NTP Notes


Mostly for RHEL5 systems. Should work on any Linux distro.

Preliminary notes

The config for ntpd is /etc/ntp.conf. These lines are of interest:

server 0.centos.pool.ntp.org  
server 1.centos.pool.ntp.org  
server 2.centos.pool.ntp.org

They must be changed to whatever you have in mind for NTP servers.
Important: You must also include these servers in the
/etc/ntp/step-tickers file! This file will be found initially empty.
If you don’t, you’ll see something like this when restarting the NTP

[root@localhost ntp]# /sbin/service ntpd start
ntpd: Synchronizing with time server:            [FAILED]
Starting ntpd:                                   [  OK  ]

The NTP port is 123 and must be open in your iptables rules to allow

The University of Iowa NTP servers

As mentioned earlier, these need to be in the step-tickers file.


How Time Zones work in Linux

Important: The NTP servers do not provide timezone-specific data!
All they provide is UTC time. This data is then adjusted by ntpd in
conjunction with /etc/localtime.

The /etc/localtime file has a list of the zone offsets (going till
2038; the 32-bit problem) and, importantly, the dates on which the clock
is moved forward and backward (for the Daylight Savings crap.) To see
the contents of this file, you have to use zdump:

zdump -v /etc/localtime | grep 2010

This will show how the clock is adjusted w.r.t. timezone and DST:

/etc/localtime  Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600  
/etc/localtime  Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000  
/etc/localtime  Sun Nov  7 06:59:59 2010 UTC = Sun Nov  7 01:59:59 2010 CDT isdst=1 gmtoff=-18000  
/etc/localtime  Sun Nov  7 07:00:00 2010 UTC = Sun Nov  7 01:00:00 2010 CST isdst=0 gmtoff=-21600

Comprehensive timezone resources are located in /usr/share/zoneinfo.
It’s the directory utilized by, for example, Anaconda to set your
timezone at install. Here’s a sample tree view of the contents of this

|-- Anguilla  
|-- Antigua  
|-- Araguaina  
|-- Argentina  
|   |-- Buenos_Aires  
|   |-- Catamarca  
|   |-- ComodRivadavia  
|   |-- Cordoba  
|   |-- Jujuy  
|   |-- La_Rioja  
|   |-- Mendoza  
|   |-- Rio_Gallegos  
|   |-- Salta  
|   |-- San_Juan  
|   |-- San_Luis  
|   |-- Tucuman  
|   -- Ushuaia   
|-- Aruba  
|-- Asuncion  
|-- Atikokan  

To manually adjust your timezone, you just need to symlink the correct
resource file to /etc/localtime.

For US zones, the naming scheme is slightly weird. Here’s a table of
files for four US timezones. They are at the root of

Time Zone File in /usr/share/zoneinfo
Eastern EST5EDT
Central CST6CDT
Mountain MST7MDT
Pacific PST8PDT

Again, use zdump to view/verify these files.

Adjusting Time Zones

The simple way

You just have to symlink the correct time resource file to
/etc/localtime. For instance, if I’m in the America/Chicago timezone:

mv /etc/localtime /etc/localtime.backup  
ln -s /usr/share/zoneinfo/America/Chicago /etc/localtime  
service ntpd restart


If you don’t want to bother with pesky symlinking to update
/etc/localtime, you can always use tzselect. It’s a small,
interactive command-line utility which will generate a string like this
that you can add to ~/.bash_profile

TZ='Africa/Porto-Novo'; export TZ

Fixing Daylight Savings Issues

A zdump of the /etc/localtime file will yield what the local system
does at certain dates.

zdump -v /etc/localtime

Look at the output. If you see the correct dates for Daylight Savings,
you’re good. For example,

/etc/localtime  Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600  
/etc/localtime  Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000  
/etc/localtime  Sun Nov  7 06:59:59 2010 UTC = Sun Nov  7 01:59:59 2010 CDT isdst=1 gmtoff=-18000  
/etc/localtime  Sun Nov  7 07:00:00 2010 UTC = Sun Nov  7 01:00:00 2010 CST isdst=0 gmtoff=-21600

Which seems correct, since the time jumped ahead an hour on March 14.
But here’s some worrisome output:

/etc/localtime  Sun Apr  4 07:59:59 2010 UTC = Sun Apr  4 01:59:59 2010 CST isdst=0 gmtoff=-21600
/etc/localtime  Sun Apr  4 08:00:00 2010 UTC = Sun Apr  4 03:00:00 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime  Sun Oct 31 06:59:59 2010 UTC = Sun Oct 31 01:59:59 2010 CDT isdst=1 gmtoff=-18000
/etc/localtime  Sun Oct 31 07:00:00 2010 UTC = Sun Oct 31 01:00:00 2010 CST isdst=0 gmtoff=-21600

In this case, you’ll need to update the timezone files in

Note: The timezone files are all compiled with zic

The easy way - Use YUM

One way to do it is to update the tzdata RPM.

yum update tzdata

This should fetch and install the correct files.

The hard way - Compile with Zic

Download the latest data. This is the file as of 2010:

wget ftp://elsie.nci.nih.gov/pub/tzdata2010f.tar.gz

They don’t extract to a directory, so make sure you create one! Now
compile zone files using zic with this syntax:

zic -d <directory to extract to> <name of zone file>

Let’s try it for North America. The file to compile is called
northamerica. Let’s compile the results to tempdir

zic -d tempdir northamerica

Now verify that the newly compiled files have the correct DST offsets:

[root@localhost tempdir]# zdump -v tempdir/CST6CDT | grep 2010  
CST6CDT  Sun Mar 14 07:59:59 2010 UTC = Sun Mar 14 01:59:59 2010 CST isdst=0 gmtoff=-21600  
CST6CDT  Sun Mar 14 08:00:00 2010 UTC = Sun Mar 14 03:00:00 2010 CDT isdst=1 gmtoff=-18000  
CST6CDT  Sun Nov  7 06:59:59 2010 UTC = Sun Nov  7 01:59:59 2010 CDT isdst=1 gmtoff=-18000  
CST6CDT  Sun Nov  7 07:00:00 2010 UTC = Sun Nov  7 01:00:00 2010 CST isdst=0 gmtoff=-21600

Woohoo! Now you can copy these over to /usr/share/zoneinfo:

[root@localhost tempdir]# cp -r * /usr/share/zoneinfo/

Now remove the old /etc/localtime and symlink the newly compiled zone

[root@localhost tempdir]# rm -f /etc/localtime  
[root@localhost tempdir]# ln -s /usr/share/zoneinfo/CST6CDT /etc/localtime

Restart ntpd and you’re good to go!


  1. Update Linux and FreeBSD systems for new Daylight Saving Time
  2. Date, Time and Time Zones for Red Hat
    - Excellent
    overview of NTP and date and time utilities.