Safari on IPv6 ?

Ron Broersma ron at
Mon Feb 1 14:22:31 CET 2010


The confusing behavior you're seeing in OSX 10.6 is most likely the mDNSResponder issue reported earlier by Bernhard.  Apple switched to mDNSResponder for unicast DNS resolution in 10.6, but that broke IPv6 address selection because mDNSResponder takes whatever is the first response (A or AAAA) and drops (rejects) all other answers.  So, depending on how you ask, and depending on what address comes back first, there is no way for the rest of the system or applications to see all the responses and make appropriate address choices.  This seriously impacts IPv6 interactions, because it no longer deterministically prefers AAAA over A, but rather uses only the response that arrives first.

A detailed bug report (#733104) was filed last October.

As always, multiple bug reports to Apple will get their attention.


On Feb 1, 2010, at 3:50 AM, Marco Hogewoning wrote:

> On 1 feb 2010, at 11:44, Bernhard Schmidt wrote:
>> On Mon, Feb 01, 2010 at 11:21:53AM +0100, Marco Hogewoning wrote:
>> Note: this is only hearsay, some colleagues have this issue.
>>> This should be easy, however I can't seem to find the answer online as
>>> most articles are about turning off IPv6 instead of one. I noticed
>>> Apple's Safari 4.04 defaults to IPv4 instead of IPv6 when both A and
>>> AAAA are present. Now you can discuss wether this is a safe default or
>>> not, I would really like to know where the little button is to switch
>>> this behavior to AAAA first, which I recall used to be the case on
>>> earlier editions.
>> We have seen a similar behaviour with most applications after upgrading
>> to MacOS X 10.6. It seems that the mdns-responder (?) of the system
>> which is responsible for all DNS lookups is broken. It sends out both A
>> and AAAA queries at the same time, but only seems to accept the first
>> (successful?) response it gets.  I've even heard that some people see
>> the second UDP response from the cache is rejected with "Port
>> unreachable".
> I can confirm it is either 10.6 or safari 4...not that I dig through traffic logs every day, just happen to notice it.
>> The effect is not a preference of IPv4, but no preference at all.
>> Extremely hard to debug.
> ssh seems to do the right thing, my ssh sessions are on 6 even tough the machine is listed with both AAAA and A but I might just got lucky on that one.
>> Bugs have been filed with Apple several times, I can ask for the
>> bug-ids.
> I'll try and kick them around a bit as well. In the meantime, Matthias suggestion seems to help, but again that might just be a lucky hit.
> MarcoH

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