Single Packet Authorization with fwknop
Single Packet Authorization (SPA) using "fwknop" is probably one of the coolest recent innovations in server and network access control technology. Just what is SPA, you ask? SPA is a method of limiting access to server and network resources by cryptographically authenticating users before any type of TCP/IP stack access is allowed.
In itís simplest form, your Linux server can have an inbound firewall rule that by default drops all access to any of itís listening services. Nmap scans will completely fail to detect any open ports, and zero-day attacks will not have any effect on vulnerable services since the firewall is blocking access to the applications.
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