XDMCP

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This page intends to allow remote sessions using "X Display Manager Control Protocol" (XDMCP). See the GDM documention for more information on the parameters: https://help.gnome.org/admin/gdm/3.26/gdm.html#xdmcpsection.

Setup graphical logins

XDM

Modify /etc/X11/xdm/xdm-config and comment out: 

DisplayManager.requestPort:    0

So it will be: 

!DisplayManager.requestPort:    0

Then modify /etc/X11/xdm/Xaccess to allow any host to get a login window. Look for a line that looks like this: 

*             #any host can get a login window

and remove the hash '#' sign at the beginning of the line.

In case you have multiple network interfaces also add a line like this: 

LISTEN 192.168.0.10

Where 192.168.0.10 should be you server IP address.

Then reboot or restart your X server and the xdm daemon.

GDM

Modify /etc/gdm/custom.conf to include: 

[xdmcp]
Enable=true
Port=177

Then restart gdm.service.

Or if using the inittab method, login as root on another tty and 

telinit 3
telinit 5 && exit

KDM

Support for this feature seems to be very buggy. Lightdm is probably an easier choice.

Edit kdmrc ( /opt/kde/share/config/kdm/kdmrc [KDE 3x] or /usr/share/config/kdm/kdmrc (KDE 4x] ) and at the end there should be something like this: 

[Xdmcp]
Enable=true

Then you need to restart kdm.service so the change you just made takes effect.

LightDM

Modify /etc/lightdm/lightdm.conf

Enable the XDMCP Server: 

[XDMCPServer]
enabled=true
port=177

On a headless system, disable the automatic start of one seat so that LightDM can run in the background: 

[LightDM]
start-default-seat=false

Then restart lightdm.service.

Accessing X from a remote machine on your LAN

You can access your login manager on the network computer 192.168.0.10 via the following command. TCP and UDP streams are opened. So it is not possible to access the login manager via an SSH connection. 

$ Xnest -query 192.168.0.10 -geometry 1280x1024 :1

Or, with Xephyr, if you experience refreshing problems with Xnest: 

$ Xephyr -query 192.168.0.10 -screen 1280x1024 -br -reset -terminate :1

Or, if you are on runlevel 3 

$ X -query server address

Xserver should recognize your monitor and set appropriate resolution.

Note: You can enable XDMCP Direct/Query and Broadcast connections from remote hosts without XDM starting a local X server.

After allowing XDMCP access as described above, edit /etc/X11/xdm/Xservers and comment out: 

#:0 local /usr/bin/X :0

Then launch XDM as root, e.g. xdm -config /etc/X11/xdm/archlinux/xdm-config

Thin client setup

First of all one should setup dhcp and tftp server. Dnsmasq has both of them. For network boot image check thinstation project. If your network card does not support PXE, you can try Etherboot

Example GUI-based Clients

Troubleshooting

Session declined: Maximum Number of Sessions Reached

Edit /etc/gdm/custom.conf and add/increase the maximum sessions. 

[xdmcp]
Enable=true
MaxSessions=2