The error "DNS server is not available" indicates a network connectivity failure between the Azure NetApp Files (ANF) service and the configured DNS server. In Azure Virtual Networks (VNets), Network Security Groups (NSGs) function as stateful firewalls. If an NSG is applied to the subnet containing the DNS server, it might have rules that block inbound DNS queries (typically on UDP/TCP port 53) originating from the ANF delegated subnet. The administrator must verify that the NSG rules explicitly allow this traffic to establish connectivity and resolve the error.

B. Make sure that the Volume and DNS server are in the same subnet:

This is incorrect because Azure NetApp Files requires a dedicated, delegated subnet where no other resources (including DNS server VMs) can be deployed.

C. Create a VNet service endpoint to the DNS server:

VNet service endpoints are used to secure and direct traffic to specific Azure PaaS services (e.g., Azure Storage), not for enabling connectivity to a VM-based DNS server.

D. Configure subnet delegation for the VNet:

Subnet delegation is a prerequisite for creating an ANF capacity pool and volumes. This action would have already been completed; it does not resolve a runtime network connectivity issue like a blocked port.

1. Microsoft Docs, "Guidelines for Azure NetApp Files network planning":

Under the "Considerations" section, it states: "Although you can't apply NSGs directly to the delegated subnet, you can apply an NSG to the VNet that contains the delegated subnet... You need to ensure that the necessary ports are open for Azure NetApp Files to function correctly." It further clarifies that firewalls or NSGs must not block traffic to domain controllers, which includes DNS services. This directly supports checking NSGs for connectivity issues.

2. Microsoft Docs, "Requirements and considerations for Active Directory Domain Services site design and planning for Azure NetApp Files":

In the "Network topology and configuration" section, it emphasizes the need for proper network connectivity. It states, "Firewalls or Network Security Groups (NSGs) can affect communication... Ensure that your firewalls or NSGs are not blocking traffic to and from the AD DS domain controllers and DNS servers." This confirms that NSGs are a primary checkpoint for DNS connectivity problems.