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
Time critic operations that needed to be syncronized
Stratum levels
Level 1 most accurate, many public exist
Level 2 next accurate, often companies timeservers
Lower levels
LW radio carriers as reference
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.

3. Download and Install The NTP Package
Check if NTP is there and then download it
Install NTP after it is downloaded
NTP is redhat standard, however it is not started.
First time chkconfig and service will open iptables firewall as well, ntp used port 123 UDP
# ftp ftp.redhat.com
ftp> cd /pub/redhat/linux/9/en/os/i386/RedHat/RPMS
ftp> ls ntp*
ftp> mget ntp*
# rpm –ivh ntp i386.rpm
# chkconfig ntpd on
# service ntpd start
Most RedHat and Fedora Linux software products are available in the RPM format. Downloading and installing RPMs isn't hard.
When searching for the file, remember that the NTP RPM's filename usually starts with the word "ntp" followed by a version number like this: ntp i386.rpm.
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

4. 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  otherntp.server.org    # A stratum 1 server
server  ntp.research.gov       # A srtatum 2 server
restrict otherntp.server.org mask nomodify notrap noquery
restrict ntp.research.gov mask nomodify notrap noquery
Fetch time from somewhere
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
restrict mask notrust nomodify notrap
restrict mask notrust nomodify notrap
restrict

5. How To Get NTP Started To get NTP configured to start at boot:
The demon is called ntpd
To start/stop/restart NTP after booting:
If you like with init scripts:
Logging in /var/log/messages
Logging in /var/log/ntpd.log
# chkconfig ntpd on
# service ntpd start
# service ntpd stop
# service ntpd restart
# /etc/init.d/ntpd start
# /etc/init.d/ntpd stop
# /etc/init.d/ntpd restart
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.

6. 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.

7. 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.

8. 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.
Fedora Core 2 syncing to Feodora 2 client has a known bug
# 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
Feodora bug workaround
Fedora Core 2 has a bug in which NTP clients will not be able to synchronize with a Fedora Core 2 time server unless the "notrust nomodify notrap" keywords in the restrict statement for the NTP client network is removed.
The example below shows a restrict statement that only has the client network defined without any keywords. The configuration line that works with other NTP versions has been commented out.
# -- CLIENT NETWORK
#restrict mask notrust nomodify notrap
restrict mask
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.

9. Fedora / RedHat File Permissions
The /etc/ntpd catalog must have correct owner and group
This catalog holds the drift file and network delay correction!
chown ntp:ntp /etc/ntp
All the Fedora / RedHat NTP daemons write temporary files to the /etc/ntp directory. Unfortunately, in Fedora Core 2, the permissions on this directory don't allow this to be done. You will have to set the group and owner of the directory to be "ntp”
You'll get errors like this in the /var/log/messages file if you don't change permissions.
Sep 7 00:29:45 smallfry ntpd[2097]: can't open /etc/ntp/drift.TEMP: Permission denied

10. Configuring Cisco Devices 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 An explanation of the commands used follows.

11. Configuring Cisco Devices To Use An NTP Server
CAT 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 An explanation of the commands used follows.

12. 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
Stratalevel 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