doorknob rattling
n. Probing a computer that is connected to the Internet to see if it has any vulnerabilities that can be exploited.
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On April 29, system administrators detected a "doorknob rattling incident" in which an interloper unsuccessfully tried to enter and retrieve account information from Chretien's Web server, the latest in a string of occurrences detailed in the records.
—Jim Bronskill, “Hackers target Chretien,” Calgary Herald (Alberta, Canada), December 03, 2000
The first one rattled the doorknob at 3 a.m. Wednesday morning, trying to see if the door was open. The next came by an hour later, then one at 8 a.m., another at 10:30 and yet another at noon…The fortress these thieves were trying to enter? A simple home computer, hooked up to the Internet with the cable modem service @Home.

The owner happens to be a computer security expert [who] knows about the attacks because his computer has a fire wall, which keeps intruders out and tells him when they have come by. Yet he was nonplussed by the number of attacks on that single day last week. 'I think it's fairly typical,' he said.

Others agree. 'It's called doorknob rattling,' said Steven Bellovin, a researcher in Internet security at AT&T Laboratories in Florham Park, N.J.
—Gina Kolata, “Check Your Doors,” The New York Times, February 20, 2000
1995 (earliest)
Overall statistics on hacking incidents or attempts are impossible to come by, but some universities estimate that they get from 10 to 30 "doorknob-rattling" attempts a week.
—Ellen Germain, “Guarding against Internet intruders,” Science, February 03, 1995
Thanks to subscriber James Callan for spying this phrase.

If you examine the second citation, you'll notice that the inexplicable misuse of the word nonplussed. The writer — veteran journalist Gina Kolata of The New York Times — is using nonplussed as a synonym for "unfazed," when it really means the opposite: "bewildered and at a loss as to what to think." This is a common error and I think the reason it happens is that the non- ("not") prefix makes it sound as though nothing happened ("the guy didn't get plussed"). It doesn't help that the -plussed part comes from the Latin root plus, which means "more." So the Latin phrase non plus means, "no more." The way I remember it is to think of some poor, bewildered soul throwing up his hands and saying "No more! No more!"
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