MX Records

[Barry Margolin]

In article <dvcj5p$a8a$1 at sf1.isc.org>, Ron Hall <ron.hall at mcgill.ca> 
wrote:

> Kris McElroy wrote:
> >
> > This may be the wrong place to ask this, so if it is just let me know 
> > and I
> > will ask elsewhere. 
> >
> > What is the difference between:
> >
> > 10 mx0.domain.tld
> > 10 mx1.domain.tld
> >
>     The above defines two machines of equal preference in which the 
> first will be used

Why do you say that?  RFC 2821 recommends:

   If there are multiple destinations with the same preference
   and there is no clear reason to favor one (e.g., by recognition of an
   easily-reached address), then the sender-SMTP MUST randomize them to
   spread the load across multiple mail exchangers for a specific
   organization.

> 
>     If you truly want round-robin load-balancing use the second method.

   The destination host (perhaps taken from the preferred MX record) may
   be multihomed, in which case the domain name resolver will return a
   list of alternative IP addresses.  It is the responsibility of the
   domain name resolver interface to have ordered this list by
   decreasing preference if necessary, and SMTP MUST try them in the
   order presented.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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