Before we discuss the details of firewalls, it's important to understand some of the basic strategies employed in building firewalls and in enforcing security at your site. These are not staggering revelations; they are straightforward approaches. They're presented here so that you can keep them in mind as you put together a firewall solution for your site.
Perhaps the most fundamental principle of security (any kind of security, not just computer and network security) is that of least privilege. Basically, the principle of least privilege means that any object (user, administrator, program, system, whatever) should have only the privileges the object needs to perform its assigned tasks — and no more. Least privilege is an important principle for limiting your exposure to attacks and for limiting the damage caused by particular attacks.
Some car manufacturers set up their locks so that one key works the doors and the ignition, and a different key works the glove compartment and the trunk; that way, you can enforce least privilege by giving a parking lot attendant the ability to park the car without the ability to get at things stored in the trunk. Many people use splittable key chains, for the same reason. You can enforce least privilege by giving someone the key to your car but not the key to your house as well.
In the Internet context, the examples are endless. Every user probably doesn't need to access every Internet service. Every ...