NTP Md5 or AutoKey?

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Tue Nov 4 00:52:05 CST 2008


On Mon, 03 Nov 2008 22:23:07 PST, Paul Ferguson said:

> I'm just wondering -- in globak scheme of security issue, is NTP
> security a major issue?

The biggest problem is that you pretty much have to spoof a server that
the client is already configured to be accepting NTP packets from.  And *then* you have to
remember that your packets can only lie about the time by a very small number
of milliseconds or they get tossed out by the NTP packet filter that measures
the apparent jitter. Remember, the *real* clock is also sending correct
updates.  At *best*, you lie like hell, and get the clock thrown out as
an "insane" timesource.  But at that point, a properly configured clock
will go on autopilot till a quorum of sane clocks reappears, so you don't
have much chance of wedging in a huge time slew (unless you *really* hit
the jackpot, and the client reboots and does an ntpdate and you manage to
cram in enough false packets to mis-set the clock then).

So in most cases, you can only push the clock around by milliseconds - and
that doesn't buy you very much room for a replay attack or similar, because
that's under the retransmit timeout for a lost packet.  It isn't like you
can get away with replaying something from 5 minutes ago.

Now, if you wanted to be *dastardly*, you'd figure out where a site's
Stratum-1 server(s) have their GPS antennas, and you'd read the recent
research on spoofing GPS signals - at *that* point you'd have a good chance
of controlling the horizontal and vertical....

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