"We all know and love Squid, the versatile HTTP caching proxy.
Squid conserves bandwidth, speeds up Web surfing, and comes with
all kinds of controls to rein in unruly users: bandwidth
throttling, domain filtering, and user access controls, to name a
few. But no matter how skillfully you configure your Squid server,
it's easy to bypass it. All your users have to do is delete the
references to it in their Web browser configurations. If all you're
doing with Squid is caching, this makes no sense, but then some
folks just like to get away with stuff. If you're using Squid for
filtering, bandwidth control or any other restrictions, you will
certainly have a rebel underground to deal with.
"Unless you set up Squid as a transparent proxy, that is. Then
you don't have to hassle with configuring individual browsers at
all, and your users cannot escape your iron fist. To set this up
all you need is Squid and iptables..."