Safari on IPv6 ?

Sam Wilson Sam.Wilson at
Thu Feb 4 11:29:11 CET 2010

On 2 Feb 2010, at 13:13, Martin Millnert wrote:

> On Tue, 2010-02-02 at 13:57 +0100, Shane Kerr wrote:
>> As I understand it, Safari does the equivalent of:
>> 1. DNS lookup
>> 2. TCP connection
>> 3. HTTP request
>> In both IPv4 and IPv6 at the same time. Whichever gets to step #3
>> first goes to completion "wins", and the other is canceled.
>> Note that once the A and AAAA record are in the DNS cache, then you
>> are really just looking at connecting on whichever TCP session opens
>> first.
> Shane,
> this might very well be their goal, but it doesn't seem to be what's
> going on, since Ron showed us on Monday that the mDNSResponder just
> shuts down the remaining queries (one lingering AAAA in the example),
> after having received its first reply.
> In other words, the actual implementation, intended or not, is  
> right now
> that whoever accomplishes #1 first, may continue.
> I would be more inclined to agree with that whoever performs #3 first
> wins, than #1.  DNS lookup speed says nothing definite about IPv4 vs
> IPv6 network/routing topologies and round trip times.

Just to be explicit about some cross-thread fertilisation.  The  
thread has an example DNS lookup which returns a CNAME record with  
both A and
AAAA in the additional section. In that case - which will be a large
proportion of cases, if not a clear majority - there should be no  
at #1 even with the current mDNSResponder implementation.


Sam Wilson
Network Team, IT Infrastructure
Information Services, The University of Edinburgh
Edinburgh, Scotland, UK

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

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