tcpdump

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10. The tcpdump program

Tcpdump is a program that allows root to “sniff” TCP/IP packets and display the contents in a number of different formats.  This is a useful tool if you want to see exactly what is happening on an Ethernet segment. The source code and installation information can be found at ftp://ftp.ee.lbl.gov/tcpdump.tar.Z.  The output can be somewhat verbose and you really need to understand what you’re looking at.  To help with that the book “UNIX Network Programming – Volume 1, Second Edition” by W. Richard Stevens is an excellent place to start.