I have two Active Directory domains in two different forests; each domain has two DCs (all of them Windows Server 2008 R2). The domains are also in different networks, with a firewall connecting them.

I need to create a two-way forest trust between the two domains and forest.

How do I configure the firewall to allow this?

I found this article, but it doesn't explain very clearly which traffic is required between DCs, and which traffic (if any) in needed instead between domain computers in one domain and DCs for the other one.

I'm allowed to permit all traffic between the DCs, but allowing computers in one network to access DCs in the other one would be a little more difficult.