The port forwarding was normal. But after a shutdown of the frontend. It did not work anymore. I have met this before. And what I did was to reinstall Virtualbox, which wasted a lot of time.
After testing for a long time, I found a solution at http://superuser.com/questions/323424/unable-to-do-port-forwarding-in-virtual-box.
In my case, the ip was 184.108.40.206. So in the Port Forwarding Rules, fill it in Guest IP. Leave Host IP empty. Then it works!