Howto: Linux set up the X Window System to allow remote X sessions via the XDMCP

To set up the X Window System to allow remote X sessions via the X Display Manager Control Protocol (XDMCP) on Linux, follow these steps:

1. Install the X Window System and a display manager, such as XDM or GDM.
2. Edit the configuration file for your display manager, which is typically located at /etc/X11/xdm/xdm-config or /etc/X11/gdm/gdm.conf.
3. Enable XDMCP by setting the Enable option to true. For example:

[xdmcp]

Enable=true

4. Restart the display manager.

5. On the client machine, install an X Window System client, such as xhost or xclock.

6. Connect to the remote X server using the -query option. For example:

$ xclock -query remote_server

This will initiate an XDMCP session with the remote server, and the display manager on the remote server will prompt you for a login.

Note that you may need to configure the firewall on the server to allow incoming connections on the XDMCP port (UDP 177). You may also need to modify the /etc/hosts.allow file to allow connections from specific client machines.