linux is a part of Amiga now.. you get a linux copywith AmigaONE so.. And Linux is pretty much the only OS it currently run ;-) So i cant see anything wrong with asking linux related questions..
But to reply to the question... i dont think domain controller is very good idea to run under linux.. Though a simple samba fileserver works great. If you need a windows domain controller, you better use WinNT or 2k :\