In Microsoft Forefront Threat Management Gateway 2010, you use the HTTP request connection method in a connectivity verifier to check web server availability. After the connectivity verifier determines that a web server is offline, the connectivity verifier may not always detect when the web server comes back online.
This problem may occur under certain timing conditions. When a web server goes offline, the connectivity verifier may enter a condition in which it cannot detect when the web server comes back online.
To resolve this issue, install the hotfix package that is described in the following Microsoft Knowledge Base article:
2689195 Rollup 2 for Forefront Threat Management Gateway (TMG) 2010 Service Pack 2
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about Forefront Threat Management Gateway connectivity verifiers, go to the following Microsoft TechNet website: