A colleague of mine pointed me to an article on Port Knocking, more specifically, Single Packet Authorization. I wasn’t too familiar with either but once I started reading, some thoughts came to mind. Does this look far to cumbersome and “pain in the butt” to implement for such a small gain to anyone else? This is just another method of implementing the doomed “security by obscurity”.
First off, Port Knocking “is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports.” [1] Single Packet Authorization is similar, but requires only one encrypted packet. While this may impress some people with it’s technical savvy, this solution should be thoroughly evaluated before implementing. As far as enterprise usability goes – limited at best. Talking amongst ourselves here we did think of one implementation that would actually be useful. That is to prevent your ISP from knowing you’re hosting a service without having to create extensive black or white lists. You could host an ftp server for example without the port ever showing as open to an overly intrusive ISP. Of course we do not condone the breaking of any agreements with an ISP.
However, for enterprise environments Port Knocking and Single Packet Authorization are in my opinion no way a replacement for good security practices These include keeping the service up to date with any patches/updates provided by the vendor. Be aware of any newly developed or developing threats to the service you’re hosting. Implement proper ACLs at the firewall. Block all of Eastern Asia from accessing your SSH service if need be. Use VPN clients. This is critical, there’s no real reason to have remote access ports opened without protection. Use VPN clients. Just about every enterprise firewall comes with some sort of VPN option. Last but not least, do not forget the importance of a strong password policy. Brute force attacks really become a non issue with a complicated enough password.
In conclusion, PK and SPA sound good in practice, and implemented as part of a greater defense in depth solution could work; otherwise, stand alone PK and SPA in my opinion are less than ideal.

[1] http://en.wikipedia.org/wiki/Port_knocking