Great question — answering as the UphillTech team:
The default firewall rules won't allow 'new' traffic from the outside. You can open certain ports, which correspond to certain server protocols, and permit new incoming traffic on those. 21 ftp 80, 81, 443, 8080 http 23 telnet Here's the full list: IP Port Assignments
Full answer by awesome14: https://unix.stackexchange.com/a/672031 (CC BY-SA 4.0).
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