4.4BSD/usr/share/man/cat8/rshd.0

RSHD(8)                   BSD System Manager's Manual                  RSHD(8)

NNAAMMEE
     rrsshhdd - remote shell server

SSYYNNOOPPSSIISS
     rrsshhdd [--aallnnLL]

DDEESSCCRRIIPPTTIIOONN
     The rrsshhdd server 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.

     The rrsshhdd server 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 512-1023, 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 _s_t_d_e_r_r. 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 512-1023.

     4.   The server checks the client's source address and requests the cor-
          responding host name (see gethostbyaddr(3),  hosts(5) and named(8)).
           If the hostname cannot be determined, the dot-notation representa-
          tion of the host address is used.  If the hostname is in the same
          domain as the server (according to the last two components of the
          domain name), or if the --aa option is given, the addresses for the
          hostname are requested, verifying that the name and address corre-
          spond.  If address verification fails, the connection is aborted
          with the message, ``Host address mismatch.''

     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 iden-
          tity on the _c_l_i_e_n_t'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 identi-
          ty 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 up-
          per bound on the size of the system's argument list.

     8.   RRsshhdd then validates the user using ruserok(3),  which uses the file
          _/_e_t_c_/_h_o_s_t_s_._e_q_u_i_v and the _._r_h_o_s_t_s file found in the user's home di-
          rectory.  The --ll option prevents ruserok(3) from doing any valida-
          tion based on the user's ``.rhosts'' file, unless the user is the
          superuser.

     9.   If the file _/_e_t_c_/_n_o_l_o_g_i_n exists and the user is not the superuser,
          the connection is closed.

     10.  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 rrsshhdd.


     Transport-level keepalive messages are enabled unless the --nn option is
     present.  The use of keepalive messages allows sessions to be timed out
     if the client crashes or becomes unreachable.

     The --LL option causes all successful accesses to be logged to syslogd(8)
     as auth.info messages.

DDIIAAGGNNOOSSTTIICCSS
     Except for the last one listed below, all diagnostic messages are re-
     turned 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 10 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.

     RRuusseerr 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.

     RReemmoottee ddiirreeccttoorryy..
             The chdir command to the home directory failed.

     PPeerrmmiissssiioonn ddeenniieedd..
             The authentication procedure described above failed.

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

     CCaann''tt ffoorrkk;; ttrryy aaggaaiinn..
             A fork by the server failed.

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

SSEEEE AALLSSOO
     rsh(1),  rcmd(3),  ruserok(3)

BBUUGGSS
     The authentication procedure used here assumes the integrity of each
     client machine and the connecting medium.  This is insecure, but is use-
     ful in an ``open'' environment.

     A facility to allow all data exchanges to be encrypted should be present.

     A more extensible protocol (such as Telnet) should be used.

4.2 Berkeley Distribution        June 4, 1993                                2