How to make X.org listen to remote connections on port 6000?

(Here follows an almost verbatim copy of a self-answer from an identical question on serverfault which I'd forgotten about; askubuntu wasn't yet created).

Based on information found in this page about enabling XDCMP and the file /etc/gdm/gdm.schemas, I managed to create a /etc/gdm/custom.conf file:

# /etc/gdm/custom.conf
[xdmcp]

[chooser]

[security]
DisallowTCP=false

[debug]

Take care with letter case: it won't work, if you write "disallowTCP=false"... I also changed the /etc/X11/xinit/xserverrc file to:

exec /usr/bin/X11/X

i.e. I removed the -nolisten tcp options to the X executable. I don't know if I needed to. You might want to try avoiding this edit.

If you only change the xserverrc file, X will nevertheless start with "-nolisten TCP".

After that, all that is needed is a restart of the gdm process:

sudo service gdm restart

You can verify the success as:

tzot@tzot-laptop:/etc/X11
$ netstat -an | grep -F 6000
tcp        0      0 0.0.0.0:6000            0.0.0.0:*               LISTEN
tcp6       0      0 :::6000                 :::*                    LISTEN

Update

After an upgrade to 12.04, I had the same issue. This time, the culprit is the lightdm that the system uses. The file that needs to be updated is /etc/lightdm/lightdm.conf and the required addition is a xserver-allow-tcp=true in the [SeatDefaults] section.

And afterwards, I also found that answer. :)

Synopsis

So, in 10.10 this still works: create /etc/gdm/custom.conf with contents as specified above and restart gdm.

This answer is related to Kubuntu 17.04 and 20.04 and I added it, because none of the other answers (related to gdm or lightdm) helped me. In my case sddm was running. To check this, run for instance

ps -eal | grep sddm

If it is is running, processes sddm and sddm-helper are shown. In this case add a configuration file /etc/sddm.conf with content

[X11]
ServerArguments=-listen tcp

for 20.04 or

[XDisplay]
ServerArguments=-listen tcp

for 17.04. After creation of this file reboot your system (may be a sddm restart is sufficient). As a consequence

ps ax | grep sddm 

shows the desired Xorg option -listen tcp and the X Server is ready for incoming connections (don't forget to add the remote host with xhost).

Ubuntu 18.04 running as lubuntu 18.04 with lightdm 1.26.0, this is what I had to do (only took me a couple weeks to figure it all out). You have to create both /etc/lightdm/lightdm.conf and /etc/lightdm.conf.d/50-xserver-command.conf as follows:

sudo vi /etc/lightdm/lightdm.conf:

[Seat:*]

xserver-allow-tcp=true
xserver-command=X -listen tcp

sudo vi /etc/lightdm/lightdm.conf.d/50-xserver-command.conf:

[Seat:*]

xserver-command=X -core -listen tcp

This will cause lightdm to run Xorg with the following command line:

Xorg -listen tcp :0 -seat seat0 -auth /var/run/lightdm/root/:0 -listen tcp vt7  
   -novtswitch

This has "-listen tcp" twice, but it doesn't hurt anything. If you don't have both files, one of the Xorg options will be "-nolisten tcp" which overrides the other "-listen tcp". I finally found the clues to this here.

Then add: xhost + to your .bashrc

Restart and check that Xorg is now listening on port 6000:

$ netstat -nal | grep 6000
tcp        0      0 0.0.0.0:6000            0.0.0.0:*               LISTEN     
tcp6       0      0 :::6000                 :::*                    LISTEN     

Enjoy...