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BSD 2.11 - man page for rshd (bsd section 8)

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RSHD(8) 										  RSHD(8)

NAME
       rshd - remote shell server

SYNOPSIS
       rshd [-aln]

DESCRIPTION
       Rshd 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.

       Rshd  listens for service requests at the port indicated in the ``cmd'' service specifica-
       tion; see services(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
	      512-1023, the server aborts the connection.

       2)     The server reads characters from the socket up to a null (`\0') byte.   The  resul-
	      tant 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  cre-
	      ated 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 verifi-
	      cation fails, the connection is aborted  with  the  message,  ``Host  address  mis-
	      match.''

       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   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)     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 estab-
	      lished 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 lead-
       ing 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.

       ``remuser 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 sys-
       tem).

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

       ``No remote directory.''
       The chdir command 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 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), ruserok(3)

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.

4.2 Berkeley Distribution		September 11, 1989				  RSHD(8)
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