Re: IPv4 and IPv6 addresses in resolv.conf

[Rich Felker <dalias@aerifal.cx>]

On Sat, Nov 30, 2013 at 05:23:22PM +0000, Justin Cormack wrote:
> On Sat, Nov 30, 2013 at 5:01 PM, Rich Felker <dalias@aerifal.cx> wrote:
> > On Sat, Nov 30, 2013 at 09:16:54AM +0000, Justin Cormack wrote:
> >> On 30 Nov 2013 03:59, "Rich Felker" <dalias@aerifal.cx> 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 <dalias@aerifal.cx> 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
> > work)?
> 
> It is EAFNOSUPPORT if no kernel support at all.
> 
> Actually I don't think there can be any cases where sending to the
> v4-mapped address (ie ::ffff:1.2.3.4) can fail where an ipv4 socket
> will succeed because those are basically ipv4 sockets with just ipv6
> notation, those addresses can't be routed by the ipv6 stack. So it

One thing I'm confused about is the addresses on the actual packets.
If we've already called bind for address :: and gotten assigned port
N, does this also reserve port N on 0.0.0.0, which will be needed when
sending from (and receiving back) IPv4 packets? Also, is there some
kernel option we might need to worry about that prevents :: from
receiving packets sent to IPv4 addresses, or does that only apply to
TCP, not UDP?

Rich