Cannot go out the firewall from internal network (NAT)

Marco Nicoloso nicolm at gmail.com
Tue Nov 14 20:58:13 CET 2006


> #Forward delle catene
> iptables -A FORWARD -i eth1 -o eth0 -j laninet
> iptables -A FORWARD -i eth0 -o eth1 -j inetlan
>
> Above shows eth1 is your LAN interface and eth0 is your WAN interface.
>
> but....
>
> #Setting up NAT
> iptables -t nat -A POSTROUTING -o eth1 -s 192.168.7.0/24 -j SNAT --to
> xxx.xxx.xxx.xxx
>
> this shows eth1 is your WAN interface, please clarify on this thing.
>
> Pradeep
>

Yes, thnk you very much Pradeep, I was wrong, I corrected it but...

...anyway still it doesn't work as I want. For now it is acting only
like a NAT, I want to understand how NAT + Packet Filtering work
together, I read the howtos provided by netfilter.org. And
particularly I didn't understand:

1) POSTROUTING chain is processed after the FORWARD chain, isn't it?
But do I really need the FORWARD chain? If yes, do I need to setup my
rules for filtering the packets coming from my LAN which I want to
pass through the firewall (using a DROP policy) in the FORWARD or just
in the INPUT chain, or in both of them?

2) Which is the right place for the NATting rules in my script? That
is, NAT rules must be placed before or after PF rules (for me after,
but as NAT controls different chains because `nat' is a different
table... maybe it's the same)

3) Someone can verify the following sentence if I understood
correctly: "Packet coming from any network connected to the firewall
enters the chain INPUT: rules are processed from the first in order to
the end, if a suiting rule for it is found, then the faith of the
packet depends only on that rule; if not packet is either dropped or
accepted, depending on the general policy of the chain above
mentioned"

Thanks.

Marco Nicoloso



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