Failed to start firewalld.service: Unit is masked

centos7 启动防火墙失败：Failed to start firewalld.service: Unit is masked.

解决：

执行” systemctl start firewalld”启动，出现”Failed to start firewalld.service: Unit is masked.”错误。需要对服务进行解锁：

systemctl unmask firewalld.service
systemctl start firewalld