rshd(8)

NAME

     rshd - remote shell server

SYNOPSIS

     shell stream tcp nowait root /usr/sbin/in.rshd in.rshd
     tcpd shell /usr/sbin/in.rshd

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 0-1023, 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 1 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 0-1023.

     4)   The server checks the  client's  source  address  and  requests  the
          corresponding   host   name  (see  gethostbyaddr(3N),  hosts(5)  and
          named(8)).  If the hostname cannot be determined,  the  dot-notation
          representation of the host address is used.

     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 according to the following steps.   The
          local (server-end) user name is looked up in the password file and a
          chdir is performed to the user's  home  directory.   If  either  the
          lookup  or chdir fail, the connection is terminated.  If the user is
          not the super-user,  (user  id  0),  the  file  /etc/hosts.equiv  is
          consulted  for  a  list  of hosts considered ``equivalent''.  If the
          client's host name is present in this file,  the  authentication  is
          considered  successful.   If  the  lookup  fails, or the user is the
          super-user, then the file .rhosts  in  the  home  directory  of  the
          remote user is checked for the machine name and identity of the user
          on the client's machine.  If this lookup fails,  the  connection  is
          terminated.

     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.

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.

     ``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).

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 should be used.