Wireshark (aka Ethereal) is a free packet sniffer computer application. It is
used for network troubleshooting, analysis, software and communications 
protocol development, and education. In June 2006 the project was 
renamed from Ethereal due to trademark issues.

The functionality Wireshark provides is very similar to tcpdump, but it has 
a graphical front-end, and many more information sorting and filtering 
options. It allows the user to see all traffic being passed over the network
(usually an Ethernet network but support is being added for others) by 
putting the network interface into promiscuous mode.

Wireshark uses the cross-platform GTK+ widget toolkit. Its powerful features 
make it the tool of choice for network troubleshooting, protocol development, 
and education worldwide.
