12.9. A Pass-Through ISAPI Authentication Filter

Web servers process requests in phases—user authentication, HTTP header processing, URL-to-filename mapping. Each phase creates the opportunity for a server extension—which can be a Netscape Server API (NSAPI) plug-in, or an Apache module, or an ISAPI filter—to alter the server's behavior in some useful way. In this case we need to hook into the user authentication phase. Microsoft'sVisual C++ compiler makes that very easy to do, because it includes a wizard that knows about the phases of IIS request processing and can generate the framework for a filter that deals with one or more of these phases. To create a custom authentication handler, start the MS Developer Studio and do File → New → Projects → ISAPI Extension Wizard. Check the Generate a Filter Object box, uncheck Generate a Server Extension Object, and then complete the ensuing dialog box, as shown in Figure 12.1.

Figure 12-1. Generating the framework for an ISAPI filter