rshd(1)                                                         rshd(1)

  rshd

  NAME

    rshd - remote shell server

  SYNOPSIS

    rshd [-alnL]

  DESCRIPTION

    The rshd(1) server 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.

    The following options are supported by rshd(1):

    -a
        Ask host name for verification.

    -l
        Prevent any authentication based on the user's .rhosts file.

    -n
        Disable keep-alive messages.

    -L
        Log successful accesses very verbosely.

    The rshd(1) 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 ends 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 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 computer. 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(2)). If the host name
          cannot be determined, the dot-notation representation of the host
          address is used. If the host name 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 host name are
          requested, verifying that the name and address correspond. If
          address verification fails, the connection is ended with the
          message, "Host address mismatch."
   5.     A null terminated user name of at most 128 characters is retrieved
          on the initial socket. This user name is interpreted as the user
          identity on the client computer.
   6.     A null terminated user name of at most 128 characters is retrieved
          on the initial socket. This user name is interpreted as a user
          identity to use on the server computer.
   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.     The rshd(1) utility then validates the user using iruserok(3), which
          uses the file /etc/hosts.equiv and the .rhosts file found in the
          user's home directory. If the daemon is not running as the SYSTEM
          user, the password of the user account used to run the daemon must
          have been previously stored using the regpwd(1) utility. The -
          l option prevents ruserok(3) from doing any validation based on the
          user's .rhosts file, unless the user is the superuser. If the
          iruserok(3) function reports that the address might have been
          spoofed, the connection is refused.
          Connection is also refused if the file /etc/nologin exists.
          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(1).

    Transport-level keep-alive messages are enabled unless the -n option is
    present. The use of keep-alive messages allows sessions to be timed out if
    the client crashes or becomes unreachable.

    The -L option causes all successful accesses to be logged to syslogd(1) as
    messages.

  Interix authentication

    On traditional systems, a user can run rsh(1) from a system identified in
    the user's .rhosts file without providing a password. This is because on a
    traditional system, the rshd(1) program is run as the superuser, and can
    take any login identity. On Interix, this is not possible. Instead, the
    user must run the regpwd(1) utility on th host to store the user's
    password before running rsh.

  DIAGNOSTICS

    Except for the last diagnostic message listed in this section, 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 computer is longer than 128
        characters.

    Ruser too long.
        The name of the user on the remote computer is longer than 128
        characters.

    Command too long.
        The command line passed exceeds the size of the argument list (as
        configured into the system).

    Remote directory.
        The chdir(1) command to the home directory failed.

    Permission denied.
        The authentication procedure described above failed or there is no
        password file entry for the specified user.

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

    Can't fork; try again.
        A fork(2) 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.

  FILES

    The rsh(1) and rshd(1) programs make use of the following files, if they
    exist:

    /etc/hosts.equiv
        On the target system, this file contains the names of systems and
        users allowed to login. See rlogind(1) for more information.

    $HOME/.rhosts
        On the target system, this file can contain the names of systems (and
        user names) allowed to login.

  BUGS

    The authentication procedure used here assumes the integrity of each
    client computer 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.

  SEE ALSO

    nologin(4)

    rsh(1)

    rcmd(3)

    iruserok(3)