NIST NTP servers

Chris Adams cma at
Wed May 11 00:17:50 UTC 2016

Once upon a time, Mel Beckman <mel at> said:
> Boss: So how did a hacker get in and crash our accounting server, break our VPNs, and kill our network performance?
> IT guy: He changed our clocks.

So, this has been repeated several times (with how bad things will go if
your clocks get changed by years).  It isn't that easy.

First, out of the box, if you use the public pool servers (default
config), you'll typically get 4 random (more or less) servers from the
pool.  There are a bunch, so Joe Random Hacker isn't going to have a
high chance of guessing the servers your system is using.

Second, he'd have to guess at least three to "win".

Third, at best, he'd only be able to change your clocks a little; the
common software won't step the clock more than IIRC 15 minutes.  Yes,
that can cause problems, but not the catastrophes of years in the future
or Jan 1, 1970 mentioned in this thread.

Is it possible to cause problems?  Yes.  Is it a practical attack?  I'm
not so sure, and I haven't seen proof to the contrary.
Chris Adams <cma at>

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