Packet sniffing

What it is

Packet sniffing is essentially eavesdropping on a network interface.

tcpdump

The granddaddy of all packet sniffers, tcpdump will be found on just about any unix system you're likely to encounter. The output is terse and hard to understand if you don't know what you're looking at or for; for this reason many folks prefer friendlier front-ends to tcp, such as ethereal.