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On a Ubuntu 10.04 system I noticed following strange NTP sync events:

Jul  3 02:19:51 hst ntpd[1432]: no servers reachable
Jul  3 02:36:55 hst ntpd[1432]: synchronized to 91.189.94.4, stratum 2
Jul  3 02:53:48 hst ntpd[1432]: time reset -10.407942 s
Jul  3 02:53:48 hst ntpd[1432]: kernel time sync status change 6001
Jul  3 02:53:48 hst dovecot: dovecot: Fatal: Time just moved backwards by 10 seconds. This might cause a lot of problems, so I'll just kill myself now. http://wiki.dovecot.org/TimeMovedBackwards
Jul  3 02:58:37 hst ntpd[1432]: synchronized to 91.189.94.4, stratum 2
Jul  3 02:58:37 hst ntpd[1432]: kernel time sync status change 2001
Jul  3 03:08:15 hst ntpd[1432]: no servers reachable
Jul  3 03:16:49 hst ntpd[1432]: synchronized to 91.189.94.4, stratum 2
Jul  3 03:17:01 hst CRON[28221]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jul  3 03:18:04 hst ntpd[1432]: time reset +10.403648 s
Jul  3 03:22:41 hst ntpd[1432]: synchronized to 91.189.94.4, stratum 2

Where 91.189.94.4 europium.canonical.com and the only server line in ntp.conf is:

server ntp.ubuntu.com

The update at 2:36 seems pretty bogus because it is canceled out 25 minutes later.

What could be possible reasons for this?

I can think of:

  • remote NTP server just provides the wrong time
  • network problems (could a high latency introduce such drifts?)
  • leap second induced bug (this should induce a crash instead, right?)

If the first alternative was the problem how can I protect against this?

Is NTPD smart enough to consult multiple NTP servers (when multiple server lines are available in ntp.conf) and detect if different answers deviate too much from each other?

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I had two separate doveceots crash tonight (at about 02:32 and 02:40:54 CEST) in the same way. –  miracle2k Jul 3 '12 at 10:19
    
A couple of my servers (with Ubuntu 8.04) also jumped 10 seconds into the past last night (detected because Dovecot crashed). It makes me wonder if these serverfault.com/questions/403732/… has anything to do (not with my servers but with some NTP peers). Also note, this server only had two servers listed in ntp.conf, that means if both disagree local ntp would not know which one to believe. (Still today I can see some NTP peers with >50 seconds offset). –  MV. Jul 3 '12 at 12:17
    
The first comment on a related lwn article reports on a jump of exactly 10 h on several servers. –  maxschlepzig Jul 14 '12 at 10:38
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1 Answer

I've seen syslog entries like that on a Slackware machine a few years ago. I believe I bought the machine in question in 2002, and pretty much ran it 24/7 for years: it was my SSH, SMTP and HTTP server. The NTP failures came on slowly, and gradually increased in frequency.

I fixed it the first time by changing the "CMOS RAM" battery, which was one of those coin-sized (US quarter) CR2032 batteries on the motherboard.

After another year or two of operation, that machine just absolutely quit keeping time accurately, and I had to regularly restart ntpd. As I understand it, ntpd keeps a "skew file" based on past data of how the local clock differs from the network clock(s). My guess was that the motherboard in question never had a great clock, and the clock finally became so bad that the "skew file" just couldn't keep up with its wild variance.

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AFAIK, the hardware clock is only used during shutdown (write current time) and power-up (read and initialize system time). Thus a broken hardware clock may result in a bogus drift file if (and only if) you reboot from time to time. In any case, broken hardware may result in broken hardware timers - which may lead to funny interactions with ntpd, too. –  maxschlepzig Jul 3 '12 at 19:19
    
That's what I thought (and still think) too. I can't explain why changing the CMOS RAM battery helped. Maybe it didn't, and I just saw what I wanted to see. –  Bruce Ediger Jul 3 '12 at 21:17
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