# Introduction
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 DKIM.

# What is DKIM?
DKIM stands for **D**omain**K**eys **I**dentified **M**ail and is described in [RFC 6376](https://tools.ietf.org/html/rfc6376) with updates in [RFC 8301](https://tools.ietf.org/html/rfc8301) and [RFC 8463](https://tools.ietf.org/html/rfc8463). It is meant to provide the owner of a domain with the means to claim that a message has actually been send by the domain's e-mail server and should therefore be considered legitimate. It works by signing every individual e-mail message with a specific key (private key), so that the receiving party can use a corresponding key (public key) published in the sending domain's DNS record to validate the e-mail authenticity and to check whether the e-mail has not been tampered with. 

# Why use DKIM?
A common used technique used by spammers is to trick the receiving party into believing an e-mail is legitimate by using a forged sender address. This is also known as e-mail spoofing. DKIM has been designed to protect against spoofing. If an incoming e-mail does not have a DKIM signature or when it's DKIM signature does not validate, the receiving e-mail server should consider the e-mail to be SPAM.

# Tips, tricks and notices for implementation
* Use a DKIM key (RSA) of [at least 1024 bits](https://tools.ietf.org/html/rfc6376#section-3.3.3) to minimize the successrate of offline attacks. Don't go beyond a key size of 2048 bits since this is not mandatory according to the RFC.
* Make sure you to change your DKIM keys regularly. A rotation scheme of 6 months is recommended. 
* Parked domains should be explicitly configured to not use e-mail. For DKIM this is done with an empty policy: "v=DKIM1; p=".

# Outbound e-mail traffic
DKIM for outbound e-mail traffic can be accomplished by publishing a DKIM policy as a TXT record in a domain name's DNS zone, and by configuring the e-mail server to sign outbound e-mails.   

## Creating a DKIM policy

Signing in Postfix

## Implementing DKIM in Postfix with SpamAssassin
**Specifics for this setup**
* Linux Debian 9.8 (Stretch) 
* SpamAssassin version 3.4.2 (running on Perl version 5.28.1)
* Postfix 3.4.5
* BIND 9.10.3-P4-Debian
* OpenDKIM v2.11.0

**Assumptions**
* DNSSEC is used
* Mail server is operational
* Software packages are already installed