NAME

     rshd — remote shell server

SYNOPSIS

     rshd [-an]

DESCRIPTION

     The rshd 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 rshd 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 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 512-1023.

     4.   The server checks the client's source address and requests the corresponding host name
          (see gethostbyaddr(3), hosts(5) and named(8)).  If the hostname cannot be determined,
          the dot-notation representation 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 -a option is given, the addresses for the hostname are requested, verifying that
          the name and address correspond.  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 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 using PAM, which uses the file /etc/hosts.equiv and the
          .rhosts file found in the user's home directory.  Due to the way the protocol was
          designed, it is not possible to exchange information other than those specified here
          for authentication.  Thus PAM authentication modules such as pam_unix should not be
          used with this service.

          Also note that the design of the .rhosts system is COMPLETELY INSECURE except on a
          carefully firewalled private network. Under all other circumstances, rshd should be
          disabled entirely.

     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.

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

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.

     Ruser 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).

     Remote directory.
             The chdir command to the home directory failed.

     Permission denied.
             The authentication procedure described above failed, or the user requested did not
             exist. (These conditions are intentionally conflated.)

     Can't make pipe.
             The pipe needed for the stderr, wasn't created.

     Can't fork; 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(1), rcmd(3), pam(8)

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 (such as Telnet) should be used.