启动防火墙报错：Failed to start firewalld.service: Unit is masked.

一、故障现象

# systemctl start firewalld
Failed to start firewalld.service: Unit is masked.


二、解决方法

执行命令：
# systemctl unmask firewalld
输出如下结果：
Removed symlink /etc/systemd/system/firewalld.service.

启动firewalld：
# systemctl start firewalld

查看firewalld状态：
# systemctl status firewalld
输出如下结果：
● firewalld.service - firewalld - dynamic firewall daemon
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; disabled; vendor preset: enabled)
   Active: active (running) since 四 2019-06-06 14:59:42 CST; 6s ago
     Docs: man:firewalld(1)
Main PID: 31046 (firewalld)
    Tasks: 2
   Memory: 25.9M
   CGroup: /system.slice/firewalld.service
           └─31046 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid

6月 06 14:59:41 node-3 systemd[1]: Starting firewalld - dynamic firewall daemon...
6月 06 14:59:42 node-3 systemd[1]: Started firewalld - dynamic firewall daemon.

Active: active (running)  表示防火墙已经启动。