Configuring OpenDKIM for Postfix SMTP Server
OpenDKIM is an open-source implementation of the DKIM (DomainKeys Identified Mail) sender authentication system used to verify the authenticity of email messages. By configuring OpenDKIM, we can sign outgoing email messages from our Postfix SMTP server to ensure that they are not spoofed or tampered with.
Here is a step-by-step guide to configuring OpenDKIM for your Postfix SMTP server:
Step 1: Install OpenDKIM To get started, you need to install OpenDKIM and its tools. You can do this by running the following command:
sudo apt install opendkim opendkim-tools
Step 2: Add Postfix User to OpenDKIM Group Next, add the postfix user to the opendkim group so that Postfix can access the OpenDKIM service. You can do this by running the following command:
sudo gpasswd -a postfix opendkim
Step 3: Configure OpenDKIM Main Configuration File Open the OpenDKIM main configuration file using the following command:
sudo nano /etc/opendkim.conf
Find the following line and uncomment it:
Syslog yes
By default, OpenDKIM logs are saved in the /var/log/mail.log file. To generate more detailed logs for debugging, add the following line:
Logwhy yes
Locate the following lines and uncomment them:
#Canonicalization simple #Mode sv #SubDomains no
Replace “simple” with “relaxed/simple” in the “Canonicalization” line.
Then, add the following lines below the “#ADSPAction continue” line:
AutoRestart yes AutoRestartRate 10/1M Background yes DNSTimeout 5 SignatureAlgorithm rsa-sha256
Finally, add the following lines at the end of the file:
#OpenDKIM user # Remember to add user postfix to group opendkim UserID opendkim # Map domains in From addresses to keys used to sign messages KeyTable refile:/etc/opendkim/key.table SigningTable refile:/etc/opendkim/signing.table # Hosts to ignore when verifying signatures ExternalIgnoreList /etc/opendkim/trusted.hosts # A set of internal hosts whose mail should be signed InternalHosts /etc/opendkim/trusted.hosts
Save and close the file.
Step 4: Restart OpenDKIM and Postfix Finally, restart the OpenDKIM and Postfix services using the following commands:
sudo systemctl restart opendkim sudo systemctl restart postfix
Now, your Postfix SMTP server is configured to sign outgoing email messages using OpenDKIM. The next time you send an email from your server, OpenDKIM will automatically sign the message to verify its authenticity.