Mac OS X IPv6 behaviour update

Tore Anderson tore.anderson at
Mon Sep 20 20:02:26 CEST 2010

Hi guys,

I've managed to get hold of an OS X 10.6.5 beta (build 10H542) and play
a bit with it in my lab.  It looks like Apple have been listening;  the
IPv6 behaviour is much improved.  Worth noting:

- 6to4 is no longer preferred over IPv4.  I've tried both setting up a
local 6to4 interface on the OS X host, advertising an autoconf prefix
with RA, using both global and RFC 1918 IPv4 addresses on the Ethernet
interface, using Safari/Firefox/Chrome - and I've so far not been able
to reproduce any dualstack brokenness.

- The local 6to4 adapter now deactivates itself automatically when the
Ethernet interface is numbered with RFC 1918 addresses (unlike OS X

It's not 100% perfect though, some issues still remain:

- Teredo is still preferred over IPv4, at least when it is configured
with using SLAAC (OS X has no built-in support for Teredo, as far as I

- If OS X has a default IPv6 route, but only link-local addresses (for
example if it has received an RA without any prefix information),
Firefox fails to connect to dual-stack sites.  tcpdump doesn't see any
IPv6 traffic on the wire though, so I'm not sure what's going on here.
Chrome and Safari handles the situation fine - they don't bother to ask
for AAAAs at all.  In any case, this is an improvement over 10.6.4,
where Safari too had the same problem as Firefox still do.

I believe, however, that the vast majority of brokenness from the OS X
clients are caused by the 6to4 issues.  OS X 10.6.5 is therefore likely
to have a very positive impact on the dualstack brokenness content
providers struggle with these days.

THANK YOU for fixing this, Apple!  :-D

Now I'm just hoping that their update notification service is aggressive
enough to make most of their users upgrade to 10.6.5 shortly after it's
been released...

Best regards,
Tore Anderson
Redpill Linpro AS -
Tel: +47 21 54 41 27

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