I've been using LXC and LXD for a while now, and was surprised to see that in Ubuntu 17.10, you no longer have to create a static bridge in
/etc/network/interfaces and assign it using
lxc profile edit default.
Turns out the new way to do it, after much googling, is to simply assign a bridge of a physical interface to your default template, and it handles it using the macvtap driver from there.
E.g. my LAN is connected to enp3s0:
lxc network attach-profile enp3s0 default eth0 eth0