VMM (http://vmm.localdomain.org/) DKIM configuration (only relevant parts shown)

archlinux.sh

sudo pacman -S opendkim
sudo cp /etc/opendkim/opendkim.conf{.sample,}
sudo mkdir /var/lib/dkim
sudo chown opendkim:mail /etc/opendkim/opendkim.conf /var/lib/dkim
sudo chmod 700 /var/lib/dkim
sudo chgrp mail /var/spool/postfix/private
sudo -u opendkim opendkim-genkey -D /var/lib/dkim -b 2048 -s mail
sudo usermod -a -G mail postfix

master.cf

# add
  -o smtpd_milters=unix:private/opendkim
# to each of your mail submission smtpds (smtp inet, smtps inet, submission inet)

opendkim.conf

Domain dsn:pgsql://opendkim:password@localhost/mailsys/table=opendkim_domains?keycol=domainname?datacol=domainname
KeyFile /var/lib/dkim/mail.private
Selector mail
Socket local:/var/spool/postfix/private/opendkim
UserID opendkim
UMask 0117

schema.sql

# As superuser
CREATE USER opendkim WITH IN ROLE mailsys PASSWORD 'password';

# As vmm
ALTER TABLE service_set ADD COLUMN dkim BOOLEAN NOT NULL DEFAULT true;
ALTER TABLE service_set DROP CONSTRAINT ukey_service_set;
ALTER TABLE service_set ADD CONSTRAINT ukey_service_set UNIQUE (smtp, pop3, imap, sieve, dkim);
INSERT INTO service_set(smtp, pop3, imap, sieve, dkim)
  SELECT smtp, pop3, imap, sieve, false FROM service_set;
CREATE VIEW opendkim_domains AS
SELECT domainname
FROM domain_name
  JOIN domain_data ON domain_name.gid = domain_data.gid
  JOIN  service_set ON service_set.ssid = domain_data.ssid
WHERE dkim = true;
GRANT SELECT ON opendkim_domains TO opendkim;