Virtual guest: NTP sync with host

Duncan Innes asked:

Is there any way to get a virtual guest to only query it’s current host for NTP?

I’m not talking about configuring ntp.conf with the host’s IP here either as VLAN’s & firewalls may block that traffic. Also migration to a different host would require ntp reconfig & restart. A direct (intercepted?) query from the guest to the host.

I’ve tried various NTP solutions and it seems to me (theoretically) that it might be best for a guest to query only it’s current host for an NTP sync (notwithstanding the VMware advice not to use their sync-with-host option).

It might/would require a version of NTP that is virt-aware.

Only one NTP source would be required as the guest wouldn’t be running if the host wasn’t available.

My answer:

You can’t really use NTP here because you can’t know where the “host” is on the network.

Some hypervisors provide a way to synchronize time with the host. For instance, VMware provides a userspace tool in VMware Tools for this purpose. KVM has a paravirtualized RTC. And so on.

Though in my experience these are all less than perfect, and trusting the host to have the correct time is a bad idea, necessitating running NTP in every VM (and having it use reliable servers) anyway.

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