I recently encountered a situation where users attempting to login to a SharePoint 3.0 site when using the “Sign In” link were recieving an “Internet Explorer cannot display the Web Page” error. The Web Application was configured to allow Anonymous Access and to also use Integrated Windows Authentication.
No errors showed in either the Application or System logs. The IIS Logs displayed a 401 error:
2008-06-20 14:55:01 W3SVC96982807 ServerIP GET /_layouts/Authenticate.aspx Source=%2Fdefault%2Easpx 80 - ClientIP Mozilla/4.0+(BrowserIdentificationString) 401 1 0. Examination of the HTTP traffic with Fiddler showed the same 401.1 error.
Investigation of the Web Site’s properties in IIS showed that HTTP Keep-Alives were disabled — enabling them resolved the problem.
Integrated Windows Authentication (NTLM) requires HTTP Keep-Alives; this is because Microsoft’s NTLM for HTTP authenticates connections, not requests. This means that the HTTP connection must be kept open while the NTLM handshake completes. More technical information can be found here. There is no method that I know of to get around this limitation other than to not use NTLM.
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