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Date: Sat, 30 Nov 2013 12:01:12 -0500
From: Rich Felker <>
Subject: Re: IPv4 and IPv6 addresses in resolv.conf

On Sat, Nov 30, 2013 at 09:16:54AM +0000, Justin Cormack wrote:
> On 30 Nov 2013 03:59, "Rich Felker" <> wrote:
> >
> > On Fri, Nov 29, 2013 at 10:51:16PM -0500, Rich Felker wrote:
> > > On Fri, Nov 29, 2013 at 10:45:26PM -0500, Strake wrote:
> > > > On 29/11/2013, Rich Felker <> wrote:
> > > > > But that would mean complete unconditional DNS failure on systems
> > > > > lacking IPv6.
> > > >
> > > > We could do so iff system has IPv6. Switching on whether system has
> > > > IPv6 rather than whether resolv.conf has any IPv6 nameservers means
> > > > * no check whether resolv.conf includes v6 server
> > > > * that adding a v6 server to resolv.conf can not break DNS even on
> > > > systems lacking v6
> > > > which seems saner.
> > >
> > > OK, so how do we detect if the system "has IPv6"? I don't think it's
> >
> > BTW, short of an answer to this question, I think the approach I
> > already suggested is rather safe. I can't imagine how an IPv6
> > nameserver address would end up in resolv.conf on a system completely
> > lacking IPv6 support at the kernel level.
> I can imagine how it got there eg if you have a standard config or you
> compile a new kernel and omit ipv6...

Indeed, this is conceivable. However, if the system is intended to be
used on an IPv6 network and you compile without IPv6 in the kernel,
lots of things will break and you probably just need to fix the
kernel. Still I'd like to avoid more breakage here than necessary.

Can you (or anyone) fill me in on how things fail on a system built
without IPv6 support or with broken IPv6 configuration? I assume the
original socket() call will fail (with what errno?) if IPv6 support is
not even compiled into the kernel, but are there other cases where
socket() might succeed but then sending to a v4-mapped address would
fail (where sending to the same v4 address with a v4 socket would

The fallback scheme I'm thinking of using is something like:

	if (have_any_v6_nameservers) {
		if (socket(PF_INET6, ...) && errno=EAFNOSUPPORT) {
			disable any v6 nameservers
			open and use v4 socket
		v4-map all v4 nameservers
		use v6 socket
	} else {
		open and use v4 socket

Does this look reasonable?


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