I'm using Ubuntu 11.10, lightdm. It appears my XDMCP service is setup correctly, but it won't connect.

I have the following in /etc/lightdm/lightdm.conf:

[XDMCPServer]
enabled=true

To restart lightdm the following command is issued:

sudo restart lightdm

This produces the following log file in /var/log/lightdm/lightdm.log:

[+0.01s] DEBUG: Launching X Server
[+0.01s] DEBUG: Launching process 6947: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+0.01s] DEBUG: Waiting for ready signal from X server :0
[+0.01s] DEBUG: Starting XDMCP server on UDP/IP port 177
[+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.17s] DEBUG: Got signal 10 from process 6947

No errors.

So far so good.. looks like the XDMCP service is started.

However, even though it looks like it's started without any error, it fails to connect. The problem may be related to nothing listening on TCP port 6000?

netstat -l reveals the following:

udp        0      0 *:xdmcp                 *:*
udp        0      0 *:34000                 *:*
udp        0      0 *:mdns                  *:*
udp6       0      0 [::]:xdmcp              [::]:*

which shows the xdmcp 177 port listening, but nothing in netstat shows tcp port 6000 listening:

netstat -l | grep 6000

returns nothing.

Isn't XDMCP supposed to be listening on port 6000?

A client trying to connect just times out.

The funny part is it was working before, and we can't think of any configuration change which would have effected XDMCP. Trouble shooting seems to point to the 6000 port not listening, but we never validated this port was being used earlier when it was working.

You also need

[SeatDefaults]
xserver-allow-tcp=true

I tried your exact configuration on PC 192.168.1.6, gone to another PC with ip 192.168.1.7 and run

Xephyr -query 192.168.1.6 :1

the Lightdm greeter is shown, I login with a user in an awesome session, then on the first PC I find

$ sudo netstat -atunp
...
tcp        0      0 192.168.1.6:40264       192.168.1.7:6001        ESTABLISHED 16555/dbus-launch
tcp        0      0 192.168.1.6:40265       192.168.1.7:6001        ESTABLISHED 16512/awesome   
tcp        0      0 192.168.1.6:40263       192.168.1.7:6001        ESTABLISHED 16555/dbus-launch
tcp       64      0 192.168.1.6:40252       192.168.1.7:6001        ESTABLISHED 16232/lightdm   
...

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