Oct 10 2011

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that looks like a scam to me

The volume of spam backscatter I am receiving at the baldric.net domain currently runs at around 18-20,000 emails per month, nearly all of which is aimed at the info@ address I have mentioned before.

My mail server is currently configured to reject mail to non-existent users at the SMTP level with a permanent failure message like so: “550 5.1.1 : Recipient address rejected: User unknown in virtual mailbox table:” Rejecting mail at this stage, rather than accepting it only to bounce it later is the “correct thing to do”. This way the sending MTA gets a failure message in its logs and the mail it was attempting to send to me never leaves its queue. If the mail admin at the other end is in any way savvy, then he or she is given enough information to allow them to investigate and, perhaps, cure the problem. But of course that assumes two things: one she /is/ savvy; and two, she actually cares enough to do anything.

Now there is nothing I can do about the second problem, but if there is any way I can provide additional information which might help the hard pressed admin understand why they might have a problem, then that would aid them, me, and any of the likely hundreds or thousands of other people out there who will be receiving crud in response to mails they didn’t send.

One possible way forward might be to add some additional information to the SMTP rejection message – something along the lines of “hey, you might have a configuration problem here, please consider investigating”. Now I dislike re-inventing wheels (and I’m lazy) so I spent a short while searching for possible modifications to my own postfix configuration which would do the trick. Sure enough, I quickly discovered backscatterer.org and its suggested modification to main.cf (though note that it assumes that postfix is using the dbm database library – not all of them do, particularly on the default debian install). Hey, that looks cool, so if I modify my configuration slightly I will be able to run a lookup against backscatterer’s DNSRBL and in cases of a hit I will send an SMTP reject message that looks like this: “554 5.7.1 Service unavailable; Client host [] blocked using ips.backscatterer.org; Sorry is blacklisted at http://www.backscatterer.org/?ip=;” instead of the much less informative message above. Now the sysadmin at mx2.infopac.ru ( will get a much more useful log message. Won’t they?

But hold on a moment, where does backscatterer.org get its RBL? Can I trust it? And am I being fair on the sending domain if I block all mail coming from there based on the simple fact that they are listed in some third party RBL? That feels a little like SORBS to me. Turn the question around. Would I, as admin for the baldric.net domain (and a dozen others) be happy if mail from my domain to some servers were blocked because I had chosen to implement something like “sender callouts” (unlikely as that might be). Worse, backscatterer.org “offers” to de-list any server from its database if you pay them 85 euros (OK, so that will only be about threepence halfpenny in a few weeks time when the eurozone finally tanks, but it is still extortion, whatever the actual sum).

So I think I’ll stay away from backscatterer – it looks like a scam to me. I’ll just have to find another way of telling my Russian sysadmin friends that their servers are “misconfigured”.

Permanent link to this article: http://baldric.net/2011/10/10/that-looks-like-a-scam-to-me/