Ubuntu Manpage: rlogind — remote login server

NAME

       rlogind — remote login server

SYNOPSIS

       rlogind [-afhlLn]

DESCRIPTION

Rlogind is the server for the rlogin(1) program. The server provides a remote login facility with
authentication based on privileged port numbers from trusted hosts.

Options supported by rlogind:

-a This option exists for compatibility with historical systems.

-f Disable the Nagle algorithm. This is only needed to work around some broken operating systems.

-h Permit use of superuser “.rhosts” files.

-l Prevent any authentication based on the user's “.rhosts” file. If the user is logging in as the
superuser and the -h option is used, “.rhosts” processing is still enabled.

-L Prevent any authentication based on “.rhosts” or “hosts.equiv” information.

-n Disable keep-alive messages.

The -h, -l, and -L flags are not used if PAM (Pluggable Authentication Module) support is in use.

The -h and -l options should also not be trusted without verifying that they work as expected with the
particular version of libc installed on your system (and should be tested again after any libc update)
because some versions of libc may not honor the internal flags used by rlogind. As the -L option
bypasses the libc functions entirely, it is not subject to this problem.

Also note that the design of the .rhosts system is COMPLETELY INSECURE except on a carefully firewalled
private network. Always use the -L option under all other circumstances. Also, since rlogind does not
encrypt communications, it should not, in general, be used at all. Consider ssh(8).

Rlogind listens for service requests at the port indicated in the ``login'' service specification; see
services(5). 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 (see
gethostbyaddr(3), hosts(5) and named(8)). If the hostname cannot be determined, the dot-notation
representation of the host address is used. 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 in rshd(8). It then allocates a pseudo terminal (see pty(4)), 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(1) 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(4) is invoked to provide ‘^S/^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’; see environ(7). 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 present. The use of keepalive
messages allows sessions to be timed out if the client crashes or becomes unreachable.

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.

BUGS

       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.

       A facility to allow all data exchanges to be encrypted should be present.

       A more extensible protocol should be used.

HISTORY