Configure DomainKeys (OpenDKIM) with Postfix on CentOS 7

OpenDKIM is method to digitally sign & verify emails on the mail servers using public & private keys. In other words opendkim implements the DKIM (DomainKeys Identified Mail) standard for signing and verifying email messages on a per-domain basis. DomainKeys are implemented to reduce the chances of outgoing mails to be marked as SPAM.

In this post we will demonstrate how to install & configure DomainKeys with postfix (MTA) on CentOS 7, i am assuming Postfix is already installed with following domain and hostname.

  • Hostname = mail5.freshdaymall.com
  • Domain = freshdaymall.com

Also Read : How to install and Configure Postfix Mail Server on CentOS 8

Step:1 Set EPEL Repository using below rpm command

OpenDKIM package is not available in the default yum repositories but available in CentOS 7 EPEL repositories.

[[email protected] ~]# rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

Step:2 Install OpenDKIM Package using yum

[[email protected] ~]# yum install -y opendkim

Step:3 Run below Command to create keys

Execute the below command to create public & private keys under folder “/etc/opendkim/keys

[[email protected] ~]# opendkim-default-keygen
Generating default DKIM keys:
Default DKIM keys for freshdaymall.com created in /etc/opendkim/keys.
[[email protected] ~]#
[[email protected] ~]# cd /etc/opendkim/keys/
[[email protected] keys]# ll
total 8
-rw-r----- 1 root opendkim 891 Nov 29 08:42 default.private
-rw-r--r-- 1 root opendkim 320 Nov 29 08:42 default.txt
[[email protected] keys]#

default.private is the private key for the domain and default.txt is public key that we will publish in DNS record (TXT) in the domain. A Selector ( default ) is created while generating keys, a selector can be unique keyword which is associated in keys and included in DKIM signature.

Step:4 Edit the Following Files :

  • /etc/opendkim.conf —- Config file of opendkim
  • /etc/opendkim/KeyTable —- As name suggest it defines the path of private key for the domain
  • /etc/opendkim/SigningTable — This file tells OpenDKIM how to apply the keys.
  • /etc/opendkim/TrustedHosts — This file defines which hosts are allowed to use keys.

Edit the file “/etc/opendkim.conf” & set the below parameters.

opendkim-conf-file

Edit the KeyTable file and replace the example.com with your domain name.

[[email protected] ~]# cat /etc/opendkim/KeyTable
# OPENDKIM KEY TABLE
# To use this file, uncomment the #KeyTable option in /etc/opendkim.conf,
# then uncomment the following line and replace example.com with your domain
# name, then restart OpenDKIM. Additional keys may be added on separate lines.

#default._domainkey.example.com example.com:default:/etc/opendkim/keys/default.private
default._domainkey.freshdaymall.com freshdaymall.com:default:/etc/opendkim/keys/default.private
[[email protected] ~]#

Edit the SigningTable file and define who will sign the outgoing mails.

[[email protected] ~]# cat /etc/opendkim/SigningTable 
# WILDCARD EXAMPLE
# Enables signing for any address on the listed domain(s), but will work only if
# "refile:/etc/opendkim/SigningTable" is included in /etc/opendkim.conf.
# Create additional lines for additional domains.

*@freshdaymall.com default._domainkey.freshdaymall.com

As i am using * in above parameter which means all the users on domain are allowed to sign the emails.

Edit the TrustedHosts file , add Server’s FQDN and domain name below localhost ip (127.0.0.1)

[[email protected] ~]# cat /etc/opendkim/TrustedHosts 
# OPENDKIM TRUSTED HOSTS
# To use this file, uncomment the #ExternalIgnoreList and/or the #InternalHosts
# option in /etc/opendkim.conf then restart OpenDKIM. Additional hosts
# may be added on separate lines (IP addresses, hostnames, or CIDR ranges).
# The localhost IP (127.0.0.1) should always be the first entry in this file.

127.0.0.1
mail5.freshdaymall.com
freshdaymall.com
[[email protected] ~]#

Step:5 Edit Postfix Config File (/etc/postfix/main.cf)

Add the below lines at end of /etc/postfix/main.cf file.

[[email protected] ~]# vi /etc/postfix/main.cf
smtpd_milters = inet:127.0.0.1:8891
non_smtpd_milters = $smtpd_milters
milter_default_action = accept

Step:6 Start OpenDKIM & postfix Service

[[email protected] ~]# hash -r
[[email protected] ~]# systemctl start opendkim ; systemctl enable opendkim ; systemctl restart postfix
ln -s '/usr/lib/systemd/system/opendkim.service' '/etc/systemd/system/multi-user.target.wants/opendkim.service'
[[email protected] ~]#

Step:7 Update the TXT DNS record of your domain.

Use the output of default.txt and update the DNS Record (TXT) of the Domain.

domain-public-key-opendkim

Step:8 Send a Test email and view the logs.

sendmail-with-telnet

Check whether email is signed or not.

signed-by-email

Wow , Our email is signed and domainKeys configuration task is completed now.

22 thoughts on “Configure DomainKeys (OpenDKIM) with Postfix on CentOS 7”

  1. How do you make a keygen for another domain besides whatever is returned by hostname?

    Also, how do you do this for multiple domains?

  2. After entering ‘cat /etc/opendkim/keys/default.txt’ I got a ‘DKIM key default for com’ and not my domain name at the end of the message.

    At the start when I ran the command ‘opendkim-default-keygen’, I got a message ‘Default DKIM keys for com created in /etc/opendkim/keys.’ instead of one with my domain name. There isn’t any step defined in your tutorial to change this.

  3. To me, step 3 is incomplete as no domain is being specified here.
    Maybe the use of these would be relevant (where mydomain.com is your 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

  4. Hi, I got a problem to start openDKIM. It comes out error for “Starting OpenDKIM Milter: opendkim: /etc/opendkim.conf: refile:/etc/opendkim/TrustedHosts: dkimf_db_open(): Permission denied

    I try to change permission for 664. It seem not work. Do you have any idea and suggestion for solve this problem?

  5. I am baffled.
    I cannot get past your Step 2: “yum install -y opendkim”

    I get an error message:
    “No package opendkim available”

    Do you know why this is happening?

    1. Hi Rodney,

      opendkim package is available in the epel repository. So first enable or configure epel repo in your system and then hit the above mentioned yum command.

  6. I have problem not authenticated but DKIM verification successful and no signing table match for but DKIM verification successful how to fix this problem?

  7. Our web server has a Thawte EV SSL certificate. Should I link to those keys or generate our own opendkim keys? How do I include the IntermediateCA.crt?

  8. Hi Pradeep,

    You have a small error – you have not removed the # from line 5 of the highlighted lines on step 4. This is for the “opendkim.conf”

    Regards 😉

  9. how send email without any relayhost

    #relayhost = [smtp.sendgrid.net]:587
    smtp_tls_security_level = encrypt
    header_size_limit = 4096000
    smtp_use_tls = yes
    smtp_sasl_auth_enable = yes
    smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
    smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt
    smtp_sasl_security_options = noanonymous
    smtp_sasl_tls_security_options = noanonymous

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

    error when do that realy access denied

  10. The mail gets sent but it is not signed . The logs don’t show anything about dkim either. Dkim is running so i’m not sure why there are no errors displayed

  11. Ok I found the problem why it wasn’t signing, but now when checking the header in e-mail it says invalid format dkim-neutral

  12. I’m out of ideas. Any help would help. what is the selector i add to my dns.. for instance.. when i do a test both show vaild records so i’m not sure why i keep getting dkim=neutral (bad format) [email protected] header.s=default header.b=lQ1YkIEB;

    ‘domain1._domainkey.domain1.com:v=DKIM1;k=rsa;p=MIGfMs…………………………..

    or

    ‘domain1.com._domainkey.domain1.com:v=DKIM1;k=rsa;p=MIGfMs…………………………..

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