Port knocking is an authentication method used by network administrators to control access to computers or other network devices behind a firewall. Port knocking takes advantage of firewall rules to allow a client who knows the "secret knock" to enter the network through a particular port by performing a sequence of connection attempts (called a knock sequence). The correct knock sequence for any given port is created for specific IP addresses by the network administrator. A small program called a daemon monitors the firewall log files for connection requests and determines whether or not a client seeking the network is on the list of approved IP addresses and has performed the correct knock sequence. If the answer is yes, it opens the associated port and allows access. Of course, if unauthorized personnel discover the knock sequence, then they, too, can gain access.

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This was last updated in September 2005
Posted by: Margaret Rouse

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