Hi all,
i've got a linux box (Mandrake 9.1) acting as a server - it has 2 NICs, each of them is connected to a windoze client via a crossed UTP cable. There is also a modem in the linux box, and i am able to acces outside world from the clients (by means of NAT). Everything works fine, except one thing - the two clients don't see each other!
To be specific, i can e.g. ping both client boxes from the linux box, i can also ping the linux box from each client, and i can ping machines in the internet from the clients as well as from the server. But a ping sent from one client to another (ment to be forwarded throug the server) always time outs, as well as any other TCP/IP connection attempt.
This is not a matter of disabled IP forwarding (note that accessing the outside world from the clients works fine) as well as not iptables issue - clients don't see each other with iptables disabled...

Any clue?

There's probably a better way, but in a router, I'd put this in the iptables script:


iptables -A FORWARD -i $LAN_NIC1 -o $LAN_NIC2 -j ACCEPT
iptables -A FORWARD -i $LAN_NIC2 -o $LAN_NIC1 -j ACCEPT

Hmmm it's still the same (actually i'd be kinda surprised if that worked, the FORWARD chain policy has been set to ACCEPT all the time). Anyway, thanks for your effort to help me je_fro...
Seems i gotta try tcpdump and see what's actually happening on the interfaces...

So, the problem has been solved. I assigned both clients IPs within the same subnetwork (192.168.1.X) and that was the problem - when a client wanted to send a packet to the other one, it logically did't know it should be send to the gateway (the linux box) and sent an ARP request instead (which, of course, could have not been answered). So i changed IP of one of the clients to 192.168.2.X and it works.
Thanks everyone for reading my post (and especially je_fro for trying to help).