Comment # 11 on bug 986395 from Marius Tomaschewski

(In reply to Neil Brown from comment #9)
> But how often?  RFC4941 doesn't list a minimum for TEMP_VALID_LIFETIME or 
> TEMP_PREFERRED_LIFETIME, just recommended defaults of 1 week and 1 day.
> Cycling every 6 hours would probably be safe

IMO, it would not be safe at all.

The defaults are veeeery long (and the defaults are similar to e.g. IEEE
defaults for bridge STP causing up to 50s delay before forwarding packets
or sleep(random(1..10s)) before dhcp4 starts to do anything.
In practice, it is probably more common to use e.g. valid lifetime of 1hour.

I'd not make any assumption about the times, but read the lifetime from the
address. What would be probably doable is to a) prefer to use non-temporary
addresses and (if there is none [a clear corner case]) b) start to switch
over to a new temporary-address when the preferred lft reached 0 (still
usable for existing connections until valid lft goes to 0).

The temporary / privacy addresses are assigned additionally to non-temporary.
IMO there is currently no autoconf option in the kernel to assign a privacy
address only, that is, there is basically always a non-temporary / renewable
[e.g. MAC based EUI64] address. When the user is using not using autoconf,
but only dhcp6 and assigns only a temp addr, you can IMO assume he knows
what he is doing and does not want to use nfs/persistent connections at all
or he misconfigured the box.