[Apple M1] IPv6 doesn't route to internet

[Vayu]

Describe the bug
VM gets assigned IPv6 and can reach the host via it but not the Internet. IPv4 works fine connecting to internet. Host itself has IPv6 connectivity.

Cheeked on 2 different machines (m1 and m1 pro) with the latest macos

To Reproduce
How, and what happened?

1. `multipass shell
2. ping6 www.google.con

Expected behavior
Ipv6 routes to internet like ipv4

Additional info

• multipass version 1.8.1+mac
• `multipass info --all'
  Name: media
  State: Running
  IPv4: 192.168.64.2
  10.9.10.103
  Release: Ubuntu 20.04.4 LTS
  Image hash: add2f33bf439 (Ubuntu 20.04 LTS)
  Load: 0.13 0.05 0.01
  Disk usage: 2.2G out of 4.7G
  Memory usage: 146.0M out of 970.0M
  Mounts: --

[Vayu]

Looks like traffic from inside VM gets NATed into ULA address of the host instead of GUA. E.g. when I send pings from the host, I see:

IP6 (flowlabel 0x90e00, hlim 64, next-header ICMPv6 (58) payload length: 16) 2603:....... > 2607:f8b0:4002:c08::63: [icmp6 sum ok] ICMP6, echo 

But when I do it from within the VM

IP6 (flowlabel 0xd41ac, hlim 63, next-header ICMPv6 (58) payload length: 64) fd16:....... > 2607:f8b0:4002:c08::63: [icmp6 sum ok] ICMP6, echo request, 

And the router doesn't route fd16 ULA address to the Internet.
Not sure where to see NAT settings that control this behavior.

PS if i manually delete ULA addresses from the host interfaces then the VM has IPv6 connectivity

[ricab]

There is also #3832 for this, which has a little more info. Let's continue tracking there.