sink.arpa question

Ted Hardie hardie at oakthorn.com
Fri Dec 18 18:25:20 UTC 2009


> 
> I wouldn't call it impossible...difficult, maybe.  Do metrics exist on 
> how many current installs still rely on the implicit MX?  Is the abuse 
> of the implicit MX causing more harm than the effort it would take 
> legacy DNS admins to specify an MX?
> 
> 

If I understand your question, you're asking how many sites don't
bother with an MX record, but count on the fallback to A to get their
mail delivered.  I have to say I don't know and don't know of anyone
who has checked.   I'm not sure that's its even possible to know
without starting with a full knowledge of the mail-sending entities
out there.  Given that many entities allow for subdomain-level
mail address (research.example.com, cs.example.edu), the number of
extant domain names at some level of the hierarchy would only be
a predictor.  Possibly someone with a very large mail installation
could run statistics to show how often they fell back; that wouldn't
be perfect, but it would be somewhat useful.

But I think the key question is actually different.  Look at this
text in RFC 2821:


   If one or more MX RRs are found for a given
   name, SMTP systems MUST NOT utilize any A RRs associated with that
   name unless they are located using the MX RRs; the "implicit MX" rule
   above applies only if there are no MX records present.  If MX records
   are present, but none of them are usable, this situation MUST be
   reported as an error.

If I put in an MX record pointing to a guaranteed non-present 
FQDN, someone complying with that text will throw an error rather than
keep seeking for an A/AAAA.  Is *that* useful?  If so, then
sink.arpa/1.0.0.257.in-addr.arpa as an MX record entry may be.

regards,

Ted Hardie





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