RSHD(8) 						      System Manager's Manual							   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 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 (`') 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 con-
	      nection 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 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 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.

       ``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 system).

       ``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)