Re: [anti-spam-wg] Domains with MX set to localhost

  • From: der Mouse mouse@localhost
  • Date: Thu, 12 Jan 2006 16:11:44 -0500 (EST)

>> How is the second one possible?  An MX record isn't capable of
>> containing anything *but* a FQDN.

> In theory ;-)

In practice, too.  Go look up the format of the MX RR; it is not
possible to carry anything else in one.

> I've seen more than once records like
>     example.com	 IN  MX  10  10.0.0.1
> i.e. an IP address instead of a FQDN in the RHS of the record

No, that's a FQDN; it just happens to be in a nonexistent TLD "1".
(And it's probably actually "IN MX 10 10.0.0.1." by the time you query
it, that trailing dot being a tipoff that it's really a FQDN that, when
converted to textual form in the canonical way, happens to look like a
dotted-quad IPv4 address.  I've seen records that look (when queried)
like

	example.com. IN MX 10 10.0.0.1.example.com.

which is a pretty sure indication that a zone file contains it without
the trailing dot.)

> and mailservers even connecting to the correct host :)

That's an indication that the mailservers in question are just plain
busted.  They should be failing for those MX records same as they would
for an MX record naming a domain in a non-all-numeric nonexistent TLD.
(It probably indicates sloppy coding, depending on textual hacks to
tell the difference between a dotted-quad address and a FQDN.  But that
busted software is common should surprise nobody here....)

/~\ The ASCII				der Mouse
\ / Ribbon Campaign
 X  Against HTML	       mouse@localhost
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B