Estou usando essa configuração e funciona muito bem.
# cat /etc/network/interfaces
auto br0
iface br0 inet dhcp
bridge_ports eth0
bridge_stp off
bridge_fd 0
bridge_maxwait 0
# cat /var/lib/lxc/<CONTAINER NAME>/config
lxc.network.type = veth
lxc.network.flags = up
lxc.network.link = br0