Whenever I mention the Hail Mary Cloud online, two suggestions always turn up: The iptables example, and "Why not use port knocking?". Well, consider this:
Port knocking usually means having all ports closed, but with a deamon reading your firewalls logs for a predetermined sequence of ports. Knock on the correct ports in sequence, your're in.
Another dirty little secret: It's possible to implement port knocking with only the tools in an OpenBSD base system. No, I won't tell you how.
Executive Summary: Don't let this keep you from keeping your system up to date.
More to follow -