tcpdump http traffic

tcpdump  -s 0 -i eth0 dst or src host mydomain.com  and port 80
tcpdump  -A -s 0 -i eth0 dst or src host mydomain.com  and port 80

other commands

tcpdump src 192.168.11.200
tcpdump src 192.168.11.200 and port tftp
tcpdump -n dst 192.168.11.1 and udp
tcpdump -n dst 192.168.0.1 or  dst 192.168.0.10 and udp # display ip
tcpdump -nn dst 192.168.0.1 or  dst 192.168.0.10 and udp # display both ip and port
tcpdump -nn ip multicast  and not broadcast

see also http://www.rationallyparanoid.com/articles/tcpdump.html