NTP

From HerzbubeWiki

Contents 1 Debian packages 2 References 3 Glossary 4 System time vs. hardware clock 5 NTP daemon 5.1 Local time sync 5.2 Provide time to clients 5.3 pool.ntp.org 6 ntpdate 7 ntpq

Debian packages

Debian package required to run an NTP daemon that synchronizes local time, but also provides a time sync service to clients:

ntp

Debian package required to simply synchronize local time:

ntpdate

Documentation package:

ntp-doc

References

• NTP documentation @ pelargir.herzbube.ch (available only if the ntp-doc package is installed): Main page with overview information, the Sitemap is more useful to quickly find stuff
• A list of public time servers : http://www.eecis.udel.edu/~mills/ntp/servers.html

Glossary

• Stratum 1 = Primary Server
• Stratum 2 = Secondary Server
• [...]

System time vs. hardware clock

Edit

/etc/init.d/hwclock.sh

• Comment (= disable) the line where hwclock is run with the parameter --adjust, in order to prevent conflicts between hwclock and NTP
• Uncomment (= enable) the line where hwclock is run with the parameter --systohc, in order to write the system time to the hardware clock on reboot

Note: I have not enabled this option.

NTP daemon

Local time sync

To activate an NTP server

• Add the following lines to /etc/ntp.conf if they don't exist

driftfile /var/lib/ntp/ntp.drift
server ntp.foobar.org

• Restart the NTP server
• Wait some time until system time has been updated. When I first experimented with NTP on my old server osgiliath I had to wait up to half an hour until the time sync occurred. On my new server pelargir, however, time sync happened immediately after the daemon restart.
• It is important that the system time is not out-of-sync more than 1000 seconds, otherwise ntp refuses to automatically correct the system time ("time correction exceeds sanity limit")

These days on my new server pelargir I simply use the default configuration in /etc/ntp.conf which seems to work out of the box. The servers configured there are

0.debian.pool.ntp.org
1.debian.pool.ntp.org
2.debian.pool.ntp.org
3.debian.pool.ntp.org

Provide time to clients

The configuration file /etc/ntp.conf mentions that, in order to provide time to clients a "broadcast" line needs to be enabled. For instance

broadcast 192.168.1.255
broadcast 192.168.2.255

However, I did not enable these commands because so far my Mac OS X clients were perfectly able to synchronize, once I told them where they were supposed to connect to (System Preferences > Date & Time).

Note: I added a "time-servers" option to my DHCP configuration (e.g. "time-servers 192.168.1.11" on the 192.168.1.0 subnet), but at the time of writing Mac OS X did not honor this option (Mac OS X 10.6). I have not yet tested how Windows behaves.

pool.ntp.org

A long time ago, on my old server osgiliath I tried to use the server pool.ntp.org - which is supposed to assign a different NTP server to my client every time my client is restarted. Unfortunately, pool.ntp.org never seemed to work, even after several attempts to use it. The system time never got corrected - the drift grew to 40 minutes and more until I had to manually re-adjust the system time. Nowadays pool.ntp.org may work better, but when I had trouble with it I instead connected to

time.euro.apple.com

ntpdate

The Debian package ntpdate provides the utility ntpdate which can be used to immediately adjust the system date at the time when the utility is run. For instance:

ntpdate time.euro.apple.com

For continuous time sync, I add the following crontab entry:

pelargir:~# ls -l /etc/cron.hourly/pelargir-ntpdate 
-rwxr-xr-x 1 root root 56 2011-02-09 19:15 /etc/cron.hourly/pelargir-ntpdate

pelargir:~# cat /etc/cron.hourly/pelargir-ntpdate 
#!/bin/sh

/usr/sbin/ntpdate -u -s time.euro.apple.com

There is also the utility ntpdate-debian which uses the configuration in the following file

/etc/default/ntpdate

ntpq

ntpq is the standard NTP query program. For instance, it can be used to check the current time source

ntpq -pn

(the source is the line with the asterisk)

For example:

root@pelargir:~# ntpq -pn
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+217.147.208.1   194.242.34.149   2 u   61   64    3    9.450  -10.368  10.087
+192.33.214.57   129.194.216.1    3 u   55   64    7   13.198  -10.532   8.899
+94.23.99.155    94.23.99.153     3 u   31   64    7   27.351   -5.038   7.588
*192.33.96.102   .PPS.            1 u   24   64   17    9.889  -12.321  11.285