Viktor Dukhovni:
> On Sun, Apr 06, 2014 at 02:25:28PM -0400, Rick Zeman wrote:
>
> > I now have IPv6 routing working along with "inet_protocols = all" in
> > my main.cf (Postfix 2.9.4). I've noticed that using my outbound relay
> > (which is load balanced across many machines) will connect with either
> > protocol with no discernible pattern even to the same destination.
>
> This is a feature, not a bug. If one of IPv4 or IPv6 works poorly
> for a given destination, the message is eventually delivered via
> the other.
>
> > What would dictate the smtp client's fallback (for lack of a better
> > word) to ipv4 assuming the same destination supports both protocols
> > (confirmed with telnet when I round-robined back to the same smtp
> > server)?
>
> Postfix determines the IPv4 vs. IPv6 priority randomly, so some
> connections will prefer IPv4 and later connections will prefer
> IPv6, at random.
>
> This has proved to be the most reliable way to ensure mail delivery
> (after a modest number of retries in some cases).
Some Linux distributors turn on Postfix IPv6 indiscriminately; when
Postfix IPv6 had precedence over IPv4, this resulted failure to
connect to sites with many IPv6 addresses (for sanity reasons Postfix
will not try every IP address that is listed for a site).
Protocol selection is configurable but this is one of those things
that you should not tinker unless all deliveries go to the same site.
Wietse
