DNSBL Safety Report 1/23/2011

UPDATE: See the latest DNSBL Safety Report for current recommendations.

SpamTips.org occasionally looks at the results of Spamassassin's nightly masscheck at RuleQA in order to analyze the performance and safety of add-on DNSBL's.  It is vitally important to know how a DNSBL is performing before deciding if it is a good idea to use it.  Many of the below DNSBL's were tested because they indicated strong performance in other comparisons.  Our analysis demonstrates that raw detection numbers can be misleading, as ham safety ratings and overlaps with other rules must be taken into consideration.

Today's report examines Hostkarma, SpamEatingMonkey, Tiopan, UCEProtect, Mailspike, and Nix Spam and Lashback UBL.  Recommended scores below are what I personally use in production.

Spamassassin 3.2.x is Unsupported

Upstream has not made any official announcement yet, but it is apparent that continuing to use spamassassin 3.2.x is a bad idea and you should really upgrade to 3.3.x.  Why?

• Rule updates for 3.2.x effectively stopped late 2008.  The last time an update was pushed was January 1st, 2010 only for the year 2010 bug.
• Thousands of other bugs were fixed, and 3.3.x has far more effective rules than 3.2.x.  3.3.x continues to receive regular rule updates via its sa-update channel.
• There is no intent upstream to fix serious problems like RCVD_ILLEGAL_IP in spamassassin-3.2.x.

RHEL5/CentOS5 users may be interested in the custom RPM's that I personally use on production servers.  These builds are essentially identical to the RHEL6 version, but built and tested on RHEL5.

If you are forced to use Spamassassin 3.2.x for some reason, then here are all custom rules that I recommend for your local.cf.  Please be sure that you have run sa-update at least once to get the last official rule updates.

# Disable Broken Rules
score    RCVD_ILLEGAL_IP 0

# SpamTips.org approved DNSBL's

header   RCVD_IN_PSBL eval:check_rbl('psbl-lastexternal', 'psbl.surriel.com.')
describe RCVD_IN_PSBL Received via a relay in PSBL
score    RCVD_IN_PSBL 2.3
header   RCVD_IN_MSPIKE_BL eval:check_rbl('mspike-lastexternal', 'bl.mailspike.net.')
score    RCVD_IN_MSPIKE_BL 2.1

Disable rfc-ignorant.org Rules

rfc-ignorant.org is helpful to detect 0.000008% of spam, and does slightly more harm than good.  While the effect is negligibly rare, you might as well disable the rule entirely to avoid a useless DNS query per e-mail scanned.

# Add these lines to your local.cf then restart your spamd
score __RFC_IGNORANT_ENVFROM    0
score DNS_FROM_RFC_DSN          0
score DNS_FROM_RFC_BOGUSMX      0
score __DNS_FROM_RFC_POST       0
score __DNS_FROM_RFC_ABUSE      0
score __DNS_FROM_RFC_WHOIS      0

DNSWL - Please List your Mail Server

If you operate a legitimate mail server, please file a request to have your IP address listed at DNSWL.org.  If your buddy's MTA IP address is not listed, suggest that they get themselves listed.  DNSWL is useful for multiple purposes like:

• Spamassassin adds a negative score if the sending IP address is listed in DNSWL.  This can be both good and bad in different ways, but recent measures indicate that it makes almost no difference to spamassassin's determination.  This is because spamassassin is carefully balanced, and DNSWL is rarely wrong.  If you do see cases where DNSWL is wrong, please report it.
• Some major servers use DNSWL as a means to avoid greylisting for "known good" IP addresses.  This eliminates delays during delivery of mail from your server.
• Some DNSBL's use DNSWL as additional input in their reputation decision.  Not exactly a "stay out of DNSBL" pass but it does help, assuming you really are not sending spam.

DNSBL Safety Report 1/8/2011

UPDATE: See the latest DNSBL Safety Report for current recommendations. 

Here is a quick look at the safety and efficacy of a few DNSBL's for SpamAssassin.  Today's report looks into Hostkarma, Spam Eating Monkey, MailSpike, NiX Spam and PSBL.
NEW: This week's analysis looks closer at safety when taking into consideration overlaps with established rules.  See last week's analysis for more details about the masscheck process used to collect the weekly statistical data in RuleQA.

Usage Limits of Spamassassin Network Tests

UPDATED: 1/8/2011

This article describes free usage limits of network test providers used by Spamassassin, along with recommendations if they are worthwhile to pay for service for sites large enough where a data feed is necessary.  Recommendations are based upon statistical data in Spamassassin's weekly masscheck as collected at RuleQA.

It is important for Spamassassin sysadmins to know the limits and usage restrictions of the various network test providers.  If those providers deem that you are abusing their service they might choose to silently block your IP address.  This can cause significant problems like mail delivery slowdown as Spamassassin waits until DNS timeout during each mail scan, along with test failure which can cripple your spam filter.

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Disable DNS_FROM_AHBL_RHSBL

Apparently AHBL_RHSBL has been performing very poorly, detecting 0.072% spam during the August 2009 rescore masscheck and 0.02% spam in recent masschecks.  This is not worth a DNS query for every mail you scan.  Well, this rule is not harmful, but you may want to disable it if you want a little more efficiency.  Insert this line below into your local.cf and restart your spamd daemon.

score DNS_FROM_AHBL_RHSBL 0

DNSBL Safety Report 1/2/2011

UPDATE: See the latest DNSBL Safety Report for current recommendations.

This blog will occasionally look at the weekly DNSBL masscheck statistics.  Our measures indicate that the performance and safety of the smaller DNSBL's can vary wildly from month to month.  If you depend on DNSBL's, you should pay attention to these safety reports in order to protect your users from the likelihood of false positives and losing mail to the spam folder.  This should help you as a SpamAssassin sysadmin to decide which add-on DNSBL's to use, and what score to assign with the goal of maximizing spam filter safety.

Here is a quick look at the safety and efficacy of a few add-on and existing DNSBL's for SpamAssassin.  Today's report looks into Hostkarma, Spam Eating Monkey, Tiopan, MailSpike, NiX Spam and PSBL.