Asp – Stop ASP.NET from handling 404 errors for files with extensions (not IIS)

The direct question was whether or not there are options to assign the ASPX handler to all request extensions: Yes, there is. I'll discuss how to do that shortly.

First, I think the "hidden" question -- the answer you really want -- is whether or not there's a way to redirect all 404 errors for pages other than ASPX, ASMX, etc. Yes, there is, and this is the better choice if it'll solve the issue you're having.

To redirect all 404s in IIS 6, right click your web application root (whether it be its own site or a virtual directory in the main site), and choose "Properties." From there, choose the "Custom Errors" tab. Find 404 in the list and change it to the redirect you want.

Now, if that won't suffice -- and I really hope it does -- yes, you can run every page through the ASPX handler. However, doing so comes at a fairly high cost in terms of efficiency -- raw HTML/image serving is considerably faster than anything dynamic.

To do this, right click your web application root and choose "Properties." Choose the "Home Directory" tab. Click "Configuration;" a new window will pop up. Copy the path from one of the ASP.NET page serves, and then use it for a wildcard application map.

Bear in mind, again, this is the wrong answer most of the time. It will negatively impact your performance, and is the equivalent of using a chainsaw to carve a turkey. I highly recommend the first option over this one, if it will work out for you.

Set existingResponse to PassThrough in system.webServer/httpErrors section:

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

Default value of existingResponse property is Auto:

Auto tells custom error module to do the right thing. Actual error text seen by clients will be affected depending on value of fTrySkipCustomErrors returned in IHttpResponse::GetStatus call. When fTrySkipCustomErrors is set to true, custom error module will let the response pass through but if it is set to false, custom errors module replaces text with its own text.

More information: What to expect from IIS7 custom error module