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Anti-SPAM Techniques: Black Listing (RBL) |
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Anti-SPAM Techniques: Black Listing (RBL) |
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This article is a part of the series on undesired email (spam, phishing, viruses, etc.). The material covers the Poisons and the Remedies.
By Stas Bekman.
Another approach to undesired email filtering is to use black listings, known as RBL (Realtime Blackhole List). It's maintained by system administrators who, using various spam detection tools, report bad-behaving IP addresses (e.g. open relays or hosts that were detected to spend undesired email, have no registered DNS record, etc.). This information goes into a central database, and is then shared by those who want to use it. So rather than trying to filter each email separately, here all email coming from a blacklisted IP is rejected as soon as the connection is established.
There are many RBLs available. Some are more aggressive (blocking whole net blocks), whereas others are more flexible. One way to deal with false-positives here is to try to query several RBLs and then make a decision based on whether they all agree or not.
Most RBLs are free for moderate usage, however if you issue too many queries in a short period of time you may get throttled by the service provider. If you pay them, they will provide you an unlimited access.
This is a very good approach, as it requires almost no resources from the receiving system, since the rejection happens before any data is received. The main problem is that sometimes a legitimate IP is reported and legitimate traffic can't make it through. Usually the reason for this is that someone has sent SPAM mail from that IP hurting all other users who also use that domain.
Here are some vendors providing Black Lists (including open-source solutions):
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MAPS Relay Spam
Stopper (RSS) Spamhaus The SpamCop Blocking List
(SCBL) NJABL.ORG is Not Just
Another Bogus List Composite Blocking List
(CBL) |
Please notify me if you know of others.
Here are some vendors providing support for Black Lists (including open-source solutions):
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Kaspersky
Internet Security SpamAssassin MailChannels'
TrafficControl SpamPal |
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Please notify me if you know of others.
And here are some pointers for additional information on the subject:
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Blacklists
Compared Dotcomeon.com Spam blocking
on DNS blacklist criteria alone Can
DNS-Based Blacklists Keep Up with Bots? |
Continue reading about other Remedies or jump to the email-related Poisons section.
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