well, my server is in a lan environment, with the internet connection as eth0 and the lan connection as eh1. im currently using iptables with nat to forward the internet to all the computers on my network. i want to be able to have 3 or so computers have internet, and the others have internet only when i choose...i wouldnt prefer the filtering to be by ip address, as a user can simply just change it...maybe some sort of mac address filtering (a list of mac addresses that are allowed)? the computers still need to be able to access the network at all times though.

im not quite sure how to go about doing this...

you might want to look into squid
proxy server

hmm....i tried out squid and the only problem i have with it is how you have to configure the client applications to use a proxy server...some applications don't support proxy servers...

have you ever been to a internet cafe? some of them somehow redirect you to a web page when you try to access the internet...then you have to authenticate yourself and you gain access to their internet....any idea on how thats done? it didnt require any authentication at all and all my programs worked just fine.

thats more what im geared towards...