The sid Keyword Rule Options

The Snort Configuration File 113 var HOME_NET 192.168.1.024 Later on you can use this variable HOME_NET in your rules: alert ip any any - HOME_NET any ipopts: lsrr; \ msg: “Loose source routing attempt”; sid: 1000001; As you can see, using variables makes it very convenient to adapt the configura- tion file and rules to any environment. For example, you don’t need to modify all rules when you copy rules from one network to another; you just need to modify a single variable.

3.7.1.1 Using a List of Networks in Variables

You can also define variables that contain multiple items. Consider that you have multiple networks in the company. Your intrusion detection system is right behind the company firewall connecting to the Internet. You can define a variable as a list of all of these networks. The following variable shows that HOME_NETWORK consists of two networks, 192.168.1.024 and 192.168.10.024. var HOME_NET [192.168.1.024,192.168.10.024] All networks in the variable name are separated by a comma.

3.7.1.2 Using Interface Names in Variables

You can also use interface names in defining variables. The following two state- ments define HOME_NET and EXTERNAL_NET variables on a Linux machine. var HOME_NET eth0_ADDRESS var EXTERNAL_NET eth1_ADDRESS The HOME_NET variable uses the IP address and network mask value assigned to interface eth0 and EXTERNAL_NET uses the IP address and network mask assigned to network interface eth1. This arrangement is more convenient since you can change IP addresses on the interfaces without modifying rules or even variables themselves.

3.7.1.3 Using the any Keyword

The any keyword can also be a variable. It matches to everything, just as it does in rules such as addresses and port numbers. For example, if you want to test packets regardless of their source, you can define a variable like the following for EXTERNAL_NET. var EXTERNAL_NET any There are many variables defined in the snort.conf file that come with the Snort distribution. While installing Snort, you need to modify these variables according to your network.