IP networking single socket, both IPv4 and V6?

[Lewis Donzis]

> On Jan 4, 2018, at 10:32 AM, Lewis Donzis <lew at perftech.com> wrote:
> 
> 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.

I forgot about one other option, which we used to get around the regression in 11.1 until the kernel gets fixed.  Because libc functions are generally published with a weak reference, you can overload the socket() function in your own code, like this:

int socket (int domain, int type, int protocol)
{
   extern int _socket(int domain, int type, int protocol);
   int s = _socket(domain, type, protocol);
   if (s >= 0 && domain == PF_INET6)
      setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, &(int){0}, sizeof (int)); // Turn off v6-only
   return s;
   }

We put that in one of our own shared libraries that we bind all of our programs to, and that solves it without having to change a lot of code.