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This question has come up a number of times in a localized way, this question is meant to provide the preferred/best method for synchronizing a system's clock using NTP.

The solution should handle multiple issues correctly, such as:

  1. Correct for time at boot-up quickly where the clock has a large skew.

  2. Provide a configuration that guards and/or corrects for situations where the clock can sometimes develop a large skew over time.

  3. A robust solution that can handle and sync the time quickly when certain problems arise such as: "the time server wasn't accessible during boot" or "the internet is inaccessible during boot".

The ideal solution would be a single NTP configuration file that is able to handle all this.

References

Many of the pieces that will provide the "ultimate" solution are spread across the U&L site in questions such as these:

There are bound to be others but these are the ones that I've seen that come to mind as being relevant.

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FYI: Planning on working on a comprehensive answer to this. See the chat log starting at chat.stackexchange.com/transcript/message/11350138#11350138 –  derobert Sep 23 '13 at 16:32
    
#2 should not happen if ntpd is running –  dfc Sep 28 '13 at 0:48
    
@dfc - this can happen when a VM is paused and then later resumed, there are other situations where it can happen too. –  slm Sep 28 '13 at 0:51
    
@slm "develops" means aone time jump in the skew after VM restart? It seems that within a couple of hours after restart ntpd should get the skew under control. Definitely not make it worse. What are the other situations? –  dfc Sep 28 '13 at 1:02

3 Answers 3

What you're looking for is ntpd with the --panicgate option.

The panicgate option allows the first adjustment after ntpd starts to be any size. This is exactly for the use case you described where a machine comes up and it's clock is wildly inaccurate. When ntpd starts with this option enabled, it can take a moment for it find a server and establish synchronization with it.

That option in itself solves your item #1.

#2 is vanilla ntpd. Ntpd keeps a drift file which is the rate your system's clock skews.

#3 is also the same as #1. The --panicgate option isn't limited to immediately when ntpd starts, it is limited to "the first adjustment", whenever that adjustment is.

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Since you cannot correct large deviations in time using ntp (unless you have a few hours for the clock to catch up or slow down) I do this:

service ntpd stop
ntpdate us.pool.ntp.org
service ntpd start

I cron it for once a day, everyday. I also put ntpdate in an init script to run before ntp starts after bootup, since reboots and power cycles are the most likely/frequent events that mess with the system time.

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If ntpd is running successfully why do you need to stop and start it? This seems like a terrible idea. –  dfc Oct 5 '13 at 3:29
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ntpdate won't work if ntpd is running. –  Grizly Oct 10 '13 at 4:29

Use chronyd/chronyc instead of ntp/ntpdate. It's already default method in fedora and, I suppose, will be in RHEL 7.0 as soon as it ready.

Documentation can be found at http://chrony.tuxfamily.org/

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