Tcpdump command is a famous network packet analysing tool that is used to display TCPIP & other network packets being transmitted over the network attached to the system on which tcpdump has been installed. Tcpdump uses libpcap library to capture the network packets & is available on almost all Linux/Unix flavors. Tcpdump command can read the contents from a network interface or from a previously created packet file or we can also write the packets to a file to be used for later. One must use the tcpdump command as root or as a user with sudo privileges.