Linux Server Security, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Snort

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Using Snort as a Packet Logger

You can, if you wish, run Snort in Sniffer mode and redirect its output into a text file.

But this isn’t recommended. If you want to minimize dropped packets, you should

forego writing them to the screen and instead tell Snort to write directly to a log

directory. You can do so by invoking Snort like this:

bash-# snort -d -l ./snort/ -h 10.10.20.0/24

As with Sniffer mode, the -d flag tells Snort to decode packets’ data payloads. The -l

flag, however, specifies a directory to log to and puts Snort into Packet Capture

mode. If the directory you specify doesn’t exist, Snort will exit with an error.

The

-h flag allows you to specify your “home network.” Snort creates a new direc-

tory for each host it observes and prefers to do so in a “client-centric” manner. For

example, if you tell Snort that addresses within 10.10.20.0/24 are the local network,

Snort will consider all other host IP addresses to be “clients” in any given transac-

tion and will name host directories after those IP addresses. If both hosts in a given

transaction are local, Snort will name a directory after the IP address using the higher

listening port or, if those are the same, after the higher IP address.

This sounds very abstract and maybe even arbitrary, but remember that Snort ...