X11Forward and xauth problems

Thu Mar 26 22:17:00 GMT 2015

On 3/26/2015 12:12 PM, Jon TURNEY wrote:
> 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.

Yes.

>
> 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)

I'm not using startx - I just do C:\Cygwin\bin\XWin.exe -multiwindow 
-listen tcp

Note I don't see a ~/.serverauth. Maybe this file is created just for 
this purpose and quickly removed?

> - 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?

Sounds plausible. Is that configurable?

> 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)

After I ssh -X to this system I do see ~/.Xauthority and 
~/.Xauthority-n. They are the same size but differ binarily. I can do mv 
~/.Xauthority-n ~/.Xauthority without issue. Why can't sshd do that?

Once I rename the file X clients work! From that machine...

So I thought this might be a crude workaround and put this in my .bashrc:

# Hack to fix ~/.Xauthority
if [ -f $HOME/.Xauthority-n ]; then
   mv $HOME/.Xauthority-n $HOME/.Xauthority
   echo "Fixed ~/.Xauthority"
fi

and then thought, "What happens when I ssh into yet another Linux 
machine that shares this home directory" and when I tried to ssh -X to 
another machine I got (with a little instrumentation):

Adefaria-lt:id
uid=20393(adefaria) gid=10513(Domain Users) groups=10513(Domain 
Users),0(root),545(Users)
Adefaria-lt:ssh -X cm-app-lprod01
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
/usr/bin/xauth:  unable to link authority file 
/home/adefaria/.Xauthority, use /home/adefaria/.Xauthority-n
Attempting mv /home/adefaria/.Xauthority-n /home/adefaria/.Xauthority
mv: cannot move `/home/adefaria/.Xauthority-n' to 
`/home/adefaria/.Xauthority': Permission denied
adefaria
uid=80176(adefaria) gid=701(Domain Users) groups=701(Domain 
Users),10013(CMAdmins),10014(CMMachines-admin.sec) 
context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
-rw-------. 1 adefaria Domain Users 223 Mar 26 13:25 
/home/adefaria/.Xauthority
-rw-------. 1 adefaria Domain Users 223 Mar 26 13:46 
/home/adefaria/.Xauthority-n
Cm-app-lprod01:mv .Xauthority-n .Xauthority
Cm-app-lprod01:

So the plot thickens... Why was mv denied permission when I can easily 
do it once I get a prompt?

Could it be because on Cygwin my uid is 20393 and on Linux it's 80176 
(and probably equivalenced by some mapping in the Netapp of 20393 <-> 80176?

> 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?

But I did rename it once I got a prompt without a problem.

> 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)

Again, I don't use startx, I simply run Xwin directly. In fact the 
startx command is not found! Setting XAUTHORITY didn't do anything different

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

Since I don't even have startx, this is not really an option.

I need to read that googling on 'Xauthority hostname nfs'...vi

Any idea why setting ForwardX11 yes and ForwardX11Trusted don't seem to 
work? I thought it was that setting ForwardX11 yes is equivalent to 
specifying -X and setting ForwardX11Trusted yes is equivalent to 
specifying -Y but they are not behaving that way!

Adefaria-lt:echo "ForwardX11 yes" > ~/.ssh/config
Adefaria-lt:ssh cm-db-ldev01 "echo DISPLAY = \'\$DISPLAY\'"
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
X11 forwarding request failed on channel 0
DISPLAY = ''
Adefaria-lt:echo "ForwardX11Trusted yes" > ~/.ssh/config
Adefaria-lt:ssh cm-db-ldev01 "echo DISPLAY = \'\$DISPLAY\'"
DISPLAY = ''
Adefaria-lt:

Also why does this work:

Adefaria-lt:ssh -X cm-app-ldev01 "echo DISPLAY = \'\$DISPLAY\'"
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
DISPLAY = 'localhost:11.0'
Adefaria-lt:

But this fail:

Adefaria-lt:ssh -X cm-db-ldev01 "echo DISPLAY = \'\$DISPLAY\'"
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
X11 forwarding request failed on channel 0
DISPLAY = ''
Adefaria-lt:

I find all of this behavior erratic and unreliable.
-- 
Andrew DeFaria
http://defaria.com

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