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This would be related to Linux Essentials, I’ll recommend going through courses like LPIC-1. You can purchase some courses from PluralSight or Udemy. There are additional some PATHs you can follow.
Usually the rule is, if the port shouldn’t be public, then close it via the firewall.
One other thing, you can check DigitalOcean’s documentation about configuring Firewall rules and when would one need to do so on DigitalOcean Cloud Firewalls. The link to the said documentation is
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