What is a Firewall?A firewall is a tool that monitors communication to and from your computer. It sits between your computer and the rest of the network, and according to some criteria, it decides which communication to allow, and which communication to block. It may also use some other criteria to decide about which communication or communication request to report to you (either by adding the information to a log file that you may browse whenever you wish, or in an alert message on the screen), and what not to report.What Is It Good For?Identifying and blocking remote access Trojans. Perhaps the most common way to break into a home computer and gain control, is by using a remote access Trojan (RAT). (sometimes it is called "backdoor Trojan" or "backdoor program". Many people simply call it a "Trojan horse" although the term "Trojan horse" is much more generic). A Trojan horse, is a program that claims to do something really innocent, but in fact does something much less innocent. This goes to the days where the Greek soldiers succeeded to enter through the gates of Troy by building a big wooden horse, and giving it as a present to the king of Troy. The soldiers allowed the sculpture to enter through their gates, and then at night, when the soldiers were busy guarding against an outside attack, many Greek soldiers who were hiding inside the horse went out and attacked Troy from the inside. This story, which may or may not be true, is an example of something which looks like something innocent and is used for some less innocent purpose. The same thing happens in computers. You may sometimes get some program, via ICQ, or via Usenet, or via IRC, and believe this program to be something good, while in fact running it will do something less nice to your computer. Such programs are called Trojan horses. It is accepted to say that the difference between a Trojan horse and a virus, is that a virus has the ability to self-replicate and to distribute itself, while a Trojan horse lacks this ability. A special type of Trojan horses, is RATs (Remote Access Trojans, some say "remote admin Trojans"). These Trojans once executed in the victim's computer, start to listen to incoming communication from a remote matching program that the attacker uses. When they get instructions from the remote program, they act accordingly, and thus let the user of the remote program to execute commands on the victim's computer. To name a few famous RATs, the most common are Netbus, Back-Orifice, and SubSeven (which is also known as Backdoor-G). In order for the attacker to use this method, your computer must first be infected by a RAT. Prevention of infections by RATs is no different than prevention of infection by viruses. Antivirus programs can identify and remove most of the more common RATs. Personal firewalls can identify and block remote communication efforts to the more common RATs and by thus blocking the attacker, and identifying the RAT.Blocking/Identifying Other Types of Trojans and WQorms?There are many other types of Trojan horses which may try to communicate with the outside from your computer. Whether they are e-mail worms trying to distribute themselves using their own SMTP engine, or they might be password stealers, or anything else. Many of them can be identified and blocked by a personal firewall. Identifying/Blocking Spyware's/Adbots?The term "spyware" is a slang which is not well defined. It is commonly used mainly for various adware (and adware is a program that is supported by presenting advertisements to the user), and that during their installation process, they install an independent program which we shall call "adbot". The adbot runs independently even if the hosting adware is not running, and it maintains the advertisements, downloads them from the remote server, and provides information to the remote server. The adbot is usually hidden. There are many companies that offer adbots, and advertisements services to adware. The information that the adbots deliver to their servers from the computer where the adbot is installed, is "how much time each advertisement is shown, which was the hosting adware, and whether the user clicked on the advertisement. This is important so that the advertisements server will be able to know how much money to get from each of the advertised companies, and how much from it to deliver to each of the adware maintainers. Some of the adbots also collect other information in order to better choose the advertisements to the users. The term "spyware" is more generic, but most of the spyware fall into this category. Many types of adbots can be identified and blocked by personal firewalls. Blocking Advertisements?Some of the better personal firewalls can be set to block communication with specific sites. This can be used in order to prevent downloading of advertisements in web pages, and thus to accelerate the download process of the web sites. This is not a very common use of a personal firewall, though. Preventing Communication to Tracking Sites? Some web pages contain references to tracking sites. e.g. instruct the web browser to download a small picture (sometimes invisible) from tracking sites. Sometimes, the pictures are visible and provide some statistics about the site. Those tracking sites will try to save a small text either as a small file in a special directory, or as a line in a special file (depending on what is your browser), and your browser will usually allow the saving site to read the text that it saved on your computer. This is called "web cookies" or sometimes simply "cookies". Cookies allow a web site to keep information that it saved some time when you entered it, to be read whenever you enter the site again. This allow the web site to customize itself for you, and to keep track on everything that you did on that site. It does not have to keep that information on your computer. All it has to save on your computer is a unique identifying number, and then it can keep in the server's side information regarding what has been done by the browser that used that cookie. Yet, by this method, a web site can get only information regarding your visits in it. Some sites such as "doubleclick" or "hitbox" can collect information from various affiliated sites, by putting a small reference in the affiliated pages to some picture on their servers. When you enter one of the affiliated web pages, your browser will communicate with the tracking site, and this
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will allow the tracking site to put or to read a cookie that identifies your computer uniquely, and it can also know what was the web page that referred to it, and any other information that the affiliated web site wanted to deliver to the tracking site. This way tracking sites can correlate information from many affiliated sites, to build information that for example will allow them to better customize the advertisements that are put on those sites when you browse them. Some personal firewalls can be set to block communication to tracking sites. It is not a common use ...

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