UPDATE THE SYSTEM

Before going any further, make sure you’re in a screen session and your system is fully up-to-date by running:

## screen -U -S opendkim-screen
## yum update

ENABLE EPEL REPOSITORY

OpenDKIM is available in the EPEL repository, so we need to enable it on the system before we can install OpenDKIM

## wget -P /tmp http://mirror.pnl.gov/epel/6/i386/epel-release-6-8.noarch.rpm
## rpm -Uvh /tmp/epel-release-6-8.noarch.rpm
## rm -f /tmp/epel-release-6-8.noarch.rpm

INSTALL OPENDKIM

Install the package using yum:

## yum install opendkim

CONFIGURE OPENDKIM

Next thing to do is to configure OpenDKIM. Its main configuration file is located in /etc/opendkim.conf, so before making any changes create a backup and add/edit the following:

## cp /etc/opendkim.conf /etc/opendkim.conf.bal
## vim /etc/opendkim.conf

Paste the below configuration parameters:

AutoRestart Yes
AutoRestartRate 10/1h
LogWhy Yes
Syslog Yes
SyslogSuccess Yes
Mode sv
Canonicalization relaxed/simple
ExternalIgnoreList refile:/etc/opendkim/TrustedHosts
InternalHosts refile:/etc/opendkim/TrustedHosts
KeyTable refile:/etc/opendkim/KeyTable
SigningTable refile:/etc/opendkim/SigningTable
SignatureAlgorithm rsa-sha256
Socket inet:8891@localhost
PidFile /var/run/opendkim/opendkim.pid
UMask 022
UserID opendkim:opendkim
TemporaryDirectory /var/tmp

SET-UP PUBLIC/PRIVATE KEYS

Generate set of keys for your mydomain.com domain name:

## mkdir /etc/opendkim/keys/mydomain.com
## opendkim-genkey -D /etc/opendkim/keys/mydomain.com/ -d mydomain.com -s default
## chown -R opendkim: /etc/opendkim/keys/mydomain.com
## mv /etc/opendkim/keys/mydomain.com/default.private /etc/opendkim/keys/mydomain.com/default

add mydomain.com to OpenDKIM’s key table by adding the following record in /etc/opendkim/KeyTable

default._domainkey.mydomain.com mydomain.com:default:/etc/opendkim/keys/mydomain.com/default

next, edit /etc/opendkim/SigningTable and add the following record to OpenDKIM’s signing table:

*@mydomain.com default._domainkey.mydomain.com

and add your domain and your hostname as trusted hosts in /etc/opendkim/TrustedHosts:

127.0.0.1
mydomain.com
host.mydomain.com

assuming the domain in question is ‘mydomain.com’ and server’s hostname is set to ‘host.mydomain.com’

finally, edit your mydomain.com DNS zone and add the TXT record from /etc/opendkim/keys/mydomain.com/default.txt

default._domainkey IN TXT ( "v=DKIM1; k=rsa; "
"p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDApHRr7ZmXRaAB+RQRbP4VdMwIrIHIP18KFtXRsv/xpWc0Gix6ZXN13fcG03KNGKZo2PY+csPkGC5quDnH5V0JEhDZ78KcDWFsU6u4fr9ktVAdt6P7jWXjcyqdHOZ8+YN4cAeU4lRFNgQvdupIcByYwzPYMgBFHfJm9014HvRqhwIDAQAB" ) ; ----- DKIM key default for mydomain.com

You can verify your dkim TXT record is valid using dig for example:

## dig +short default._domainkey.mydomain.com TXT

"v=DKIM1\; k=rsa\; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDApHRr7ZmXRaAB+RQRbP4VdMwIrIHIP18KFtXRsv/xpWc0Gix6ZXN13fcG03KNGKZo2PY+csPkGC5quDnH5V0JEhDZ78KcDWFsU6u4fr9ktVAdt6P7jWXjcyqdHOZ8+YN4cAeU4lRFNgQvdupIcByYwzPYMgBFHfJm9014HvRqhwIDAQAB"

CONFIGURE POSTFIX

In order to integrate OpenDKIM with Postfix we need to add the following few lines in /etc/postfix/main.cf:

smtpd_milters = inet:127.0.0.1:8891
non_smtpd_milters = $smtpd_milters
milter_default_action = accept
milter_protocol = 2

(RE)START SERVICES

Add OpenDKIM to your system’s start-up and start opendkim and restart postfix using the following commands:

## service opendkim start
## chkconfig opendkim on
## service postfix restart

TEST THE SET-UP

To test the set-up simply send an email to check-auth@verifier.port25.com and you should receive back an email containing something like this:

==========================================================
Summary of Results
==========================================================
SPF check: pass
DomainKeys check: neutral
DKIM check: pass
DKIM check: pass

Src: https://www.rosehosting.com/blog/how-to-install-and-integrate-opendkim-with-postfix-on-a-centos-6-vps/