was set to match on the 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 [*].

Figure: 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 [*] for an example of an IP list in action.

Figure: IP Address Lists

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 [*].

Figure: 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 [*].

Figure: Example of Port Negation

3.2.5 The Direction Operator

The direction operator -3#3 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 13#13 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 [*].

Also, note that there is no 2#2- 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 2#2- does not exist is so that rules always read consistently.

Figure: Snort rules using the Bidirectional Operator

3.2.6 Activate/Dynamic Rules


Activate and Dynamic rules are being phased out in favor of a combination of tagging ([*]) and flowbits ([*]).

Activate/dynamic rule pairs give Snort a powerful capability. You can now have one rule activate another when its 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 [*].

Figure: 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.