IP networking single socket, both IPv4 and V6?

[Lewis Donzis]

On Jan 4, 2018, at 10:17 AM, Karl Denninger <karl at denninger.net> wrote:
> 
> I've written a fair bit of code that binds to both Ipv4 and v6 for
> incoming connections, using two sockets (one for each.)
> 
> Perusing around the 'net I see an implementation note written by IBM
> that implies that on their Unix implementation you can set up an INET6
> listener and it will open listeners on *both* IPv4 and v6; you code it
> as an Ipv6 socket/bind/listen/accept, and if an Ipv4 connection comes in
> you get a prefix'd IPv4 address back when you call getpeername().
> 
> This would obviously shorten up code and remove the need to open the
> second listener socket, but playing with this on FreeBSD it doesn't
> appear to work -- I only get the IPv6 listener in "netstat -a -n"
> showing up and as expected a connection to a v4 address on that port
> fails (refused, since there's no listener.)
> 
> Is this something that *should* work on FreeBSD?

It works.  We do it all the time.  You either have to set the sysctl:

   net.inet6.ip6.v6only=0

which you can do in /etc/sysctl.conf or with the sysctl utility, or, in your program, use setsockopt to turn off the V6ONLY option, e.g.:

   setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, &(int){0}, sizeof (int)); // Turn off v6-only

We use the first method, which is broken in FreeBSD 11.1 prior to patch level 5 or 6, I can’t remember which, but works in all others.  The second method is considered to be more portable.

FWIW, Linux, by default, sets v6only off, so it doesn't require anything special.