Re: question about x11 forwarding in ssh

Andre Charbonneau wrote:
> 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?

The listening is done on the SSH server, at the request of the client.

In OpenSSH, the server normally starts at a port greater than 6000
(6010, but it's controlled by the X11DisplayOffset keyword in
sshd_config(5)) and tries to bind to the port, incrementing the port by
one until the bind succeeds.  Once it knows the port, it sets the
$DISPLAY variable to the value corresponding to the port it bound to (eg
for port 6010, $DISPLAY is localhost:10 [1] and so on) and adds the
xauth "cookie" to the .Xauthority file corresponding to that $DISPLAY.

The X11 client looks up $DISPLAY in its environment and the xauth cookie
to that $DISPLAY in the .Xauthority file, then connects to localhost
and, hopefully, the connection is forwarded to your X server.

> Is there a document online that I can read that explains in details how
> ssh does the x11 forwarding?

Not that I know of.  If it helps, you can think of it as a special case
of remote TCP port forwarding with the addition of the xauth authentication.

[1] Actually, sometimes it doesn't use localhost but it doesn't matter
for the purposes of this discussion.  See X11UseLocalhost in sshd_config(5).

-- 
Darren Tucker (dtucker at zip.com.au)
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    Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.