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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 :
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
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.
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.
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 (126.96.36.199), 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.
Several additional terms are used when describing NTP-related topics:
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.
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.
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.
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
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.
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: chronyd version 1.29.1 starting Jan 11 10:20:15 localhost.localdomain chronyd: Linux kernel major=3 minor=10 patch=0 Jan 11 10:20:15 localhost.localdomain chronyd: 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: Generated key 1 Jan 11 10:20:15 localhost.localdomain systemd: Started NTP client/ server. Jan 11 10:20:20 localhost.localdomain chronyd: Selected source 188.8.131.52 Jan 11 10:20:20 localhost.localdomain chronyd: System clock wrong by 11.256802 seconds, adjustment started Jan 11 10:20:31 localhost.localdomain chronyd: System clock was stepped by 11.257 seconds Jan 11 10:20:33 localhost.localdomain chronyd: Selected source 184.108.40.206
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.
If your server is configured with a graphical interface, you can use the graphical tool to manage time. To do so, follow these steps:
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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