ntp - makes the clock show the correct time
How to get started with ntpdate, the tool for constantly adjusting the system clock to the internet
- Installing ntpd
- Configuration files for running ntp as daemon
- Setting the clock at boot or using cron
ntp is the Network Time Protocol suite.
1. Installing ntpd
First, make sure ntp/ntpdate are installed. The package is by default installed on Mandrake, Linux and Redhat. If you do not have ntp installed you will find packages on your distributions install-cd(s).
On Gentoo Linux, install by typing
emerge ntp
2. Configuration files for running ntp as daemon
2.1. Client Setup
Now, edit the following files:
/etc/ntp.conf
The only important thing is that the line contains server some.thing.org.
Example:
server ntp.uio.no driftfile /etc/ntp/drift broadcastdelay 0.008 authenticate no
Make sure you add ntpd as a boot (init) service.
Gentoo Linux: rc-update add ntpd default , /etc/init.d/ntpd start
2.2. (LAN) Server setup
To run a ntp server to sync the clocks on your lan, edit
/etc/ntp.conf
Example:
server fartein.ifi.uio.no prefer server ntp.uio.no server ntp.eunet.no fudge 10.0.0.50 stratum 10 driftfile /etc/ntp/drift broadcastdelay 0.008 authenticate no
fudge tells the server to pass on the correct time to the given network interface. stratum is the level of time servers, 10 being the lowest. You only need a higher number if you will be acting as a public internet timeserver with fast bandwidth.
It makes sence to add a few servers. Select the best as prefer
Make sure you add ntpd as a boot (init) service.
Gentoo Linux: rc-update add ntpd default , /etc/init.d/ntpd start
2.3. Firewalls
ntp uses port 123, both UDP and TCP. Make sure this port is not blocked by a firewall.
If you are using iptables, you can open this port with
$IPTABLES -A INPUT -j ACCEPT -p tcp --dport 123
2.3.1. Known valid and working ntp servers
| server adress | Location |
|---|---|
| ntp.ipv6.viagenie.qc.ca | IPV6 ONLY |
| clock.via.net | |
| server fartein.ifi.uio.no | Norway |
| server ntp.uio.no | Norway |
| server ntp.eunet.no | Norway |
| ntp.demon.co.uk | UK |
| ntp.nasa.gov | USA |
| bigben.cac.washington.edu | USA |
| time-b.nist.gov | USA |
| montpelier.ilan.caltech.edu | USA |
| nist1.aol-ca.truetime.com | USA |
| nist1.datum.com | USA |
| time-a.timefreq.bldrdoc.gov | USA |
| time-b.timefreq.bldrdoc.gov | USA |
| time-c.timefreq.bldrdoc.gov | USA |
| time.nist.gov | USA |
| utcnist.colorado.edu | USA |
| tick.usno.navy.mil | USA |
| tock.usno.navy.mil | USA |
| mizbeaver.udel.edu | USA |
2.3.2. Server List
3. Setting the clock at boot or using cron
I good idea is _not_ to run the ntpd daemon on clients as it uses about 1,7 MB memory, unless you are a server or have a box with a broken clock. Many boxes clocks do drift along on their own, a check every hour or day is generally a good idea.
3.1. rdate
rdate is a simple 3 kB tool for syncing the system clock to a server. It's options are -p to print the date on the given server, -s to set the system clock according to it (must be done as root) and -u to use the UDP protocol. Example:
rdate -p sntp.lth.se
3.2. clockspeed
clockspeed is a very small tool for setting the clock and is, for clients, a much better alternative than installing the 4+ MB ntp package.
3.3. cron & ntp
You can simply add ntpdate as a cron job every now and then:
echo 'ntpdate -b server.dot.com' > /etc/cron.hourly/time.cron
chmod a+x /etc/cron.hourly/time.cron
Most distributions come with a cron. dcron and Vixie cron are nice.
3.4. Gentoo & ntp-client
You can set the clock using ntp at boot (without starting the daemon) on Gentoo Linux by editing:
/etc/conf.d/ntp-client
The file should contain:
NTPDATE_CMD="ntpdate" NTPDATE_OPTS="-b your.server.here"
Then add ntp-client to the services started at boot:
rc-update add ntp-client default
You can check what services are started at boot with rc-status.
