NTP
Network Time
Goals of this lab:
 Learn how to maintain synchronized clocks in a network.
Prerequisites: LXB, NET
REVISION: 2.0 [2015-08-28]
©2005-2015 DAVID BYERS
Table of Contents
PREPARATION ......................................................................................................................................................... 1
Exercise 1: Review and preparation ............................................................................................................ 1
MAIN LAB ................................................................................................................................................................ 3
Part 1: Network time .......................................................................................................................................... 3
Exercise 2: The Network Time Protocol ...................................................................................................... 4
Exercise 3: Install and configure NTP server and clients ............................................................................. 4
IDA/ADIT
NTP: NETWORK TIME
IDA/ADIT
NTP: NETWORK TIME
PREPARATION
Complete these exercises before starting with the main lab. The package documentation and on-line
sources should be sufficient to answer the questions.
Exercise 1: Review and preparation
Documentation for nt p is available in /usr/share/doc/ntp-doc on marsix.
1-1
Make sure you understand roughly how ntp works.
1-2
Make sure you know what an ntp peer is.
1-3
Read the documentation for nt pdat e, nt pd, nt pq and ntp.conf.
1-4
What command would you use to display the current status of ntp on a host?
Report:
No report is required.
IDA/ADIT
NTP: NETWORK TIME
1
MAIN LAB
Keeping clocks in a network tightly synchronized is surprisingly important. Although most applications
will function even if clocks are not synchronized, you can run in to unexpected problems such as:
•
It becomes impossible to correlate events in log files on different computers.
•
It becomes impossible to determine the true time of events in the system (such as crashes, breakins or other nasty problems).
•
Tools that use the modification time on files may come to different conclusions on different clients
that use the same shared filesystem (for example, mak e might think files are modified in the future,
or that files that have been modified actually haven’t).
•
Single sign-on systems such as Kerberos will not function properly if clocks diverge too much.
•
Users get confused and start complaining.
Keeping clocks synchronized is easy, and should always be done, even if there is no apparent need.
Time taken 2006: 1.5-7 hours, average 3.5 hours.
Past problems: For most groups this lab was pretty straightforward. One problem is that NTP takes
quite a long time to synchronize, which means that test cases take a long time to run. For this reason,
it’s important to be careful with configuration, so it works right the first time – multiple rounds of
testing will consume a lot of time. It might also be a good idea to work on this lab and on some other lab
in parallel. If you find that this lab is taking you more than six hours, please make sure you talk to the lab
assistant to get some help.
Part 1: Network time
There are several tools that will enable a system administrator to keep clocks in a network synchronized.
Fundamentally, they all are based on the concept of a shared reference clock, a clock that every system
turns to for the correct time, but how the reference clock is used differs widely.
The simplest method is to set the system clock at boot time and then let it run freely. This method is
only adequate when fairly large errors are acceptable and when systems are restarted frequently.
A slightly more sophisticated method is to have the computer set its clock periodically from a reference
clock. This avoids high levels of clock skew, but has the distinct disadvantage of introducing
discontinuous time. Every time the clock is set, time jumps, and some software can react very badly to
jumps in time.
The network time protocol (NTP), which is used in this lab, operates at a completely different level. It
establishes relationships with one or more reference clocks and maintains synchronization with them by
speeding up or slowing down the system clock, thereby avoiding discontinuities.
A typical NTP system will set its clock at startup using one of its NTP servers as a reference, and then use
NTP to keep it synchronized. Setting the clock at startup is necessary, since NTP will refuse to adjust the
system clock if the difference between it and the reference clock is too large.
IDA/ADIT
NTP: NETWORK TIME
3
Exercise 2: The Network Time Protocol
2-1
Explain the concept of “stratum” in a clock hierarchy (e.g. NTP, but also synchronized
networks and other clock hierarchies).Why is it necessary to have a hierarchy of clocks rather
than just a bunch of reference clocks?
2-2
How large a difference between the reference clock and the system clock does NTP accept
and attempt to adjust?
2-3
NTP usually works by speeding up or slowing down the clock, not setting it outright. Why?
Report:
Answers to the questions above.
The time service is the one service that makes sense to run on the router, the reason being that many
commercial routers include an NTP server.
Exercise 3: Install and configure NTP server and clients
The NTP server should be installed on your router. Placing the NTP server on your router is reasonable
as many NTP servers today include NTP servers.
3-1
Install the necessary software and configure your router as an NTP server. It should use idagw.sysinst.ida.liu.se as its reference clock. It should allow no other peers to update its clock.
Anyone should be allowed to read the clock.
3-2
Configure your clients and your server as NTP clients of your router. They should either get
the time directly from the router or accept broadcast or multicast time announcements from
the router.
3-3
Explain the output of nt pq –p.
3-4
Verify that NTP works.
Report:
Automated test cases that show that NTP works as expected. An explanation of the entire
output of nt pq –p.
Note that NTP can take quite some time before it starts adjusting the clock. When testing, be patient
and allow NTP several minutes to do anything. You can run the ntp server with debugging enabled in the
router console to see more information about what it is doing.
IDA/ADIT
NTP: NETWORK TIME
4
FEEDBACK FORM
NTP
Complete this feedback form individually at the end of the lab and hand it to the lab assistant when you
finish. Your feedback is essential for improving the labs. Each student should hand in a feedback form.
Do not cooperate on completing the form.
You do not need to put your name on the feedback form. Your feedback will be evaluated the same way
regardless of whether your name is on it or not. Your name is valuable to us in case you have made and
comments in the last section that need clarifications or otherwise warrant a follow-up.
For each section, please rate the following (range 1 to 5 in all cases).
 Difficulty: Rate the degree of difficulty (1=too easy, 5=too difficult)
 Learning: Rate your learning experience (1=learned nothing, 5=learned a lot).
 Interest: Rate your interest level after completing the part (1=no interest, 5=high interest).
 Time: How long did the part take to complete (in minutes)?
Difficulty
Learning
Interest
Time
(minutes)
Preparation
Part 1: Network time
Overall
Please answer the following questions:
 What did you like about this lab?
 What did you dislike about this lab?
 Make a suggestion to improve this lab.
IDA/ADIT
NTP: NETWORK TIME
FEEDBACK FORM
NTP
Complete this feedback form individually at the end of the lab and hand it to the lab assistant when you
finish. Your feedback is essential for improving the labs. Each student should hand in a feedback form.
Do not cooperate on completing the form.
You do not need to put your name on the feedback form. Your feedback will be evaluated the same way
regardless of whether your name is on it or not. Your name is valuable to us in case you have made and
comments in the last section that need clarifications or otherwise warrant a follow-up.
For each section, please rate the following (range 1 to 5 in all cases).
 Difficulty: Rate the degree of difficulty (1=too easy, 5=too difficult)
 Learning: Rate your learning experience (1=learned nothing, 5=learned a lot).
 Interest: Rate your interest level after completing the part (1=no interest, 5=high interest).
 Time: How long did the part take to complete (in minutes)?
Difficulty
Learning
Interest
Time
(minutes)
Preparation
Part 1: Network time
Overall
Please answer the following questions:
 What did you like about this lab?
 What did you dislike about this lab?
 Make a suggestion to improve this lab.
IDA/ADIT
NTP: NETWORK TIME