The view from a blacklist operator

We run top-level DNS servers for several blacklists including the CBL, the blacklist of infected machines that the SpamHaus XBL is based on. We don’t run the CBL blacklist itself (so we aren’t the right people to contact about a CBL listing) we just run some of the DNS servers – but that means that we do get to see how many different ways people mess up their spam filter configurations.
This is what a valid CBL query looks like:

  • “14.23.177.10.cbl.abuseat.org”

It’s just the IP address being queried (10.177.23.14) with the numbers reversed, with “.cbl.abuseat.org” added on the end. Not rocket science.
Here’s a tiny sample of some of the invalid queries:

  • “70.46.6.10.abuseat.org”
  • “202.204.219.10cbl.abuseat.org”
  • “252.94.193.10.ns1-cbl.abuseat.org”
  • “255.190.244.10 cbl.abuseat.org”
  • “166.193.222.10#cbl.abuseat.org”
  • “214.6.224.10.*@cbl.abuseat.org”
  • “212.9.185.10.http://cbl.abuseat.org”
  • “76.207.80.10.bl.abuseat.org”
  • “185.124.73.10.cbb.abuseat.org”
  • “201.54.179.10.cbl-xbl.abuseat.org”
  • “54.191.254.10.opm.abuseat.org”
  • “181.4.133.10.sbl-xbl.abuseat.org”
  • “176.33.165.10.cbl.abuseat.orgcbl.abuseat.org”
  • “101.126.133.10.cbl.abuseat.org:Mail from %IP% refused by blackhole site cbl.abuseat.org”

Those are just 15 of about 1800 different misconfigurations I have on file, just for queries to the CBL. I’ve seen similar things at other domains I host, and I’ve heard of just the same sort of thing from other people who own domains that are similar in some way to a domain used by a blacklist. It’s not unusual.
What happens when someone misconfigures a blacklist lookup in this way? Because of the way DNS based blacklists work the response to any of these invalid queries will be “no, that IP address isn’t listed”. So all these people are attempting to use the CBL to filter out spam and haven’t noticed that it’s never actually stopped any email. And all the time they’re doing this, they’re hammering my DNS servers (and many other peoples) with millions of pointless queries every day.
What can the DNS server operators do about that? Because of the way DNS works, blocking the broken queries will actually increase the amount of traffic they have to deal with by several times. Contacting all the people making the queries and pointing out the problem would be a huge task, and even when I have tracked down contact information and notified people by email I’ve never had a response and the problem has never been fixed.
So the only remaining option is to make the misconfiguration more obvious to the user – by responding to the invalid queries with “yes, that IP address is listed” and hoping that causing them to reject all the mail sent to their users will encourage them to fix their configuration. I check my nameserver statistics every so often and add “poison” entries for the more obvious misconfigurations I find. I did that for a bunch of misconfigurations manually yesterday, which will probably cause a lot of domains to reject a bunch of email they didn’t want to this morning.
There are fairly simple ways to make sure you’re querying a real blacklist – pretty much all of the legitimate blacklists include the IP address “127.0.0.2” as a test entry. You can use that to check that a blacklist is live manually – if the blacklist domain is sbl.spamhaus.org then a dns lookup for “2.0.0.127.sbl.spamhaus.org” should return an answer (typically 127.0.0.2) while a dns lookup for “1.0.0.127.sbl.spamhaus.org” should return “not found” / “NXDOMAIN”. If either of those tests fails, the blacklist is broken in some way, and you shouldn’t use it.
The choice of 127.0.0.2 for the test entry wasn’t arbitrary: 127.0.0.2 is a “local” address that’s always available on machine, though it’s usually never used for anything. But you can use it – if you open a commandline on your mailserver you can run an SMTP transaction by hand (as I discussed yesterday) from 127.0.0.2 using “telnet -b 127.0.0.2 your.hostname 25” (on Linux-ish systems, anyway – some other telnets use “-s” instead of “-b”). That way you can see whether you’re really rejecting based on a blacklists, and what error you’re giving. (It would be nice if every blacklist also had another test entry in 127.* as well as 127.0.0.2, so you could check them individually, but they don’t. Hint to blacklist operators.).
It’s very easy for spam filter authors to check those test entries once a day for each of the blacklists they were configured to use, and to disable the ones that failed. If you’re a postmaster who uses blacklists as part of your spam filter (and you probably should) you should check with the people who provide the filter whether it makes those checks – and if it doesn’t, ask them to add them. That will protect you from misconfigurations, blacklists being shut down, blacklists being abandoned and bought up by domain squatters and all sorts of other things that can cause you to lose a lot of mail.

Basic email delivery using telnet
Spam lawsuits: new and old

Related Posts

Basic email delivery using telnet

Whenever we’re working with someone to diagnose some obscure delivery issue one of the things we usually have them try is to “run a transaction by hand”. Being able to do that is a trick that everyone working with email should be able to do. I was drafting a blog post today and wanted to refer to running a transaction by hand and I realized that we hadn’t actually explained it anywhere. So here we are.
When you’re running a transaction by hand you’re doing everything your mailserver would do to deliver an email, but you’re doing it yourself. That means that you get to see all the responses from the mailserver you’re sending the mail to, and also any delays or errors in much more detail than you can usually get from mailserver delivery logs.
I want to send some email to playingwithtelnet@gmail.com. There are two main steps to doing this – first I need to find out which mailserver I need to talk to to send mail to gmail, then I need to actually send the mail.
To find the mailserver I have to look up the MX record for gmail.com. From a unix / linux / mac command prompt you can do that like this (the bits you type are in orange):

Read More

It doesn't matter what you say

“What should we tell the ISP?” is a frequent question from my customers. The answer is pretty simple. It doesn’t usually matter what you tell the ISP. What matters are your actions.
If a sender is having delivery problems then the solution is not to call the ISP and talk to them about why the sender’s mail should not be delivered to the bulk folder. Instead, the solution is to evaluate the email and the address acquisition process and the list hygiene process. Identify where potential problems are and then resolve those problems.
Typically, the ISPs won’t need to be contacted. The changes to the email will register and delivery will improve. In some cases, particularly when there’s been some major mistake, contacting the ISP and explaining the mistake and what steps have been taken to stop the mistake from happening in the future may help resolve the issue faster. But if nothing has changed, then there’s no reason for the ISP to expect anything to change.
It doesn’t matter what you say. It matters what you do.

Read More

Troubleshooting the simple stuff

I was talking with one of my Barry pals recently and was treated to a rant regarding deliverability experts that can’t manage simple things. We’ve been having an ongoing conversation recently about the utterly stupid and annoying questions some senders ask. Last week, I was ranting about a delivery person asking what “5.7.1. Too many receipts this session” meant. This morning I got an IM.

Read More