I hope that most of you have already seen the upside-downternet. This hack has been around for a while, but that doesn't make it any less brilliant: thanks to some Linux trickery, every user connected to your network gets served their images upside down. It's pretty clever.

It's also similar to a related but not-quite-identical application I've had in mind. So how does it work? The magic arrives in the form of transparent proxying, a technique whereby a router sends its traffic through a proxy server without the user responsible for that traffic noticing anything (or having to change their configuration). This is called "transparent proxying". In the case of the upside-downternet the Proxy server was Squid, an awesomely powerful, sprawling application. In this case Squid runs on its own machine; the router is configured to reroute its traffic through the Squid server before letting it out onto the net. The Squid server keeps an eye on the URLs being requested and uses a custom script to rewrite any that look like images. The replacement URLs point at a web-accessible script on the Squid server that knows to fetch the original image, flip it upside down, then serve it back.

As I said, I wanted to do approximately the same thing. But I wanted to do it all on the router. And I don't mean router in the systems engineer sense of "router". I don't mean a box sitting in a datacenter with fiber channel running into the back. I mean a box sitting on a shelf in Best Buy with a peel-off coupon on the front.