[ubuntu] Block single LAN PC from accessing certain websites while allowing others

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http://ubuntuforums.org – I have an Ubuntu server acting as a firewall/gateway to a lan consisting of 30 PC's. Of these PC's, only some may access the internet. I now need to allow a single PC to access the internet, but I need to be able to block certain websites such as twitter.com and Facebook and some others (as its proving to be counter-productive). On the Ubuntu server, I have squid running (not transparent) that allows PC's to browse the net after authenticating. After setting up some rules in squid, I'm finding that its not blocking the sites I've designated. Also, how does https factor into this? Before I (Hardware)