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# Table of contents
- [Executive Summary](#summary)
- [Executive Summary](#executive-summary)
- [Introduction](#introduction)
- [What is DANE?](#what-is-dane-)
- [Why use DANE for SMTP?](#why-use-dane-for-smtp-)
- [Guaranteeing a valid TLSA record](#guaranteeing-a-valid-tlsa-record)
* [DANE explained by illustrations](#dane-explained-by-illustrations)
+ [Mail delivery: TLS without DANE](#mail-delivery--tls-without-dane)
+ [Mail delivery: TLS with MITM using evil certificate](#mail-delivery--tls-with-mitm-using-evil-certificate)
+ [Mail delivery: TLS with MITM using evil certificate](#mail-delivery--tls-with-mitm-using-evil-certificate-1)
+ [Mail delivery: TLS with DANE](#mail-delivery--tls-with-dane)
- [Reliable certificate rollover](#reliable-certificate-rollover)
* [Current + next details](#current---next-details)
- [Tips, tricks and notices for implementation](#tips--tricks-and-notices-for-implementation)
- [Outbound e-mail traffic (DNS records)](#outbound-e-mail-traffic--dns-records-)
- [Inbound e-mail traffic (publishing DANE DNS records)](#inbound-e-mail-traffic--publishing-dane-dns-records-)
* [Generating DANE records](#generating-dane-records)
* [Publishing DANE records](#publishing-dane-records)
* [Generating DANE roll-over records](#generating-dane-roll-over-records)
* [Publishing DANE roll-over records](#publishing-dane-roll-over-records)
- [Implementing DANE for SMTP on Postfix (inbound e-mail traffic)](#implementing-dane-for-smtp-on-postfix--inbound-e-mail-traffic-)
- [Implementing DANE for SMTP on Postfix (inbound & outbound e-mail traffic)](#implementing-dane-for-smtp-on-postfix--inbound---outbound-e-mail-traffic-)
* [Configuring Postfix](#configuring-postfix)
- [Implementing DANE for SMTP on Exim (inbound & outbound e-mail traffic)](#implementing-dane-for-smtp-on-exim--inbound---outbound-e-mail-traffic-)
* [Configuration for inbound e-mail traffic](#configuration-for-inbound-e-mail-traffic)
@ -53,6 +59,24 @@ DANE addresses these shortcomings because:
* This allows sending mail servers to unconditionally require STARTTLS with a matching certificate chain. Otherwise, the sending mail server aborts the connection and tries another server or defers the message.
* Receiving servers with published TLSA records, are therefore no longer vulnerable to "STARTTLS stripping".
## DANE explained by illustrations
### Mail delivery: TLS without DANE
The illustration below shows two TLS capable e-mail servers without using DANE.
![](dane-example-1-no-dane.png)
### Mail delivery: TLS with MITM using evil certificate
The illustration below shows what happens when an attacker performs a man in the middle (MITM) attack and inserts its own certificate into the connection process.
![](dane-example-1-evilcert.png)
### Mail delivery: TLS with MITM using evil certificate
The illustration below shows what happens when an attacker performs a man in the middle (MITM) attack and forces an unsecure connection by stripping the TLS capability from the receiving e-mail server.
![](dane-example-1-striptls.png)
### Mail delivery: TLS with DANE
The illustration below shows how the use of DANE can protect against man in the middle (MITM) attacks by addressing the shortcomings of TLS without DANE.
![](dane-example-1-with-dane.png)
# Reliable certificate rollover
It is a good practice to replace certificates and keys from time to time, but this need not and should not disrupt email delivery even briefly.
* Since a single TLSA record is tied to a particular certificate or (public) key, the TLSA records that match a server's certificate chain also change from time to time.
@ -102,8 +126,8 @@ This section describes several pionts for attention when implementing DANE for S
* Implement monitoring of your DANE records to be able to detect problems as soon as possible.
* Make sure your implementation supports the usage of a CNAME in your MX record. There are some inconsistencies between multiple RFC's. According to [RFC 2181](https://tools.ietf.org/html/rfc2181#section-10.3) a CNAME in MX records is not allowed, while [RFC 7671](https://tools.ietf.org/html/rfc7671#section-7) and [RFC 5321](https://tools.ietf.org/html/rfc5321#section-5.1) imply that the usage of a CNAME in MX records is allowed.
# Outbound e-mail traffic (DNS records)
This part of the how-to describes the steps that should be taken with regard to your outbound e-mail traffic. This enables other parties to use DANE for validating the certificates offered by your e-mail servers.
# Inbound e-mail traffic (publishing DANE DNS records)
This part of the how-to describes the steps that should be taken with regard to your inbound e-mail traffic, which primairily involves publishing DANE DNS records. This enables other parties to use DANE for validating the certificates offered by your e-mail servers.
## Generating DANE records
**Primary mail server (mail1.example.nl)**
@ -178,7 +202,7 @@ With this information we can create a rollover DNS record for DANE:
> _25._tcp.mail.example.nl. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde
> _25._tcp.mail2.example.nl. IN TLSA 2 1 1 c784333d20bcd742b9fdc3236f4e509b8937070e73067e254dd3bf9c45bf4dde
# Implementing DANE for SMTP on Postfix (inbound e-mail traffic)
# Implementing DANE for SMTP on Postfix (inbound & outbound e-mail traffic)
**Specifics for this setup**
* Linux Debian 9.8 (Stretch)
@ -238,7 +262,7 @@ In Exim you should configure TLS by adding the following to **main/03_exim4-conf
tls_privatekey = /path/to/private.key
## Configuration for outbound e-mail traffic
This part of the how-to describes the steps that should be taken with regard to your outbound e-mail traffic. This enables other parties to use DANE for validating the certificates offered by your e-mail servers.
This part of the how-to describes the steps that should be taken with regard to your outbound e-mail traffic. This enables your e-mail environment to use DANE for validating the certificates offered by other e-mail servers.
### DNSSEC validating resolvers
Make sure to configure DNSSEC validating resolvers on the mail server. When using the locale systemd resolver, make sure to add the following to **/etc/systemd/resolved.conf**.