To enable on-premises DNS servers to resolve hostnames within an Azure private DNS zone, a DNS forwarder or proxy within the Azure VNet is required. Azure Firewall can fulfill this role. The solution involves two steps:

1. Enable the DNS Proxy feature on the Azure Firewall (FW1). This configures the firewall to listen for DNS queries on its private IP address and forward them to the Azure-provided DNS service, which can resolve records in the linked private zone.

2. Configure the on-premises DNS servers with a conditional forwarder for the contoso.com zone. This forwarder must point to the private IP address of the Azure Firewall (FW1), ensuring that queries for contoso.com are sent to the firewall's DNS proxy for resolution.

B. The IP address 168.63.129.16 is a virtual IP for Azure platform resources and is not routable or reachable directly from an on-premises network.

C. Modifying the VNet's DNS settings affects name resolution for VMs inside the VNet, but it does not establish the required forwarding path from on-premises.

E. Configuring a custom DNS server on the firewall would bypass the Azure-provided DNS needed to resolve the private zone, unless that custom server was also configured to forward to Azure.

1. Microsoft Learn. (2023). Azure Firewall DNS settings. "To use DNS proxy for on-premises networks

configure on-premises DNS servers to forward queries to the firewall's private IP address." and "When you enable DNS Proxy... Azure Firewall... process and forward DNS queries from a Virtual Network(s) to your desired DNS server." This document directly supports enabling the proxy (D) and configuring on-premises forwarders to the firewall's IP (A).

Source: https://learn.microsoft.com/en-us/azure/firewall/dns-settings

2. Microsoft Learn. (2023). Azure Private DNS scenarios. Under the section "On-premises workloads using a DNS forwarder

" the document explains the architectural pattern: "To resolve records from a private DNS zone from on-premises

you need to deploy a DNS forwarder in the virtual network... You can then create a conditional forwarder on your on-premises DNS server that points to the forwarder in Azure." Azure Firewall's DNS proxy feature serves as this forwarder.

Source: https://learn.microsoft.com/en-us/azure/dns/private-dns-scenarios

3. Microsoft Learn. (2023). What is IP address 168.63.129.16?. "The IP address 168.63.129.16 is used in all regions and all national clouds. Microsoft owns this special public IP address... The IP address can't be accessed from outside Azure." This confirms that on-premises servers cannot forward queries directly to this address

invalidating option B.

Source: https://learn.microsoft.com/en-us/azure/virtual-network/what-is-ip-address-168-63-129-16