is the server for the
program. The server provides a remote login facility
with authentication based on privileged port numbers from trusted hosts.
Options supported by
Ask hostname for verification.
Prevent any authentication based on the user's
file, unless the user is logging in as the superuser.
Disable keep-alive messages.
listens for service requests at the port indicated in
the ``login'' service specification; see
When a service request is received the following protocol
The server checks the client's source port.
If the port is not in the range 512-1023, the server
aborts the connection.
The server checks the client's source address
and requests the corresponding host name (see
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
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,
proceeds with the authentication process described in
It then allocates a pseudo terminal (see
and manipulates file descriptors so that the slave
half of the pseudo terminal becomes the
for a login process.
The login process is an instance of the
program, invoked with the
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
program. In normal operation, the packet protocol described
is invoked to provide
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,
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
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
is invoked, a null byte is returned as in indication of success.
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.
A facility to allow all data exchanges to be encrypted should be
A more extensible protocol should be used.
command appeared in
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