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Re: The "such responses" extension sentence (was: Re: MX to CNAME and (mis)interpretation of 2821)

2008-02-25 08:42:09

At 12:42 24-02-2008, John C Klensin wrote:
The current sentence reads

   "Such responses could, in principle, be covered by an SMTP
   extension in the future."

Since there is a normative DNS prohibition on CNAME targets (in RFC2181), that sentence is, strictly speaking, untrue. It could be corrected by saying:

   "Such responses could, in principle, be covered by DNS
   changes or extensions and an SMTP extension in the future."

Or we could drop it. My instinct at the moment (i.e., I reserve the right to change my

I suggest dropping that line as taking about DNS changes and SMTP extensions makes the it convoluted. Suggested text is as follows:

  When a domain name associated with an MX RR is looked up and
  the associated data field obtained, the data field of that
  response MUST contain a fully-qualified domain name.  That
  fully-qualified domain name, when queried, MUST return at
  least one address record (e.g., A or AAAA RR) that gives the
  IP address of the SMTP server to which the message should
  be directed.  Any other response,  specifically including a
  value that will return a CNAME record when queried, lies
  outside the scope of this standard.  The prohibition on CNAME
  RRs is discussed in more detail in RFC 2181, Section 10.3 [29].