Collection of various OpenSMTPD filters

This collection of OpenSMTPD filters is described in my blog posts Adding DKIM support to OpenSMTPD with custom filters and Converting incoming emails on the fly with OpenSMTPD filters. No support is provided, use at your own risk.

Installing

These scripts are most easily installed via pipx:

pipx install git+https://github.com/palant/opensmtpd-filters.git

Once installed, you can run the dmarc2html-cli command for example.

dkimverify

This filter will perform DKIM and SPF verification on incoming mail. It can be used in smtpd.conf like this:

filter dkimverify proc-exec "/home/user/.local/share/pipx/venvs/opensmptd_filters/bin/dkimverify.py example.com"
listen on eth0 tls filter dkimverify

It takes a single command line parameter: the host name to appear in the Authentication-Results email header. It will add a header like Authentication-Results: example.com; dkim=pass; spf=fail (sender is example.com/1.2.3.4) smtp.mailfrom=me@example.org to emails, this header can then be considered in further processing.

dkimsign

This filter will add a DKIM signature to outgoing emails. It can be used in smtpd.conf like this:

filter dkimsign proc-exec "/usr/local/bin/dkimsign.py example.com:mydkim:/etc/mail/dkim/mydkim.key"
listen on eth0 port 587 tls-require auth filter dkimsign

It takes one or multiple parameters of the form domain:selector:keyfile on the command line. Instead of configuring all domains on the command line, you can also pass this script -c /etc/mail/dkim/dkim.conf parameter, with the file /etc/mail/dkim/dkim.conf containing domain configurations in the same format, one per line.

dmarc2html

This filter helps to simplify handling of DMARC aggregate reports for low-volume email servers. It can be used in smtpd.conf like this:

filter dmarc2html proc-exec "/home/user/.local/share/pipx/venvs/opensmptd_filters/bin/dmarc2html.py dmarc"

For any email to the dmarc@… account (or any other account specified as command line parameter), it will process the attachment and replace the email’s main part by the resulting HTML code.

There is also a script that will convert a DMARC aggregate report on the command line:

dmarc2html-cli dmarc.tar.gz