A. Validate that the Compute Engine instances are using the Metadata Service IP address as their resolver. Configure an outbound forwarding zone for the on-premises domain pointing to the on-premises DNS server. Configure Cloud Router to advertise the Cloud DNS proxy range to the on-premises network.

B. Validate that there is network connectivity to the on-premises environment and that the Compute Engine instances can reach other on-premises resources. If errors persist, remove the VPC Network Peerings and recreate the peerings after validating the routes.

C. Review the existing Cloud DNS zones, and validate that there is a route in the VPC directing traffic destined to the IP address of the DNS servers. Recreate the existing DNS forwarding zones to forward all queries to the on-premises DNS servers.

D. Ensure that the operating systems of the Compute Engine instances are configured to send DNS queries to the on-premises DNS servers directly.