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<H2><A NAME="s6">6.</A> <A HREF="packet-filtering-HOWTO.html#toc6">How Packets Traverse The Filters</A></H2>

The kernel starts with three lists of rules in the `filter' table;
these lists are called firewall chains or just
chains. The three chains are called INPUT,
OUTPUT and FORWARD.

The three circles represent the three chains mentioned above. When
a packet reaches a circle in the diagram, that chain is examined to
decide the fate of the packet. If the chain says to DROP the packet,
it is killed there, but if the chain says to ACCEPT the packet, it
continues traversing the diagram.

A chain is a checklist of rules. Each rule says `if the packet
header looks like this, then here's what to do with the packet'. If
the rule doesn't match the packet, then the next rule in the chain is
consulted. Finally, if there are no more rules to consult, then the
kernel looks at the chain policy to decide what to do. In a
security-conscious system, this policy usually tells the kernel to
DROP the packet.

<OL>
<LI>When a packet comes in (say, through the Ethernet card) the kernel
first looks at the destination of the packet: this is called
`routing'.
</LI>
<LI>If it's destined for this box, the packet passes downwards
in the diagram, to the INPUT chain. If it passes this, any processes
waiting for that packet will receive it.
</LI>
<LI>Otherwise, if the kernel does not have forwarding enabled, or it
doesn't know how to forward the packet, the packet is dropped. If
forwarding is enabled, and the packet is destined for another network
interface (if you have another one), then the packet goes rightwards
on our diagram to the FORWARD chain. If it is ACCEPTed, it will be
sent out.
</LI>
<LI>Finally, a program running on the box can send network packets.
These packets pass through the OUTPUT chain immediately: if it says
ACCEPT, then the packet continues out to whatever interface it is
destined for.</LI>
</OL>

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