Firewall Information and Hardware Setup Guide

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Ever wondered what a firewall is and whether you need one? Or perhaps you know what a firewall is and are interested in building a firewall appliance? Read on to answer your questions about hardware firewalls and software firewalls, and see how to set up a sophisticated firewall hardware appliance.

What is a firewall?

A term borrowed either from construction—aircraft or automobile design--a firewall is a barrier that segregates two areas to protect one space from the environment of the other.  In buildings or airframes, it is designed to prevent fire from spreading from one section to another. In racing, it protects the driver from a possible fuel tank fire.  Also in automobiles, the bulkhead separating the engine compartment from the passenger compartment is called a firewall.

In computing terms, a firewall isolates a computer or network from another computer or network.  Most often, this creates a so-called "trusted zone" on the inside of the firewall (your local network), which is protected from the untrusted zone outside (the internet).  Some network firewalls sit between sections of the network; this creates DMZs, or De-Militarized Zones, referring to the military term for areas that separate two opposing factions to reduce the risk of war.  Certain devices, such as public web servers, that need to interface more with untrusted zones will be in the DMZ with a firewall between them and the local network, offering more protection for that network.

As with firewalls in buildings, a certain amount of penetration of the firewall is allowed, but these penetrations, or ports, are controlled and safeguarded against bad stuff trying to get in.

Ports

In networking, one will often hear the term port.  Ports, according to the Internet Assigned Numbers Authority (IANA, which coordinates functions for the internet), "name the ends of logical connections which carry long term conversations.  For the purpose of providing services to unknown callers, a service contact port is defined."  Essentially, this is an addressing scheme that allows the computer to assign meaning to incoming and outgoing information.

Ports fall into three categories:

• Port numbers that range from 0 through 1023 are called Well Known Ports.  On most systems, they can only be used by system (or root) processes or by programs executed by privileged users.  The IANA has assigned specific uses for most of these ports.
• The Registered Ports are those from 1024 through 49151 and can be used by ordinary user processes or programs executed by ordinary users.  Many of these ports are also assigned.
• The Dynamic and/or Private Ports are those from 49152 through 65535.  The name is self-explanatory; they are not assigned.

So what firewalls do is filter the data coming into them, allowing information for certain ports to go through and rejecting others, according to preset rules.  There are three different ways this is done:

Packet filtering - Packets (small chunks of data) are analyzed against a set of filters

Proxy service - Doesn't accept packets coming in from the untrusted zone unless they were specifically requested by a computer in the trusted zone. 

Stateful inspection - Doesn't examine the entire incoming packet, but compares certain key parts of that packet to defining characteristics derived from packets traveling inside the firewall to the outside.

Next: Software Firewalls and Hardware Firewalls >>
 

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