BSD rlogin — a remote shell session that trusts the client host instead of asking for a password.
rlogin, documented in RFC 1282, gives a terminal session much like Telnet, but its authentication is host-based: the server trusts a connection from a privileged source port if the client host and user appear in /etc/hosts.equiv or the target user's ~/.rhosts. That trust model is the reason it died — anyone who can spoof an address or compromise one trusted host inherits accounts everywhere. The traffic, including anything typed into the session, is unencrypted. UDP 513 is a different service entirely, rwho, which broadcasts who is logged in and the machine's load average.
Do not expose
Cleartext interactive login with address-based trust — a .rhosts entry plus a spoofed or compromised peer is a remote shell. SSH replaced this in the 1990s.
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