Port forwarding via single packet authorization
Most port knocking or Single Packet Authorization implementations offer the ability to passively authenticate clients for access only to a locally running server (such as SSHD). That is, the daemon that monitors a firewall log or that sniffs the wire for port knock sequences or SPA packets can only reconfigure a local firewall to allow the client to access a local socket. This is usually accomplished by allowing the client to connect to the server port by putting an ACCEPT rule in the INPUT chain for iptables firewalls, or adding a pass rule for ipfw firewalls for the client source IP address. For local servers, this works well enough, but suppose that you ultimately want to access an SSH daemon that is running on an internal system?
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