Is it possible to implement a simple code lock triggered by connecting ports with iptables? So for example a visitor need to connect ports 1001,1009,1008,1004 in this order to get access to port 80. Any other port hit out of order in the range 1000-1009 would reset the lock.
This strategy is known as "port-knocking" and there are numerous implementations.
See this link for a large list, pretty linux-centric: http://portknocking.org/view/implementations
And the Wikipedia entry: https://en.wikipedia.org/wiki/Port_knocking
Here's an iptables-only version (which I personally find revolting): http://www.debian-administration.org/articles/268