Re: [v6ops] AWS ipv6-only features

[Owen DeLong <[e-mail]>]

> On Nov 30, 2021, at 01:47 , Philip Homburg <[e-mail]> wrote:
> 
>>   On Nov 29, 2021, at 10:33 , Nick Buraglio <[1][e-mail]>
>>   wrote:
>> 
>>   "ULAs are preferred over GUAs, so when a host is presented with
>>   both a ULA and GUA as possible ways to reach a destination, the
>>   host will select the ULA. Once the ULA destination address is
>>   chosen, the host will then choose its ULA as a source address
>>   to reach the ULA destination. This preference of ULA addressing
>>   over GUA addressing is the mechanism that provides internal
>>   network connectivity independence from concurrent external
>>   Internet connectivity."
>> 
>>   Yep... The moral of the story is that GUA works as intended and
>>   ULA is a bit of a mess.
> 
> I'm a bit confused about this scenario.
> 
> Typically a hosts gets addresses from DNS. So this suggests that people
> create DNS RR sets that contains both GUAs and ULAs.
> 
> Is this common practice somewhere? Do people expect that something sensible
> will happen if you try that? Is it documented what should happen?

Probably more common in mDNS than DNS, but yes, something sensible SHOULD
happen as documented in the RFCs:

AAAA record sorting in getaddrinfo() or getnameinfo() should return the ULA records
before the GUA records in the linked list.

As a result, an application that sensibly iterates through the list in order (as is expected
behavior) should connect via GUA if possible (if not, it should rapidly receive an error
and move on to the next item in the list, so no extraordinary processing or coding
is required in the application).

Owen