At 07:44 -0400 on 04/07/2008, John C Klensin wrote about Re: current
usage of AAAA implicit MX?:
--On Sunday, 06 April, 2008 22:44 -0400 "Robert A. Rosenberg"
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
Someone would need to check with Craig Partridge, but I believe
that "only" (or "ONLY") in the above is not strictly correct.
It may have been the primary reason, but, no matter how often
"only" is repeated a desire to permit minimal configurations
did, I am quite sure, figure in to the equation.
Why, since MX has been in existence for 20 years and most (if not
all) currently deployed DNS and Resolver software support it, do you
suggest the need to allow the use of A records in the absence of a MX
to be allowed as opposed to being treated as a definition error? I am
asking why NEW definitions of an IPv4 MTA should not use a MX record
not why we should allow grandfather'ed definitions to not need to be
upgraded via the addition of the MX.
BTW: You are quoting me out of context. The above quote of mine was
NOT justifying any suggestion to deprecate A-Fallback support but
only to justify initially banning AAAA-Fallback (until we can address
the issue of allowing AAAA-Fallback) since the use of MX records to
point at IPv6 MTAs can work since there is no pre-MX set of IPv6 MTAs
to be supported (unlike the A-Only MTAs 20 years ago). I regard ANY
MX-Less IPv6 MTAs as a definition error since the MX could (and
SHOULD) have been defined (along with the legacy A-ONLY IPv4 MTAs
being added to the MX when it is defined). Once you initially define
a IPv6 MTA it is not any extra effort to add the requisite MX when
you define the AAAA.