Applications /

Finalizing fine filtering

By Mark Gibbs
Network World, 10/08/01

Following up on our recent discussions of Trivial FTP, Stuart Berman wrote to point out some other graphical user interface-based TFTP implementations: "Weird Solutions makes a great [for pay] TFTP server application called Turbo TFTP [we like the features and security it offers]. And Cisco distributes a rather basic but free TFTP implementation from Network TeleSytems."

But where were we? Oh yeah, Proxomitron, a "universal" Web filter proxy. First, some feedback. Zeke Crater wrote: "I enjoyed reading your article on Proxomitron, but using Proxomitron (or any other proxy server like WebWasher) doesn't fix the problem. Instead, Proxomitron only circumvents the problem on individual PCs, leaving the other, less gearheaded Internet users to continue to suffer. To fix the problem would require eliminating browser and Java support for such features."

Zeke, we're with you but eliminating browser and Java support won't happen so Gearhead will happily settle for a reasonable solution such as Proxomitron.

Our old friend Phil Daley wrote, "But does it work in conjunction with an already installed company proxy server?"

You bet your URLs it can. Proxomitron has an option that lets you define multiple proxy servers to relay through and forwarding to any proxy or switching off proxy chaining can be done on the fly any time.

A curious feature is the ability for automatic rotation between multiple proxy servers. If anyone can explain why this is desirable we would be most grateful.

A. Nonymous asked how proxying works. When you configure your browser to use a proxy (including Proxomitron), the browser knows that it needs to include the full URL in an HTTP request (such as "GET //www.nwfusion.com/columnists/gearhead.html") instead of just the tail ("GET/columnists/gearhead.html") as the browser would normally do. Without the server name (or its IP address), the proxy wouldn't know where to direct the request.

When the proxy receives the full URL, it forwards the tail to the target server and receives the response (an error or the requested content). The proxy then relays the response (possibly modified as in the case of Proxomitron) back to the original requesting browser.

Proxomitron can be configured to listen for HTTP requests on any port you like, and can listen in "promiscuous" mode - the latter feature letting you run a copy of Proxomitron on a PC that will provide proxy services to a group of PCs (normally Proxomitron will only accept connections to "localhost" or 127.0.0.1).

Proxomitron can't distinguish between internal and external nets. If you're thinking of using Proxomitron as a gateway between your LAN and the Internet you will have to front Proxomitron with a tool such as Zone Alarm.

If you don't do something to restrict access, people on the 'Net can use your Proxomitron as an anonymous proxy to hide their IP address while surfing. One of the biggest consequences of this would be the erosion of your bandwidth.

You can configure Proxomitron to filter Web page content, both incoming and outgoing HTTP headers and freeze GIF animation. You can even kill any given connection on the basis of the requested URL - so if you have a centrally shared Proxomitron server it will be easy to block requests for "naughty" sites.

The supplied filters are extensive and include such gems as "Banner Blaster," which "takes any image that looks like an advertisement and replaces it with a plain text link."

Another interesting filter is "Hide Browser's Referrer from JS." The author Scott Lemon notes, "The 'referrer' is by far the most revealing data your browser sends out. By looking at referrer lists, Webmasters can tell not only the site you visited last, but if you clicked a link from e-mail or a page on your hard drive; possibly more personal information as well! Note this is normally revealed in an HTTP header named 'Referrer' but JavaScript can be used to grab this information as well."

There's so much more to this product that we strongly recommend you check it out. Without doubt, Proxomitron is an outstanding tool and collects nine gear teeth out of 10, with a commendation for being free.

Although we don't proxy requests here, send 'em anyway to gearhead@gibbs.com.