> nmap -Pn -A -T4 nsagate.apple.com
Starting Nmap 6.40 ( http://nmap.org ) at 2014-03-23 21:27 CLST
Nmap scan report for nsagate.apple.com (17.254.0.48)
Host is up.
All 1000 scanned ports on nsagate.apple.com (17.254.0.48) are filtered
"In computer networking, 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). A variant called Single Packet Authorization exists, where only a single "knock" is needed, consisting of an encrypted packet.[1][2]
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."
Comments
What ports are open?
Services could be hidden through the use of port knocking.
Cool, never heard of it.
"In computer networking, 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). A variant called Single Packet Authorization exists, where only a single "knock" is needed, consisting of an encrypted packet.[1][2]
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."
http://en.wikipedia.org/wiki/Port_knocking