Service Principal Name (SPN) checklist for Kerberos authentication with IIS 7.0/7.5

Service Principal Name (SPN) checklist for Kerberos authentication with IIS 7.0/7.5

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In continuation to one of my earlier posts which focused on IIS 6.0 this post is more about the confusion that may arise around SPNs for setting up Kerberos authentication in IIS 7.0. IIS 7.0 has a new Kernel-mode authentication feature using which the ticket for the requested service is decrypted using Machine account (Local system) of the IIS server. It no longer depends upon the application pool Identity for this purpose by default and in turn improves the performance.

Here is how it looks like.

image

image

So what does this mean?

You no longer need to worry about the correlation between HTTP SPNs and the Application pool Identity that was required in the earlier version i.e. IIS 6.0. But that's not blindly true. There has been some confusion whether we don't have to care at all about SPNs or may have to depending upon the settings. Here is a checklist to give more clarity for different scenarios that you may fall under:

SCENARIO 1a

IIS 7.0 Web Site/Application  
Authentication Integrated Windows authentication
Application Pool Identity NETWORK SERVICE
Kernel-Mode authentication Enabled (<attribute name="useKernelMode" type="bool" defaultValue="true" /> in the ApplicationHost.config file)
Site URL Accessed with the NetBIOS name, like http://<myIISserver-NetBIOS-name>/Default.aspx

SPNs will be required ONLY for the IIS machine account:

HOST/<myIISserver-NetBIOS-name>
HOST/<myIISserver-NetBIOS-name.fully-qualified-domainname> for e.g. HOST/myIISserver.mydomain.com

***Note: By default HOST/<myIISserver-NetBIOS-name> and HOST/<myIISserver-NetBIOS-name.fully-qualified-name> is already added for the machine account when a machine is added to a domain and HTTP forms a part of HOST. So you may not have to do anything special here for SPNs. Everything should be set by default.

You can check the set of existing SPNs for the machine account by running the following command:

> Setspn.exe -L <myIISserver-NetBIOS-name> or directly using a Snap-in like Adsiedit.msc.

SCENARIO 1b

IIS 7.0 Web Site/Application  
Authentication Integrated Windows authentication
Application Pool Identity Custom account for e.g. Domain1\Username1
Kernel-Mode authentication Enabled (<attribute name="useKernelMode" type="bool" defaultValue="true" /> in the ApplicationHost.config file)
Site URL Accessed with the NetBIOS name, like http://<myIISserver-NetBIOS-name>/Default.aspx

The SPN requirements remain the same as above. You don't have to add SPNs like http/<myIISserver-NetBIOS-name> for the Domain1\Username1 unlike in IIS 6.0 (where we had to add an SPN of the form http/<myIISserver-NetBIOS-name> for the Application Pool identity).

SPNs will be required ONLY for the IIS machine account:

HOST/<myIISserver-NetBIOS-name>
HOST/<myIISserver-NetBIOS-name.fully-qualified-domainname> for e.g. HOST/myIISserver.mydomain.com

***Note: By default HOST/<myIISserver-NetBIOS-name> and HOST/<myIISserver-NetBIOS-name.fully-qualified-name> is already added for the machine account when a machine is added to a domain and HTTP forms a part of HOST. So you may not have to do anything special here for SPNs. Everything should be set by default.

You can check the set of existing SPNs for the machine account by running the following command:

> Setspn.exe -L <myIISserver-NetBIOS-name> or directly using Snap-in like Adsiedit.msc.

 

SCENARIO 2a

IIS 7.0 Web Site/Application  
Authentication Integrated Windows authentication
Application Pool Identity NETWORK SERVICE
Kernel-Mode authentication Enabled (<attribute name="useKernelMode" type="bool" defaultValue="true" /> in the ApplicationHost.config file)
Site URL Accessed with a Custom Host name, like http://www.mysite.com


SPNs will be required ONLY for the IIS machine account in the following format:

HTTP/<site-custom-name> for e.g. HTTP/www.mysite.com

You can add an SPN using Setspn.exe like

> Setspn -a http/<site-custom-name> <myIISserver-NetBIOS-name> 

where <myIISserver-NetBIOS-name> is the IIS machine account and <site-custom-name> is the custom host/host header name for the Web Site URL.

e.g. > Setspn -a http/www.mysite.com <myIISserver-NetBIOS-name>
*The command is NOT case sensitive

You can check the existing set of SPNs for the machine account by running the following command:

> Setspn.exe -L <myIISserver-NetBIOS-name>

SCENARIO 2b

IIS 7.0 Web Site/Application  
Authentication Integrated Windows authentication
Application Pool Identity Custom account for e.g. Domain1\Username1
Kernel-Mode authentication Enabled (<attribute name="useKernelMode" type="bool" defaultValue="true" /> in the ApplicationHost.config file)
Site URL Accessed with a Custom host/Host header name, like http://www.mysite.com


SPNs will be required ONLY for the IIS machine account and NOT for Domain1\Username1 account unlike in IIS 6.0.

HTTP/<site-custom-name> for e.g. HTTP/www.mysite.com

You can add an SPN using Setspn.exe like

> Setspn -a http/<site-custom-name> <myIISserver-NetBIOS-name> where <myIISserver-NetBIOS-name> is the IIS machine account and <site-custom-name> is the custom host/host header name for the Web Site URL.

e.g. > Setspn -a http/www.mysite.com <myIISserver-NetBIOS-name>
*The command is NOT case sensitive  

You can check the existing set of SPNs for the machine account by running the following command:

> Setspn.exe -L <myIISserver-NetBIOS-name>

Special case of running IIS 7.0 in a WEB FARM

If you are running IIS 7.0 server in a Web farm the KDC will not know in advance which individual server the request may go to and hence ticket decryption may fail. Hence in such a scenario instead of registering SPNs under a specific machine account use a domain account. I am not a SharePoint guy but based on what I have read on the Web this scenario is also applicable to a single SharePoint server configuration.

There are two ways to go:

Either

Disable Kernel mode authentication and follow the general steps for Kerberos as in the previous IIS 6.0 version. Refer this

Or,

[Recommended for Performance reasons]

Let Kernel mode authentication be enabled and the Application pool's identity be used for Kerberos ticket decryption. The only thing you need to do here is:

1. Run the Application pool under a common custom domain account.

2. Add this attribute "useAppPoolCredentials" in the ApplicationHost.config file.

<system.webServer>
   <security>
      <authentication>
         <windowsAuthentication enabled="true" useKernelMode="true" useAppPoolCredentials="true" />
      </authentication>
   </security>
</system.webServer>

Remember there is no GUI setting for this. You need to modify the ApplicationHost.config file from

<%SystemDrive%>/Windows/System32/inetsrv/config folder on the IIS 7.0 machine.

3. Add the SPNs in the form:

http/<virtualhost-name> and

http/<virtualhost-name.fully-qualified-name>  for the Application Pool Identity.

Ensure that we don't have such an entry for SPNs for any other account including IIS server machine account.

***If we have the same SPN mapped to multiple accounts (be it a machine or an user account) it leads to Duplicate SPNs and will break Kerberos.

Hope this helps!

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  • This post is more about the confusion that may arise around SPNs for setting up Kerberos authentication

  • There has been a known issue around Server going into a blue screen (OS crash) when you have Windows Authentication and Kernel Mode authentication enabled plus you have the useAppPoolCredentials attribute set to true in the authentication section in the Applicationhost.config file.

    *Ensure that you apply the following hotfix to the IIS 7.0 Server.

    http://support.microsoft.com/kb/962943

  • Dear Saurabh,

    Thanks a lot for these useful info.

    We would like to deploy on the same server an IIS web server (7 or 7.5) and a Tomcat web server, both using Windows Authentication/Spnego.

    For Tomcat, we use http://spnego.sourceforge.net/ and it works fine but we have to define a SPN (HTPP/<server-NetBIOS-name.fully-qualified-domainname>).

    In that case, can we still use Kernel-mode authentication or should we disable it and configure the application pool identity to use the system account linked to the SPN?

    Thanks,

    Benoit

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