Network troubleshooting tools
Here's information on sniff and ngrep.
Let's start off with ngrep. I won't go into details about getting ngrep because there should be ready-made packages for most *nix systems easily available ó if not installed by default.
As the name suggests, ngrep stands for "network grep." Like grep, ngrep searches for a string or regular expression ó but does so on the network layer. This can be particularly useful in a number of cases when you're trying to troubleshoot problems, watch for interactions between programs on the network level, or just if you're nosey. It's also a good idea to play around with ngrep a bit just to see what kind of information other people might be able to glean by sniffing packets coming from your system.
By Joe "Zonker" Brockmeier at Unix Review.
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