John C Klensin <john-ietf@xxxxxxx> wrote: > --On Saturday, 22 March, 2008 23:02 -0700 Douglas Otis > <dotis@xxxxxxxxxxxxxx> wrote: > >> The "update" of RFC2821 is making a _significant_ >> architectural change to SMTP by explicitly stating AAAA >> records are within a list of SMTP server discovery records. > > Well, to be very precise, 2821 is ambiguous on the subject. Agreed. > Some people have read (and implemented) it as if the text said > "address records", implying a default MX for the AAAA case as > well as the A one. Others have read it more narrowly and only > supporting the default for A RRs. To the extent to which > 2821bis is expected to eliminate ambiguities that were present > in 2821, it had to say something on this subject. It might, however, be best to simply document the ambiguity. I suspect that implementation reports would show some implementations querying for both AAAA and A RRs, while others query only for A RRs. I am not convinced that 2821bis _should_ try to resolve this. > If it says "address records" (as the current text does), Actually, it says "address RR" (if I understand what text we're discussing). I believe we're discussing Section 5.1 of http://www.ietf.org/internet-drafts/draft-klensin-rfc2821bis-09.txt where it says " " The lookup first attempts to locate an MX record associated with the " name. If a CNAME record is found instead, the resulting name is " processed as if it were the initial name. If no MX records are " found, but an address RR (i.e., either an IPv4 A RR or an IPv6 AAAA " RR, or their successors) is found, the address RR is treated as if it " was associated with an implicit MX RR, with a preference of 0, " pointing to that host. (Please correct me if I misunderstand what text we're discussing.) > you (and perhaps Mark and others) dislike the consequences and > claim "significant architectural change". If it is changed to > explicitly indicate that only A RRs can be used to imply an > MX record, then I assume that those who read 2821 the other way > and are now supporting AAAA records to generate an implied MX > would claim "significant architectural change". Indeed, that seems likely (though not demonstrated). But regardeless, we're on shaky ground if we try to force either kind of implementation to change. May I suggest a different starting point: I think there's very strong consensus that the presence of an MX RR demonstrates the intention to receive email addressed to the domain in question. I don't think there's any consensus that the presence of an AAAA or A RR demonstrates such an intent. There is, however, considerable history that the presence of an address RR _in_combination_with_ a process listening on port 25 of the IP address in question indicates a willingness to receive email for a domain identical to the domain of that address RR. Whether or not we have any consensus that this historical practice should be deprecated (I would vote YES!), rfc2821-bis is not, IMHO, the right place to deprecate it. (If I may digress a bit, let me explain that this implied-MX rule is a real pain to me as an ISP, in that we maintain a SMTP server on the same IP address as a number of virtual web services; and the implied-MX rule brings us rather significant spam traffic that I'd _much_ rather be sending to a different IP address than the web- server for the domain in question.) Getting back to my point, what would be wrong with changing this language in Section 5.1 to document the ambiguity instead of trying (probably unsuccessfully) to prescibe a single way of resolving it? Thus, I suggest: " ... " If no MX records are " found, but an address RR (i.e., either an IPv4 A RR or an IPv6 AAAA " RR, or their successors) is found, the address RR " is treated as if it ^^ s/is/MAY be/ " was associated with an implicit MX RR, with a preference of 0, " pointing to that host. I think that is a much better description of actual practice. > Given that, the document can't win. The choices are between > ambiguity (which I hope is the lowest preference for all of us) > and picking an option that some people won't like. I do not share John Klensin's distaste for ambiguity in evolving standards. Were we starting afresh, I would put a lot of effort into clarity, but when progressing a decades-old animal like SMTP to Draft Standard, I think compatibility rates higher (and that we're only fooling ourselves to claim that all existing implementations will be changed to reflect what we write). Thus, I believe we'll be better served leaving the choice of whether to infer a non-existent MX record to implementors and administrators. As currently practiced, this implied-MX rule leads to temporary errors (not reported for many hours) on typos, instead of immediate permanent errors. Giving the immediate error, IMHO, should be an option available to administrators. > Even without the anti-spam argument that you raise, I think > there are mail performance and DNS reasons (one cited by Mark) > for declaring the utility of implicit MX records, even those > built on A RRs, to be at an end. I quite agree. (But I don't think 2821-bis can go there.) > ... I'd recommend the BCP path I comments on in an earlier note. To tell truth, I dislike writing I-Ds. But I'd be willing to help in the writing of such a document. Any other volunteers? -- John Leslie <john@xxxxxxx> _______________________________________________ IETF mailing list IETF@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf