参考：
CentOS7安装iptables防火墙 https://www.cnblogs.com/kreo/p/4368811.html
CentOS7使用firewalld打开关闭防火墙与端口 https://www.cnblogs.com/moxiaoan/p/5683743.html
CentOS7查看和关闭防火墙 https://blog.csdn.net/ytangdigl/article/details/79796961
解决CentOS7，/etc/sysconfig/下没有iptables的问题 https://blog.csdn.net/aafeiyang/article/details/81557191
Iptables与Firewalld防火墙。 https://www.linuxprobe.com/chapter-08.html
CentOS中防火墙相关的命令(CentOS7中演示) https://www.cnblogs.com/yunman/p/7919096.html
CentOS7和6的默认防火墙的区别 https://blog.csdn.net/qq_31666147/article/details/80025937
Linux系统扫描技术及安全防范 https://www.imooc.com/learn/344
用iptables搭建一套强大的安全防护盾 https://www.imooc.com/learn/389