Internet TCP port 513 is primarily used by two different services: the Remote Login a la Berkeley rlogin, and the WHO protocol. The rlogin service allows a user to log in on another host via a network, executing commands on that machine. The WHO protocol, on the other hand, is used to deliver system user information.
TCP port 513 is associated with the rlogin protocol, which is used for remote login to Unix-like operating systems. In the past, hackers have exploited this port to gain unauthorized access to systems. They typically do this by guessing or cracking weak passwords, or by exploiting vulnerabilities in the rlogin protocol itself. Once they gain access, they can execute commands, steal data, or use the compromised system as a launchpad for further attacks. This is why it's crucial to secure TCP port 513, for instance by disabling rlogin if it's not needed, or by using secure alternatives like SSH.
