So one of our cloud providers offers a lower-latency, zero-rated "Internal Networking" option between VMs. The interfaces on the internal network are in the 10.x.x.x IP Space. When one of our servers queries the hostname of another server on the internal network, we'd like it to resolve to the address on the internal network rather than the public routable address which incurs transfer accounting.
We've implemented a small server that does exactly this, when the source IP is a 10.x.x.x IP, then it'll return the internal address.
What we don't want to do is have our internal server be the sole primary DNS server for our VMs. We would have it be the first server on the client list, followed by a public dns service like 1.1.1.1.
So if the internal DNS is queried for google.com (which we don't own), what is the proper response we should return to the client? Should we simply ignore the request or should we return an NxDomain response?
Thank you for any advice!