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Re: current usage of AAAA implicit MX?

2008-04-05 22:34:17

At 22:03 +0200 on 04/05/2008, Arnt Gulbrandsen wrote about Re: current usage of AAAA implicit MX?:

Peter J. Holzer writes:
On 2008-04-04 17:30:29 -0400, Robert A. Rosenberg wrote:
RFC 3974
It, BTW, REQUIRES an MX if there are any IPv6 MTAs and only supports A-Fallback in the absence of an MX


"For the transition period, all mail domains should have MX records such that MX targets with IPv4 and IPv6 addresses exist"?

I will point out the existence of an MX for the FQDN makes the question of AAAA-Fallback (or the existence of AAAA records for FQDN when you simulate the existence of a FQDN MX 0 FQDN record) a moot point since there is no AAAA record to fall-back on since it is LISTED in the actual MX as a valid MTA. Also the quote above says "SUPPLY MX RECORDS for your MTAs".

 It also states that in the presence of an MX containing IPv6 address
 references, an IPv4-Only stack should ignore the IPv6 addresses

Yes, of course. It cannot reach them anyway. Just as IPv6-only hosts can ignore A records since they cannot reach them either.

An IPv6-only host must use the presence/absence of A records to generate the DSN. The DSN status code is 5.1.2 if there's no A and 5.4.x if there is an A but no way to reach it (ie. no configured relay).