Step by Step Kerberos Authentication for SharePoint 2010

Introduction

Kerberos authentication, created at MIT and named after Hades’ three-headed guard dog Cerberus (according to Wikipedia), has been around for decades.  The latest version 5, implemented currently by Active Directory, was released in 1993.  The protocol is designed to provide rapid, secure authentication to users on a multi-system network, or “farm” as we like to call them.

Advantages over Traditional Windows Authentication

The main advantage of Kerberos over NTLM or forms-based authentication is the ability for a user’s identity to securely traverse multiple servers without requiring a re-key of the user’s credentials.  This concept is referred to as single sign-on: login once to access everything.

A secondary advantage is speed.  Authenticating connections with Kerberos tokens is considerably faster than other methods.

Platform Uniformity

Another advantage is platform uniformity.  Any application, that you wrote, or Microsoft wrote, or anyone wrote, which uses Windows Authentication can automatically use Kerberos.  It’s built in to Windows and Active Directory.  It doesn’t require custom code like a forms-based or claims-aware provider.  Enabling it is as simple as telling the web.config to use it.

Necessity

Many farm scenarios do not warrant Kerberos authentication.  How can you tell if yours does?  There is a simple test: the double-hop.  Draw a quick diagram of your farm topology.  If you have any servers which are more than two degrees of separation away from your client, you will need Kerberos authentication only if you need to delegate access to those resources.  The figure below shows the double-hop scenario.

Figure 1: The Double-Hop

Each connection, or “hop,” must be authenticated.  Thus, the SharePoint server must establish a secure, authenticated connection to SQL in order to return data for the user.  If the data connections above need to impersonate the user, the connections must use delegation.  Kerberos authentication allows SharePoint and SQL Server to implement delegation.

Real-World Examples

The most common example of Kerberos in practice involves Reporting Services.  A user browses to a SharePoint document library to run a Report with data in a SQL Server database.  SharePoint and SQL Server both implement Kerberos authentication to allow the user to view the Report using the user’s own credentials.  No login prompts, no proxy accounts, no stored credentials.

Setup

Setting up Kerberos authentication for SharePoint and SQL Server takes only a few minutes.  Follow the steps below to get it running in your farm.  We will assume that SharePoint requires classic mode authentication for the Web Application.  (Obviously, you will need to change CONTOSO to your Domain name and use your actual service accounts.)

1. Configure SQL Server

Configuring SQL Server to use Kerberos is easy.  Create a Service Principal Name for your SQL Server by running the setspn.exe utility from the command-line.  NOTE: you will need to be a Domain Administrator to do this:

Figure 2: setspn.exe Syntax

Service Principal Names

You will need to become familiar with Service Principal Names to setup Kerberos.  They are composed of the following pieces:

Service			Principal
Service Class	Endpoint	Port	Domain	User
MSSQLSvc	DB-SRV-01	1433	CONTOSO	SqlServer
This is the unique class name of the service. It differs between different types of services.	This is the DNS address where the service is accessed. In this case, it’s the server name, but it can also be the fully-qualified domain name like: db-srv-01.contoso.local - or an alias like - database.contoso.local	The port is needed if it is not a standard port for the Service Class.	This is the NetBIOS domain name of the Active Directory where the service account resides.	This is the login name for the service account itself.

As far as I know, the Service Class is case-sensitive.

For good measure, Microsoft recommends creating multiple Service Principal Names.  The reason why: the client application creates the Service Principal Name when it sends it to the server.  If the client application choses to include the port number, or not include the port number, you should be ready.  The solution: create all of the following SPNs for SQL Server:

·         MSSQLSvc/DB-SRV-01 CONTOSO\SqlServer

·         MSSQLSvc/DB-SRV-01:1433 CONTOSO\SqlServer

·         MSSQLSvc/DB-SRV-01.contoso.local CONTOSO\SqlServer

·         MSSQLSvc/DB-SRV-01.contoso.local:1433 CONTOSO\SqlServer

Note the variation in the Endpoint and Port.  We do this to ensure that we cover all the possible combinations that a client application could throw at SQL Server.  This is the best practice.

2. Create a Web Application

Create a new Web Application in SharePoint 2010 to use with Kerberos authentication.  Pick Classic Mode Authentication and make sure NTLM is used.  This Web Application will be created as the Default Zone.  We want to put this on a non-standard port and use NTLM authentication to ensure that we can always access it from the SharePoint server itself.

Note: you must use a Domain Account for the application pool identity.

Figure 3: New Web Application

3. Extend the Web Application to use Kerberos Authentication

Extend the Web Application you just created.  Set the Zone to Intranet and put the site on Port 80.  Use the host header intranet.contoso.local:

Figure 4: Web Application Extension

When you click OK you will get a warning about Kerberos.  Don’t worry: the Service Principal Name can be created before or after the Web Application Extension.

6. Create the DNS Record

Your server needs a static IP address and a DNS record to be accessed by users.  When Kerberos is involved, you must be sure that you create an A (for address) record and not a CNAME (canonical name, or alias) record for the SharePoint Web Application Extension:

Figure 5: New DNS Record

Enter the IP address of the SharePoint server and hostname of the Web Application Extension into the box and click Add Host to save the new DNS record.  The automatically generated FQDN should read intranet.contoso.local.

4. Create a Service Principal Name

Just like we did for SQL Server, create a Service Principal Name for the SharePoint Web Application Extension:

Figure 6: SharePoint SPN

The SharePoint Service Principal Name breakdown is as follows:

Service			Principal
Service Class	Endpoint	Port	Domain	User
HTTP	intranet.contoso.local		CONTOSO	SP_WebApp
HTTP works for http and https connections.	This is the DNS address where SharePoint is accessed. In this case, it’s the URL of the Web Application Extension	80 is a standard port, therefore we don’t need to include it.	This is the NetBIOS domain name of the Active Directory where the service account resides.	This is the login name for the SharePoint Application Pool account.

5. Enable Constrained Delegation

If this were SharePoint 2007, we’d be done.  But SharePoint 2010 requires Constrained Delegation.  In order to enable constrained delegation you have to connect to the Domain Controller and enable Delegation on the account used to host the SharePoint Web Application Pool.

Remote Desktop into the Domain Controller, open Active Directory Users and Computers, then locate the SharePoint Web Application Pool account.  Double-click on the account and locate the Delegation tab:

Figure 7: Delegation

Pick Trust this user for delegation to any service and click OK.  SharePoint will now authenticate clients using Kerberos authentication to http://intranet.contoso.local

Workarounds

A common work-around to the Real-World Scenario above, when Kerberos authentication is not involved, is a proxy account: hard-code the Report Server credentials into the Report itself.  When the user accesses the Report, SharePoint connects to SQL using the stored credentials.  This is also what the Secure Store service does.  This is also a form a delegation, but does not pass the user’s actual credentials to the data store: it uses a proxy account.  Thus, all users get the same rights on the data store and the password is saved in clear-text in the Report’s connection string.  If this doesn’t meet your requirements, you need to call in Kerberos to handle the connection.

Looking Ahead

Even though Kerberos is not always needed, or possible like with extranets, the introduction of External Content Types in SharePoint 2010 as a reporting tool will greatly increase the need for it.  The increased maturity and new features in PerformancePoint, PowerPivot, and Reporting Services in SharePoint mode, if your data is not on the SharePoint server itself you will need to use delegation.  The best choice which provides the lowest maintenance overhead, the highest level of security, and the lowest processor overhead, is Kerberos authentication.  Try it out in a VM farm on your local computer.  It’s a great tool to have in your SharePoint architect’s toolbox.

Reference:

http://www.thesharepointblog.net/Lists/Posts/Post.aspx?List=815f255a-d0ef-4258-be2a-28487dc9975c&ID=43