upsidedownternet.rb

#!/usr/bin/env ruby

require 'webrick'
require 'webrick/httpproxy'

callback = proc do |request, response|
  if response.content_type =~ /jpe?g|png|gif/
    f = Tempfile.new("upside-down")
    f.write(response.body)
    f.close
    system("mogrify -flip #{f.path}")
    response.body = File.read(f.path)
    response.content_length = response.body.size
  end
end

server = WEBrick::HTTPProxyServer.new(:BindAddress => '0.0.0.0', :Port => 8080, :ProxyContentHandler => callback)
trap("INT") { server.shutdown }
server.start