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Using Kerberos with SQL Server

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Kerberos is a widely accepted network authentication protocol that is used to provide a highly secure method to authenticate users. Reliance is placed upon a trusted third party called the Key Distribution Center (KDC) to facilitate the generation and secure distribution of authentication tokens and symmetric session keys. In a Windows environment, the operation of the KDC is assumed by the Domain Controller (DC) using the Active Directory (AD). Furthermore, all Windows domain users are effectively Kerberos principals and are capable of engaging in Kerberos authentication.

Among the key benefits of Kerberos authentication that make it popular are:

  • Mutual authentication (the client can validate the identity of the server principal)

  • Secure authentication tickets (only encrypted tickets are used and passwords are never included in the ticket)

  • Integrated authentication (single sign-on, i.e., once the user is logged on, s/he does not need to log on again to access any service that supports Kerberos authentication)

Kerberos with SQL Server

SQL Server 2005 (and 2000) supports Kerberos indirectly through the Windows SSPI interface when using Windows integrated authentication (as opposed to SQL authentication). However, Kerberos will only be used under certain circumstances as SQL Server allows SSPI to negotiate the authentication protocol to use; if Kerberos cannot be used, then Windows will fall back NTLM authentication. Kerberos authentication is far more desirable than NTLM from a security (and, to a lesser degree, performance) point of view and I think it’s important to understand how to ensure Kerberos is used for remote connections when possible. If you're using integrated auth, you need to make sure that the follow are done:

  1. Both the client and server machines must be part of the same Windows domain, or else trusted domains.

  2. The server's Service Principal Name (SPN) must be registered with the Active Directory (I'll explain this in more detail below)

  3. The client must connect to the server using TCP/IP. Assuming that the server has TCP/IP enabled, this can be accomplished by either placing TCP/IP at the top of the client's protocol order or else prefixing the connection string with "tcp:"

Registering an SPN

 

The SPN is essentially a mapping between a principal name and the Windows account that started the server instance service. This is needed because the client will use the server’s hostname and the TCP/IP port to which it connects to compose an SPN. If the SPN mapping has not been performed, then the Windows security layer will be unable to determine the account associated with the SPN and Kerberos authentication will not be used. In an attempt to facilitate this, the SQL Server 2005 instance will automatically try to register the SPN with the AD at startup if TCP/IP is enabled. The problem is, however, that only a domain administrator or a Local System account has the authority to register an SPN. Therefore, in the majority of cases where the service is started under a normal account, SQL Server will be unable to register the SPN for the instance. This will not prevent the instance from starting but you will see the following entry in the error log:

 

The SQL Network Interface library could not register the Service Principal Name (SPN) for the SQL Server service. Error: 0x2098. Failure to register an SPN may cause integrated authentication to fall back to NTLM instead of Kerberos. This is an informational message. Further action is only required if Kerberos authentication is required by authentication policies.

 

The alternative is to have a domain admin manually register the SPN for the instance. The format for an SPN is MSSQLSvc/FQDN:tcpport, where FQDN is the fully qualified domain name of the server and tcpport is the TCP/IP port number. To register the SPN, the administrator will need to use the SetSPN.exe tool which is available from the Windows server resource kit (for Windows 2K3, you can find it here, https://support.microsoft.com/default.aspx?scid=kb;en-us;892777). An example of the command is:

 

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

 

If an SPN already exists, then it must be deleted before it can be re-registered. This is accomplished by a domain admin using the setspn -D command.

 

To verify that Kerberos authentication is being used, you may query the sys.dm_exec_connections DMV and look under the auth_scheme column, e.g.

 

select auth_scheme from sys.dm_exec_connections where session_id=@@spid

 

If Kerberos is being used, then it will display “KERBEROS”.

 

I should also mention that if the instance automatically registered an SPN at startup, then it will unregister it when the instance is stopped.

 

Potential Problems

 

If instance is configured to listen to a different port, then the SPN will need to be deleted and recreated using the new port number. The problem is worse if the server is configured to use dynamic IP addresses as, potentially, a new SPN must be configured every time the server is started. I recommend that a static IP address be used so that the SPN need only be registered once. In addition, using a static IP address provides the additional benefit that the client can specify the TCP/IP directly in the connection string. Doing this will prevent the need to rely upon SQL Browser to determine the port number (SQL Browser communication uses an unauthenticated UDP channel).

 

A couple of other considerations that are documented in the BOL are (i) the Dedicated Admin Connection (DAC) only uses NTLM so no SPN is registered for the DAC connection, and (ii) if the instance is configured to listen to multiple IP addresses, then the server will only attempt to automatically register the SPN using the first port that it identifies.

Il-Sung Lee
Program Manager, SQL Server Protocols

Disclaimer: This posting is provided "AS IS" with no warranties, and confers no rights

Comments

  • Anonymous
    January 03, 2006
    Is the select statement "to verify that Kerberos authentication is being used" for SQL 2005? If so, is there an equivilant for SQL 2000?
    Thanks,
    Al.

  • Anonymous
    January 03, 2006
    Unfortunately, querying resulting SSPI auth package of SPNEGO, whether it is NLTM or Kerberos, is not available in SQL 2000. This is new feature added in SQL 2005.

  • Anonymous
    March 16, 2006
    Will this also work when using the NT AuthorityNetwork Service account to control the SQL Server instances?  

  • Anonymous
    May 09, 2006
    The “Cannot generate SSPI context” issue is described by http://support.microsoft.com/?id=811889 in general....

  • Anonymous
    June 01, 2006
    After manually registering the spn for the sql server service account we still are receiving this error. The results of the above query are still displaying NTLM. Is there a work around to this problem?

  • Anonymous
    June 07, 2006
    The comment has been removed

  • Anonymous
    June 27, 2006
    I think I may have a firewall issue, but if someone could look at the following and shed some light or toss a tip I would be highly appreciative.

    I have a server ServerA which resolves internally to ServerA.windomain.co and externally to Somename.Outside.Com
    I am using a Domain Account and verified the SPN's are correct via setspn -L and can log in using a trusted connection via any of the following internally:
    tcp:ServerA.windomain.co,1433
    ServerA.windomain.co
    Somename.Outside.Com
    tcp:Somename.Outside.Com,1433
    tcp:ComputerName,1433
    Each of the above returns 'Kerberos' when checking the sys.dm_exec_connections system view.

    However, from outside, Somename.Outside.Com (and all variations) returns 'Cannot Generate SSPI Context'.  ping -a resolves correctly and connecting to Somename.Outside.com / direct IP via SQL authentication works fine. Also connects fine (as assumed) when using a VPN.

    I've done Matt's (http://blogs.msdn.com/sql_protocols/archive/2005/10/15/481297.aspx) delete SPN trick successfully, checked every setting under the KB articles and 'Cannot Generate SPPI Context' blog and the outside clients are members of the same domain.
    Is there an additional port (besides 1433) that is required when using SSPI?

    Server: SQL 2005 Server Standard Edition SP1
    Windows 2003 SP1
    Clients: Windows XP SP2 w/ SQL 2005 Express SP1

  • Anonymous
    June 27, 2006
    The comment has been removed

  • Anonymous
    June 28, 2006
    We're having an issue with linked servers/Kerberos and SPN's.

    SPN is registered. Everything works ok.
    However it seems like every day or every other day the linked server breaks reverting back to NTLM. What gives? Checked SPN -l...service is there.
    Kerberos debugging on....no noticeable errors.
    Able to receive tgt and service tickets.
    Checked everything under the sun. Any suggestions as to why it works one day and then breaks?

  • Anonymous
    August 02, 2006
    I'd suggest taking a network trace from the middle tier server for request from both client and the backend server.

    Also, check the settings according to the article "Troubleshooting Kerberos Delegation" http://www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/security/tkerbdel.mspx

  • Anonymous
    August 10, 2006
    SQL Server 2005: “Login failed for user 'NT AUTHORITYANONYMOUS LOGON'”.
    SQL Server 2000: “Login failed...

  • Anonymous
    September 01, 2006
    We just had a similar issue to hte one described above. In the end we resolved it by drop/creating the linked servers on both sides.

  • Anonymous
    September 01, 2006
    Take a look at Nan's recent blog about the "Login failed for user 'NT AUTHORITYANONYMOUS LOGON'” error message with DQ.

    Thanks,
    Il-Sung.

  • Anonymous
    November 01, 2006
    The comment has been removed

  • Anonymous
    November 16, 2006
    In my company we use 2 SQL servers 2000 and the link works fine... Both servers are mixed mode. Everyone but my NT Login... When I'm executing something that has to read data from the other server : I get the Error Login Failed for user null. My Nt login has rights on both servers. I've tried  in Server Client Network Util to use names pipes / tcp/ip nothing worked. We checked with Network Admin settings of my account to another that works and we have no differences!!! I'm Lost !!! Thx

  • Anonymous
    November 27, 2006
    The Windows Security layer always falls back to NTLM when connections are made locally.  This appears to be the design of SSPI when NEGOTIATE is used so what you are seeing is expected.

  • Anonymous
    November 27, 2006
    The comment has been removed

  • Anonymous
    January 01, 2007
    The comment has been removed

  • Anonymous
    May 08, 2007
    I can connect to my SQL server using KERBEROS from any server on the network (Win 2000, Win 2003) But when i connect from my Win XP SP2 PC i always connected as NTLM? Any idea why? Any local policy to check? anything else? My Setspn return the correct results.

  • Anonymous
    May 10, 2007
    where should SetSPN command executed, Active Directory machine or sql server machin

  • Anonymous
    May 11, 2007
    The comment has been removed

  • Anonymous
    June 21, 2007
    I wounder if i can use Kerberos without Active Directory? We've got samba domain controller. I need to install WSUS server and i want to use our dedicated SQL 2005 server, but it now runs only with NTLM, because we don't have AD.

  • Anonymous
    June 22, 2007
    Hi, Mikhail    Please check out following blog: http://blogs.msdn.com/sql_protocols/archive/2006/12/02/understanding-kerberos-and-ntlm-authentication-in-sql-server-connections.aspx Good Luck! Ming.

  • Anonymous
    July 13, 2007
    i have a error in the installation process like "logon account cannot be validated for the sql service.please verify user name and password".what is this

  • Anonymous
    July 18, 2007
    I can verify that the sql server is using Kerberos authentication by running the select statement: select auth_scheme from sys.dm_exec_connections where session_id=@@spid However, I can not see the spn name when I run Setspn.exe -L "service-account-name". I am still getting:Login failed for user 'NT AUTHORITYANONYMOUS LOGON'

  • Anonymous
    August 24, 2007
    Не первый раз встречаюсь с тем, что настройка делегирования для связанных серверов (так в русском BOL

  • Anonymous
    November 23, 2007
    Is only the SQL service needed as SPN? If not, how is the syntax for SQLAgent service, which seems to need Kerberos in CU3+ updates or is sharing this: setspn -A MSSQLSvc/FQDN:1433 accountname

  • Anonymous
    January 09, 2008
    Is this error related to this discussion?? Thanks! SSPI handshake failed with error code 0x8009030c while establishing a connection with integrated security

  • Anonymous
    February 28, 2008
    sir, i have to link postgre sql from sql server 2005 express. (sp_addlinkedserver) i tried. but its not working but it is possible in msaccess 97.... pls help me.. regards seenu

  • Anonymous
    June 19, 2008
    We've configured this sometime ago. The problem we have is we get inconsistent results when connecting using the same account. Client's OS: Windows XP 2002 SP2 SQL Server: SQL 2005 SP2 Result 1 - If I logon to the client PC with my Windows domain account, and then register SQL Server Management Studio directly with Windows Authentication, the result is 'NTLM'. Result 2 - If I logon to the client PC with my Windows account, and then register SQL Server Management Studio with Windows Authentication again but using Run As my Windows domain account, the result is 'KERBEROS'.   As a result of this, we get different results here and there depending on how the user is connecting and how the connection string is set up in the application and how the application is deployed. It looks like the KERBEROS is configured right for SQL Server. What's wrong here? We need it to be KERBEROS consistently. Thank you in advance. Sarah

  • Anonymous
    August 22, 2008
    We have the same SSPI issue and asked the system admin to add SPN to AD, which he did successfully. But we still get "Cannot generate SSPI context" error message. Do we need to restart SQL Server service? Thanks, Ben

  • Anonymous
    September 05, 2008
    PingBack from http://sqldbpool.wordpress.com/2008/09/05/%e2%80%9ccannot-generate-sspi-context%e2%80%9d-error-message-more-comments-for-sql-server/

  • Anonymous
    September 08, 2008
    Hi, is there anything else should be done if client is a JDBC application? thanks Vlad.

  • Anonymous
    September 08, 2008
    The comment has been removed

  • Anonymous
    October 28, 2008
    I am getting error 0x20b5/8373 "Failed to assign SPN on account..." Does anyone have suggestions for solving this issue? v/r, Mike

  • Anonymous
    December 01, 2008
    I am getting the SSPI error only when users are connected to the Network via Cisco VPN.  Would the fix mentioned above fix my problem?

  • Anonymous
    December 31, 2008
    To verify that Kerberos authentication is being used, you may query the sys.dm_exec_connections DMV and look under the auth_scheme column, e.g. select auth_scheme from sys.dm_exec_connections where session_id=@@spid The above statement works in SQL 2005. What is the way to know the auth scheme in SQL 2000 for connections.

  • Anonymous
    December 31, 2008
    Not using a system table or DMV. There are several ways you can determine what is used.  The easiest is to enable security auditing for successful logons on the server side, this will create an event log for each logon and this event log entry will tell you whether or not Kerberos was used. This article tells you how to turn it on: http://support.microsoft.com/kb/300549

  • Anonymous
    January 21, 2009
    PingBack from http://www.keyongtech.com/2202082-spn-requirement

  • Anonymous
    March 17, 2009
    Hello, I'm using SQL Server 2008 for MOSS 2007. We're using Kerberos. Is there any reason why I shouldn't disable all protocols on the server side other than TCP/IP? There is no SQL Server internal requirements for the Shared Memory or Named Pipes protocols to be enabled are there?

  • Anonymous
    May 20, 2009
    We've successfully set up Kerberos on our SQL Servers by manually setting the SPN's for each.  All of our SQL Servers are linked using window authentication.  This all seems to be working correctly, however, we have a old VB6 app that runs under a domain account that executes a distributed query and we are getting an anonymous login error on the hop to the second sql server.  This worked previously when the linked servers used a sql login to connect.  Also, when running the query from the same machine the app runs on with the same credentials (from query analyzer) the query runs without any problems.  Does anyone have any ideas?   Also, with manually registered SPN's... do they persist if the DC is rebooted?  Is there any point at which manually registered SPN's need to be re-registered. Thanks!

  • Anonymous
    July 20, 2009
    Hi Il-Sung Lee, thanks for such extreme good article! Appreciated, Mike

  • Anonymous
    July 23, 2009
    Client Tier is Management Studio or Query Analyzer Middle Tier x64 2003 SP1 - SQL 2000 SP4 Destination is x86 2003 SP1 - SQL 2000 SP4 SPN is set for Domain Service Account on middle tier & destination, TCP/IP is the only protocol enabled, linked server does not work on double-hop. However, if I remote to the middle tier and use linked server it works (obviously) and while that "ticket" is still live, I am able to successfully double-hop from the Client.  What am I missing???

  • Anonymous
    October 23, 2009
    The comment has been removed

  • Anonymous
    March 08, 2010
    Kiran, try with. setspn MSSQLSvc.3/machinename.redmond.corp.microsoft.com:1433 domainname<account under which sql service runs> Instead of: setspn MSSQLSvc/machinename.redmond.corp.microsoft.com:1433 domainname<account under which sql service runs> For SQL 2005 you need to use MSSQLSvc.3, to work properly.

  • Anonymous
    October 23, 2010
    I see the below error my SQL Logs after I restart cluster service. The SQL Network Interface library could not register the Service Principal Name (SPN) for the SQL Server service. Error: 0x2098, state: 15. Failure to register an SPN may cause integrated authentication to fall back to NTLM instead of Kerberos. This is an informational message. Further action is only required if Kerberos authentication is required by authentication policies. http://www.shop2guntur.com/

  • Anonymous
    June 26, 2013
    I have a SPN registered to two accounts: the server SQL 2000 is installed on and a domain account The current Server replaced an older server that had the same name.  From what I am told, the old server was removed from Active Directory and the new one was brought into AD with the same name.  The old server used a Domain account (the same one used to register the SPN) to start the SQL Server service.  The new server was configured to use the Local System account to start the service.   I noticed a Event ID 11 pointing to the SPN.  Which SPN can I remove?  I am taking an educated guess that I can remove the one registered under the Domain account used for the SQL Service

  • Anonymous
    August 17, 2015
    server reboot need or not? if we add/remove SPN. Thanks Senthilkumar S