# apt-get install gdm
# dpkg-reconfigure lightdm
# cat /etc/X11/default-display-manager
# /etc/init.d/lightdm stop
# /etc/init.d/lightdm start
toSwitchback
----------------------------
# dpkg-reconfigure lightdm
# cat /etc/X11/default-display-manager
# /etc/init.d/gdm stop
# /etc/init.d/lightdm start