Hi there @LAiM,
You can expose port 9000 on CentOS by running the following commands. The first adds the port to the firewall, saving it to the configuration. The second command then reloads the firewall to ensure it has applied.
sudo firewall-cmd --permanent --add-port=9000/tcp
sudo firewall-cmd --reload
To check that these worked correctly, you can list all ports permitted in the firewall by running:
sudo firewall-cmd --list-ports
You can also view existing services that are permitted through the firewall (such as http or ssh) by running this:
sudo firewall-cmd --list-services
Hope that helps!