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Re: History of fallback to A

2008-03-30 15:10:31

Alex van den Bogaerdt <alex(_at_)ergens(_dot_)op(_dot_)het(_dot_)net> writes:
On Sun, Mar 30, 2008 at 10:51:25AM -0700, Russ Allbery wrote:

In general, you should count on human laziness.  If mail works without
adding an MX record, people aren't going to add one.  If nothing breaks
when the MX record is missing, no one will notice it is missing.

Then the opposite also holds. As soon as people find out mail doesn't
work as soon as they are on IPv6 only, they will add an MX record.

I am not arguing that the implicit MX should go for IPv4(*). I am just
arguing that it shouldn't appear for IPv6.

Ah, okay.  Thank you for the clarification (and I probably should have
understood originally that's what you meant).

My gut feeling: In IPv6 there will be a lot more hosts, and a
significantly smaller percentage of those hosts will receive mail.
Saving in DNS lookups will outweight the inconvenience of publishing MX
records for those "few" hosts which require it and wouldn't with an
implicit MX.

Certainly if I were designing DNS from scratch today, it would make sense
to not assume mail services without an MX record.

The issues around upgrading from IPv4 to IPv6 and what can and cannot be
changed in that process is way outside my area of expertise, and I'm happy
to leave it to others to decide whether a fallback to AAAA is appropriate.
I only have strong opinions about the existing A record fallback, which I
don't think anyone wants to drop anyway.  :)

Russ Allbery (rra(_at_)stanford(_dot_)edu)             

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