At 02:13 -0400 on 04/07/2008, Hector Santos wrote about Re: How does
SMTP IPv4 and IPv6 work together:
Robert A. Rosenberg wrote:
My error in not stating my real question but assuming the caveat by
implication. To reword my query (*CAPS* are the omitted caveat):
What justification do you offer to allow an IPv6 MTA to run *VIA
IPv6* without an EXPLICATE MX record? IOW: If it is dual stack and
willing to use IPv4 then just allow the A-Fallback but why allow
AAAA-Fallback to allow it to talk via IPv6? Note: I was assuming
that an EXPLICATE MX would point not only to all the needed AAAA
records but also the A records that would be used by a IPv4-Only
Stack in Fallback mode (ie: Since the existence of the MX precludes
A-Fallback , the A records MUST be referenced by the MX).
>
I guess I will need to be more explicate in the future and avoid
unstated assumptions <g>.
Side question: Are you using term EXPLICATE intentional? because I
think everyone is on sync using "Explicit" vs "Implicit". Implicit
MX is the term used in 2821.
I am using the term explicate to mean "A MX was found in the DNS in
response to a MX request" to distinguish it from the 2821 "implicate"
case of pretending that the MX request returned a "FQDN MX 0 FQDN"
since there was no MX there to be returned.
That said, we have a IPv4 SMTP working model for the last 20+ years.
By extension, IMV, any reasonable engineer would model IPv6 in the
same way. Another way to say that is if you are attempting to fit
IPv6 into the IPv4 world, then it would be common sense that IPv6
will behave with all the expectations of a IPv4 model. In other
words, IPv6 should not alter IPv4 behavior.
This model ignores the reason why IPv4 works this way NOW. IOW: To
allow pre-MX-era defined MTAs that used the A record to continue to
work without requiring a MX to be defined and the failure in the 20
years since to depreciate the use of no longer needed (since a MX
will work) A records as well as not cracking down on admins who to
this day define new IPv4 MTAs via A not MX records. If all the
missing MX records were suddenly/magically be defined, IPv4 support
would still work as it currently does (ie: The IPv4 MTAs would still
get located). Thus REQUIRING that a MX (pointing at both the new IPv6
MTA and the legacy IPv4 MX-Less MTA) be defined once you have a IPv6
MTA is does not break the current model (ie: MX is optional for
IPv4-Only MTAs but CAN be defined).
That would be my justification from an engineering standpoint why an
IPv6 client "may" be able to do a AAAA lookup, simply because an
IPv4 client can do a A lookup. So this means, the IPv6 MTA does not
have to have a MX record.
But why should it NEED to be able look at AAAA records when a MX
record can and will point at both the IPv4 and IPv6 MTA's IPNs? Enter
the 21st century and just define the MX record when you create your
IPv6 MTA.
--
Sincerely
Hector Santos, CTO
http://www.santronics.com
http://santronics.blogspot.com