NTP 服务可以用来当做客户端所在服务器的时间，也可以作为服务端当做时间校准的服务。

安装 NTP

$ yum -y install ntp
# 开机自启
$ systemctl enable ntpd
# 先同步一下时间，此处使用的是阿里云的 NTP 校准服务
$ ntpdate -u time1.aliyun.com

配置 NTP

　　配置文件目录：/etc/ntp.conf
　　

# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).

driftfile /var/lib/ntp/drift

# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default nomodify notrap nopeer noquery

# Permit all access over the loopback interface.  This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
restrict ::1

# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
# 修改内容 --- 上级时间服务器，阿里云的时间服务
server time1.aliyun.com
server time2.aliyun.com
server time3.aliyun.com
server time4.aliyun.com
server time5.aliyun.com
server time6.aliyun.com
server time7.aliyun.com

#broadcast 192.168.1.255 autokey        # broadcast server
#broadcastclient                        # broadcast client
#broadcast 224.0.1.1 autokey            # multicast server
#multicastclient 224.0.1.1              # multicast client
#manycastserver 239.255.254.254         # manycast server
#manycastclient 239.255.254.254 autokey # manycast client

# 修改内容 --- 允许上层时间服务器主动修改本机时间
#restrict time1.aliyun.com nomodify notrap noquery
#restrict time2.aliyun.com nomodify notrap noquery
#restrict time3.aliyun.com nomodify notrap noquery

# 修改内容 --- 只有在上级时钟源失效时，NTP 才会使用127.127.1.0的本地时钟，将local时间作为ntp服务器时间提供给ntp客户端。NTP把本地主机的时钟也看作外部时钟源来处理，分配的地址是127.127.1.0
server 127.127.1.0
# 修改内容 --- 设置本地时钟源的层次为 5，这样如果NTP服务从本地时钟源获取时间的话，NTP对外宣布的时间层次为 6
fudge  127.127.1.0 stratum 5


# Enable public key cryptography.
#crypto

includefile /etc/ntp/crypto/pw

# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography.
keys /etc/ntp/keys

# Specify the key identifiers which are trusted.
#trustedkey 4 8 42

# Specify the key identifier to use with the ntpdc utility.
#requestkey 8

# Specify the key identifier to use with the ntpq utility.
#controlkey 8

# Enable writing of statistics records.
#statistics clockstats cryptostats loopstats peerstats

# Disable the monitoring facility to prevent amplification attacks using ntpdc
# monlist command when default restrict does not include the noquery flag. See
# CVE-2013-5211 for more details.
# Note: Monitoring will not be disabled with the limited restriction flag.
disable monitor

　　配置完毕之后重启 NTP 服务

$ systemctl restart ntpd

# 查看上级连通状态
$ ntpq -p

# 查看时间同步状态
$ ntpstat
synchronised to NTP server (203.107.6.88) at stratum 3
   time correct to within 50 ms
   polling server every 1024 s

　　这里 NTP 服务配置已经完毕，它既是客户端也是服务端，如果要当服务端可以开放 123 端口，其他服务器上安装 NTP 服务并把上级时间服务器的配置项更改到本服务器的 IP/域名/主机名 即可。