If it doesn’t know you, it transparent-proxies all your outgoing port 80 traffic to the local webserver’s port 81, where you could put a redirect to a polite message or something.

In my case I’d just like to put some sort of guestbook page that logs your MAC and adds it automatically to the firewall rules after you leave a comment. I don’t mind sharing my bandwidth from time to time, but I’d love to know if someone benefitted from it.

I was impressed with the power of iptables, once I got my head round it. I found an ascii-art diagram that helped me understand the flow through the various tables and targets.