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Re: RFC 2821 Address Resolution

2004-01-05 11:37:10

Hector Santos wrote:
In regards to RFC2821, section 5, saying:
... There seems to be a conflict between this and the following itemized rule in
      5.3.4  Reliable Mail Transmission
         (1)  Multiple MX Records -- these contain a preference
              indication that should be used in sorting.  If there are
              multiple destinations with the same preference and there
              is no clear reason to favor one (e.g., by address
              preference), then the sender-SMTP SHOULD pick one at
              random to spread the load across multiple mail exchanges
              for a specific organization; note that this is a
              refinement of the procedure in [DNS:3].

RFC2821 seems to suggest to include all equal preference MX records, while
RFC1123 seems to suggest, unless I am reading it wrong, to choose 1 (at
random) of the equal preference MX records.

I think RFC 1123 is ambiguous here. You are right that it says to choose one. But note that it does not say what to do next -- if the first one picked fails -- and that is the question. Your interpretation can follow from what it says.

One possible interpretation (which I believe is yours) is:
1. to skip over all the remaining destinations with the same preference (that is to pick one -- and only one -- destination with this preference).

But other interpretations can also follow, such as:
2. to pick another one at random from the remaining destinations with the same preference as the first one that failed (that is to pick first one at random, and then another one at random if the first fails).


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