RLOGIND(8) BSD System Manager's Manual RLOGIND(8)
rlogind — remote login server
Rlogind is the server for the rlogin(1) program. The server provides a
remote login facility with authentication based on privileged port num‐
bers from trusted hosts.
Options supported by rlogind:
-a Ask hostname for verification.
-D Disable reverse client checking. Security is lower
-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”
-n Disable keep-alive messages.
The -h, -l, and -L flags are not used if PAM (Pluggable Authentication
Module) support is in use. In this case the same effects can be achieved
by editing /etc/pam.conf.
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. Con‐
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 cor‐
responding host name (see gethostbyaddr(3), hosts(5) and named(8)).
If the hostname cannot be determined, the dot-notation representa‐
tion 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 corre‐
spond. Normal authentication is bypassed if the address verifica‐
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 suc‐
ceeded. 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 facili‐
ties 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.
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 indica‐
tion of success.
A fork by the server failed.
login(1), ruserok(3), rshd(8)
The authentication procedure used here assumes the integrity of each
client machine and the connecting medium. This is insecure, but is use‐
ful in an ``open'' environment.
A facility to allow all data exchanges to be encrypted should be present.
A more extensible protocol should be used.
The rlogind command appeared in 4.2BSD.
Linux NetKit (0.17) March 16, 1991 Linux NetKit (0.17)