Richard Hicks' Forefront TMG Blog

Immediately after configuring Forefront TMG 2010 to support Kerberos authentication with NLB, you may encounter a scenario where the Forefront TMG management console fails to communicate with the members of the array and includes the following error message:

Unable to retrieve data from: <array_members>

In addition, an Event ID 4 from the Security-Kerberos source is recorded in the system event log:

The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server tmgsvc2. The target name used was tmg3$@richardhicks.net. This indicates that the target server failed to decrypt the ticket provided by the client. This can occur when the target server principal name (SPN) is registered on an account other than the account the target service is using. Please ensure that the target SPN is registered on, and only registered on, the account used by the server. This error can also happen when the target service is using a different password for the target service account than what the Kerberos Key Distribution Center (KDC) has for the target service account. Please ensure that the service on the server and the KDC are both updated to use the current password. If the server name is not fully qualified, and the target domain (RICHARDHICKS.NET) is different from the client domain (RICHARDHICKS.NET), check if there are identically named server accounts in these two domains, or use the fully-qualified name to identify the server.

When making the changes to support Kerberos authentication in load balanced scenarios, the Forefront TMG firewall service is configured to run in the context of a domain user account. When the Forefront TMG management console was first opened, it authenticated to the array members using the Service Principal Name (SPN) registered to the machine (computer) account. As the changes are applied and the services are restarted, the array members are now running in the context of a domain user account. However, the management console continues to send a Kerberos ticket with the SPN registered to the machine account. The Forefront TMG firewall service running in the context of the domain user account cannot decrypt this Kerberos ticket and replies with a Kerberos error.

Resolution in this case is quite simple. Closing the Forefront TMG management console and opening it again will force the client to re-authenticate, this time using the correct SPN. The Kerberos event log errors are anomalous and can safely be ignored.