Confused about packages and dns

I got a domain master, a domain backup and some domain slaves, all with the package Active Directory Domain Controller installed. This works as expected. So I try to expand my network and installed a slave server in my DMZ. As I don’t need Samba there, I left the package Active Directory Domain Controller unticked. But now I have some DNS issues on this machine. I think the join did not work properly.
So I’m a little bit confused, which packages do I need on my servers and why I cannot run this slave server without Samba installed.
Any explanations?