fwknop-server - 2.6.11-2+b1 main

The FireWall KNock OPerator implements an authorization scheme called
Single Packet Authorization (SPA), based on Netfilter and libpcap.
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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.
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The authorization server passively listens for authorization packets via
libpcap, thus preventing any connections from being processed on the
traditional port. Access to a protected service is only granted after a
valid encrypted and non-replayed packet is detected.