[ubuntu] Limit available users on unity-greeter to AD users

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http://ubuntuforums.org – Hi, I am going to deploy ubuntu 12.04 in a secondary school. Students have a shared account depending on their grade (4) which is managed via Windows/AD. The requirement is to allow authentication via four AD accounts and two local accounts. The four AD accounts should be available on the greeter screen, the two local accounts should be available via the 'other' login fields. I know how to connect to AD with likewise-open and I know how to make local accounts as well as the 'other' form available on the greeter. However, I have no idea how to limit the available accounts to the four AD accou (Hardware)