Network Time Protocol(NTP),NTP协议由在用户空间中运行的守护程序实现，即ntp和chrony。

RHEL7和RHEL8同步系统时间区别

RHEL7中，可以选择ntp或chrony，在RHL8中，不支持ntp，使用chronyd守护进程来实现NTP。

chrony和ntp之间的差异

1.ntp服务器在控制客户端访问方式默认响应来自任何地址的请求，相对不安全
2.chronyd默认不允许访问，要想提供时间同步服务端需要在allow中指定
3.ntpd和chrony在系统时钟校正方面的默认行为也不同
4.客户端同步的工作方式有差异

RHEL8配置时间同步服务

1.配置软件仓库(dnf源)
2.了解配置文件（man 5 xxx.conf）
3.启动服务，开机自启动

时间同步服务器端

[xiaoshuai.zhu@rhel8 ~]$ sudo dnf install chrony -y
[xiaoshuai.zhu@rhel8 ~]$ sudo vim /etc/chrony.conf

pool 2.rhel.pool.ntp.org iburst
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
**pool ntp1.aliyun.com iburst**
# Record the rate at which the system clock gains/losses time.
driftfile /var/lib/chrony/drift

# Allow the system clock to be stepped in the first three updates
# if its offset is larger than 1 second.
makestep 1.0 3

# Enable kernel synchronization of the real-time clock (RTC).
rtcsync

# Enable hardware timestamping on all interfaces that support it.
#hwtimestamp *

# Increase the minimum number of selectable sources required to adjust
# the system clock.
#minsources 2

# Allow NTP client access from local network.
#allow 192.168.0.0/16

# Serve time even if not synchronized to a time source.
#local stratum 10

# Specify file containing keys for NTP authentication.
keyfile /etc/chrony.keys

# Get TAI-UTC offset and leap seconds from the system tz database.
leapsectz right/UTC

# Specify directory for log files.
logdir /var/log/chrony

# Select which information is logged.
#log measurements statistics tracking

[xiaoshuai.zhu@rhel8 ~]$ sudo systemctl start chronyd.service && sudo systemctl enable chronyd.service
[xiaoshuai.zhu@rhel8 ~]$ sudo chronyc sources -v

210 Number of sources = 1

  .-- 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   6    17     3   -207us[-1457us] +/-   16ms