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.

ngrep was originally developed to:

As well, it could be used to do plaintext credential collection, as with HTTP Basic Authentication, FTP or POP3 authentication. Like all useful tools, it can be used for good and for bad.

Visit EXAMPLES to learn more about how ngrep works and can be leveraged to see all sorts of neat things.

If you need help, have constructive feedback, or would like to submit a patch, please visit ngrep's project at GitHub and use the online tools there. It will help the author better manage the various requests and patches so that nothing is lost or missed (as has been the case in the past, unfortunately).

Please see CREDITS for a partial list of the many people who helped make ngrep what it is today. Also, please note that ngrep is released under a simple BSD-style license, though depending on which regex library you compile against, you'll either get the GPL (GNU regex) or Artistic (PCRE).