Download and Install The NTP Package
Most RedHat and Fedora Linux software products are available in the RPM format.
Downloading and installing RPMs isn't hard. If you need a refresher, Chapter 6, "Installing
Linux Software", has all the details.
When searching for the file, remember that the NTP RPM's filename usually starts with the word
ntp followed by a version number as in ntp-4.1.2-5.i386.rpm.
The /etc/ntp.conf File
The /etc/ntp.conf file 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 are the steps to create a
configuration file using a pair of sample Internet-based NTP servers:
1) First we specify the servers you're interested in:
server otherntp.server.org # A stratum 1 server at server.org
server ntp.research.gov # A stratum 2 server at research.gov
2) Restrict the type of access you allow these servers. In this example the servers are not allowed
to modify the run-time configuration or query your Linux NTP server.
restrict otherntp.server.org mask 255.255.255.255 nomodify notrap noquery
restrict ntp.research.gov mask 255.255.255.255 nomodify notrap noquery
The mask 255.255.255.255 statement is really a subnet mask limiting access to the single IP
address of the remote NTP servers.
3) 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. You do so with a modified restrict statement
removing the noquery keyword to allow the network to query your NTP server. The syntax is:
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
In this case the mask statement has been expanded to include all 255 possible IP addresses on the
local network.
4) 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 127.0.0.1
5) Save the file and restart NTP for these settings to take effect. You can now configure other
Linux hosts on your network to synchronize with this new master NTP server in a similar
fashion.
How To Get NTP Started
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.
To get NTP configured to start at boot, use the line:
[root@bigboy tmp]# chkconfig ntpd on
To start, stop and restart NTP after booting, follow these examples:
[root@bigboy tmp]# service ntpd start
[root@bigboy tmp]# service ntpd stop
[root@bigboy tmp]# service ntpd restart
Testing And Troubleshooting NTP
After configuring and starting NTP, you should test it to make sure it is working. Here are some
guidelines you can follow to get NTP working correctly.
Verifying NTP is Running
To test whether the NTP process is running use the command
[root@bigboy tmp]# pgrep ntpd
You should get a response of plain old process ID numbers.
Doing An Initial Synchronization
If the time on the local server is very different from that of its primary time server your NTP
daemon will eventually terminate itself leaving an error message in the /var/log/messages file.
You should run the ntpdate -u command to force your server to become instantly synchronized
with its NTP servers before starting the NTP daemon for the first time. The ntpdate command
doesn't run continuously in the background, you will still have to run the ntpd daemon to get
continuous NTP updates.
Take a look at some sample output of the ntpdate command in which a server whose initial time
was set to midnight, was correctly set to 8:03 am.
The date was originally set to midnight which was verified by using the date command.
[root@smallfry tmp]# date
Thu Aug 12 00:00:00 PDT 2004
[root@smallfry tmp]#
The ntpdate command is run three times to synchronize smallfry's clock to server
192.168.1.100, but it must be run while the ntpd process is stopped. So you'll have to stop
ntpd, run ntpdate and then start ntpd again.
[root@smallfry tmp]# service ntpd stop
[root@smallfry tmp]# ntpdate -u 192.168.1.100
Looking for host 192.168.1.100 and service ntp
host found : bigboy.my-site.com
12 Aug 08:03:38 ntpdate[2472]: step time server 192.168.1.100 offset
28993.084943 sec
[root@smallfry tmp]# ntpdate -u 192.168.1.100
Looking for host 192.168.1.100 and service ntp
host found : bigboy.my-site.com
12 Aug 08:03:40 ntpdate[2472]: step time server 192.168.1.100 offset 2.467652
sec
[root@smallfry tmp]# ntpdate -u 192.168.1.100
Looking for host 192.168.1.100 and service ntp
host found : bigboy.my-site.com
12 Aug 08:03:42 ntpdate[2472]: step time server 192.168.1.100 offset 0.084943
sec
[root@smallfry tmp]# service ntpd start
[root@smallfry tmp]#
The date is now corrected.
[root@smallfry tmp]# date
Thu Aug 12 08:03:45 PDT 2004
[root@smallfry tmp]#
Determining If NTP Is Synchronized Properly
Use the ntpq command to see the servers with which you are synchronized. It provided 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.
[root@bigboy tmp]# ntpq -p
Here is some sample output of the command:
remote refid st t when poll reach delay offset
jitter
=============================================================================
=
-jj.cs.umb.edu gandalf.sigmaso 3 u 95 1024 377 31.681 -18.549
1.572
milo.mcs.anl.go ntp0.mcs.anl.go 2 u 818 1024 125 41.993 -15.264
1.392
-mailer1.psc.edu ntp1.usno.navy. 2 u 972 1024 377 38.206 19.589
28.028
-dr-zaius.cs.wis ben.cs.wisc.edu 2 u 502 1024 357 55.098 3.979
0.333
+taylor.cs.wisc. ben.cs.wisc.edu 2 u 454 1024 347 54.127 3.379
0.047
-ntp0.cis.strath harris.cc.strat 3 u 507 1024 377 115.274 -5.025
1.642
*clock.via.net .GPS. 1 u 426 1024 377 107.424 -3.018
2.534
ntp1.conectiv.c 0.0.0.0 16 u - 1024 0 0.000 0.000
4000.00
Your Linux NTP clients cannot Synchronize Properly
A telltale sign that you haven't got proper synchronization is when all the remote servers have
jitter, delay and reach values of 0. In some older versions of Fedora, the jitter values will be
4000.
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 0.0.0.0 16 u - 64 0 0.000 0.000 0.000
snvl-smtp1.trim 0.0.0.0 16 u - 64 0 0.000 0.000 0.000
nist1.aol-ca.tr 0.0.0.0 16 u - 64 0 0.000 0.000 0.000
This could be caused by the following:
Older versions of the NTP package that don't work correctly if you use the DNS name for
the NTP servers. In these cases you will want to use the actual IP addresses instead.
A firewall blocking access to your Stratum 1 and 2 NTP servers. This could be located on
one of the networks between the NTP server and its time source, or firewall software
such as iptables could be running on the server itself.
The notrust nomodify notrap keywords are present in the restrict statement for the NTP
client. In some versions of the Fedora Core 2's implementation of NTP, clients will not be
able to synchronize with a Fedora Core 2 time server unless the notrust nomodify notrap
keywords are removed from the NTP client's restrict statement.
In this example the restrict statement has only the client network defined without any
keywords and the configuration line that works with other NTP versions has been
commented out:
# -- CLIENT NETWORK -------
#restrict 172.16.1.0 mask 255.255.255.0 notrust nomodify notrap
restrict 172.16.1.0 mask 255.255.255.0
Fedora Core 2 File Permissions
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 writing of
temporary files. Instead you have to set the group and owner of the directory to be ntp.
[root@bigboy tmp]# chown ntp:ntp /etc/ntp
If you don't, you'll get errors like this in the /var/log/messages file.
Aug 12 00:29:45 smallfry ntpd[2097]: can't open /etc/ntp/drift.TEMP:
Permission denied
Configuring Cisco Devices To Use An NTP
Server
You can use NTP to synchronize time on a variety of devices including networking equipment. I
have included the necessary NTP commands for a variety of Cisco Systems products because it
is one of the most popular manufacturers of networking equipment and would feature in the
overall architectures of many home office/small office (SOHO) environments and corporate
departments.
Cisco IOS
To make your router synchronize with NTP servers with IP addresses 192.168.1.100 and
192.168.1.201, use the commands:
ciscorouter> enable
password: *********
ciscorouter# config t
ciscorouter(config)# ntp update-calendar
ciscorouter(config)# ntp server 192.168.1.100
ciscorouter(config)# ntp server 192.168.1.201
ciscorouter(config)# exit
ciscorouter# wr mem
The ntp server command forms a server association with another system, and ntp update-
calendar configures the system to update its hardware clock from the software clock at periodic
intervals.
CATOS
To make your router synchronize with NTP servers with IP addresses 192.168.1.100 and
192.168.1.201, use the commands:
ciscoswitch> enable
password: *********
ciscoswitch# set ntp client enable
ciscoswitch# ntp server 192.168.1.100
ciscoswitch# ntp server 192.168.1.201
ciscoswitch# exit
The ntp server command forms a server association with another system, and set ntp client
enable activates the NTP client.
NTP Security
You should always be aware of how NTP can be affected by your network's security policy.
Here are some common areas of concern.
Firewalls and NTP
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 (above 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.
A sample Linux iptables firewall script snippet is in Appendix II, "Codes, Scripts, and
Configurations".
NTP Authentication
There may be cases where you want to not only restrict NTP synchronization to specific
networks but also to require a synchronization password. This is beyond the scope of this book,
but is covered in detail at the NTP website www.ntp.org.
Configuring A Windows NTP Client
Windows clients that are part of an Active Directory domain automatically get their time
synchronized from the domain server. If your client is not part of a domain you can add your
new NTP server to your Windows client. Here's how:
1. Click on the time at the bottom right hand side of your screen.
2. Click on the "Internet Time" tab of the dialog box
3. Click the check box labeled "Automatically synchronize with an Internet time server" and
enter the name or IP address in the box underneath it.
4. Click on the "Update Now" button
You will get a message saying "Your time has been successfully synchronized" when the
operation is complete.
Conclusion
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.