Introduction of IP CHAINS
May 14th, 2003
The kernel starts with three lists of rules; these lists are called firewall chains or just chains. The three chains are called input, output and forward. When a packet comes in (say, through the Ethernet card) the kernel uses the input chain to decide its fate. If it survives that step, then the kernel decides where to send the packet next (this is called routing). If it is destined for another machine, it consults the forward chain. Finally, just before a packet is to go out, the kernel consults the output chain.
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