callback

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.

3. communications cost control callback.

(2003-07-13)