GW (LAN1, LAN2, ADSL) config

[j.halifax .]

Hi All,

please help me kindly to reconfig my default GW (Fedora 12).

The GW has 3 active Ethernet cards:
eth0 - connected to Internet ADSL router (incl. VPN, pptpd)
eth1 - not used
eth2 - LAN1
eth3 - LAN2.

I had everything working fine but due to some problems I lost 
the configuration of the GW and I can't get it working again.

(1) The Internet access (LAN1 -> GW -> Internet) is working fine.
(2) The access of  (Internet -> GW -> LAN1 / LAN2) is ok incl. VPN
(3) I can not access LAN2 neither from LAN1 nor from GW box

Traceroute shows that for (3) packets don't go to eth3 (LAN2) as
they should, but they fall down to eth0 (default gw).

Can you please advise me what I need to set-up (iptables) in the GW?

Thank you so much for your kind help.

Regards,
jh