NTP installation and configuration

NTP installation and configuration

STEP 1 (Test the current server):

Try issuing the following command:

$ ntpq -pn

remote refid st t when poll reach delay offset jitter
===================================================
tock.usno.navy 0.0.0.0 16 u - 64 0 0.000 0.000 4000.00

The above is an example of a problem.
Compare it to a working configuration.

$ ntpq -pn

remote refid st t when poll reach delay offset jitter
========================================================
+128.4.40.12 128.4.40.10 2 u 107 128 377 25.642 3.350 1.012
127.127.1.0 127.127.1.0 10 l 40 64 377 0.000 0.000 0.008
+128.91.2.13 128.4.40.12 3 u 34 128 377 21.138 6.118 0.398
*192.5.41.41 .USNO. 1 u 110 128 377 33.69 9.533 3.534

STEP 2 (Configure the /etc/ntp.conf):

$ cat /etc/ntp.conf

# My simple client-only ntp configuration.
server timeserver1.upenn.edu
# ping -a timeserver1.upenn.edu shows the IP address 128.91.2.13
# which is used in the restrict below
restrict 128.91.2.13
server tock.usno.navy.mil
restrict 192.5.41.41
server 128.4.40.12
restrict 128.4.40.12
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

STEP 3 (Configure /etc/ntp/step-tickers):

The values for server above are placed in the "/etc/ntp/step-tickers" file

$ cat /etc/ntp/step-tickers

timeserver1.upenn.edu
tock.usno.navy.mil
128.4.40.12

The startup script /etc/rc.d/init.d/ntpd will grab the servers in this
file and execute the ntpdate command as follows:

/usr/sbin/ntpdate -s -b -p 8 timeserver1.upenn.edu

Why? Because if the time is off ntpd will not start. The command above set the
clock. If System Time deviates from true time by more than 1000 seconds, then,
the ntpd daemon will enter panic mode and exit.

STEP 4 (Restart the service and check):

Issue the restart command

/etc/init.d/ntpd restart

check the values for "ntpq -pn",
which should match step 1.

ntpq -pn

SPECIAL NOTE:

Time is always stored in the kernel as the number of seconds since
midnight of the 1st of January 1970 UTC, regardless of whether the
hardware clock is stored as UTC or not. Conversions to local time
are done at run-time. So, it's easy to get the time in different
timezones for only the current session as follows:


$ export TZ=EST
$ date
Mon Aug 2 10:34:04 EST 2004

$ export TZ=NET
$ date
Mon Aug 2 15:34:18 NET 2004

The following are possible values for TZ:

Hours From Greenwich Mean Time (GMT) Value Description
0 GMT Greenwich Mean Time
+1 ECT European Central Time
+2 EET European Eastern Time
+2 ART
+3 EAT Saudi Arabia
+3.5 MET Iran
+4 NET
+5 PLT West Asia
+5.5 IST India
+6 BST Central Asia
+7 VST Bangkok
+8 CTT China
+9 JST Japan
+9.5 ACT Central Australia
+10 AET Eastern Australia
+11 SST Central Pacific
+12 NST New Zealand
-11 MIT Samoa
-10 HST Hawaii
-9 AST Alaska
-8 PST Pacific Standard Time
-7 PNT Arizona
-7 MST Mountain Standard Time
-6 CST Central Standard Time
-5 EST Eastern Standard Time
-5 IET Indiana East
-4 PRT Atlantic Standard Time
-3.5 CNT Newfoundland
-3 AGT Eastern South America
-3 BET Eastern South America
-1 CAT Azores

DST timezone


0 BST for British Summer.
+400 ADT for Atlantic Daylight.
+500 EDT for Eastern Daylight.
+600 CDT for Central Daylight.
+700 MDT for Mountain Daylight.
+800 PDT for Pacific Daylight.
+900 YDT for Yukon Daylight.
+1000 HDT for Hawaii Daylight.
-100 MEST for Middle European Summer,
MESZ for Middle European Summer,
SST for Swedish Summer and FST for French Summer.
-700 WADT for West Australian Daylight.
-1000 EADT for Eastern Australian Daylight.
-1200 NZDT for New Zealand Daylight.

The following is an example of setting the TZ environment variable
for the timezone, only when timezone changes go into effect.

$ export TZ=EST+5EDT,M4.1.0/2,M10.5.0/2

Take a look at the last line "M10.5.0/2". What does it mean? Here is the
documentation


Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of
month m (1 <= m <= 12). Week 1 is the first week in which day d
occurs and week 5 is the last week in which day d occurs. Day 0
is a Sunday.

The time fields specify when, in the local time currently in
effect, the change to the other time occurs. If omitted,
the default is 02:00:00.

So this is what it means. M10 stands for October, the 5 is the fifth week
that includes a Sunday (note 0 in M10.5.0/2 is Sunday). To see that it is
the fifth week see the calendar below. The time change occurs a 2am in
the morning. (Special Note: In 2007, DST was extended. See TIP 230).

October
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31

Prove it. Take the following program sunrise, which can calcuates sunrise
and sunset for an latitude and longitude. This program can be downloaded
from the following location:
http://sourceforge.net/direct-dl/mchirico/souptonuts/working_with_time.tar.gz

Below is a bash script that will run the program for the next 100 days.

#!/bin/bash
# program: next100days Mike Chirico
# download:
# http://sourceforge.net/direct-dl/mchirico/souptonuts/working_with_time.tar.gz
#
# This will calculate the sunrise and sunset for
# latitude 39.95 Note must convert to degrees
# longitude 75.15 Note must convert to degrees
lat=39.95
long=75.15
for (( i=0; i <= 100; i++))
do
sunrise `date -d "+$i day" "+%Y %m %d"` $lat $long
done

Take a look at the following sample output.

$ export TZ=EST+5EDT,M4.1.0/2,M10.5.0/2
$ ./next100days

Sunrise 08-24-2004 06:21:12 Sunset 08-24-2004 19:43:42
Sunrise 08-25-2004 06:22:09 Sunset 08-25-2004 19:42:12
Sunrise 08-26-2004 06:23:06 Sunset 08-26-2004 19:40:41
Sunrise 08-27-2004 06:24:03 Sunset 08-27-2004 19:39:09
Sunrise 08-28-2004 06:25:00 Sunset 08-28-2004 19:37:37
Sunrise 08-29-2004 06:25:56 Sunset 08-29-2004 19:36:04
Sunrise 08-30-2004 06:26:53 Sunset 08-30-2004 19:34:31
Sunrise 08-31-2004 06:27:50 Sunset 08-31-2004 19:32:57
Sunrise 09-01-2004 06:28:46 Sunset 09-01-2004 19:31:22
Sunrise 09-02-2004 06:29:43 Sunset 09-02-2004 19:29:47
..[values omitted ]
Sunrise 10-28-2004 07:25:31 Sunset 10-28-2004 18:02:34
Sunrise 10-29-2004 07:26:38 Sunset 10-29-2004 18:01:19
Sunrise 10-30-2004 07:27:46 Sunset 10-30-2004 18:00:06
Sunrise 10-31-2004 06:28:53 Sunset 10-31-2004 16:58:54
Sunrise 11-01-2004 06:30:01 Sunset 11-01-2004 16:57:44
Sunrise 11-02-2004 06:31:10 Sunset 11-02-2004 16:56:35

Compare 10-30-2004 with 10-31-2004. Sunrise is an hour earlier because
daylight saving time has ended, just as predicted.

There is an easier way to switch between timezones. Take a look at the
directory zoneinfo as follows:

$ ls /usr/share/zoneinfo

Africa Chile Factory Iceland Mexico posix UCT
America CST6CDT GB Indian Mideast posixrules Universal
Antarctica Cuba GB-Eire Iran MST PRC US
Arctic EET GMT iso3166.tab MST7MDT PST8PDT UTC
Asia Egypt GMT0 Israel Navajo right WET
Atlantic Eire GMT-0 Jamaica NZ ROC W-SU
Australia EST GMT+0 Japan NZ-CHAT ROK zone.tab
Brazil EST5EDT Greenwich Kwajalein Pacific Singapore Zulu
Canada Etc Hongkong Libya Poland SystemV
CET Europe HST MET Portugal Turkey

TZ can be set to any one of these files. Some of these are directories and contain
subdirectories, such as ./posix/America. This way you don not have to enter the
timezone, offset, and range for dst, since it has already been calculated.

$ export TZ=:/usr/share/zoneinfo/posix/America/Aruba
$ export TZ=:/usr/share/zoneinfo/Egypt


Reference:
http://prdownloads.sourceforge.net/cpearls/date_calc.tar.gz?download

Also see (TIP 27).
Also see (TIP 103) using chrony which is very similiar to ntpd.
Note time settings can usually be found in /etc/sysconfig/clock.