Sometime back I came across a very good question on ServerFault and I thought of doing some research to understand how IIS handles Custom Errors setting when all requests are passed through aspnet_isapi.dll.

Here is a brief background:

Scenario 1:

We have a Wildcard mapping configured for aspnet_isapi.dll with “Verify that file exists” unchecked.

We also have Custom Errors configured for 404. We have configured it as a URL and the path is /404/. Remember the path 404 does not exists in wwwroot (or the Web site content folder).

Here’s my observation:

1. When you add "Wildcard mapping" for aspnet_isapi.dll with "Verify that file exists" unchecked, all the request actually passes through the .net pipeline
2. When you access a fake directory, .net actually looks for all .config files.. i.e. machine.config, root web.config (that's in C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG) and your Web site root directory to look for a possible HttpHandler that can handle your request
3. Since none of the handler's are configured to handle "directory" request, the request falls back on IISGeneral: GENERAL_STATIC_FILE_HANDLER and fails with 0x80070002 i.e. FILENOTFOUND
4. Since you have IIS Custom Errors 404 configured as /404/, it looks into the directory c:\inetpub\wwwroot\404\, fails with another 0x80070002 and sends a generic The system cannot find the file specified. error.
5. When you access a fake main.aspx page, .aspx request is handled in the root web.config file and hence you see a .net specific (The resource cannot be found.) error.

Scenario 2:

We now change the Wildcard mapping configured for aspnet_isapi.dll with “Verify that file exists” as checked.

Here’s what changes:

1. Now, when you check "Verify that file exists" for the "Wildcard mapping", the server checks that the requested script exists before mapping an extension to an application
2. Hence, the request never passes through the .net pipeline and gives you the error as stated in point 4 above.

Hope this helps,
Vivek Kumbhar

Quote of the day:
Philosophy is a battle against the bewitchment of our intelligence by means of language. - Ludwig Wittgenstein