
  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 and
  UDP across ethernet, ppp and slip interfaces, and understands bpf
  filter logic in the same fashion as more common packet sniffing
  tools, like tcpdump and snoop.  

