The first set of variables in a typical snort.conf look like this:
var HOME_NET any
var EXTERNAL_NET any
var DNS_SERVERS $HOME_NET
var SMTP_SERVERS $HOME_NET
var HTTP_SERVERS $HOME_NET
var SQL_SERVERS $HOME_NET
var TELNET_SERVERS $HOME_NET
var SNMP_SERVERS $HOME_NET
var AIM_SERVERS [a long list of AOL IM servers]
These variables allow an operator to define IP addresses that Snort will reference when inspecting traffic. It's customary to set the HOME_NET variable to be the range of IP addresses that the operator wishes to monitor. For example:
var HOME_NET 192.168.2.0/24
Many leave EXTERNAL_NET as any, or chose it to be all those IP addresses not associated with the HOME_NET:
var EXTERNAL_NET !$HOME_NET
Specific servers or ranges of IPs can be defined for each of the following variables. For example:
var DNS_SERVERS [172.16.3.2,4.2.2.1]
The more accurately one defines these variables (listing specific Web servers, SMTP servers and so on) the more relevant the alerts Snort will produce. You may leave the default variables in place, but expect more alerts than you might wish to inspect.
Snort: Understanding the configuration file
Introduction: Upgrade to Snort 2.6.1.2
The snort.conf file
Defining IP ranges of interest
Defining ports of interest
Core preprocessors
Non-dynamic preprocessors
Conclusion
About the author
Richard Bejtlich is founder of TaoSecurity, author of several books on network security monitoring, including Extrusion Detection: Security Monitoring for Internal Intrusions, and operator of the TaoSecurity blog.
