BSD rexec — run a command on a remote host, with the username and password sent in cleartext.
IANA registers TCP 512 as "exec": remote process execution with authentication performed using passwords and UNIX login names. A client opens the connection, sends a NUL-separated username, password, and command line, and the rexecd server runs the command and streams back its output. It is the oldest and weakest of the three BSD r-services (512 exec, 513 rlogin, 514 rsh) and virtually every modern distribution ships rexecd disabled or not at all. On UDP the same number is comsat/biff, the local new-mail notifier, which is a different and equally obsolete service.
Do not expose
The password crosses the wire in plaintext on every invocation and the whole session is unencrypted — there is no configuration that makes this safe. Replace it with SSH.
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