Part 7 of the security fundamentals series. Firewalls, intrusion detection and prevention, VPNs and forward and reverse proxies compared by what they can see, what they can stop, where they sit, and the misconfigurations we find most often.
Part 2 of the security fundamentals series. The networking concepts you need to reason about exposure: IP and MAC addresses, ports, TCP versus UDP, common services, and how to see what your own hosts are listening on.
Firewall rules describe what you meant to expose; a scan from outside shows what you actually expose. This is the repeatable nmap routine we run against our own hosts, including the Docker port that slipped past ufw and how we diff results week to week.