How to Install and Configure SSL Certificate on Postfix?

What is Postfix?

Postfix is a free and open-source mail transfer agent (MTA) that is an SMTP-based electronic mail transfer agent. It was originally designed as a replacement for the Sendmail program and that the primary goals were ease of configuration, security and performance.

Postfix is said to be secure and fast and it is implemented in a modular form so that it can be extended in order to meet the requirements of one or the other mail server.

Postfix works by collecting messages from mail user agents (MUAs) or other mail servers, after which it forwards these messages to their target users or even other mail servers if they are local users or not, respectively.

It employs one or a sequence of processes together with corresponding queues to process email messages, guaranteeing that they are delivered securely.

Postfix works with all major protocols and standards such as Simple Mail Transfer Protocol (SMTP) and is compatible with other additional tools and services like spam filters, antivirus, and email authentication methods such as SPF, DKIM, DMARC.

Prerequisites for Installing SSL on Postfix

Before going for SSL installation, confirm that the certificate that has been issued is a valid SSL certificate reacting to the mail server’s hostname.

If you have not done so yet, you shall be able to buy an SSL certificate from a proper CA such as Certera. Holders of the SSL certificate can download the SSL certificate files through their registered email or through their CAs account control panel.

Step-by-Step Guide to Install SSL on Postfix

Step 1: Upload and Concatenate Certificate Files

Once you have the SSL certificate files, copy it to the directory of your server where the SSL certificates are placed. These files are often found in /etc/ssl/certs/ for the certificates and in /etc/ssl/private/ for certificate’s keys.

If your SSL certificate provider has provided separate CA bundle files, kindly upload them to the same directory.

In order to concatenate these files into a single certificate file, open a terminal session and execute one of the following commands:

# Concatenate yourdomainname.crt and yourdomainname.ca-bundle into certificate.crt
cat /etc/ssl/certs/yourdomainname.crt /etc/ssl/certs/yourdomainname.ca-bundle >> /etc/ssl/certs/certificate.crt

Or, if you have multiple CA bundle files:

# Concatenate yourdomainname.crt, COMODORSADomainValidationSecureServerCA.crt,
# COMODORSAAddTrustCA.crt, and AddTrustExternalCARoot.crt into certificate.crt

cat /etc/ssl/certs/yourdomainname.crt /etc/ssl/certs/COMODORSADomainValidationSecureServerCA.crt /etc/ssl/certs/COMODORSAAddTrustCA.crt /etc/ssl/certs/AddTrustExternalCARoot.crt >> /etc/ssl/certs/certificate.crt

Make sure, in the concatenated file (certificate.crt), you include the full chain of trust, right from your domain certificate up to the root CA certificate.

Step 2: Edit Postfix Configuration for SSL/TLS

Postfix is in charge of mail delivery as well as the mail transfer within and out of the system. To set up SSL/TLS encryption for Postfix, the main configuration file is found at /etc/postfix/main.cf using a text editor:

sudo nano /etc/postfix/main.cf

Find the following directives and modify them to point to your concatenated certificate file (certificate. crt) and your private key file (yourdomainname. key):

smtpd_tls_cert_file = /etc/ssl/certs/certificate.crt
smtpd_tls_key_file = /etc/ssl/private/yourdomainname.key
smtpd_use_tls = yes

These directives tell Postfix to employ SSL/TLS encryption in SMTP connections Short for Simple Mail Transfer Protocol. Make sure that smtpd_use_tls is configured to ‘yes’ in order to allow TLS encryption on arriving mails.

Step 3: Secure Other Mail Ports

Postfix employs different ports for communicating in mails and each port can be configured to use SSL/TLS options. Open the Postfix master. cf file is stored in /etc/postfix/master to configure postfix.cf

sudo nano /etc/postfix/master.cf

Remove the  Comments (#) from the following lines and make the necessary modifications to enable the SSL/TLS encryption for the Submission and SMTPS services that run on TCP port 587 and 465 respectively.

For Submission (587):

submission inet n - y - - smtpd
            -o syslog_name=postfix/submission
            -o smtpd_tls_security_level=may
            -o smtpd_sasl_auth_enable=yes

For SMTPS (465):

smtps inet n - y - - smtpd
            -o syslog_name=postfix/smtps
            -o smtpd_tls_wrappermode=yes
            -o smtpd_sasl_auth_enable=yes

Save and close the master.cf file after making the changes.

Step 4: Configure Dovecot for SSL/TLS

POP3 and IMAP are arranged by Dovecot for retrieving incoming mails. To set up the use of SSL/TLS encryption for Dovecot, it is required to edit the ‘10-ssl’ configuration file. dovecot configuration file is of type conf and can be found in /etc/dovecot/conf. d/:

sudo nano /etc/dovecot/conf.d/10-ssl.conf

Replace the content of ssl_cert with the location of your concatenated certificate file (certificate. crt) and replace the content of ssl_key with the path to your private key file (yourdomainname. key).

ssl_cert = </etc/ssl/certs/certificate.crt
ssl_key = </etc/ssl/private/yourdomainname.key
ssl = yes

These directives allow using SSL/TLS encryption for IMAP and POP3 in Dovecot. Save and close the 10-ssl.conf file after the changes have been made as well.

Step 5: Advanced Tweaks

If you require additional, more specific SSL/TLS configuration settings then you may wish to review further Postfix [main. cf] and Dovecot [10-ssl. conf].

The fine tuning configurations are available for SSL handshake implementation, disabling plaintext authentication, and choice of cipher preference for secure response.

Step 6: SSL Installation Verification

The validity of the SSL certificate installation can also be checked using the tools such as OpenSSL or SSL checkers. For instance, to check SSL/TLS connection availability for some given ports with the help of OpenSSL

openssl s_client -connect yourdomain.com:port_number -servername yourdomain.com -starttls smtp

Replace yourdomain. com with your real domain name and port_number with the port number like 587 for Submission). This command verifies that you have SSL/TLS encryption correctly installed and working for your mail server.

Conclusion

This way, you will be able to install & configure SSL/TLS encryption on your Postfix-Dovecot mail server to enable secure channels for incoming and outgoing mails. From web hosting to mail servers and any other platforms that require protection, Certera has the best solutions for your organization.