Locating and Using an External Time Source

You can find lists of public stratum 1 and stratum 2 NTP servers at http://www.eecis.udel.edu/~mills/ntp/servers.html. The U.S. Naval Observatory operates a set of public stratum 1 servers; check http://tycho.usno.naw.mil/ntp.html for a list. In theory, you can use any of these servers as a source for your own main NTP server. In practice, some work better than others, and there's an etiquette involved in using these servers. All other things being equal, the best server to use is the one closest to you in a network sense. NTP works, in part, by measuring the round-trip latency between two systems. When the latency is low, the accuracy of the protocol is improved. As a general rule, network latency increases with physical distance, although there are exceptions to this rule. Therefore, if you're located in, say, California, you're almost certainly better off using an external server in the western half of the United States than you would be using a server in Australia or Europe.

Tip You can use ping to determine the round-trip latency between your network and a potential external time source. You may also find traceroute to be informative in determining the number of network hops between yourself and another site.

After perusing the lists, you should have a set of potential external time sources. Be sure to read the website and especially the description of the sites you might use. Many sites are open to public use but request that potential users contact the NTP server maintainer before using the server. Be sure to do so if a site asks this courtesy. Also, as noted on the main NTP servers page, you shouldn't use a stratum 1 server as a source unless your site serves many computers—the most frequently cited cutoff value is 100 clients.

In addition to the public servers listed on the NTP servers page, you may be able to find other servers. One potentially very good source is your own ISP. Many ISPs operate NTP servers and are happy to let their customers point to them. This feature is seldom widely advertised, though, so you may need to ask about it. An ISP's server is likely to be an excellent candidate because it's likely to be close to you in network topology. If you're configuring an NTP server for a department in a larger organization, consult with your colleagues within the organization; it's possible that another department is running an NTP server to which you could synchronize.

However you locate external servers, you enter them in the/etc/ntp.conf file using server lines:

server time.abigisp.net server ntp.pangaea.edu

You can use just one server, but listing two or three may be beneficial in case one server goes down or produces an inaccurate time source. Using three or more enables NTP to spot and exclude false tickers—NTP servers whose time signal is just plain wrong. If you trust your main external time source, though, using just one will help to reduce the load on other servers.

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