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| Running opensmtpd-filter-dkimsign on OpenBSD
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To use filter-dkimsign, you must first generate a private key:

  doas -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/mail/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.

To generate the private key:

  doas -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\n" "$(doas -u _dkimsign openssl pkey -outform DER -pubout -in /etc/mail/dkim/private.ed25519.key | tail -c +13 | openssl base64)"

Edit the /etc/mail/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).
