email-toolbox-wiki/STARTTLS-how-to.md

UNDER CONSTRUCTION!!!

STARTTLS how-to

This how-to is created by the Dutch Internet Standards Platform (the organization behind internet.nl) and is meant to provide practical information and guidance on implementing STARTTLS.

Table of contents

Under construction

What is STARTTLS?

Under construction

Why use STARTTLS?

Under construction

Tips, tricks and notices for implementation

• Use the RFC 7919 defined DH groups: https://raw.githubusercontent.com/internetstandards/dhe_groups/main/ffdhe4096.pem)

Implementing STARTTLS in Postfix

Specifics for this setup

• Ubuntu 24.04
• Postfix 3.10.x
• OpenSSL 3.0.3

Assumptions

• Mail server is using DANE
• Software packages are already installed

Configuring Postfix

/etc/postfix/main.cf

tls_high_cipherlist = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-CCM:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-CCM:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256
tls_config_name = postfix
tls_preempt_cipherlist = yes

smtpd_tls_cert_file=...
smtpd_tls_key_file=...
smtpd_tls_security_level=may
smtpd_tls_auth_only=yes
smtpd_tls_protocols = >=TLSv1.2
smtpd_tls_mandatory_protocols = >=TLSv1.2
smtpd_tls_ciphers = high
smtpd_tls_mandatory_ciphers = high
smtpd_tls_loglevel = 1
smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_tls_session_cache
smtpd_tls_received_header = yes

smtp_tls_note_starttls_offer = yes
smtp_tls_protocols = >=TLSv1.2
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_ciphers = high
smtp_tls_mandatory_ciphers = high
smtp_tls_loglevel = 1
smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_tls_session_cache
smtp_dns_support_level = dnssec
smtp_tls_security_level = dane

/etc/ssl/openssl.cnf

postfix = postfix_settings
....
[postfix_settings]
ssl_conf = postfix_ssl_settings
[postfix_ssl_settings]
system_default = baseline_postfix_settings
[baseline_postfix_settings]
SignatureAlgorithms = ECDSA+SHA256:ECDSA+SHA384:ECDSA+SHA512:RSA-PSS+SHA256:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA+SHA512:RSA+SHA256:RSA+SHA384

To enable PQC:

tls_eecdh_auto_curves =
tls_ffdhe_auto_groups =