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Chapter 3     Working with Snort Rules

3.7 The Snort Configuration File

Snort uses a configuration file at startup time. A sample configuration file

snort.conf is included in the Snort distribution. You can use any name for the con 

figuration file, however snort.conf is the conventional name. You use the  c com 

mand line switch to specify the name of the configuration file. The following command

uses /opt/snort/snort.conf as the configuration file.

/opt/snort/snort  c /opt/snort/snort.conf

You can also save the configuration file in your home directory as .snortrc, but

specifying it on the command line is the most widely used method. There are other

advantages to using the configuration file name as a command line argument to Snort.

For example, it is possible to invoke multiple Snort instances on different network inter 

faces with different configuration. This file contains six basic sections:

  Variable definitions, where you define different variables. These variables are

used in Snort rules as well as for other purposes, like specifying the location of

rule files.

  Config parameters. These parameters specify different Snort configuration

options. Some of them can also be used on the command line.

  Preprocessor configuration. Preprocessors are used to perform certain actions

before a packet is operated by the main Snort detection engine.

  Output module configuration. Output modules control how Snort data will be

logged.

  Defining new action types. If the predefined action types are not sufficient for

your environment, you can define custom action types in the Snort

configuration file.

  Rules configuration and include files. Although you can add any rules in the

main snort.conf file, the convention is to use separate files for rules. These

files are then included inside the main configuration file using the include

keyword. This keyword will be discussed later in this chapter.

Although the out of the box configuration file works, you need to modify it to

adapt it to your environment. A sample configuration file is presented later on.

3.7.1

Using Variables in Rules

In the configuration file, you can use variables. This is a very convenient way of cre 

ating rules. For example, you can define a variable HOME_NET in the configuration file.