My mail servers return 5xx on NXDOMAIN. If my little shop can spend not too much money for three-9s reliability in the DNS servers, other shops can as well. When I first deployed the system, the overwhelming majority of the rejects were from otherwise known spam locations (looking at Spamhaus, Spamcop, and a couple of other well-known DNSBLs). The number of false positives were so small that whitelisting was easy and simple to maintain. If a shop is not multihomed, they can contract with one or more DNS hosts to provide high-availability DNS, particularly for their in-addr.arpa zones. It's not hard. Nor expensive. Paul Ferguson wrote:
Re-sending due to Merit's minor outage.
- ferg
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- -- Robert Blayzor <rblayzor@inoc.net> wrote:
The fact that they're rejecting on a 5xx error based on no DNS PTR is a=
bit harsh. While I'm all for requiring all hosts to have valid PTR records, there are times when transient or problem servers can cause a DNS lookup failure or miss, etc. If anything they should be returning a=
4xx to have the remote host"try again later".
Oh, wait till you realize that some of the HTTP returns are bogus altogether -- and actually still serve malware.
It's pretty rampant right now. :-/
- - ferg
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-- "Fergie", a.k.a. Paul Ferguson Engineering Architecture for the Internet fergdawg(at)netzero.net ferg's tech blog: http://fergdawg.blogspot.com/