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.
To use a host attribute table, Snort must be configured with the -enable-targetbased flag.