The FireWall KNock OPerator implements an authorization scheme called Single Packet Authorization (SPA), based on Netfilter and libpcap.
Its main application is to protect services such as OpenSSH with an additional layer of security in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) much more difficult.
This is the client program responsible for accepting password input from the user, constructing SPA packets that conform to the fwknop packet format, and encrypting packet data.
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