This is where it really helps to have an ISP/Host that is following IPv6 deployment recommendations, and providing an option to route addition subnet(s) to you host machine. Then your host can act as a router and you can then allocate the additional subnets to LXD’s network. That way there’s no need for messing about with proxy NDP (this is where you tell the host to artificially reply to NDP enquiries for a particular IP on a particular external interface that is not actually bound on the LXD host).
Proxy NDP can be used to ‘advertise’ specific IPs that LXD is using on its internal network to the wider external network when the ISP only provides a single /64 subnet at L2 to the host. This allows traffic for those IP to arrive at the LXD host’s external interface, at which point if the LXD host has the correct routes locally it will then route the packets into the LXD network as normal.
We provide a routed NIC type that will allow this occur. However because Linux’s proxy NDP support only allows single IPs to be advertised, rather than an entire subnet, the routed NIC doesn’t support SLAAC/DHCPv6, and IPs must be statically allocated in LXD and configured inside the container.
See
The other alternative is to run a separate daemon, something like ndppd which can be configured to artificially respond to all NDP queries for a particular subnet:
https://manpages.ubuntu.com/manpages/focal/man1/ndppd.1.html