WebFilter is a patch to Cern's httpd web server. This server can act as a proxy, which means that your web browser (e.g. Mosaic, Netscape or Lynx), when asked to bring up a certain web page, doesn't contact the remote web server directly, but queries the proxy server (which runs on your own computer, or nearby) instead; this proxy then turns around, fetches the page from the remote web server and forwards it back to the browser. This is commonly used in order to hop over a security firewall or to implement caching of web pages, but it can be used to filter the web as well.

The WebFilter extension to httpd allows you to provide the proxy server with a list of URL templates and corresponding filter scripts. Whenever the proxy is asked to fetch a web page whose address matches one or more of the URL templates, it pipes the page through the corresponding filter scripts and presents the result to the browser. The advantage of this approach is that it works transparently with every browser, and that there is extreme flexibility about what you can do to a page, since the filter script can be any program whatsoever (for most cases, a sed, awk or perl script will do, however).

The idea is to run your own personalized WebFilter proxy server, tell your browser about it, and off you go. The proxy does the filtering and the browser doesn't even know about it. That's why this approach works with every browser.