DNS Blocklists
DNS Blocklists are a common form of network-accessible database used in spam detection. They're also referred to as "DNSBLs", "DNS Blacklists" and "RBLs". (The latter usage is incorrect; see ["RBL"].)
SpamAssassin includes support for many of the bigger DNSBLs, with optimal scores (or at least, optimal as determined by the GeneticAlgorithm).
Support for these is built-in, and shipped in the default configuration.
All of the free BL services are enabled by default. However, note that if you're using mail-abuse.org, and you're installing SpamAssassin for site-wide use, you must license their services; mail-abuse.org is now only free for personal use.
Note that SpamAssassin still works very well without using these two services, however.
Live accuracy figures for most of the DNSBLs used in SpamAssassin, based on the Oct 2003 mail feed for one user, can be found [http://taint.org/2003/11/07/184247a_mail.html here].
Other places to find out about DNS blacklists / blocklists:
- Dr. Jørgen Mash's DNS database list checker http://moensted.dk/spam/
- List of All Known DNS-based Spam Databases (blacklists) http://www.declude.com/junkmail/support/ip4r.htm
- Current Blacklist Comparison (note: no statistics for false positive rates) http://www.sdsc.edu/~jeff/spam/cbc.html
Note that it's extremely important to compare false positive rates (nonspam messages marked as spam), as well as spam hit-rates, when evaluating any anti-spam system, include DNS blocklists. (For example, a blocklist that returned a match for every single mail would 'catch all the spam', but would also mark every nonspam mail too.) Some of the above pages omit this information, so take with a pinch of salt.
[http://www.blocklisting.com/faq.html news.admin.net-abuse.blocklisting] is a newsgroup devoted to discussion of subjects related to the use, administration, and effects of blocklists in ameliorating the problem of unsolicited bulk email and other unwanted or abusive network traffic. It is also accessible through [http://groups.google.com/groups?group=news.admin.net-abuse.blocklisting groups.google.com].
Comments:
Q: This documentation doesn't seem to cover how to configure dns-blocklists. It says "Support for these is built-in" but I can't believe that all free BL's is called each time a mail is beeing checked. There must be a way to configure which to use.
A: See the Mail::SpamAssassin::Conf documentation page, and the rules file 20_dnsbl_tests.cf , for details and examples of how to add a new DNSBL lookup rule.
Q: The dns-blocklists just don't appear to be used. What is going wrong?
A: First, make sure Net::DNS for perl is installed. Without this the blocklists will not be used.
A: Second, do some tests with Net::DNS to make sure it is resolving names (see the Net::DNS site for examples). A common mistake for client machines (such as Mandrake 9.2) is to have 127.0.0.1 in the </etc/resolv.conf> file – Net::DNS does not check multiple nameservers it appears, so you need to comment this line out for Net::DNS to work. (Anybody with a better solution, other than running a local nameserver?)
Q: Does anybody know of a good way to use the cluecentral.net country lists? I'd like to penalize certain countries from which I get a lot of spam and almost no real mail. I can't seem to get it working with multiple countries: http://www.cluecentral.net/rbl/showcountries.php