Criminals, The Network, and You [Was: Something Else]
Jason J. W. Williams
williamsjj at digitar.com
Wed Sep 12 16:13:00 UTC 2007
It seems to me reverse DNS just isn't an acceptable anti-spam measure.
Too many broken reverses exist with smaller companies (try getting a 3rd
party to fix it). It's not that hard for a bot to figure out a DSL's
reverse entry and use that for its HELO. And there are a lot more
effective pre-processing anti-spam measures, including greylisting (with
its own problems) and reputation-based systems.
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of
Sent: Wednesday, September 12, 2007 9:55 AM
To: nanog at nanog.org
Subject: Re: Criminals, The Network, and You [Was: Something Else]
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
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 at inoc.net> wrote:
>> The fact that they're rejecting on a 5xx error based on no DNS PTR is
> 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
> 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
> ferg's tech blog: http://fergdawg.blogspot.com/
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