Accurate Time is Important

The real time clock built in to your computer is accurate enough for most applications. Setting it from your watch can mean it’s off by a few minutes, but it will maintain that (incorrect) time fairly well. Join a network with that computer, and suddenly the correct time becomes important. Here’s why:

1. Network Protocols Rely on Accurate Time – Many network protocols, including encryption, data transport, and data caching require the sending and receiving hardware to have accurate time to function well.
2. Log Analysis Needs Accurate Time – Analyzing data across machines in multiple log files is impossible without those machines agreeing on the time.
3. Legal Records Require Accurate Time – When you are being cross-examined and are asked “Well, how can you be sure that was the exact time in your logs?” you will be able to demonstrate the automatic process that keeps all of your computer clocks accurate.
4. Contractual Records Require Accurate Time – Knowing when a request was fulfilled is needed for billing and royality payment purposes.

Setup the NTP Server

Most *nix distributions will have NTP available to install if not installed by default. Once installed, configuring the server is as easy as:

1. Select one or two close servers from which to obtain accurate time. See The NTP Servers Web for a list of public servers. List these servers with the “server” directive in your ntp.conf file.
2. Use the “restrict” directive. List the network(s) allowed to query the time.
3. Run ntpdate several times. Until the “offset” returned is less than 1.
4. Start the ntpd service. And make sure it runs after a reboot.

You are going to set up more than one of these, right? Single Point of Failure (SPoF) is usually a bad thing…

Setup the NTP Clients

Setting up the client is almost the same procedure as setting up the server.

1. Setup your client to use your local NTP servers. List these servers with the “server” directive in your ntp.conf file.
2. Run ntpdate several times. Until the “offset” returned is less than 1.
3. Start the ntpd service. And make sure it runs after a reboot.

You may also want to change the startup script to include ntpdate -b server at start up (and before starting the daemon). This will cause the time to jump to the correct time instead of being slowly adjusted toward the correct time. Handy if your clock is way off.

Setup a Stratum 1 Server

Want to setup NTP, but on an isolated network? No problem – just setup a stratum 1 server. In NTP, “stratum” describes the distance from a stable time source – higher number is further away. Stratum 0 is the reference time source itself, like WWV or a cesium clock. Stratum 1 is the computer directly connected to the reference clock. Most reference clocks are too expensive to run yourself, but there is cheap time source that is easy to setup and connect to your NTP server.

1. Obtain a Stratum 0 time reference – Use your favorite internet search engine to find a “pulse-per-second” GPS receiver. In January of 2009, Garmin has an OEM model for about $70. Since a GPS knows it’s location by comparing it’s (derived) hyper-accurate clock against the time broadcast from GPS satelites, it can tell you the exact time, as well as it’s location. Learn more details in this great tutorial.
2. Connect the GPS to the Server – The pulse-per-second feature sends a signal every second down a wire. Plug that wire into your serial port, and NTP can use that time source as an NTP server.

This post certainly isn’t enough information to set up a stratum 1 server, but it really is pretty easy. To really set it up, you need to read the NTP docs, search the ‘net, and read the great articles that you find.