Public bug reported:

My network has both DHCPv6 and SLAAC for IPv6. From both a privacy
perspective and readability reasons, DHCPv6 should *always* be preferred
over SLAAC addresses when available.

NetworkManager has always been able to adhere to that by simply setting
ip6.privacy=0 for the connection.

So if you would - for instance - run `curl`, the DHCPv6
address would be used to connect to the outside world.

Since the update to 1.36.6, this is no longer the case. NetworkManager
now routes outgoing traffic through the SLAAC address, even if
ip6.privacy=0 is set for the connection. Setting
net.ipv6.conf.all.use_tempaddr = 0 and
net.ipv6.conf.<interface>.use_tempaddr = 0 with sysctl also no longer
has any effect.

Physically removing the SLAAC addresses or disabling RA's altogether is
the only way to stop NetworkManager from prefering SLAAC over DHCPv6

Looking at the changelog of NetworkManager 1.36.6, things regarding IP
address order and temporary addresses have been changed in that release:

Unfortunately, this introduced this bug, which is really breaking a lot
of my use cases.

I should note that the bug is not present in NetworkManager 1.38.0 on
Debian sid. That just prefers DHCPv6 addresses when available, like it

** Affects: network-manager (Ubuntu)
     Importance: Undecided
         Status: New

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  DHCPv6 addresses are no longer preferred over SLAAC addresses

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