Hi all.
I've found this problem solved everywhere and I tried a lot of suggestions. But none of them worked.
This is the situacion.
- There is a server running a Debian. It has a user, let's say myuser.
- This myuser needs to execute a python library that makes use of libqt4 and it requires an active X server to perform almost any graphic operation. This python program does not need to pop up any window or so, it just creates a svg file, but the X server is a dependence that can't be avoided.
- But when I try to run this python I get the same as when I run, for instance, xclock:
$ xclock
No protocol specified
Error: Can't open display: :0.0


These are the things I tried to solve it unsuccesfully:
- login as myuser with ssh -X:
$ xhost +localhost
xhost: unable to open display "000.000.00.000:0.0"

Also tried login as root.

- xauth merge ~/.Xauthority

-In /etc/ssh/ssh_config set ForwardX11 yes

- xhost root logged as root. Also logged as myuser -I'm not sure who is the server and the client in this situation-.

- xhost + from both user and root
# xhost root
No protocol specified
xhost: unable to open display ":0"

- Installed network manager and added nm-system-settings.conf:
[keyfile]
hostname = <your hostname>

- In /etc/X11/Xwrapper.config allowed_users=anybody

After the changes sudo /etc/init.d/gdm restart

This may help:
$ startx
Fatal server error:
Server is already active for display 0
If this server is no longer running, remove /tmp/.X0-lock
and start again.
Please consult the The X.Org Foundation support
at http://wiki.x.org
for help.
^CInvalid MIT-MAGIC-COOKIE-1 keygiving up.
xinit: Resource temporarily unavailable (errno 11): unable to connect to X server
xinit: No such process (errno 3): unexpected signal 2.


Any idea? Thanks in advance.

Post here output of 'ps aux|grep X'. It will show the options you server started with as well as if your X server is really up and running. /var/log/Xorg.*.log could be useful too.

There it goes:

# ps aux|grep X
root 934 0.0 0.5 94516 2928 tty7 Ss+ Apr25 7:36 /usr/bin/X :0 -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
root 9370 0.0 0.1 15520 768 ? S 04:23 0:00 xinit /etc/X11/xinit/xinitrc -- /usr/bin/X :1 -auth /tmp/serverauth.cUFnJhfnCb
root 9371 0.0 1.9 100028 9552 tty8 S<s+ 04:23 0:01 /usr/bin/X :1 -auth /tmp/serverauth.cUFnJhfnCb
root 10273 0.0 0.0 5912 484 ? Ss 06:33 0:00 /usr/lib/gdm/gdmopen -l /bin/sh -c /usr/bin/whiptail --yesno 'There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.)' 16 70
root 10274 0.0 0.1 3956 628 tty9 Ss+ 06:33 0:00 -sh -c /usr/bin/whiptail --yesno 'There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.)' 16 70
root 10280 0.0 0.3 17904 1608 tty9 S+ 06:33 0:00 /usr/bin/whiptail --yesno There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.) 16 70
root 10468 0.0 0.0 5912 488 ? Ss 06:53 0:00 /usr/lib/gdm/gdmopen -l /bin/sh -c /usr/bin/whiptail --yesno 'There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.)' 16 70
root 10469 0.0 0.1 3956 624 tty10 Ss+ 06:53 0:00 -sh -c /usr/bin/whiptail --yesno 'There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.)' 16 70
root 10474 0.0 0.3 17904 1616 tty10 S+ 06:53 0:00 /usr/bin/whiptail --yesno There already appears to be an X server running on display :0. Should another display number by tried? Answering no will cause GDM to attempt starting the server on :0 again. (You can change consoles by pressing Ctrl-Alt plus a function key, such as Ctrl-Alt-F7 to go to console 7. X servers usually run on consoles 7 and higher.) 16 70
root 12541 0.0 0.1 7552 836 pts/2 S+ 10:37 0:00 grep X


The .log is LONG, not sure if I should paste all of it here... The last thing I see on that log are about devices, the mouse, concretely. (Is a quite fresh installation.)

Ok. I'm not a big specialist in X, but may be there is a good idea to try to stop GDM, kill all the X you started before (listed in ps output) and then start X from console?

Well I did it and this is what I get now:

# ps aux|grep X
root 934 0.0 0.5 94516 2820 tty7 Ss+ Apr25 7:49 /usr/bin/X :0 -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
root 9370 0.0 0.1 15520 752 ? S May15 0:00 xinit /etc/X11/xinit/xinitrc -- /usr/bin/X :1 -auth /tmp/serverauth.cUFnJhfnCb
root 9371 0.0 1.9 100028 9828 tty8 S<s+ May15 0:01 /usr/bin/X :1 -auth /tmp/serverauth.cUFnJhfnCb
root 16778 0.0 0.1 7552 832 pts/1 S+ 05:52 0:00 grep X


Also did this logged as user: $ export DISPLAY=localhost:0
And now the "No protocol specified" disappeared, and now only get the error:

ETE: cannot connect to X server localhost:0

If I try this as user I get:
$ xhost +
xhost: unable to open display "localhost:0"

Hmm. You still have two X servers run. My primary idea was avoid using GDM-started session. Also, try to set DISPLAY to ':1' where non-GDM X running. Does this help?

I rebooted and this is what I get now:

# ps aux|grep X
root 927 0.4 1.3 93084 6748 tty7 Ss+ 02:06 0:00 /usr/bin/X :0 vt7 -nolisten tcp -auth /var/lib/xdm/authdir/authfiles/A:0-DRBDdQ

It seems there's only one X now. Then I guess that of :1 would not be necessay, am I wrong?
If I make this logged as root:
# echo $DISPLAY
:0.0

If I make this as user:
$ echo $DISPLAY
:0.0

And as user I still get:
$ xclock
No protocol specified
Error: Can't open display: :0.0

If I specify as user that of localhost, the protocol message disappears-I guess this should be good:
$ DISPLAY=localhost:0
$ xclock
Error: Can't open display: localhost:0

And I'm completely stuck now. :/

Other things I tried:

# grep X11Forwarding /etc/ssh/sshd_config
X11Forwarding yes

(then login as ssh -Y user@hostname)

$ xhost + localhost

xhost: unable to open display :0.0

export XAUTHORITY=/home/usersnamegoesinhere/.Xauthority

sudo apt-get install --reinstall xserver-xorg

sudo dpkg-reconfigure xserver-xorg

Start X like this:
/usr/bin/X :0 vt7 -ac (this turns off authentication)

And run client with DISPLAY=localhost:0

I this work, all the problem is in authentication.

1 members found this post helpful.

Thanks for helping me to discard causes, nyshtyak, I finally found it!
In /etc/X11/xdm/xdm-config I needed to set:

DisplayManager*authorize: false
DisplayManager*authComplain: false

I hope it helps somebody, it took me lots of hours to find it.

1 members found this post helpful.