[ntp:questions] iburst and NIST servers

Charles Elliott elliott.ch at comcast.net
Tue Aug 4 23:57:18 UTC 2015


I use minpoll 4 maxpoll 5 on the client that accesses external servers and
minpoll 4 maxpoll 4 on the LAN.  The results justify the means: consistent
sub-100 millisecond offsets on the LAN (presently -0.011 on this computer).
The only time I have been knocked off an NIST server was when I switched
from one computer to another as the external gateway.  Because of the NAT,
time-d and time-d.nist.gov picked up the fast request rate right away when
both clients were briefly active, and would not let me back on for a day or
so.

Charles Elliott

> -----Original Message-----
> From: questions [mailto:questions-
> bounces+elliott.ch=comcast.net at lists.ntp.org] On Behalf Of Charles
> Swiger
> Sent: Sunday, August 2, 2015 10:34 AM
> To: Mike Cook
> Cc: Questions List
> Subject: Re: [ntp:questions] iburst and NIST servers
> 
> On Aug 2, 2015, at 2:31 AM, Mike Cook <michael.cook at sfr.fr> wrote:
> >  Can anyone confirm that this is an issue?
> >
> > I habitually put an burst directive in my ntp.conf server statements.
> ex:
> >
> >  server 129.6.15.30 noselect iburst minpoll 4 maxpoll 6
> >  server 128.138.140.44 noselect iburst minpoll 4 maxpoll 6
> >  server 98.175.203.200 noselect iburst minpoll 4 maxpoll 6
> >
> > But in the case of these NIST servers, sometimes they never get out
> of INIT state.
> 
> iburst isn't usually a problem, but minpoll 4 / maxpoll 6 would be
> considered abusive without prior arrangements.  minpoll 6 is the
> fastest
> rate you should query other NTP servers without explicit permission.
> 
> To be more specific, folks who implement per-client firewall rate rules
> tend to block clients who exceed ~100 packets per hour.
> 
> 
> The main point of iburst is to quickly get a downed NTP server back up
> and serving valid time.  That matters most for isolated stratum-2+
> servers; if you've already got S1 timesources available and multiple
> redundant NTP servers locally, using iburst is superfluous.
> 
> Sure, use iburst on one remote server entry if you want and/or against
> all of the other NTP peers on your local subnet, but it's not obviously
> helpful to use iburst everywhere.
> 
> Regards,
> --
> -Chuck
> 
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