A war dialer is a computer program that will call a preset list of telephone numbers looking for computers, fax machines and other data devices. These programs are generally used for illegal or unethical practices and can be a common hacking tool. Since the basic technology uses phones and modems, the practice is less common than it was in the 80s and 90s when those communication devices were more common. A war dialer is similar to another obsolete hacking method called demon dialing, as well as to other processes that are still in use, such as port scanning and war driving.
The war dialer became a staple of hacking in the early 80s. In the movie WarGames, a program was used to dial all the phones in a single town. While mass phone dialing programs existed before that, the "WarGames dialer" became a popular name at the time. Due to Internet shorthand and the eight-character limit on file names, the name eventually became war dialer, with the programs often named ‘wardial.exe.'
A war dialer works like an automatic telephone. The program receives a preset listing of numbers, often all of the phone numbers in a single area code or city. The program then dials the numbers, one after another, looking for data receivers. Common data devices, such as a facsimile, or fax, machine or computer modem, will answer after a single ring. If the phone rings twice, the dialer simply hangs up and calls the next number; if a data device answers, the number is recorded and the dialer disconnects.
Programs like this are not illegal on their own, but many people consider them annoying. The dialer calls numbers in sequence, so offices with sequentially-numbered phones can have several minutes of fake telephone calls. If the dialer works long enough, it will call numbers all night as well. When the dialer finishes, its user will have a listing of data devices. These lists are also legal, but they are a stepping stone to other operations that are not.
Several other hacking processes are similar to war dialing. The nearly obsolete process called demon dialing involves calling a single number over and over. Dial-up servers have code that prevents drops when a signal is lost for a small amount of time. If the demon dialer redialed the number right when someone else disconnected, the dialer would gain access to his account.
There are two active hacking methods similar to war dialing. When a computer is port-scanned, a computer’s ports are pinged one after another as they look for ones that are open. The hacker can use this list to find out if specific programs are running on the machine or what the likely access point is should they decide to invade. War driving involves going up and down streets looking for wireless Internet hot spots. These spots are cataloged and saved for later use.