[ntp:questions] ntpd on dial up machine: no DNS resolving => giving up on all servers

Danny Mayer mayer at ntp.org
Sun Jun 7 16:35:39 UTC 2009

Richard B. Gilbert wrote:
> Harlan Stenn wrote:
>>>>> In article <h0enbo$cme$1 at newsreader2.netcologne.de>, Arno Wald <arno.wald at netcologne.de> writes:
>> Arno> ntpd is started at boot time and automatically can handle interfaces
>> Arno> that are brought up or down while ntpd is running since
>> Arno> 4.2.4p0+dfsg-1. But the problem is, that ntpd cannot resolve the
>> Arno> ntp-server names at startup (because the PC is offline) and gives up
>> Arno> on them. When the ppp interface is brough up, ntpd does recognize
>> Arno> this, but does not retry to resolve the servers.
>> There is an option to have ntpd retry failed DNS lookups for "longer".  This
>> is in ntp-dev however (which is about to become ntp-4.2.6).
>> Arno> I cannot find a configuration option to change this. Is there
>> Arno> something I can do to make ntpd look up the servers again when the PC
>> Arno> is going online?
>> Use IP numbers instead of names.
>> Get a reasonably inexpensive GPS device and use that for time (or if
>> geographically convenient, one of the radio refclocks).
>> Arno> An alternative was to (re)start ntpd on ip-up instead of starting it
>> Arno> at boot time. The problem with this is that (if I am not wrong) ntpd
>> Arno> does a big time step instead of just little time adjustments
>> Arno> once. This can make the mail server dovecot stop if time is set back
>> Arno> by too many seconds. So is there an option to forbid ntpd such a big
>> Arno> time step at startup?
>> You really only want to prevent ntpd from stepping the time backwards.
>> This topic is one for wider discussion.  Ideally, we'd get something useful
>> out of POSIX or some other standard.  Or we could all agree on a useful spec
>> and motivate OS vendors and Standards groups to implement it.
> If you need to SET the time, do it before starting anything else.
> Also, keep in mind that NTPD can need many hours to set both phase 
> (time) and frequency accurately!  It works best if you run it 24x365.
> Once set, NTPD should maintain the correct time.  Some sites, for one 
> reason or another, cannot run 24x365.  If this is the case with your 
> site you should investigate tools other than NTP.

No, that's no true. You can get good time and frequency in seconds if
you use iburst on the server lines. What does take a much longer time is
what it takes for the time and frequency to reach the point in which it
is most stable.


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