Request: Select non-temporary (non-private) global ipv6 if available

[robobenklein]

I see the latest commit does address the issue of local addresses being assigned as permanent addresses on the interface, but I wouldn't consider all other addresses to be "permanent" or even suitable for inclusion in a DNS AAAA record.

Here's an example:

$ ip -6 addr show scope global
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2601:844:4000:750:XXXX:XXXX:XXXX:9431/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 86383sec preferred_lft 14383sec
3: eno2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2601:844:4000:750:ffff:74:8:10/128 scope global dynamic noprefixroute 
       valid_lft 4470sec preferred_lft 1770sec
    inet6 2601:844:4000:750:XXXX:XXXX:XXXX:9432/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 86383sec preferred_lft 14383sec

Each interface has a private, temporary address: marked by mngtmpaddr which means:

(IPv6 only) make the kernel manage temporary addresses created from this one as
template on behalf of Privacy Extensions (RFC3041). For this to become active, the
use_tempaddr sysctl setting has to be set to a value greater than zero. The given
address needs to have a prefix length of 64. This flag allows to use privacy
extensions in a manually configured network, just like if stateless auto-
configuration was active.

And I think that if an address is intended to be 'private' and 'temporary' we should instead prefer a non-private address, even if it might still be labeled as 'temporary' by the lifetime valid/preferred timers or 'dynamic'. ('dynamic' addresses can still be assigned to something unchanging by the router, like it is in my case, which also has SLAAC)

[robobenklein]

Simplest implementation seems to be to run ip -6 addr show scope global -mngtmpaddr -temporary first, and if it has no matches, then continue with the existing implementation.