DKIM (DomainKeys Identified Mail) is a method of signing electronic emails using public-private key. DKIM is used by receiving mail server for identifying email, that they are sent by authorized mail servers. It also minimizes the possibility of getting emails SPAM.
This tutorial will provide you a quick and easy way to set up DomainKeys with your POSTFIX running on CentOS and RHEL systems.
How DKIM Works ?
When we configured DKIM on sending servers. First, we generated a public/private key pair for signing outgoing messages. The public key is configured as TXT record on a domains name server, and the private key is configured in the outbound email server. When an email is sent by an authorized user of the email server, the server uses the stored private key to generate a digital signature of the message, which is inserted in the message as a header, and the email is sent as normal.
Step 1 – Install DKIM-milter
First make sure you have enabled EPEL repository in your system. After that install dkim-milter package using following command.
yum install postfix opendkim
Step 2 – Generate Key Pair
Now create DKIM key pair using dkim-genkey command line utility provided by dkim-milter package. For this tutorial we are using domain name “example.com”, Change this name with your actual names.
MYDOMAIN=example.com mkdir -p /etc/opendkim/keys/$MYDOMAIN cd /etc/opendkim/keys/$MYDOMAIN opendkim-genkey -r -d $MYDOMAIN
Above command will generate two files default.private and default.txt. You can created multiple DKIM keys for different-2 domains and configure with your postfix server.
Now set the proper permissions on Keys directory.
chown -R opendkim:opendkim /etc/opendkim chmod go-rw /etc/opendkim/keys
Step 3 – Configure OpenDKIM
Edit the Opendkim configuration file and Add/Update following entries in file.
Mode sv Socket inet:8891@localhost Domain example.com #KeyFile /etc/opendkim/keys/default.private
### comment this lineKeyTable /etc/opendkim/KeyTable SigningTable refile:/etc/opendkim/SigningTable ExternalIgnoreList refile:/etc/opendkim/TrustedHosts InternalHosts refile:/etc/opendkim/TrustedHosts
Then edit the domain keys lists setting file /etc/opendkim/KeyTable and add following entry.
After that edit /etc/opendkim/SigningTable file and update following entry.
And edit /etc/opendkim/TrustedHosts file and update following entry.
Step 4 – Configure Postfix
Now edit POSTFIX configuration file /etc/postfix/main.cf and add following values at the end of file
smtpd_milters = inet:127.0.0.1:8891 non_smtpd_milters = $smtpd_milters milter_default_action = accept
finally start DKIM service using following command
service opendkim start
Step 5 – Configure DNS Entry
After configuring private key in postfix server. there will be another file /etc/opendkim/keys/example.com/default.txt/strong> generated by opendkim-genkey. Edit your DNS zone file and add this as TXT record found in default.txt. In my case this is like below.
default._domainkey IN TXT ( "v=DKIM1; k=rsa; s=email; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCdTtEqM8FqndiFYOderzljMMMqBdEp+wJKP+VUbhc9GigmK34ZjrSqqdKjIEWr2q9DvSVp1H1bZs4t050m0HZxJqknDz2yoDJ6W4mCaSCHesRde5V44V/L65Gqm/rvBz1d6CCp8A2515eveWrIAocOD6pKJ4tnXHz3uwV2ZtgQiQIDAQAB" ) ; ----- DKIM key default for example.com
Step 6 – Verify DKIM
To verify that DKIM is working properly. Let’s send a test email through command line
mail -vs "Test DKIM" [email protected] < /dev/null
In the received email in our mailbox, open the source of the email and search for "DKIM-Signature". You will find something like below
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=default.private; t=1402388963; bh=fdkeB/A0FkbVP2k4J4pNPoe23AvqBm9+b0C3OY87Cw8=; h=Date:From:Message-Id:To:Subject; b=M6g0eHe3LNqURha9d73bFWlPfOERXsXxrYtN2qrSQ6/0WXtOxwkEjfoNTHPzoEOlD i6uLLwV+3/JTs7mFmrkvlA5ZR693sM5gkVgVJmuOsylXSwd3XNfEcGSqFRRIrLhHtbC mAXMNxJtih9OuVNi96TrFNyUJeHMRvvbo34BzqWY=