This architecture shows how to extend an on-premises Active Directory domain to Azure to provide distributed authentication services.
Download a Visio file of this architecture.
If your application is hosted partly on-premises and partly in Azure, it may be more efficient to replicate Active Directory Domain Services (AD DS) in Azure. This can reduce the latency caused by sending authentication requests from the cloud back to AD DS running on-premises.
This architecture is commonly used when the on-premises network and the Azure virtual network are connected by a VPN or ExpressRoute connection. 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.
For additional considerations, see Choose a solution for integrating on-premises Active Directory with Azure.
This architecture extends the hybrid network architecture shown in Connect an on-premises network to Azure using a VPN gateway. It has the following components.
- 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 are domain controllers implementing directory services (AD DS) running as VMs in the cloud. These servers can provide authentication of components running in your Azure virtual network.
- Active Directory subnet. The 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 Gateway and Active Directory synchronization. The Azure gateway provides a connection between the on-premises network and the Azure VNet. This can be a VPN connection or 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.
The following recommendations apply for most scenarios. Follow these recommendations unless you have a specific requirement that overrides them.
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. Do not store these items on the same disk as the operating system. By default, data disks that are attached to a VM use 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 an availability set.
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.
Do not 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.
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 together AD DS objects that are located close to one another and are 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 that you create an AD DS site including the subnets defined for your application in Azure. Then, 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 masters
The operations masters 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: 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.
We recommend you do not assign operations masters roles to the domain controllers deployed in Azure.
Monitor the resources of the domain controller VMs as well as the AD DS Services and create a plan to quickly correct any problems. 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.
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 to configure the VMs running AD DS with the correct size for your network load requirements, monitor the load on the VMs, and scale up or down as necessary.
Deploy the VMs running AD DS into an availability set. 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 be used in place of the primary operations masters server during failover.
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 may not be in a consistent state when it's copied, making it impossible to restart the database.
It is not recommended to shut down a domain controller VM using the Azure portal. Instead, shut down and restart from 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:
- Resets the
invocationIDof the Active Directory repository.
- Discards the current Active Directory relative identifier (RID) pool
- 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 very long time, or is used for workflows requiring repetitive creation and deletion of accounts, the domain may already be nearing RID pool exhaustion. It is a good practice to monitor the domain for RID pool exhaustion warning events – 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 shutdown and deallocated, on restart each DC 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.
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 acting 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.
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 jumpbox, and Azure AD 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 ADDS on the virtual machines.
Consider using Azure DevOps or any other CI/CD solutions to automate your deployments. Azure Pipelines is the recommended component of Azure DevOps Services that brings automation for solution builds and deployments, it's also highly integrated in 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.
Here are cost considerations for the services used in this architecture.
AD Domain Services
Consider having Active Directory Domain Services as a shared service that is consumed by multiple workloads to lower costs. For more information, see Active Directory Domain Services pricing.
Azure VPN Gateway
The main component of this architecture is the VPN gateway service. You are charged based on the amount of time that the gateway is provisioned and available.
All inbound traffic is free, all outbound traffic is charged. Internet bandwidth costs are applied to VPN outbound traffic.
For more information, see VPN Gateway Pricing.
Azure Virtual Network
Azure Virtual Network is free. Every subscription is allowed to create up to 50 virtual networks across all regions. All traffic that occurs within the boundaries of a virtual network is free. So, communication between two VMs in the same virtual network is free.
- Learn the best practices for creating an AD DS resource forest in Azure.
- Learn the best practices for creating an Active Directory Federation Services (AD FS) infrastructure in Azure.