[ipv6-wg] Real-world IPv6 SMTP experience

On Wed, Dec 21, 2005 at 04:08:16PM +0100, Florian Weimer wrote:
> * Gert Doering:
> 
> > So if I understand this correctly, having a v4-only backup MX should
> > solve all problems commonly experienced?
> 
> As far as I know, the primary MX needs an A record (to satisfy
> Sendmail), and there needs to be a MX which has an A record only (and
> no AAAA or A6 record; this is done to satisfy Exim installations).  My
> initial question to this list was whether if you can do this with just
> a single IPv4 address, and still prefer IPv6 over IPv4 (assuming that
> IPv6-capable hosts prefer IPv6 hosts over IPv4 hosts if they have the
> same priority).

At least at uninett.no this is done with only one IPv4 address. The
primary MX has both A and AAAA, and the secondary is only A, but with
same IPv4 address as primary. Well, you can check mx's for uninett.no
yourself.

I know several sites that have both A and AAAA for all MXs. It doesn't
sound safe to me, but I assume it can't be that problematic then...?

Stig