NTP 意即网络时间协议Network Time Protocol，它经过网络同步计算机系统之间的时钟。NTP 服务器可使组织中的全部服务器保持同步，以准确时间执行基于时间的做业。NTP 客户端会将其时钟与 NTP 服务器同步。

一.验证NTP设置同步是否正常工作的方法，可使用ntpq、ntpstat、timedatectl命令。

一、使用命令 ntpq -p 查看状态

检查同步状态

[root@a /]# ntpq -premote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xpingless.com    36.224.68.195    2 u   28   64  363  225.252   10.164  12.366

使用NTP服务时间同步

1.安装ntp

 yum -y install ntp

2.启动ntp服务

systemctl start  ntpd
systemctl enable  ntpd
Created symlink from /etc/systemd/system/multi-user.target.wants/ntpd.service to /usr/lib/systemd/system/ntpd.service.

3.查看ntp服务

systemctl status ntpd

4.修改ntp.conf文件

[root@a /]# vim /etc/ntp.conf
[root@a /]# grep server /etc/ntp.conf 
# Use public servers from the pool.ntp.org project.
#server 0.centos.pool.ntp.org iburst
server ntp.aliyun.com iburst
#broadcast 192.168.1.255 autokey	# broadcast server
#broadcast 224.0.1.1 autokey		# multicast server
#manycastserver 239.255.254.254		# manycast server

5.重启服务

[root@a /]# systemctl restart ntpd

6.检查同步状态

[root@a /]# ntpq -premote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xpingless.com    36.224.68.195    2 u   28   64  363  225.252   10.164  12.366

二、使用命令 ntpstat 查看状态

如果提示

-bash: ntpstat : command not found

yum -y install ntpstat

[root@a /]# ntpstatsynchronised to NTP server (192.168.1.8) at stratum 3time correct to within 508 mspolling server every 64 s

三、使用命令 timedatectl或者timedatectl status查看状态

[root@a /]# timedatectlLocal time: Fri 2022-10-28 15:32:35 CSTUniversal time: Fri 2022-10-28 07:32:35 UTCRTC time: Fri 2022-10-28 07:32:35Time zone: Asia/Shanghai (CST, +0800)NTP enabled: yes
NTP synchronized: yesRTC in local TZ: noDST active: n/a

四、关于NTP客户端的替代品Chrony

Chrony可以更快地同步系统时钟，提高时间精度，对于一直不在线的系统尤其有用，同时chronyd较小，它使用较少的内存，只在必要时才唤醒CPU，这样可以更好地节省电能，即使网络拥塞较长时间，它也能很好地运行。

可以使用以下任何命令来检查Chrony状态：

[root@a /]# chronyc tracking
Reference ID    : 64643D58 (100.100.61.88)
Stratum         : 2
Ref time (UTC)  : Fri Oct 28 07:28:02 2022
System time     : 0.000020753 seconds slow of NTP time
Last offset     : +0.000098698 seconds
RMS offset      : 0.000065608 seconds
Frequency       : 41.289 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.004 ppm
Root delay      : 0.000177195 seconds
Root dispersion : 0.010756997 seconds
Update interval : 1031.0 seconds
Leap status     : Normal

 运行sources命令以显示有关当前时间源的信息：

[root@a /]# chronyc sources
210 Number of sources = 15
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^+ 120.25.115.20                 2  10   277   245   +821us[ +821us] +/-   19ms
^? 10.143.33.49                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^+ 100.100.3.1                   2  10   377   699  +4361us[+4459us] +/-   12ms
^+ 100.100.3.2                   2  10   377   728   -273us[ -175us] +/- 8575us
^+ 100.100.3.3                   2  10   325   467   -263us[ -263us] +/- 8956us
^+ 203.107.6.88                  2   4   377     8   -462us[ -462us] +/- 6250us
^? 10.143.33.50                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.33.51                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.44                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.45                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.46                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^+ 100.100.5.1                   2  10   377   875    -60us[  +38us] +/-   14ms
^+ 100.100.5.2                   2  10   377   799  +2370us[+2468us] +/-   12ms
^+ 100.100.5.3                   2  10   377   566  -2313us[-2313us] +/-   12ms
^* 100.100.61.88                 1  10   377   587   +249us[ +348us] +/-   10ms

[root@a /]# chronyc sources -v
210 Number of sources = 15.-- Source mode  '^' = server, '=' = peer, '#' = local clock./ .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^+ 120.25.115.20                 2  10   277   260   +821us[ +821us] +/-   19ms
^? 10.143.33.49                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^+ 100.100.3.1                   2  10   377   710  +4361us[+4459us] +/-   12ms
^+ 100.100.3.2                   2  10   377   739   -273us[ -175us] +/- 8575us
^+ 100.100.3.3                   2  10   325   478   -263us[ -263us] +/- 8956us
^+ 203.107.6.88                  2   4   377     3   -685us[ -685us] +/- 6322us
^? 10.143.33.50                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.33.51                  0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.44                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.45                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^? 10.143.0.46                   0  10     0     -     +0ns[   +0ns] +/-    0ns
^+ 100.100.5.1                   2  10   377   887    -60us[  +38us] +/-   14ms
^+ 100.100.5.2                   2  10   377   811  +2370us[+2468us] +/-   12ms
^+ 100.100.5.3                   2  10   377   578  -2313us[-2313us] +/-   12ms
^* 100.100.61.88                 1  10   377   599   +249us[ +348us] +/-   10ms