TCPDump is a tool for network monitoring and data acquisition. It can save lots of time and can be used for debugging network or server related problems. Tcpdump prints out a description of the contents of packets on a network interface that match the boolean expression.
Monitor all packets on eth0 interface
tcpdump -i eth0
Monitor all traffic on port 22 ( SSH )
tcpdump -i eth0 'port 22'
Monitor all traffic on port 25 ( SMTP )
tcpdump -vv -x -X -s 1500 -i eth0 'port 25'