It’s a good idea to have an accurate clock on any computer you access. Beyond the obvious convenience, consistent timestamps across your infrastructure make log analysis and event replays far more reliable. Unfortunately, clocks drift over time. NTP – the Network Time Protocol – keeps things honest by synchronising your clock against a group of reference servers on the internet.
There’s a wrinkle, though: Xen guests (such as those provided by Xeriom Networks) have their clocks tied to the Xen host by default. Here’s how to break free and get NTP running properly.
Taking a shortcut
If you’re running a Ubuntu-based VM and you use the package host at Xeriom Networks, one command does the lot:
sudo apt-get install xeriom-ntp-client --yes --force-yes
Gaining independence
To stop your VM’s clock being slaved to the host, tell the kernel the clock is independent:
sudo su -c "echo 1 > /proc/sys/xen/independent_wallclock"
To make this persist across reboots, add the following line to /etc/sysctl.conf:
xen.independent_wallclock = 1
Installing and configuring NTPD
Install the NTP daemon via apt-get:
sudo apt-get install ntp --yes
If you’re on one of Xeriom’s VMs you can use time.xeriom.net as a time source. Edit /etc/ntp.conf to include it alongside a few servers from your nearest NTP pool:
server time.xeriom.net prefer
server 0.uk.pool.ntp.org
server 1.uk.pool.ntp.org
server 2.uk.pool.ntp.org
Restart NTP and you’re done:
sudo /etc/init.d/ntp restart
What’s the time, Mr. Wolf?
NTP takes roughly 15 minutes to settle down and select the best time source. You can check its progress with ntpq -p:
ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
*time.xeriom.net 212.13.194.87 3 u 39 64 377 0.402 -45.729 7.496
+dns1.rmplc.co.u 195.66.241.3 2 u 39 64 377 3.443 -54.808 6.142
+ntpt1.core.thep 194.152.64.68 3 u 40 64 377 0.723 -53.765 5.965
+weevil.pwns.ms 249.240.53.144 2 u 38 64 377 9.110 -57.739 11.427
The * marks the currently selected source, and + indicates candidates NTP might switch to. For a more detailed breakdown of this output, the Novell documentation is worth a read.
Now there’s no excuse for being late
That’s it – your Xen guest now keeps its own time, synchronised properly via NTP. No more mysterious timestamp drift in your logs.