Log in to Google Compute Engine instance if ssh port is disabled by firewall

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One of our developers forgot to add port 22 into allow list of firewall (Ubuntu Ufw). There are 2 way to resolve this problem:

  1. If you still have user/password of that VM, connect to the VM via Serial Port then login to open ssh port again.  
  2. If you don't have user/password (My case), add a Startup Script to that VM then restart it. Start up script responsible is open port when the VM starts.  
Then ... You have your VM back, Cheer!