Docker bypasses UFW: why and how to fix it
Docker publishes container ports with iptables rules that skip UFW, so a denied port still answers the internet. See the mechanism and the fixes that work.
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UFW and IPv6: the open door on your VPS
Your UFW rules and cloud firewall may only cover IPv4, leaving services wide open on IPv6. See why it happens on a VPS and how to close the gap.
UFW firewall basics: set up a VPS firewall
UFW is the simple way to run a firewall on a VPS. Set a default-deny policy, allow only the ports you need, and turn it on without locking out SSH.
What ports are and how Linux services listen
A port is how a server sorts incoming traffic to the right service. Learn to read what is listening with ss, and tell a local port from a public one.