Getting X11 forwarding through ssh working after running su

Posted by daveseff on Mon 29 Jan 2007 at 06:06

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X authentication is based on cookies -- secret little pieces of random data that only you and the X server know... So, you need to let the other user in on what your cookie is. One way to do this is as follows:
Before you issue the su or sudo (but after having ssh'ed into the remote system), request the cookie for the current DISPLAY that's connecting to your X server:

$ xauth list $DISPLAY
You'll get something like

somehost.somedomain:10 mit-magic-cookie-1 4d22408a71a55b41ccd1657d377923ae

Then, after having done su, tell the new user what the cookie is:

$ xauth add somehost.somedomain:10 MIT-MAGIC-COOKIE-1 4d22408a71a55b41ccd1657d377923ae

(just copy'n-paste the output of the above 'xauth list' onto 'xauth add') That's it. Now, you _should_ be able to start any X application.
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#1
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (82.241.xx.xx) on Mon 29 Jan 2007 at 06:28
You can use the sux package too (with the command of the same name).

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#4
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (74.104.xx.xx) on Mon 29 Jan 2007 at 17:35
And similarly there is a gksu package that provides gksu and gksudo.

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#11
Re: Getting X11 forwarding through ssh working after running su
Posted by k8to (64.142.xx.xx) on Wed 31 Jan 2007 at 12:22
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Although it is a crying shame that gksu as packaged on debian is in the same package as gksudo, which pulls in sudo with all the nasty configuration and ugly behavior. NO sudo on my computers, please!

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#2
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (212.23.xx.xx) on Mon 29 Jan 2007 at 09:47
Another technique I discovered recently (after getting stuck on a SLES system with no sux packages) is to set the XAUTHORITY environment variable to $USER/.Xauthority (where $USER is the username you ssh'd in as).

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#10
Re: Getting X11 forwarding through ssh working after running su
Posted by tong (69.63.xx.xx) on Wed 31 Jan 2007 at 03:03
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Another technique ... is to set the XAUTHORITY

Please elaborate when and where do you do that, and under what conditions.

because I really doubt that it will work. ~/.Xauthority is most often of "-rw-------" attribute. nobody else except root can access it. Let alone we are talking about ssh to another box here.

but I maybe wrong, and would like to listen.

thanks

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#17
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (194.178.xx.xx) on Tue 6 Feb 2007 at 17:59
"~/.Xauthority is most often of "-rw-------" attribute. nobody else except root can access it"

You are not right. Nobody else but root and the _user_ owning the file can access it. And the user owning the file is .... who?

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#16
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (194.178.xx.xx) on Tue 6 Feb 2007 at 17:55
I'm pretty sure you do not mean "$USER/.Xauthority", but "$HOME/.Xauthority". Where $HOME is the home of $USER. And $USER is the username you ssh'd in....

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#3
Re: Getting X11 forwarding through ssh working after running su
Posted by lpenz (200.213.xx.xx) on Mon 29 Jan 2007 at 15:30
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Another really really simple alternative is to ssh again to root@localhost.

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#15
Re: Getting X11 forwarding through ssh working after running su
Posted by busfault (69.205.xx.xx) on Fri 2 Feb 2007 at 18:58
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Would that be a good idea though? That would require enabling root login through ssh. At which point why not just ssh into the machine with root to begin with? Is it possible to deny foreign ssh as root but accept local ssh as root?

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#5
Re: Getting X11 forwarding through ssh working after running su
Posted by drgraefy (128.59.xx.xx) on Mon 29 Jan 2007 at 18:51
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My first reaction to this is "ack! why are you opening x clients as root!?" but i guess you could be su'ing to another non-root user.

I definitely agree with the previous commenter, though, that the easiest thing to do is to just ssh again into the account of the local user with whom you want to open the x clients.

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#6
Re: Getting X11 forwarding through ssh working after running su
Posted by daveseff (208.251.xx.xx) on Mon 29 Jan 2007 at 19:03
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I didn't say anything about root. Root is easy. You usually can run 'sudo xapplication' and be happy with that. As an administrator, which is what this site is about, sometimes you have to help troubleshoot things for other users and you need to 'sudo su - username' to see the problem from their perspective. When you sudo su, or su in general you loose your DISPLAY and Xauth settings.

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#9
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (24.10.xx.xx) on Tue 30 Jan 2007 at 22:10
"My first reaction to this is "ack! why are you opening x clients as root!?" but i guess you could be su'ing to another non-root user."

Erm... Because some X clients are root only? Such as some system configuration GUI tools for example...

Or sometimes you need to use a GUI tool (for convenience sake) such as kompare on files only accessible by root?

Admittedly, MOST of the time it's unwise to run X clients as root, but that doesn't mean it's ALWAYS a bad idea. It's not utterly taboo or anything. Just not always the best plan as a first option. Better avoided if other options are available.

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#7
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (198.45.xx.xx) on Mon 29 Jan 2007 at 21:24
I don't believe the timing of this article! :)

I'd been struggling with this recently:

I can't login as user B directly on a Solaris box.

I login as user A, then su to B. Of course, X forwarding stops working. And I need X working to install some proprietary software.

Then I see this article that explains precisely how!

Thanks!

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#8
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (194.78.xx.xx) on Tue 30 Jan 2007 at 06:35
#Asssuming you did
ssh -X user@hostname

# Gain root privileges,
su -

# and merge the Xauth information like this:
xauth merge /home/user/.Xauthority

Cheers,


Benoît

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#12
Re: Getting X11 forwarding through ssh working after running su
Posted by tong (63.243.xx.xx) on Wed 31 Jan 2007 at 14:38
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will the trick works on chroot as well, provided everything eg /dev /proc are properly bind mounted?

Having done "xauth add" my DISPLAY is still empty, and

$ xauth list

seems to takes years to report the cookie...

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#13
Re: Getting X11 forwarding through ssh working after running su
Posted by daveseff (208.251.xx.xx) on Wed 31 Jan 2007 at 14:43
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I believe xauth is for X11 authentication. Once you run dchroot, you may have to set you DISPLAY to :0.0 or localhost:10.0 depending on where you are. Once the variable is set, you should be ok.

I may be wrong as my experience with chroot is limited.

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#14
Re: Getting X11 forwarding through ssh working after running su
Posted by tong (63.243.xx.xx) on Wed 31 Jan 2007 at 14:44
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Yep, it works, after I bind mount /tmp as well!!!

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#18
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (82.180.xx.xx) on Fri 23 Feb 2007 at 22:39
on debian etch testing I did...

ssh ip -X -l snot
echo $DISPLAY
copy the output
su -
cat /home/snot/.Xauthority >.Xauthority
export DISPLAY="paste the output of echo $DISPLAY here... without quotes"

not pretty but it works...

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#19
Re: Getting X11 forwarding through ssh working after running su
Posted by Anonymous (134.226.xx.xx) on Mon 26 Feb 2007 at 15:33
You'll need to set the DISPLAY environmental variable after doing the su command in order for this to work.

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