Problems with NAT

["Jose Luis Hime" <jhime@synchro.com.br>]

Dear all:

I have the following network:
              :
              :    /---------\
/-------\  Leased  | Router  |  Leased  /----------\
| LAN B |----------| without |----------| Internet |
\-------/  Line 1  | NAT     |  Line 2  \----------/
              :    \---------/
              :         |
              :         |
              :   /----------\
              :   | Firewall |      /-------\
              :   | Linux    |------| LAN A |
              :   | with NAT |      \-------/
              :   \----------/
              :
   CITY "B"   :     CITY "A"

1. The router, the firewall and LAN A are in city "A"
2. LAN B is in another city (city "B")
3. LAN A must access the internet, LAN B must not;
4. Unfortunately my router does not support NAT;
5. Both the router and the linux firewall have real internet IP addresses;
6. So:
   - The linux firewall must NAT packets from LAN A to the internet;
   - The linux firewall must not NAT packets from LAN A to LAN B;

I created rules in table "filter" allowing communication between LAN A and
LAN B:
   -t filter -A INPUT   -s LAN A -d LAN B -j ACCEPT
   -t filter -A INPUT   -s LAN B -d LAN A -j ACCEPT
   -t filter -A FORWARD -s LAN A -d LAN B -j ACCEPT
   -t filter -A FORWARD -s LAN B -d LAN A -j ACCEPT

After that, I created one rule in table "nat" in order to allow LAN A
accessing the internet:
   -t nat -A POSTROUTING -s LAN A -d 0/0 -j SNAT --to Firewall_IP_address

The problem is that LAN A is making NAT to LAN B.

Is there a way to prevent the firewall from NATing from LAN A to LAN B? The
problem is that both traffics (LAN A->internet and LAN A->LAN B) are going
through the same interface...

With ipchains, after reaching the INPUT and FORWARD rules the firewall
would stop and would not reach the NAT rules. This behavior changed in
iptables and it always check both tables (filter and nat).

Thanks in advance,
Jose Hime