ntpd服务作为既可以作ntpServer，又可以作ntpClient ，只是配置上稍有不同。
ntpd服务运行时需要开启双向（服务器端和客户端） udp/123端口。
iptables：
# yum install iptables iptables-services
# iptables -A INPUT -p udp -m udp --dport 123 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
# iptables -A OUTPUT -p udp -m udp --dport 123 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
# systemctl restart iptables.service
firewalld：
# yum -y install firewalld
# firewall-cmd --zone=public --add-port=123/udp --permanent
# firewall-cmd --reload
What are the iptables rules to permit ntp?
https://superuser.com/questions/141772/what-are-the-iptables-rules-to-permit-ntp
Why does NTP require bi-directional firewall access to UDP port 123?
https://superuser.com/questions/762579/why-does-ntp-require-bi-directional-firewall-access-to-udp-port-123
Troubleshooting NTP
http://support.ntp.org/bin/view/Support/TroubleshootingNTP
【chrony】CentOS7.2 上chrony的安装与配置
https://www.jianshu.com/p/a8976ed21422
【ntp】CentOS7.x ntpd作为ntpServer ntpClient 的安装配置
https://www.jianshu.com/p/8513a4bbd257
【systemd+iptables】用systemd定义一个iptables防火墙系统服务
https://www.jianshu.com/p/96d179f4e444
【firewalld】CentOS 上的 firewalld 简明指南
https://www.jianshu.com/p/2cd238f6df71
