How do I configure lightdm to work with XDMCP?

How to use XDMCP+GDM and Xnest?

To enable the XDMCP server in LightDM edit /etc/lightdm/lightdm.conf and add the following section:

[XDMCPServer]
enabled=true

Then restart LightDM with sudo restart lightdm from a text console (logout from any graphical sessions first).

You can set the XDM-AUTHENTICATION-1 key by setting key=value (value is in the same format as is used in the X -cookie option). If the key is not set then no authentication is used.

You can change the UDP/IP port by setting port=1234.

All the available LightDM settings are shown in the example configuration in/usr/share/doc/lightdm/lightdm.conf.

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I have been trying to enable XDMCP on GDM without much success. Following some instructions I have edited /etc/gdm/custom.conf and added:

[daemon]
RemoteGreeter=/usr/lib/gdm/gdm-xdmcp-chooser-slave
[xdmcp]
Enable=true

Then restarted gdm and tried to connect both locally and from a remote system with:

Xnest :1 -query localhost
Xnest :1 -query remote_system_hostname

I just get a black screen instead of the GDM window as expected. I am missing something ?