November 10, 2009

How Firewalls Work ?

To understand what a firewall is, you need to first understand what a network connection is. Even though you have only one skinny set of wires connecting your computer to the Internet (through a phone line or cable outlet), that connection actually consists of 65,535 ports. Each port can simultaneously carry on its own conversation with the outside world. So, theoretically, you could have 65,535 things going on at a time. Of course, nobody ever has that much going on all at one time. A handful of ports is more like it.



The ports are divided into two categories: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is generally used to send text and pictures (Web pages and e-mail), and includes some error checking to make sure all the information that’s received by a computer matches what the sending computer sent. UDP works more like broadcast TV or radio, where the information is just sent out and there is no error checking. UDP is  generally used for real-time communications, such as voice conversations and radio broadcasts sent over the Internet.

Each port has two directions: incoming (or ingress) and outgoing (or egress). The direction is in relation to stuff coming into your computer from the outside: namely the Internet. It’s the stuff coming into your computer that you have to watch out for. But you can’t close all ports to all incoming traffic. If you did, there’d be no way to get the good stuff in. But you don’t want to let everything in, either. You need a way to separate the wheat from the chaff, so to speak — a way to let in the good stuff while keeping out the bad stuff.

Antispyware and antivirus software are good tools for keeping out viruses and other bad things that are attached to files coming into your computer. But hackers can actually sneak worms and other bad things in through unprotected ports without involving a file in the process. That’s where the firewall comes into play. A stateful firewall, such as the one that comes with Windows 7, keeps track of everything you request. When traffic from the Internet wants to come in through a port, the firewall checks to make sure the traffic is something you requested. If it isn’t, the firewall assumes this is a hacker trying to sneak something in without your knowing it, and therefore prevents the traffic from entering your computer. Picture illustrates how it works.



So, there’s really more to it than just having a port open or closed. It’s also about filtering. About making sure that data coming into an open port is something you requested and not some rogue uninvited traffic sent by some hacker. Many of the worms that infected so many computers in the 1990s did so by sneaking in undetected through unfiltered ports. These days, you really want to make sure you have a firewall up whenever you go online to prevent such things.

What a firewall doesn’t protect against

It’s important to understand that a firewall alone is not sufficient protection against all Internet threats. A firewall is just one component in a larger defense system. Specifically:

- Windows firewall doesn’t protect you from spyware and viruses.

- Windows firewall doesn’t protect you from attacks based on exploits.

- A firewall doesn’t protect you from pop-up ads.

- A firewall doesn’t protect you from phishing scams.

- Windows firewall doesn’t protect you from spam (junk e-mail).

So, a firewall isn’t a complete solution. Rather, it’s an important component of a larger security strategy.

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