> Port 25 blocked on link B, through IPTables, but opened at Link A. > Would the request try to go through link A, or would it stop because It has > reached Link B and was denied?
The traffic will use link B (default gateway), except if it needs a host in the network of link A. So the traffic is simply blocked. But I also have some set-ups like you described. I used the Linux Advanced Routing & Traffic Control HOWTO [http://lartc.org/lartc.html] as knowledge. Look at 'Chapter 11. Netfilter & iproute - marking packets' [http://lartc.org/lartc.html#LARTC.NETFILTER]. I think that's what you need: use iptables to mark specified packets [`iptables -A PREROUTING -i $INT_IF -t mangle -p tcp --dport 25 -j MARK --set-mark 1`] and add the route for all the traffic marked by the iptables (using the `ip` command!). Don't forget to check for the right kernel options! All this needs some knowledge of iproute [`apt-cache show iproute`], also explained in the HOWTO. And some kernel compiling ... Kind regards, Fred
