What is an ISAPI Filter?

Internet Information Server (IIS) supports a programming interface called the Internet Server Application Programming Interface (ISAPI). While ISAPI extensions generate responses to client requests, ISAPI filters allow developers to inspect or modify requests and responses as they pass through IIS.

An ISAPI filter is implemented as a Windows DLL that is loaded by IIS. Once installed, the filter receives notifications for selected events during request processing.

Web browser %gt; ISAPI Filter gt; IIS > static file or CGI or ASP

ISAPI filters can be used for a variety of purposes, including authentication, request logging, custom headers, URL processing, and response modification.

Filters are configured through the Internet Service Manager by specifying the DLL and the events the filter should receive.

Because the filter executes inside the IIS process, it should be thoroughly tested. Errors in an ISAPI filter may affect all requests being processed by the web server.