Coming back to the topic of computer security, the TCP Wrapper is an example of such a safety net. I wrote it when my systems were under attack by someone who appeared to walk through walls.

Wietse Zweitze Venema (born 1951) is a Dutch programmer and physicist best known for writing the Postfix email system. He also wrote TCP Wrapper and collaborated with Dan Farmer and Samuel Johnson to produce the computer security tools SATAN and The Coroner's Toolkit.