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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsConfigure IIS’s <httpErrors> section under <system.webServer>. For a safe troubleshooting setup, use errorMode="DetailedLocalOnly": requests from the server itself receive details, while remote clients receive your custom response. Set this in ApplicationHost.config for server-wide behavior or in an application or site Web.config when delegation permits it.
First identify which layer generated the error
IIS-generated HTTP errors are controlled by <system.webServer><httpErrors>. ASP.NET’s <customErrors> section is separate and applies to framework-generated errors. Changing one does not automatically change the other.
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Before editing configuration, reproduce the failure and note the HTTP status and IIS substatus. A 404 substatus, for example, can distinguish a missing file from an unmapped extension, missing handler, filtering rule, or hidden file. The status and substatus tell you which error entry IIS must match and whether the underlying problem is routing, configuration, security, or application code.
Choose who should see details
| Goal | Setting | Result |
|---|---|---|
| Debug locally without exposing internals | errorMode="DetailedLocalOnly" |
Detailed errors for local requests; configured custom errors for external requests. This is the documented default. |
| Show a friendly page to every client | errorMode="Custom" |
Uses custom errors for local and remote requests, including when the request originates on the server. |
| Temporarily diagnose a problem from any client | errorMode="Detailed" |
Sends detailed errors to all clients. Do not leave this enabled on a publicly reachable site because the response can disclose internal information. |
Use Detailed only for a controlled, short-lived investigation. Return to DetailedLocalOnly or Custom after testing.
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Add an <httpErrors> element to the relevant configuration scope. This example keeps details local and serves a static file for HTTP 500 responses:
<configuration>
<system.webServer>
<httpErrors errorMode="DetailedLocalOnly" defaultResponseMode="File">
<remove statusCode="500" />
<error statusCode="500"
path="C:inetpubcusterr500.htm"
responseMode="File" />
</httpErrors>
</system.webServer>
</configuration>- Confirm that
C:inetpubcusterr500.htmexists and is readable by IIS. <remove statusCode="500" />removes an inherited entry before defining the site’s version.- The path is a file-system path because the response mode is
File.
Microsoft’s configuration reference also documents language-specific files through prefixLanguageFilePath. Use that pattern when the error page must vary by language.
Understand the three custom-response modes
File: serve static content
Use responseMode="File" when the value of path identifies a file on the server, such as an HTML error document. This is appropriate for a self-contained static page.
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ExecuteURL: run an internal URL
Use responseMode="ExecuteURL" when IIS should execute a server-relative URL internally. The target is an application path, not a file-system path and not an external address.
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Redirect: send the client elsewhere
Use responseMode="Redirect" when the client should receive a redirect. The target must be an absolute URL. A redirect causes a new client request, so it is different from serving or executing an error page inside the current request.
An <error> entry can match a status code and, where needed, a substatus. If an inherited entry prevents the desired match, remove that entry or clear inherited entries at the appropriate scope.
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Control whether IIS replaces an application response
An application, managed module, or framework may already have produced an error body. The existingResponse setting determines whether IIS preserves that body or substitutes the configured custom response.
| Value | Behavior |
|---|---|
PassThrough |
Preserves the existing response. |
Replace |
Replaces the existing response with the IIS custom error response. |
Auto |
Lets IIS apply its ordered rules based on the error mode, whether a response body exists, and whether the producing module set the fTrySkipCustomErrors flag. |
If your application returns a carefully designed 500 or 404 body but IIS shows a different page, inspect existingResponse and the application’s response handling before changing the page path. Conversely, if IIS should enforce one standard page, Replace is the explicit choice.
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- Reproduce and record the code. Capture the HTTP status and IIS substatus from the response, browser tools, or logs.
- Identify the producer. Decide whether IIS, ASP.NET, another framework, or application code generated the response. Configure
<httpErrors>only for the IIS layer. - Check exposure settings. Review
errorModeandexistingResponse. Also check whether application code supplied a body or asked IIS to skip custom errors. - Verify the matching entry. Confirm that the required status and substatus entry exists, and that its
responseModeagrees with the path or URL format you supplied. - Validate scope and inheritance. A server-level setting in
ApplicationHost.configcan be inherited by sites and applications. AWeb.configchange may be blocked when the section is not delegated; in that case, have the server administrator make the change or adjust delegation deliberately. - Trace failures that are hard to reproduce. Configure IIS Failed Request Tracing for the relevant failure condition, then inspect its events. It can capture useful diagnostics for intermittent failures.
Common reasons the custom page does not appear
The request is local
With DetailedLocalOnly, a request made on the IIS server is expected to show detailed information rather than the remote custom page. Test from a separate client when checking the external experience, or temporarily choose Custom.
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The application supplied its own body
existingResponse="Auto" may preserve an application response, especially when the module indicates that IIS should skip custom errors. Inspect the response producer and choose PassThrough or Replace intentionally.
The target value uses the wrong kind of path
A file-system path belongs with File; an internal server-relative URL belongs with ExecuteURL; an absolute URL belongs with Redirect. Mixing these formats prevents the intended action.
An inherited entry wins
Remove the specific inherited status entry, or clear inherited entries where appropriate, before adding the site-level rule. Confirm that the configured status and substatus actually match the failure.
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The error is not an IIS error
If ASP.NET or application code generated the response, configure that framework or code path instead of expecting <httpErrors> to control it.
Version and deployment scope
The <httpErrors> section was introduced in IIS 7.0 and the Microsoft reference lists it as unchanged in IIS 8.0, 8.5, and 10.0. IIS 6.0 uses a different metabase property. Configuration support and delegation still depend on how the server is administered, so verify the target IIS version and whether the section is allowed at the scope where you are editing.
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