inetutils.info: rexecd invocation

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24 'rexecd': server for 'rexec'
*******************************

'rexecd' is the server for the 'rexec' routine.  The server provides
remote execution facilities with authentication based on user names and
passwords.  It passes error messages and notices to the 'syslog'
facility 'LOG_DAEMON'.

     rexecd [OPTION]...

   'rexecd' listens for service requests at the port indicated in the
'exec' service specification.  When a service request is received the
following protocol is initiated:

  1. The server reads characters from the socket up to a NUL ('\0')
     byte.  The resultant string is interpreted as an ASCII number, base
     10.

  2. 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.

  3. A NUL terminated user name of at most 16 characters is retrieved on
     the initial socket.

  4. A NUL terminated, unencrypted password of at most 16 characters is
     retrieved on the initial socket.

  5. A NUL 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.

  6. 'rexecd' then validates the user as is done at login time and, if
     the authentication was successful, changes to the user's home
     directory, and establishes the user and group protections of the
     user.  If any of these steps fail the connection is aborted with a
     diagnostic message returned.

  7. A NUL 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 rexecd.

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

The only option is as follows:

'-l'
'--logging'
     Raise logging level for this service; use more than once for
     increased verbosity.  The 'syslog' facility in use is 'LOG_DAEMON'.

   Should 'rexecd' have been built with PAM support, it reads any
setting specified for a service named 'rexec'.

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 7 above upon successful completion of all the steps
prior to the command execution).

'username too long'
     The name is longer than 16 characters.

'password too long'
     The password 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.

'Password incorrect.'
     The wrong password was supplied.

'No remote directory.'
     The chdir command to the home directory failed.

'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
     ...

   _Note_, that indicating 'Login incorrect' as opposed to 'Password
incorrect' is a security breach which allows people to probe a system
for users with null passwords.