NAME
rshd - remote shell server
SYNOPSIS
shell stream tcp nowait root /usr/sbin/in.rshd in.rshd
tcpd shell /usr/sbin/in.rshd
DESCRIPTION
Rshd is the server for the rcmd(3) routine and, conse-
quently, for the rsh(1) program. The server provides remote
execution facilities with authentication based on privileged
port numbers from trusted hosts.
Rshd listens for service requests at the port indicated in
the ``cmd'' service specification; see services(5). When a
service request is received the following protocol is ini-
tiated:
1) The server checks the client's source port. If the
port is not in the range 0-1023, the server aborts the
connection.
2) The server reads characters from the socket up to a
null (`\0') byte. The resultant string is interpreted
as an ASCII number, base 10.
3) If the number received in step 1 is non-zero, it is
interpreted as the port number of a secondary stream to
be used for the stderr. A second connection is then
created to the specified port on the client's machine.
The source port of this second connection is also in
the range 0-1023.
4) The server checks the client's source address and
requests the corresponding host name (see
gethostbyaddr(3N), hosts(5) and named(8)). If the
hostname cannot be determined, the dot-notation
representation of the host address is used.
5) A null terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is
interpreted as the user identity on the client's
machine.
6) A null terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is
interpreted as a user identity to use on the server's
machine.
7) A null terminated command to be passed to a shell is
retrieved on the initial socket. The length of the
command is limited by the upper bound on the size of
the system's argument list.
8) Rshd then validates the user according to the following
steps. The local (server-end) user name is looked up
in the password file and a chdir is performed to the
user's home directory. If either the lookup or chdir
fail, the connection is terminated. If the user is not
the super-user, (user id 0), the file /etc/hosts.equiv
is consulted for a list of hosts considered
``equivalent''. If the client's host name is present
in this file, the authentication is considered success-
ful. If the lookup fails, or the user is the super-
user, then the file .rhosts in the home directory of
the remote user is checked for the machine name and
identity of the user on the client's machine. If this
lookup fails, the connection is terminated.
9) A null byte is returned on the initial socket and the
command line is passed to the normal login shell of the
user. The shell inherits the network connections esta-
blished by rshd.
DIAGNOSTICS
Except for the last one listed below, all diagnostic mes-
sages are returned on the initial socket, after which any
network connections are closed. An error is indicated by a
leading byte with a value of 1 (0 is returned in step 9
above upon successful completion of all the steps prior to
the execution of the login shell).
``locuser too long''
The name of the user on the client's machine is longer than
16 characters.
``remuser too long''
The name of the user on the remote machine is longer than 16
characters.
``command too long ''
The command line passed exceeds the size of the argument
list (as configured into the system).
``Login incorrect.''
No password file entry for the user name existed.
``No remote directory.''
The chdir command to the home directory failed.
``Permission denied.''
The authentication procedure described above failed.
``Can't make pipe.''
The pipe needed for the stderr, wasn't created.
``Try again.''
A fork by the server failed.
``<shellname>: ...''
The user's login shell could not be started. This message
is returned on the connection associated with the stderr,
and is not preceded by a flag byte.
SEE ALSO
rsh(1), rcmd(3).
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.