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Re: X11Forward and xauth problems


On 25/03/2015 17:40, Andrew DeFaria wrote:
Prediction: This problem probably will end up having something to do
with the permissions and file system that ~/.Xauthority resides on,
which is, I believe, a NetApp. This file system is the file system for
the Linux Home directories (Windows "home" directories are somewhere
else). In an attempt to have a transparently workable environment I set
my Cygwin home directory to access the same directory my Linux servers
use for the home directory - this NetApp. If you need more information
about that then let me know and perhaps tell me how I can get that.

This seems very plausible.

If I am understanding you correctly, ~/.Xauthority is the same file on the NetApp at both ends. I think perhaps that is somehow the cause of the problem.

The sequence of actions is something like:

- startx(|win) generates a random cookie and stores it in ~/.serverauth.<pid> and uses that file as the server -auth option - it also uses 'xauth add' to put that cookie into ~/.Xauthority for the display (e.g. :0) - ssh reads that cookie out of ~/.Xauthority using 'xauth list' and sends it to the far end - sshd tries to store that cookie using xauth for the proxy display (e.g :10)

Reading the source of xauth [1], it does try to lock the ~/.Xauthority file for up to 20 seconds before giving up, which perhaps corresponds to the delay you see?

However, the "unable to link authority file .Xauthority, use .Xauthority-n" message indicates that the working file .Xauthority-n cannot renamed as .Xauthority (xauth tries both to hard-link it as .Xauthority, and to rename it)

Of course, sshd doesn't understand it's helpful advice to use a different filename, so things don't work out so well. :)

Given that it works the first time, when there is no existing ~/.Xauthority, perhaps the NetApp doesn't permit this file to be renamed over an existing file, for some reason?

You can tell startx to use a different file by using the XAUTHORITY env var, so setting that to something like ~/.Xauthority-$HOSTNAME might be a workaround. (Some googling on 'Xauthority hostname nfs' might be informative)

Or editing startx and changing enable_xauth to 0 might also be a workaround.

As you can see with my current ~/.Xauthority file things don't work. But
if I remove them, the ~/.Xauthority* files one is created at the next
login and everything works fine. Log out and back in however and it
breaks again.

If you want this to work, you will now (since X server 1.17) need to
start the server with the option '-listen tcp'.

Restarted Xwin with -multimonitor and -listen tcp. Now I get:

Sorry for any ambiguity, but you have misunderstood what I wrote.

If you want explicitly setting DISPLAY and allowing access using xhost
to work, you must start the server with the option '-listen tcp'.

Sorry I misunderstood. This works for me and is a work around. But I
wish I could get that xauth thing working correctly.

Good.

[1] http://cgit.freedesktop.org/xorg/app/xauth

--
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

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