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Re: current usage of AAAA implicit MX?

2008-04-06 22:20:14

At 20:44 -0400 on 04/05/2008, Hector Santos wrote about Re: current usage of AAAA implicit MX?:

Consider a world where there is only IPv6 and everyone is talking IPv6
only.  I believe we will have the same issues with missing MX record and
a implicit MX AAAA lookup behavior for the domain.

Only if we do NOT mandate NOW that a MX record is REQUIRED to locate/access an IPv6 MTA. Remember that the A-Fallback behavior is ONLY due to the needs (back in the 1980s) of MTAs that were following the pre-MX rule that you found an MTA by looking at the FQDN A-Record group. Once the use of MX records become the standard method of locating/accessing MTAs the direct use of A Records was (or rather should have been) depreciated.

The only way that AAAA fallback can be eliminated, is if we use the
opportunity for an IPv6 only system to mandate proper MX record.

Since the MX record exists NOW (and predates IPv6) there is NO NEED to EVER use an AAAA record to directly find an MTA and we should make the requirement to have a MX for IPv6-Only and Mixed IPv4/IPv6 MTAs a MUST. If we put this off, we will just get into the same "can not disallow fall-back" mess as we have now with MX-less A records.

Note that this keeps the current status quo in that we are NOT requiring an MX for IPv4-Only MTAs although a SHOULD (or MAY?) clause could be included to discourage this practice and advice admins to start to add any missing MXs since if/when they add an IPv6 MTA for the FQDN (going to dual stack) they are going to have to add the MX at that time (so get ready now).