ipv6 stable privacy (rfc 7217) as default ipv6 address for outgoing traffic

[Serge Pouliquen <sp31415@free.fr>]

Hi,

According to my understanding, it's possible to have multiple ipv6 
addresses for an interface.
But I didn't find any option to select the address related to stable 
privacy (rfc 7217) as the default for outgoing address.

In /etc/network/interfaces:
iface enp6s0 inet6 dhcp

In /etc/sysctl.d/00-sp31415-sysctl.conf:
net.ipv6.conf.xxxx.stable_secret = 1111:2222:3333:4444:5555:6666:7777:8888
net.ipv6.conf.xxxx.addr_gen_mode = 2

I'm getting 2 addresses : one from slaac with stable privacy and one 
from dhcpv6.
It looks like the one from dhcpv6 is used as the default for outgoing 
traffic.

How can I indicate that I want to use the one related to stable privacy 
as the default outgoing address ?

With privacy extension (rfc 4941), I can choose the outgoing ipv6 
address with use_tempaddr=2 (or 1 to enable but not use it for outgoing 
traffic)

I can also disable dhcpv6, but I would like to keep dhcpv6.
  iface enp6s0 inet6 auto

Regards,
Serge