Automagic proxy arp?

[Chinh Nguyen <cnguyen@certicom.com>]

Hi,

I have a machine M that is 'walled' off from the rest of the local subnet
similar to this.

        .1        .2        .3       .4-.254
+-------+         +---------+
+   M   + ------ eth1  FW  eth0 ---- local subnet
+-------+         +---------+

With ip_forward on and using standard forward rules on FW (e.g., -A FORWARD
--in-interface eth1 -j ACCEPT, -A FORWARD --in-interface eth0 -m state --state
ESTABLISHED,RELATED -j ACCEPT), M can reach the local subnet.

However, I need to add a routing entry in M to send all local traffic to FW.
Otherwise, M will attempt to arp the destination as they are all on same subnet.
The linux arp man page claims that linux will "automagic proxy arp when a route
exists and it is forwarding".

Does anyone know how to set up iptables on FW to enable this "automagic"?

I've also tried using explicit forward rules such as "--in eth1 -d !.1 -j
ACCEPT", "--in eth1 -d .4 -j ACCEPT, --in eth1 -d .5 -j ACCEPT, etc." to no effect.

Thanks.