odd dns behavior on osx

Tore Anderson tore.anderson at redpill-linpro.com
Sun Jun 20 15:52:48 CEST 2010

* Ryan Rawdon

> 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.

I can think of three possible options:

- A queries are given preferential treatment, so that a negative AAAA
response doesn't cause OS X to stop waiting for the A response, and/or:
- OS X might optimise away AAAA queries if the machine doesn't have a
global IPv6 address, thus masking the problem for most people on the
internet today, and/or:
- A responses are generally answered more rapidly than AAAA queries due
to them being more likely to be pre-cached.

I don't have a Mac, so I can't confirm any of them.  Feel free to do so
though, it shouldn't be more difficult than to look at the DNS traffic
with tcpdump as the problem is being reproduced.

Best regards,
Tore Anderson
Redpill Linpro AS - http://www.redpill-linpro.com/
Tel: +47 21 54 41 27

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