Re: DNS & IPv6

On Sat, 22 May 2010 17:04:49 -0600, Alex Rousskov
<rousskov_at_measurement-factory.com> wrote:
> On 05/14/2010 01:38 PM, Henrik Nordström wrote:
>> As you may have noticed I have been looking at the IPv6 support in the
>> last days, and now I am looking at how we perform DNS queries.
>>
>> Today we first do an AAAA query, followed by an A query. As we anyway
do
>> both queries it would be better if we ran both in parallell. The

Except when configured with dns_v4_fallback == off. In which case the A
look starting requires a NXDOMAIN result from the AAAA query.

>> benefits of this is twofold:
>>
>> a) Lower latency.
>>
>> b) Easier policy on re-transmission
>>
>> But needs a bit of reshuffling to get done unfortunately, separating
the
>> actual queries a bit from the lookup.
>
> Is there an RFC or somesuch that recommends parallel lookups over
> sequential ones or vice versa?
>
> Thank you,
>
> Alex.

No RFCs that I'm aware of (though I have not been following the newest
ones). Just whitepapers about methods of encouraging and helping the
world-wide v6 transition.

For comparison the OS resolver does them sequentially with A first unless
the options "inet6" entry is set. That default encourages non-v6 traffic
which we want to reverse.

Doing them in parallel I think is a good idea from the speed viewpoint. We
will need to be careful over which lookup uses, such as forwarding, can go
ahead with just ONE set and want the fastest path; versus lookups like
config and ACL which require BOTH to be completed before it is reliably
answered.

Amos
Received on Sat May 22 2010 - 23:37:16 MDT