[time] ntpd 4.2.7 DNS and the pool

Dave Hart davehart
Mon Feb 8 10:58:33 UTC 2010


On Mon, Feb 8, 2010 at 8:40 UTC, Adrian von Bidder wrote:
> On Sunday 07 February 2010 22.28:29 Dave Hart wrote:
>> Is once a day often enough to re-resolve non-pool hostnames?
>
> I wonder if it wouldn't be better to generally only re-resolve hostnames if
> a server goes "bad" (unreachable or outlier/falseticker.)
>
> This would nicely sidestep the issue of server associations changing all the
> time in the case of pool.ntp.org configured as "server" and not "pool".

Yes it does, and I suspect that is how the revamped "pool" directive
will work, because it results in so-called ephemeral associations
which are removed if a server goes "bad".  The removal will indirectly
trigger a DNS query if the results of the last query have been
exhausted.

The "server" directive in ntp.conf configures a persistent association
that never is removed no matter how long the peer is nonresponsive or
useless.  Given the current design, it would be a bit of work to
trigger lookups based on some quality criteria for configured
associations, simply because ntpd doesn't currently score (rate) them
as it does ephemeral associations.  The result will likely be
persistent associations have their hostnames re-resolved on some
schedule, whether fixed or configurable with a new ntp.conf knob.
That doesn't mean there will be gratuitous clock hopping introduced --
so long as the currently used peer address shows up among the results,
I would expect it to continue to be used.

Currently ntpd is very primitive in using DNS.  If a hostname resolves
to multiple IPs, ntpd doesn't attempt any but the first (within any
restriction from -4 or -6 options).  Although it may not happen at the
same time as the pool directive and re-resolving changes, it may make
sense for persistent associations configured with a hostname that
resolves to multiple addresses to use all of them in turn (changing
after some period of simple unreachability, likely without the
subtlety of the decision to demobilize an ephemeral association.

Cheers,
Dave Hart



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