I would like to run OSD and domain join in a Untrusted Forest/Domain

I would like to run OSD and domain join in a Untrusted Forest/Domain. There is no site system in the Untrusted Forest/Domain. So is it enough to publish site resources in the Untrusted DNS? 
Or do I need to extend the Active Directory?

Is it also possible to use discovery Methodes if you not have extended the Active Directory?