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Register a Service Principal Name for Kerberos connections

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Applies to: SQL Server

To use Kerberos authentication with SQL Server, both of the following conditions must be true:

  • The client and server computers must be part of the same Windows domain, or in trusted domains.

  • A Service Principal Name (SPN) must be registered with Active Directory, which assumes the role of the Key Distribution Center in a Windows domain. The SPN, after it's registered, maps to the Windows account that started the SQL Server instance service. If the SPN registration hasn't been performed or fails, the Windows security layer can't determine the account associated with the SPN, and Kerberos authentication isn't used.

    Note

    If the server can't automatically register the SPN, the SPN must be registered manually. See Manual SPN Registration.

You can verify that a connection is using Kerberos by querying the sys.dm_exec_connections dynamic management view. Run the following query and check the value of the auth_scheme column, which is KERBEROS if Kerberos is enabled.

SELECT auth_scheme
FROM sys.dm_exec_connections
WHERE session_id = @@SPID;

Tip

Microsoft Kerberos Configuration Manager for SQL Server is a diagnostic tool that helps troubleshoot Kerberos related connectivity issues with SQL Server. For more information, see Microsoft Kerberos Configuration Manager for SQL Server.

The role of the SPN in authentication

When an application opens a connection and uses Windows Authentication, SQL Server Native Client passes the SQL Server computer name, instance name and, optionally, an SPN. If the connection passes an SPN, it's used without any changes.

If the connection doesn't pass an SPN, a default SPN is constructed based on the protocol used, server name, and the instance name.

In both of the preceding scenarios, the SPN is sent to the Key Distribution Center to obtain a security token for authenticating the connection. If a security token can't be obtained, authentication uses NTLM.

An SPN is the name by which a client uniquely identifies an instance of a service. The Kerberos authentication service can use an SPN to authenticate a service. When a client wants to connect to a service, it locates an instance of the service, composes an SPN for that instance, connects to the service, and presents the SPN for the service to authenticate.

Note

The information in this article also applies to SQL Server configurations that use clustering.

Windows Authentication is the preferred method for users to authenticate to SQL Server. Clients that use Windows Authentication are authenticated by either NTLM or Kerberos. In an Active Directory environment, Kerberos authentication is always attempted first.

Permissions

When the Database Engine service starts, it attempts to register the Service Principal Name (SPN). Suppose the account starting SQL Server doesn't have permission to register an SPN in Active Directory Domain Services. In that case, this call fails, and a warning message is logged in both the Application event log and the SQL Server error log.

To register the SPN, the Database Engine must be running under a built-in account, such as Local System (not recommended), or NETWORK SERVICE, or an account that has permission to register an SPN. You can register an SPN using a domain administrator account, but this isn't recommended in a production environment. You can run SQL Server using a virtual account or a managed service account (MSA). Both virtual accounts and MSAs can register an SPN. If SQL Server isn't running under one of these accounts, the SPN isn't registered at startup, and the domain administrator must register the SPN manually.

SPN formats

The SPN format supports Kerberos authentication on TCP/IP, named pipes, and shared memory. The supported SPN formats for named and default instances are as follows.

Named instance

  • MSSQLSvc/<FQDN>:[<port> | <instancename>], where:

    • MSSQLSvc is the service that is being registered.
    • <FQDN> is the fully qualified domain name of the server.
    • <port> is the TCP port number.
    • <instancename> is the name of the SQL Server instance.

Default instance

  • MSSQLSvc/<FQDN>:<port> | MSSQLSvc/<FQDN>, where:

    • MSSQLSvc is the service that is being registered.
    • <FQDN> is the fully qualified domain name of the server.
    • <port> is the TCP port number.
SPN format Description
MSSQLSvc/<FQDN>:<port> 1 The provider-generated, default SPN when TCP is used. <port> is a TCP port number.
MSSQLSvc/<FQDN> The provider-generated, default SPN for a default instance when a protocol other than TCP is used. <FQDN> is a fully qualified domain name.
MSSQLSvc/<FQDN>:<instancename> The provider-generated, default SPN for a named instance when a protocol other than TCP is used. <instancename> is the name of an instance of SQL Server.

1 The SPN format doesn't require a port number. A multiple-port server, or a protocol that doesn't use port numbers, can still use Kerberos authentication.

For a TCP/IP connection, where the TCP port is included in the SPN, SQL Server must enable the TCP protocol for a user to connect by using Kerberos authentication.

Automatic SPN registration

When an instance of the SQL Server Database Engine starts, SQL Server tries to register the SPN for the SQL Server service. When the instance is stopped, SQL Server tries to unregister the SPN. For a TCP/IP connection, the SPN is registered in the format MSSQLSvc/<FQDN>:<tcpport>. Both named instances and the default instance are registered as MSSQLSvc, relying on the <tcpport> value to differentiate the instances.

For other connections that support Kerberos the SPN is registered in the format MSSQLSvc/<FQDN>:<instancename> for a named instance. The format for registering the default instance is MSSQLSvc/<FQDN>.

To give permissions to the SQL Server startup account, to register and modify the SPN, perform the following steps:

  1. On the Domain Controller machine, navigate to Active Directory Users and Computers.

  2. Select View > Advanced.

  3. Under Computers, locate the SQL Server computer, and then right-click and select Properties.

  4. Select the Security tab and select Advanced.

  5. In the list, if SQL Server startup account isn't listed, select Add to add it. Once it's added, perform the following steps:

    1. Select the account and select Edit.

    2. Under Permissions select Validated Write servicePrincipalName.

    3. Scroll down and under Properties select:

      • Read servicePrincipalName
      • Write servicePrincipalName

    4. Select OK twice.

  6. Close Active Directory Users and Computers.

Manual intervention might be required to register or unregister the SPN if the service account lacks the permissions that are required for these actions.

Manual SPN registration

To register the SPN manually, you can use the setspn tool that is built into Windows. setspn.exe is a command-line tool that enables you to read, modify, and delete the Service Principal Names (SPN) directory property. This tool also enables you to view the current SPNs, reset the account's default SPNs, and add or delete supplemental SPNs.

For more information on the setspn tool, required permissions and examples on how to use it, review setspn.

The following example illustrates the syntax used to manually register an SPN for a TCP/IP connection using a domain user account:

setspn -S MSSQLSvc/myhost.redmond.microsoft.com:1433 redmond\accountname

Note

If an SPN already exists, it must be deleted before it can be reregistered. You do this by using setspn with the -D switch. The following examples illustrate how to manually register a new instance-based SPN. For a default instance using a domain user account, use:

setspn -S MSSQLSvc/myhost.redmond.microsoft.com redmond\accountname

For a named instance, use:

setspn -S MSSQLSvc/myhost.redmond.microsoft.com:instancename redmond\accountname

For more information about Always On availability group configurations, see Listeners and Kerberos (SPNs).

Client connections

User-specified SPNs are supported in client drivers. However, if an SPN isn't provided, it's generated automatically based on the type of a client connection. For a TCP connection, an SPN in the format MSSQLSvc/FQDN:[<port>] is used for both the named and default instances.

For named pipes and shared memory connections, an SPN in the format MSSQLSvc/<FQDN>:<instancename> is used for a named instance and MSSQLSvc/<FQDN> is used for the default instance.

Use a service account as an SPN

Service accounts can be used as an SPN. They're specified through the connection attribute for the Kerberos authentication and take the following formats:

  • username\@domain or domain\username for a domain user account

  • machine$\@domain or host\FQDN for a computer domain account such as Local System or NETWORK SERVICE.

To determine the authentication method of a connection, execute the following query.

SELECT net_transport, auth_scheme
FROM sys.dm_exec_connections
WHERE session_id = @@SPID;

Authentication defaults

The following table describes the authentication defaults that are used, based on SPN registration scenarios.

Scenario Authentication method
The SPN maps to the correct domain account, virtual account, MSA, or built-in account. For example, Local System or NETWORK SERVICE. Local connections use NTLM, remote connections use Kerberos.
The SPN is the correct domain account, virtual account, MSA, or built-in account. Local connections use NTLM, remote connections use Kerberos.
The SPN maps to an incorrect domain account, virtual account, MSA, or built-in account Authentication fails.
The SPN lookup fails or doesn't map to a correct domain account, virtual account, MSA, or built-in account, or isn't a correct domain account, virtual account, MSA, or built-in account. Local and remote connections use NTLM.

Note

Correct means that the account mapped by the registered SPN is the account that the SQL Server service is running under.

Remarks

The Dedicated Administrator Connection (DAC) uses an instance name-based SPN. Kerberos authentication can be used with a DAC if that SPN is registered successfully. As an alternative a user can specify the account name as an SPN.

If SPN registration fails during startup, this failure is recorded in the SQL Server error log, and startup continues.

If SPN deregistration fails during shutdown, this failure is recorded in the SQL Server error log, and shutdown continues.

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