What are the Snort HOME_NET and EXTERNAL_NET Variables?! To know that let’s see how Snort rules work. Snort rules rely on variables to know what traffic they should inspect and what to ignore. Each Snort rule has a header where a bunch of variables are defined such as the action to be taken, protocol, source IP, source port, destination IP and destination port. The most important two bits among these variables are the source and destination IP addresses.