Troubleshooting HTTP 401 errors in IIS

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Hello. My name is Lou Prete. I have been supporting Microsoft Internet Information Services (IIS) for the past five years and have been IIS content lead for the past two years.

HTTP 401 errors are among the most common errors you may have to deal with in IIS. While the causes for these errors can vary greatly, the causes fall into a finite number of categories. Correctly identifying the category of the cause for your HTTP 401 error can decrease the amount of time needed to identify the root cause of the error.

This article describes the troubleshooting steps when you encounter HTTP 401.x errors (401.1, 401.2, 401.3, 401.4, 401.5) in Microsoft Internet Information Services (IIS):

Troubleshooting steps

For IIS 6.0 or the later versions of IIS 6.0

Starting in IIS 6.0, the substatus code is logged in the
Web logs. The Web logs are located in the following location:
In the Web logs, the last three numbers in each entry represent
the status, the substatus, and the Win32 status.
#Fields: date time s-sitename s-ip cs-method cs-uri-stem cs-uri-query s-port cs-username c-ip cs(User-Agent) sc-status sc-Sub-status sc-win32-status2006-03-06 20:37:42 W3SVC1 GET /default.aspx - 80 - Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+SV1;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+InfoPath.1) 401 2 21480742542006-03-06 20:37:42 W3SVC1 GET /default.aspx - 80 - Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+SV1;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+InfoPath.1) 401 1 02006-03-06 20:38:36 W3SVC1 GET /default.aspx - 80 DOMAIN\user Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1;+SV1;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+InfoPath.1) 200 0 0

Useful tools and resources

  • WFetch
    284285 How to use Wfetch.exe to troubleshoot HTTP connections

  • Network Monitor
    148942 How to capture network traffic with Network Monitor

  • Auditing/Security log
    300549 How to enable and apply security auditing in Windows 2000

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