1. Network Time Protocol Objectives Contents to learn how to setup ntp
The NTP Server
What is NTP?
Download and Install The NTP Package
The /etc/ntp.conf File
How To Get NTP Started
Determining If NTP Is Synchronized Properly

2. What is NTP? Network Time Protocol Stratum levels
Used to keep clock’s syncronized within your nodes
Important for logservers and logfiles
Transaction servers
Various applications, position system,mobilephones like GSM,transaction systems
Time critical operations that needed to be syncronized
Stratum levels
Level 1 most accurate, directly connected to reference clock
Level 2 next accurate,connected to more than one Level 1
Lower levels is connected to only one Level 1 or to Level 2
DCF77 receiver’s, LW radio carriers as reference
Available at very low price from various places
GSM time reference
Uses GPS satellites as timebase, all GPS satellites have atomic clocks
Atomic clock
Usally based on rugby or cesium Atomic Clock, based on atomic oscillations
Proposed 1943, first built 1949, practical 1955
Important atomic clock servers that supports NTP
Network Time Protocol (NTP) is a protocol used to help synchronize your system clock with an accurate time source. There are a number of "Stratum 1" (NTP sites using an atomic clock for timing) and "Stratum 2" (NTP sites with slightly less accurate time sources) sites that allow the general public to synchronize with them. It is good practice to have at least one server on your network be the local time server for all your other devices, this makes the correlation of system events on different systems much easier.
Rubidium
The Rubidium atomic clock, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.
Cesium
The most accurate commercial atomic clock available today uses the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beams, reducing small changes in frequency due to the Doppler effect.

3. Download and Install The NTP tar-boll
Check if NTP is there, remove if it is there, keep yast
NTP comes from ISC Internet Systems Consortium
Download and unzip the tar ball
Configure the source package
Make the source package and install
# rpm -qa | grep -i ntp
xntp-4.2.0a-35
yast2-ntp-client
# rpm –e xntp-4.2.0a-35
# cd /usr/local/src
# wget
# tar xvfz ntp-dev-4.2.0b tar.gz
When setting up NTP it is often nessesary to install from sources, since the time reference you might have as hardware in form of DCF77, GPS-time, Atomic clock need some special configuration at step ”configure the source package” which is not available if you use the RPM.
Example 1)
You have GPS timepice: ./configure - -help | grep –i gps
--enable-GPSVME TrueTime GPS receiver/VME interface
--enable-ARBITER Arbiter 1088A/B GPS receiver
--enable-AS Austron 2200A/2201A GPS receiver
--enable-FG Forum Graphic GPS
. . .
Example 2)
You have DCF timepice: ./configure - -help | grep –i dcf
--enable-NEOCLOCK4X NeoClock4X DCF77 / TDF receiver
--enable-DCF s ELV/DCF7000 clock
--enable-RAWDCF s DCF77 raw time code
However if you do not have hardware time reference, you can saftely use the RPM or precompiles binaries.
# cd ntp-dev-4.2.0b ; ./configure
# make && make test && make install

4. Download and Install The NTP rpm Package
Check if NTP is there, if not then download it
Install NTP after it is downloaded
NTP is SuSE standard, however it is not preconfigured and started.
First time insserv and rcxntpd will open iptables firewall as well, ntp used port 123 UDP
# rpm -qa | grep -i ntp
xntp-4.2.0a-35
yast2-ntp-client
# cd /usr/local/src
# wget
# rpm –ivh xntp-4.2.0a-35.i586.rpm
Most SuSE, RedHat and Fedora Linux usally have ntp or xntp installed by default. SuSE makes by unknown reason a softlink from ntpd to xntpd. Xntpd was originally planned to replace ntpd as extended ntp but did not materialize to all plateforms.
When searching for the file, remember that the NTP RPM's filename usually starts with the word "ntp" or xntp followed by a version number like this:
SuSE SuSE10.0
(*) xntp-4.2.0a-35.i586.rpm (*) xntp-4.2.0a-46.i586.rpm
xntp-debuginfo-4.2.0a-35.i586.rpm xntp-debuginfo-4.2.0a-46.i586.rpm
xntp-doc-4.2.0a-35.i586.rpm xntp-doc-4.2.0a-46.i586.rpm
(*) = Needed for NTP
Remember to download ntp –rpm file in a ”desent” catalog on your upcoming NTP enabled host, a good place could be: mkdir /usr/src/extras ; cd /usr/src/extras
# insserv xntpd
# rcxntpd start

5. The /etc/ntp.conf File
First we specify the sample servers we're interested in:
Servers are used in order, average time can be calculated
Then we restrict the type of access you allow these servers
Declare the networks this NTP server should serve
This NTP server should serve itself unrestricted
server ntp1.gbg.netnod.se # A stratum 1 server
server gps.dix.dk # A stratum 2 server
server clock.isc.org # unknown strata
restrict ntp1.gbg.netnod.se mask nomodify notrap noquery
restrict gps.dix.dk mask nomodify notrap noquery
restrict clock.isc.org mask nomodify notrap noquery
This is the main configuration file for Linux NTP in which you place the IP addresses of the stratum 1 and stratum 2 servers you want to use. Here is a sample of a configuration using a pair of sample Internet based NTP servers.
The mask statement is really a subnet mask limiting access to the single IP address of the remote NTP servers.
Serve time to some nets If this server is also going to provide time for other computers, such as PCs, other Linux servers and networking devices, then you'll have to define the networks from which this server will accept NTP synchronization requests. This is done with a modified restrict statement with the "nomodify" replaced with a "notrust" keyword. In this case the mask statement has been expanded to include all 255 possible IP addresses on our local network.
NTP Server yourself as server We also want to make sure that localhost (The universal IP address used to refer to a Linux server itself) has full access without any restricting keywords
Fetch time from somewhere on the net:
restrict mask notrust nomodify notrap
restrict mask notrust nomodify notrap
restrict

6. How To Get NTP Started (not RPM)
To get NTP configured to start at boot:
To start/stop/restart NTP after booting:
To see if ntp is running:
If you like write your own start stop script and put in runlevel 3 and 5
Logging in /var/log/messages
Logging in /var/log/ntpd.log
Add in /etc/ntp.conf
# echo ”/usr/local/bin/ntpd” >> /etc/init.d/boot.local
# pkill -9 ntpd
# pkill -HUP ntpd
# /usr/local/bin/ntpd
# pgrep ntpd
You have to restart the NTP process every time you make a change to the configuration file for the changes to take effect on the running process.
driftfile /var/lib/ntp/drift/ntp.drift
logfile /var/log/ntp.log

7. Testing And Troubleshooting NTP
Verifying NTP is Running
you should get a response of plain old process ID numbers
Doing An Initial Synchronization against one server
First check current idea of time
Then syncronize against NTP server
Last check that time was adjusted
If time it was to big difference in time you might want to set the time as exact you can manually and do the sync again
# pgrep ntpd
# date
Thu Sep 7 00:00:00 PDT 2004
# ntpdate –u ntp.research.gov 
Looking for host ntp.research.gov and service ntp
host found : ntp.research.gov
7 Sep 08:03:38 ntpdate[2472]: step time server ntp.research.gov offset sec     
Sep 7 08:03:38 PDT 2004
NTP sync is sensitive to to large timedeltas, if more than two hours local time you will be refused sync.
Bad motherboard or motherboard there BIOS batteries are worn down will have svere problems syncronizing with NTP, also time can drift to much in some system.
Your NTP daemon will eventually terminate itself leaving an error message in the /var/log/messages file if the time on the local server is very different from that of its time server. You should run the "ntpdate -u" command to rapidly force it to synchronize. Doesn't run continuously in the background, and you will still have to run the ntpd daemon to get continuous NTP updates.

8. Determining If NTP Is Synchronized Properly
With ntpq command see the servers you sync with
Jitter should be less than 100
Try to use NTP server close to you with high strata level
# ntpq -p
     remote          refid       st t when poll reach  delay   offset   jitter ============================================================================== -jj.cs.umb.edu   gandalf.sigmaso  3 u      377  31.681       1.572 milo.mcs.anl.go  ntp0.mcs.anl.go  2 u     125          mailer1.psc.edu ntp1.usno.navy.  2 u     377     19.589    dr-zaius.cs.wis ben.cs.wisc.edu  2 u     357      3.979     taylor.cs.wisc. ben.cs.wisc.edu  2 u     347      3.379     ntp0.cis.strath harris.cc.strat  3 u         -5.025    1.642 *clock.via.net   .GPS.            1 u        -3.018    2.534 ntp1.conectiv.c            16 u    - 1024    0   0.000    0.000  
Use the ntpq command to see the servers with which you are synchronized. It will provide you with a list of configured time servers and the delay, offset and jitter that your server is experiencing with them. For correct synchronization, the delay and offset values should be non-zero and the jitter value should be under 100.

9. Your Linux NTP clients cannot Synchronize Properly
Your test show something like this
This could be caused by the following
Older versions of NTP must have IP addresses, not FDQN
A firewall blocking access to your Stratum 1 and 2 NTP servers, port 123 UDP must be opened.
NTPd is not running on server or is not syncronized yet
# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        LOCAL(0)        10 l    -   64    7    0.000    0.000   0.008
 ntp-cup.externa          16 u    -   64    0    0.000    
 snvl-smtp1.trim          16 u    -   64    0    0.000    
 nist1.aol-ca.tr          16 u    -   64    0    0.000    
A telltale sign that you haven't got proper synchronization is when all the remote servers have jitters of 4000 with delay and reach values of zero
NTP servers communicate with one another using UDP with a destination port of 123. Unlike most UDP protocols, the source port isn't a high port (ie. greater than 1023), but 123 also. You'll have to allow UDP traffic on source/destination port 123 between your server and the Stratum 1/2 server with which you are synchronizing.

10. Configuring Cisco router To Use An NTP Server
Cisco IOS
ntp server: Forms a server association with another system.
ntp update-calendar: Configures the system to update its hardware clock from the software clock at periodic intervals.
ciscorouter> enable
password: *********
ciscorouter# config t
ciscorouter(config)# ntp update-calendar ciscorouter(config)# ntp server ciscorouter(config)# ntp server ciscorouter(config)# exit
ciscorouter# wr mem
Here are the commands you would use to make your router synchronize with NTP servers with IP addresses and

11. Configuring Cisco switch To Use An NTP Server
Cisco CATalyst OS
ntp server: Forms a server association with another system.
set ntp client enable: Activate the NTP client
ciscoswitch> enable password: ********* ciscoswitch# set ntp client enable ciscoswitch# ntp server ciscoswitch# ntp server
ciscoswitch# exit
Here are the commands you would use to make your router synchronize with NTP servers with IP addresses and

12. Configuring A Windows NTP Client
You can add your new NTP server to your Windows client. Here's how:
Click on the time at the bottom right hand side of your screen.
Click on the "Internet Time" tab of the dialog box
Click the check box labeled "Automatically synchronize with an Internet time server" and enter the name or IP address in the box underneath it.
Click on the "Update Now" button
You will get a message saying "Your time has been successfully synchronized" when the operation is complete

13. Summary NTP server is adjust time and date on nodes
Configuration sit in /etc/ntpd.conf
Calibration files sit in /etc/ntpd/
Strata levels tells system accuracy
Strata level 1 is most accurate
Strata level 2 is next accurate
Many applications rely on exact time
NTPD is the name of client and server demon
Show ntp status with ntpq –p
Update time manually: ntpdate –u ntp.research.gov
Most routers/Switches has NTP timesettings
It is important that all the systems under your control have the same accurate time. It can help to give a very clear indication of a chain of events that involve multiple devices and it can also help in the synchronization of time sensitive-transactions.
Having an NTP server on your local network can make this easier to do. Sometimes it isn't desirable for all your NTP clients to have access to the Internet to synchronize with stratum 1 and 2 servers, even when they all have access there is the risk of them losing synchronization if the central connection to the Internet is lost. The maintenance of firewall rules for multiple NTP connections to the Internet can also be daunting especially if the management of the firewall is handled by another group.
A local NTP server can ensure that the clients all have the same time relative to the server even when Internet connectivity is temporarily lost thereby reducing the problems of them being out of synchronization with each other. The firewall rules can also be greatly simplified. A local NTP server is frequently a good thing to have for these reasons.