Configuring the NTP Daemon

Consumer hardware is notoriously bad at keeping accurate time. NTP is a service that can query other computers over the network and synchronize your computer's clock to their time. It's very complex to configure, since the network itself isn't instantaneous, and two or more computers will usually disagree as to exactly what time it is.

There is a list of publically accessible NTP servers at http://www.eecis.udel.edu/~mills/ntp/servers.htm, or see the actual NTP informational homepage at http://www.eecis.udel.edu/~ntp/ for other details.

Red Hat Linux comes with a sample /etc/ntp.conf, but if you just want your machine to synchronize with a public server, I recommend removing that file and starting with a far simpler script. Remove the existing file and create one that looks like the following.

/etc/ntp.conf (complete sample file)

# A very simple client-only ntp configuration. server 127.127.1.0 # local clock fudge 127.127.1.0 stratum 10 driftfile /etc/ntp/drift restrict default ignore restrict 127.0.0.0 mask 255.0.0.0 authenticate no

Choose your preferred NTP servers. Note that some servers are not available to the public, and some servers are too busy to handle new requests. Please do your homework in choosing appropriate servers that are geographically near you, and which will accept new public connections, and whether you must obtain their permission first.

You should select two or three servers for best results. For each server you find, discover its hostname and its fixed numerical address. Then add lines like the following to the /etc/ntp.conf file you created above.

/etc/ntp.conf (additional lines for each server)

server ntp5.someserver.etc restrict 123.123.123.123 nomodify # ntp5.someserver.etc

(The over-simplified Red Hat Linux time and date settings application allow a single-server configuration, but usually mangles the lines in the file, unfortunately, so don't use that utility once you've edited your own /etc/ntp.conf manually.)

Next, a separate ticker file must be created that lists the servers. The following command will read the /etc/ntp.conf file you've made, and create the proper list of servers for the ticker file. This finds the hostnames listed on lines beginning with 'server', except for the initial 127.127.1.0 local clock, and lists them in the ticker file.

$ su - # awk '/^server/ {print $2}' /etc/ntp.conf | grep -v '127.127.1.0' > /etc/ntp/step-tickers

Before starting the service, roughly synchronize your clock using one of the servers with ntpdate. Ensure your configuration is set to run the NTP daemon ntpd to track and alleviate clock drifting.

$ su - # chkconfig --list ntpd ntpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off # ntpdate -u ntp5.someserver.etc 9 Dec 15:15:15 ntpdate[12345]: step time server 123.123.123.123 offset 7.879896 sec # chkconfig --level 345 ntpd on

If your system clock is likely more than a few minutes off, it is best to shut down as many applications as possible before performing gross clock corrections. Some programs may have bugs which cause them to lose data or crash. A minimal environment such as running at runlevel 1 (single user mode) is the ideal situation for drastic clock changes.

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