4.3BSD-Tahoe/usr/man/cat8/rshd.0

RSHD(8C)	    UNIX Programmer's Manual		 RSHD(8C)



NNAAMMEE
     rshd - remote shell server

SSYYNNOOPPSSIISS
     //eettcc//rrsshhdd

DDEESSCCRRIIPPTTIIOONN
     _R_s_h_d is the server for the _r_c_m_d(3X) routine and, conse-
     quently, for the _r_s_h(1C) program.	The server provides
     remote execution facilities with authentication based on
     privileged port numbers from trusted hosts.

     _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(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 2 is non-zero, it is
	  interpreted as the port number of a secondary stream to
	  be used for the ssttddeerrrr.  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
	  _g_e_t_h_o_s_t_b_y_a_d_d_r(3N), _h_o_s_t_s(5) and _n_a_m_e_d(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 cclliieenntt'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 sseerrvveerr'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.



Printed 7/9/88		October 25, 1987			1






RSHD(8C)	    UNIX Programmer's Manual		 RSHD(8C)



     8)   _R_s_h_d then validates the user according to the following
	  steps.  The local (server-end) 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 success-
	  ful.	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 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 _r_s_h_d.

DDIIAAGGNNOOSSTTIICCSS
     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).

     ````llooccuusseerr ttoooo lloonngg''''
     The name of the user on the client's machine is longer than
     16 characters.

     ````rreemmuusseerr ttoooo lloonngg''''
     The name of the user on the remote machine is longer than 16
     characters.

     ````ccoommmmaanndd ttoooo lloonngg ''''
     The command line passed exceeds the size of the argument
     list (as configured into the system).

     ````LLooggiinn iinnccoorrrreecctt..''''
     No password file entry for the user name existed.

     ````NNoo rreemmoottee ddiirreeccttoorryy..''''
     The _c_h_d_i_r command to the home directory failed.

     ````PPeerrmmiissssiioonn ddeenniieedd..''''
     The authentication procedure described above failed.

     ````CCaann''tt mmaakkee ppiippee..''''
     The pipe needed for the ssttddeerrrr, wasn't created.




Printed 7/9/88		October 25, 1987			2






RSHD(8C)	    UNIX Programmer's Manual		 RSHD(8C)



     ````TTrryy aaggaaiinn..''''
     A _f_o_r_k by the server failed.

     ````<<sshheellllnnaammee>>:: ......''''
     The user's login shell could not be started.  This message
     is returned on the connection associated with the ssttddeerrrr,
     and is not preceded by a flag byte.

SSEEEE AALLSSOO
     rsh(1C), rcmd(3X)

BBUUGGSS
     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.



































Printed 7/9/88		October 25, 1987			3