Robert D. French

Using rdate(8) for VM Time Correction

If you suspend an OpenBSD virtual machine in VMWare Fusion, its clock will not automatically update when you resume. To fix this by hand, you can use the rdate(8) command (as root).

rdate will update the system's clock using an NTP server that you provide as an argument. For example, to sync your VM with Cloudflare Time Services, you can run the following:

# rdate time.cloudflare.com

But why should you have to do this yourself? Shouldn't this be easy for the computer to automate? Well, yes, but as with most time problems, the answer is tricky. If you call rdate as mentioned above, you will make the computer skip forward in time. If your VM has been suspended for a long time, this could cause problems with time-sensitive applications, like databases. This is usually not an issue on a desktop or a development host.

To safeguard against this scenario in a production environment, OpenBSD's ntpd(8) server will slowly adjust the time until it matches the time of the outside world. To see this in action, you can compare the UNIX timestamp of your OpenBSD virtual machine with that of your workstation:

echo "$(date +%s) - $(ssh your-openbsd-vm date +%s)" | bc

This will run date(1) on your local machine to get the current UNIX timestamp (which we are assuming is up-to-date), and then it will run the same command on your OpenBSD VM. It will then pass those two figures as a subtraction problem to the bc(1) calculator command, which will print the difference (in seconds).

If you run this command again in 10-20 minutes, you will see that the difference between the two hosts will have shrunk by a few seconds. This is because ntpd is bringing the virtual machine into sync very slowly in order to minimize confusion for time-sensitive applications. ntpd accomplishes this by using the adjtime(2) system call, which requires root privileges.

If you want to learn more about ntpd, your best resource is the ntpd.conf(5) man page. This is pretty common for OpenBSD daemons: the manual for the daemon contains only basic information about the flags you'd use in rc.conf(8), but the manual for the config file contains much more information about how the daemon works, what features it supports, etc.