OpenDkim

To setup open dkim a couple of steps have to be made to install and configure the service and get it running. The example domain is noordwijk.nl, change it to yours.

mkdir /etc/dkimkeys chown opendkim.opendkim /etc/dkimkeys

A key needs to be generated per domain
sudo -u opendkim opendkim-genkey -D /etc/dkimkeys -d noordwijk.nl -s 2020

Domain                  noordwijk.nl
Selector                2020
KeyFile                 /etc/dkimkeys/2020.private
Socket                  inet:8891@localhost

Make a backup of the original config
cp -rp /etc/postfix/main.cf /etc/postfix/main.cf_bak

Add the new config parts
vi /etc/postfix/main.cf

smtpd_milters = inet:localhost:8891
non_smtpd_milters = $smtpd_milters

Restart services to activate
systemctl restart opendkim systemctl restart postfix

Following text record needs to go into the DNS Zonefile
user@host:~# cat /etc/dkimkeys/2020.txt

2020._domainkey IN      TXT     ( "v=DKIM1; k=rsa; "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqcS4IGfYnGeToHq7lziTCithEA5esxonnQjtp8hbVSGNDXeAAXQvdzg34QkpS/GCtsRa28XmnTw451dUDei/IE3CUrVet4SaYurERKzpwoBHyAMGCQgqgUzhGXpAOcL50XHXTtjZ9h1rdwyfTlg9YNosXrEXyF7Rt5YJOa/n3UQIDAQAB" )  ; ----- DKIM key 2020 for noordwijk.nl

Put the keys in the keytable
vi /etc/dkimkeys/keytable

2020._domainkey.noordwijk.nl noordwijk.nl:2020:/etc/dkimkeys/2020.private
2022._domainkey.lostlemon.nl lostlemon.nl:2022:/etc/dkimkeys/2022.private

Create the signingtable
vi /etc/dkimkeys/signingtable

*@lostlemon.nl 2022._domainkey.lostlemon.nl
*@test03lostlemon.nl 2022._domainkey.lostlemon.nl
*@noordwijk.nl 2020._domainkey.noordwijk.nl

Decide which hosts are allowed, localhost has to be included!
vi /etc/dkimkeys/trustedhosts

127.0.0.1/8
85.90.72.92/27
172.16.1.1/24

Have OpenDkim allowing domains
vi /etc/opendkim/opendkim.conf

KeyTable file:/etc/dkimkeys/keytable
SigningTable refile:/etc/dkimkeys/signingtable 
InternalHosts refile:/etc/dkimkeys/trustedhosts