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Re: Registration model, 2821bis-06

2008-04-02 05:51:57



Arnt Gulbrandsen wrote:

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.

first rule is fine - really I think that fallback to A should eventually be deprecated (probably as of some flag day) but 2821bis clearly is not the place to do this.

second rule - MAY should be SHOULD NOT, perhaps even MUST NOT.

third rule is a bad idea, IMHO. the 974 fallback means that no MX records + an A record implies authorization to deliver mail to that IPv4 address. it doesn't imply any authorization to deliver mail to any IPv6 address.

the principle of least surprise should dictate that AAAA use is consistent from one conforming sending MTA implementation to another. the presence or absence of an A record should not affect whether an AAAA record is used.

I envision an MTA administrator getting IPv6 support for his server and then testing to see if it works for receiving mail. IPv6 should work for receiving mail if the DNS is configured correctly - i.e. an MX record is present - and not otherwise.

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