Until kernel 2.4.17 it had two built-in chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally-generated packets before routing).

mangle: This table is used for specialized packet alteration.

It consists of three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering locally-generated packets before routing), and POSTROUTING (for altering packets as they are about to go out).

nat: This table is consulted when a packet that creates a new connection is encountered.

It contains the built-in chains INPUT (for packets destined to local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets).

filter: This is the default table (if no -t option is passed).

If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there.

-t, -table table This option specifies the packet matching table which the command should operate on.

There are currently three independent tables (which tables are present at any time depends on the kernel configuration options and which modules are present). If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target specified by the chain policy determines the fate of the packet. Please also see the NFQUEUE target as described later in this man page.) RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. Packets with a target of QUEUE will be sent to queue number '0' in this case.

Kernels 2.6.14 and later additionally include the nfnetlink_queue queue handler. 2.4.x and 2.6.x kernels up to 2.6.13 include the ip_queue queue handler. (How the packet can be received by a userspace process differs by the particular queue handler. QUEUE means to pass the packet to userspace. DROP means to drop the packet on the floor. If the packet does not match, the next rule in the chain is the examined if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain or one of the special values ACCEPT, DROP, QUEUE or RETURN.ДЄCCEPT means to let the packet through.

A firewall rule specifies criteria for a packet and a target.

Iptables netmap dnat