opensmtpd-filter-dkimsign adds a dkim signature to the email message.
It supports the rsa and ed25519 signing algorithms.

mandoc is needed only at build time.

It is recommended to run filter-dkimsign as a non root user. For
example, create a user and group "_dkimsign" (this name has come from
an example in smtpd.conf(5)):

  groupadd -g 376 _dkimsign
  useradd -u 376 -g 376 -s /sbin/nologin -M _dkimsign

The private key must be readable by this user.
