Encontrei a solução aqui: link
Eu precisava do pacote:
sudo apt-get install bridge-utils
e o /etc/network/interfaces
teve que ser:
auto lo
iface lo inet loopback
auto br0
iface br0 inet static
address 192.168.0.10
network 192.168.0.0
netmask 255.255.255.0
broadcast 192.168.0.255
gateway 192.168.0.1
bridge_ports eth0 eth1
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off
e a interface br0
precisava ser ativada usando:
sudo ifup br0