odd dns behavior on osx

Ryan Rawdon ryan at u13.net
Sat Jun 19 10:52:02 CEST 2010

Firstly I'll point out that I'm not denying the existence of the bug 
described, it is something I've observed at least since Snow Leopard was 

However, if this is the case for v6-only hosts, then why wouldn't this 
cause IPv4-only hostnames to suffer the same sporadic brokenness that 
the OP describes with v6-only hostnames?  e.g.  the same dual queries 
for a v4-only host like mozilla.org go out, comes back with the AAAA 
NXDOMAIN first, thus causing the A answer to be refused and subsequently 
result in the browsing failure.


On 6/18/10 1:48 AM, Tore Anderson wrote:
> Hi Alan,
> * Alan Batie
>> I've been using www.v6.facebook.com since I found out about it earlier
>> this week, and OSX is exhibiting some odd behavior: after a short time
>> (the cache timeout period?), gui apps, specifically firefox and safari
>> complain "can't find host".  From a terminal window, I can ping6 the
>> site, after which firefox and safari can again find it themselves.  Very
>> odd...
> You're experiencing a known OS X bug.  It will send out the A and AAAA
> queries in parallel, and will use the first answer it gets back and
> ignore the second one.  You probably get an NXDOMAIN response to your A
> request first, and therefore the AAAA response you're actually
> interested in gets ignored.
> Using ping6 will force an AAAA-only query to be sent, that way you end
> up getting the correct response cached in the OS for some time.
> See:  http://openradar.appspot.com/7333104
> Best regards,

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