How to Install and Configure OpenDKIM in Ubuntu

Introduction

DomainKeys Identified Mail ( DKIM ) is an email authentication method designed to detect forged sender addresses in emails, a technique often used in phishing and email spam. DKIM allows an organization to claim responsibility for a message in a way that can be validated by the recipient. OpenDKIM is an open-source implementation of the DKIM specification that is widely used to add this layer of security to email systems. The goal of implementing OpenDKIM is to improve email deliverability and protect your domain’s reputation.

This guide will walk you through the steps of installing and configuring OpenDKIM on an Ubuntu server. We will cover the installation process, configuration details, integration with the Postfix mail server, and testing to ensure everything is set up correctly. Properly configuring OpenDKIM is crucial for maintaining a trustworthy email presence.

Prerequisites

Before we begin, ensure that you have the following:

1. An Ubuntu server (version 20.04 or later is recommended).
2. Root or sudo privileges on the server.
3. A registered domain name.
4. A working Postfix mail server.
5. Access to your domain’s DNS settings.

Step 1: Update System Packages

Start by updating the package lists on your Ubuntu server to ensure you have the latest versions available.

$ sudo apt update
$ sudo apt upgrade -y

Next, install OpenDKIM and its dependencies using the following command:

$ sudo apt install opendkim opendkim-tools -y

Create OpenDKIM Configuration Directory

Create a directory for OpenDKIM configuration and key files.

$ sudo mkdir /etc/opendkim
$ sudo mkdir /etc/opendkim/keys

Configure OpenDKIM

Open the main configuration file for OpenDKIM.

$ sudo nano /etc/opendkim.conf

Add the following configuration settings to the file:

Syslog                  yes
UMask                   002
Mode                    sv
Canonicalization        relaxed/simple
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts           refile:/etc/opendkim/TrustedHosts
KeyTable                refile:/etc/opendkim/KeyTable
SigningTable            refile:/etc/opendkim/SigningTable
Socket                  inet:12345@localhost
PidFile                 /var/run/opendkim/opendkim.pid
UserID                  opendkim:opendkim
TemporaryDirectory      /var/tmp

Configure Trusted Hosts

Edit the TrustedHosts file to include your local network and mail server.

$ sudo nano /etc/opendkim/TrustedHosts

Add the following lines:

127.0.0.1
localhost
192.168.0.1/24  # Replace with your local network
*.yourdomain.com  # Replace with your domain

Configure Key Table

Edit the KeyTable file to specify the location of your DKIM keys.

$ sudo nano /etc/opendkim/KeyTable

Add the following line (replace yourdomain.com with your actual domain name):

mail._domainkey.yourdomain.com yourdomain.com:mail:/etc/opendkim/keys/yourdomain.com/mail.private

Configure Signing Table

Edit the SigningTable file to define which domains should be signed.

$ sudo nano /etc/opendkim/SigningTable

Add the following line:

*@yourdomain.com mail._domainkey.yourdomain.com

Step 4: Generate DKIM Keys

Navigate to the keys directory and create a directory for your domain.

$ cd /etc/opendkim/keys
$ sudo mkdir yourdomain.com
$ cd yourdomain.com

Generate a new DKIM key pair using the following command:

$ sudo opendkim-genkey -s mail -d yourdomain.com
$ sudo chown opendkim:opendkim mail.private

This will generate two files:

• mail.private: The private key, used by OpenDKIM to sign emails.
• mail.txt: The public key, which needs to be added to your DNS records.

Step 5: Add DKIM Public Key to DNS

Open the mail.txt file and copy its contents.

$ cat mail.txt

You will see something like this:

mail._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDmt+8iyK2xwLth..."

Log in to your DNS provider and create a new TXT record with the following details:

• Name/Host/Record Name: mail._domainkey
• Type: TXT
• Value/Text: The entire content of the mail.txt file, including the quotes.

Step 6: Configure Postfix to Use OpenDKIM

Edit the Postfix main configuration file to integrate OpenDKIM.

$ sudo nano /etc/postfix/main.cf

Add the following lines at the end of the file:

milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:12345
non_smtpd_milters = inet:localhost:12345

Save and close the file.

Step 7: Start and Enable OpenDKIM

Start the OpenDKIM service and enable it to start on boot.

$ sudo systemctl start opendkim
$ sudo systemctl enable opendkim

Restart Postfix to apply the changes.

$ sudo systemctl restart postfix

Step 8: Test Your Configuration

Send a Test Email

Send a test email to an external email address (e.g., Gmail or Yahoo) to check if DKIM signing is working.

After receiving the test email, check the email headers for the DKIM-Signature header. This header indicates that the email has been signed by OpenDKIM.

Verify DKIM Signature

Use an online DKIM verification tool (e.g., DKIMCore, MXToolbox) to verify the DKIM signature. Enter the selector (mail) and your domain (yourdomain.com) to check if the public key is correctly configured in your DNS.

Troubleshooting

Check OpenDKIM Logs

If you encounter issues, check the OpenDKIM logs for error messages.

$ sudo journalctl -u opendkim

Ensure Correct Permissions

Ensure that the opendkim user has the correct permissions for the key files.

$ sudo chown -R opendkim:opendkim /etc/opendkim/keys

Validate DNS Configuration

Double-check your DNS records to ensure the DKIM public key is correctly added.

Restart Services

If changes are made to the configuration files, restart the OpenDKIM and Postfix services.

$ sudo systemctl restart opendkim
$ sudo systemctl restart postfix

Conclusion

Setting up DKIM with OpenDKIM on an Ubuntu server enhances your email security by verifying the authenticity of your emails. By following the steps outlined in this guide, you can successfully install, configure, and integrate OpenDKIM with your Postfix mail server. Regularly monitor and update your DKIM keys and DNS records to maintain a secure email environment.

Alternative Solutions for DKIM Implementation

While OpenDKIM provides a robust solution for DKIM signing, alternative methods exist, catering to different needs and infrastructure setups. Here are two alternative approaches:

1. Using a Commercial Email Service Provider (ESP) with Built-in DKIM

Instead of self-hosting a mail server and managing DKIM manually, leveraging a commercial ESP like SendGrid, Mailgun, or Amazon SES offers a simplified approach. These services typically handle DKIM configuration automatically or provide user-friendly interfaces for setup.

Explanation:

ESPs are designed to manage email infrastructure, including authentication protocols like DKIM, SPF, and DMARC. By using an ESP, you offload the complexity of managing these protocols to a specialized provider. They typically provide clear instructions on how to configure your DNS records to delegate signing authority to them. This is a great solution for those who do not want to self-manage an email server.

How it Works:

1. Account Creation: Sign up for an account with a reputable ESP.
2. Domain Verification: Verify ownership of your domain through DNS records (usually by adding a TXT record provided by the ESP).
3. DKIM Configuration (Automatic or Guided): The ESP either automatically configures DKIM for your domain or provides step-by-step instructions for adding specific DNS records (usually CNAME records) to delegate signing authority.
4. Sending Emails: Send emails through the ESP’s API or SMTP server. The ESP automatically signs your emails with DKIM using its own infrastructure.

Example (Conceptual – Varies by ESP):

Let’s say you choose Mailgun. After domain verification, they might provide the following DNS records to add:

Type: TXT
Name: krs._domainkey.yourdomain.com
Value: v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAyC/....

You add this TXT record to your domain’s DNS. When you send emails through Mailgun, they will use this key (and their corresponding private key) to sign your emails.

Advantages:

• Simplified setup and management.
• Scalability and reliability provided by the ESP’s infrastructure.
• Automatic handling of DKIM key rotation and updates.
• Improved deliverability due to the ESP’s reputation and best practices.

Disadvantages:

• Cost associated with the ESP’s services.
• Dependency on a third-party provider.
• Less control over the underlying DKIM implementation.

2. Using a DKIM Proxy Server (e.g., dkimproxy)

DKIM proxy servers act as intermediaries between your mail server and the outside world, adding DKIM signatures to outgoing emails. dkimproxy is a popular option.

Explanation:

A DKIM proxy sits between your Mail Transfer Agent (MTA) (like Postfix) and the internet. It intercepts outgoing emails, signs them with DKIM, and then forwards them to their destination. This provides a centralized signing solution that can be easier to manage than configuring DKIM directly on the MTA. This is also helpful if your MTA does not natively support DKIM signing.

How it Works:

1. Installation: Install dkimproxy on a server (can be the same as your mail server or a separate one).
2. Configuration: Configure dkimproxy with your domain, selector, and private key.
3. Postfix Integration: Configure Postfix to forward outgoing emails to dkimproxy.
4. Signing: dkimproxy signs the emails and forwards them to their destination.

Example (Illustrative):

First, install dkimproxy (the exact installation method will depend on your OS; here’s a conceptual apt-get):

sudo apt-get install dkimproxy

Then, configure dkimproxy.conf:

domain yourdomain.com
signature_algorithm rsa-sha256
selector mail
key /etc/dkimproxy/private.key

Next, generate a private key (similar to the OpenDKIM process, but dkimproxy might have its own key generation tool). Place this private key at /etc/dkimproxy/private.key.

Finally, configure Postfix to route outgoing mail through dkimproxy. In /etc/postfix/main.cf, you would add (or modify) settings to use dkimproxy as a transport:

smtp_sasl_auth_enable = no
smtp_generic_maps = hash:/etc/postfix/generic
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport:
yourdomain.com dkimproxy:
*               smtp:[next-hop-mail-server]

/etc/postfix/generic:
root@yourdomain.com   info@yourdomain.com

You would also need to configure dkimproxy to listen for connections from Postfix. This usually involves configuring firewall rules and ensuring dkimproxy is listening on the correct port. [next-hop-mail-server] would be your actual next-hop mail server. You’d also need to run postmap /etc/postfix/transport and postmap /etc/postfix/generic after making these changes.

Advantages:

• Centralized DKIM signing for multiple mail servers.
• Simpler integration with existing mail server infrastructure.
• Can be used with MTAs that don’t natively support DKIM.

Disadvantages:

• Requires additional server and software management.
• Potential performance overhead due to the proxy server.
• Increased complexity compared to using an ESP.