Re: DNS question, null MX records

[Douglas Otis <dotis () mail-abuse org>]

On 12/15/09 8:06 AM, Andy Davidson wrote:
> Eric J Esslinger wrote:
> > I have a domain that exists solely to cname A records to another domain's websites.
> [...]
> > I found a reference to a null MX proposal, constructed so:
> > example.com    IN    MX 0 .
> [...]
> > Question: Is this a valid dns construct or did the proposal die?
>
> It's "valid", but you will probably find people still try to spam to
> machines on the A records, and all of the other weird and wonderful things
> that spambots try to do to find a path that will deliver mail...

SRV records documented the hostname "." as representing "no service". 
However, errors made by non-RFC-compliant clients still generate a fair 
amount of root traffic attempting to resolve A records for ".".  The MX 
record never defined a hostname "." to mean "no service" so it would be 
unwise to expect email clients will interpret this as a special case 
meaning "no service" as well.  One might instead consider using:

example.com.    IN MX 0 192.0.2.0
                IN MX 10 192.0.2.1
                ...
                IN MX 90 192.0.2.9

where 192.0.2.0/24 represents a TEST-NET block.
                
This should ensure traffic will not hit the roots or your servers. 
Assuming a sender tries all of MX addresses listed, they may still 
attempt to resolve A records for example.com.  This MX approach will 
affect those failing to validate email prior to acceptance, and, of 
course, spammers.

-Doug