Iis – 404 when page exists – IIS 5, ASP.NET 4.0

To test your logic, I actually walked through the complete process. I used ETW tracing for IIS and ASP.NET and Process Monitor to see what's happening. Here is what I find:

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 Error 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.
   
   
6. 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
7. Hence, the request never passes through the .net pipeline and gives you the error as stated in point 4.

Hope this helps.

Based on the log file, you are getting a 404.2 error message. This typically means "web service extension lockdown policy blocked the request".

Since you are trying to load an ASPX page, go into IIS manager and go to the "Web Service Extensions" category. Ensure that the ASP.Net extensions are Allowed.