tcpdump

This is an open-source sniffing tool that is used for packet capture and analysis. tcpdump runs using a command line interface. tcpdump has also been custom-designed for packet capturing as it does not have a GUI that enables the analysis and display of data. It is a tool with one of the most powerful packet-filtering capabilities and can even selectively capture packets. This differentiates it from most other sniffing tools that have no means of filtering packets during capture. Following is a screenshot of the tcpdump tool. In the screenshot, it is listening to the ping commands being sent to its host: