catflap

https://github.com/nyk/catflap

A simple solution to provide on-demand service access (e.g. port 80 on webserver), where a more robust and secure VPN solution is not available. Essentially, it is a more user-friendly form of "port knocking". The original proof-of-concept implementation was run for almost three years by Demotix, to protect development and staging servers from search engine crawlers and other unwanted traffic.