Configuring Time Services in RHEL 7

• Introduction
• Server hardware clock
• Using NTP for synchronizing time for all managed server
• New utilities and daemon for managing time on RHEL7
• Using date command for checking and converting system time
• Setting hardware clock time via hwclock command
• Using timedatectl to check if you system is configured correctly
• Cheatsheet for configuring NTP in RHEL7
• Checking Cronyd status via systemctl
• Setting proper time zone
• Using Graphical Tools to Manage Time

Introduction

RHEL 7 changes default provide of NTP to chrony, which create the need to learn this new daemon. The idea behind chrony is that most sever do not need full fledged NTP daemon as they are consumer of those servers and never the providers.

Most client-server applications depends on synchronization of time and will not work property without correctly configured ntp daemon.

Unsynchronized NTP server is a source of very difficult to troubleshoot errors, so the correctness of NTP configuration should be changed first in any such troubleshooting. So it is very important that this daemon is configured correctly and NTP works as expected. That main difficulties happen if you are behind firewall or proxy.

as RHEL7 introduced a new chrony daemon the configuration file for this new daemon is now in /etc/chrony.conf Parameters are similar to those in the /etc/ntp.conf file. In this file you can specify your corporate servers. There is a new command line utility timedatectl  to contol the daemon.

# chronyc tracking
Reference ID : 192.0.2.1 (192.0.2.1) 
Stratum : 12
Ref time (UTC) : Fri Aug 05 19:06:51 2016
System time     : 0.000823375 seconds fast of NTP time
Last offset     : 0.001989304 seconds
RMS offset      : 0.060942811 seconds
Frequency       : 1728.043 ppm slow
Residual freq   : 1.100 ppm
Skew            : 94.293 ppm
Root delay	    : 0.000207 seconds 
Root dispersion : 0.016767 seconds
Update interval : 65.1 seconds
Leap status     : Normal

Some of the important fields are :

• Reference ID: This is the reference ID and name (or IP address) if available, of the server to which the computer is currently synchronized.
• Stratum: The stratum indicates how many hops away from a computer with an attached reference clock you are.
• Ref time: This is the time (UT C) at which the last measurement from the reference source was processed.

To check youor NTP servers run

# chronyc sources
210 Number of sources = 1
MS Name/IP address    Stratum    Poll   Reach   LastRx   Last sample 
=============================================================================
^* 192.0.2.1           11        6      377      63      +1827us[+6783us]...

To start/shutdown and restart it you use systemctl

systemctl start chronyd

TIP:

While on RHEL 7, chrony is the default solution to manage network time. NTP can still be installed via RPM. If you have services that need ntpd, you are free to use it, but you should realize that in that case you cannot use the timedatectl command.

Server hardware clock

Each server contains a hardware clock. Typically, it is an integrated circuit on the system board that is completely independent of the current state of the operating system and keeps running even when the computer is shut down. From the hardware clock, the system gets its initial time setting during the boot.

The time on the hardware clock on Linux servers is usually set to universal time coordinated (UTC). UTC is a time that is the same everywhere on the planet, and based on UTC, the current local time is calculated. To do this we need to provide offset called the time zone.

System time is a time maintained by the operating system. Once the system has booted, the system clock became independent of the hardware clock. The system also can synchronize hardware c lock with the external source of precise time -- the time server. Time server can be external or internal. For large corporation it is typically internal.

System time is a time that is maintained by the operating system and it is kept in UTC. Applications running on the server are converting system time into local time. Local time is the actual time in the current time zone. Moreover there are twice a year changes (to and from winter time), which are called daylight savings time (DST) which moves the local time one hour and creates a lot of difficulties.

Using NTP for synchronizing time for all managed server

Network Time Protocol (NTP). NTP is a method of maintaining system time that is provided through NTP servers on the Internet. It is an easy solution to provide an accurate time to servers, because most servers are connected to the Internet anyway.

NTP was developed by David Mills and others at the University of Delaware as a means for solving the problem of time synchronization between different servers connected to the Internet. Detailed information about NTP, version of the protocol and corresponding RFCs can be found at www.ntp.org

NTP runs on UDP port 123. The NTP servers advertise every 64 seconds, by means of a multicast address (224.0.1.1), that they are NTP servers. Any NTP client that is not configured with the unicast address of an NTP server multicasts . The NTP client sends request packets to all the NTP servers that it knows using their unicast addresses. Included in the request packet is the client’s local time. The NTP server replies by inserting UTC time into the return packet. The client compares its original request time with its own time when it receives the response from the server. This allows the client to determine how long the packet was in transit on the network.

Despite trivial semantic NTP is a rather complex protocol. Servers are organized into hierarchical levels called stratum that act in a hierarchy.

• Stratum 1 is the highest hierarchical level and those servers are directly attached to highly accurate reference sources such as atomic clocks. This is backbone of NTP and regular servers should not interface with backbone directly, but using NTP servers on lower stratums .
• Stratum 2 servers tend to operate out of academic institutions and large corporations. Those can be used by "regular" Internet-connected servers but any sizable organization usually has its own "local" NTP server(s) that interfaces with Stratum 2 servers.
• Below Stratum 2 you will find local NTP server that most organizations on the Internet now have. In the past most organization used free xntp daemon, but some prefer using appliances. Appliances often provide alternative synchronization case of lost internet connectivity using GPS.

Several additional terms are used when describing NTP-related topics:

• clock A clock that provides current time by accurately following a time standard, such as Universal Time Coordinate (UTC).
• Strata NTP servers are arranged in a hierarchy of levels, called strata. A stratum-1 server is more accurate than a stratum-2 server. There are 15 strata, but only first three are usually used.
• Resolution The smallest increment in time that a clock offers. For example, a wristwatch usually has a resolution of one second.
• Precision The smallest increase in time that a computer program can use.
• Jitter The difference of the differences experienced when repeatedly measuring time.
• Accuracy How close a clock follows an official time reference, such as UTC.
• Reliability The length of time that a clock can remain accurate within a specified range.
• Wander All clocks suffer from frequency variations. This variation is called wander.
• Drift file A file that contains the frequency offset of the local system’s clock oscillator. Drift file contents can be used by protocols, like NTP, to cause a system’s clock to be more accurate.
• Discipline A general term used for various actions carried out by some protocol, which helps keep a local clock better synchronized to an official time source, such as UTC.
• Tick. The Sun system central processing units (CPUs) generate the regular interrupts. By default, 100 interrupts are generated per second. For the system’s counter to increment, the CPUs interrupt must be processed by the kernel. Each interrupt that gets processed is known as a clock tick.

NTP client software is essentially nothing more then an intelligent corrector of the latency errors due to transmission via IP. All modern OSes provide daemons for NTP protocol but the devil in in details. RHEL 6 used different daemon then RHEL 7.

Setting up a server to use NTP time on RHEL 7 means that you need to put into the file /etc/chrony.conf the list of NTP servers that should be used.

You can switch on NTP and provided initial set of NTP servers in Anaconda during the installation and this is preferable way to doing this task. If you made a mistake or the situation changed you need manually switch on NTP, by using timedatectl set-ntp 1 and correct the set proper NTP servers. This is especially important if the server is behind firewall or proxy, because in this case default server do not work -- they are not assessable.

New utilities and daemon for managing time on RHEL7

RHEL 7 introduced a new set of command for managing NTP. Old books describing RHEL6 techniques of managing NTP for the most part are no longer valid

On a Linux system, time is calculated as an offset of epoch time. Epoch time is the number of seconds since January 1, 1970, in UTC. In some logs (such as /var/log/audit/audit.log), you’ll find time stamps in epoch time and not in human time. To convert such an epoch time stamp to human time, you can use the --date option, followed by the epoch string that is starting with an @:

date --date '@1420987251'

The use of epoch time is also creating a potential timing problem on Linux. On a 32-bit system, the number of seconds that can be counted in the field that is reserved for time notation is finished in 2037. But at this point hardware that uses 32 bit presentation will long be obsolete so this is not a problem.

Using date command for checking and converting system time

Date command is a lone survivor from older version on Linux and UNIX. While most sysadmin know about the date command existence, they often do not suspect what additional capabilities it has.

Like most Linux commands it can do more than a typical sysadmin expects. For example, you can also use it to show the current time in different formats. Some useful examples of date are listed here:

 date              # Shows the current system time
 date +%d-%m-%y    # Shows the current system day of month, month, and year
 date -s 16:03     # Sets the current time to 3 minutes past 4 p.m.

Setting hardware clock time via hwclock command

The date command enables you to set and show the current system time. Using the date command will not change the hardware time that is used on your system.

To synchronize hardware clock with system clock , you can use the hwclock command. The hwclock command has many options. Among most useful:

 hwclock -c         # shows the difference between hardware time and system time. The output refreshed every 10 seconds
 hwclock --systohc  # synchronizes current system time to the hardware clock.
 hwclock --hctosys  # synchronizes current hardware time to the system clock.

[root@server1 ~]# hwclock -c
hw-time      system-time         freq-offset-ppm   tick
1428584002   1428584002.011018
1428584012   1428584012.033019              2200     22
1428584022   1428584022.054953              2197     22
1428584032   1428584032.083572              2418     24
1428584042   1428584042.111683              2517     25

Using timedatectl to check if you system is configured correctly

This is new command introduced in RHEL 7. It enables you to manage many aspects of time is When used without any arguments, timedatectl shows detailed information about the current time and date. It also displays the time zone your system is in, in addition to information about the use of NTP network time and information about the use of DST.

Click here to view code image

[root@localhost ~]# timedatectl
      Local time: Sun 2015-01-11 10:02:41 EST
  Universal time: Sun 2015-01-11 15:02:41 UTC
        RTC time: Sun 2015-01-11 15:02:51
        Timezone: America/New_York (EST, -0500)
     NTP enabled: n/a
NTP synchronized: no
 RTC in local TZ: no
      DST active: no
 Last DST change: DST ended at
                  Sun 2014-11-02 01:59:59 EDT
                  Sun 2014-11-02 01:00:00 EST
 Next DST change: DST begins (the clock jumps one hour forward) at
                  Sun 2015-03-08 01:59:59 EST
                  Sun 2015-03-08 03:00:00 EDT

The timedatectl command works with commands to perform time operations.

The timedatectl command was developed as a generic solution to manage time on RHEL 7. It has some functions that are offered through other commands, but the purpose of the command is that eventually it will replace other commands used for managing time and date settings.

When timedatectl is used to switch on NTP time, it talks to the chronyd process.

Cheatsheet for configuring NTP in RHEL7

1. Open a root shell and type date.
2. Now type hwclock and see whether both commands are showing more or less the same time.
3. Type hwclock -c. Notice that this shows differences between system time and hardware time in much more detail. Use Ctrl+C to interrupt.
4. You have just seen that the current time displayed by hwclock -c is in epoch time.
5. Use date -d ‘@12345678’ (in which you replace 12345678 with the epoch time hwclock -c has just shown) to translate the current epoch time to human-readable time.
6. Type timedatectl status to show current time settings.
7. Use timedatectl list-timezones to show a list of all time zone definitions.
8. Use timedatectl set-timezone Europe/Amsterdam to set the current time zone to Amsterdam.
9. Type timedatectl show and notice the differences with the previous output.
10. Type timedatectl set-ntp 1 to switch on NTP use. You might see the error “failed to issue method call.” If you get this message, type yum -y install chrony and try again.
11. Open the configuration file /etc/chrony.conf and look up the server lines. These are used to specify the servers that should be used for NTP time synchronization.
12. Type systemctl status chronyd and verify that the chrony service is started and enabled. If this is not the case, use systemctl start chronyd; systemctl enable chronyd to make sure that it is operational.
13. Type systemctl status -l chronyd and read the status information.

Checking Cronyd Status via systemctl

Click here to view code image

[root@localhost system]# systemctl status -l chronyd
chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled)
   Active: active (running) since Sun 2015-01-11 10:20:15 EST; 2min 14s
ago
  Process: 13938 ExecStartPost=/usr/libexec/chrony-helper add-dhclient-
servers (code=exited, status=0/SUCCESS)
  Process: 13935 ExecStart=/usr/sbin/chronyd -u chrony $OPTIONS
(code=exited, status=0/SUCCESS)
 Main PID: 13937 (chronyd)
   CGroup: /system.slice/chronyd.service
           13937 /usr/sbin/chronyd -u chrony

Jan 11 10:20:15 localhost.localdomain chronyd[13937]: chronyd version
1.29.1 starting
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: Linux kernel
major=3 minor=10 patch=0
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: hz=100 shift_hz=7
freq_scale=1.00000000 nominal_tick=10000 slew_delta_tick=833 max_tick_
bias=1000 shift_pll=2
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: Generated key 1
Jan 11 10:20:15 localhost.localdomain systemd[1]: Started NTP client/
server.
Jan 11 10:20:20 localhost.localdomain chronyd[13937]: Selected source
178.21.23.127
Jan 11 10:20:20 localhost.localdomain chronyd[13937]: System clock
wrong by 11.256802 seconds, adjustment started
Jan 11 10:20:31 localhost.localdomain chronyd[13937]: System clock was
stepped by 11.257 seconds
    Jan 11 10:20:33 localhost.localdomain chronyd[13937]: Selected
    source 87.195.109.207

Setting proper time zone

Between Linux servers, time is normally communicated in UTC. This allows servers across different time zones to all use the same time settings, which makes managing time in large organizations a lot easier. To make it easier for end users, though, the local time must also be set. To do this, the appropriate time zone needs to be selected.

On Red Hat Enterprise Linux 7, you have four approaches to setting the correct local time zone:

Use the system-config-date utility as discussed in the next section of this chapter.

Go to the directory /usr/share/zoneinfo. In this directory, you’ll find different subdirectories containing files for each of the time zones that has been defined. To set the local time zone on a server, you can create a symbolic link with the name /etc/localtime to the time zone file that is involved. If you want to set local time to Los Angeles time, for instance, use ln -sf /usr/share/zoneinfo/America/Los_Angeles /etc/localtime.

Use the tzselect utility. This tool starts the interface , from which the appropriate region and locale can be selected.

Use timedatectl to set the time zone information.

Using Graphical Tools to Manage Time

If your server is configured with a graphical interface, you can use the graphical tool to manage time. To do so, follow these steps:

1. On the graphical display, click the current time that is shown in the upper-right corner.
2. On the screen that opens, click Date & Time Settings.
3. A new screen opens. To get access to the options that allow you to change time, click Unlock.
4. From the screen you see now, you can switch network time on and off and change the current time zone setting as well as the current time.

Alternatively, you can start the graphical utility to manage time by using the system-config-date command. The interface of this utility is similar to what you see in Anaconda during the initial installation of Red Hat.

[root@server1 ~]# tzselect
Please identify a location so that time zone rules can be set
correctly.
Please select a continent or ocean.
 1) Africa
... ... ... 
11) none - I want to specify the time zone using the Posix TZ
format.
#? 2
Please select a country.
 1) Anguilla                 28) Haiti
 ... ... ... 

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Last modified: December 13, 2020