In version v11.5.31.0 we’ve added an option called Port Knocking. In this version it’s available with FTP, SFTP & SMTP. We’ll enhance this in future version.
Port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port(s). The primary purpose of port knocking is to prevent an attacker from scanning a system for potentially exploitable services by doing a port scan because unless the attacker sends the correct knock sequence, the protected ports will appear closed.
More information about Port Knocking can be found here:
http://en.wikipedia.org/wiki/Port_knocking
http://linux.die.net/man/1/knockd
http://blog.chipx86.com/2011/02/10/i-invented-port-knocking/
