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Re: ntp/ntpdate and NAT

On Tue, Jun 04, 2013 at 11:20:21AM +0000, Michael van Elst wrote:
> NTP doesn't use ICMP but UDP and client and servers exchange time
> information directly within the NTP protocol. The gateway is
> nothing more than a router in the path that causes some delay,
> with NAT or without.

OK. But do you know if the offset is taken as the mean of the time (in
client's time) for the round-trip, and if the out-going translation
(allocation of port) on the gateway is more time consuming than the
translation when the packet returns i.e. the mean is inaccurate due to
some asymetry between going and returning? (Or more generally because
the NAT gateway can take a variable amount of time treating the

The question arised because I have bad values (more than 2 seconds
offset) for a node on which I have made a "ntpdate -b" (so no adjtime) a
day before: the (new, dual-core x86_64, NetBSD 6.1) PC clock is not an
atomic clock, but when the system is running it does not shift by
several seconds a day, or is it?

(It is more to understand what is going on, since this network I have a
node into is intermittently connected to the Internet, so ntpd is not an
option; and as long as all the LAN nodes have the same time, it does not
matter if it is not exactly in sync with external. But there was an
appliance---NAS---whether taking time from some Windows node serving 
Active Directory (? I'm not a Windows administrator...) if it was
not using ntpd, or using ntpd and going insane when left without a 
connection for 2 days. So I'm trying to see with the NetBSD the accuracy
of ntpdate/ntpd in this topology.)
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
Key fingerprint = 0FF7 E906 FBAF FE95 FD89  250D 52B1 AE95 6006 F40C

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