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HTTP 404 errors are resolved by verifying file paths, checking virtual directory configurations, and customizing error pages via web.config. 2. HTTP 500 errors require enabling detailed errors, reviewing Event Viewer and Failed Request Tracing logs, validating web.config syntax, and ensuring proper application pool permissions and .NET configuration. 3. HTTP 403 errors are fixed by enabling default documents or directory browsing, configuring IP and domain restrictions, and confirming proper NTFS permissions for the IIS user. 4. HTTP 503 errors are addressed by starting or stabilizing the application pool, adjusting recycling settings, verifying .NET CLR version, and ensuring the app pool identity has adequate privileges. 5. HTTP 401 errors are resolved by configuring appropriate authentication methods in IIS, setting correct NTFS ACLs, and verifying domain connectivity for Windows Authentication. 6. Proactive management includes using custom error pages, monitoring IIS logs, enabling Failed Request Tracing, validating configurations with AppCmd, and ensuring all dependencies like .NET Framework and modules are properly installed and registered, with troubleshooting guided by logs rather than guesswork.

Handling IIS (Internet Information Services) web errors effectively is essential for maintaining a reliable and user-friendly web server. These errors can range from simple configuration issues to deeper application or security problems. Below are common IIS errors and practical steps to diagnose and resolve them.

1. HTTP 404 – Not Found

This error occurs when the requested page or resource doesn’t exist on the server.

Common causes:

• Incorrect URL or broken links
• Missing files or misconfigured virtual directories
• URL Rewrite rules redirecting incorrectly

How to fix:

• Verify the file exists in the correct directory under the site root.
• Check if the requested page has been moved or renamed.
• Ensure virtual directories or applications are configured properly in IIS Manager.
• Use IIS Failed Request Tracing to see which path was attempted.
• Customize the 404 error page to guide users back (via web.config):

<configuration>
  <system.webServer>
    <httpErrors errorMode="Custom">
      <remove statusCode="404" />
      <error statusCode="404" path="/errors/404.html" responseMode="ExecuteURL" />
    </httpErrors>
  </system.webServer>
</configuration>

2. HTTP 500 – Internal Server Error

A generic server-side error indicating something went wrong during processing.

Common causes:

• Misconfigured web.config
• Application pool crashes
• Permission issues
• .NET runtime errors

How to troubleshoot:

• Enable Detailed Errors in IIS:
  • In IIS Manager → select site → Error Pages → Edit Feature Settings → choose "Detailed errors".
• Check Event Viewer (Windows Logs → Application) for ASP.NET or IIS-related errors.
• Enable Failed Request Tracing:
  • In IIS Manager → Failed Request Tracing Rules → Add a rule for status code 500.
  • Review the generated XML logs in C:\inetpub\logs\FailedReqLogFiles.
• Validate web.config syntax (especially <system.webServer> and <handlers> sections).
• Ensure the Application Pool identity has proper permissions to access files and databases.
• If using ASP.NET, check customErrors mode:

<system.web>
  <customErrors mode="Off" />
</system.web>

3. HTTP 403 – Forbidden

Access is denied, even though the server is reachable.

Common variations:

• 403.14 – Directory browsing is disabled and no default document is found.
• 403.6 – IP address rejected.
• 403.7 – Client certificate required.

Solutions:

• For 403.14:
  • Enable directory browsing (not recommended for production) or
  • Add default documents (e.g., index.html, default.aspx) in IIS → Default Document.
• For 403.6 / 403.7:
  • Check IP Address and Domain Restrictions in IIS.
  • Ensure client certificates are properly configured if required.
• Confirm the IIS user (e.g., IIS_IUSRS) has read (and execute) permissions on the site folder.

4. HTTP 503 – Service Unavailable

The server is temporarily unable to handle the request, often due to the application pool being stopped.

Most common cause:

• Application Pool is stopped or crashing.

Steps to resolve:

• Open IIS Manager → Application Pools → check if the pool is Started.
• If it keeps stopping:
  • Check Event Viewer for .NET errors, memory leaks, or recycling issues.
  • Adjust application pool recycling settings (e.g., disable rapid-fail protection temporarily).
  • Set the application pool identity to a user with proper privileges (e.g., ApplicationPoolIdentity is usually sufficient).
  • Ensure the app pool is set to the correct .NET CLR version (e.g., v4.0 for modern apps).
• Test by manually starting the pool and accessing the site.

5. HTTP 401 – Unauthorized

Authentication failed or required credentials weren't provided.

Common subcodes:

• 401.1 – Logon failed
• 401.2 – Authentication failed due to server configuration
• 401.3 – Access denied due to ACLs

How to fix:

• Check Authentication settings in IIS:
  • For public sites: Enable Anonymous Authentication, disable others.
  • For intranet: Enable Windows Authentication.
• Ensure the anonymous user (usually IIS_IUSRS) has proper NTFS permissions.
• If using Windows Auth, verify domain connectivity and user access rights.
• For 401.3, review file/folder ACLs — ensure the app pool identity can read the content.

6. Other Tips for Proactive Error Management

• Use Custom Error Pages: Provide user-friendly messages without exposing server details.

• Monitor Logs Regularly: IIS logs are located by default in %SystemDrive%\inetpub\logs\LogFiles. Use tools like LogParser or ELK stack for analysis.

• Enable Failed Request Tracing: Helps capture detailed info on requests that result in errors (especially 4xx and 5xx).

• Validate Configuration with AppCmd: Use command-line tool to test config:

  %windir%\system32\inetsrv\appcmd validate config

• Check Dependencies: Ensure .NET Framework, ASP.NET registration (aspnet_regiis.exe), and required modules are installed.

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Handling IIS errors efficiently comes down to enabling proper logging, understanding HTTP status codes, and systematically checking configuration, permissions, and application health. Most issues can be resolved by combining IIS Manager, Event Viewer, and failed request logs.

Basically, don’t guess — trace the request and let the logs guide you.

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