Re: Routing and PREROUTING

["John A. Sullivan III" <John.Sullivan@nexusmgmt.com>]

Erik Wikström wrote:
> Hi again
> 
> I'm probably thinking to much here but I've got a bit of a problem with
> allowing traffic from my local network out to the Internet. Currently
> I've got the following rules which should do that:
> 
> # Allow traffic from LAN to WAN
> $IPT -t nat -A PREROUTING -i $LAN -s $LOCAL_NET -j ACCEPT
> $IPT -t filter -A FORWARD -i $LAN -o $WAN -s $LOCAL_NET -j ACCEPT
> $IPT -t nat -A POSTROUTING -o $WAN -s $LOCAL_NET -j MASQUERADE
> 
> My problem is with the first rule, where I'm uding the "nat" table but
> I'm not doing any NATing, more like filtering since I only pass some
> packets. And filtering is not supposed to be done in the "nat" table but
> on the other hand there is no filter table in PREROUTING.
> 
> But the alternative would be to have a ACCEPT policy in the
> PREROUTING-chain and do all the filtering in the FORWARD-chain, which is
> kind of unnecessary since a number of packets would then have to travel
> through a number of rules (larger than the number of rules in the
> PREROUTING-chain) just to be droped in the end. So is my rule OK to use
> or would you do in any other way?
> 
> --
> Erik Wikström
Hmmm . . . someone correct me if I'm wrong but I think your first rule 
will end processing in the nat table but the packet will still pass to 
the FORWARD table where, if it does not match a rule, it will be handled 
by your FORWARD chain policy - John
-- 
John A. Sullivan III
Chief Technology Officer
Nexus Management
+1 207-985-7880
john.sullivan@nexusmgmt.com
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