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71 lines
3.1 KiB
Markdown
71 lines
3.1 KiB
Markdown
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<img align="right" src="images/logo-internetnl-en.svg">
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# UNDER CONSTRUCTION!!!
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# STARTTLS how-to
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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.
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# Table of contents
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To-Do
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# What is STARTTLS?
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# Why use STARTTLS?
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# Tips, tricks and notices for implementation
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* 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.
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* E-mail forwarding is not supported, since the e-mail is often forwarded by another e-mail server.
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* SPF does not work between domains that use the same e-mail server.
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* 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".
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* 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.
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* 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.
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## Implementing STARTTLS in Postfix
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**Specifics for this setup**
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* Linux Debian 10 (Buster)
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* Postfix 3.4.5
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* OpenSSL 1.1.1d
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**Assumptions**
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* Mail server is using DANE
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* Software packages are already installed
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### Configuring Postfix
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# use DANE
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smtp_dns_support_level = dnssec
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smtp_tls_security_level = dane
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smtp_host_lookup = dns
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smtp_tls_note_starttls_offer = yes
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# TLS protocol config
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smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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lmtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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lmtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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# TLS cipher config
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smtpd_tls_mandatory_ciphers=high
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smtpd_tls_ciphers=high
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tls_ssl_options = NO_COMPRESSION, 0x40000000
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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
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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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# Enable server cipher-suite preferences
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tls_preempt_cipherlist = yes
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# Forward secrecy (use the RFC 7919 defined DH group:https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem)
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smtpd_tls_eecdh_grade=ultra
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smtpd_tls_dh1024_param_file = /etc/postfix/ssl/ffdhe4096.pem
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# log the ciphers that are used
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smtp_tls_loglevel = 1
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smtpd_tls_loglevel = 1
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