Bring an Atomic Clock to Your Home with Chrony

Here is a device that finally provides what generations of scientists have dreamed about: an ultra-precise time reference, a timekeeping piece of incredible accuracy. These are not simple gadgets; some physics experiments, such as the verification of gravitation theories, require measurement of very small time intervals.

The National Institute of Standards and Technology (NIST) has a lab in Boulder, Colorado, devoted to running atomic clocks and providing official US time. In this lab, the NIST-F1 cesium fountain atomic clock provides a time reference with a precision of 2 × 10-15 (two parts per millionth of a billionth) by counting the vibrations of cesium atoms at about 9GHz. An even better clock is in the works. It will measure the resonance of a single mercury ion at about 100,000 times that frequency, and it will provide a thousand-fold increase in precision.

Figure 1. Atomic Clock

The sacred duty of true precision-obsessed geeks is now clear. They simply have to synchronize the real-time clock of their Linux machine(s) with such an insanely precise clock.

Of course, you cannot simply go to a computer store and buy an atomic clock. (Not that I didn't try—sheesh, the brazen gall of that sales guy, trying to saddle me with a radium dial alarm clock.) The next best thing is a radio-synchronized clock, and a variety of models are available. They can be connected to the serial port of a PC and provide time signals synchronized on the NIST clock.

But why buy hardware when well-designed, free software would do the trick? The Network Time Protocol (NTP) has been created to synchronize computers and distribute time references across networks. An NTP server keeps time as close to the official reference as possible. Remote NTP clients query these servers and sync the local real-time clock (RTC) of the machine. This timekeeping is a complex problem due to the nature of distributed computing. Propagating packets between machines takes a nonzero, variable time. Various correction schemes are integrated in NTP to take variable latency into account.

There are several NTP clients and servers available for Linux. The simplest way of using NTP would be to fire up a program, such as xntpd, and point it to an NTP server. However, this program and most other NTP clients work best when they are connected to the Internet continuously. Unfortunately, an intermittent connection through a modem is still the way most homes access the Internet.

That's where chrony comes in. Chrony is a program that explicitly supports intermittent connections. It is comprehensive but a tad intimidating, so we'll walk through an installation and configuration for the most common case: a home user with a modem connection.

Chrony is composed of chronyc, a text-based client program; chronyd, an NTP server running as a dæmon in the background; and various configuration files. To interact with the chronyd dæmon (chronyd), you run the chronyc client and issue commands.

Some Linux distributions include a version of chrony. Chances are that this version is an older one, e.g., 1.15 or less. In that case, you can uninstall the chrony package before installing the new version.

First, download the chrony tarball from its home page (see Resources). At the time of this writing, the current version is 1.16.1. It is composed of the 1.16 version completed by a patch to 1.16.1. We extract the source from the tarball and apply the patch:

tar -zxvf chrony-1.16.tar.gz # extracts source
cd chrony-1.16  # dir created from tarball
gunzip < ../chrony-1.16-1.16.1-patch.gz | patch -p1
patching file NEWS
patching file configure
patching file rtc_linux.c
patching file version.txt

The program uses a configure script, which makes customization a snap. The only option that you need to specify manually is the installation directory, with the --prefix option. By default, chrony will install the client chronyc into /usr/local/bin and the dæmon chronyd into /usr/local/sbin. It is the equivalent of:

# In the same chrony-1.16 dir as before
./configure --prefix /usr/local

warning: extra tokens at end of #endif directive

#endif GOT_REPORTS_H

#endif /* GOT_REPORTS_H */

Now, do:

# In the same chrony-1.16 dir as before
make
su root  # You need to be root to install
install

The next step is to make sure that chronyd starts up at boot time. If your distribution came with an older version of chrony, then you are all set; just make sure that the newer version was installed in the same location as the old one. Otherwise, there are several methods. The simplest is to add a paragraph supplied by the chrony doc in your /etc/rc.d/rc.local.