ngrep: network grep

"ngrep strives to provide most of GNU grep's common features, applying them to the network layer. ngrep is a pcap-aware tool that will allow you to specify extended regular or hexadecimal expressions to match against data payloads of packets. It currently recognizes TCP, UDP, ICMP, IGMP and Raw protocols across Ethernet, PPP, SLIP, FDDI, Token Ring, 802.11 and null interfaces, and understands bpf filter logic in the same fashion as more common packet sniffing tools, such as tcpdump and snoop." read more...

Website: http://www.packetfactory.net/projects/ngrep