Wireshark

Wireshark is the most commonly used network protocol analyser and monitoring tool. It is also used to perform certain network exploits like sniffing, password hacking, etc. Network sniffing also allows systems admins to analyse the various security loopholes in the network and undertake proper remedial measures to overcome them. This tool is supported on UNIX/Linux as well as on Windows.

Download link: https://www.wireshark.org/download.html