Description:

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.