On my firewall I've three interfaces : eth0 : public IP (46.X.X.X.) eth0:0 public IP (46.X.X.Y.) eth1 : public IP (88.X.X.X.) eth2 : private LAN (172.X.X.X) I've setup a basic NAT which works great until I turn on the eth1 interface, I basically loose the connectivity. When I turn off the interface (ifconfig eth1 down) then the NAT re-work. I've added some policy routing via iproute, which makes my three public IP's available. I don't understand why turning on eth1 on makes the LAN unavailable. PS : weirder ; when I turn on eth1 BUT remove the NAT, then the firewall is accessible by using the public IPS. So to me it's exclusively a NAT issue, since without the NAT the network works while with the NAT without the second public interface, the NAT does work. Regards EDIT : I've been able to make it work by using iproute2 rules. That was definitely a routing issue. Here is what I did : ip rule add prio 50 table main ip rule add prio 201 from ip1/netmask table 201 ip rule add prio 202 from ip2/netmask table 202 ip route add default via gateway1 dev interface1 src ip1 proto static table 201 ip route append prohibit default table 201 metric 1 proto static ip route add default via gateway2 dev interface2 src ip2 proto static table 202 ip route append prohibit default table 202 metric 1 proto static # mutipath ip rule add prio 221 table 221 ip route add default table 221 proto static \ nexthop via gateway1 dev interface1 weight 2\ nexthop via gateway2 dev interface2 weight 3