This guide assumes you have a working Postfix server and want it to sign with DKIM.

There are a few things we  have to understand to make all of this work though, which require you to be familiar with DNS as well.

1.) Install OpenDKIM

apt install opendkim

systemctl enable opendkim

2.) Edit /etc/opendkim.conf

Syslog   yes
SyslogSuccess yes
Mode     sv
OmitHeaders .
Socket   inet:8891@localhost
Domain   yourdomain.com
KeyTable        /etc/opendkim/KeyTable
SigningTable   refile:/etc/opendkim/SigningTable
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts   refile:/etc/opendkim/TrustedHosts
UserID  opendkim:opendkim


3.) Create your first OpenDKIM Key for your domain

yourdomain=yourdomain.com


mkdir -p /etc/opendkim/keys/$yourdomain
cd /etc/opendkim/keys/$yourdomain
opendkim-genkey -r -d $yourdomain


4.) Configure KeyTable and SigningTable

#Add this line to /etc/opendkim/KeyTable

The "default" is what is called the selector, it could be nearly anything but we're calling it default here.  Be sure to change "yourdomain.com" to the actual domain you want DKIM signing for.

default._domainkey.yourdomain.com yourdomain.com:default:/etc/opendkim/keys/yourdomain.com/default.private

#Add this to line to /etc/opendkim/SigningTable

*@yourdomain.com default._domainkey.yourdomain.com

The * allows all e-mails from the domain to be signed, change it if this should not be the case.

Again, be sure you change yourdomain.com to your actual domain that you setup in the KeyTable in the previous step.

5.) Edit your Your DNS zonefile for yourname.com
 

Here's an example using bind/named:

#edit bind/named DNS entry for domain/zone

cat /etc/opendkim/keys/yourdomain.com/default.txt

Take the output of the above and then add it to your zonefile like below:

default._domainkey    IN    TXT    ( "v=DKIM1; k=rsa; s=email; "
      "p=blablabla" )  ; ----- DKIM key default for yourdomain.com

Be sure to change the blablabla to the output of /etc/opendkim/keys/yourdomain.com/default.txt

_dmarc.yourdomain.com.      IN     TXT    "v=DMARC1; p=quarantine; rua=mailto:someaddress@yourdomain.com"
 

Be sure to change someaddress@yourdomain.com to a real address for your domain, you will receive DMARC reports to this address.

Set permissions to be sure:

chown -R opendkim.opendkim /etc/opendkim

5.) Enable OpenDKIM milter in Postfix

# edit /etc/postfix/main.cf
cp -a /etc/postfix/main.cf /etc/postfix/main.cf-`date +%Y%m%d-%s`
 

#edit /etc/postfix/main.cf

#enable dkim
smtpd_milters           = inet:127.0.0.1:8891
non_smtpd_milters       = $smtpd_milters
milter_default_action   = accept

systemctl restart postfix

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