opensmtpd-filter-dkimsign for Debian
------------------------------------

The instructions below are based on the README file included in the OpenBSD
opensmtpd-filter-dkimsign port [0]. These instructions assume that the openssl
package is installed. The opensmtpd-filter-dkimsign package has already created
the _dkimsign user and group for your convenience.

Debian users should note the following changes from OpenBSD:

 * The instructions assume that dkim keys will be stored in /etc/mail/dkim, but
   you may use a different directory. If you would like to use /etc/mail/dkim,
   you will need to create it. It should be owned by _dkimsign:_dkimsign and
   have mode 770:

      sudo install -d -m 770 -o _dkimsign -g _dkimsign /etc/mail/dkim

[0] https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/ports/mail/opensmtpd-filters/dkimsign/pkg/README

 -- Ryan Kavanagh <rak@debian.org>  Thu, 10 Jun 2021 09:43:35 -0400

$OpenBSD: README,v 1.2 2021/06/10 10:58:51 martijn Exp $

+-----------------------------------------------------------------------
| Running opensmtpd-filter-dkimsign on OpenBSD
+-----------------------------------------------------------------------

To use filter-dkimsign, you must first generate a private key:

  sudo -u _dkimsign openssl genrsa -out /etc/mail/dkim/private.rsa.key 2048

To generate the public key ready for dns:

  openssl rsa -in /etc/mail/dkim/private.rsa.key -pubout | \
    sed '1s/.*/v=DKIM1;p=/;:nl;${s/-----.*//;q;};N;s/\n//g;b nl;'

This value needs to be placed in a DNS txt record with the following syntax:
   <selector>._domainkey.<domain>

Edit the /etc/smtpd.conf file to declare the filter:

  filter dkimsign_rsa proc-exec "filter-dkimsign -d <domain> -s <selector> -k /etc/mail/dkim/private.rsa.key" user _dkimsign group _dkimsign

Then add the filter to each listener that should be signed:

  listen on all filter dkimsign_rsa

To use Ed25519 similar steps must be taken.

  Make sure the ed25519 flavor of opensmtpd-filter-dkimsign is installed.

To generate the private key:

  sudo -u _dkimsign openssl genpkey -algorithm ed25519 -outform PEM -out /etc/mail/dkim/private.ed25519.key

To generate the public key ready for dns:

  printf "v=DKIM1;k=ed25519;p=%s" "$(openssl pkey -outform DER -pubout -in /etc/mail/dkim/private.ed25519.key | tail -c +13 | openssl base64)"

Edit the /etc/smtpd.conf file to declare the filter:

  filter dkimsign_ed25519 proc-exec "filter-dkimsign -a ed25519-sha256 -d <domain> -s <selector> -k /etc/mail/dkim/private.ed25519.key" user _dkimsign group _dkimsign

To add both filters to each listener that should be signed:

  filter dkimsign chain { dkimsign_rsa, dkimsign_ed25519 }
  listen on all filter dkimsign

For a full list of options see filter-dkimsign(8).