Troubleshooting HTTP 401 errors in IIS

Introduction

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.

A great tool for troubleshooting these issues is Authentication and Access Control Diagnostics, or AuthDiag. You can download this tool from the following Microsoft Download Center Web site:This tool is also part of the IIS Diagnostics Toolkit that you can download from the following Microsoft Download Center Web site: In almost every situation described in this column, AuthDiag can provide quick answers to the HTTP 401 error at hand.

In this column, I will outline an effective approach to identifying and correcting the common problems that lead to HTTP 401 errors. I will also point out a number of Microsoft Knowledge Base articles that will be useful and a number of tools that will help you along your way.

Troubleshooting steps

Identify the substatus code of the HTTP 401 error

There are two common ways to identify the substatus code:

• Starting in IIS 6.0, the substatus code is logged in the Web logs. The Web logs are located in the following location:
  %SYSTEMROOT%\System32\LogFiles\W3SVC###\
  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-status
  2006-03-06 20:37:42 W3SVC1 192.168.1.101 GET /default.aspx - 80 - 192.168.17.45 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 2148074254
  2006-03-06 20:37:42 W3SVC1 192.168.1.101 GET /default.aspx - 80 - 192.168.17.45 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 0
  2006-03-06 20:38:36 W3SVC1 192.168.1.101 GET /default.aspx - 80 DOMAIN\user 192.168.17.45 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

• In versions of IIS earlier than IIS 6.0, the substatus code is not logged in the Web logs. In these cases (or in cases where you don't have access to the Web logs), you can use the information sent back to the browser. In Microsoft Internet Explorer, you will have to disable the Show Friendly HTTP Error messages setting. With this change, you should see an error page similar to the one below. In this case, we got an HTTP 401.2 error, and the page even gives a brief description of what the error means:
  You are not authorized to view this page
  
  You do not have permission to view this directory or page using the credentials that you supplied because your Web browser is sending a WWW-Authenticate header field that the Web server is not configured to accept.
  
  Please try the following:
  Contact the Web site administrator if you believe you should be able to view this directory or page.
  Click the Refresh button to try again with different credentials.
  
  HTTP Error 401.2 - Unauthorized: Access is denied due to server configuration. Internet Information Services (IIS)
  
  Technical Information (for support personnel)
  Go to Microsoft Product Support Services and perform a title search for the words HTTP and 401.
  Open IIS Help, which is accessible in IIS Manager (inetmgr), and search for topics titled About Security, Authentication, and About Custom Error Messages.

Note You can also use tools, such as WFetch and Network Monitor, to gather substatus codes. For more information about these tools, click the following article numbers to view the articles in the Microsoft Knowledge Base:

Base your troubleshooting on the substatus code

Once you know the HTTP substatus code, focus on issues related to that particular substatus. All others can be ignored.

HTTP 401.1: Denied by invalid user credentials

Description

IIS failed to log on a user to execute the request. All requests must be associated with a user, even if the request is anonymous.

Common reasons

• The wrong user name or password is provided. Identify the user who failed to log on, and correct the user name or password.
• Kerberos authentication fails. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
  326985

  (http://support.microsoft.com/kb/326985/ )

  How to troubleshoot Kerberos-related issues in IIS
  Other useful Kerberos articles are as follows:
  871179

  (http://support.microsoft.com/kb/871179/ )

  You receive an "HTTP Error 401.1 - Unauthorized: Access is denied due to invalid credentials" error message when you try to access a Web site that is part of an IIS 6.0 application pool
  Configuring Application Pool Identity with IIS 6.0 (IIS 6.0)
  http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/f05a7c2b-36b0-4b6e-ac7c-662700081f25.mspx

  (http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/f05a7c2b-36b0-4b6e-ac7c-662700081f25.mspx)

  
  
  Integrated Windows Authentication (IIS 6.0)
  http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/523ae943-5e6a-4200-9103-9808baa00157.mspx

  (http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/523ae943-5e6a-4200-9103-9808baa00157.mspx)

  
  
  Configuring Constrained Delegation for Kerberos (IIS 6.0)
  http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/df979570-81f6-4586-83c6-676bb005b13e.mspx

  (http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/df979570-81f6-4586-83c6-676bb005b13e.mspx)
• The local or domain policy or the user rights assignment prevents the user from accessing the server. If the server is configured to audit logon failures, there may be additional information in the Security log. Refer to the following articles for the required user rights:
  812614

  (http://support.microsoft.com/kb/812614/ )

  Default permissions and user rights for IIS 6.0
  271071

  (http://support.microsoft.com/kb/271071/ )

  How to set required NTFS permissions and user rights for an IIS 5.0 Web server
  832981

  (http://support.microsoft.com/kb/832981/ )

  Users cannot access Web sites when the security event log is full
  300549

  (http://support.microsoft.com/kb/300549/ )

  How to enable and apply security auditing in Windows 2000
• This error may also occur when anonymous access is configured. This may occur if the user name or password for the anonymous account that is stored in the IIS metabase differs from the actual information stored in the local user database (or the Active Directory directory service, if a domain account is used). Resetting the password for the account and in IIS resolves this problem.
• After you upgrade a server running IIS 5.0 to IIS 6.0, IIS is running in IIS 5.0 compatibility mode. Once the server is switched to IIS 6.0 isolation mode, you may see HTTP 401.1 errors on anonymous requests. This occurs because of IIS 5.0 anonymous password synchronization. To resolve this problem, set the AnonymousPasswordSync metabase key to false, and reset the anonymous user's password for the account and in IIS.
• For more information about this error, click the following article numbers to view the articles in the Microsoft Knowledge Base:
  896861

  (http://support.microsoft.com/kb/896861/ )

  You receive error 401.1 when you browse a Web site that uses Integrated Authentication and is hosted on IIS 5.1 or IIS 6
  304201

  (http://support.microsoft.com/kb/304201/ )

  Cannot access Web sites or cannot start IIS services that run under non-local system account and use Windows authentication with IIS
  263140

  (http://support.microsoft.com/kb/263140/ )

  Anonymous and Basic authentication fail when you connect to IIS 5.0 on a domain controller

HTTP 401.2: Denied by server configuration

Description

The client browser and IIS could not agree on an authentication protocol.

Common reasons

• No authentication protocol (including anonymous) is selected in IIS. At least one authentication type must be selected. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
  253667

  (http://support.microsoft.com/kb/253667/ )

  Error message: HTTP 401.2 - Unauthorized: Logon failed due to server configuration with no authentication
• Only Integrated authentication is enabled, and an older, non-Internet Explorer client browser tries to access the site. This happens because the client browser cannot perform Integrated authentication. To resolve this problem, use one of the following methods:
  • Configure IIS to accept Basic authentication. This should only occur over SSL for security purposes.
  • Use a client browser that can perform Integrated authentication. Internet Explorer and new versions of Netscape Navigator and Mozilla Firefox can perform Integrated authentication.
• Integrated authentication is through a proxy. This happens because the proxy doesn't maintain the NTLM-authenticated connection and thus sends an anonymous request from the client to the server. Options to resolve this problem are as follows:
  • Configure IIS to accept Basic authentication. This should only occur over SSL for security purposes.
  • Don't use a proxy.

HTTP 401.3: Denied by resource ACL

Description

This error is returned when the user successfully authenticated to the server, but the user does not have NTFS permissions to the content requested.

Common solutions

• Set the NTFS permissions correctly on the content. Review the "NTFS Permissions" section in the following articles:
  812614

  (http://support.microsoft.com/kb/812614/ )

  Default permissions and user rights for IIS 6.0
  271071

  (http://support.microsoft.com/kb/271071/ )

  How to set required NTFS permissions and user rights for an IIS 5.0 Web server
• Verify that the correct authentication method is set. For example, when you use Integrated authentication, users are not prompted for authentication credentials. In this instance, it may be unclear if the request is authenticating or not.
• If the content is located on a remote share, verify that users have sufficient NTFS and Share permissions. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
  332142

  (http://support.microsoft.com/kb/332142/ )

  NTLM requests for content on UNC share may be returned with 401 error messages

HTTP 401.4: Denied by custom ISAPI filter

Description

An ISAPI filter loaded denied the request.

Solution

Identify which ISAPI filter denied the request, and contact the developer or vendor to determine a solution.

HTTP 401.5: Denied by custom ISAPI/CGI Web application

Description

An ISAPI extension or CGI application denied the request.

Solution

Identify which ISAPI extension or CGI application denied the request, and contact the developer or vendor to determine a solution.

Summary

In summary, when you troubleshoot HTTP 401 errors, the first step should always be to determine the substatus code.

• 401.1: Authentication was attempted, but failed.
• 401.2: Authentication was not attempted because the server and client could not agree on an authentication protocol.
• 401.3: Authentication was successful, but the account that authenticated does not have sufficient permissions to access the requested resource or content.
• 401.4: An ISAPI filter denied the request.
• 401.5: An ISAPI extension or CGI application denied the request.

Useful tools and resources

Microsoft tools

• Authentication and Access Control Diagnostics 1.0
  http://www.microsoft.com/downloads/details.aspx?FamilyID=e90fe777-4a21-4066-bd22-b931f7572e9a&DisplayLang=en

  (http://www.microsoft.com/downloads/details.aspx?FamilyID=e90fe777-4a21-4066-bd22-b931f7572e9a&DisplayLang=en)
• IIS Diagnostics Toolkit
  http://www.microsoft.com/downloads/details.aspx?FamilyID=9bfa49bc-376b-4a54-95aa-73c9156706e7&DisplayLang=en

  (http://www.microsoft.com/downloads/details.aspx?FamilyID=9bfa49bc-376b-4a54-95aa-73c9156706e7&DisplayLang=en)
• WFetch
  284285

  (http://support.microsoft.com/kb/284285/ )

  How to use Wfetch.exe to troubleshoot HTTP connections
• Network Monitor
  148942

  (http://support.microsoft.com/kb/148942/ )

  How to capture network traffic with Network Monitor
• Auditing/Security log
  300549

  (http://support.microsoft.com/kb/300549/ )

  How to enable and apply security auditing in Windows 2000

Third-party tools

• Filemon
  http://technet.microsoft.com/en-us/sysinternals/bb896642.aspx

  (http://technet.microsoft.com/en-us/sysinternals/bb896642.aspx)
• Regmon
  http://technet.microsoft.com/en-us/sysinternals/bb963881.aspx

  (http://technet.microsoft.com/en-us/sysinternals/bb963881.aspx)

Until next time, thank you for your time, and have a great day. As always, feel free to submit ideas on topics you want addressed in future columns or in the Knowledge Base using the Ask For It form.

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