[Bug 24828] Re: IPv6 should be disabled by default

[Rémi Denis-Courmont]

I have serious doubts that blacklisting IPv6 really solves the problem.
glibc's getaddrinfo() will still query AAAA regardless, unless
AI_ADDRCONFIG flag is set. And it is very often *not* set, if only
because it is a "proprietary" GNU extension.

In other words, the problem is now fixed merely for some applications,
but surely not all of them, and particularly probably a bunch of apps in
Ubuntu universe.

IMHO, the proper solution would rather go along the lines of what MacOS
X does, i.e. only try AAAA if there is any non-loopback non-link-local
IPv6 address (and only try A is there is any non-loopback IPv4 address).
That means hacking glibc, and has nothing to do with the *kernel* IPv6
plugin. Also, I think it is much less likely that an application will
want to resolve AAAA in absence of IPv6 connectivity, than it is likely
that some Ubuntu users will want IPv6 connectivity, even though both are
uncommon situation in any case.

Finally, adding such a blacklist looks like an incoming source of
headaches if IPv6 is to ever be restored, and a nice source of Ubuntu
bashing from advanced users once Feisty is released.

To sum up, the real problems are that glibc only checks IPv6
connectivity if AI_ADDRCONFIG is set (this is a trivial hack to
sysdeps/posix/getaddrinfo.c), and then glibc's check_pf considers IPv6
is available even if there is only ::1 or fe80::something which is
clearly wrong (this is a not-so-trivial to make_request from
sysdeps/unix/sysv/linux/check_pf.c).

My 0,02€. Regards.

-- 
IPv6 should be disabled by default
https://launchpad.net/bugs/24828