# Saturday, November 21, 2009

Creating linked servers between two instances of SQL Server may not always be as straight forward as hoped. To make it more complicated, interference may be caused by matters beyond the control of the database administrator; Active Directory. After creating a linked server, being able to successfully use it may be the real challenge, it may even work for one user and fail for another.

Service Account

To communicate successfully in a domain environment, the service account for your database engine is the first to check/configure. Possible choices for the service account are;

  • Domain User (recommended)
  • Local User
  • LocalSystem
  • LocalService
  • NetworkService (recommended)

From those options, a local user or LocalService for the service account will eventually stop you. In both cases a SQL Server has the permission set of user on the machine where SQL Server operates, with no identity on the network. In case the SQL Server has to connect to any remote resource, it will do so as an anonymous user (null session). Unless you want to be absolutely sure that SQL Server should never do anything beyond anonymous on the network, you can go this route... but hey, you're reading about linked servers, so just drop the thought of using a local user or LocalService.

Valid options to enable SQL Server to operate on the network are LocalSystem, NetworkService and Domain User. All have the potential to work well, but LocalSystem is not recommended from a security perspective. LocalSystem has the permission set of an administrator on the system where SQL Server operates, anyone obtaining control over your SQL Server has full access to the underlying operating system (think about the damage that could be done when xp_cmdshell is enabled). From a network perspective, LocalSystem and NetworkService are identical; when the SQL Server operates over the network, it will do so under the credentials of the Windows machine. With a domain user as service account, SQL Server has user permissions locally (as is the case with NetworkService, LocalService and a local user), on the network SQL Server will present the credentials of the domain user.

So two good options remain; NetworkService and a domain user. Which one you chose mainly depends on the advantages of one over the other;

  • NetworkService:
    • No account or password management in Active Directory
    • Service Principal Names are automatically generated
  • Domain User
    • Multiple instances on different machines can have the same identity which is required for clustering.
    • Transparent security for Database Mirroring. 
    • Usually SQL Agent will be the same account, this makes things easier with Log Shipping and Replication. ChangeServiceAccount
    • Improved access to Service Master Key (in case you don't have a backup of the key itself). -->

Personally, I favor using a domain user.

Should you find that you need to change the Service Account; use the "SQL Server Configuration Manager"

Configuring and using a SQL Server linked server for Windows Authentication

NewLinkedServer1There are two ways to go about the configuration; graphically and via stored procedure. The graphical approach brings up a dialog, where just a little configuration is needed.

On the General page, select the server type to be SQL Server (choosing SQL Server Native or OLEDB drivers will work too, but require extra configuration) and type the name of the target instance using the flat name or FQDN followed by a backslash and instance name in case of a named instance.

On the Security page, select the option to "Be made using the login's current security context", which effectively tells SQL Server to impersonate or delegate the login requesting access though the linked server.

On the Server Options page, verify that Data Access is set to True. To see the stored procedures involved, either use the Script button on top of the dialog instead of pressing ok or right-click a configured linked server and "Script Linked Server as >".

To avoid running into (later explained) problems when creating linked servers, create the linked server from a shell (interactive or remote desktop) on the Windows machine where the linked server is configured.

To use the linked server, issue the query

SELECT TOP 50 * FROM [HOST.NET.LOCAL\INSTANCE].AdventureWorks.Sales.SalesOrderDetail

Note that the linked server HOST.NET.LOCAL\INSTANCE is enclosed in braces, which is required when the server name is fully qualified and/or when it is a named instance. If you don't want to expose host and/or instance names, consider creating an alias in "SQL Server Configuration Manager" under client configuration and point the linked server to the alias.

Impersonation and delegation

During the configuration (Security page) impersonation and delegation were mentioned. Also to avoid problems while configuring a linked server, the best way to do so was on the server where the linked server was to be created. The reason to do so is to avoid delegation. Below are a summary of impersonation and delegation (and links to a more complete overview). Windows accounts will be impersonated if necessary, Windows accounts will not be delegated unless both account and delegating system are configured to do so.

Impersonation happens when a process connects to resources pretending to have the identity of it's caller. Like when a user Bobbie is connected to SQLINST1 and issues a query for a table via a linked server on SQLINST2. Depending on the configuration of the linked server, SQLINST1 will pretend to be Bobbie when connecting on Bobbies behalf to SQLINST2. For a more detailed overview of impersonation, check the Pluralsight-wiki on impersonation. Note that impersonation is limited to the  system where process making the call is active.

Delegation is much like impersonation, with the additional feature that delegation is not limited to the system of the process making the call. Because the process now has to forward the credentials to another system and prove to operate on behalf of someone else than the system itself, the Kerberos protocol is required (that or knowledge of the password of the delegated user). For a more detailed overview of delegation, check the Pluralsight-wiki on delegation.

UserDelegationPropertyFrom the perspective of Windows systems, Bobbie (DOMAIN\Bobbie) is logged in on a typical client system (Windows XP, Vista or 7) and uses an application that connects to SERVER1\SQLINST1. In SQLINST1 two linked servers are configured; SERVER1 (being the default instance on SERVER1) and SERVER2\SQLINST1. The context of the application is DOMAIN\Bobbie and thus an access token for DOMAIN\Bobbie is handed to SERVER1 (for logging in on the SQL Server service SERVER1\SQLINST1). When the application issues a query for an object on the default instance of SERVER1, one local process on SERVER1 has to impersonate DOMAIN\Bobbie to the other local process on SERVER1. This will succeed and can work both with NTLM and Kerberos, because SERVER1 trusts itself and the domain; DOMAIN\Bobbie was authenticated by the domain and is only used by SERVER1 locally.

Next the application calls for SERVER2 and now the challenge is on! Bobbie get's a

Msg 18456, Level 14, State 1, Line 1
Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.

SystemDelegationPropertySERVER2 does not trust SERVER1 and therefor will not accept the claim of SERVER1 that it acts on Bobbies behalf, SERVER2 wants to see PROOF! One way of proof would be if SERVER1 had the password of Bobbie, indicating that Bobbie fully trusts SERVER1... but Lesson number 1; never give anyone your password. Lesson number 2; don't write it on a Post-It... Lesson number 3; don't trust computers that violate lessons 1 or 2. The other option is to turn to a mutually trusted third party: the domain, but now the configuration needs to be right for Kerberos.

For Kerberos to do it's trick, the user (Domain\Bobbie) and the machine delegating (SERVER1) the user have to be trusted for delegation. This is sufficient if the SQL Server services on SERVER1\SQLINST1 and SERVER2 run as NetworkService.

In case SERVER1\SQLINST1 is running under a domain user account, trust that user account for delegation, not the server.

Service Principal Name

Kerberos distinguishes three roles;

  • Key Distribution Center (KDC), in Active Directory this role is held by the Domain Controllers.
  • Client, these are the users.
  • Server, these are the computers in your network.

With this basic division of roles, things work well in a domain, until a service account like that of the SQL Server database engine is a domain user. If so, that particular domain user should be made a server too. To mark an account as a service, a Service Principal Name must be registered. With NetworkService, this was done by the server (because servers can write SPN's on their own behalf), for any other account, by default only Domain Administrators can write an SPN. Use the SetSPN tool to mark an account as Service Principal Name, SetSPN can be downloaded, is part of the Windows Server 2003 Support Tools and is installed on Windows Server 2008 with the AD DS role.

To register an SPN for a service account, use the following syntax:

SetSPN -a MsSqlSvc/server1.domain.local:1433 domain\user

SetSPN -a MsSqlSvc/server1:1433 domain\user

Substitute the blue parts in the syntax for the server names (FQDN and NetBIOS), the port where SQL Server listens (this could be different from 1433, especially when registering an SPN for additional instances) and the domain user account used as SQL Server service account.

When running more SQL Server instances under the same domain account, it may be useful to check the approach listed in Step 3 of How to use Kerberos authentication in SQL Server, so the AD-people have to be called upon only once for the service account, not for every instance installation.

Reference Environment

ReferenceEnvironment As reference environment (may be a layout for a test-network), the following machines, users and services are used in a single domain configuration;

  • Windows Server 2003 or 2008: DC1 (domain controller), SERVER1 (member), SERVER2 (member)
  • Windows XP, Vista or 7: CLIENT (member)
  • SQL Server 2005 or 2008: SERVER1 (default instance) tcp 1433 static, SERVER1\SQLINST1 tcp dynamic (named instance), SERVER2\SQLINST1 (named instance) tcp 1433 static

To make it work, user Domain\Bobbie has to be trusted for delegation in all four scenario's. Depending on the service accounts for SERVER1\SQLINST1 and SERVER2\SQLINST2, use the following settings

Service account for SERVER1\SQLINST1 Service account for SERVER2\SQLINST1 Users and or computers to be trusted for delegation SPN's to create
NetworkService NetworkService SERVER1
MSSQLSvc/server2.domain.local:1433 DOMAIN\SRV_SQL2
MSSQLSvc/server2.domain.local:1433 DOMAIN\SRV_SQL2

Things to consider

AdjustKerberosIntervals When working with Kerberos, tickets are granted for a longer period of time (typically 10 hours), so configuration changes usually take a longer time to apply. Use the KERBTRAY and/or KLIST resource kit utilities to view and purge tickets, in order to speed things up (avoids rebooting systems or logging off/on users). Alternatively, consider shortening the ticket lifetimes to the minimum; 10 minutes for services and 1 hour for users.

When configuring the linked servers, it was best to do so at the server where the linked server is configured from. Depending on your role you may be a domain admin, so should domain admins be delegated in order to make setting up linked servers easier? Rather not! Imagine someone installed malicious program or script on a system that is trusted for delegation and somehow a domain administrator executes this program or script. It could hurt any system or user in your network.

Saturday, November 21, 2009 8:56:50 PM (W. Europe Standard Time, UTC+01:00)