Bring an Atomic Clock to Your Home with Chrony

Among all the techno-toys that make a true geek salivate, few are as cool as an atomic clock.
Running the Client

So now that chrony is installed, verify that chronyd runs in the background (start it if necessary). Remember that the configuration file specifies (with the offline keyword) that chronyd should not query the servers without your permission. Start your modem connection, verify that you are connected to your ISP and then start the chronyc client. Figure 2 shows a sample session.

Figure 2. Sample chronyc Session:

  1. Password command—notice that your password doesn't echo.

  2. The online command tells chronyd to start using the NTP servers.

  3. The source command lists the NTP servers known by chronyd: ^ means a server, * indicates the source to which chronyd is currently synchronized and + indicates other acceptable sources.

  4. Stratum 2 is good enough.

  5. The base-2 logarithm of the number of seconds between two polls of the server: 7 is 128 sec, 8 is 256.

  6. Time since the lastsample was received from the source (in seconds unless you see m, h, d or y for minutes, hours, days or years).

  7. Offset measurement from the last sample. First comes the original measurement, then the actual offset between brackets, then the margin of error.

The first command you enter in chronyc should be the password command. Then, order the dæmon to start talking to the NTP servers with the online command. List the NTP servers (sources -v, which is the verbose form of the sources command). See the tilde (~) in the second column? It says that the server cannot be used yet. It's too early; the dæmon needs a couple of minutes to accumulate timestamps and make sure the responses of the NTP servers aren't delirious. By some cosmic quirk, the difference between my machine's clock and the NTP timestamps happens to be 42 seconds (all hail Douglas Adams!).

Wait a moment and issue another sources command. After a while, you'll see that one of the servers has been selected by chronyd (a star appears in the second column) and that the offset of your machine is decreasing:

2    6     54  +2999ms[+2999ms] +/- 3653ms

Chrony slowly accelerates or slows your clock to make it reflect the NTP time. So over the course of a few minutes, by gradual correction, any offset will disappear.

Other useful commands include:

  • tracking: shows how your system clock is doing, that is, how fast or slow it is with respect to NTP sources.

  • sourcestats -v: shows what chronyd thinks of the sources from the measurements it has obtained so far.

  • makestep: immediately sets your system's clock to the NTP time instead of gradually skewing the clock. This is the equivalent of setting the time. Some versions of X11 can freeze if you set the time back brutally.

Finally, remember to issue an offline command in chronyc before you disconnect your modem. Otherwise, chrony will believe the source it has picked has become unreachable and frantically will try to pick a new one.

Automated Sync

As you can guess, chronyc begs for automated operation. You can easily create two little scripts that will set chrony on-line and off-line. The on-line script:

#! /bin/sh
# This script is called after connect
/usr/local/bin/chronyc <<EOF
password  zack
online EOF

should be called after the modem connection has been established, and the off-line script:

#! /bin/sh
# This script is called before disconnect
/usr/local/bin/chronyc <<EOF
password zack
should be called right before you disconnect.

If you use a special dialer, check if it has options to allow post-connect and pre-disconnect commands. I am using the ATT Global Network dialer, and it allows me to put such scripts in its /opt/attdial/bin. If you are using the plain vanilla PPP, you can insert the on-line script in the /etc/ppp/ip-up file and the off-line script in /etc/ppp/ip-down. Some distributions want you to leave ip-up and ip-down alone and modify only ip-up.local and ip-down.local (check to see if these files exist).


I found chrony the ideal tool for syncing my machine through a modem connection that is only up a few hours a week. I'd like to thank chrony's author, Richard Curnow, who sent me valuable tips and answered many questions quickly.



Fred Mora has been a UNIX system administrator and developer since 1990. He has published and coauthored several books and technical manuals. He is doing his best to lose the rest of his sanity by tinkering with Linux and writing more books, with the encouragement of his techie wife. He works at IBM.



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Excellent article, thanks

Steve's picture

Excellent article, thanks will try to implement it here


Great technology. A lot of

AtomicClock Guru's picture

Great technology.
A lot of new atomic clocks are coming out that are SMALL and even some nice trendy ones.
Check some out at Atomic !!

But what about atomic watches?