Security Basics mailing list archives
Re: educating rDNS violators
From: "Chris Olave" <chrisfocus () saber net>
Date: Mon, 23 Aug 2004 20:21:17 -0700
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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Current thread:
- Re: educating rDNS violators, (continued)
- Re: educating rDNS violators Niek (Aug 26)
- Re: educating rDNS violators Derek Schaible (Aug 30)
- Re: educating rDNS violators JGrimshaw (Aug 24)
- Re: educating rDNS violators James Kelly (Aug 25)
- Re: educating rDNS violators Bryan S. Sampsel (Aug 25)
- Re: educating rDNS violators SMiller (Aug 26)
- Re: educating rDNS violators Derek Schaible (Aug 25)
- Re: educating rDNS violators Mark Reis (Aug 28)
- Re: educating rDNS violators Derek Schaible (Aug 30)
- Re: educating rDNS violators Bryan S. Sampsel (Aug 30)
- Re: educating rDNS violators James Kelly (Aug 25)