Wireshark, the Live Network Analyser

Formally known as Ethereal, Wireshark is a popular Network analyser that displays all data packets that get in and out of your computers network interface (LAN, Wi-Fi etc).
 
Wireshack is useful tool for security experts and users who want to control their network interface and find security holes. Even the Linux platform is vulnerable to attacks. This open source network packet sniffer would watch on DNS, TCP, UDP, HTTP and most (if not all) the known network protocols. This cross-platform utility could be installed in Linux by downloading and compiling the source code.
In Ubuntu it might be necessary to first install the libc6-dev g++ gcc for the C compiler. Just do

sudo apt-get install libc6-dev g++ gcc.

Wireshark is released under the GNU General Public License and it is developed and maintained by a global team of protocol experts.