The "default-user" option has been removed from lightdm. Why? Nobody knows. It's been five years since this question was originally asked and it's still broken. Lightdm now saves the state of who last logged in in a hidden file here:
So, here's a three step workaround:
Create a shell script called
/usr/local/bin/lightdm-default-user with the following contents:
# LightDM removed the default-user option.
# The only recourse now is an ugly kludge.
# Note that if you want to default to the "Guest Session",
# you need to specify the last user as "*guest".
/bin/echo -e '[greeter]\nlast-user=*guest' > /var/lib/lightdm/.cache/lightdm-gtk-greeter/state
Make it executable:
chmod 755 /usr/local/bin/lightdm-default-user
Have lightdm automatically run the script upon start up by editing
/etc/lightdm/lightdm.conf and adding a line in the
This is horrifically ugly but it is, unfortunately, the best solution at the moment.