IPTables : How to force data coming from ethX being output by the same device

["Yves DUF" <yves.duf@gmail.com>]

Hello World.

Not totally dumb with iptables (I know how to build a simple
firewall), I'm far from being an expert. I got a quite simple need,
but the more I try to build it, the less I understand how to do it :={

==============================
Let me explain my configuration :
==============================
I got a GNU/Linux server, with two Ethernet boards, for hosting on FTP server.
Here is a simplified diagram of my network :

    FTP Server     <=>                Netasq FireWall Router
    <=>       FTP client
   _________                ________________________________
  | eth0/ IP1a | _______ |  Dev 1
       |                   _________
  |                 |              |  + IP1b
                 |                  |   Client    |
  |                 |              |
     Dev 3       |  ________  |  + IP3a    |
  | eth1/ IP2a |________|  Dev 2                              + IP3b
   |                  |_________|
  | _________|              |  + IP2b
          |
                                  |________________________________|

The 3 sub-networks IP1 IP2 and IP3 are different. All the routing are
direct (no NAT/DNAT).

Some others constraints:
- I can not use two hosts for FTP server, neither another hardware
- I can not use NAT/DNAT inside the Netasq Firewall.

==============================
The issue :
==============================
The FTP client from IP3a arrives to router IP3b. It redirect the
packet to the good aimed wire (IP1a or IP1b). So the FTP server
receive the connection from the good link.
When the FTP server wants to answer, it aims IP3a. But it doesn't know
which device to use (eth0 or eth1). So it use the default gateway (if
that case let say eth0).
The whole stuff works if I do ftp to IP1a. But when I do ftp IP2a, the
answer comes back through IP1b. And the firewall blocks it because
it's not an authorized transfer.

==============================
The mighty solution :
==============================
I think that iptables on the GNU/Linux FTP server would be a good
solution, to do a sort of "ftp contracking". But I don't manage to
write a simple rule as "All traffic that comes from ethX will output
by ethX"
Does somebody got ideas on this subject (iptables or whatever else)?

Regards.
Yves