[ih] MX and A records

[Craig Partridge]

Hi John:

This is from memory, and I wrote RFC 974 over 27 years ago, but
my recollection is that the fallback to A was motivated by transition but
also included as a permanent fallback/bootstrap feature.  That is,
as I was crafting how MX RRs would work, it was clear we needed to
fallback to A records as a transitional matter, but some person (don't
recall who) observed that there was no reason to sunset the fallback scheme,
as it provided robustness -- if someone forgot to create an MX for a new host,
email could still get there.

Thanks!

Craig


> MX records were invented in 1986 in RFCS 973 and 974, replacing MD and
> MF and as far as I can tell obsoleting MB, MG, and MR.
> 
> It looks like there wasn't a rule to fall back to A if there were no
> MD and MF, or if there was it didn't make it into the RFC.  RFC 974
> does describe fallback from MX to A.
> 
> Was that expected to be a transitional thing until MX was more widely
> implemented, or was it expected to be permanent?  Or didn't anybody
> think about it either way?
> 
> R's,
> John
********************
Craig Partridge
Chief Scientist, BBN Technologies
E-mail: craig at aland.bbn.com or craig at bbn.com
Phone: +1 517 324 3425