Upside-Down-Ternet:

if you want to run an open wireless network, but are having frustration with having a persistent freeloader, you may appreciate this hack which allows you to send a “knock it off” message, without having to close down your blessed open network.

The Upside-Down-Ternet works like this:

  1. You set up a DHCP server to assign addresses from one IP netblock to known MAC addresses, and another “untrusted” netblock to unknown MACs.
  2. The trusted netblock is routed normally, but the untrusted netblock gets all port 80 traffic forwarded to a transparent squid proxy using iptables.
  3. The squid proxy filters all HTTP traffic, looking for URLs ending in jpg or gif.
  4. If a jpg or gif is encountered, the image is flipped using morgrify and the untrusted user is sent the upside down image instead of the original.
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