[ntp:questions] NTP client with 4 servers lost sync

Nuno Pereira nuno.pereira at g9telecom.pt
Thu Jul 2 19:59:27 UTC 2015

Hi Harlan,

> De: Harlan Stenn [mailto:stenn at ntp.org]
> "Nuno Pereira" writes:
> > Following last night's leap second, we had some issues with our NTP
> > servers, especially in a clients with 4 servers configured, but not in
> > clients with 1 source configured.
> We do our best to describe how to choose the right number of servers.
You mean in the list, or in the docs?
> 4 is not a good nuumber - you really need more than that.
And why does the configuration files of the CentOS ntp packages (as I know)
have 4 sources. It's because they're from the pool?

I read that:
 * 1 is definitely not good because if it fails, well, no source is available;
 * 2 it's not good because they can fall into a situation where they don't
intersect and none of them can be considered valid;
 * 3 it's also not a good choice as it can fall into a 2 sources config if one
of them fails or goes out of the interval;
 * 4 is now not a good number, and I need more? Can you elaborate a little on
that, or point me to some site?

If I need more than 4 sources, and as we prefer to use local sources, in order
to have all of our clients with better accuracy between them,
in that case we would need to have 5 NTP servers! It's a little insane, in our
And now with the virtualization, it's becoming more and more difficult to have
bare metal servers.

> Also, if you are using a fixed set of servers and you do not control
> them you need to make sure you know how they will behave during leap
> seconds.
> It's also much easier to maintain a leapsecond file now than it was
> before.
In our local sources I've added a leap second file, and that worked well. Saw
some external sources in the 24 hours before, and they seemed good.
I've thought that if all our clients used our local NTP server with leap
armed, then all would go well. Bad assumption, as it became proved.

> We're working on a system to analyze ntp.conf files and make
> recommendations on how to improve an ntp.conf file.  This is a HUGE
> project and it will probably not be usable until at least late this
> year.
That sounds very, very interesting to see.
> --
> Harlan Stenn <stenn at ntp.org>
> http://networktimefoundation.org - be a member!

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