The PREROUTING/POSTROUTING chains exists for manipulating packets, not
filtering purposes.

You cannot use the 'nat' table for filtering. This table only receives
the first packet of any new connection. The 'nat' table is specifically
designed for NAT rules, nothing else.

There is also a PREROUTING chain in the 'mangle' table. This is intended
for setting nfmark values and similar things, not really filtering. But
theoretically you can do filtering there if you like.

Officially filtering should be done in the FORWARD/INPUT/OUTPUT chains
of the 'filter' table. FORWARD for bridged packets, INPUT for packets
addressed to the bridge itself and OUTPUT for packets sent from the
bridge itself.

Until NAT is fully working in bridging the PREROUTING/POSTROUTINGchains
 is of quite limited use. Just stick to FORWARD for bridge filtering
rules and you should be set. When NAT is working then
PREROUTING/POSTROUTING of the 'nat' table will become quite important
for managing NAT rules as outlined in the netfilter HOWTO. There should
be no difference between routing or a bridging for how you implement
netfilter rulesets.

--
Henrik Nordstrom
MARA Systems AB
Sweden


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