Diving Deeper into Azure Private DNS Zones

Introduction

A DNS service offers a platform to manage various DNS records, such as A, CNAME, MX, etc. Azure DNS, part of the Azure Cloud, is a cloud-based DNS service that enables the management of DNS records. With Azure DNS, we can create either Public or Private DNS zones. Public DNS Zones manage domain names accessible over the internet, while Private DNS Zones are for use within Azure Virtual Networks (VNets). In the blog post, Understanding Azure DNS, we learned about Azure DNS in general and the Azure Public DNS Zone. This blog post will dive deeper into Azure Private DNS Zones.

What is Azure Private DNS?

Azure Private DNS is a dedicated DNS service tailored for Azure Virtual Networks (VNets). It simplifies managing and resolving domain names within VNets, enabling us to utilize custom domain names in our deployments instead of relying on default Azure-provided names. This facilitates the development of a network architecture specifically configured to meet our organization's unique requirements.

Azure Private DNS service is particularly beneficial for providing name resolution for virtual machines (VMs) and other resources within a VNet and across interconnected VNets. It supports features such as automatic registration of VMs, automatic host record management, forward DNS resolution, and reverse DNS lookup. It even has split-horizon DNS views, where private and public DNS zones share the same name.

Setting Up Azure Private DNS Zones

Setting up Private DNS Zones in Azure can be accomplished in a few key steps:

1. Create a Private DNS Zone: Start by establishing a Private DNS Zone for our internal domain, such as internal.cloudericks.com, for managing our internal DNS records.

2. Configure DNS Record Sets: Like public zones, configure various DNS record sets within your private zone. This can include the record set types A, AAAA, CNAME, MX, PTR, SOA, SRV, and TXT.

3. Link the Private Zone to an Azure VNet: To resolve records in a private DNS zone from a Virtual Network (VNet), it's essential to establish a link between the DNS zone and the VNet. Once linked, the VNet gains full access to all DNS records in the private zone. Additionally, enabling autoregistration on this link can automatically register DNS records for virtual machines within the linked VNet into the private zone. Moreover, Azure DNS actively updates these records in response to events like the VM creation, IP address modification, or VM deletion, ensuring that the DNS information remains current and accurate.

Azure Private DNS Benefits and Capabilities

Azure Private DNS provides many advantages, such as the following:

1. Eliminates the Need for Custom DNS Solutions: Simplifies DNS management by using Azure's native infrastructure, thus reducing complexity and overhead.
2. Familiar Tools and User Experience: Employs well-known Azure DNS tools such as the portal, CLI, etc., ensuring an efficient and user-friendly experience.
3. Support for Common DNS Record Types: Handles various DNS record types like A, AAAA, CNAME, MX, PTR, SOA, SRV, and TXT.
4. Global Availability: Accessible across all Azure public cloud regions, it provides widespread reliability and availability.

Additionally, the effectiveness of Azure Private DNS is further increased by several key capabilities, such as the following:

1. Automatic Registration of Virtual Machines: This feature involves Azure Private DNS automatically creating and removing DNS records as VMs are added to or deleted from the virtual network. Example: Suppose we deploy a new VM named 'Server1' in your Azure VNet. With automatic registration, Azure Private DNS will immediately create a corresponding DNS record, like 'Server1.yourdomain.internal', which maps to the VM's private IP address. If we later delete 'Server1', Azure Private DNS automatically removes its DNS record, keeping our DNS zone current and accurate.

2. Automatic Hostname Record Management: Azure private DNS can automatically update DNS records to reflect changes in VM hostnames or IP addresses within the virtual network. Example: Imagine a VM named 'DataServer' in our VNet, whose DNS record is 'DataServer.yourdomain.internal'. If we change the hostname of this VM to 'BackupServer' or if its IP address changes, Azure Private DNS will automatically update the DNS record. The new record will now point to the correct hostname or IP, such as 'BackupServer.yourdomain.internal', ensuring that DNS queries are always resolved to the right resource.

3. Hostname Resolution Across Virtual Networks: Azure Private DNS facilitates hostname resolution across different Azure Virtual Networks (VNets), regardless of whether they are interconnected via peering or functioning independently. This is crucial for scenarios ranging from simple resource identification and monitoring in unpeered VNets to seamless, direct communication in peered VNets.It supports both forward DNS resolution – allowing resources in one VNet to resolve the DNS names of resources in another – and reverse DNS lookup, which enables verifying the identities of network resources. For an in-depth exploration of hostname resolution across VNets, refer to the blog post: Achieving Cross-VNet DNS Resolution with Azure Private DNS.
4. Split-Horizon DNS Support: Split-Horizon DNS support is a feature that enables a single domain name to resolve to different IP addresses based on the query's origin – within an Azure VNet or the public internet. This capability is helpful for organizations that manage distinct internal and external services under a unified domain. For a comprehensive understanding of Split-Horizon DNS, read the blog post: Demystifying Split-Horizon DNS with Azure Private DNS Zones.

Azure Private DNS Limitations and Other Considerations

While Azure Private DNS offers a range of benefits and capabilities, it's important to be aware of certain limitations, too. Here's a breakdown of these limitations:

1. Linking Virtual Networks to Private Zones:

   • When automatic registration of VM DNS records is enabled, one virtual network can only be linked to one private DNS zone. However, multiple VNets can be linked to a single DNS zone. This means if we need automated DNS record management for VMs in a VNet, that VNet cannot be linked to multiple private zones.

2. Reverse DNS Lookup Limitations and Considerations:

   • Reverse DNS functionality is confined to private IP addresses within the linked virtual network. It doesn't extend to public IP addresses or IPs outside the VNet scope.
   • For reverse DNS lookups, a private IP address in a linked VNet typically returns a default suffix like 'internal.cloudapp.net' for VMs. In VNets linked to a private zone with autoregistration enabled, a reverse lookup may yield two Fully Qualified Domain Names (FQDNs): one with this default suffix and another with the private zone's suffix. Example: Imagine a VM in an Azure VNet, say 'VNet-A', with a private IP address of 10.0.0.4. This VM is not part of any private zone with autoregistration enabled. When we perform a reverse DNS lookup on the IP address 10.0.0.4, Azure Private DNS returns a Fully Qualified Domain Name (FQDN) with the default suffix, such as vm1.internal.cloudapp.net. Now, let's consider a VM in a VNet, 'VNet-B', linked to a private DNS zone with autoregistration enabled, say 'mycompany.local'. Assume this VM also has the private IP address 10.0.1.4. Performing a reverse DNS lookup on 10.0.1.4 in this scenario yields two FQDNs. One is the default Azure suffix, like vm2.internal.cloudapp.net, and the other reflects the private zone's suffix, for example, vm2.mycompany.local.

3. Integration with On-Premises Networks and Conditional Forwarding:

   • While Azure Private DNS is robust within the Azure ecosystem, integrating it with on-premises networks for DNS resolution requires additional setup.
   • The Azure DNS Private Resolver can be used for conditional forwarding between Azure and on-premises environments. Conditional forwarding is a DNS feature that allows us to specify conditions under which DNS queries are forwarded to different DNS servers. In conditional forwarding, if the DNS server receives a query for a specific domain or set of domains, it forwards these queries to a designated DNS server rather than trying to resolve them. In the context of Azure DNS Private Resolver, it is particularly relevant for hybrid environments where we have resources both in Azure and on-premises networks.

Conclusion

Azure Private DNS offers a suite of tools and features that greatly enhance DNS management within Azure VNets. Key highlights like the ability to use custom domain names, automatic registration and management of VM DNS records, and advanced features like Split-Horizon DNS support provide the flexibility and control needed in today's dynamic cloud environments. Whether ensuring seamless communication in peered VNets or enabling efficient resource identification and monitoring in unpeered VNets, Azure Private DNS proves invaluable.

However, it's also crucial to acknowledge the limitations of Azure Private DNS, such as the constraints in linking VNets to private zones and the specificities of reverse DNS lookups. As we continue to navigate the evolving landscape of cloud networking, Azure Private DNS Zones offer a reliable and sophisticated platform for managing our DNS needs. For those looking to delve deeper into the capabilities of Azure Private DNS and explore practical use cases, our detailed blog posts on topics like Cross-VNet DNS Resolution and Split-Horizon DNS provide valuable insights and examples.