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Re: RFC 974 - Mail Routing and the Domain System

  • From: Tim Wolfe
  • Date: Tue Sep 15 17:17:30 1998

On Tue, 15 Sep 1998, Studded wrote:

> Michael J. Maravillo wrote:
> > domain.com. MX 10 mx1.domain.com.
> > domain.com. MX 10 mx2.domain.com.
> > domain.com. MX 10 mx3.domain.com.
> > ;...
> > mx1.domain.com. IN A 10.0.0.1
> > mx2.domain.com. IN A 10.0.0.2
> > mx3.domain.com. IN A 10.0.0.3
> > 
> > and:
> > 
> > domain.com. MX 10 mx.domain.com.
> > ;...
> > mx.domain.com. IN A 10.0.0.1
> > mx.domain.com. IN A 10.0.0.2
> > mx.domain.com. IN A 10.0.0.3
> > 
> > >From RFC 974:
> > 
> > "...Note that multiple MXs may have the same preference value.  In
> > this case, all MXs at with a given value must be tried before any of a
> > higher value are tried..."
> > 
> > Is the behaviour the same with the round-robined configuration?
> 
> 	My imperfect understanding of how most mail agents handle MX records is
> that with the first configuration, mx1 will always be tried first,
> followed by mx2, etc. With the second configuration, a BIND server will
> return the A addresses in an effectively random pattern, so load should
> be distributed across the 3 servers. 
> 
> 	For future reference you are better off asking at bind-users@vix.com,
> which seems to be back up and running. 

One small detail that you seem to have left out.  If you use solution number
2 I believe the following will happen.

Assuming for whatever reason 10.0.0.2 goes down.  SMTP1 attempts to send
mail to domain.com  Domain.com's DNS server says that all mail goes to
mx.domain.com and says that mx.domain.com points to 10.0.0.2.  At this
point, SMTP1 can not send any mail to domain.com until cached zone for
domain.com expires on SMTP1 at which point, SMTP1 will requery ns.domain.com
and may or may not get a different address for mx.domain.com

I'm not sure as to the first scenario, but I believe from my own experience
sending mail to domains with multiple mail handlers (ie mx1.domain.com
mx2.domain.com etc.) that it will send to different servers each time it
hits them.  Once again, I do NOT know this to be true, this is just what I
seem to have experienced.

Also, I think this type of thing would be better posted on
list@inet-access.net not a bind list since it relates to Sendmail operation
as well as bind..

Tim

----------------------------------------------------
Timothy M. Wolfe       | Why surf when you can Sail? 
tim@clipper.net	       | Join Oregon's Premier
Sr. Network Engineer   | Wireless Internet Provider!
ClipperNet Corporation | http://www.clipper.net/
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