[policy] When Tech Meets Policy...

Mark Andrews Mark_Andrews at isc.org
Wed Aug 15 05:22:11 UTC 2007

> On Wed, 2007-08-15 at 11:58 +1000, Mark Andrews wrote:
>  > > Accepting messages from a domain lacking MX records might be risky
>  > > due to the high rate of domain turnovers.  Within a few weeks,
>  > > more than the number of existing domains will have been added and
>  > > deleted by then.  Spammers take advantage of this flux.  SMTP
>  > > server discovery via A records is permitted and should be
>  > > deprecated.
>  >
>  >  All it would require is a couple of large ISP's to adopt
>  >  such a policy.  "MX 0 <self>" really is not hard and benefits
>  >  the remote caches.
> Agreed.  While some suggest deprecating A record discovery requires
> adoption by a standards body, it really only requires a few ISPs to make
> their intentions public.  A small minority of domains lacking an MX
> record are likely to comply quickly.  At that point, adoption by a
> standards body becomes possible.  It is rare to find a standards body
> willing impose additional requirements on email, but this is a case
> where such a requirement is clearly necessary.
> That point forward, spammers would be less able to take advantage
> of domains in flux, and policy schemes would be far less perilous for
> roots or second level domains.
>  > > Once MX records are adopted as an _acceptance_
>  > > requisite, domains not intended to receive or send email would be
>  > > clearly denoted by the absence of MX records.  SMTP policy
>  > > published adjacent to MX records also eliminates a need for email
>  > > policy "discovery" as well.  Another looming problem.
>  >
>  >  Better yet use MX records to signal that you don't want to
>  >  receive email e.g. "MX 0 .".  It has a additional benefits
>  >  in that it is *much* smaller to cache than a negative
>  >  response.  It's also smaller to cache than a A record.
>  >
>  >  Since all valid email domains are required to have a working
>  >  postmaster you can safely drop any email from such domains.
> Use of root "." as a name for a target may create undesired non-cached
> traffic when applications unaware of this convention then attempt to
> resolve an address for servers named root.

	All modern iterative resolvers are required to support
	negative caching.

> The use of root as a convention will complicate a general strategy
> identifying adoption of a protocol by publication of a discovery
> record.  The use of root as a target name in SRV records has been
> problematic, although this convention was defined for SRV records at the
> outset.

> Using an MX record to mean "no email is accepted" by naming the
> target 'root' changes the meaning of the MX record.

	Not really.  It's entirely consistant with existing DNS
	usage where "." is a domain name / hostname place holder.

	Lots of RR types use "." to indicate non-existance.

> It is also not clear
> whether the root target would mean "no email is sent" as well.

	That is, I'll agree, more of a issue but no one can reasonably
	expect people to accept non-repliable email.
> A clearer and safer strategy would be to insist that anyone who cares
> about their email delivery, publish a valid MX record.  Especially when
> the domain is that of a government agency dealing with emergencies.  At
> least FEMA now publishes an MX record.  This requirement should have
> been imposed long ago. : )

	I much prefer positive data vs the absence of data to make a
	decision.  "MX 0 ." is a definative response saying you don't
	want email.

> -Doug
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at isc.org

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