Acabei tendo:
auto eth0
iface eth0 inet static
address 10.0.3.2
gateway 10.0.3.1
dns-nameservers 8.8.8.8
em /etc/network/interfaces
. Com /var/lib/lxc/*/config
sendo:
...
lxc.network.type = veth
lxc.network.link = lxcbr0
lxc.network.flags = up
lxc.network.hwaddr = 00:16:3e:d6:21:a6