Socket listener for IPv6 and IPv4

Question

I ported an application to support IPv6, using popular tutorials. I decided to use only one socket listener for both protocols. Now I realized I have to set IPV6_V6ONLY properly (it's only working on my x86 linux out of the box, but not on my ARM).

Is this really the way to go? Some say IPV6_V6ONLY shouldn't be used (apparently it's outdated, because of IPv4 mapping over the wire), some say using one socket for both protocols is fine.

So, I'm confused. What is the current state about this problem? Did I misunderstand the problem?

Answer 1

You should always bind both sockets explicitly, with the IPv6 socket bound with IPV6_V6ONLY.

Why? Cross-platform compatibility.

Windows by default requires that you explicitly bind on IPv4 and IPv6. Binding only to IPv6 will not implicitly bind to IPv4 as well.

Linux by default will implicitly bind to IPv4 as well when you bind on IPv6, only if the net.ipv6.bindv6only sysctl is set to 0. Distributions such as Debian change this default to 1, breaking your assumption.

I can't remember what Mac OS X does here (someone chirp in the comments please?), but the point is that explicitly binding to both protocols leaves no surprises.

Answer 2

It is fine either way.

If you bind v4 and v6 explicitly, you need to set IPV6_V6ONLY, otherwise, you need to clear it. The default setting varies between platforms.

Some platforms do not support accepting v4 connections on v6 sockets, so for maximum compatibility I'd go with the "two sockets" approach.