How to Create an Application Filter

This section describes how to create an SMTP application filter, which captures and analyzes data sent by external clients.

A step-by-step example shows you how to create an SMTP filter. To simplify the procedure, this particular filter does not have complete SMTP filtering functionality. The filter can search for a particular string, but it does not act based on that string. A fully functional SMTP filter is provided with ISA Server, and a sample SMTP filter is provided with the ISA SDK. Comparison of the code in this section with the code in the sample SMTP filter will highlight where the filter code should be placed.

The sample SMTP filter demonstrates the use of a data filter factory, not shown in this step-by-step example.

The example consists of the sequential steps in the following sections:

• Creating the Filter
• Attach a Session Filter to a Firewall Session
• Add Registration Code and Register the Filter
• Add SMTP Content Filtering Using the IFWXDataFilter Interface
• Reading Configuration from Vendor Parameter Sets
• Raising an Event When a String is Found