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Email Services: TLS Configuration with Postfix

Securing `postfix` is an important part of managing an email server that is addressable on the Internet these days. One step in doing so is making sure that all connections use TLS. This hands-on lab will allow you to build a certificate chain and configure `postfix` to use that chain to encrypt data.

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Path Info

Clock icon Beginner
Clock icon 10m
Clock icon Sep 18, 2020

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Table of Contents

  1. Challenge

    Generate the SSL Certificate

    1. We need to create the key and CSR (Certificate Signing Request). We'll also need to work as the root user, so elevating now will save time.
    sudo -i
    openssl req -nodes -newkey rsa:2048 -keyout privatekey.key -out mail.csr

    This will ask for a bunch of information. Since we’re doing a self-signed certificate, we don’t need to fill all of it out — accept the defaults.

    1. Now we need to sign that request using the key we just generated.
    openssl x509 -req -days 365 -in mail.csr -signkey privatekey.key -out secure.crt
    1. Now we copy the key and certificate over to where postfix can use them.
    cp {privatekey.key,secure.crt} /etc/postfix
  2. Challenge

    Configure Postfix to use TLS

    1. Run:
    vim /etc/postfix/
    1. Insert these lines at the end:
    smtpd_use_tls = yes
    smtpd_tls_cert_file = /etc/postfix/secure.crt
    smtpd_tls_key_file = /etc/postfix/privatekey.key
    smtp_tls_security_level = may
    1. Restart postfix:
    systemctl restart postfix
    1. Test the configuration:
    openssl s_client -connect localhost:25 -starttls smtp
    1. Scroll back and look at the certificate information. If it matches what you put in, everything worked correctly!

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