Register a Service Principal Name for Kerberos Connections

Applies to: yesSQL Server (all supported versions)

To use Kerberos authentication with SQL Server requires both the following conditions to 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 will be "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.

A service principal name (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 that is provided in this topic 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 using NTLM or Kerberos. In an Active Directory environment, Kerberos authentication is always attempted first. Kerberos authentication isn't available for SQL Server 2005 (9.x) clients using named pipes.

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 the Application event log as well as 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 is not recommended in a production environment. When SQL Server runs on the Windows 7 or Windows Server 2008 R2 operating system, you can run SQL Server using a virtual account or a managed service account (MSA). Both virtual accounts and MSA's 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.

Note

When the Windows domain is configured to run at less than the Windows Server 2008 R2 Windows Server 2008 R2 functional level, then the Managed Service Account will not have the necessary permissions to register the SPNs for the SQL Server Database Engine service. If Kerberos authentication is required, the Domain Administrator should manually register the SQL Server SPNs on the Managed Service Account.

Additional information is available at How to Implement Kerberos Constrained Delegation with SQL Server 2008

SPN Formats

Beginning with SQL Server 2008, the SPN format is changed in order to support 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.

    Note

    The new SPN format doesn't require a port number. This means that a multiple-port server or a protocol that doesn't use port numbers can use Kerberos authentication.

SPN format Description
MSSQLSvc/<FQDN>:<port> 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.

Note

In the case of 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>.

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, the administrator must use the Setspn.exe tool that is provided with the Microsoft Windows Server 2003 Support Tools. For more information, see the Windows Server 2003 Service Pack 1 Support Tools KB article.

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.

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

setspn -A 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 the setspn command together 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 -A MSSQLSvc/myhost.redmond.microsoft.com redmond\accountname  

For a named instance, use:

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

Client Connections

User-specified SPNs are supported in client drivers. However, if an SPN isn't provided, it will be 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.

Using 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 SERVICES.

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.

Comments

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 de-registration fails during shutdown, this failure is recorded in the SQL Server error log, and shutdown continues.

See Also