[ntp:hackers] Receiving smeared time

Miroslav Lichvar mlichvar at redhat.com
Tue Jun 30 09:00:14 UTC 2015


On Mon, Jun 29, 2015 at 06:36:23PM +0000, Harlan Stenn wrote:
> Miroslav Lichvar writes:
> > To me it makes more sense to use a fixed refid (e.g. 127.127.?.?), so
> > a leap smearing server can still be detected and false positives are
> > unlikely.
> 
> An IPv6 hash can start with 127. just as easily.  127.127 is better, but
> we need more than 16 bits for the data.

I meant to fix all 32 bits of the refid, i.e. don't encode any offset
there as it's not very useful. I think a chance of 1 in 4 billions is
acceptable.

> I've got code ready to go to change the IPv6 refid hash to 255.<3 bytes
> of the MD5 hash of the IPv6 address) and an update to the RFC to go
> along with it.

That would be an incompatible change in NTPv4.

In NTPv5 the NTP packet could contain two refid values, the server's
own refid (a randomly generated number) and the refid of its
synchronization source.

-- 
Miroslav Lichvar


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