Which DNSBLs work well?

This is a question I get quite often and it’s a tough one to answer. I don’t really bother with running my own mail system any more, as I’m tired of the headache and happy to leave the server-level spam prevention to somebody else.

And I'm tired of taking other peoples' word for it that a certain blocklist works well or doesn't work well -- I've been burned a number of times by people listing stuff on a blocklist outside of a list's defined charter. It's very frustrating. And lots of people publish stats on how much mail they block with a given list, which is an incomplete measure of whether or not a list is any good. Think about it. If you block all mail, you're going to block all spam. But you're going to block all the rest of your inbound mail, too. And when you block mail with a DNSBL, you don't always have an easy way to tell if that mail was actually wanted or not.

So, I decided to tackle it a bit differently than other folks have. See, I have my own very large spamtrap, and the ability to compare lots of data on the fly.

For this project, I've created two feeds. One is a spam feed, composed of mail received by my many spamtrap addresses, with lots of questionable mail and obvious non-spam weeded out. I then created a non-spam feed. In this “hamtrap” I am directed solicited mail that I signed up for from over 400 senders, big and small. Now, I just have to sit back, watch the mail roll in, and watch the data roll up.

For the past week or so, I’ve been checking every piece of mail received at either the spamtrap or hamtrap against a bunch of different blocklists. I wrote software to ensure that the message is checked within a few minutes of receipt, a necessary step to gather accurate blocklist “hit” data.

After that first week, here’s what I’ve found. It might be obvious to you, or it might not: Spamhaus is a very accurate blocklist, and some others...aren't. Spamhaus’s “ZEN” blocklist correctly tagged about two-thirds of my spam, and tagged no desired mail incorrectly. Fairly impressive, especially when compared to some other blocklists. SORBS correctly tagged 55% of my spam mail, but got it wrong on the non-spam side of things ten percent of the time. If you think throwing away ten percent of the mail you want is troublesome, how about rejecting a third of desired mail? That’s what happens if you use the Fiveten blocklist. It correctly would block 58% of my spam during the test period, but with a false positive rate of 34%, that would make it unacceptable blocklist to use in any corporate environment where you actually want to receive mail your users asked to receive.

One fairly surprising revelation is that Spamcop’s blocklist is nowhere as bad as I had previously believed it to be. I’ve complained periodically here about how Spamcop’s math is often wrong, how it too often lists confirmed opt-in senders, how it is too aggressive against wanted mail, but...my data (so far) shows a complete lack of false positives. This is a nice change, and it makes me very happy to see. Assuming this trend keeps up, I think you'll see me rewriting and putting disclaimers in front of some of my previous rants on that topic.

NJABL Dynablock List Now Obsolete

With the advent of Spamhaus's new PBL anti-spam blocking list, it appears that the NJABL Dynablock list is now obsolete. I just saw the following post on the public SPAM-L mailing list, from the NJABL folks: The following text was sent to list AT njabl.org on Jan 19, 2007. Judging from the number of DNS queries still being handled for dynablock.njabl.org, the message doesn't seem to have made it to a wide enough audience.

If you use or know people who use dynablock.njabl.org, this is important information:

With the advent of Spamhaus's PBL (http://spamhaus.org/pbl/), dynablock.njabl.org has become obsolete. Rather than maintain separatesimilar DNSBL zones, NJABL will be working with Spamhaus on the PBL. Effective immediately, dynablock.njabl.org exists as a copy of the Spamhaus PBL. After dynablock users have had ample time to update their configurations, the dynablock.njabl.org zone will be emptied.

Other NJABL zones (i.e. dnsbl, combined, bhnc, and the qw versions) will continue, business as usual, except that combined will eventually lose its dynablock component.

If you currently use dynablock.njabl.org we recommend you switch immediately to pbl.spamhaus.org.

If you currently use combined.njabl.org, we recommend you add pbl.spamhaus.org to the list of DNSBLs you use.

You may also want to consider using zen.spamhaus.org, which is a combination zone consisting of Spamhaus's SBL, XBL, and PBL zones.

(Editor's note: I'm very happy with ZEN so far. See this post detailing my recent experiences.)

Spamcop Roundup

5/22/2007: This information is out of date. Please click here for my latest take on Spamcop's SCBL.

My most recent take on Spamcop, from February 2007, can be found here. In that commentary, I talk about the history of the Spamcop spam reporting service, its current corporate ownership, and my take on how this type of DNSBL works, especially as to how it relates to to the impact against solicited (wanted) mail.

In February 2007, I found that Microsoft is using Spamcop to filter inbound (corporate) mail. By corporate mail, I mean mail to microsoft.com users, not mail to MSN/Hotmail users. This surprised me, because of what I believe are aggressive listing practices on the part of Spamcop. Indeed, how the issue was brought to my attention was by an unhappy person mad because he couldn't send one-to-one mail to Microsoft, because Spamcop blocked it.

Also, back in 2003, I published an article about the ongoing issues I was having with Spamcop blocking opt-in confirmation requests. Back then I found (through some admittedly unscientific survey techniques) that admins using the SCBL seemed to assume that all blocked mail must be spam because Spamcop blocked it. Not a very encouraging find. It was also a bit insulting to be lectured on how confirmed opt-in worked by people who were blocking confirmed opt-in requests, especially considering I've been pushing senders to implement and utilize confirmed opt-in/double opt-in for many years.

Spamhaus ZEN: Recommended

Look for a longer article from me in the near future on Spamhaus; I'm collecting a ton of data against a large spam corpus and hope to summarize and publish my findings within the next month or so.

Until then, feel free to bop on over to Spam Resource, where I talk about my experience using the Spamhaus ZEN list to tag and filter inbound mail to our abuse desk. I've been quite pleased with the results.

Also of note is that Microsoft is using both Spamcop and Spamhaus to reject mail to their corporate users. (They're NOT using it on MSN Hotmail.)

Update: Find my full review of Spamhaus ZEN here on DNSBL Resource.

DCC: Spam filter?

The Distributed Checksum Clearinghouse (DCC), created by Vernon Schryver, is a very powerful tool to help system administrators identify and block bulk mail. The project's website suggests a strong correlation between "bulk" and "spam," but as I do a bit more research, I don't think it's always that simple.

There's a common misconception in the spam filtering world (and the sending world) -- people think DCC is a spam blocking list. It's not, though. It's a tool to help users block bulk mail, not spam mail. That's an important distinction.

Think about it. There are a lot of types of bulk mail you might have signed up for and might want, things like newsletters you actually subscribed to, messages from companies you've done business with and actually want to hear from, or news, weather and traffic alerts you might be waiting for. (I don't need an email message to warn me that it's snowing outside, but I know that lots of people sign up for these.)

DCC tells you whether or not the mail attempting to be delivered was sent to lots of people besides you. Sure, spam is sent to lots of people all at once, but so is a bunch of solicited mail. What defines spam is whether or not you signed up to receive it. If you signed up to receive it, whether or not other people are getting it too has no bearing on the fact that you asked for it.

If a filter like DCC rejects a piece of mail you actually solicited and wished to receive, I would consider that a "false positive." To help prevent false positives, proper DCC usage dictates that you whitelist, ahead of time, all the sources of legitimate list or bulk mail you wish to receive. They include this sample file to get started, and they recommend this whitelist of example small messages that are most likely to be caught up in the filtering, even if solicited.

As Vernon Schryver himself said on the DCC mailing list recently, false positives "speak to a misuse or misunderstanding of [DCC]." He says that in a sense, there's no such thing as a DCC false positive. My interpretation of his comments is that he means that it's up to users of DCC to know what they're getting in to. DCC blocks mail sent to multiple recipients, and it's up to you to whitelist any mail sources you want to receive mail from.

DCC is a very powerful tool. That's both a plus and a minus. If you know what you're doing, comfortable working without a safety net, manually compiling lists of sites you want to receive any sort of bulk or list mail from, then maybe it can work for you to help reduce spam.

But, if you're not clear on the difference between bulk and spam, are not clear on what sites are sending you bulk or list mail that you or your users will want, then it's not going to work the way you think, and it's going to reject mail that you or your users asked for.

Internet Service Providers (ISPs), when deciding whether or not to accept a sender's mail, do measure whether or not your message is being sent to multiple people. It's not the only thing they look at, though. The smarter ISPs tie in a reputation measurement to that process. Meaning, is this mail coming from a good sender, or a bad sender? Does this sender generate spam complaints? Does this sender generate an above average percentage of bounces? Wrap that all up together, and an ISP has good info available to them to decide what mail to accept. Don't measure any of those things, and you're left with an incomplete view -- no easy way to tell the good mail from the bad. It's up to you to know about and whitelist the good senders ahead of time. If you don't, you're going to reject mail from them, presumably mail that you or your users wanted to receive.

Spamcop BL: A blacklist with a hair trigger

The Spamcop Blocking List (SCBL) is a DNSBL populated with data obtained from spamtrap hits and spam reports from users of the popular Spamcop spam reporting service. The Spamcop spam reporting service was originally created by Julian Haight. It was later purchased by Ironport Systems. Ironport has since been purchased by networking and communications technology company Cisco. (In spite of this transition to corporate ownership, the Spamcop site's front page contains a prominent legal defense fund link, and contains further information on the fund in the Spamcop FAQ.) 

Unlike the more privately-run CBL, which is designed to minimize the impact on legitimate mail, the SCBL regularly blocks sources of mail that some feel are legitimate. It has been described as having a "hair trigger" by respected anti-spam and internet guru John Levine, and I related some of the issues I've had with Spamcop from 2003 over here on spamresource.com. In fact, back around that time, the SCBL information page said this regarding using the list: "This blocking list is somewhat experimental and should not be used in a production environment where legitimate email must be delivered." As I look at the same page today, in February, 2006, I can see that guidance has since been modified somewhat. Spamcop now recommends "use of the SCBL in concert with an actively maintained whitelist of wanted email senders. SpamCop encourages SCBL users to tag and divert email, rather than block it outright." Both then and now, they go on to add, "The SCBL is aggressive and often errs on the side of blocking mail." 

Translated: "Don't block mail with this blocking list, it will block mail you want." 

Like ISP feedback loops, the spam complaints lodged by Spamcop users are sometimes found to be erroneous. That's not to say that where there's smoke, there's never a fire. But just like with feedback loop reports, significant spam issues generate far more reports than than the day-to-day noise of people lodging spam reports about email from a company they previously did business with, or otherwise had a potentially legitimate reason to be contacted by a given sender. (As an example, I noted my issues with confirmed opt-in/double opt-in systems being listed by Spamcop in 2003; I don't believe I'm the only one to ever have observed that kind of issue.) My experience with Spamcop has taught me that it's not always that good at drawing the line between blocking spam and blocking wanted mail. 

Spamcop's probably really good at blocking spam-in-progress from infected servers spewing illegal spam. (Though, the CBL isn't too shabby at that, either.) The problem is, Spamcop will block mail in a number of edge cases, like if an email service provider is tasked with serving mail on behalf of some e-commerce or travel site. If you want to ensure that you're always going to receive your follow up emails from the department store you ordered that purse from, or the hotel reservation from a booking site that outsources their confirmation email, choosing to outright block mail from servers listed on the SCBL may not be your best choice.

Status of block.blars.org: DEAD

The “Blars” DNSBL (block.blars.org) appears to have gone on walkabout.

Created in 2002, the “Blars Block List” was an aggressive, semi-private blocking list run by a gentleman known to the greater internet community only by the pseudonym of “Blars.”

The "BlarsBL" had a broad criteria for listing. This included spam sending domains, open relays, sites with disagreeable spam reporting policies, sites lacking abuse addresses, those who host spammer dropboxes or websites, those who have threatened Blars or others with legal action, and sites originating break-in attempts and other exploits (open proxy, open relay, etc.).

The list has been criticized for implying that payment was required for removal. From the site: "If you would like a site be added or removed from BlarsBL, you may hire Blars at his normal consulting rates (currently $250/hour, 2 hour minimum, $1000 deposit due in advance for non-established customers) to investigate your evidence about the site. If it is found that the entry was a mistake, no charge will be made and the entire deposit will be refunded."

The list appears to be no more. The websites www.blars.org and block.blars.org both resolve to a “This domain is parked free with GoDaddy” placeholder page.

Note: I confirmed today that all lookups against block.blars.org DSNBL will result in a match. This is the “Osirusoft solution,” also known as “listing the whole world.” Intentional or not, this means that if you continue to use this blocking list, you will receive no incoming mail whatsoever. If you are using this list to reject mail, I recommend you cease doing so immediately. It will block all of your inbound mail. See this page at MXToolbox.com further confirmation of BLARS mysterious disappearance. This post from the newsgroup news.admin.net-abuse.email indicates that it has likely been out of operation since approximately December 18, 2006.

Status of relays.ordb.org: DEAD

Created by Thomas Jensen in 2001, the Open Relay Database (ORDB) was one of the multitude of open relay spam blocking lists to come about in the wake of the legal troubles of Alan Brown and his New Zealand-based ORBS DNSBL.

The ORDB service ceased operation on December 18, 2006. The website was retired on December 31, 2006.

The website indicated that blocking open relays is no longer as effective as it once was.

"It's been a case of a long goodbye as very little work has gone into maintaining ORDB for a while. Our volunteer staff has been pre-occupied with other aspects of their lives. In addition, the general consensus within the team is that open relay RBLs are no longer the most effective way of preventing spam from entering your network as spammers have changed tactics in recent years, as have the anti-spam community.”

If you have checks against relays.ordb.org configured in your mail server or spam filtering software, please stop querying the list immediately. Use of the list will no longer block any unwanted spam, and the nameservers listed in the domain registration are likely overwhelmed with traffic. This is especially heightened due to the fact that the list was in wide, popular use, and also that it was so recently retired.

3/26/08 Update: ORDB has "listed the entire world" -- returning any query with a "listed" response. The result is that if you still have ORDB in your mail server config files, you're now blocking 100% of your inbound mail. For anyone still trying to "use" ORDB, you're not going to receive any inbound mail until you disable queries to it.

Status of opm.blitzed.org: DEAD

The primary project of the “Blitzed” group is the Blitzed Internet Relay Chat (IRC) network.

They also operated a DNSBL zone called opm.blitzed.org. This was the Blitzed Open Proxy Monitor (OPM). This popular open proxy DNSBL was run in such a way as to not probe a remote server to determine its open proxy status unless the server was implicated in reports of abuse. It did not list open relays.

The Blitzed group seems to have suffered a database or server failure as of May, 2006. This email to the “OPM Announce” mailing list details the situation, and explains that the OPM list would not be resurrected.

The list is not active at this time.

Based on this information, I would recommend that you remove opm.blitzed.org from the list of DNSBLs being checked in your mail server. It will no longer block any spam, and the potential exists for unpredictable results to be returned. Additionally, you'll be generating unnecessary DNS query traffic to the Blitzed network.