DNS question, null MX records

Mark Andrews marka at isc.org
Thu Dec 17 23:44:39 CST 2009

In message <6eb799ab0912172126g1eac7e49ve8f803552f6dbd82 at mail.gmail.com>, James
 Hess writes:
> On Thu, Dec 17, 2009 at 6:54 AM, Tony Finch <dot at dotat.at> wrote:
> > On Wed, 16 Dec 2009, Douglas Otis wrote:  > more polite to use a nonexisten
> t name that you control, but that doesn't   allow the source MTA to skip furt
> her DNS lookups
> If you want to be kind,  point the MX to an  A record that resolves to
> Common MX'es should immediately reject, and report a "configuration
> error"/MX loop with the domain.
> Your intent will also be clear, to just about everyone,  it will be
> obvious the MX is intentionally broken.  Other tricks may be more
> obscure,  will be less obvious  that you don't want mail, and may look
> like a mistake  --  you might even get visitors to your domain
> contacting you  to report the broken MX record.
> An alternative to resolving MX to an invalid IP might be to cut to the
> chase and just  make further  DNS lookups impossible altogether...
> @                604800 IN MX                   MX.BOGUSMX
> BOGUSNS  604800 IN  A            
> BOGUSMX 604800  IN  NS                   BOGUSNS
> Or  for that matter  delegate the subdomain to
> The recursive resolvers  already have to immediately reject DNS
> delegation to broadcast addresses and the like.
> Though  i'd be afraid of finding that some obscure resolver didn't......
> [EG] "Gee thanks... some spammer exploited my open relay,  and your
> broadcast NS delegation,  caused  my LAN to get swamped  by my mail
> servers'  DNS lookups while it was trying to send the  10 million
> spams to you...."
> --
> -J

Just document "MX 0 ." and be done with it.  MTA and MUA vendors
will update their products.  Most caching nameserver negatively
cache the non-existance of address records so the traffic is mostly
between the non-updated MTA and the recursive server.

2 queries (A and AAAA) every 3 hours won't kill the roots.


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

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