Bring an Atomic Clock to Your Home with Chrony

Now, it's time to configure chrony. Because you want to sync your machine's clock on an NTP server, you need to pick one. Actually, you'll need to pick a few different servers in case one of them is unreachable. See the URL for the list of NTP servers in Resources.

The list separates the NTP servers into several strata. Now, what's a stratum? We are not talking about a geological layer of rock here. Think onion rings instead. Stratum 1 is composed of servers that are directly synced with an atomic clock. Stratum 2 is a set of NTP servers that are fed timestamps by stratum 1 and so on. Keep away from stratum 1 unless you run a physics lab or your private network has hundreds of machines. Stratum 1 machines should be reserved for distributing time references to secondary servers or to machines that cannot settle for the few microseconds of imprecision incurred by using stratum 2. For our purpose, choosing servers in stratum 2 or even 3 will be perfectly fine.

Notice that the administrators of some NTP servers require you to e-mail them if you want to sync with their machine. Please do so. If a thousand home machines suddenly start requesting timestamps from a poor university NTP server, the administrators need to know that it's actually for syncing clocks and not some new form of flood attack.

Ideally, you should pick an NTP server that is not too far away from your machine (IP-wise). This is not always the same as geographically. A rule of thumb is to check the output of traceroute, which lists the systems traversed by packets between your machine and the destination. For instance, since I am a New York dweller, I picked the following machines: ntp.ctr.columbia.edu from Columbia University, clock.psu.edu from Pennsylvania State University and ntp0.cornell.edu from Cornell University (send an e-mail to pgp1@cornell.edu if you use this server).

Become root and edit your /etc/chrony.conf file to add these definitions:

server ntp.ctr.columbia.edu offline
server
server ntp0.cornell.edu offline

Of course, replace these server names with the names of NTP servers located close to you. Note that the servers are declared off-line. Most modem-connected machines do not start a connection at boot. So when chronyd starts up, it should not start querying servers. Also, note that the chrony doc insists that you should use the TCP/IP numerical address of the NTP servers to alleviate a dependency on DNS. Well, the administrators of most NTP servers want you to use only the DNS-declared names so that they retain the ability to move the servers around. Besides, your modem connection hopefully can reach your ISP's DNS, so I recommend that you use the NTP server names in the chrony.conf file.

The NTP protocol supports packet authentication. After all, if you run a company, you don't want wrongdoers to set your machines' clocks to an arbitrary time. Financial records with an incorrect timestamp can throw your auditors into a loop.

Chrony supports that authentication with a simple password scheme. You can define several chrony users identified by a number and give them different passwords. The relevant statements in the chrony.conf file are:

commandkey 9
keyfile /etc/chrony.keys

This specifies that this machine uses key number 9 to be read from the passwords stored in /etc/chrony.keys. The latter contains, for instance:

9 zack

Also, chrony needs to store the rate at which your machine's clock deviates, or drifts, from the NTP server time. Specify a location for the drift file with:

driftfile /etc/chrony.drift

This way, chrony does not have to accumulate measurements and recalculate the drift every time you start it.

You might have several computers in your home. In that case, it's a good idea to sync their clocks, too. By default, chronyd acts strictly as an NTP client with respect to the servers you define in the server statements. But you can set chronyd to act like a server for your own subnetwork. Just add an allow statement to your chrony.conf, and specify either some machines or a subnetwork. For example, my home machines' Ethernet cards have addresses in the (nonroutable) 192.168 subnet, and the machine acting as a server has the statement:

allow 192.168

in its chrony.conf file. This way, other machines in my home can sync with my server (address 192.198.0.1) by running chronyd with this simple configuration file:

server 192.198.0.1
keyfile /etc/chrony.keys
commandkey 9
driftfile /etc/chrony.drift

server ntp.ctr.columbia.edu offline
server
server
commandkey 9
keyfile /etc/chrony.keys
driftfile /etc/chrony.drift