(2015-02-04)
Problem
Lightdm only lets me log in as a locally defined user or a "guest" user. I need to log in as NIS or Kerberos or LDAP. (Presuming said authentication method is already set up -- ie I can ssh into the system with network credentials, but can't log into the GUI with them.)
Solution
Create
/etc/lightdm/lightdm.conf as so:
[SeatDefaults]
greeter-show-manual-login=true
greeter-hide-users=true
Reboot, and you should be able to log in using network credentials.
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Source)