Microsoft KB Archive/902160

= You receive &quot;HTTP Error 401&quot; error messages, and you intermittently cannot connect to a Web site that is hosted on IIS 6.0 =

Article ID: 902160

Article Last Modified on 12/3/2007

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APPLIES TO


 * Microsoft Internet Information Services 6.0

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SYMPTOMS
You intermittently cannot connect to a Web site that is hosted on Microsoft Internet Information Services (IIS) 6. When this behavior occurs, you may receive an error message that is similar to the following:

HTTP Error 401 - Unauthorized: Access is denied due to invalid credentials.

This behavior occurs if the following conditions are true:
 * The Web site that you try to view uses Integrated Windows authentication.
 * The application pool for the Web site recycles during the authentication process.



CAUSE
The NTLM protocol that is used for Integrated Windows authentication requires a two-step process to authenticate clients. The behavior that is mentioned in the &quot;Symptoms&quot; section occurs when the following conditions are true:
 * The application pool recycles after the first step of the Integrated Windows authentication process.
 * However, the application pool recycles before the second step of the Integrated Windows authentication process.



WORKAROUND
To work around this behavior and to reduce the frequency of these error messages, configure the application pool to recycle less frequently. To configure application pool recycling, follow these steps:
 * 1) Click Start, click Run, type inetmgr, and then click OK.
 * 2) Expand the name of the computer on which you want to configure application pool recycling, and then expand Application Pools.
 * 3) Right-click the application pool that you want to configure, and then click Properties.
 * 4) On the Recycle tab, configure one of the metrics so that IIS recycles less frequently.

Note IIS 6 application pools support several metrics. These metrics include the time elapsed, the number of requests, and the specified time to recycle an application pool. If an application pool has a metric that causes the application pool to recycle frequently, you will experience this issue more frequently.
 * 1) Click OK.



MORE INFORMATION
For more information about how to use application pool recycling, visit the following Microsoft Web site:

http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/18b3a3cd-ff5f-4a48-bc63-810be04b131d.mspx

For more information about how IIS authenticates browser clients by using Windows Integrated authentication, click the following article number to view the article in the Microsoft Knowledge Base:

264921 How IIS authenticates browser clients

Keywords: kbtshoot kbprb KB902160

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