Not the law, but a good idea.

Port knocking: a stealthy system for network authentication across closed ports

Port Knocking has not been seen on TV

port knocking > about > summary

firewall primer details knock lab download implementations documentation FAQ

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Port Knocking Web Article in Linux Journal

Port Knocking - Network Authentication Across Closed Ports Article in SysAdmin Magazine

The port knocking Perl prototype is licensed under the GPL license. GPL License

Port Knocking (c) 2002-2013 Martin Krzywinski

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Port Knocking (c) 2002-2013 Martin Krzywinski