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

     rshd -- remote shell server

     rshd [-aLln]

     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

     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 cor-
	  responding host name (see getnameinfo(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 -a 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 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 iden-
	  tity 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 up-
	  per bound on the size	of the system's	argument list.

     8.	  rshd then validates the user using ruserok(3), which uses the	file
	  /etc/hosts.equiv and the .rhosts file	found in the user's home di-
	  rectory.  The	-l option prevents ruserok(3) from doing any valida-
	  tion based on	the user's ".rhosts" file, unless the user is the su-

     9.	  If the file /etc/nologin 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 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.

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

     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).

     Locuser too long.
	     The name of the user on the client's machine is longer than 16

     Ruser too long.
	     The name of the user on the remote	machine	is longer than 16

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

     Login incorrect.
	     No	password file entry for	the user name existed.

     Remote directory.
	     The chdir(2) to the home directory	failed.

     Permission	denied.
	     The authentication	procedure described above failed.

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

     Can't fork; try again.
	     A fork(2) by the server failed.

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

     rsh(1), ssh(1), rcmd(3), ruserok(3), hosts_access(5), login.conf(5),

     The authentication	procedure used here assumes the	integrity of every ma-
     chine and every network that can reach the	rshd/rlogind ports on the
     server.  This is insecure,	but is useful in an "open" environment.
     sshd(8) or	a Kerberized version of	this server are	much more secure.

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

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

     rshd intentionally	rejects	accesses from IPv4 mapped address on top of
     AF_INET6 socket, since IPv4 mapped	address	complicates host-address based
     authentication.  If you would like	to accept connections from IPv4	peers,
     you will need to run rshd on top of an AF_INET socket, not	an AF_INET6

BSD				 March 9, 2005				   BSD


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