Ok, I'm writing this answer to try and clarify the security model of the X server, as I understand it. I'm not an expert on the subject, so I may have got some (many?) things wrong. Also, many things are different in different distributions, or even different versions of the same distribution, as the OP noted.
There are two main ways to get authorized to connect to the X server:
- The
xhost way (Host Access): The server maintains a list of hosts, local users, groups, etc. that are allowed to connect to the server.
- The
xauth way (Cookie based): The server has a list of randomly generated cookies, and anybody showing one of these cookies will be granted access.
Now, the distribution specific stuff...
When the X server is launch by the start-up system, it is usually passed a command line of the form -auth <filename>. This file contains a list of initial cookies to be used for authorization. It is created before the X server is run using the xauth tool. Then just after the X server, the login manager is launch, and it is instructed to read the cookie from this same file, so it can connect.
Now, when user rodrigo logs in, it has to be authorized to connect to the server. That is done by the login manager, and it has two options:
- It does the equivalent to:
xhost +si:localuser:rodrigo.
- It generates another cookie, adds it to the server and passes it to the user. This passing can be done in two ways:
- It is written in the file
$HOME/.Xauthority (home of the new user).
- It is written somewhere else (
/var/run/gdm/auth-for-rodrigo-xxxx) and the environment variable XAUTHORITY is set to the name of that file.
Also, it can do both things. Some login managers even add the root user to the list of authorized users by default (as if xhost +si:localuser:root).
But note that if you are not authorized to connect to the X server, you cannot add yourself to the list (running xhost + for example). The reason is the same as why you cannot open a house doof from the outside without a key... That's true even if you are root!
Does it mean that the root user cannot connect to the server? Absolutely not! But to get to that first you have to know how is the logged user configured to connect to the server. For that run as the logged user:
$ xhost
It will show a message and the list of authorized users, hosts or groups, if any:
access control enabled, only authorized clients can connect
SI:localuser:rodrigo
Then run:
$ echo $XAUTHORITY
To see where the authorization file is saved. If it is empty, then it will be ~/.Xauthority. Then:
$ xauth list :0
To see the list of your authorized cookies.
Now, if there are any cookie in the server, the root user should be able to connect making the XAUTHORITY environment variable point to the right cookie file. Note that in many setups, the cookie of the login manager is also kept around. Just look for it!
Another possibility for root access is to modify the Xsession files to add the command xhost +si:localuser:root and get permanent access. The details vary with the particular program used, but for gdm you would simply add an executable script in /etc/gdm/Init/ with the xhost command and it will be run automatically in the next boot.
PS: You can check your root access to the X server with sudo -i, but note that some sudo configurations may keep the DISPLAY, XAUTHORITY or HOME variables and modify the results of the tests.
EXAMPLE: This script should be able to connect you to the X server as root
export DISPLAY=:0
export XAUTHORITY=`ls /var/run/gdm/auth-for-gdm-*/database`
xrandr #just for show
Naturally, the path for the XAUTHORITY variable will depend on what login manager you are using (greeter). You can use the user file (you say it is in /home/redsandro/.Xauthority but I'm not so sure). Or you can use the greeter cookie. To get the greeter cookie you can use the following command:
$ pgrep -a Xorg
Which in my system gives:
408 /usr/bin/Xorg :0 -background none -verbose -auth /var/run/gdm/auth-for-gdm-gDg3Ij/database -seat seat0 -nolisten tcp vt1
So my file is /var/run/gdm/auth-for-gdm-gDg3Ij/database. The gDg3Ij is random and changes every time the server is restarted, that's why the ls ... trick.
The nice thing of using the GDM cookie instead of the user is that it does not depend on the user logged in. It will even work with no user at all!
UPDATE: From your latest comment I see that your X server command is:
/usr/bin/X :0 -audit 0 -auth /var/lib/mdm/:0.Xauth -nolisten tcp vt8
So there is the name of the cookie used to start the login manager. If I'm correct, that should be available all the time, if you are able to read the file. And you are root, so, the following lines should be enough to get you access to the display as root:
export DISPLAY=:0
export XAUTHORITY=/var/lib/mdm/:0.Xauth
zenity --info --text 'Happy New Year'
