Wireshark is a network protocol analyzer that lets users capture and interactively browse traffic running on a computer network.
The long list of features include:
-- Deep inspection of hundreds of protocols, with more being added all the time
-- Live capture and offline analysis
-- Standard three-pane packet browser
-- Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others
--Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility