


      RRRRSSSSHHHHDDDD((((1111))))                     ((((bbbbssssdddd 4444....2222))))                     RRRRSSSSHHHHDDDD((((1111))))



      NNNNAAAAMMMMEEEE
           rshd - remote shell server

      SSSSYYYYNNNNTTTTAAAAXXXX
           ////eeeettttcccc////rrrrsssshhhhdddd

      DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
           _R_s_h_d is the server for the _r_s_h(1) program.  The server
           provides remote execution facilities with authentication
           based on privileged port numbers.

           _R_s_h_d listens for service requests at the port indicated in
           the ``cmd'' service specification; see _s_e_r_v_i_c_e_s(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 resulting 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 ssssttttddddeeeerrrrrrrr.  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.  If the
                address is associated with a host for which no
                corresponding entry exists in the host name data base
                (see _h_o_s_t_s(4)), the server aborts the connection.

           5)   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 sssseeeerrrrvvvveeeerrrr'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 the user identity on the cccclllliiiieeeennnntttt'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)   _R_s_h_d then validates the user according to the following



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                steps.  The remote user name is looked up in the
                password file and a _c_h_d_i_r is performed to the user's
                home directory.  If either the lookup or _c_h_d_i_r fail,
                the connection is terminated.  If the user is not the
                super-user, (user id 0), the file /_e_t_c/_h_o_s_t_s._e_q_u_i_v 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 ._r_h_o_s_t_s 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 connection associated
                with the ssssttttddddeeeerrrrrrrr and the command line is passed to the
                normal login shell of the user.  The shell inherits the
                network connections established by _r_s_h_d.

      DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS
           All diagnostic messages are returned on the connection
           associated with the ssssttttddddeeeerrrrrrrr, 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 command
           execution).

           ````````llllooooccccuuuusssseeeerrrr ttttoooooooo lllloooonnnngggg''''''''
           The name of the user on the client's machine is longer than
           16 characters.

           ````````rrrreeeemmmmuuuusssseeeerrrr ttttoooooooo lllloooonnnngggg''''''''
           The name of the user on the remote machine is longer than 16
           characters.

           ````````ccccoooommmmmmmmaaaannnndddd ttttoooooooo lllloooonnnngggg ''''''''
           The command line passed exceeds the size of the argument
           list (as configured into the system).

           ````````HHHHoooossssttttnnnnaaaammmmeeee ffffoooorrrr yyyyoooouuuurrrr aaaaddddddddrrrreeeessssssss uuuunnnnkkkknnnnoooowwwwnnnn....''''''''
           No entry in the host name database existed for the client's
           machine.

           ````````LLLLooooggggiiiinnnn iiiinnnnccccoooorrrrrrrreeeecccctttt....''''''''
           No password file entry for the user name existed.

           ````````NNNNoooo rrrreeeemmmmooootttteeee ddddiiiirrrreeeeccccttttoooorrrryyyy....''''''''
           The _c_h_d_i_r command to the home directory failed.

           ````````PPPPeeeerrrrmmmmiiiissssssssiiiioooonnnn ddddeeeennnniiiieeeedddd....''''''''
           The authentication procedure described above failed.




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      RRRRSSSSHHHHDDDD((((1111))))                     ((((bbbbssssdddd 4444....2222))))                     RRRRSSSSHHHHDDDD((((1111))))



           ````````CCCCaaaannnn''''tttt mmmmaaaakkkkeeee ppppiiiippppeeee....''''''''
           The pipe needed for the ssssttttddddeeeerrrrrrrr, wasn't created.

           ````````TTTTrrrryyyy aaaaggggaaaaiiiinnnn....''''''''
           A _f_o_r_k by the server failed.

           ````````////bbbbiiiinnnn////sssshhhh:::: ............''''''''
           The user's login shell could not be started.

      SSSSEEEEEEEE AAAALLLLSSSSOOOO
           rsh(1)

      BBBBUUUUGGGGSSSS
           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.




































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