NTP users are strongly urged to take immediate action to ensure that their NTP daemons are not susceptible to being used in distributed denial-of-service (DDoS) attacks. Please also take this opportunity to defeat denial-of-service attacks by implementing Ingress and Egress filtering through BCP38.
ntp-4.2.8p15 was released on 23 June 2020. It addresses 1 medium-severity security issue in ntpd, and provides 13 non-security bugfixes over 4.2.8p13.
Are you using Autokey in production? If so, please contact Harlan - he's got some questions for you.
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See GettingStartedDev for discussion of this topic.
3.1. Getting Started With NTP
Note: If you are
really impatient, the
NTP Quick Start Page (or
http://www.eecis.udel.edu/~mills/ntp/html/quick.html) is an even shorter quick start.
3.1.1. What NTP is and what it does
The internal clocks of most computers are rather inaccurate. You can
correct the clock by hand on occasion, or you can use NTP to regulate
your clock to be much more accurate.
NTP stands for "Network Time Protocol". It is a way for computers to
exchange information about the exact time, so that they can regulate
their clocks.
NTP also stands for a particular daemon (continuously running program)
that uses the Network Time Protocol to determine the correct time and
regulate your computer's clock to show the correct time. The NTP
daemon software can be obtained from
http://www.ntp.org. It runs on
Unix and Windows/NT systems. (There are other programs that use the
Network Time Protocol for setting your computer's clock as well -- see
3.1.10. Further resources for information.)
NTP can regulate your clock using a "reference clock", a device that
obtains accurate time information from an outside source and delivers
it to your computer. More commonly, NTP can regulate your clock by
obtaining time information from another NTP daemon. NTP can also
provide time information to other NTP daemons.
Once you install the NTP software and start it running, you must
provide it with a source of time information, either a reference
clock, or with the addresses of other NTP daemons. This document
describes the critical steps of installing the software and
configuring NTP to obtain time from other NTP daemons. Information about
using a reference clock with NTP is at
http://www.eecis.udel.edu/~mills/ntp/html/refclock.html.
3.1.2. The names of the programs: Is it ntpd or xntpd?
Some operating systems supply the NTP programs (
ntpd,
ntpdate,
ntpdc,
ntpdq,
etc.) with filenames starting with "x" (
xntpd,
xntpdate, etc.).
Others use filenames without the "x".
This document uses the names without the "x".
Try to run the program using the name without the "x" first.
If you receive a "command not found" error, try the name with an "x".
The "x" versions of the programs are from version 3 of NTP. They are old, but some OS vendors still distribute these versions of NTP. The versions without the "x" are from version 4. The commands and options used in this document should work for either version.
3.1.3. Pick your servers
Find some public servers that you can sync with. Two is good enough.
Here is a list of public NTP servers.
These sites are kind enough to perform this service for you, you should follow any guidelines they request (asking permission first, donations, thank-you notes, whatever.)
Let's suppose you've picked
clock.example.org and
timekeeper.sample.com. Those aren't real sites, but I'll use them as examples in the rest of this document.
3.1.4. "Manually" set the clock correctly just once
HMS: This section is best for xntp (V3).
ntpdate is deprecated in ntp4, see
StartingNTP for more information.
Let's "manually" set the clock to the correct time just to start things off. When ntpd is running and finds your clock is wrong, it makes tiny little changes until the clock is right. This way applications don't get confused. However, on reboot, the clock may have lost a lot of time and since no applications are running yet, we can make a big timeleap to correct the clock without risk of confusing them. To do this, we use a different utility called
ntpdate. It can't run at the same time as ntpd. We even give it the "-b" option so that it knows to leap forward or backwards in time to set the clock correctly.
Here's the command that does this:
ntpdate -b clock.example.org timekeeper.sample.com
(NOTE: Substitute the 2 NTP servers that you'd like to sync with. Those aren't real.)
Try this once from the command line (as "root") to make sure you're got it right.
# ntpdate -b clock.example.org timekeeper.sample.com
5 Jun 08:50:18 ntpdate[23777]: step time server 10.10.10.10 offset -111.106949 sec
This means that your clock was off by -111.106949 seconds, but now it's been brought up to date. The good news is that for an instant you were in sync. The bad news is that by the time you read this, your clock has drifted. Darn computers!
3.1.5. Set the clock correctly on every boot up
On reboot, you want to tell the system to sync up quickly. While your machine was down the CPU gets bored and plays with the clock. Haven't you ever been in a clock store waiting for your mother to make her purchase and started setting the clocks to funny times? It's like that, only your mother has nothing to do with this (I'm sure she's a very nice person, you just don't appreciate her enough).
Let's configure your system to run this command on boot up. Here's how to do this on various operating systems:
This is done automatically if the
ntpdate package has been installed and configured.
Simply add this line to the end of
/etc/rc.conf (if not running ntpd):
ntpdate_enable="YES"
(If you are running ntpd,
-g option specified below means we don't need to run
ntpdate, and do NOT add the line above.)
(Release 8.0) This is done automatically if the
ntp package has been installed and configured.
This is done automatically if a properly configured
/etc/inet/ntp.conf exists.
(people can volunteer the easiest way to do this on their favorite OS)
3.1.6. Keep the clock in sync permanently and continuously.
Having your clock set properly every time you boot is nice, if you reboot constantly. Of course, you could put that command in
cron but we have a solution that is so much better you'll thank me in the morning. We'll run
ntpd which will constantly keep your clock correct by making micro-adjustments all day long. Heck,
ntpd is so smart that if you lose contact with all your NTP servers it will remember how bad your clock was and keep making adjustments based on past bad performance. Pretty cool, eh?
The NTP documentation has volumes about the various features and entire books can be written about the theory of operation of NTP. It's really quite amazing how the system works so precisely, so accurately, does the right thing through outages, detects and avoids misconfigured servers, conserves bandwidth, and as Jim Trocki once said, "fixes problems that you don't yet realize you have". However, you don't care about that, you just want a simple configuration that works. Here's one.
ntpd 's configuration file is different on different systems.
/etc/ntp.conf
/etc/inet/ntp.conf
Put these 3 lines in your configuration file:
driftfile /etc/ntp.drift
server clock.example.org iburst
server timekeeper.sample.com iburst
Now start
ntpd:
ntpd is automatically started after package installation or reconfiguration.
ntpd may be started manually with
/etc/init.d/ntp start
ntpd is started automatically by adding the
xntpd_enable and
xntpd_flags described below.
(Release 8.0)
/sbin/service ntpd start
/etc/rc2.d/S74xntpd start
It will now sync with the 'servers' in
/etc/ntp.conf. Once an hour it will record a little information in
/etc/ntp.drift. (
/etc/ntp.drift contains what it needs to know if it loses contact with all your servers and needs to "go it alone").
Of course, you'll want to make sure that this command is run on bootup, after
ntpdate is run (they can't run at the same time). Here's how to do that on various operating systems:
This is done automatically if one of the
ntp packages has been installed and configured.
Simply add these lines to the end of
/etc/rc.conf:
xntpd_enable="YES"
xntpd_flags="-p /var/run/ntpd.pid -g -N high"
(Release 8.0) This is done automatically if the
ntp package has been installed and configured.
This is done automatically if a properly configured
/etc/inet/ntp.conf exists.
3.1.7. What about stratum 1, 2, 3 and all that?
You don't need to specify that in the configuration file. If you list a mixture of stratum 1s, 2s, and 3s ntpd will do the right thing. Your stratum is determined dynamically based on what you've synced to. You don't need to worry about this. However, if you want to know more, there is info in the documentation.
3.1.8. Check your work.
ntpdc is a command that lets you talk to your ntpd (just like
lp has
lpc,
ntpd has
ntpdc). It has a zillion features, but the one you really need to know is:
ntpdc -c peers
Here's a sample run:
> ntpdc -c peers
remote local st poll reach delay offset disp
=======================================================================
=clock.example.o 5.0.0.0 16 64 0 0.00000 0.000000 0.00000
=timekeeper.samp 5.0.0.0 16 64 0 0.00000 0.000000 0.00000
Output like that (all zeros) means that we haven't synced with those two servers yet.
When things get synced, the numbers will fill out like this:
> ntpdc -c peers
remote local st poll reach delay offset disp
=======================================================================
*clock.example.o 64.46.24.66 3 64 377 0.00018 0.000329 0.00255
=timekeeper.samp 61.26.45.88 3 64 377 0.00017 0.002122 0.00342
st -- the stratum of that server (calculated automatically)
poll -- how often you poll that site (backs off dynamically)
ntpdc can sh ow the status of other machines if you have the proper access set up. Just list the hostname on the command line:
Let's see how the server at ntp-2.cso.uiuc.edu is doing:
> ntpdc -c peers ntp-2.cso.uiuc.edu
remote local st poll reach delay offset disp
=======================================================================
+ntp-1.gw.uiuc.e 0.0.0.0 2 1024 172 0.00627 -0.000044 0.00761
+tick.usno.navy. 0.0.0.0 1 1024 373 0.09023 -0.023207 0.15714
*truechimer.cso. 0.0.0.0 1 1024 377 0.00401 -0.000054 0.00006
+ntp-0.gw.uiuc.e 0.0.0.0 2 512 332 0.01059 0.001216 0.00497
3.1.9. Running NTP on many machines in a network.
If you have a lot of machines that need to be kept in sync, it is rude to point them all at public NTP servers. It's also a waste of your internet bandwidth. A much better solution is to set up 1-2 NTP servers on your network that point to public NTP servers, then point all your other machines at these NTP servers. This greatly reduces the amount of load you put on the public NTP servers.
At my company, we have 2 NTP servers:
ntp1.mycompany.com and
ntp2.mycompany.com. Those are actually aliases ("CNAME" in DNS) that point to the 2 machine that I want everyone to use for NTP service. We do another trick that makes the aliases point to different hosts outside our firewall. That way the same configuration file works if a machine is connected inside or outside our firewall.
These 2 NTP servers need to have a custom configuration, that includes each other:
driftfile /etc/ntp.drift
# public NTP servers we sync to: (use these 2 lines ONLY on ntp1 and ntp2!)
server clock.example.org
server timekeeper.sample.com
# Use these lines on all machines in mycompany.com:
server ntp1.mycompany.com
server ntp2.mycompany.com
I find that it is easier to have the exact same file on both of these servers. That means that
ntp1 will try to sync with itself, and
ntp2 will try to sync with itself. Luckily, this is ok. NTP will realize that its talking to itself and do the right thing (ignore itself).
All the other hosts have this
ntp.conf:
driftfile /etc/ntp.drift
# Use these lines on all machines in mycompany.com:
server ntp1.mycompany.com
server ntp2.mycompany.com
3.1.10. Further resources
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--
TomLimoncelli - 05 Jun 2003
--
BradKnowles - 28 Aug 2003
--
HarlanStenn - 04 Sep 2003
--
DaleWorley - 09 Oct 2003
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