Mailinglist Archive: opensuse (1599 mails)

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[opensuse] Re: ntp and wireless
  • From: Joachim Schrod <jschrod@xxxxxxx>
  • Date: Fri, 30 Oct 2009 10:23:23 +0100
  • Message-id: <hcebab$b18$1@xxxxxxxxxxxxx>
Roger Oberholtzer wrote:
On Fri, 2009-10-30 at 00:18 +0100, Carlos E. R. wrote:

Exactly what I said would happen. NTP runs "happily" with no network, but
it quits, aborts, when it finds the time difference is too big.

It must be the presence of both factors at the same time (no network,
difference too big).

Difference too big is the actual reason that ntpd will die.
That difference is 1000 secs, btw.

AFAIR, SUSE adds local clock to its ntp sources, and thus ntp will
have an available time source and will not die if the other time
sources are not reachable. (I have my own ntp configs and can not
check SUSE's default config.)

A big problem at start up of xntpd and ntpd is that the mitigation
algorithms are initially crippled in the interest of a fast start,
and may not detect a falseticker and will happily sync to it if it
is the first server that becomes available. In the SUSE case, the
falseticker is the local clock, which may be more than 1000 secs
off. Suppose that you have no network at first, and then add one.
Then (maybe) four servers come available. After 5 polls, the
truechimers will vote the false ticker off the island, and the
system will now want to reset to the correct time, but since this
is not the first sync, the 1000 second limit will come into play
and ntpd will exit.

This is an architectural issue; I don't know how to handle that
problem for mobile systems. For all kind of configurations, one can
find counter scenarios where it doesn't work. The basic premise of
ntp is `always online' and doesn't take into account wild straying
systems that are not regularly connected (and long enough to sync!)
to the network. I happen to leave off local clocks in our ntp
client configuration -- not on the ntp servers, but there one has
to take care that the local clocks *must* be in a different
stratum, otherwise this will create havoc. But that still gives
problems in some cases, so YMMV.

Configuring ntp properly is a minefield; it looks easy at first and
is very difficult in the details. And it doesn't help that the
documentation does not care for common cases.

HTH nevertheless,

Joachim Schrod Email: jschrod@xxxxxxx
Roedermark, Germany

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