FIM 2010: Kerberos Authentication Setup

The goal of this article is to provide some background information regarding

the Kerberos related configuration steps of the FIM Portal and FIM Service. The

article has been written in such a way so that most of the points can in fact be

used for any application requiring Kerberos. This article will not discuss the

various possible FIM Topologies. All information should be valid regardless

whether all roles are combined on a single server or split across multiple

servers.

Throughout the article a demo domain will be used. The domain which will be

referenced as an example is contoso.com (NetBIOS name: CONTOSO).

Before we can start configuring SPN’s (Service Principal Names) we have to

determine what services we want to enable for Kerberos authentication. A typical

FIM Portal deployment has the following services:

Database for the FIM Service (SQL Service)

FIM Service

FIM Portal (Windows Sharepoint Services (WSS))

Note

In the above overview we’re leaving the FIM Synchronization Service and the

databases for the WSS aside. They don’t bring any added value to this

article.

The following picture provides an overview of these services.

Kerberos is all about authenticating principals to a service. Each principal

is represented by an account in AD. This can either be a computer or a user

account. Before Kerberos can take place, each service should be represented by

an account in AD. Again this can either be a computer or a user account.

Therefore it’s important to determine which account represents a given

service.

A typical Windows Service has its identity configured in the Services MMC. A

website however has its identity configured in the IIS Management Console (below

the Application Pools section)

The list below provides an overview of our services and their associated

identities.

Database for the FIM Service: the user account running the sqlservr.exe

process of the SQL Instance hosting that database

FIM Service: the user account running the FIM Service service

FIM Portal: Application Pool identity in IIS for the FIM Portal site

This information is displayed in the following picture.

Besides the principal representing a service, we also need to determine a

name to access the service. Choosing names can be rather important when actual

people are involved. Check the following examples:

The FIM Service is configured to access its database on

SPRDL2FIMSQL01B.contoso.com

Users visit the FIM Portal by browsing to SPRDL3FIMPOR01.contoso.com

The first one is in fact not a problem at all. Nobody will mind that a name,

for which IT probably has an explanation, is configured for a service to use. In

the second example your users will by no means be able to remember the URL.

Something like fimportal.contoso.com is way more feasible.

Important

Choose your service names carefully and always keep in mind whether end-users

will use them.

In the picture above several client-server communication arrows have been

pictured. In our example we will go with the following names to access the

services:

Database for the FIM Service: fimsql.contoso.com

FIM Service: fimsvc.contoso.com

FIM Portal: fimportal.contoso.com

There’s nothing wrong with choosing the actual server name of the SQL server

to associate with your SQL service.

Clients have to be able to resolve the names for these services. We can

register these records in DNS. It might seem convenient to use an alias (CNAME)

record for some of the services. However this is a bad idea as explained in the

following paragraph. Using a CNAME record would ensure that updating the server

its IP has no influence on the service name record. However CNAME records

resolve in another way than A records. A client requesting a Kerberos ticket for

a given service will ask AD a ticket for whatever the name resolves to.

This is how a client will resolve those names:

fimsvc.contoso.com (CNAME) -> server01.contoso.com

-> IP_of_FIM_Server

fimsvc.contoso.com (A) -> IP_of_FIM_Server

In bold the names are shown for which a Kerberos

authentication attempt will be performed. In the first example you can clearly

see that our client will request a Kerberos ticket for the wrong service as our

service is coupled to fimsvc.contoso.com. So things will go wrong. For more

information check

Register A records to ensure the correct service name is used in the Kerberos

authentication attempt

So we got a name and an identity for our service. How do we tell AD that

these belong together? Ahah! Now we get to the Service Principal Names (SPN‘s).

Whenever someone wants to use Kerberos to authenticate to a given service, they

contact the Key Distribution Centre (KDC) and ask for a service ticket. The KDC

is running on each domain controller. It knows which ticket to hand out because

the client specified the service it wants a ticket for. The service was in fact

specified by its name. More particularly by using the Service Principal Name

(SPN).

An SPN is based upon the following format

<service>/<fqdn>:<port>

In our example we will execute the following commands:

Setspn –S MSSQLsvc/fimsql.contoso.com:1433 sa_sqlsvc

Setspn –S MSSQLsvc/fimsql:1433 sa_sqlsvc

Setspn –S FIMService/fimsvc.contoso.com sa_fimsvc

Setspn –S FIMService/fimsvc sa_fimsvc

Setspn –S HTTP/fimportal.contoso.com sa_wss

Setspn –S HTTP/fimportal sa_wss

Never register a given service (<service>/<fqdn>:<port>) on

multiple accounts. Whenever multiple accounts are responsible for the same

service, AD cannot determine which account to use to hand out the Kerberos

service ticket. As such Kerberos authentication breaks. This issue is called

Duplicate SPNs. You can do a quick check in your domain for duplicate SPN‘s by

executing Setspn -X.

Always register both short and long (domain fqdn) for a service. This will

ensure Kerberos is available at all times.

SQL always requires an SPN of the format MSSQLsvc/<fqdn>:<port>,

even when using the default (1433) port. If your port is dynamic you have to

configure it to be static or give the SQL Server service account permissions to

update its own SPN‘s.

A lot of guides will tell you to use Setspn –A instead of setspn –S. The

advantage of using the –S option is that it will check the domain prior to

adding the SPN. This will avoid setting duplicate SPNs.

When the above steps have been implemented, both the FIM Service and SQL will

start accepting Kerberos. However IIS is slightly different. In fact skipping

this particular step will often break your configuration all together. One of

the symptoms when having a bad Kerberos implementation is the following: you

type the URL of your website, you get presented with an authentication prompt,

and no matter how many times you correctly enter your credentials, you keep

getting prompted over and over again.

This issue occurs because by default IIS uses the account of the server to

validate service tickets instead of the Application Pool identity. We can force

IIS to use the identity of the application pool by configuring this in the

applicationHost.config configuration file.

The applicationHost.config is typically located in

c:\windows\system32\inetsrv\config\ Remember to take a backup when

modifying this file.

The following steps are required to configure Kerberos Authentication to work

with a custom Application Pool Identity.

Launch an elevated command prompt and execute the following commands:

cd c:\Windows\System32\inetsrv\config

copy applicationHost.config

applicationHost.config.dateOfToday.bak

notepad applicationHost.config

Search for windowsAuthentication enabled="true" if you are below:

<code>&lt;</code><code>location</code>

<code>path</code><code>=</code><code>"SharePoint - 80"</code><code>&gt;</code>

The above might actually be different in your environment. You need to locate

the path of the IIS site which represent your FIM Portal WSS site.

Add useAppPoolCredentials="true" so the line looks like:

<code>&lt;</code><code>windowsAuthentication</code>

<code>enabled</code><code>=</code><code>"true"</code>

<code>useAppPoolCredentials</code><code>=</code><code>"true"</code><code>&gt;</code>

Save the file and exit notepad

Execute the following command: iisreset

Now that we got Kerberos authentication working for all of the involved

services we have to determine whether additional configuration is required.

Sometimes it’s obvious that Kerberos delegation has to be configured, sometimes

it’s less obvious. Either way, it’s advised to check the product specific

documentation to be sure. Kerberos delegation will allow a service to

impersonate a visiting user and authenticate to another service as if it were

the user himself who visits that service.

From the we know that the following delegation scenarios are required:

FIM Portal to FIM Service

FIM Service to FIM Service

This is explained in the "Establish SPNs for FIM 2010" section of the

installation guide.

To allow a given service to delegate to an other service, we have to

configure delegation on the service its service account to the delegated service

its SPN. Delegation can be configured using Active Directory Users &amp;

Computers (ADUC). As explained in the previous section we have to configure the

following delegation scenario‘s:

For the Portal to be able to delegate to the FIM Service we would have

to:

Open ADUC and locate the service account for the Portal (sa_wss)

Open the properties of sa_wss and choose the delegation tab

Check Trust this user for delegation to the specified services only

Check Use Kerberos only

Click Add...

Click users or Computers...

Type the name of your FIM Service service account: sa_fimsvc

Click Check Names and Click Ok

Select the FIMService entry and Click Ok

Click Ok to close the account properties

Some screenshots to aid in the process: FIMService selection screen.

And the resulting Delegation tab for the sa_wss account:

For the FIM Service to be able to delegate to the FIM Service we would have

Open ADUC and locate the service account for the FIM Service (sa_fimsvc)

Open the properties of sa_fimsvc and choose the delegation tab

The delegation tab on a user is only visible when an SPN has been registered

for that account.

The above procedure assumes your domain is in 2003 DFL or higher. Windows

2000 DFL only has unconstrained delegation available.

Optionally you can configure the FIM Portal to only accept Kerberos. This is

explained in the FIM Installation Guide  &gt; Installing The FIM 2010

Server Components &gt; Activating The Kerberos Protocol Only ()

The following steps are required to force Kerberos Authentication for the FIM

Portal.

cd c:\inetpub\wwwroot\wss\VirtualDirectories\80

copy web.config web.config.dateOfToday.bak

notepad web.config

Locate the element

<code>&lt;</code><code>resourceManagementClient</code> <code>. . . /&gt;</code>

Add requireKerberos=”true” so that it reads

<code>&lt;</code><code>resourceManagementClient</code>

<code>requireKerberos</code><code>=</code><code>"true"</code> <code>. . . /&gt;</code>