Trawl for packets with Wireshark
If you want to keep your network secure then you need to know what traffic is passing through it. To do that you'll be hard pressed to find a better tool than the excellent open source network protocol analyzer called Wireshark (previously known as Ethereal).
Wireshark runs on many platforms including Windows, OS X, Linux and Solaris, and once up and running on a machine attached to your network it presents a live window on much of the traffic flowing over it.
[ Read more ]