rshd - remote shell server
shell stream tcp nowait root /usr/sbin/in.rshd in.rshd
tcpd shell /usr/sbin/in.rshd
Rshd is the server for the rcmd(3) routine and, consequently, 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 initiated:
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
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 successful. 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
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 established by rshd.
Except for the last one listed below, all diagnostic messages 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
``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).
No password file entry for the user name existed.
``No remote directory.''
The chdir command to the home directory failed.
The authentication procedure described above failed.
``Can't make pipe.''
The pipe needed for the stderr, wasn't created.
A fork by the server failed.
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
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.