Sabahattin Gucukoglu wrote:
On 31 Mar 2008 at 11:32, Arnt Gulbrandsen <arnt(_at_)oryx(_dot_)com> said:
Adding IPv6 support to internet mail forces a question: If there isn't an
MX RR for a given domain, does the SMTP client fall back to looking up an A
RR, A or AAAA, or A and AAAA? We have to choose, and all of them represent
Why? As John has pointed out, all 2821bis is saying you have to do as an
Internet mailer is to imagine that there's an MX record of preference 0
with the target of the recipient domain for that same recipient domain
when there isn't one already. That's how it's always been, and all this
bother about whether or not it's appropriate to deny mail to recipients
without an MX record in the IPv6 case is apparently appealing only to
people who want it that way for purely anti-abuse motives
false. this isn't just about deterring abuse.
- the vast majority of hosts on the Internet today don't want to receive
mail. this will be even more true in an IPv6 world where every toaster,
light switch, etc. can have an IP address.
at the time RFC 974 was published fallback to A records was needed for
backward compatibility with existing practice. we don't have that
problem now with AAAA records today, because today, nobody can expect to
reliably receive mail via IPv6 without setting up a MX that will accept
incoming mail from IPv4 and relay it over IPv6.
- some of those hosts that don't want to receive mail actually have SMTP
servers because they are configured that way by default by the OS
vendor. any mail sent to those servers just takes up disk space -
nobody ever reads it.
- mail queues get clogged up trying to send mail to hosts that don't
support SMTP. because "connection refused" is a temporary error to
SMTP, any mail sent to a host that doesn't support SMTP will sit in a
queue for days before bouncing or being discarded.
(note that mail is fairly unique among applications in this respect.
most applications don't persistently retry when they see "connection
because most hosts don't want to receive mail, the "right thing" to do
is to have the default behavior - i.e. the behavior when no MX record is
present - be to not deliver mail to that host. we still need to
fallback to A records for backward compatibility with hosts that rely on
that. but we don't need to fallback to AAAA records.