Hello Agent 1994,
Thanks again very much for the help. I've got around to setting up the FGT VM on a VMWare free trial. I've spent a fair amount of time trying to get it up on VirtualBox and gotten to the point where the VM is loaded, but it doesn't seem that the kernel will boot for VirtualBox probably due to differing virtualization protocols implemented. In proceeding forwards, I've configured a simple virtual environment of one domain controller and one client connected to the FGT VM.
I've used 2 port interfaces (Port 1 = external/internet, Port 2 = internal), one for internet/external access and the second for internal access by the VM client/server I've attached. The server forwards it's DNS requests to the FGT VM, which is then configured with it standard FGT DNS servers and a policy to route external traffic through Port 1. I've tested the setup without specifying the Internal DNS to my domain controller as per my production setup and the domain lookup checks out.
When I do introduce a virtual server with a single real server as my domain controller however, I've noticed that pings from my clients failed to ping/see the virtual server, but I can ping it just fine from the FGT device. My attempts to resolve the internal domain from the client using the virtual server DNS have all failed hence, I'm wasn't sure if there are any firewall rules/policies I would need to configure for the virtual server to be accessible from the internal interface.
After a little tinkering in consultation to the manual and sacrifices of coffee to the networking gods, I was able to get it to work by adding a firewall policy which allowed internal DNS traffic to go through to the virtual server. From here, query lookups seemed to work fine unless I had disabled the real server within the Load Balancing settings.
All pings from the client to the virtual server however still fails probably due to the virtual server being set not to respond. I'm not entirely sure whether this is the right approach but I hope this also helps anyone who's in the same shoes as I am. The challenge now is to somehow allow inter-site queries of the DNS servers through the virtual server. I'm not able to for some reason get the FGT devices to ping inter-site clients yet. If anyone else has any additional input or advice on this it'd be greatly appreciated.