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 firstname.lastname@example.org 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 zackZack is the name of my cat. Before I started using chrony, the beast was the closest thing to a precise clock I had in the house. Every morning at 7:30, he meows pathetically until I feed him—including weekends. He quickly became very good at ducking pillows.
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:
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:
in its chrony.conf file. This way, other machines in my home can sync with my server (address 18.104.22.168) by running chronyd with this simple configuration file:
server 22.214.171.124 keyfile /etc/chrony.keys commandkey 9 driftfile /etc/chrony.driftTo summarize, the chrony.conf file of your modem-connected machine is given below. Note: replace the NTP servers in this example with the ones that you pick in the list of NTP servers (see Resources):
server ntp.ctr.columbia.edu offline server server commandkey 9 keyfile /etc/chrony.keys driftfile /etc/chrony.drift
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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|SUSE LLC's SUSE Manager||Jul 21, 2016|
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|Non-Linux FOSS: Caffeine!||Jul 19, 2016|
|Murat Yener and Onur Dundar's Expert Android Studio (Wrox)||Jul 18, 2016|
- Stunnel Security for Oracle
- The Firebird Project's Firebird Relational Database
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- SUSE LLC's SUSE Manager
- Managing Linux Using Puppet
- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- Google's SwiftShader Released
- Doing for User Space What We Did for Kernel Space
- SuperTuxKart 0.9.2 Released
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide