Microsoft Developer Network

Troubleshooting Internet Information Services (IIS)

This article provides a comprehensive guide to diagnosing and resolving common issues encountered with Microsoft Internet Information Services (IIS).

1. Understanding IIS Architecture and Logs

Before diving into troubleshooting, it's crucial to understand the basic components of IIS and where to find critical information. Key components include:

• Worker Processes (w3wp.exe): The process that hosts your web applications.
• Application Pools: Collections of one or more URLs that are grouped together for the purpose of using a specific set of Apache, .NET, or PHP resources.
• Event Viewer: A system utility that records application and system events.
• IIS Log Files: Located typically in %SystemDrive%\inetpub\logs\LogFiles, these logs record HTTP requests.

Tip: Ensure that logging is enabled for your websites and application pools. Regularly review IIS logs for patterns of errors, unusual request volumes, or specific error codes.

2. Common IIS Errors and Their Solutions

2.1 HTTP 500 Internal Server Error

This is a generic error indicating that something went wrong on the server. Common causes include:

• Application Errors: Unhandled exceptions in your web application code (.NET, ASP, PHP). Check the application's specific error logs.
• Configuration Issues: Incorrect settings in web.config, applicationHost.config, or IIS metabase.
• Permissions Problems: The IIS worker process (w3wp.exe) lacks the necessary permissions to access files or resources.

Troubleshooting Steps:

1. Enable detailed error messages in IIS (temporarily, for development environments).
2. Review the Windows Application and System Event Logs for specific error details.
3. Check your application's logs for exceptions.
4. Verify file and folder permissions for the IIS user account (e.g., IUSR, Application Pool Identity).

2.2 HTTP 404 Not Found

The requested resource could not be found. This usually means:

• The file or directory does not exist at the specified location.
• The URL is misspelled.
• IIS is not configured to serve the requested file type (e.g., missing handler mapping for a specific extension).
• URL Rewrite rules are incorrectly configured.

Troubleshooting Steps:

1. Verify the existence and correct path of the requested file/directory on the server.
2. Check for typos in the URL.
3. Ensure the Handler Mappings are correctly configured in IIS for the requested file extension.
4. Review your URL Rewrite rules if applicable.

2.3 HTTP 403 Forbidden

Access to the requested resource is denied. This can occur due to:

• Directory Browsing Disabled: If there's no default document (like index.html or default.aspx) in a directory, and directory browsing is disabled, you'll get a 403.
• File/Directory Permissions: The IIS user account lacks read permissions.
• IP Address Restrictions: Specific IP addresses might be blocked.
• Authentication Issues: Failed authentication attempts.

Troubleshooting Steps:

1. Ensure a default document exists in the requested directory or enable Directory Browsing (use with caution).
2. Verify file and directory permissions for the IIS user.
3. Check IP Address and Domain Restrictions in IIS.
4. Examine authentication settings.

2.4 Application Pool Crashes / Recycles

Frequent application pool recycles or crashes can lead to intermittent site unavailability.

Causes:

• Application exceptions or unmanaged code errors.
• Memory leaks.
• Configuration errors.
• Overloaded server resources.

Troubleshooting Steps:

1. Configure the application pool to log w3wp.exe failures in the Event Viewer.
2. Check the application pool's recycling settings (e.g., maximum number of worker processes, idle time-out).
3. Use tools like Debug Diagnostic (DebugDiag.exe) to capture crash dumps for w3wp.exe.
4. Monitor server resources (CPU, Memory).

3. Advanced Troubleshooting Tools and Techniques

3.1 IIS Failed Request Tracing

IIS Failed Request Tracing provides detailed, request-level information about problems occurring on your web server. It can trace every step of the request processing pipeline.

To enable:

1. Open IIS Manager.
2. Select the site or server.
3. Double-click "Failed Request Tracing Rules".
4. Click "Add..." in the Actions pane.
5. Configure rules based on status codes (e.g., 400-599) or specific event types.

Traced logs are typically found in %SystemDrive%\inetpub\logs\FailedReqLogFiles.

3.2 Sysinternals Tools

Microsoft's Sysinternals suite offers powerful tools for diagnosing system issues:

• Process Explorer: Provides detailed information about running processes, including w3wp.exe.
• ProcMon (Process Monitor): Logs file system, registry, process, and network activity in real-time, invaluable for pinpointing permission or access issues.

3.3 Performance Monitoring

Use Performance Monitor (perfmon.exe) to track key IIS counters such as:

• Web Service\Total Internet Server Connection Attempts
• Web Service\Total Errors/sec
• ASP.NET Applications\# of Application Restarts
• Process\% Processor Time and Working Set for w3wp.exe

4. Best Practices for Preventing Issues

• Keep IIS and Windows updated with the latest security patches.
• Regularly review IIS and application logs.
• Implement robust error handling in your web applications.
• Use dedicated application pools for different websites or applications to isolate them.
• Test configuration changes thoroughly in a staging environment before deploying to production.

By systematically approaching troubleshooting and leveraging the right tools, you can effectively diagnose and resolve most IIS-related problems.