the router advertisment has two flags: M (managed) and O (otherconf).
if those flags are set, we are supposed to do DHCPv6.
But with otherconf, we are not supposed to actually configure IPv6 addresses.
This broke with 1.36 rework.
Upstream report: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/953#note_1377764
Upstream fix: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1220
This is reported here, to properly track it and ensure we have CI tests for this.
Fixed upstream by https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1220
notes for testing:
1) have a IPv6 router that announces a certain prefix. The RA has the "O flag" set, which means to do stateless DHCPv6, that is, not to get an IPv6 lease.
2) configure an NM profile with `ipv6.method=auto`. Note that NM will start DHCPv6 due to the "other config" flag.
3a) In this mode, NM should do DHCPv6, but it should not ask for any addresses. I think usually the DHCPv6 server would see that, and consequently the lease would not have any addresses. The relevant part here is that NM makes a DHCPv6 in a way that requests info-only.
3b) even if NM requests no IPv6 addresses, if the DHCPv6 happens to hand out any IPv6 addresses (which would be a bug), NM should not configure them.