What's a packet sniffer, where do I get such thing and how do I use it?
Packet sniffer is another word for protocol analyzer. It's a utility that can display ethernet packets that are crossing a network. A couple *nix utilities are tcpflow and tcpdump. (MacOS X users running 10.2.n can get tcpflow from http://www.entropy.ch/) Very handy for trouble shooting network apps! There are similar tools for windows, just google protocol analyzer shareware.
Here's an example using tcpflow: if you want to watch packets from a certain host on port 80, from a command prompt you would enter the following:
[locutus:~] colet% sudo tcpflow -c -i en0 host www.google.com and port 80
tcpflow[626]: listening on en0
192.168.000.002.49193-216.239.051.099.00080: GET / HTTP/1.1
Host: www.google.com
Accept: */*
Accept-Language: en
Connection: Keep-Alive
If-Modified-Since: Sat, 12 Jul 2003 12:37:33 GMT
etc..
If your computer is on a hub, you can see the traffic from anything else connected to it. A good switch will let you config a port to do this.
Tcpdump is similar, but just displays packet headers.
Hope this helps, Cole
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