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Update STARTTLS-how-to.md

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Dennis Baaten 2020-05-28 11:01:58 +02:00
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<img align="right" src="images/logo-internetnl-en.svg">
<img align="right" src="/images/logo-internetnl-en.svg">
# UNDER CONSTRUCTION!!!
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This how-to is created by the Dutch Internet Standards Platform (the organization behind [internet.nl](https://internet.nl)) and is meant to provide practical information and guidance on implementing STARTTLS.
# Table of contents
To-Do
Under construction
# What is STARTTLS?
Under construction
# Why use STARTTLS?
Under construction
# Tips, tricks and notices for implementation
* The sender address shown to the user ("RFC5322.From") is not used when authenticating. SPF uses the invisible "RFC5321.MailFrom" header. Combining SPF with DMARC removes this disadvantage.
* E-mail forwarding is not supported, since the e-mail is often forwarded by another e-mail server.
* SPF does not work between domains that use the same e-mail server.
* Parked domains should be explicitly configured to not use e-mail. For SPF this is done with an empty policy (not mentioning any ip-adresses or hostnames which are allowed to send mail) and a hard fail: "v=spf1 –all".
* When processing incoming mail we advise to favor a DMARC policy over an SPF policy. Do not configure SPF rejection to go into effect early in handling, but take full advantage of the enhancements DMARC is offering. A message might still pass based on DKIM.
* At the same time, be aware that some operaters still allow a hard fail (-all) to go into effect early in handling and skip DMARC operations.
* http://postfix.1071664.n5.nabble.com/Disable-SSL-TLS-renegotiation-td96864.html#a96871
## Implementing STARTTLS in Postfix
**Specifics for this setup**
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### Configuring Postfix
# use DANE
# use DANE (when acting as a client)
smtp_dns_support_level = dnssec
smtp_tls_security_level = dane
smtp_host_lookup = dns
smtp_tls_note_starttls_offer = yes
# TLS protocol config
# --- TLS settings ---
smtpd_tls_security_level = may
smtpd_tls_key_file = /etc/postfix/ssl/example.nl.key
smtpd_tls_cert_file = /etc/postfix/ssl/example.nl.crt
smtpd_tls_CAfile = /etc/postfix/ssl/example.nl-cabundle.crt
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
# --- TLS protocol config ---
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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lmtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
lmtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
# TLS cipher config
# --- TLS cipher config ---
smtpd_tls_mandatory_ciphers=high
smtpd_tls_ciphers=high
# disable compression and client-initiated renegotiation
tls_ssl_options = NO_COMPRESSION, 0x40000000
smtpd_tls_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CBC3-SHA, KRB5-DES, CBC3-SHA, DHE-RSA-AES256-CCM8, AES256-CCM8, DHE-RSA-AES128-CCM8, AES128-CCM8
smtp_tls_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CBC3-SHA, KRB5-DES, CBC3-SHA, DHE-RSA-AES256-CCM8, AES256-CCM8, DHE-RSA-AES128-CCM8, AES128-CCM8
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# Forward secrecy (use the RFC 7919 defined DH group:https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem)
smtpd_tls_eecdh_grade=ultra
smtpd_tls_dh1024_param_file = /etc/postfix/ssl/ffdhe4096.pem
# log the ciphers that are used
# --- TLS logging ---
smtp_tls_loglevel = 1
smtpd_tls_loglevel = 1