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.
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.
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.
* 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.
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.
Create the the file **/etc/opendkim/trusted_hosts** and make sure it contains the following:
127.0.0.1
localhost
example.nl
mail.example.nl
Now create the directory **/etc/opendkim/keys/example.nl** and execute the following command with this directory and make sure to replace 'YYYYMM' with the number of the current year and month. For example: "selector201906". This makes it easier to determine the age of a specific key in a later stage.
`opendkim-genkey -s selectorYYYYMM -d example.nl`
There are now 2 files in **/etc/opendkim/keys/example.nl** (the key pair):
* selector201906.txt: this file contains DNS complete DKIM DNS record including the public key.
> selector201906._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP+8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2cL+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB" ; ----- DKIM key selector201906 for example.nl
* selector201906.private: this file contains the private key which is going to be used by Postfix to sign all outbound e-mails.
Now make sure that the private key can only be read by the user opendkim by executing the following command:
`chown opendkim:opendkim selector201906.private`
The next step is to create the key table file **/etc/opendkim/key_table**. This file will tell opendkim about the domains that have been configured and where to find their keys. Add the following to configure example.nl:
Start OpenDKIM and check your logfiles for possible errors.
#### Key rotation
OpenDKIM does not support the automated rotation of DKIM keys. This means that you should rotate your keys manually, write a script to do this, or use an existing script like [https://github.com/tetsuo13/OpenDKIM-Rotate-Keys](https://github.com/tetsuo13/OpenDKIM-Rotate-Keys) or (https://github.com/captbrando/dkimrotator)[https://github.com/captbrando/dkimrotator].
Make sure to add the following lines to you domain's zone file:
> selector201906._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP+8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2cL+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB"
> _adsp._domainkey IN TXT "dkim=all"
The first line publishes the selector and the associated public key. The second line tells receiving mail server that all e-mail coming from the domain example.nl are DKIM signed.
### Configure Postfix
The final step is to configure Postfix to actually sign outbound e-mail using OpenDKIM. In order to do this add the following to **/etc/postfix/main.cf**:
milter_protocol = 6
milter_default_action = accept
smtpd_milters = inet:localhost:12301
non_smtpd_milters = inet:localhost:12301
When you are ready to start using DKIM restart Postfix, but make sure you waited long enough for the DKIM DNS record to succesfully propagate.
SpamAssassin uses a scoring mechanism in order to determine if an e-mail should be considered spam. By default SpamAssassin considers an e-mail to be spam if the score at least "5". An e-mail starts with a score of 0 and points are added based on the [tests](https://spamassassin.apache.org/old/tests_3_3_x.html) performed. The tests performed can be configured by adding specific [configuration parameters](https://spamassassin.apache.org/full/3.4.x/doc/Mail_SpamAssassin_Conf.html) in **/etc/spamassassin/local.cf**.
Now here's the tricky part. The points added to the score of an incoming e-mail based on the results of a specific test, is at its core a custom job. Many variables can be taken into consideration when scoring an e-mail (which is considered the strength of a post-SMTP spam filter) and the detailed scoring depends on a domain owner's specific wishes. For the sake of this how-to, the DKIM scoring will be based on the assumption that the domain owner wants to consider an e-mail to be spam if the sending e-mail server's IP-address or host is not in the domain's SPF record.
With SpamAssassin this can be configured by adding the following scoring configuration parameters to **/etc/spamassassin/local.cf**:
