Ethernet bridge:

aptitude install bridge-utils

/etc/network/interfaces:

iface eth0 inet manual
iface eth1 inet manual
auto br0
iface br0 inet static
        bridge_ports eth0 eth1
        # bridge_maxwait 0
        address x.x.x.x
        netmask 255.255.255.0
        gateway x.x.x.x