How to do rDNS. WAS: RE: educating rDNS violators

From: Ferino Mardo (RMardo_at_ALJOMAIHBEV.com)
Date: 08/25/04

  • Next message: Erich D. Heintz: "RE: unable to join domain from dmz" 
    Date: Wed, 25 Aug 2004 09:26:16 +0300
To: "Chris Olave" <chrisfocus@saber.net>, <security-basics@securityfocus.com>

    This is a nice read. Just like to ask how does one implement rDNS from
    the mail server? Or is it done from the DNS server?

    > -----Original Message-----
    > From: Chris Olave [mailto:chrisfocus@saber.net]
    > Sent: Tuesday, August 24, 2004 6:21 AM
    > To: SMiller@unimin.com; security-basics@securityfocus.com
    > Subject: Re: educating rDNS violators
    >
    >
    > Our previous mail server setup included refusing all messages
    > coming from non-resolvable IP addresses. We had toyed with
    > the idea of imposing a full DNS check (forward to reverse
    > matching reverse to forward), however we decided that it
    > refused too much potentially-legit mail, we only allowed it
    > for about a half hour.
    >
    > We had the rDNS requirement imposed for about two years and
    > never had a problem with it. Friends and family emailing our
    > customers would get a customized refusal saying "hostname
    > lookup failed"; they seemlessly would forward it on to their
    > provider who would eventually fix the problem. We imposed
    > this restriction and noticed a fair amount of junkmail
    > missing from our inboxes the next morning.
    >
    > Another way to catch a fair amount of spam is to require that
    > the "From:" addresses on messages be MX-able. This will only
    > catch the small bit of spam that hasn't caught up to the rest
    > of the world (using fake domains). Newer spam methods include
    > using a fake address "@yourdomain.com"; you could write a
    > ruleset that will check to see if the "From:" is a valid
    > local user (only if it's a local domain, obviously) and
    > refuse to deliver the message based on the "From:" not being
    > a true local user. This will catch a fair amount of spam as well.
    >
    > Have you looked into using services such as the MAPS RBL, DUL
    > or other lists? We used these for a while and they seem to
    > catch a good amount, but not nearly enough spam.
    >
    > We eventually decided to go with a "middle-man" mail filter.
    > We pointed our MX records to the filter then the filter would
    > forward mail to our SMTP server. Then we had the problem of
    > spammers directly delivering mail to our server (ignoring
    > MX). Then we had to impose restrictions for our mail filter
    > to be "OK" to deliver mail but no one else. We are soon
    > going to be changing our refusal message from "Access denied"
    > to "Please honor our MX records and we'll accept your mail."
    >
    > Our customers have not voiced any kind of displeasure. If
    > they do, we will simply have to tell them the remote end
    > needs to honor our MX records; servers not abiding by it are
    > not abiding by SMTP protocol in which case there's probably a
    > reason they are trying to bypass the filter.
    >
    >
    > Good luck!
    >
    > ----- Original Message -----
    > From: <SMiller@unimin.com>
    > To: <security-basics@securityfocus.com>
    > Sent: Wednesday, August 18, 2004 2:49 PM
    > Subject: educating rDNS violators
    >
    >
    > > Our mail administration group recently implemented blocking of all
    > incoming
    > > messages from domains that cannot be resolved via reverseDNS, for
    > > purposes of spam prevention. Of course, there are quite a
    > number of
    > > legitimate business contacts who do not have rDNS properly
    > configured.
    > > Assuming that the rDNS criterion remains, the question
    > becomes one of
    > > who will notify and/or educate the sender(s) about this issue. The
    > > only time-efficient
    > way
    > > that I can think of to do this would be to have instructions and
    > references
    > > in the body of the bounce message itself. Anyone tried that?
    > > Results? Other suggestions? Thanks in advance.
    > >
    > > Scott
    > >
    > >
    > >
    > ----------------------------------------------------------------------
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    > ---- 

    --
>
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Computer Forensics Training at the InfoSec Institute. All of our class
sizes are guaranteed to be 12 students or less to facilitate one-on-one
interaction with one of our expert instructors. Gain the in-demand
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left behind by fraud, theft, and cybercrime perpetrators. Discover the
source of computer crime and abuse so that it never happens again.
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Computer Forensics Training at the InfoSec Institute. All of our class sizes
are guaranteed to be 12 students or less to facilitate one-on-one
interaction with one of our expert instructors. Gain the in-demand skills of
a certified computer examiner, learn to recover trace data left behind by
fraud, theft, and cybercrime perpetrators. Discover the source of computer
crime and abuse so that it never happens again.
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  • Next message: Erich D. Heintz: "RE: unable to join domain from dmz"

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