rules for 2nd inteface on gateway

[Steve <dlist@bluewin.ch>]

Hello,

I'm new to this list and new to firewall so please be kind if any
question seem obvious.

Here is my problem. I have a little home lan with one interface
connected to my modem/router (eth0), another for the lan (ath0,
wireless) and a third one (eth1) which I use to repair thing when the
wireless doesn't work (very rarely):

eth0 : 192.168.1.35 fixed
ath0: 192.168.20.1 fixed (and two boxes on that network, *.20.2 et
*.20.3) 
eth1: 192.168.2.2 fixe

and I defined two ip aliases on eth1, eth1:1 and eth1:2 as

eth1:1 : 192.168.10.1
eth1:2 : 192.168.10.2

in order to run ntp on them and have my lan get the time from it.
Everything work fine until I decided to activate on the gateway  the
iptables. Now, from the internet all of my ports are blocked, as
desired, I can surf without any problem from any of the 192.168.20.0/24
boxes, but they cannot access the 192.168.10.0 network and this is were
my question arises. Which rules do I have to put to get things working?

thank you very much


steve

ps : all boxes running debian sarge with 2.6 kernels.