On Gentoo some postfix processes run chroot’ed but others do not. In result these processes need different sock locations, but this is impossible.
Solution:
# mkdir -p /var/run/postfix/var/run/opendkim # chmod milter:milter /var/run/postfix/var/run/opendkim # usermod -a -G milter postfix # mount --rbind /var/run/opendkim /var/run/postfix/var/run/opendkim