DNS forwarder Port 53 TCP or UDP?

Hi guys,

I have two non-trusted Domains A and B and I want to configure DNS forwarder in Domain A to Domain B, so that DNS queries from Domain A should get forwarded to Domain B if the resource is in Domain B.

I understood that port 53 should be open between two domain's DNS server. My questions are:

1. Will it required Port 53 TCP or UDP?

2. Would that be bidirectional or unidirectional?

3. If unidirectional, which one will be source and which one will be destination? (I support source=Domain A and Destination=Domain B)

Regards,

Ajay K.