rexecd(ADMN)                    19 June 1992                    rexecd(ADMN)


 Name

    rexecd - remote execution server

 Syntax

    /etc/rexecd

 Description

    rexecd is the server for the rexec(SLIB) routine.  The server provides
    remote execution facilities with authentication based on user names and
    passwords.

    rexecd listens for service requests at the port indicated in the ``exec''
    service specification; see services(SFF).  When a service request is
    received, the following protocol is initiated:

    1.  The server reads characters from the socket up to a null (`\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 null-terminated user name of at most 16 characters is retrieved on
        the initial socket.

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

    5.  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.

    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 mes-
        sage returned.

    7.  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 rexecd.

    rexecd is started by the ``super-server'' inetd and, therefore, must have
    an entry in inetd's configuration file, /etc/inetd.conf.

 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 argu-
                        ment list (as configured into the system).

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

    Password incorrect. The wrong was password 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 preceded by a flag byte.


 Notes

    Indicating ``Login incorrect'' as opposed to ``Password incorrect'' is a
    security breach which allows people to probe a system for users with null
    passwords.

    A facility to allow all data and password exchanges to be encrypted
    should be present.

 See also

    rexec(SLIB), inetd(ADMN), inetd.conf(SFF), services(SFF)