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War dialing is a practice that involves calling large volumes of phone numbers to collect information about which numbers connect to modems and other devices of interest. There are a number of reasons why people may engage in war dialing and it is not legal in some regions. Many phone companies have systems that are designed to detect it. These systems will kick in to lock a user out of the phone system if he or she appears to be attempting to war dial.
In war dialing, people identify a range of telephone numbers of interest, such as all the numbers within a given prefix. Numbers can be dialed sequentially or randomly and notes are taken about the response at each number, whether it is a modem, fax machine, internal phone system, home, or business. Most war dialers use software designed for this purpose to automate the process rather than dialing numbers by hand, allowing them to scan hundreds of numbers very rapidly.
Security companies sometimes use war dialing as a tool for penetration testing, to identify weaknesses in security that might allow hackers to access a system through the phones. Sometimes war dialing reveals a vulnerable modem or other weak point that could potentially be exploited. People who are simply curious or who want to map out the numbers in a prefix can also use this technique, such as someone who wants to generate a list of all of the phone numbers owned by a given company.
Hackers and crackers can use war dialing to find systems that can be manipulated in some way. A war dialing program may be designed to try and force user names and passwords if it is able to access a modem or internal phone system, for example. Likewise, hackers can take note of anything interesting identified during the scanning session, and use this information to attempt to hack or crack a system. This may be done with simple curiosity or malice in mind.
One problem with war dialing is that it can be troublesome for people living or working in an area that is being scanned. The war dialer typically hangs up after two rings or when a person answers, rejecting the number as uninteresting. This can be annoying for people with a single phone line who think they missed a call, and it can become disruptive for businesses that may own a block of phone numbers, as each number will ring separately as the war dialer moves through the list of numbers being scanned.