如果我们从CentOS6过渡到CentOS7系统来的话，会发现原来的iptables被firewalld取代，而且相应的命令格式也稍微有些变化。今天老左有在设置添加放行端口的时候的出现”FirewallD is not running”的错误提示，应该是没有开启firewalld导致的。
[root@centos-linux ~]# systemctl start firewalld
Failed to start firewalld.service: Unit is masked.
[root@centos-linux ~]# service firewalld start
Redirecting to /bin/systemctl start firewalld.service
Failed to start firewalld.service: Unit is masked.
解决CentOS7出现的”Failed to start firewalld.service”问题
既然没有启动，于是就执行”systemctl start firewalld”启动，但是又出现”Failed to start firewalld.service: Unit is masked.”错误。继续找解决方法，然后输入”systemctl unmask firewalld.service”。
[root@centos-linux ~]# systemctl unmask firewalld.service
Removed symlink /etc/systemd/system/firewalld.service.
[root@centos-linux ~]# service firewalld start
Redirecting to /bin/systemctl start firewalld.service
解决CentOS7出现的”Failed to start firewalld.service”问题
然后再启动”systemctl start firewalld.service”即可解决问题没有报错。
再执行端口开放操作
