25 September 2005

Lose 25% of your spam with a few simple rules

Require a HELO and reject (at SMTP time) attempts to send without a HELO are not explicitly logged, only the connect/disconnect without anything happening in between.

Taking that to imply a no-HELO conversation, my server has seen ~72,000 unique conversations in the last 7 weeks, of which a mere 28 fall into this category. Not much love there, but I have no idea how many spammers try HELOless connections first then retry with a HELO on error, and it forces the caller to pass the next hurdle.

The next hurdle is to refuse HELOs claiming to be 127.0.0.1, localhost, localhost.localdomain, any of your external addresses or their reverse-DNS names, or any of your MX names. This doesn”t require any magic technology, the PostFix config for it is:

smtpd_helo_required = yes
smtpd_helo_restrictions =
reject_invalid_hostname,
check_helo_access hash:/etc/postfix/helo_access,
reject_non_fqdn_hostname,
permit

The file simply contains a list of unloved hosts accompanied by the word REJECT (and optionally a message):

127.0.0.1                  REJECT
localhost REJECT
localhost.localdomain REJECT Your server's misconfigured, find Cw in /etc/mail/sendmail.cf
169.254.73.37 REJECT
smtp.somerandomdomain.net REJECT

Remember to postmap it. These rules have so far snared 2 misconfigured SendMail installations (not bad for ~72,000 connections), and rejected ~18,000 pieces of spam, 25% of the incomings. If you wanted to get fancy, you could reject all known private IPs as well, with a handful of regexes.

If you add a reject for unknown hosts, you will rapidly build up a fine collection of misconfigured MS-Exchange servers (I have 19 so far), but reject a lot more spam (another ~28,000 pieces in my case — 39% for a running total of 64% rejected before even being queued, let alone handed to a computationally expensive spam or virus scanner). You can cope with the MSCE/MS-Exchange duds by making sure that the check_helo_access comes before the reject_unknown_hostname, and adding them to the list as they are discovered with ACCEPT instead of REJECT alongside them.

Interestingly, I scored another 2% by also blocking "ss" as a HELO, and another 1.2% by rejecting any recipient address that looks like a messageID (check_recipient_access pcre /[0-9]+\.[0-9]+\.[a-z0-9]+@/).

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