When we installed Microsoft Internet Information Services (IIS) 5.0, the installation program loaded several Internet Server API (ISAPI) filters on the server. Can we remove these filters?

The features you install on IIS 5.0 (and Internet Information Server—IIS—4.0) determine which ISAPI filters you can remove. Also, the options you choose to install on the IIS server will affect which ISAPI filters are present.

Figure 1 shows four common ISAPI filters. The following list defines the function of each of these filters:

  • sspffilt—implements Secure Sockets Layer (SSL) encryption
  • Compression—required when Compression is enabled on IIS 5.0; IIS 4.0 doesn't support this filter
  • md5filt—used for Digest authentication
  • fpexedll.dll—provides compatibility for Microsoft FrontPage Server Extensions

If you're not using the feature that the filter supports, you can safely remove the filter from the list. Removing ISAPI filters that you don't require is a good idea for security and reliability reasons.

Other filters you might see in your particular implementation include:

  • URLScan—installs with the IIS Lockdown tool; provides URL filtering based on a specified rule set
  • ASP.NET_version#—supports ASP.NET Web applications on the server

When you remove an ISAPI filter from the list, you don't remove the filter from the server but from the list of filters to be loaded. Consequently, if you need to reinstall a filter, you can simply click Add and point to the filter to replace it in the list of filters. When you make a change to the ISAPI filter list, you must restart IIS for the change to take effect.