Watching 404 Not-Found Errors

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Applies To: Windows Server 2003, Windows Server 2003 with SP1

The Web Service performance object includes a counter that displays not-found errors. Not-found errors result from client requests that were not satisfied because they included a reference to a Web page or a file that does not exist. These errors are identified by their HTTP status code number, which is 404. Many not-found errors occur because Web pages and files have been deleted or moved to another location.

You can use the Web Service\Not Found Errors/sec counter, described in Table 6.30, to track the rate at which not-found errors are occurring on your server. Alternatively, set an alert in Performance Logs and Alerts to notify the administrator when the rate of not-found errors exceeds a threshold.

Table 6.30 Counters for Not-Found Errors

Object\Counter Indicates

Web Service\Not Found Errors/sec

The number of client read requests that were not satisfied because the URL did not point to a valid file.

An increase in not-found errors might indicate that a file has been moved without its link being updated. However, it can also indicate failed attempts to access protected documents, such as user lists and file directories.

An increase in not-found errors might also be related to IIS 6.0 security measures. When you perform a clean installation of IIS 6.0, the default settings ensure that IIS is installed in a secure, locked mode. This default installation serves only static HTTP content, such as .htm files; the request handlers for dynamic content, such as ISAPI and CGI, are disabled.

IIS 6.0 provides new substatus error codes that can help you determine why an HTTP 404 error (file or directory not found) occurred. For example, if a request is denied because an ISAPI extension or CGI is locked, a 404.2 error is returned, and a 404.3 substatus code indicates a MIME map restriction policy.

For more information about analyzing substatus codes, see Troubleshooting IIS 6.0. For more information about unlocking dynamic content, see Common IIS 6.0 Administrative Tasks.