What is a firewall?

A firewall is a piece of hardware or software (or a combination of both) designed to be a first line of defense against an unauthorized user accessing a private network. More commonly, firewalls are used in connection with Internet sites. Think of firewalls as gatekeepers. All messages coming in or going out are carefully screened to meet certain select security criteria.

The Love Bug showed us just how vulnerable we are to virus attacks. But crackers (malicious hackers) are an even scarier threat, since they silently attack our computers to uncover our personal information, such as passwords and credit card numbers. Most of us don't password-protect our machines. And if you use Windows networking to share files over local networks, you should know that, by default, it also allows file and printer sharing over the Web. So anyone armed with your IP address (your location on the Net) can see anything on your machine.

You may need a firewall.

Since crackers need your IP address before they attack, DSL customers are open targets. Unlike dial-up connections, which assign new IP addresses every time you log on, DSL lines rarely, if ever, change your address. To keep the creeps out, you need firewall software.

Firewalls provide security by making ports (the channels through which data passes to and from the Net) invisible--effectively cloaking your machine. Some firewalls go a lot further than others, defining rules for specific ports and applications, rather than general settings. They monitor data that passes between your machine and the Internet, making sure that your machine accepts only the traffic you approve.

I use  ZoneAlarm which I have been told offers the most thorough Internet protection for the lowest possible cost--it's absolutely free for home users. Not only does this product block unauthorized entry into your PC, it also cloaks your machine so that crackers don't even know it's there. Plus, ZoneAlarm lets you create different protection levels for networking and for the Net so that you can, say, share files with colleagues at the next desk and still restrict remote computers on the Internet.  Don't worry, I don't know how to do this either.

Some common firewall techniques include:

Packet Filter: This technique looks at each packetentering or leaving a network, accepting or rejecting it based on established rules. Packet filering is fairly effective and transparent to users, however it is often difficult to configure. It is also vulnurable to Denial of Service attacks.

Application Gateway: This method is used for specific applications, such as FTPand Telenet. This is also effective, but can result in performance loss.

Circuit-Level Gateway: This is also used for specific applications, such as TCPor UDP. Once a connection has been established, packets can flow between the hosts without further checking.

Proxy Server: This method intercepts all messages entering and leaving a network. The proxy server essentially hides the network's true address.

Many firewalls employ two or more of the above techniques. For additional security, a company may also encrypt. its internal data so that only authorized users have the correct key.

Problems

Other peoples firewalls, specifically high tech companies and the government are a pain for the rest of us.   They use so many firewalls, you may have trouble sending people e-mails -even if you have their e-mail address.  If this occurs and the mail bounces back.  You will have to get the person to send you an e-mail first and then each time you send them an e-mail use one that you have received from them and reply to it.  

Other difficulties may be in sending attachments or post cards.

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