# yum install -y ntp

# systemctl enable ntpd
# systemctl start ntpd

# firewall-cmd --add-service=ntp --permanent
# firewall-cmd --reload
# firewall-cmd --list-all

server 0.cn.pool.ntp.org
server 1.asia.pool.ntp.org
server 2.asia.pool.ntp.org

restrict 192.168.0.0 mask 255.255.0.0

# systemctl restart ntpd

# netstat -an|grep 123

*/30 * * * * /usr/sbin/ntpdate 192.168.0.123