Wireshark is a network protocol analyzer for Unix and Windows.

Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software and communications protocol development, and education. Originally named Ethereal, in May 2006 the project was renamed Wireshark due to trademark issues.  Wireshark is cross-platform; it runs on various Unix-like operating systems including Linux, OS X, BSD, and Solaris, and on Microsoft Windows.