That alone is non-obvious. Docker adds prerouting and masquerade changes. They don't show up in the default iptables rule list (iptables -L). If you don't know

That alone is non-obvious. Docker adds prerouting and masquerade changes. They don't show up in the default iptables rule list (iptables -L). If you don't know about it creating the other tables it adds, and how to list them explicitly, you won't see these rules added. Docker binds to the public interface by default, rather than to much-safer private/localhost defaults. If you have an iptables INPUT DENY rule, it doesn't come into play at all, and that's certainly untuitive for people who aren't iptables experts. For example, if you're using ufw (which most non-power-users rely on) and have it set to DENY by default, that won't protect you from the example you provided. The way Docker works is simply wrong. It should not mess with iptables at all. It does so because that makes it "easier" for novice users, but that ease of use is a real security risk.