6.d Configure time service clients

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Large computer networks can have many devices acting as NTP servers. Where multiple NTP sources are available, NTP clients need some way of (a) judging which time sources are likely to be the most accurate, and (b) preventing timing loops*. The NTP protocol achieves these aims by including a simple measure of the synchronisation distance from the primary time source. This is known as the Stratum level.

Stratum 1 indicates a computer that has a true real-time reference directly connected to it (e.g. GPS, atomic clock, etc.), such computers are expected to be very close to real time. Stratum 2 computers are those which have a stratum 1 server; stratum 3 computers have a stratum 2 server and so on. A value of 10 indicates that the clock is so many hops away from a reference clock that its time is fairly unreliable.

An NTP server is a source of time information, and an NTP client is a system/device that is attempting to synchronize its clock to a server. Servers can be either a primary or secondary server

• A primary server receives UTC time signals directly from a very accurate source such as an atomic clock or more commonly - a GPS signal source.A secondary server receives its time signal from one or more upstream servers, and distributes its time signals to one or more downstream servers and clients.
• Secondary servers are arranged in a strict hierarchy in terms of upstream and downstream, and the stratum terminology is often used to assist in this process.

127.127.1.0 is an ntpd-specific way to enable the local refclock driver (note that this is not supported in chrony and it's only here as Reference).

Display the current time in the given FORMAT, or set the system date.

$ date
Thu Dec 10 13:58:14 EST 2020
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Use timedatectl to manage time and timezones.

Main commands:

• status - Show current settings of the system clock and RTC, including whether network time synchronization through systemd-timesyncd.service is active

• show - Show the same information as status, but in machine readable form

• set-time [TIME] - Set the system clock to the specified time

• set-timezone [TIMEZONE] - Set the system time zone to the specified value

• list-timezones - List available time zones, one per line

• set-ntp [BOOL] - Enables/disables NTP

  # timedatectl  
                 Local time: Mon 2020-11-23 17:27:11 EST
             Universal time: Mon 2020-11-23 22:27:11 UTC
                   RTC time: Mon 2020-11-23 22:27:11
                  Time zone: America/Toronto (EST, -0500)
  System clock synchronized: no
                NTP service: active
            RTC in local TZ: no
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📝NOTE: To use NTP the 'chrony' service needs to be running. You may also need to restart the service after enabling NTP with timedatectl.

You can also use 'tzselect' to select the timezone. This script will ask you questions through multiple menus.

# tzselect  
Please identify a location so that time zone rules can be set correctly.
Please select a continent, ocean, "coord", or "TZ".
1) Africa
2) Americas
3) Antarctica
4) Asia
5) Atlantic Ocean
6) Australia
7) Europe
8) Indian Ocean
9) Pacific Ocean
10) coord - I want to use geographical coordinates.
11) TZ - I want to specify the time zone using the Posix TZ format.
#?  
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hwclock is an administration tool for the time clocks.

hwclock can:

• display the Hardware Clock time
• set the Hardware Clock to a specified time
• set the Hardware Clock from the System Clock
• set the System Clock from the Hardware Clock
• compensate for Hardware Clock drift
• correct the System Clock timescale
• set the kernel's timezone, NTP timescale, and epoch (Alpha only)
• predict future Hardware Clock values based on its drift rate

The Chrony daemon, chronyd, runs in the background and monitors the time and status of the time server specified in the chrony.conf file. If the local time needs to be adjusted, chronyd does it smoothly without the programmatic trauma that would occur if the clock were instantly reset to a new time.

Configuration for chronyd can be made by editing /etc/chrony.conf.

chronyd - chronyd is a daemon for synchronisation of the system clock. It can synchronise the clockwith NTP servers, reference clocks (e.g. a GPS receiver), and manual input using wristwatchand keyboard via chronyc.

Chrony's chronyc tool allows someone to monitor the current status of Chrony and make changes if necessary. The chronyc utility can be used as a command that accepts subcommands, or it can be used as an interactive text-mode program.

chronyc - chronyc is a command-line interface program which can be used to monitor chronyd’s performance and to change various operating parameters whilst it is running