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The CGI mechanism functions by having the browser encode the URL of the page
to filter as a CGI request. The web server running the filter passes this to the CGI filter
program which can decode the URL, fetch the page from the source server as though it
were a normal client, modify the page, and return it to the filter's web server which
returns it to the client. Typically, the filter will alter the hyperlinks on the page to redirect
them back through itself, so that any links followed will also come through the CGI filter.
Proxy servers rely on the cooperation of the client software to filter pages. The
client program, typically a web broser is configured to send all of its request for pages to
the proxy server. The proxy server relays this request to the source server and gets the
page back in response. It can then modify the page before returning it to the client. In
effect, the proxy appears as a server to the web client and as a client to the source server.
This scheme offers the advantage of being able to pass forms and CGI data to the source
server, whereas the CGI filter may potentially interfere with this. Additionally, it is fairly
transparent to the user.
While these rely on an intermediate server to handle the data, some filtering can
be done at either the client or the server side. In the case of the server, it may use CGI to
customize pages for each user. Or it may use something like Microsoft's Active Server
Pages (ASP) for the same effect. On the client side, Java applets, Java script, ActiveX
controls or browser plugins may allow for the browser itself to filter the pages for the
user. While the data downloaded by the browser would be the same, these would modify
the way that the browser renders them for the user.
Description of Existing Systems
Muffin
Mark R. Boyns developed the Muffin system as part of a thesis project at San
Diego State University. The author designed it to filter out annoying elements from
WWW pages and increase security by stripping information on the users identity, as
transmitted by the browser client.
Muffin consist of 21,000 lines of pure Java code distributed across 128 classes
and 12 interfaces. As a result, it runs on any system with a Java 1.1 runtime environment.
Muffin is available both in binary form and in source form, released under the GNU
General Public License.
Individual filters descend from a base filter class and any number of them can be
invoked to alter a page. Current filters distributed with Muffin offer the ability to block
advertisements, remove Java applets and Javascript, stop animated gifs and remove
cookies, among other things. However, Muffin may easily be extended through additional
filters written in accordance with its API.
Muffin provides a remote administration feature, whereby a user can change
global preferences while it is running through a browser interface in which the system
responds to virtual URL addresses.
However, Muffin is designed for single user use and lacks the ability to set
preferences and configuration on a user by user basis and has no user authentication
capability. Moreover, Muffin does not support group usage in any form. Additionally,