Protecting SSH Servers with Single Packet Authorization

The current issue (May '07) of the Linux Journal, contains part II of my series on Single Packet Authorization (SPA). The article is entitled Protecting SSH Servers with Single Packet Authorization, and appears to not be available yet through the LJ website so I suppose that it may only be in the print edition for a while. Part II is a hands-on treatment of fwknop operations, and covers SPA with both symmetric and asymmetric ciphers. here is a short excerpt from the article:

"...This article gets away from theory and concentrates on the practical application of SPA with fwknop and iptables to protect SSHD from reconnaissance and attack. With this setup on a Linux system, no one will be able to tell that SSHD is even listening under an nmap scan, and only authenticated and authorized clients will be able to communicate with SSHD.
     To begin, we require some information about configuration and network architecture. This article assumes that you have installed the latest version of fwknop (1.0.1 as of this writing) on the same system where SSHD and iptables is running. You can download fwknop from http://www.cipherdyne.org/fwknop/ and install either from the source tar archive by running the install.pl script, or install via the RPM for RPM-based Linux distributions..."