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FORWARD … that flow through a gateway computer, coming in one
interface and going right back out another.
INPUT … just before they are delivered to a local process.
OUTPUT … just after they are generated by a local process.
POSTROUTING … just before they leave a network interface.
PREROUTING … just as they arrive from a network interface (after dropping
any packets resulting from the interface being in promiscuous
mode and after checksum validation).
