Infoworld’s Roger Grimes writes:
Many, many innovations come from the Linux and Unix world. Few are more intriguing to me than port knocking. As a global security plug-in to protect services, it has a lot going for it and few downsides. However, for one reason or another, it suffers from lack of use and understanding. A lot of administrators may have heard of it, but few know how to implement it. Even fewer have used it.
Port knocking works on the concept that users wishing to attach to a network service must initiate a predetermined sequence of port connections or send a unique string of bytes before the remote client can connect to the eventual service. In its most basic form, the remote user’s client software must first connect to one or more ports before connecting to the final destination port.
More on Port Knocking: