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Since a recent upgrade from 10.04 LTS to 12.04 LTS I can no longer login using XDMCP (ssh -XC is okay).

This is similar to an earlier question but the answer (add xserver-allow-tcp=true) didn't help me.

I have followed the instruction to setup LightDM as an XDMCP server as follows:

/etc/lightdm/lightdm.conf

[SeatDefaults]
user-session=ubuntu
greeter-session=unity-greeter
xserver-allow-tcp=true

[XDMCPServer]
enabled=true

And then restarted by

sudo restart lightdm

I have the following in /var/log/lightdm.log

[+0.01s] DEBUG: Waiting for ready signal from X server :0
[+0.01s] DEBUG: Starting XDMCP server on UDP/IP port 177
[+0.01s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.01s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.18s] DEBUG: Got signal 10 from process 4925
[+0.18s] DEBUG: Got signal from X server :0
[+0.18s] DEBUG: Connecting to XServer :0

netstat -l | grep xdmcp gives

udp        0      0 *:xdmcp                 *:*                                
udp6       0      0 [::]:xdmcp              [::]:*    

which seems okay (although others had said there should be a TCP port 6000 here as well).

The firewall port is open:

sudo ufw status

177                        ALLOW       Anywhere
6000                       ALLOW       Anywhere
22                         ALLOW       Anywhere (v6)
177                        ALLOW       Anywhere (v6)

But no go:

Xephyr :2 -query *

[dix] Could not init font path element /usr/share/fonts/X11/cyrillic, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/100dpi/:unscaled, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/75dpi/:unscaled, removing    from list!
[dix] Could not init font path element /usr/share/fonts/X11/100dpi, removing from    list!
[dix] Could not init font path element /usr/share/fonts/X11/75dpi, removing from list!
XDM: too many retransmissions, declaring session dead

Does anyone have any clues, or even how I can go about debugging this?

Many thanks!!

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1 Answer 1

I also have exactly the same problem, on Ubuntu 13.04 instead. I found if I swtich to wdm, the XDMCP problem is solved, but Unity doesn't work with WDM.

You need to have this line modified as following, in /etc/X11/wdm/wdm-config

DisplayManager.requestPort: 177

And this line modified as following in /etc/X11/wdm/Xservers :0 local /usr/bin/X :0 vt7

I think this is because Unix is evolving (I call it regressing). Anyway I don't think anyone care about XDMCP on lightdm, especially since Unity moved to 3D. It's about time you choose your wagon.

P.S. vnc simply is far less performant than X11-over-tcp, I know.

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