OpenSSH is a service to remotely access the shell of a remote server. While famous for it’s in-depth security measures and high code quality, an attacker gaining access through OpenSSH has complete control over the machine under the rights of the logged in user. Thus, it is crucial to harden any OpenSSH access and/or limit it to known networks, etc.

OpenSSH commonly runs on port 22 or ports 222/2222. In theory, port assignment is up to the administrator. If you want to verify whether OpenSSH is openly running on a specific port, you can use Netcat or Telnet:

$ nc example.com 22
SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u3
^C

$ telnet example.com 22
Trying 203.0.113.0…
Connected to example.com.
Escape character is '^]’.
SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u3
^]
telnet> quit
Connection closed.

The server responds with the banner of the currently installed OpenSSH version.