John C Klensin <[EMAIL PROTECTED]> wrote: > > Suppose one has mail addressed to [EMAIL PROTECTED] Suppose > further that one has > foo.example.com. IN A 10.0.0.1 > IN A 10.0.0.2 > IN AAAA .... > and no MX record. > > Now what 2821bis now says includes "address record" and the old > rule about making an implicit MX record with preference 0 and > then following the rules. So we pretend that we had > foo.example.com. MX 0 foo.example.com. > to complement the above.
Indeed: both 2821 ande 2821bis-09 require exactly that. The _difference_ is that 2821 _requires_ a DNS query for an A RR, while 2821bis-09 _requires_ querying for " " either an IPv4 A RR or an IPv6 AAAA RR, or their successors As I read this, it requires querying for _both_ A and AAAA whether or not the host doing the querying knows about both IPv4 and IPv6. (And it introduces that "or their successors" language, which I find frankly impossible to implement.) > Interestingly enough, in that case, even if the rule were > "implicit MX on A RRs only", exactly the same MX record would be > generated. The only time the "address record" rule that is now > present in 2821bis is an issue is if there is no A RR. Exactly. (I have been hesitating to point this out, for fear of interrupting a perfectly good flame war.) > Once that implicit MX record exists, there are fairly clear > rules about which address to use for foo.example.com. Alas, they're not all that clear to me. :^( > To be a little more precise, while the rules are clear, they > pretty much leave it up to the sending host (That's what I mean by not so clear...) Please excuse me for snipping the rest of John K's wisdom on this diversion. I'd be _very_ happy to discuss it under a different Subject heading. > If a receiving host wants to specify a priority for which of > IPv6 or IPv4 it prefers to receive traffic over, it must have > two names, one for its IPv4 interface(s) and one for its IPv6 > one(s) and then put in an explicit set of MX records that bind > separate priorities to those two names. Or, it might have two MX RRs with the same priority... > So again no issue with whether a host looks for an AAAA record > if it cannot find an MX. In this particular case: yes, substituting the 2821bis language has no effect on the outcome -- the only effect being requiring more DNS traffic. There are, of course, other cases where the different language _would_ lead to different results -- the most obvious being an IPv6-only domain. This whole flame-war seems a lot like many other IPv6 flame-wars I've observed: one side saying that differences between IPv4 and IPv6 should be prohibited, with the other side saying differences should be respected and used. Frankly, I have no use for the "differences prohibited" side (so I don't usually reply). I'd much prefer to be talking about _how_ to best respect the differences. -- John Leslie <[EMAIL PROTECTED]>
