ngrep is like GNU grep applied to the network layer.

It's a PCAP-based tool that allows you to specify an extended regular or
hexadecimal expression to match against data payloads of packets.
It understands many kinds of protocols, including IPv4/6, TCP, UDP, ICMPv4/6,
IGMP and Raw, across a wide variety of interface types, and understands BPF
filter logic in the same fashion as more common packet sniffing tools, such as
tcpdump and snoop.
