Tcpdump is a type of packet analyzer software utility that monitors and logs TCP/IP traffic passing between a network and the computer on which it is executed.
Tcpdump is an open-source network utility that is freely available under the BSD license. Tcpdump works on the command line interface and provides descriptions of packet content in several formats, depending on the command used.
Techopedia explains Tcpdump
Tcpdump is primarily a network monitoring and management utility that captures and records TCP/IP data on the run time. Tcpdump is designed to provide statistics about the number of packets received and captured at the operating node for network performance analysis, debugging and diagnosing network bottlenecks and other network oriented tasks.
Because it is a command line utility, data retrieved through tcpdump can vary. For example, when used with -A operator, it prints out each packet in ASCII format. Tcpdump is supported by most Unix-based operating systems, such as Linux, Mac OSX and BSD. The Windows variant of tcpdump is known as WinDump.