Re: question about x11 forwarding in ssh

From: Robert Hajime Lanning (robert.lanning_at_gmail.com)
Date: 09/23/05

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    Date: Fri, 23 Sep 2005 02:18:47 -0700
To: secureshell@securityfocus.com

    The SSH client is actually an X client also. It uses the local $DISPLAY and
    xlib to connect to the X server (via TCP Socket, Unix Domain Socket or
    shared memory.) It can use the MIT-Magic-Cookie from the local .Xauthority file
    to authenticate to the X server.

    If the SSH client can successfully connect to the X server, it will
    request a data
    channel within the SSH connection for the X forwarding.

    The SSH server, upon receiving the X forwarding request from the SSH client
    (part of the sequence of requesting the data channel), emulates an X server
    listening on the local loopback interface for a TCP connection. Once the
    SSH server is able to bind to a TCP port, it creates an MIT-Magic-Cookie of
    it's own and calls xauth to write it to the .Xauthority file.

    Once all this is setup, it sets the $DISPLAY and forks the shell.

    On 9/21/05, Andre Charbonneau <andre.charbonneau@nrc-cnrc.gc.ca> wrote:
    > Hi,
    > I'm currently trying to debug a SSH client implementation library which
    > tries to do x11 forwarding. In the code it tries to open a socket to
    > localhost:6000 when a x11 channel request arrives (on the ssh client
    > side). But I get a 'connection refused'. I checked my system
    > configurations and indeed my xserver is started with "-nolisten tcp", so
    > this explains why I can't connect to port 6000.
    >
    > But I've read somewhere that someone should only open port 6000 only if
    > they really really need it, and that they should use ssh x11 forwarding
    > instead.
    >
    > But my question is: How does the SSH client (openSSH for example)
    > forwards x11 when port 6000 is not open??? Does it use another, special
    > port that it can connect to?
    > Is there a document online that I can read that explains in details how
    > ssh does the x11 forwarding?
    >
    >
    > Thanks!
    > Andre
    >
    > --
    > Andre Charbonneau
    >
    > 100 Sussex Drive, Rm 2025
    > Research Computing Support, IMSB
    > National Research Council Canada
    > Ottawa, ON, Canada K1A 0R6
    >
    >
    >

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