Before moving into the article, let me tell you how this article has been written. This article starts with the introduction to knockd, and proceeds with the implementation of port knocking by using iptables. Note that the same port knocking can be achieved using knockd, as well, which will be discussed in the upcoming article.

Full article here:
Port knocking: Enhance Security Using knockd and/or Iptables From Basics (Unixmen)