light shows WARNING: Ignoring packet of wrong length when queried via XDMCP from gdm

[Sebastian Muniz]

I have installed lightdm on an armhf device.
Package: lightdm
State: installed
Automatically installed: no
Version: 1.2.2-3
Priority: optional
Section: x11
Maintainer: Debian Xfce Maintainers <email address hidden>
Architecture: armhf
Uncompressed Size: 1007 k
Depends: libc6 (>= 2.13-28), libglib2.0-0 (>= 2.28.0), libpam0g (>= 0.99.7.1),
         libxcb1, libxdmcp6, debconf (>= 0.5) | debconf-2.0, lightdm-gtk-greeter
         | lightdm-qt-greeter, dbus, consolekit, adduser
Recommends: xserver-xorg
Suggests: accountsservice
Provides: x-display-manager

Enabled xdmcp and vnc.

Using my laptop I try to connect from the gdm greeter and on the armhf device I see
root@mpg:~# tail -f /var/log/lightdm/lightdm.log
[+1836.11s] WARNING: Ignoring packet of wrong length
[+1836.65s] WARNING: Ignoring packet of wrong length
[+1836.65s] WARNING: Ignoring packet of wrong length
[+1865.21s] WARNING: Ignoring packet of wrong length
[+1865.22s] WARNING: Ignoring packet of wrong length
[+1867.21s] WARNING: Ignoring packet of wrong length
[+1867.22s] WARNING: Ignoring packet of wrong length
[+1869.21s] WARNING: Ignoring packet of wrong length
[+1869.22s] WARNING: Ignoring packet of wrong length
[+1878.29s] WARNING: Ignoring packet of wrong length

I can, however, connect using Xorg -query hostname.

Thanks
Sebastian

[Ivan Baldo (ibaldo)]

I can confirm that lightdm in Ubuntu 12.04 AMD64 produces that warning when GDM Chooser running in an Ubuntu 10.04 i386 sends a broadcast to search for XDMCP servers.
The only XDMCP servers appearing are the GDM ones from Ubuntu 10.04.
Yep, I have installed manually GDM on those old Ubuntu's, guess I will have to do the same with 12.04...
Once upon a time (8.04 I think), a simple user could configure GDM to allow XDMCP and another user could simply use the option to see what servers where on the network, no creepy text command terminal involved... those where good times indeed!
Using old computers for dumb terminals was a piece of cake, really easy to set up.
Now we are stuck with unfinished programs like this!
How one is supposed to setup an Ubuntu server to work as an application server and other terminals to just connect to that server?
And what happens if one has three application servers and wants the users to choose one freely?
The Linux Terminal Server Project is nice but I think it is a bit too complicated for small businesses, specially if you don't want to bother with the network configuration (other DHCP servers on the net, etc.).
So yes, I am disappointed and I needed to express openly my feelings.
Don't take this comment too negatively, it isn't, only just a bit ;-).

[Robert Ancell (robert-ancell)]

I tried connecting to an armh device running LightDM from Remmina using XDMCP and it worked fine. It appears there's no way to set up recent version of GDM to be and XDMCP chooser so I can't reproduce your case. I added some more logging to LightDM to report why it thinks the XDMCP packages aren't valid (and what packets they are).

My guess is they are XDMCP Query packets that either have additional data on the end or more than one packet joined together. As I read the spec these should not be allowed. If you can get a packet capture you may have more luck.

I think GDM upstream no-longer supports this case so if there is a problem a fix may not be used.