| Microsoft Internet Security and Acceleration Server 2004 SDK |
Using Web filters, you can intervene in the processing of HTTP requests. Web filters are dynamic-link libraries (DLLs) that are loaded when the Microsoft Firewall service is started and stay in memory until the service shuts down. Web filters can be configured to receive filter-event notifications that occur with each HTTP request that the ISA Server Web proxy receives and with each response returned through the Web proxy by an external Web server. For more information about the types of event notifications that are sent to Web filters, see Event Notifications.
The Web proxy sends event notifications to the Web filters that are registered to receive them by calling the applicable entry-point function. For more information about the entry-point functions that are implemented by Web filters, see Web Filter Entry-Point Functions.
In each call to an entry-point function, the Web proxy passes information to the Web filter in a structure, which also contains pointers to functions that the Web filter can use in its reaction to the call. For more information the structures used in Web filters, see Web Filter Structures.
The following illustration provides a schematic overview of the HTTP request-and-response process. A Web filter can register for and react to the events represented in the illustration. These events include both client-side events and server-side events. Client-side events occur when a client sends a request to the ISA Server computer, and when the ISA Server computer responds to the client. Server-side events occur when the ISA Server computer passes a request to a Web server, and when the Web server responds to the ISA Server computer. In this illustration, the ISA Server computer is represented by the Web proxy component of ISA Server.
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