computer and network time exercise

Timezone control

Back up the system's notion of the rules for local time setting:

cp   /etc/localtime   /etc/localtime.org

Now make the system adopt the same rules for local time setting that people use in New York:

cp   /usr/share/zoneinfo/America/New_York   /etc/localtime

Set the time to a part of the year in which daylight savings time is not in effect in New York, then to a time when it is. In each case, ask the system what time it thinks it is locally (New York) and UTC, and note how many hours' difference.

Make the system think it's February. 

date   02020200       sets 2 o'clock February 2
date
date   --utc

How many hours' difference are there between the time New York uses in Feburary and UTC? How many hours' difference are there between the times reported by the last 2 commands above, when system time is in February?

Make the system think it's August. 

date   08080800       sets 8 o'clock August 8
date
date   --utc

How many hours' difference are there between the time New York uses in August and UTC? How many hours' difference are there between the times reported by the last 2 commands above, when system time is in August?

How did the system know correctly what goes on in New York? New York's rules for setting clocks are embodied in /usr/share/zoneinfo/America/New_York. The system uses the rules in /etc/localtime. If /etc/localtime is a copy (or link to) /usr/share/zoneinfo/America/New_York then the system adopts New York's rules and time behavior:. The rules used by many other cities and regions are also supplied in /usr/share/zoneinfo/. Make the system adopt the one you want by copying it to /etc/localtime.

Restore the system's original notion of the rules:

cp   /etc/localtime.org   /etc/localtime

You can view the time in another zone without converting your system to that zone, with the zdump command. It takes as argument a timezone specification in the form of the location of that zone's file relative to /usr/share/zoneinfo/. Try

zdump   America/New_York
zdump   Japan
zdump   Iran

Some places have unconventional time rules. For example, Iran is "on the half hour" relative to most other places. The linux timezone file for it knows about such peculiarities.

Configure ntpd

The most important configuration responsibility is to supply ntpd with the identity of a time server. Visit www.ntp.org's public time server lists and choose a geographically close, stratum 2 server that's listed with access policy "Open Access" and notify policy "No". One such is ntp.ucsd.edu, another is nist1-la.ustiming.org. However you please pick a different one. Then save the original configuration file

cp   /etc/ntp.conf   /etc/ntp.conf.org

and create a new one using your editor, with the following barebones content:

server  <server you chose>
restrict  <server you chose>  mask 255.255.255.255  nomodify  notrap  noquery
restrict  127.0.0.1

Do an initial synchronization

Without checking the system time, find out from your wristwatch or your fellow students the approximate time. Try to determine a time that's rough, about 5 or 10 minutes fast or slow. We want the time to be a little off so that ntp will have some correcting to do! Let's say its May 23, more or less 9:45pm (2145h):

date 05232145

Set up 3 open terminal windows on your GUI desktop. I'll call them the ntp, sniffer, and log windows. In the sniffer window, execute:

tcpdump -nti eth0 port 123

Move over to the log window and do:

tail -f /var/log/messages

Both windows, in different ways, are now poised to dynamcially reveal the activities involved in performing a network time protocol interaction between your machine and the server you've configured. Set the 3 windows apart from each other so they are simultaneously visible with as little overlap as possible. In the ntp window execute:

ntpd -q

How far off was your time guess?

Now that you know the accurate time repeat the process but first, as the initial time, set the system clock way off by several hours. If it's 9:45 in the evening, set it to 11am or something like that:

date 05231100

Redo the test. What didn't happen? Why not? How are you going to restore accurate time to the system clock now that you've messed it up and ntp refuses to help? Restore it please before you continue.

Configure as a server

Restore the original configuration file:

cp   /etc/ntp.conf.org   /etc/ntp.conf

Note that the servers it designates for its use have names like 1.pool.ntp.org, suggesting not a single specific server but a pool of them  collectively. This is a load distribution feature. Note the section for opening up your ntpd to others who may wish to update their time from you:

# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

To activate this, uncomment the "restrict" line and edit the network designation to that of the network you want to serve (e.g., "164.67.79.0 mask 255.255.255.0"). Working with another student, have him designate you as his server, by your IP address in a "server" statement within his config file. See if you can update his time. Start your server:

service   ntpd   restart

Have him stop his if it's running and then issue "ntpd -q".

Reverse roles, adjusting your respective configuration files as needed.