the answer: tcpdump
‘tcpdump’ is a packet-sniffer written by Steve McCanne, Craig Leres, and Van Jacobson at LBL
Monitors a LAN in realtime; prints info about each packet (source/dest, sequence numbers, flags, acknowledgements, options)
Runs on most UNIX variants
The most spectacularly fantastically wonderful network debugging tool on planet Earth; my knee-jerk reaction whenever there is any problem is to fire this up first