Port 513

Remote Login (rlogin) is a classic Unix utility that allows users to establish an interactive text terminal session over a network connection, primarily TCP port 513. Designed as part of the Berkeley r-commands suite, rlogin facilitates seamless remote access between trusted Unix hosts by enabling direct login without repeated password prompts when configured with .rhosts or hosts.equiv files.

Technically, rlogin establishes a TCP-based full-duplex communication channel between a client and a server. Once connected, it forwards terminal input/output, effectively mirroring the remote shell environment. The protocol supports features like window size negotiation and control character exchange, improving interactive usability compared to raw Telnet.

However, rlogin was designed decades ago before network security was a major concern, therefore it transmits all communication, including authentication credentials and session data, in cleartext. This lack of encryption and rudimentary authentication make it obsolete in modern secure network environments, but it remains historically significant.