I have written a custom web page, to deal with a 404 page not found error. Control is redirected to my custom web page by the custom errors tag in my web.config file.

 <authentication mode="Windows"/>

<customErrors mode="On" defaultRedirect="PageNotFoundGateWay.aspx">

<error statusCode="404" redirect="PageNotFoundGateWay.aspx"/>

</customErrors>

This works fine if someone tries to access a .aspx file that does not exists. But if someone tries to access any other type of web page, or no web page at all then control is never redirected to my custome web page (PageNotFoundGateWay.aspx). Does anyone know why the redirect works if someone types in the name of an .aspx file that does not exists? But yet does not work for other types of web pages. Or if just a directory is typed in the URL.

Here is an example of what I am talking about.

If the following URL is typed in the command line and myPage.aspx does not exists, then program execution is redirected to my page PageNotFoundGateWay.aspx.

http://www.mysite.com/thefolder/myPage.aspx

Yet if any of the following URL's are accessed then I get the default .NET file not found web page.

http://www.mysite.com/thefolder/myStarPage.html     // Note myStarPage.html does not exists.  I only get the .NET default file not found web page and not my custom web page. It does not like the .html filename exstension.

http//www.mysite.com/thefolder/fakefolder                // No file name is specified at all. I only get the .NET default file not found web page and not my custom web page.

The point is that .html files are not associated with the ASP.NET aspnet_isapi.dll, but are rather served by IIS directly... you are bypassing ASP.NET runtime. If you look at the properties of your virtual directory, then select the "Directory" tab and then click the "Advanced" button, you'll see the IIS mappings: those link the file type to a dll which is able to understand and serve that kind of request: for instance .aspx, .ascx, .asmx etc... are associated with the aspnet_isapi.dll, .asp (classic ASP) is associated to asp.dll etc...
You could add a new mapping to link the html extension to aspnet_isapi.dll and therefore the ASP.NET runtime; this way it should work.

Cheers
Carlo 

Thank you for the help Carlo. I did properties in IIS on the virtual directory of the website in question. But I could not see the "Directory" tab you were talking about. Here are the names of the tabs displayed: 

HTTP Headers,Custom Errors, ASP.NET, Virtual Directory, Documents, Directory Security

Now when you say "You could add a new mapping to link the html extension to aspnet_isapi.dll and therefore the ASP.NET runtime" Are you implying that this can be done through IIS?

Will

Ok, it's the "Virtual Directory" tab (not sure why, never checked, but sometimes this is simply "Directory" and sometimes "Virtual Directory"... ).

Yes, you can add the new mapping through the IIS Manager, after you find that mapping list (you'll have "Add", "Remove" and "Edit" buttons in the dialog window).

Cheers