Denic (.de) blocking 6to4 nameservers (since begin feb 2010)

Tore Anderson tore.anderson at
Mon Feb 15 23:32:37 UTC 2010

* Nathan Ward

> You are very unlikely to get traffic from Teredo, because:
> 1) Windows only asks for AAAA if it has non-Teredo IPv6 connectivity
> 2) When Windows has non-Teredo IPv6 connectivity and so can ask for
> AAAA, preference for reaching your web content is going to be
> non-Teredo IPv6 -> IPv4 -> Teredo, due to the prefix policy table,
> unless you have an AAAA in 2001::/32 (Teredo space), in which case it
> will prefer IPv4 -> Teredo.
> With 6to4, Windows hosts will ask for AAAA, and will prefer non-6to4
> IPv6 over 6to4 over IPv4. I'm a little surprised at how little 6to4
> traffic you get.
> Teredo gets most use when an application asks for a connection to a
> certain IPv6 address, without DNS. This is most common in peer to
> peer - you're not going to levels of web traffic and P2P traffic
> using Teredo that are comparable ratios to IPv4.

When it comes to HTTP traffic, that's not always the case:  The Opera
web browser in all recent versions will unconditionally prefer IPv6
(including Teredo and 6to4) over IPv4.  Since Windows Vista and newer
automatically configure Teredo and/or 6to4, this is the biggest single
reason for regular clients being unable to access dualstacked websites
here in Norway, according to my measurements (which are done in a
similar fashion to yours).  In case you're interested, I've been posting
reports to the ipv6-ops list about it for a few months now:

Opera has fortunately improved the behaviour in their next version
(10.50) by simply using getaddrinfo() on Windows.  It is due to be
released in a month or two - hopefully then I'll be able to talk some of
my customers into dualstacking their content.

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

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