ASP.NET Application hosted on IIS7 that is ignoring custom errors and falls back to IIS errors

Question

I have a C# web forms ASP.NET 4.0 web application that uses Routing for URLs for some reason custom errors defined in the system.web section of my web.config is entirely ignored and it will fall back the IIS errors.

This gets entirely ignored

  <system.web>
    <customErrors mode="On">
      <error statusCode="500" redirect="~/Error" />
      <error statusCode="404" redirect="~/404" />
      <error statusCode="403" redirect="~/Error" />
    </customErrors>
  </system.web>

This part takes over

  <system.webServer>
    <httpErrors>
      <!--<clear />-->
      <remove statusCode="500" subStatusCode="-1" />
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" subStatusCode="-1" path="/App1/404" responseMode="Redirect" />
      <error statusCode="500" prefixLanguageFilePath="" path="/App1/Error" responseMode="Redirect" />
    </httpErrors>
  </system.webServer>

This would be a minor inconvenience except that by the fact it falls back to IIS native instead of my application it completely circumvents Elmah logging my 404 exceptions correctly.

Edit: Just to avoid any suggestions of such I only added the httpErrors configuration after customErrors stopped working so I would have anything.

Answer 1

To disable the IIS error messages you have to set

  Response.TrySkipIisCustomErrors = true;

in your error page. After that, your Error messages should show without problem.

Answer 2

Instead of using:

Response.TrySkipIisCustomErrors = true;

Use this in Web.config:

<httpErrors errorMode="Custom" existingResponse="Replace">

Answer 3

You can pass through IIS7 default error messages in two ways One is to set response.TrySkipIisCustomErrors = true

response.TrySkipIisCustomErrors = true;
response.Status = response.Status;

For some reason, TrySkipIisCustomErrors is not honoured if you don't set response.Status.

The other is to set existingResponse to "PassThrough" in web.config

<configuration>
  <system.webServer>
    <httpErrors existingResponse="PassThrough" />
  </system.webServer>
</configuration>

But this will ignore all set IIS custom error pages.