How to use Nmap as a port knocking client?

I want to use Nmap for port knocking on CTF machines, so I've read the manual for configuring it properly: nmap -p ,,... -r --max-retries 0 --max-parallelism 1 In particular, -r ensures ports are scanned in the listed order, --max-retries 0 ensures that ports are not probed more than once, and max-parallelism 1 ensures that only a single thread runs. However, the above Nmap command fails to open the hidden service. If I instead use the following port knocking client everything just works: https://github.com/grongor/knock Any ideas?
http://dlvr.it/S0qWVQ