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21 rlogind: Remote login server

rlogind is the server for the rlogin program (see rlogin invocation). The server provides a remote login facility with authentication based on privileged port numbers from trusted hosts.

rlogind listens for service requests at the port indicated in the ‘login’ service specification. When a service request is received the following protocol is initiated:

  1. The server checks the client's source port. If the port is not in the range 512-1023, the server aborts the connection.
  2. The server checks the client's source address and requests the corresponding host name. If the hostname cannot be determined, the dot-notation representation of the host address is used. If the hostname is in the same domain as the server (according to the last two components of the domain name), or if the -a option is given, the addresses for the hostname are requested, verifying that the name and address correspond. Normal authentication is bypassed if the address verification fails.

Once the source port and address have been checked, rlogind proceeds with the authentication process described rshd invocation. It then allocates a pseudo terminal, and manipulates file descriptors so that the slave half of the pseudo terminal becomes the stdin, stdout, and stderr for a login process. The login process is an instance of the login program, invoked with the -f option if authentication has succeeded. If automatic authentication fails, the user is prompted to log in as if on a standard terminal line.

The parent of the login process manipulates the master side of the pseudo terminal, operating as an intermediary between the login process and the client instance of the rlogin program. In normal operation, the packet protocol described in ‘PTY’ is invoked to provide C-S/C-Q type facilities and propagate interrupt signals to the remote programs. The login process propagates the client terminal's baud rate and terminal type, as found in the environment variable, TERM. The screen or window size of the terminal is requested from the client, and window size changes from the client are propagated to the pseudo terminal.

Transport-level keepalive messages are enabled unless the -n option is client instance of the rlogin program. The use of keepalive messages allows sessions to be timed out if the client crashes or becomes unreachable.

See ruserok, for details.

21.1 Invoking

The options are as follows:

-a
--verify-hostname
Ask hostname for verification.
-d
--daemon
Daemon mode.
-l
--no-rhosts
Ignore .rhosts file.
-L name
--local-domain=name
Set local domain name.
-n
--no-keepalive
Do not set SO_KEEPALIVE.
-k
--kerberos
Use kerberos IV authentication.
-x
--encrypt
Turns on DES encryption for all data passed via the rlogind session. This may impact response time and CPU utilization, but provides increased security.
-D[level]
--debug[=level]
Set debug level, not implemented.
-o
--allow-root
Allow the root user to login, disabled by default.
-p port
--port=port
Listen on given port (valid only in daemon mode).
-r
--reverse-required
Require reverse resolving of a remote host IP.

21.2 Diagnostics

All initial diagnostic messages are indicated by a leading byte with a value of 1, after which any network connections are closed. If there are no errors before login is invoked, a null byte is returned as in indication of success.

Try again.
A fork by the server failed.

The authentication procedure used here assumes the integrity of each client machine and the connecting medium. This is insecure, but is useful in an “open” environment.