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Overview of Azure Files identity-based authentication options for SMB access

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This article explains how Azure file shares can use domain services, either on-premises or in Azure, to support identity-based access to Azure file shares over SMB. Enabling identity-based access for your Azure file shares allows you to replace existing file servers with Azure file shares without replacing your existing directory service, maintaining seamless user access to shares.

Applies to

File share type SMB NFS
Standard file shares (GPv2), LRS/ZRS Yes No
Standard file shares (GPv2), GRS/GZRS Yes No
Premium file shares (FileStorage), LRS/ZRS Yes No

Glossary

It's helpful to understand some key terms relating to identity-based authentication for Azure file shares:

  • Kerberos authentication

    Kerberos is an authentication protocol that's used to verify the identity of a user or host. For more information on Kerberos, see Kerberos Authentication Overview.

  • Server Message Block (SMB) protocol

    SMB is an industry-standard network file-sharing protocol. For more information on SMB, see Microsoft SMB Protocol and CIFS Protocol Overview.

  • Azure Active Directory (Azure AD)

    Azure AD is Microsoft's multi-tenant cloud-based directory and identity management service. Azure AD combines core directory services, application access management, and identity protection into a single solution.

  • Azure Active Directory Domain Services (Azure AD DS)

    Azure AD DS provides managed domain services such as domain join, group policies, LDAP, and Kerberos/NTLM authentication. These services are fully compatible with Active Directory Domain Services. For more information, see Azure Active Directory Domain Services.

  • On-premises Active Directory Domain Services (AD DS)

    On-premises Active Directory Domain Services (AD DS) integration with Azure Files provides the methods for storing directory data while making it available to network users and administrators. Security is integrated with AD DS through logon authentication and access control to objects in the directory. With a single network logon, administrators can manage directory data and organization throughout their network, and authorized network users can access resources anywhere on the network. AD DS is commonly adopted by enterprises in on-premises environments or on cloud-hosted VMs, and AD DS credentials are used for access control. For more information, see Active Directory Domain Services Overview.

  • Azure role-based access control (Azure RBAC)

    Azure RBAC enables fine-grained access management for Azure. Using Azure RBAC, you can manage access to resources by granting users the fewest permissions needed to perform their jobs. For more information, see What is Azure role-based access control?

  • Hybrid identities

    Hybrid user identities are identities in AD DS that are synced to Azure AD using either the on-premises Azure AD Connect sync application or Azure AD Connect cloud sync, a lightweight agent that can be installed from the Azure Active Directory Admin Center.

Supported authentication scenarios

Azure Files supports identity-based authentication over SMB through the following methods. You can only use one method per storage account.

  • On-premises AD DS authentication: On-premises AD DS-joined or Azure AD DS-joined Windows machines can access Azure file shares with on-premises Active Directory credentials that are synched to Azure AD over SMB. Your client must have line of sight to your AD DS. If you already have AD DS set up on-premises or on a VM in Azure where your devices are domain-joined to your AD, you should use AD DS for Azure file shares authentication.
  • Azure AD DS authentication: Cloud-based, Azure AD DS-joined Windows VMs can access Azure file shares with Azure AD credentials. In this solution, Azure AD runs a traditional Windows Server AD domain on behalf of the customer, which is a child of the customer’s Azure AD tenant.
  • Azure AD Kerberos for hybrid identities: Using Azure AD for authenticating hybrid user identities allows Azure AD users to access Azure file shares using Kerberos authentication. This means your end users can access Azure file shares over the internet without requiring a line-of-sight to domain controllers from hybrid Azure AD-joined and Azure AD-joined VMs. Cloud-only identities aren't currently supported.
  • AD Kerberos authentication for Linux clients: Linux clients can use Kerberos authentication over SMB for Azure Files using on-premises AD DS or Azure AD DS.

Restrictions

  • None of the authentication methods support assigning share-level permissions to computer accounts (machine accounts) using Azure RBAC, because computer accounts can't be synced to an identity in Azure AD. If you want to allow a computer account to access Azure file shares using identity-based authentication, use a default share-level permission or consider using a service logon account instead.
  • Neither on-premises AD DS authentication nor Azure AD DS authentication is supported against Azure AD-joined devices or Azure AD-registered devices.
  • Identity-based authentication isn't supported with Network File System (NFS) shares.

Common use cases

Identity-based authentication with Azure Files can be useful in a variety of scenarios:

Replace on-premises file servers

Deprecating and replacing scattered on-premises file servers is a common problem that every enterprise encounters in their IT modernization journey. Azure file shares with on-premises AD DS authentication is the best fit here, when you can migrate the data to Azure Files. A complete migration will allow you to take advantage of the high availability and scalability benefits while also minimizing the client-side changes. It provides a seamless migration experience to end users, so they can continue to access their data with the same credentials using their existing domain-joined machines.

Lift and shift applications to Azure

When you lift and shift applications to the cloud, you want to keep the same authentication model for your data. As we extend the identity-based access control experience to Azure file shares, it eliminates the need to change your application to modern auth methods and expedite cloud adoption. Azure file shares provide the option to integrate with either Azure AD DS or on-premises AD DS for authentication. If your plan is to be 100% cloud native and minimize the efforts managing cloud infrastructures, Azure AD DS might be a better fit as a fully managed domain service. If you need full compatibility with AD DS capabilities, you might want to consider extending your AD DS environment to cloud by self-hosting domain controllers on VMs. Either way, we provide the flexibility to choose the domain service that best suits your business needs.

Backup and disaster recovery (DR)

If you're keeping your primary file storage on-premises, Azure file shares can serve as an ideal storage for backup or DR, to improve business continuity. You can use Azure file shares to back up your data from existing file servers while preserving Windows discretionary access control lists (DACLs). For DR scenarios, you can configure an authentication option to support proper access control enforcement at failover.

Advantages of identity-based authentication

Identity-based authentication for Azure Files offers several benefits over using Shared Key authentication:

  • Extend the traditional identity-based file share access experience to the cloud
    If you plan to lift and shift your application to the cloud, replacing traditional file servers with Azure file shares, then you might want your application to authenticate with either on-premises AD DS or Azure AD DS credentials to access file data. Azure Files supports using either on-premises AD DS or Azure AD DS credentials to access Azure file shares over SMB from either on-premises AD DS or Azure AD DS domain-joined VMs.

  • Enforce granular access control on Azure file shares
    You can grant permissions to a specific identity at the share, directory, or file level. For example, suppose that you have several teams using a single Azure file share for project collaboration. You can grant all teams access to non-sensitive directories, while limiting access to directories containing sensitive financial data to your finance team only.

  • Back up Windows ACLs (also known as NTFS permissions) along with your data
    You can use Azure file shares to back up your existing on-premises file shares. Azure Files preserves your ACLs along with your data when you back up a file share to Azure file shares over SMB.

How it works

Azure file shares use the Kerberos protocol to authenticate with an AD source. When an identity associated with a user or application running on a client attempts to access data in Azure file shares, the request is sent to the AD source to authenticate the identity. If authentication is successful, it returns a Kerberos token. The client sends a request that includes the Kerberos token, and Azure file shares use that token to authorize the request. Azure file shares only receive the Kerberos token, not the user's access credentials.

You can enable identity-based authentication on your new and existing storage accounts using one of three AD sources: AD DS, Azure AD DS, or Azure AD Kerberos (hybrid identities only). Only one AD source can be used for file access authentication on the storage account, which applies to all file shares in the account. Before you can enable identity-based authentication on your storage account, you must first set up your domain environment.

AD DS

For on-premises AD DS authentication, you must set up your AD domain controllers and domain-join your machines or VMs. You can host your domain controllers on Azure VMs or on-premises. Either way, your domain-joined clients must have line of sight to the domain controller, so they must be within the corporate network or virtual network (VNET) of your domain service.

The following diagram depicts on-premises AD DS authentication to Azure file shares over SMB. The on-premises AD DS must be synced to Azure AD using Azure AD Connect sync or Azure AD Connect cloud sync. Only hybrid user identities that exist in both on-premises AD DS and Azure AD can be authenticated and authorized for Azure file share access. This is because the share-level permission is configured against the identity represented in Azure AD, whereas the directory/file-level permission is enforced with that in AD DS. Make sure that you configure the permissions correctly against the same hybrid user.

Diagram that depicts on-premises AD DS authentication to Azure file shares over SMB.

To learn how to enable AD DS authentication, first read Overview - on-premises Active Directory Domain Services authentication over SMB for Azure file shares and then see Enable AD DS authentication for Azure file shares.

Azure AD DS

For Azure AD DS authentication, you should enable Azure AD DS and domain-join the VMs you plan to access file data from. Your domain-joined VM must reside in the same virtual network (VNET) as your Azure AD DS.

The following diagram represents the workflow for Azure AD DS authentication to Azure file shares over SMB. It follows a similar pattern to on-premises AD DS authentication, but there are two major differences:

  1. You don't need to create the identity in Azure AD DS to represent the storage account. This is performed by the enablement process in the background.

  2. All users that exist in Azure AD can be authenticated and authorized. The user can be cloud-only or hybrid. The sync from Azure AD to Azure AD DS is managed by the platform without requiring any user configuration. However, the client must be joined to the Azure AD DS hosted domain. It can't be Azure AD joined or registered. Azure AD DS doesn't support non-cloud VMs (i.e. user laptops, workstations, VMs in other clouds, etc.) being domain-joined to the Azure AD DS hosted domain.

Diagram of configuration for Azure AD DS authentication with Azure Files over SMB.

To learn how to enable Azure AD DS authentication, see Enable Azure Active Directory Domain Services authentication on Azure Files.

Azure AD Kerberos for hybrid identities

Enabling and configuring Azure AD for authenticating hybrid user identities allows Azure AD users to access Azure file shares using Kerberos authentication. This configuration uses Azure AD to issue the necessary Kerberos tickets to access the file share with the industry-standard SMB protocol. This means your end users can access Azure file shares over the internet without requiring a line-of-sight to domain controllers from hybrid Azure AD-joined and Azure AD-joined VMs. However, configuring directory and file-level permissions for users and groups requires line-of-sight to the on-premises domain controller.

Important

Azure AD Kerberos authentication only supports hybrid user identities; it doesn't support cloud-only identities. A traditional AD DS deployment is required, and it must be synced to Azure AD using Azure AD Connect sync or Azure AD Connect cloud sync. Clients must be Azure AD-joined or hybrid Azure AD-joined. Azure AD Kerberos isn’t supported on clients joined to Azure AD DS or joined to AD only.

Diagram of configuration for Azure AD Kerberos authentication for hybrid identities over SMB.

To learn how to enable Azure AD Kerberos authentication for hybrid identities, see Enable Azure Active Directory Kerberos authentication for hybrid identities on Azure Files.

You can also use this feature to store FSLogix profiles on Azure file shares for Azure AD-joined VMs. For more information, see Create a profile container with Azure Files and Azure Active Directory.

Access control

Azure Files enforces authorization on user access to both the share level and the directory/file levels. Share-level permission assignment can be performed on Azure AD users or groups managed through Azure RBAC. With Azure RBAC, the credentials you use for file access should be available or synced to Azure AD. You can assign Azure built-in roles like Storage File Data SMB Share Reader to users or groups in Azure AD to grant access to an Azure file share.

At the directory/file level, Azure Files supports preserving, inheriting, and enforcing Windows ACLs just like any Windows file server. You can choose to keep Windows ACLs when copying data over SMB between your existing file share and your Azure file shares. Whether you plan to enforce authorization or not, you can use Azure file shares to back up ACLs along with your data.

Configure share-level permissions for Azure Files

Once you've enabled an AD source on your storage account, you must do one of the following to access the file share:

  • Set a default share-level permission that applies to all authenticated users and groups
  • Assign built-in Azure RBAC roles to users and groups, or
  • Configure custom roles for Azure AD identities and assign access rights to file shares in your storage account.

The assigned share-level permission allows the granted identity to get access to the share only, nothing else, not even the root directory. You still need to separately configure directory and file-level permissions.

Configure directory or file-level permissions for Azure Files

Azure file shares enforce standard Windows ACLs at both the directory and file level, including the root directory. Configuration of directory or file-level permissions is supported over both SMB and REST. Mount the target file share from your VM and configure permissions using Windows File Explorer, Windows icacls, or the Set-ACL command.

Use the storage account key for superuser permissions

A user with the storage account key can access Azure file shares with superuser permissions. Superuser permissions bypass all access control restrictions.

Important

Our recommended security best practice is to avoid sharing your storage account keys and leverage identity-based authentication whenever possible.

Preserve directory and file ACLs when importing data to Azure file shares

Azure Files supports preserving directory or file level ACLs when copying data to Azure file shares. You can copy ACLs on a directory or file to Azure file shares using either Azure File Sync or common file movement toolsets. For example, you can use robocopy with the /copy:s flag to copy data as well as ACLs to an Azure file share. ACLs are preserved by default, so you don't need to enable identity-based authentication on your storage account to preserve ACLs.

Pricing

There's no additional service charge to enable identity-based authentication over SMB on your storage account. For more information on pricing, see Azure Files pricing and Azure AD Domain Services pricing.

Next steps

For more information about Azure Files and identity-based authentication over SMB, see these resources: