Options for non-virtualized network interface isolation?

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http://serverfault.com – My server has two physical NICs, interfaces are eth0 192.168.1/24 (lan) and eth1 192.168.3/24 (dmz). Both NICs are connected to a router via wire. Router has two vlans for lan and dmz with separated ports also, with a firewall between the zones (OpenWRT). Problem object is the server and the way Linux prioritizes loopback interface. I want to have dmz -> lan initiating traffic blocked, for which the router firewall takes care of on the network, but as we know traffic going over server loopback interface between local NICs doesn't even reach the router. lan -> dmz direction all must be allowed (HowTos)