详细参考：https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/chap-Managing_Services_with_systemd.html
详细参考：https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Security_Guide/sec-Using_Firewalls.html#sec-Using_iptables
详细参考：https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/sec-Introduction_to_NetworkManager.html
centos 7 三个最重要变化（systemd,firewalld,networkmanager），涉及非兼容性
原文：http://www.cnblogs.com/zhjh256/p/6675315.html