This option adds an iptables 'portkng' match, which allows you to
to open ports based on port knocking method.

Suppported options are:
--knock
  List of ports to knocked in order [xx - Port number]
--timeout
  Timeout in seconds to keep destionation port open since
  last inbound connection.
--knock-timeout
  Timeout in seconds between valid knocks.

Example:
  $ iptables -A INPUT -m portkng --knock 45,67,89 -p tcp --dport 22 -j ACCEPT
  Will open port 22 when ports 45,67 and 89 are knocked (in order).
