Wireshark

Wireshark is one of the world's foremost network protocol analyzers, and is the standard in many parts of the industry. It is the continuation of a project that started in 1998. Hundreds of developers around the world have contributed to it, and it it still under active development. Wireshark has a rich feature set which includes the following: Standard three-pane packet browser Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others Multi-interface: Along with a standard GUI, Wireshark includes TShark, a text-mode analyzer which is useful for remote capture, analysis, and scripting The most powerful display filters in the industry VoIP analysis Live capture and offline analysis are supported Read/write many different capture file formats: tcpdump (libpcap), NAI's Sniffer (compressed and uncompressed), Sniffer Pro, NetXray, Sun snoop and atmsnoop, Shomiti/Finisar Surveyor, AIX's iptrace, Microsoft's Network Monitor, Novell's LANalyzer, RADCOM's WAN/LAN Analyzer, HP-UX nettl, i4btrace from the ISDN4BSD project, Cisco Secure IDS iplog, the pppd log (pppdump-format), the AG Group's/WildPacket's EtherPeek/TokenPeek/AiroPeek, Visual Networks' Visual UpTime and many others Capture files compressed with gzip can be decompressed on the fly Hundreds of protocols are supported, with more being added all the time Coloring rules can be applied to the packet list, which eases analysis