File: inetutils.info,  Node: rshd invocation,  Next: talkd invocation,  Prev: rlogind invocation,  Up: Top

24 'rshd': Remote shell server
******************************

The 'rshd' server is the server for the 'rcmd' routine and,
consequently, for the 'rsh' (*note rsh invocation::) 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.  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.  However,
     this condition is not applied for Kerberized service.

  2. The server reads characters from the socket up to a NUL ('\0')
     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.  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 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', which uses the file
     '/etc/hosts.equiv' and the '.rhosts' file found in the user's home
     directory.  The '-l' option prevents 'ruserok' from doing any
     validation based on the user's '.rhosts' file, unless the user is
     the superuser.

  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'.

  11. 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.

  12. The '-L' option causes all successful accesses to be logged to
     'syslogd' (*note syslogd invocation::) as 'auth.info' messages.

   *Note ruserok: (libc)ruserok, for details.

24.1 Invoking
=============

The options are as follows:

'-a'
'--verify-hostname'
     Ask hostname for verification.

'-k'
'--kerberos'
     Use Kerberos authentication.

'-l'
'--no-rhosts'
     Ignore '.rhosts' file.

'-L'
'--log-sessions'
     Log successful logins.

'-n'
'--no-keepalive'
     Do not set SO_KEEPALIVE.

'-S NAME'
'--servername=NAME'
     Set Kerberos server name, overriding canonical hostname.

'-v'
'--vacuous'
     Fail any call asking for non-Kerberos authentication.

'-r'
'--reverse-required'
     Demand that the client's IP address be resolvable as a host name.

   Should 'rshd' have been built with PAM support, it reads any setting
specified for a service named either 'rsh' or 'krsh', the latter name
for clients seeking Kerberised authentication.

24.2 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 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
     characters.

'Ruser 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.

'Remote directory'
     The chdir command to the home directory failed.

'Permission denied'
     The authentication procedure described above failed, or address
     resolution was insufficient.

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

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

': ...'
     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.

   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.