Linux – time sync with ntpd

linux

I run Debian on several systems, and their times do not seem to stay in sync. I can run ntpdate manually, but I thought that I should have an ntpd running that would automate that.

I did check with apt and apt-cache but don't find any ntpd (or associated ntpq), not any such names in my system (locate…), but ntp-doc does still describe them.

Looking around I see that there is an ntpdate-debian command, and it uses /etc/default/ntpdate for servers (instead of the standard /etc/ntp.conf), but even thought that file is there and has "yes" indicated to use ntp.conf, it fails with "no servers can be used", although ntpdate works fine. Is this just a layer over ntpdate, any reason to use it instead?

So, why are they missing, do I need them, how do I automate time updates?

Associated, two of my machines are virtualized on a MSoft VM, how is it that their clocks drift, and both to different values? (The underlying Windows machine clock seems stable). I see a few old notes about time & ntp problems on VMware, didn't find anything either current or relating to MSoft VMs. Anything I did see says just to use ntpd, but as above, …?!

Best Answer

The package you are looking for is ntp, which provides daemon and utilities for time synchronization.

Related Solutions

Problem Synchronizing Server Time with ntpd – Solutions

Many distributions these days are configuring ntpd to restrict access. If restrict lines are present in your server's /etc/ntp.conf, only hosts/networks matching those lines will be permitted to connect to ntpd. You probably need to add additional restrict lines for the hosts or networks you want to allow to sync to your server. For example, to let the client you mentioned sync, add one of the following lines:

# allow just this host
restrict 10.99.84.134 nomodify notrap

# or allow the whole /24 segment
restrict 10.99.84.0 mask 255.255.255.0 nomodify notrap

After that, restart ntpd, and your clients should be able to sync.

Linux – Best way to say “sync all system clocks to this server, when and ONLY when I say so?” Mixed setup of Windows+Linux servers

Are you looking for the specific command to start/stop NTP on Linux and Windows? Those are different on different Linux distros. It will be something like "/etc/init.d/ntpd stop" on some, different on others, and I'm not sure on Windows.

There are two ways to do this, and one caveat.

Method 1: Run NTP normally but stop it for the benchmark.

Method 2: Don't run NTP but run NTP in "one-shot" mode right before the benchmark runs. (On linux that would be "ntpdate SERVERNAME" on Unix/Linux or [i think] "ntp -q" on windows)

Method 1 has some advantages in that the clock will be correct all those times that the you aren't running benchmarks. The clock will also be more accurately set for the benchmark. That is, NTP maintains the clock very accurately even between NTP packets. The NTP packets on the network are simply fine tuning. Between packets NTP uses past experience to nudge the system clock faster or slower. The disadvantage is that when you start NTP again, it will refuse to start if the clock is too far off. This is not a bug, it does this on purpose to prevent syncing to a server that has gone bad. To prevent this, do an "ntpdate" or "ntp -q" right before you restart NTP.

Method 2 has the advantage that it is more simple to implement. However, the clock will not be very accurate. "one shot" mode doesn't set the clock as accurately as running NTP. It is mostly for fixing a far-off clock before you start NTP (see above). However, if your benchmark only has to be accurate to about 1/10th second you'll be fine.

The caveat is that the benchmarks might not be benchmarking what you think they are benchmarking if you turn off NTP. The computer's clock will drift without NTP running constantly. That's ok if you are benchmarking how much drift the computer has but not much else.

(And I should note... the drift will be different not just brand-to-brand, but between particular machines.)

HTH!

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