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Computing (FOLDOC) dictionary (also found in English - English (Wordnet), )
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1. programming A scheme used in event-driven programs
where the program registers a subroutine (a "callback
handler") to handle a certain event. The program does not
call the handler directly but when the event occurs, the
run-time system calls the handler, usually passing it
arguments to describe the event.
2. communications, security A user authentication scheme
used by some computers running dial-up services. The user
dials in to the computer and gives his user name and
password. The computer then hangs up the connection and
uses an auto-dial modem to call back to the user's
registered telephone number. Thus, if an unauthorised person
discovers a user's password, the callback will go, not to him,
but to the owner of that login who will then know that his
account is under attack.
However, some PABXs can be fooled into thinking that the
caller has hung up by sending them a dial tone. When the
computer tries to call out on the same line it is not actually
dialing through to the authorised user but is still connected
to the original caller.