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Registration model, 2821bis-06

2008-04-02 04:28:42

Now that I think about it, 2821bis-06 does require something more than A/AAAA in order to operate a mail server: If a host only has an IPv6 address, 2821bis-06 section 5.1 requires the operator of that host to form an arrangement with a dual-stack relay in order to send mail to IPv4-only targets.

IMO, that's not an unreasonable requirement. I think it's perfectly reasonable and almost unavoidable. I'm pointing it out to show that the text already requires something more than getting an AAAA record and setting up some software on the host itself.

How many people would be happy if 5.1 were to say this?

 - if there is no MX record, but there is an A, then the sending
   host MUST synthesise an MX
 - if there is no MX record and no A, but there is an AAAA, then
   the sending host MAY synthesise an MX
 - even if a client doesn't use AAAA records for synthesising MX
   records, it MAY use them for delivering mail once an MX has
   been synthesised.

This permits people to set up IPv6-only mail servers without any MX or relay, and it doesn't declare any deployed 2821-compliant code to be illegal, and it doesn't tell anyone to assume that IPv6-only devices are mail servers by default.

Connectivity to/from AAAA-only mail servers will suck terribly for many decades to come, but no 2821bis wordsmithing can avoid that. Only an arrangement with a dual-stack relay can improve connectivity, and that practically requires an MX RR or more.


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