Packet Filters

I find this stuff interesting:

http://www.howtoforge.com/how-to-set-up-a-linux-layer-7-packet-classifier-on-centos5.1

This tutorial will walk you through setting up a Linux layer 7 packet classifier on CentOS 5.1, this can easily be adapted to any other Linux distribution out there.

L7-filter is a classifier for the Linux Netfilter that identifies packets based on patterns in application layer data. This allows correct classification of P2P traffics.
It can classify packets such as Kazaa, HTTP, Jabber, Citrix, Bittorrent, FTP, Gnucleus, eDonkey2000, etc., that uses unpredictable ports as well as standard protocols running on non-standard ports. It complements existing classifiers that match on IP address, port numbers and so on. http://l7-filter.sourceforge.net/