Email authentication plays a critical role in improving email deliverability and protecting domains from spoofing. DomainKeys Identified Mail (DKIM) adds a digital signature to outgoing emails, allowing receiving servers to verify that messages are legitimate and unaltered.

This guide explains how to install and configure OpenDKIM with Postfix to enable DKIM signing on a Linux server.

What is OpenDKIM?

OpenDKIM is an open-source implementation of the DKIM standard. It integrates with mail transfer agents like Postfix to automatically sign outgoing emails using cryptographic keys.

Benefits include:

• Improved email delivery rates
• Reduced spam classification
• Protection against email spoofing
• Increased domain reputation

Step 1: Install Required Packages

Install the RPMForge repository and required packages.

wget http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm
rpm -ivh rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm
yum install opendkim postfix

Step 2: Disable Sendmail

Stop Sendmail and remove it from startup services.

/etc/init.d/sendmail stop
chkconfig sendmail off

Step 3: Enable Services at Boot

Enable Postfix and OpenDKIM services.

chkconfig postfix on
chkconfig opendkim on

Step 4: Configure Postfix

Edit Postfix configuration:

vi /etc/postfix/main.cf

Update or add the following parameters:

myhostname = server.yourdomainname.com
mydomain = yourdomainname.com
myorigin = $mydomain
inet_interfaces = all
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
home_mailbox = Maildir/

Add DKIM Integration

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

Step 5: Configure OpenDKIM

Important Configuration Files

Edit OpenDKIM Configuration

vi /etc/opendkim.conf

Add or update:

PidFile /var/run/opendkim/opendkim.pid
Mode sv
Canonicalization relaxed/simple
Syslog yes
SyslogSuccess yes
LogWhy yes
UserID opendkim:opendkim
Socket inet:8891@localhost
Umask 002
Selector default
KeyTable refile:/etc/opendkim/KeyTable
SigningTable refile:/etc/opendkim/SigningTable
ExternalIgnoreList refile:/etc/opendkim/TrustedHosts
InternalHosts refile:/etc/opendkim/TrustedHosts

Step 6: Generate DKIM Keys

Create key directories and generate keys.

cd /etc/opendkim/keys
mkdir yourdomainname.com
cd yourdomainname.comopendkim-genkey -d yourdomainname.com -s default

Set proper ownership:

chown opendkim:opendkim ../yourdomainname.com -R

Step 7: Configure KeyTable

vi /etc/opendkim/KeyTable

Add:

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

Step 8: Configure SigningTable

vi /etc/opendkim/SigningTable

Add:

*@yourdomainname.com default._domainkey.yourdomainkey.yourdomainname.com

Step 9: Configure Trusted Hosts

vi /etc/opendkim/TrustedHosts

Add:

127.0.0.1
localhost
server.yourdomainname.com
yourdomainname.com

Ensure localhost is included.

Step 10: Start Services

Start OpenDKIM first, then Postfix:

/etc/init.d/opendkim start
/etc/init.d/postfix start

Monitor logs:

tail -f /var/log/maillog

You should see OpenDKIM initialization messages.

Step 11: Add DKIM DNS Record

Retrieve the public DNS key:

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

Create a TXT record in DNS:

default._domainkey IN TXT "v=DKIM1;k=rsa;p=PUBLIC_KEY"

Ensure k=rsa exists in the record.

Step 12: Configure SPF Record

Add an SPF record to improve email authentication:

v=spf1 mx a ip4:SERVER_IP ~all

Step 13: Test DKIM Setup

Send a test email:

echo "This is a test mail" | mail -s "OpenDKIM test mail" test@gmail.com

Check logs:

tail -f /var/log/maillog

Look for:

DKIM-Signature header added

Step 14: Verify DKIM Status

Open the received email headers and confirm:

dkim=pass

Conclusion

Configuring OpenDKIM with Postfix secures outgoing mail using domain authentication. Proper DKIM and SPF setup significantly improves email reputation, prevents spoofing, and ensures reliable inbox delivery.

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