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Linux Firewalls by Michael Rash

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Single Packet Authorization

Port knocking has shown us how to maximize the use of a packet filter to enforce a default-drop stance against all attempts to communicate with a protected service.[74] However, as shown earlier in this chapter, port knocking is not a panacea, and it has significant architectural limitations. In this section, we'll explore an alternative to port knocking that retains its benefits while avoiding its shortcomings.

Single Packet Authorization (SPA) combines a default-drop packet filter with a passively monitoring packet sniffer in a manner similar to port-knocking implementations. However, instead of transferring authentication data within packet header fields, SPA leverages payload data to prove possession of authentication ...

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