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.
Browse other questions tagged centos postfix dkim or ask your own question. The Overflow Blog The Overflow #42: Bugs vs. How to put machine learning. Add the following lines at the end of your Postfix main.cf file, which will make Postfix aware of OpenDKIM and allow it to sign and verify mail: vi /etc/postfix/main.cf smtpdmilters = inet:127.0.0.1:8891. I spent a while trying to set up DKIM with Postfix on CentOS 5.2. I read the HOWTOs on HOWToForge written by Andrew Colin Kissa (aka TopDog) who subsequently helped me towards getting this setup working. My setup is that I have a mail spooler and multiple mail senders.
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
Opendkim Postfix Centos 6
First make sure you have enabled EPEL repository in your system. After that install dkim-milter package using following command.
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.
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.
Step 3 – Configure OpenDKIM
Edit the Opendkim configuration file and Add/Update following entries in file.
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
finally start DKIM service using following command
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.
Step 6 – Verify DKIM
To verify that DKIM is working properly. Let’s send a test email through command line
In the received email in our mailbox, open the source of the email and search for 'DKIM-Signature'. You will find something like below
What is OpenDKIM?
DKIM (DomainKeys Identified Mail) is an important authentication mechanism to help protect both email receivers and email senders from forged and phishing email. DKIM will sign outbound message, and receiving servers will check that signature against DNS records is added to your zone to verify the signature and lower the spam score if it passes.
Here’s how to install dkim-milter and Integration with postfix.
ENABLE EPEL REPOSITORY
Download and install epel-release package as per your system version and architecture. Basically they provides noarch packages, which means they are architecture independent but still EPEL providing different packages to download. So you can download as per you choice.
1. Install EPEL repository yum configuration, if not already installed.
For CentOS/RHEL 7:
For CentOS/RHEL 6:
For CentOS/RHEL 5:
2. Test EPEL Repository
Now check if your repository is working properly on your system. Execute following command to list all packages available in epel repository
INSTALL DKIM-MILTER
% sudo yum –enablerepo=epel install -y dkim-milter
CONFIGURE DKIM-MILTER
3. Configure dk-milter by editing /etc/mail/dkim-milter/dkim-filter.conf.
A sample configuration is provided below:
,—- /etc/mail/dkim-milter/dkim-filter.conf
| # Key list file.
| KeyList /etc/mail/dkim-milter/keys/keylist
|
| # default selector
| Selector default
|
| # zero tolerance towards alteration of headers in transit.
| Canonicalization simple/simple
|
| # execute under the context of postfix user.
| UserID postfix
|
| # List of domains delimited by commas
| Domain domain.tld
|
| # Path to the socket
| Socket local:/var/run/dk-milter/dk.sock
`—-
| # Key list file.
| KeyList /etc/mail/dkim-milter/keys/keylist
|
| # default selector
| Selector default
|
| # zero tolerance towards alteration of headers in transit.
| Canonicalization simple/simple
|
| # execute under the context of postfix user.
| UserID postfix
|
| # List of domains delimited by commas
| Domain domain.tld
|
| # Path to the socket
| Socket local:/var/run/dk-milter/dk.sock
`—-
4. Alter permissions on dkim-milter socket directory.
% sudo chown postfix:dkim-milter /var/run/dkim-milter
% sudo chmod 700 /var/run/dkim-milter
% sudo chmod 700 /var/run/dkim-milter
SET-UP PUBLIC/PRIVATE KEYS
5. Now generate a 2048 bit key for DKIM in /etc/mail/dkim-milter/keys directory, and append key details to the keylist file.
% sudo su –
# cd /etc/mail/dkim-milter/keys
# dkim-genkey -b 2048 -d domain.tld -s default
# echo ‘*:domain.tld:/etc/mail/dkim-milter/keys/default’ >>keylist
% sudo su –
# cd /etc/mail/dkim-milter/keys
# dkim-genkey -b 2048 -d domain.tld -s default
# echo ‘*:domain.tld:/etc/mail/dkim-milter/keys/default’ >>keylist
6. Update permissions on keys directory.
% sudo chown -R postfix:dkim-milter /etc/mail/dkim-milter/keys
% sudo chmod 700 /etc/mail/dkim-milter/keys
% sudo chmod 700 /etc/mail/dkim-milter/keys
INTEGRATE POSTFIX
7. Configure dkim-milter service to run with ‘postfix’ user.
% echo user=postfix |sudo tee -a /etc/sysconfig/dkim-milter
8. Alter permissions on dkim-filter executable to be executable by ‘postfix’ user.
% sudo chown postfix /usr/sbin/dkim-filter
9. Add the following lines to postfix configuration main.cf:
,—-
| # Inform postfix about the DKIM milter
| smtpd_milters = unix:/var/run/dkim-milter/dkim-milter.sock
| non_smtpd_milters = unix:/var/run/dkim-milter/dkim-milter.sock
`—-
| # Inform postfix about the DKIM milter
| smtpd_milters = unix:/var/run/dkim-milter/dkim-milter.sock
| non_smtpd_milters = unix:/var/run/dkim-milter/dkim-milter.sock
`—-
(RE)START SERVICES
10. Start the service.
% sudo /sbin/service dkim-milter start
11. Configure it to auto start on boot.
% sudo /sbin/chkconfig –levels 2345 dkim-milter on
Centos 7 Postfix Dkim
12. Reload postfix service
Configure DNS Entry
13. After configuring private key in postfix server. there will be another file dd the contents of the file ‘/etc/mail/dkim-milter/keys/default.txt’ generated by dkim-genkey. Edit your DNS zone file and add this as TXT record found in default.txt. In my case this is like below.
Postfix Dkim Centos 8
14. Now send a mail with [email protected] address to your Gmail, Y!, or any DKIM compliant mail service provider. You should see dkim=pass or something like that in Authentication-Results MIME header.
Authentication-Results: mx.google.com;….[ other verification schemes ]……..; dkim=pass [email protected]
Postfix Dkim Centos Key
Known issues
Postfix Dkim Centos Command
If mail signature verification fails (dkim=hardfail) for some particular mails, then this hints that those messages are being modified in the transit after signing. If you don’t strictly care for message being modified in transit, but just care for it to be verifiable, then in that case. change ‘Canonicalization’ parameter in dkim-filter.conf to the ‘relaxed/relaxed’ which hints at relaxed verification.