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

["Ferino Mardo" <RMardo () ALJOMAIHBEV 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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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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