In message <4B284376.3000800@mail-abuse.org>, Douglas Otis writes:
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
Which will expand to: example.com. IN MX 0 192.0.2.0.example.com. IN MX 10 192.0.2.1.example.com. .... IN MX 90 192.0.2.9.example.com. MX records DO NOT take IP addresses.
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
-- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org