On 5/6/22 10:49 AM, Ted Mittelstaedt wrote:
Arg. Well I think you hit the nail on the head. And I think I may have stumbled on to a spam defeating trick.
Ya ... not running email server on IPv6 is a way of not receiving (some) spam. But I view it very similarly as not running an email server period is another way of not receiving (all) spam.
It's a short term gain that has long term negative repercussions.
The problem for them is that if there is no response from that A record then normal TCPIP stack is going to wait for a while then eventually time out.
What you are describing is the premise behind "No Listing". The two primary ways it's done is to 1) leverage TCP timeouts or 2) leverage a TCP Reset. Either way, you're tickling bugs that alter behavior of spam cannons. Things that proper SMTP servers handle much more gracefully, most without any problem at all.
I did not remove the AAAA records because the IPv6 RFCs require that if the initial connection tried with IPv6 fails you retry with IPv4.
I would encourage you to not have your host's FQDN include a AAAA record if it's not going to be utilizing it. I'd instead suggest adding an alternate name with the (bogus) AAAA record and reference that in MX records.
It is in effect a sort of tarpit I believe.
That's not tar pitting to me. Tar pitting would be answering and replying extremely slowly. Such that you burn a LOT of time for an established and arguably functioning (as in exchanging data) SMTP connection. Like one character per second or every few seconds.
You could extend this to defining multiple nonexistent numbers for an IPv4 host except that DNS does not seem to have a way to force ordering of multiple IPv4s
DNS zone files don't have a way to force ordering. Some DNS servers, BIND in particular, does have the ability to sort records.
Of course, spammers could get around this by recompiling their bots to use only IPv4.
You can apply No Listing to different IPs within a protocol and / or across protocols.
-- Grant. . . . unix || die
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