Why setting customErrors in web.config doesn't work at this case?

Question

In my ASP.NET 3.5 Website which is published in shared hosting provider , I've configured my web.config file like this :

    <customErrors mode="On" defaultRedirect="GenericErrorPage.htm">
        <error statusCode="403" redirect="AccessDenied.htm"/>
        <error statusCode="404" redirect="FileNotFound.htm"/>
    </customErrors>

If the user request pages that doesn't exist ( like "www.example.com/NotExistPage.aspx" ) , user will be redirected to FileNotFound.htm page as we expect .

But if the user request some address like : "www.example.com/NotExistDirectory" without .aspx extension , the user will encounter IIS 7.5 Error page :

HTTP Error 404.0 - Not Found The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

Detialed error information :

Module  IIS Web Core
Notification    MapRequestHandler
Handler StaticFile
Error Code  0x80070002

Requested URL   http://www.example.com:80/NotExistDirectory
Physical Path   D:\Websites\example\example.com\wwwroot\NotExistDirectory
Logon Method    Anonymous
Logon User  Anonymous

This is a yellow page which is not user friendly and we didn't expect .

I'm wondering setting customeError in webconfig doesn't support this type of address or not ? How can i prevent users seeing this yellow page .

Edit : Thanks to David's answer , But I found the actual reason and correct solution. Please see my answer.

Answer 1

That's interesting, After couple of years I suddenly figure it out what's the problem.

Thanks to @David solution, But the reason and complete solution is as bellow :

By setting customErrors mode to "On" it's only works when we get an exception in ASP.NET application, While when we trying to reach nonExistingdirectory Or notExsitingStaticResouce, IIS render 404 error and it does not reach to asp.net runtime and serverd by IIS directly.

So we need to add configuraiton for IIS as bellow in Web.config:

  <system.webServer>
    <httpErrors errorMode="Custom">
      <remove statusCode="404"/>
      <error statusCode="404" path="~/404.html" responseMode="File" />
    </httpErrors>
  <system.webServer>

It's important to set responseMode to "File", Otherwise status code automatically would change from 404 to 200 . So from the client perspective they don't get the actual 404 status code.

Answer 2

@Mostafa: I faced the exact same problem. I found out it can be solved by adding the following to the web.config file:

<system.webServer>
    <httpErrors errorMode="Custom">
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" subStatusCode="-1" prefixLanguageFilePath="" path="/MyErrorPage.aspx" responseMode="ExecuteURL" />
    </httpErrors>
  </system.webServer>