System for public internet access

view story

http://serverfault.com – I'm looking for solution for a public wifi spot.. I want that all people who connect to our wifi network should be redirected to special website(after they open any internet page) where they should reed and agree with our terms of services(by clicking a button). After that they can open any internet page. I have a router and a linux box with site(with agreement page ready), so I'm looking for some software/or hardware solution for this, preferable open-source. (HowTos)