centos7防火牆不再采用iptables命令，改用firewalld
禁用防火牆命令：
# systemctl stop firewalld.service
# systemctl disable firewalld.service
# systemctl status firewalld.service
啟動一個服務：systemctl start firewalld.service 關閉一個服務：systemctl stop firewalld.service 重啟一個服務：systemctl restart firewalld.service 顯示一個服務的狀態：systemctl status firewalld.service 在開機時啟用一個服務：systemctl enable firewalld.service 在開機時禁用一個服務：systemctl disable firewalld.service 查看服務是否開機啟動：systemctl is-enabled firewalld.service;echo $? 查看已啟動的服務列表：systemctl list-unit-files|grep enabled