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Deploy AD DS in an Azure virtual network

Microsoft Entra
Azure Virtual Network

This architecture shows how to extend an on-premises Active Directory domain to Azure to provide distributed authentication services.

Architecture

Diagram that shows a secure hybrid network architecture with Active Directory.

Download a Visio file of this architecture.

This architecture extends the hybrid network architecture shown in Connect an on-premises network to Azure using a VPN gateway.

Workflow

  • On-premises network. The on-premises network includes local Active Directory servers that can perform authentication and authorization for components located on-premises.
  • Active Directory servers. These servers are domain controllers implementing directory services (AD DS) running as VMs in the cloud. They can provide authentication of components running in your Azure virtual network.
  • Active Directory subnet. The Active Directory Domain Services (AD DS) servers are hosted in a separate subnet. Network security group (NSG) rules protect the AD DS servers and provide a firewall against traffic from unexpected sources.
  • Azure VPN Gateway and Active Directory synchronization. VPN Gateway provides a connection between the on-premises network and Azure Virtual Network. This connection can be a VPN connection or via Azure ExpressRoute. All synchronization requests between the Active Directory servers in the cloud and on-premises pass through the gateway. User-defined routes (UDRs) handle routing for on-premises traffic that passes to Azure.

Components

  • Microsoft Entra ID is an enterprise identity service that provides single sign-on, multifactor authentication, and conditional access.
  • VPN Gateway is a service that uses a virtual network gateway to send encrypted traffic between an Azure virtual network and on-premises locations over the public internet.
  • ExpressRoute enables you to extend your on-premises networks into the Microsoft cloud over a private connection with the help of a connectivity provider.
  • Virtual Network is the fundamental building block for private networks on Azure. You can use it to enable Azure resources, like virtual machines, to communicate with each other, the internet, and on-premises networks.

Scenario details

If your application is hosted partly on-premises and partly in Azure, replicating AD DS in Azure might be more efficient. This replication can reduce the latency caused by sending authentication requests from the cloud back to AD DS running on-premises.

For more considerations, see Choose a solution for integrating on-premises Active Directory with Azure.

Potential use cases

This architecture is commonly used when a VPN or ExpressRoute connection connects the on-premises and Azure virtual networks. This architecture also supports bidirectional replication, meaning changes can be made either on-premises or in the cloud, and both sources will be kept consistent. Typical uses for this architecture include hybrid applications in which functionality is distributed between on-premises and Azure and applications and services that perform authentication using Active Directory.

Recommendations

The following recommendations apply to most scenarios. Follow these recommendations unless you have a specific requirement that overrides them.

VM recommendations

Determine your VM size requirements based on the expected volume of authentication requests. Use the specifications of the machines hosting AD DS on-premises as a starting point and match them with the Azure VM sizes. Once deployed, monitor utilization and scale up or down based on the actual load on the VMs. For more information about sizing AD DS domain controllers, see Capacity Planning for Active Directory Domain Services.

Create a separate virtual data disk for storing the database, logs, and sysvol folder for Active Directory. Don't store these items on the same disk as the operating system. By default, data disks are attached to a VM using write-through caching. However, this form of caching can conflict with the requirements of AD DS. For this reason, set the Host Cache Preference setting on the data disk to None.

Deploy at least two VMs running AD DS as domain controllers and add them to different availability zones. If not available in the region, deploy in an availability set.

Networking recommendations

Configure the VM network interface (NIC) for each AD DS server with a static private IP address for full domain name service (DNS) support. For more information, see How to set a static private IP address in the Azure portal.

Note

Don't configure the VM NIC for any AD DS with a public IP address. See Security considerations for more details.

The Active Directory subnet NSG requires rules to permit incoming traffic from on-premises and outgoing traffic to on-premises. For detailed information on the ports used by AD DS, see Active Directory and Active Directory Domain Services Port Requirements.

If the new domain controller VMs also have the role of DNS servers, we recommend that you configure them as custom DNS servers at the virtual network level, as explained in Change DNS servers. This should be done for the virtual network hosting the new domain controllers and peered networks where other VMs must resolve Active Directory domain names. For more information on configuring hybrid DNS name resolution, see Name resolution for resources in Azure virtual networks.

For initial configuration, you may need to adjust the Network Interface of one of your Domain Controllers in Azure, to point to a domain controller on-premises as the primary DNS source.

The inclusion of its IP address in the list of DNS servers improves performance and increases the availability of DNS servers. However, a start-up delay can result if the DNS server is also a domain controller and points only to itself or points to itself first for name resolution. For this reason, be cautious when configuring the loopback address on an adapter if the server is also a domain controller.

This may mean overwriting the Network Interface DNS settings in Azure to point towards another Domain Controller hosted in Azure or on-premises for the primary DNS server. The loopback address should be configured only as a secondary or tertiary DNS server on a domain controller.

Active Directory site

In AD DS, a site represents a physical location, network, or collection of devices. AD DS sites are used to manage AD DS database replication by grouping AD DS objects located close to one another and connected by a high-speed network. AD DS includes logic to select the best strategy for replicating the AD DS database between sites.

We recommend creating an AD DS site, including the subnets defined for your application in Azure. Then, you can configure a site link between your on-premises AD DS sites, and AD DS will automatically perform the most efficient database replication possible. This database replication requires little beyond the initial configuration.

Active Directory operations master

The operations master role can be assigned to AD DS domain controllers to support consistency checking between instances of replicated AD DS databases. There are five operations master roles (FSMO): schema master, domain naming master, relative identifier master, primary domain controller master emulator, and infrastructure master. For more information about these roles, see Planning operations master role placement. Giving at least two of the new Azure DCs the Global Catalog (GC) role is also recommended. More details on GC placement can be found here.

Monitoring

Monitor the resources of the domain controller VMs and the AD DS Services and create a plan to correct any problems quickly. For more information, see Monitoring Active Directory. You can also install tools such as Microsoft Systems Center on the monitoring server (see the architecture diagram) to help perform these tasks.

Considerations

These considerations implement the pillars of the Azure Well-Architected Framework, which is a set of guiding tenets that you can use to improve the quality of a workload. For more information, see Microsoft Azure Well-Architected Framework.

Reliability

Reliability ensures that your application can meet your commitments to your customers. For more information, see Overview of the reliability pillar.

Deploy the VMs running AD DS into at least two availability zones. If availability zones aren't available in the region, use availability sets. Also, consider assigning the role of standby operations master to at least one server, and possibly more, depending on your requirements. A standby operations master is an active copy of the operations master that can replace the primary operations master's server during failover.

Security

Security assures against deliberate attacks and the abuse of your valuable data and systems. For more information, see Overview of the security pillar.

AD DS servers provide authentication services and are an attractive target for attacks. To secure them, prevent direct Internet connectivity by placing the AD DS servers in a separate subnet with an NSG as a firewall. Close all ports on the AD DS servers except those necessary for authentication, authorization, and server synchronization. For more information, see Active Directory and Active Directory Domain Services Port Requirements.

Use either BitLocker or Azure disk encryption to encrypt the disk hosting the AD DS database.

Azure DDoS Protection, combined with application-design best practices, provides enhanced DDoS mitigation features to provide more defense against DDoS attacks. You should enable Azure DDOS Protection on any perimeter virtual network.

Operational excellence

Operational excellence covers the operations processes that deploy and keep an application running in production. For more information, see Overview of the operational excellence pillar.

  • Use Infrastructure as Code (IaC) practice to provision and configure the network and security infrastructure. One option is Azure Resource Manager templates.

  • Isolate workloads to enable DevOps to do continuous integration and continuous delivery (CI/CD) because every workload is associated and managed by its corresponding DevOps team.

In this architecture, the entire virtual network that includes the different application tiers, management jump box, and Microsoft Entra Domain Services is identified as a single isolated workload.

Virtual machines are configured by using Virtual Machine Extensions and other tools, such as Desired State Configuration (DSC), used to configure AD DS on the virtual machines.

  • Consider automating your deployments using Azure DevOps or any other CI/CD solutions. Azure Pipelines is the recommended component of Azure DevOps Services that brings automation for solution builds and deployments and's highly integrated into the Azure ecosystem.

  • Use Azure Monitor to analyze the performance of your infrastructure. It also allows you to monitor and diagnose networking issues without logging into your virtual machines. Application Insights provides rich metrics and logs to verify the state of your infrastructure.

For more information, see the DevOps section in Microsoft Azure Well-Architected Framework.

Manageability

Perform regular AD DS backups. Don't copy the VHD files of domain controllers instead of performing regular backups because the AD DS database file on the VHD might not be consistent when copied, making it impossible to restart the database.

We don't recommend that you shut down a domain controller VM using the Azure portal. Instead, shut down and restart the guest operating system. Shutting down through the Azure portal causes the VM to be deallocated, which results in the following effects when the domain controller VM is restarted:

  1. Resets the VM-GenerationID and the invocationID of the Active Directory repository
  2. Discards the current Active Directory relative identifier (RID) pool
  3. Marks the sysvol folder as nonauthoritative

The first issue is relatively benign. Repeated resetting of the invocationID will cause minor additional bandwidth usage during replication, but this is usually not significant.

The second issue can contribute to RID pool exhaustion in the domain, especially if the RID pool size has been configured to be larger than the default. Consider that if the domain has been around for a long time or is used for workflows requiring repetitive creation and deletion of accounts, the domain might already be nearing RID pool exhaustion. Monitoring the domain for RID pool exhaustion warning events is good practice – see the Managing RID Issuance article.

The third issue is relatively benign as long as an authoritative domain controller is available when a domain controller VM in Azure is restarted. If all domain controllers in a domain are running in Azure, and they are all simultaneously shut down and deallocated, on a restart, each domain controller will fail to find an authoritative replica. Fixing this condition requires manual intervention – see the How to force authoritative and non-authoritative synchronization for DFSR-replicated sysvol replication article.

Performance efficiency

Performance efficiency is the ability of your workload to scale to meet the demands placed on it by users efficiently. For more information, see Performance efficiency pillar overview.

AD DS is designed for scalability. You don't need to configure a load balancer or traffic controller to direct requests to AD DS domain controllers. The only scalability consideration is configuring the VMs running AD DS with the correct size for your network load requirements, monitoring the load on the VMs, and scaling up or down as necessary.

Cost optimization

Cost optimization is about reducing unnecessary expenses and improving operational efficiencies. For more information, see Overview of the cost optimization pillar.

Use the Azure pricing calculator to estimate costs. Other considerations are described in the Cost section in Microsoft Azure Well-Architected Framework.

Here are cost considerations for the services used in this architecture.

AD Domain Services

Consider having Active Directory Domain Services as a shared service consumed by multiple workloads to lower costs. For more information, see Active Directory Domain Services pricing.

VPN Gateway

The main component of this architecture is the VPN gateway service. You are charged based on the time the gateway is provisioned and available.

All inbound traffic is free, and all outbound traffic is charged. Internet bandwidth costs are applied to VPN outbound traffic.

For more information, see VPN Gateway pricing.

Virtual Network

Virtual Network is free. Every subscription is allowed to create up to 50 virtual networks across all regions. All traffic within a virtual network's boundaries is free, so communication between two VMs in the same virtual network is free.

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