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(Or "wardialer") A program which uses a modem to dial a series of phone numbers (say, from 770-0000 to 770-9999), and keeps a log of what phone numbers answer with a modem carrier. The results of such a search were generally used by people looking to engage in random mischief in random machines.
Since the 1980s, wardialers have generally fallen into disuse, partly because of easily available "caller ID" technology, partly because fax machines are now in wide use and would often be logged as a carrier by a wardialer, and partly because there are so many new and more interesting venues for computerised mischief these days.