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What is Azure DNS Private Resolver?

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Azure DNS Private Resolver is a new service that enables you to query Azure DNS private zones from an on-premises environment and vice versa without deploying VM based DNS servers.

How does it work?

Azure DNS Private Resolver requires an Azure Virtual Network. When you create an Azure DNS Private Resolver inside a virtual network, one or more inbound endpoints are established that can be used as the destination for DNS queries. The resolver's outbound endpoint processes DNS queries based on a DNS forwarding ruleset that you configure. DNS queries that are initiated in networks linked to a ruleset can be sent to other DNS servers.

You don't need to change any DNS client settings on your virtual machines (VMs) to use the Azure DNS Private Resolver.

The DNS query process when using an Azure DNS Private Resolver is summarized below:

  1. A client in a virtual network issues a DNS query.
  2. If the DNS servers for this virtual network are specified as custom, then the query is forwarded to the specified IP addresses.
  3. If Default (Azure-provided) DNS servers are configured in the virtual network, and there are Private DNS zones linked to the same virtual network, these zones are consulted.
  4. If the query doesn't match a Private DNS zone linked to the virtual network, then Virtual network links for DNS forwarding rulesets are consulted.
  5. If no ruleset links are present, then Azure DNS is used to resolve the query.
  6. If ruleset links are present, the DNS forwarding rules are evaluated.
  7. If a suffix match is found, the query is forwarded to the specified address.
  8. If multiple matches are present, the longest suffix is used.
  9. If no match is found, no DNS forwarding occurs and Azure DNS is used to resolve the query.

The architecture for Azure DNS Private Resolver is summarized in the following figure. DNS resolution between Azure virtual networks and on-premises networks requires Azure ExpressRoute or a VPN.

Azure DNS Private Resolver architecture

Figure 1: Azure DNS Private Resolver architecture

For more information about creating a private DNS resolver, see:

Azure DNS Private Resolver benefits

Azure DNS Private Resolver provides the following benefits:

  • Fully managed: Built-in high availability, zone redundancy.
  • Cost reduction: Reduce operating costs and run at a fraction of the price of traditional IaaS solutions.
  • Private access to your Private DNS zones: Conditionally forward to and from on-premises.
  • Scalability: High performance per endpoint.
  • DevOps Friendly: Build your pipelines with Terraform, ARM, or Bicep.

Regional availability

See Azure Products by Region - Azure DNS.

Data residency

Azure DNS Private Resolver doesn't move or store customer data out of the region where the resolver is deployed.

DNS resolver endpoints and rulesets

A summary of resolver endpoints and rulesets is provided in this article. For detailed information about endpoints and rulesets, see Azure DNS Private Resolver endpoints and rulesets.

Inbound endpoints

An inbound endpoint enables name resolution from on-premises or other private locations via an IP address that is part of your private virtual network address space. To resolve your Azure private DNS zone from on-premises, enter the IP address of the inbound endpoint into your on-premises DNS conditional forwarder. The on-premises DNS conditional forwarder must have a network connection to the virtual network.

The inbound endpoint requires a subnet in the VNet where it’s provisioned. The subnet can only be delegated to Microsoft.Network/dnsResolvers and can't be used for other services. DNS queries received by the inbound endpoint ingress to Azure. You can resolve names in scenarios where you have Private DNS zones, including VMs that are using auto registration, or Private Link enabled services.

Note

The IP address assigned to an inbound endpoint can be specified as static or dynamic. For more information, see static and dynamic endpoint IP addresses.

Outbound endpoints

An outbound endpoint enables conditional forwarding name resolution from Azure to on-premises, other cloud providers, or external DNS servers. This endpoint requires a dedicated subnet in the VNet where it’s provisioned, with no other service running in the subnet, and can only be delegated to Microsoft.Network/dnsResolvers. DNS queries sent to the outbound endpoint will egress from Azure.

Virtual network links enable name resolution for virtual networks that are linked to an outbound endpoint with a DNS forwarding ruleset. This is a 1:1 relationship.

DNS forwarding rulesets

A DNS forwarding ruleset is a group of DNS forwarding rules (up to 1000) that can be applied to one or more outbound endpoints, or linked to one or more virtual networks. This is a 1:N relationship. Rulesets are associated with a specific outbound endpoint. For more information, see DNS forwarding rulesets.

DNS forwarding rules

A DNS forwarding rule includes one or more target DNS servers that are used for conditional forwarding, and is represented by:

  • A domain name
  • A target IP address
  • A target Port and Protocol (UDP or TCP)

Restrictions

The following limits currently apply to Azure DNS Private Resolver:

DNS private resolver1

Resource Limit
DNS private resolvers per subscription 15
Inbound endpoints per DNS private resolver 5
Outbound endpoints per DNS private resolver 5
Forwarding rules per DNS forwarding ruleset 1000
Virtual network links per DNS forwarding ruleset 500
Outbound endpoints per DNS forwarding ruleset 2
DNS forwarding rulesets per outbound endpoint 2
Target DNS servers per forwarding rule 6
QPS per endpoint 10,000

1Different limits might be enforced by the Azure portal until the portal is updated. Use PowerShell to provision elements up to the most current limits.

Virtual network restrictions

The following restrictions hold with respect to virtual networks:

  • A DNS resolver can only reference a virtual network in the same region as the DNS resolver.
  • A virtual network can't be shared between multiple DNS resolvers. A single virtual network can only be referenced by a single DNS resolver.

Subnet restrictions

Subnets used for DNS resolver have the following limitations:

  • A subnet must be a minimum of /28 address space or a maximum of /24 address space. A /28 subnet is sufficient to accommodate current endpoint limits. A subnet size of /27 to /24 can provide flexibility if these limits change.
  • A subnet can't be shared between multiple DNS resolver endpoints. A single subnet can only be used by a single DNS resolver endpoint.
  • All IP configurations for a DNS resolver inbound endpoint must reference the same subnet. Spanning multiple subnets in the IP configuration for a single DNS resolver inbound endpoint isn't allowed.
  • The subnet used for a DNS resolver inbound endpoint must be within the virtual network referenced by the parent DNS resolver.
  • The subnet can only be delegated to Microsoft.Network/dnsResolvers and can't be used for other services.

Outbound endpoint restrictions

Outbound endpoints have the following limitations:

  • An outbound endpoint can't be deleted unless the DNS forwarding ruleset and the virtual network links under it are deleted.

Ruleset restrictions

  • Rulesets can have up to 1000 rules.

Other restrictions

  • IPv6 enabled subnets aren't supported.
  • DNS private resolver does not support Azure ExpressRoute FastPath.
  • DNS private resolver inbound endpoint provisioning isn't compatible with Azure Lighthouse.
    • To see if Azure Lighthouse is in use, search for Service providers in the Azure portal and select Service provider offers.

Next steps