Troubleshooting Internet Information Services (IIS)

A comprehensive guide to diagnosing and resolving common IIS issues.

1. Common IIS Errors and Their Solutions

1.1 HTTP 404 - Not Found

This error indicates that the requested resource could not be found on the server. Common causes include:

• Incorrect URL entered by the client.
• The file or directory does not exist at the specified path.
• Permissions issues preventing IIS from accessing the file.
• Incorrect application pool identity.

Troubleshooting Steps:

1. Verify the URL is correct and the file exists.
2. Check file system permissions for the IIS process user (e.g., IUSR, ApplicationPoolIdentity).
3. Ensure the Default Document is configured correctly for the site.
4. Review the Application Pool identity and ensure it has necessary read permissions.

1.2 HTTP 500 - Internal Server Error

A generic error indicating that something went wrong on the server. This can be caused by:

• Errors in ASP.NET or other server-side code.
• Issues with the application pool (e.g., crashing, recycling).
• Configuration errors in web.config.
• Insufficient permissions.

Troubleshooting Steps:

1. Enable Failed Request Tracing (FREB) for detailed error logging.
2. Check the Windows Event Viewer for Application and System logs.
3. Review your application's code and web.config for syntax errors or logical issues.
4. Restart the IIS application pool or the entire IIS service.

1.3 HTTP 403 - Forbidden

This error means the server understood the request but refuses to authorize it. Reasons include:

• Lack of read permissions on the requested file or directory.
• Anonymous access is disabled and no authentication is provided.
• Directory browsing is disabled, and no default document is present.

Troubleshooting Steps:

1. Check file/directory permissions.
2. Configure authentication settings in IIS Manager.
3. Ensure a default document is available if directory browsing is off.

2. IIS Logging and Diagnostics

2.1 IIS Log Files

IIS generates detailed log files that are crucial for troubleshooting. By default, they are located in:

%SystemDrive%\inetpub\logs\LogFiles\W3SVC[site_id]

Key fields to examine include:

• date, time: Timestamp of the request.
• s-ip: Server IP address.
• cs-method: HTTP method (GET, POST, etc.).
• cs-uri-stem: Requested URL path.
• cs-uri-query: Query string.
• sc-status: HTTP status code (e.g., 200, 404, 500).
• sc-substatus: IIS sub-status code for more specific error information.
• time-taken: Time in milliseconds to process the request.

2.2 Failed Request Tracing (FREB)

FREB is a powerful tool that captures detailed information about individual requests, including the steps IIS takes to process them. To enable it:

1. Open IIS Manager.
2. Select the server or site you want to configure.
3. In the Features View, double-click "Failed Request Tracing Rules".
4. Click "Add..." in the Actions pane.
5. Configure the rules to capture specific status codes (e.g., 400-599).

FREB logs are typically found in:

%SystemDrive%\inetpub\logs\FailedReqLogFiles\W3SVC[site_id]

3. Application Pool Issues

3.1 Application Pool Recycling

Application pools can recycle periodically or when certain conditions are met (e.g., memory limits, errors). While normal, frequent recycling might indicate underlying problems.

Troubleshooting:

• Check the "Advanced Settings" of the Application Pool for recycling triggers.
• Monitor memory usage of the application pool.
• Investigate application code for memory leaks or unhandled exceptions that might cause crashes.

3.2 Application Pool Identity

The identity under which an application pool runs is critical for file system and network access. Common identities include:

• ApplicationPoolIdentity: A virtual account specific to the application pool. Recommended for security.
• NetworkService, LocalService: Built-in accounts with limited privileges.
• LocalSystem: Highly privileged account; use with extreme caution.
• Custom user accounts.

Troubleshooting:

Ensure the application pool identity has the necessary read/write permissions on the IIS content directories and any other resources your application needs to access.

4. Configuration Errors (web.config)

Incorrectly formatted or conflicting settings in web.config files are a frequent source of IIS problems.

Troubleshooting:

• Validate your web.config syntax using an XML validator or an online tool.
• Use IIS Manager to check site/application configurations, which often override or complement web.config settings.
• Temporarily rename web.config to see if the error is resolved, indicating an issue within the file.

5. Performance Issues

Slow response times can be caused by various factors:

• Inefficient application code or database queries.
• High CPU or memory usage by the server or application pool.
• Network bottlenecks.
• Under-provisioned server resources.

Troubleshooting:

• Use IIS Performance Monitor counters.
• Analyze application logs and profiler tools.
• Check server resource utilization (Task Manager, Performance Monitor).
• Implement caching mechanisms where appropriate.