If your host system uses Time Stamp Counter and Virtual Machine guests use kvm-clock, then you should not use ntp on the guest. But what if you sync the physical host to a good Internet time source and just sync the virtualized guest to itself?

I hate configuring things like GPS devices that run super restricted verisons of linux or some other OS. They never seem to deal with error handling very well. For example, here’s the oddball command for fetching ntp.conf and ntp.keys from a ntp server onto a Symmetricom GPS receiver. This is what you want to see, it just works. But in the many failures leading up to this configuration, it was finding problems fetching the files or having the correct access but it was happily coasting right along, overwriting its own configuration with jibberish and rebooting it self only to find the configuration was bollocks.

I was curious about using NTP to keep a few stand-alone machines synchronized. None of them have access to the outside world, or access to another host that has access to the outside world - they’re completely cut off. So how do you run a NTP server with no upstream time source? Easy!