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Does anyone know of a way to [effectively] have a user or group in one forest acquire the privileges of the Domain Admin group in another forest?

The obvious approach of adding Domain Admins@OneForest into Domain Admins@OtherForest isn't an option, because the Domain Admins group is a Global group (and thus, can't have members from other forests, due to the scope of Global groups).

You can add the Domain Admins@OneForest into a domain-local group in OtherForest, but then you can't add a domain-local group as a member of a Global (or Universal) group, which seems to result in something of dead-end using this approach to the problem.

I came across something of a partial workaround (that having typed out, I'll put as an answer to unclutter the question), the problem being that it provides Administrative rights on domain computers, but not the domain itself - for example, it doesn't allow the cross-forest account to edit GPOs.

The other approach I've considered, and had basically no luck in researching is to replicate/clone/duplicate the Domain Admins group (but as a domain-local group, so it can accept members from another domain), but I can't seem to locate a resource on what permissions this cloned group would need, and to what resources. Seeing as how it's not a trivial task to determine what permissions a given Active Directory group has, I was hoping there would be some Microsoft documentation on what permissions the built in and default groups have, but all I can find is the descriptions of their permissions, which do me no good in trying to configure another group to match.

Long question short, does anyone know how to apply domain admin privileges in one forest to an account from another forest?

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What I've discovered, that I'm hoping someone else can beat (by having these rights applies to existing objects) is:

  1. Establish proper DNS communication between the two forests.
    • In my case, this required a DNS delegation zone and properly configured conditional forwarders.
  2. Create a two-way, forest trust with forest-wide authentication.
  3. Add the Domain Admins@OneForest group to the Builtin\Adminstrators@OtherForest group.
    • This effectively grants user-level privileges on OtherForest domain computers, and administrative privileges on the domain controllers for OtherForest.
  4. Create a domain-local group in OtherForest and add the Domain Admins@OneForest group to it as members.
  5. Create a GPO/GPP to add the group created in step 4 to the local administrators group on all your domain computers.
    • Image of the above GPP
  • So, it turns out I'm retarded. This is the way to create cross-forest Domain Admins, but permissions aren't "retroactive" - so many of the default objects in my forest didn't allow this groups full control, but new ones do. Live and learn. – HopelessN00b May 24 '14 at 1:24
  • STEP 4 cannot be done as in the process of adding the ONEFOREST group the AD only sees the OTHERFOREST Domain. – JukEboX Jan 16 '18 at 16:07

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