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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsTo make IIS show custom HTTP error messages, configure the <httpErrors> section under <system.webServer>. For a safer production setup, use errorMode="DetailedLocalOnly": requests from the server itself can show diagnostic details, while remote visitors receive configured custom pages. Avoid leaving detailed errors enabled for every visitor.
Choose the right IIS error setting
IIS’s <httpErrors> section controls HTTP errors generated by IIS. ASP.NET’s <customErrors> section is separate, so first identify which layer produced the response. Microsoft documents <httpErrors> for IIS 7.0 and later; IIS 6.0 uses a different metabase property. See the IIS HTTP Errors configuration reference.
The errorMode value determines who sees detailed errors and who sees custom responses:
| Setting | Effect | When to use it |
|---|---|---|
DetailedLocalOnly |
Detailed errors for local requests; custom errors for remote requests. This is the documented default. | Recommended for troubleshooting while limiting information shown to visitors. |
Custom |
Custom errors for all requests, including local ones. | When friendly error pages should be shown universally. |
Detailed |
Detailed errors for all requests. | Only for temporary troubleshooting when access is controlled; detailed responses can expose sensitive information. |
Microsoft explains that DetailedLocalOnly permits local troubleshooting without exposing sensitive information to external browsers in its HTTP errors troubleshooting guide.
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Configure a custom error page
You can configure IIS at the server level in ApplicationHost.config, or at site or application scope in Web.config, subject to the configuration delegation and inheritance rules for that server. The following example keeps detailed errors local and serves a static file for HTTP 500:
<configuration>
<system.webServer>
<httpErrors errorMode="DetailedLocalOnly" defaultResponseMode="File">
<remove statusCode="500" />
<error statusCode="500" path="C:inetpubcusterr500.htm" responseMode="File" />
</httpErrors>
</system.webServer>
</configuration>- Save a valid error page at the specified path, or change the path to the file you intend to serve.
- Add the
<httpErrors>configuration at the appropriate server, site, or application scope. - Use
<remove statusCode="500" />when an inherited entry for that status needs to be removed before adding your own mapping. - Trigger a 500 error and check the response locally and from a remote client to confirm each receives the intended content.
For language-specific files, Microsoft’s configuration reference also documents prefixLanguageFilePath and corresponding error-file entries. Make sure paths and configuration scope are valid for the deployed IIS version.
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Select how IIS delivers the page
Each <error> entry can match a status code and, where needed, a substatus. Its responseMode determines what IIS does with the configured path:
File: serves a static file.ExecuteURL: executes an internal, server-relative URL.Redirect: redirects the client to an absolute URL.
These path formats are not interchangeable. Use a filesystem path for a file response, an internal server-relative path for ExecuteURL, and an absolute URL for Redirect. See Microsoft’s configuration reference for the accepted settings and examples.
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Decide whether IIS replaces an existing response
If application code or a module has already supplied an error response, the existingResponse setting controls whether IIS keeps or replaces it. The setting is especially important when the custom page appears not to take effect:
| Value | Behavior |
|---|---|
PassThrough |
Preserves the existing response. |
Replace |
Replaces the existing response with the configured error response. |
Auto |
Lets IIS apply its rules based on the error mode, existing response content, and whether the module requested that IIS skip custom errors. |
Microsoft documents the behavior in the HTTP Errors reference. If the application returns its own error body, check this setting and the application’s error handling rather than assuming IIS will overwrite the response.
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Troubleshoot when the custom message does not appear
- Record the status and substatus. Reproduce the problem locally and note both values. A 404 substatus can distinguish causes such as an unmapped extension, missing handler, filtering rule, or hidden file.
- Identify which layer generated the response. IIS
<httpErrors>and ASP.NET<customErrors>are separate settings; changing one will not necessarily change errors produced by the other. - Check error handling and response preservation. Review
errorModeandexistingResponse, and determine whether application code supplied a body or asked IIS to skip custom errors. - Validate the matching entry. Confirm the status code and any substatus match, and that the entry’s response mode and path use the correct format.
- Trace failures that are hard to reproduce. Configure Failed Request Tracing for the relevant failure condition, then inspect the captured events. Microsoft notes that it can capture useful tracing information for intermittent or difficult-to-reproduce errors in its Failed Request Tracing guide.
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