taint.org: Justin Mason’s Weblog

Back in January, I wrote about how I deal with email backscatter nowadays. Since then, I’ve made a notable tweak.

This is that I no longer reject “null-sender” traffic during the SMTP transaction. It turned out that it broke Exim’s implementation of Sender Address Verification, which performs the SAV check using a MAIL FROM of <>, rendering it indistinguishable from a bounce during the SMTP transaction.

Now, I’ve complained about SAV, but I have to be pragmatic anyway (Postel’s law and all that!) — so it was better to just allow other sites to perform SAV lookups against our server, and fix the anti-bounce stuff some other way.

The new method (below) does this, by allowing null-sender SMTP traffic just fine; it detects bounces in Postfix if they arrive via SMTP in RFC-3464 format, and bounces that slip past are then dealt with in a more CPU-intensive manner using the SpamAssassin “VBounce” ruleset (which is part of the now-released SpamAssassin 3.2.0, btw).

This increases the load, since some bounces cannot be rejected at MAIL FROM time now, and instead we have to wait ’til DATA — but CPU hasn’t been a problem recently, so this is ok.

Here are the updated instructions:

In Postfix

In my Postfix configuration, on the machine that acts as MX for my domains – edit ‘/etc/postfix/header_checks’, and add these lines:

/^Content-Type: multipart\/report; report-type=delivery-status\;/  REJECT no third-party DSNs
/^Content-Type: message\/delivery-status; /     REJECT no third-party DSNs

Edit ‘/etc/postfix/main.cf’, and ensure it contains:

header_checks = regexp:/etc/postfix/header_checks

Then run:

sudo /etc/init.d/postfix restart

This catches most of the bounces — RFC-3464-format Delivery-Status-Notification messages from other mail servers.

In SpamAssassin

As before, install the Virus-bounce ruleset and set it up. This will catch challenge-response mails, “out of office” noise, “virus scanner detected blah” crap, and bounce mails generated by really broken groupware MTAs — the stuff that gets past the Postfix front-line.

Tags: anti-spam, backscatter, bounces, email, howto, smtp, spamassassin, vbounce, virus-bounces, viruses

As I’ve noted before, we still have a major problem with sites generating bounce/backscatter storms in response to forged mail — whether deliberately targeted, as a “Joe-Job”, or as a side-effect of attempts to evade over-simplistic sender address verification as seen in spam, viruses, and so on.

Sites sending these bounces have a broken mail configuration, but there are thousands remaining out there — it’s very hard to fix an old mail setup to avoid this issue. As a result, even if your mail server is set up correctly and can handle the incoming spam load just fine, a single spam run sent to other people can amplify the volume of response bounces in a Smurf-attack-style volume multiplication, acting as a denial of service. I’ve regularly had serious load problems and backlogs on my MX, due solely to these bounces.

However, I think I’ve now solved it, with only a little loss of functionality. Here’s how I did it, using Postfix and SpamAssassin.

(UPDATE: if you use the algorithm described below, you’ll block mail from people using Sender Address Verification! Use this updated version instead.)

Firstly, note that if you adopt this, you will lose functionality. Third party sites will not be able to generate bounces which are sent back to senders via your MX — except during the SMTP transaction.

However, if a message delivery attempt is run from your MX, and it is bounced by the host during that SMTP transaction, this bounce message will still be preserved. This is good, since this is basically the only bounce scenario that can be recommended, or expected to work, in modern SMTP.

Also, a small subset of third-party bounce messages will still get past, and be delivered — the ones that are not in the RFC-3464 bounce format generated by modern MTAs, but that include your outbound relays in the quoted header. The idea here is that “good bounces”, such as messages from mailing lists warning that your mails were moderated, will still be safe.

OK, the details:

In Postfix

Ideally, we could do this entirely outside Postfix — but in my experience, the volume (amplified by the Smurf attack effects) is such that these need to be rejected as soon as possible, during the SMTP transaction.

Update: I’ve now changed this technique: see this blog post for the current details, and skip this section entirely!

(If you’re curious, though, here’s what I used to recommend:)

In my Postfix configuration, on the machine that acts as MX for my domains – edit ‘/etc/postfix/header_checks’, and add these lines:

/^Return-Path: <>/                              REJECT no third-party DSNs
/^From:.*MAILER-DAEMON/                         REJECT no third-party DSNs

Edit ‘/etc/postfix/null_sender’, and add:

<>              550 no third-party DSNs

Edit ‘/etc/postfix/main.cf’, and ensure it contains these lines:

header_checks = regexp:/etc/postfix/header_checks
smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/null_sender

(If you already have an ’smtpd_sender_restrictions’ line, just add ‘check_sender_access hash:/etc/postfix/null_sender’ to the end.) Finally, run:

sudo postmap /etc/postfix/null_sender
sudo /etc/init.d/postfix restart

This catches most of the bounces — RFC-3464-format Delivery-Status-Notification messages from other mail servers.

In SpamAssassin

Install the Virus-bounce ruleset. This will catch challenge-response mails, “out of office” noise, “virus scanner detected blah” crap, and bounce mails generated by really broken groupware MTAs — the stuff that gets past the Postfix front-line.

Once you’ve done these two things, that deals with almost all the forged-bounce load, at what I think is a reasonable cost. Comments welcome…

Tags: anti-spam, backscatter, bounces, email, howto, smtp, spamassassin, vbounce, virus-bounces, viruses