Endpoint address dns resolution - option to prefer IPv6 or IPv4

[Luis Ressel]

On Fri, Mar 13, 2020 at 03:52:19PM +0100, Torsten Krah wrote:
> I've got an endpoint with a DNS name which has both, an A and AAAA
> record.
> I would like to configure the client, so that I can tell him, to either
> prefer IPv6 or IPv4 for the vpn connection.

The relevant source code can be found here:
https://git.zx2c4.com/wireguard-tools/tree/src/config.c#n242

As you can see, wg resolves host names through the libc's getaddrinfo()
function, and simply uses the first ip address returned by that
function.

A standards-compliant implementation should return an ipv6 address in
the first position if one is available and the host has a working
connection to the v6 internet. As far as I know, both glibc and musl do
this (though the behaviour of glibc can be modified by editing
/etc/gai.conf); I'm not sure what other libcs do.

It would of course be possible to make this behaviour configurable for
wg; we just need to figure out if that's an useful feature, and what the
exact semantics should be.

Cheers,
Luis