I started FirewallD service but didn't allow SSH port, now I can't login my VM through SSH - authentication

Question

I have a Centos VM on Google Cloud, I am using a custom SSH port and private SSH file to manage my VM.It worked fine for a long time but yesterday I started the "FirewallD" service in CentOS but I forgot to add a rule to allow my custom SSH port.
Now I can't connect to my VM through SSH, I also have tried to connect my VM through Web Console ("Open in browser window on custom port" and Open in browser window using provided private SSH key),neither of them works.
Are there any other solutions?

Answer 1

If you don’t have access through ssh at your vm, you could use the serial port to login
Go to the VM instances page in Google Cloud Platform console.
Click on the instance for which you want to add a startup script.
Click the Edit button at the top of the page.
Click on ‘Enable connecting to serial ports’
Click Save and then click RESET on the top of the page. You might need to wait for some time for the instance to reboot.
Click on 'Connect to serial port' in the page.
If you don’t have a root password for the serial console, you could use a startup script to add it to your instance, the script would be like this:
Go to the VM instances page in Google Cloud Platform console.
Click on the instance for which you want to add a startup script.
Click the Edit button at the top of the page.
Click on ‘Enable connecting to serial ports’
Under Custom metadata, click Add item.
Set 'Key' to 'startup-script' and set 'Value' to this script:
#! /bin/bash
useradd -G <an existing group in the instance> USERNAME
echo USERNAME:PASSWORD | chpasswd
Click Save and then click RESET on the top of the page. You might need to wait for some time for the instance to reboot.
Click on 'Connect to serial port' in the page.
In the new window, you might need to wait a bit and press on Enter of your keyboard once; then, you should see the login prompt.
Login using the USERNAME and PASSWORD you provided.
In this way you can login at the vm and stop the firewall service to modify your rules before to start the service again