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.
Testing Kerberos
There are tools available for testing Kerberos but it’s quite easy to determine if it is running properly.
When it’s enabled but not working the following symptoms may be present
- Login prompts may appear when the previously did not under NTLM Authentication
- Login Errors appear in the Windows Security Event Log typically stating that Kerberos authentication failed
- Users are required to login using Office applications when their
machines are domain members and the logged in user should have rights.
When Kerberos is first configured for the application pool account a
message will appear in the Windows Security Logs stating that a ticket
was requested.
Open SharePoint in a browser using the URL where Kerberos is now
configured and then refresh the security log. If Kerberos is running
properly messages similar to the one below will appear in the logs on a
regular basis.
For particular users logged in, events will appear similar to the one below
In addition, many messages similar to the one below will appear in the event log.
By: Saravanan.J
