fyi, if you prefer a dedicated user approach, just need to make sure you're
consistent,
groupdel opendkim
groupadd opendkim
useradd opendkim -g opendkim -G "" -s /bin/false -d /var/run/opendkim -M
usermod -a -G opendkim postfix
id opendkim
uid=5117(opendkim) gid=5117(opendkim) groups=5117(opendkim)
id postfix
uid=5001(postfix) gid=5001(postfix)
groups=5001(postfix),12(mail),5002(postdrop),...,5117(opendkim),...
cat /etc/systemd/system/opendkim.service
...
[Service]
User=opendkim
Group=opendkim
Type=forking
PIDFile=/var/run/opendkim/opendkim.pid
ExecStart=/opt/opendkim/sbin/opendkim -l -x
/usr/local/etc/opendkim/opendkim.conf -u opendkim
...
cat /usr/local/etc/opendkim/opendkim.conf
...
UserID opendkim:opendkim
Socket local:/var/run/opendkim/opendkim.sock
PidFile /var/run/opendkim/opendkim.pid
...
cat /usr/local/etc/opendkim/key_table
dkim-56..._domainkey.example1.com example1.com:
dkim-56...:/usr/local/etc/sec/dkim/dkim-146...example1.com.key.pem
dkim-0e..._domainkey.example2.com example2.com:
dkim-0e...:/usr/local/etc/sec/dkim/dkim-146...example2.com.key.pem
...
ls -alr /var/run/opendkim
total 4.0K
srwxrwxr-x 1 opendkim opendkim 0 Sep 2 09:33 opendkim.sock=
-rw-r--r-- 1 opendkim opendkim 5 Sep 2 09:33 opendkim.pid
drwxr-xr-x 42 root root 1.2K Sep 3 08:06 ../
drwxr-xr-x 2 opendkim opendkim 80 Sep 2 09:33 ./
ls -alr /usr/local/etc/opendkim
total 40K
-rw-rw-r--+ 1 opendkim opendkim 93 May 30 2016 trusted_hosts
-rw-r-----+ 1 opendkim opendkim 2.1K May 30 2016 signing_table
-rw-r-----+ 1 opendkim opendkim 7.6K May 30 08:26 opendkim.conf
-rw-r-----+ 1 opendkim opendkim 4.1K May 30 2016 key_table
drwxrwxr-x+ 32 root root 4.0K Aug 28 07:30 ../
drwxr-xr-x+ 2 opendkim opendkim 4.0K May 30 2016 ./
ls -al /usr/local/etc/sec/dkim
total 384K
drwxr-xr-x 2 opendkim opendkim 12K May 30 2016 ./
drwxr-xr-x 10 root root 4.0K Aug 28 07:32 ../
-rw------- 1 opendkim opendkim 1.7K May 30 2016
dkim-14...example1.com.key.pem
-rw------- 1 opendkim opendkim 451 May 30 2016
dkim-14...example1.com.pubkey.pem
-rw------- 1 opendkim opendkim 1.7K May 30 2016
dkim-14...example2.com.key.pem
-rw------- 1 opendkim opendkim 451 May 30 2016
dkim-14...example2.com.pubkey.pem
...
cat /usr/local/etc/postfix/master.cf
...
[127.0.0.1]:10005 inet n - n - - smtpd
-o smtpd_milters=...,unix:/var/run/opendkim/opendkim.sock,...
...
[int.mx.MYDOMAIN.COM]:587 inet n - n - - smtpd
-o smtpd_milters=...,unix:/var/run/opendkim/opendkim.sock,...
...
cat /usr/local/etc/postfix/main.cf
...
authorized_submit_users = ..., opendkim, ...
...
works well here.
hth.
