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rshd(8)								      rshd(8)



NAME

  rshd - The remote shell server

SYNOPSIS

  rshd [-aeln]

OPTIONS

  -a  The addresses for	the hostname are requested, verifying that the name
      and address correspond.

  -e  Causes rshd to check for the /etc/nologin_hostname and /etc/nologin
      files.  If either	exists,	rshd prints its	contents and exits.

  -l  Prevents the ruserok command from	doing any validation based on the
      user's .rhosts file, unless the user is the superuser.

  -n  Disables transport-level,	keep-alive messages.

DESCRIPTION

  The rshd daemon 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 daemon 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 512 to 1023, the server aborts the	connection.

   2.  The server reads	bytes 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 nonzero, it is interpreted as the
       port number of a	secondary stream to be used for	the stderr option. 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	to 1023.

   4.  The server checks the client's source address and requests the
       corresponding hostname (see gethostbyaddr(3), hosts(4), 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 username of at	most 16	bytes is retrieved on the
       initial socket.	This username is interpreted as	the user identity on
       the client 's machine.

   6.  A null-terminated username of at	most 16	bytes is retrieved on the
       initial socket.	This username 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.  The rshd	daemon then validates the user using ruserok(3), which uses
       the file	/etc/hosts.equiv and the .rhosts file found in the user's
       home directory.	The -l option prevents ruserok(3) from doing any
       validation based	on the user's .rhosts file, unless the user is the
       superuser.

   9.  If rshd was started with	the -e option, the user	is not the superuser,
       and either the /etc/nologin_hostname or /etc/nologin file exists, rshd
       prints the contents of the first	file found and aborts the connection.
       If the file has a zero length, rshd prints a "logins disabled" mes-
       sage.

   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 rshd.

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

NOTES

  This security-sensitive command uses the SIA (Security Integration Archi-
  tecture) routine as an interface to the security mechanisms. See the
  matrix.conf(4) reference page	for more information.

RESTRICTIONS

  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.

ERRORS

  Except for the last diagnostic message listed, 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 charac-
      ters.

  Remuser too long.
      The name of the user on the remote machine is longer than	16 charac-
      ters.

  Command too long.
      The command line passed exceeds the size of the argument list (as	con-
      figured into the system).

  Login	incorrect.
      No password file entry for the username existed.

  Logins disabled.
      The server is currently not accepting connections.

  No remote directory.
      The chdir	command	to the home directory failed.

  Permission denied.
      The authentication procedure previously described	failed.

  Can't	make pipe.
      The pipe needed for the stderr option, but it was	not 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 option, and is not pre-
      ceded by a option	byte.

FILES

  /usr/sbin/rshd
      Specifies	the command path

  /etc/nologin
      Stops logins.  In	a cluster, there is also /etc/nologin_hostname.

SEE ALSO

  Commands: rsh(1)

  Functions: rcmd(3), ruserok(3)