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Linux Firewalls by Michael Rash

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Chapter 13. INTRODUCING FWKNOP

The FireWall KNock OPerator (fwknop, see http://www.cipherdyne.org/fwknop) was released as an open source project under the GNU Public License (GPL) in June 2004. It was the first port-knocking implementation to combine encrypted port knocking with passive OS fingerprinting, making it possible to allow only Linux systems to connect to your SSH daemon. (The TCP stack of the port-knocking client system acts as an additional authentication parameter.) fwknop's port-knocking component is based on iptables log messages, and it uses iptables as the default-drop packet filter.

In May 2005, I released the Single Packet Authorization mode for fwknop, so fwknop became the first publicly available SPA software. As of this writing, ...

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