Azure Active Directory Domain Services (AD DS) provides managed domain services such as domain join, group policy, lightweight directory access protocol (LDAP), and Kerberos / NTLM authentication. You use these domain services without the need to deploy, manage, and patch domain controllers (DCs) in the cloud.
An Azure AD DS managed domain lets you run legacy applications in the cloud that can't use modern authentication methods, or where you don't want directory lookups to always go back to an on-premises AD DS environment. You can lift and shift those legacy applications from your on-premises environment into a managed domain, without needing to manage the AD DS environment in the cloud.
Azure AD DS integrates with your existing Azure AD tenant. This integration lets users sign in to service and applications connected to the managed domain using their existing credentials. You can also use existing groups and user accounts to secure access to resources. These features provide a smoother lift-and-shift of on-premises resources to Azure.
How does Azure AD DS work?
When you create an Azure AD DS managed domain, you define a unique namespace. This namespace is the domain name, such as aaddscontoso.com. Two Windows Server domain controllers (DCs) are then deployed into your selected Azure region. This deployment of DCs is known as a replica set.
You don't need to manage, configure, or update these DCs. The Azure platform handles the DCs as part of the managed domain, including backups.
A managed domain is configured to perform a one-way synchronization from Azure AD to provide access to a central set of users, groups, and credentials. You can create resources directly in the managed domain, but they aren't synchronized back to Azure AD.
Applications, services, and VMs in Azure that connect to the managed domain can then use common AD DS features such as domain join, group policy, LDAP, and Kerberos / NTLM authentication.
In a hybrid environment with an on-premises AD DS environment, Azure AD Connect synchronizes identity information with Azure AD, which is then synchronized to the managed domain.
Azure AD DS replicates identity information from Azure AD, so it works with Azure AD tenants that are cloud-only, or synchronized with an on-premises (AD DS environment. The same set of Azure AD DS features exists for both environments.
If you have an existing on-premises AD DS environment, you can synchronize user account information to provide a consistent identity for users. To learn more, see How objects and credentials are synchronized in a managed domain.
For cloud-only environments, you don't need a traditional on-premises AD DS environment to use the centralized identity services of Azure AD DS.
You can expand a managed domain to have more than one replica set per Azure AD tenant. Replica sets can be added to any peered virtual network in any Azure region that supports Azure AD DS. Additional replica sets in different Azure regions provide geographical disaster recovery for legacy applications if an Azure region goes offline. Replica sets are currently in preview.