RSHD(1M)                                                 RSHD(1M)



     NAME
          rshd - remote shell server

     SYNOPSIS
          /etc/rshd

     DESCRIPTION
          Rshd is the server for the rcmd(3X) routine and,
          consequently, for the rsh(1C) 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(4).  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 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(4) and named(1M)).  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.



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     RSHD(1M)                                                 RSHD(1M)



          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 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
               established by rshd.

     DIAGNOSTICS
          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 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.




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     RSHD(1M)                                                 RSHD(1M)



          ``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(1C), rcmd(3X)

     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.

     ORIGIN
          4.3 BSD
































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