Re: IPv4 and IPv6 addresses in resolv.conf

[Rich Felker <dalias@aerifal.cx>]

On Sat, Nov 30, 2013 at 05:33:35PM +0000, Rob wrote:
> Rich Felker, Sat, 30 Nov 2013:
> >>
> >>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?
> 
> I've been seeing this output consistently from mpd at startup:
> 
> 	listen: bind to '0.0.0.0:6600' failed: Address already in use
> 	(continuing anyway, because binding to '[::]:6600' succeeded)
> 
> mpd is the only program on my machine that binds to 6600 so it would
> appear that :: port bindings reserve the ipv4 port too. Could be a
> kernel configuration option though...

It looks like this depends on the value of the socket option
IPV6_V6ONLY, which on Linux gets its default from
/proc/sys/net/ipv6/bindv6only, which in turn is 0 (off) by default. So
on systems with non-default configuration, the proposed musl strategy
could badly break use of v4 nameservers (in a mixed environment).

I think manually setting IPV6_V6ONLY to 0 with setsockopt before the
bind should make it possible to reliably get the behavior you
described (and which we want).

Rich