ngrep

grep for network traffic
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 expressions to match
against data payloads of packets. It currently recognizes TCP, UDP
and ICMP across Ethernet, PPP, SLIP and null interfaces, and
understands bpf filter logic in the same fashion as more common
packet sniffing tools, such as tcpdump and snoop.