如何防止意外重启 Linux（基于 systemd）

说是有一个 RedHat 的客户提出需求:为了防止意外重启 Linux，要求 root 用户明确地执行某条命令后才允许对 Linux 进程重启操作。我们都知道 systemd linux 中的 reboot 是基于 reboot.target 来实现的，因此最直观的做法应该就是 mask 掉这个 target 了。

lujun9972:~/ $ sudo systemctl mask reboot.target
[sudo] lujun9972 的密码：
Created symlink /etc/systemd/system/reboot.target → /dev/null.
lujun9972:~/ $ sudo systemctl reboot
Failed to reboot system via logind: Access denied
Failed to start reboot.target: Unit reboot.target is masked.
lujun9972:~/ $ reboot now
Failed to write reboot parameter file: 权限不够
lujun9972:~/ $ sudo reboot now
Failed to talk to init daemon.

然后 RedHat 工程师玩出了另一种花样（下面内容摘自于 access.redhat 商业网/solutions/1580343）：

1. 创建一个 unit 文件 /etc/systemd/system/reboot-guard.service

   [Unit]
   Description=Reboot Guard
   [Service]
   ExecStart=/bin/true
   [Install]
   RequiredBy=shutdown.target
   

   这一步是核心操作了，由于 /bin/true 执行后立即停止，因此这个被 shutdown.target 依赖的单元被显示停止，那么 shutdown.target 单元本身也就跟着立即停止了。
2. 再创建一个 unit 文件 /etc/systemd/system/start-reboot-guard.service

   [Unit]
   Description=Start Reboot Guard
   [Service]
   ExecStart=/bin/systemctl enable reboot-guard
   [Install]
   WantedBy=multi-user.target
   

   这一步保证Linux启动后自动enable reboot-guard 服务
3. 执行 systemctl daemon-reload 重新加载配置
4. 执行 systemctl enable reboot-guard start-reboot-guard 启用新服务
5. 再尝试重启就会失败了

   [root]# systemctl reboot
   Failed to issue method call: Transaction contains conflicting jobs 'stop' and 'start' for reboot.target. Probably contradicting requirement dependencies configured.
   [root]# shutdown -h now
   Failed to issue method call: Transaction contains conflicting jobs 'stop' and 'start' for poweroff.target. Probably contradicting requirement dependencies configured.
   
   Broadcast message from root@example.com on pts/1 (Mon 2015-08-17 13:37:10 EDT):
   
   The system is going down for power-off NOW!
   

6. 如果需要重启，那么只需要先执行 systemctl disable reboot-guard 禁止 reboot-guard 命令即可

   [root]# systemctl poweroff
   Failed to issue method call: Transaction contains conflicting jobs 'stop' and 'start' for poweroff.target. Probably contradicting requirement dependencies configured.
   [root]# systemctl disable reboot-guard.service 
   rm '/etc/systemd/system/shutdown.target.requires/reboot-guard.service'
   [root]# systemctl poweroff
   Broadcast message from root@example.com on pts/1 (Mon 2015-08-17 13:37:50 EDT):
   
   The system is going down for system halt NOW!
   [  OK  ] Started Show Plymouth Power Off Screen.
   [  OK  ] Started Restore /run/initramfs.
   [  OK  ] Stopped LSB: Bring up/down networking.
            Stopping Network Manager...
   [  OK  ] Stopped Network Manager.
   ...
   [  OK  ] Reached target Unmount All Filesystems.
   [  OK  ] Stopped target Local File Systems (Pre).
            Stopping Remount Root and Kernel File Systems...
   [  OK  ] Stopped Remount Root and Kernel File Systems.
   [  OK  ] Reached target Shutdown.
   Cannot finalize remaining file systems and devices, giving up.
   dracut Warning: Killing all remaining processes
   Powering off.
   [ 4653.288570] Power down.