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Re: How does SMTP IPv4 and IPv6 work together

2008-04-07 13:47:23

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.


Hector Santos, CTO