Comment 3 for bug 1030328

At least for me, /etc/default/lxdm.conf is not a file, it is a symlink, pointing to /etc/alternatives/lxdm.conf -- which is itself a symlink that points to /etc/xdg/lubuntu/lxdm/lxdm.conf

So there is actually only one such configuration file. We're just using the alternatives system to manage it.

Checking which package creates this file may help understanding its purpose and origin:

  dpkg -S /etc/xdg/lubuntu/lxdm/lxdm.conf

outputs

  lubuntu-default-settings: /etc/xdg/lubuntu/lxdm/lxdm.conf

My suspicion is that leaving this file in that package could help ensure that users who decide to revert back to lxdm instead of lightdm, for whatever reason, get a default configuration that works well for Lubuntu. It is also possible this one configuration file is a leftover from when Lubuntu used lxdm by default and can now be removed.

Based on the value of XDG_CONFIG_DIRS, which for me on Lubuntu 12.04 is

  /etc/xdg/lubuntu/:/etc/xdg-Lubuntu:/etc/xdg

the file is in the right place for an Lubuntu-specific lxdm configuration file.