Bug#268631: force preference of IPv4 over IPv6

[Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr>]

>>  There are excellent reasons why IPv6 is preferred to IPv4 by
>> default, and this is not going to change.
>
> I'm very interested in this!  What are the reasons?

The basic idea is that we want to make sure that either there's no IPv6
at all, or IPv6 works fine.  This is important, since it's easy to write
software that detects and works around the lack of IPv6, but it's next
to impossible to write software that will reliably detect broken IPv6.

By preferring IPv6 to IPv4, we make sure that broken IPv6 is something
that people notice, and hence we have a good chance that there won't be
to many broken IPv6 deployments.  If IPv4 were preferred to IPv6, then
people could add RAs to their network even though their IPv6 routing is
broken -- most clients wouldn't notice, they'd just continue using IPv4.

Now your particular deployment of IPv6 doesn't fit the ``IPv6 at least
as good as IPv4'' model that we are promoting; hence, you need to hack
your gai.conf files.  Which is fine -- what you do on your private
network is your private business.  What is not okay is suggesting that
Debian change the default, and break a deployment model that the
networking community has agreed on -- if you don't agree with a stan-
dard, you work on getting it revised, you don't just change your
distribution unilaterally.

By the way, this is a somewhat unsatisfactory solution: manually hacking
/etc/gai.conf doesn't scale.  The networking community is aware of that,
and are working on a solution -- see RFC 5220 and 5221.

                                        Juliusz