Microsoft KB Archive/288323

= Web browsers receive &quot;HTTP 401.5&quot; or &quot;Unable to Find Server or DNS Error&quot;error when you use Windows NT Challenge/Response =

Article ID: 288323

Article Last Modified on 12/3/2007

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APPLIES TO


 * Microsoft Internet Information Server 4.0
 * Microsoft Internet Information Services 5.0
 * Microsoft Internet Information Services 5.1
 * Microsoft Internet Information Services 6.0

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This article was previously published under Q288323



We strongly recommend that all users upgrade to Microsoft Internet Information Services (IIS) version 6.0 running on Microsoft Windows Server 2003. IIS 6.0 significantly increases Web infrastructure security. For more information about IIS security-related topics, visit the following Microsoft Web site:

http://www.microsoft.com/technet/security/prodtech/IIS.mspx



SYMPTOMS
This error can occur when Internet Information Server is set to require Microsoft Windows NT Challenge/Response on a Web site or virtual directory.

Windows NT 4.0:

After you install Microsoft Windows NT 4.0 Service Pack (SP) 6a, you may receive the following error message:

HTTP 401.5

Unauthorized: Authorization failed by ISAPI/CGI app.

After you install Microsoft Internet Information Services (IIS) 5.0 on a computer that is running Windows Server 2000, or IIS 6.0 on a computer that is running Windows Server 2003, you may receive the following message:

Unable to Find Server or DNS Server Error.



CAUSE
All versions HTTP Keep-Alives are required for Windows NT 4.0 Challenge/Response to work properly. If the HTTP Keep-Alives Enabled check box has been cleared, the IIS metabase has not been updated with the correct setting, and therefore, Keep-Alives are still enabled. Windows NT Challenge/Response continues to work properly.

Windows NT 4.0 Pre-SP6a After you install Windows NT 4.0 SP 6a, the metabase setting is corrected to match the HTTP Keep-Alives Enabled check box. Because Windows NT Challenge/Response requires HTTP Keep-Alives, the now disabled setting prevents Windows NT Challenge/Response from working correctly.



RESOLUTION

 * 1) In the Internet Services Manager or in the IIS Manager, right-click the Web site for which you want to enable Windows NT Challenge/Response, and then click Properties.
 * 2) Click the Web Site tab, and then click to select the HTTP Keep-Alives Enabled check box or the Enable HTTP Keep-Alives check box.
 * 3) Click OK.



MORE INFORMATION
For more information about Windows NT Challenge/Response, click the following article numbers to view the articles in the Microsoft Knowledge Base:

102716 NTLM user authentication in Windows

286128 Windows Integrated Authentication (NTLM) doesn't work, Basic does

