Update DKIM-how-to.md

DKIM-how-to.md

@@ -1,5 +1,5 @@
 # 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.
+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. 
@@ -15,11 +15,7 @@ A common used technique used by spammers is to trick the receiving party into be
 # 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
+## Implementing DKIM with OpenDKIM for Postfix with SpamAssassin
-
-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)
@@ -31,3 +27,85 @@ Signing in Postfix
 * DNSSEC is used
 * Mail server is operational
 * Software packages are already installed
+
+### Set up OpenDKIM and created key pair for your domain
+Make sure the file ***/etc/opendkim.conf** has a least the following configuration options.
+
+    UMask                   002
+    Canonicalization        relaxed/simple
+    Mode                    sv
+    AutoRestart             Yes
+    AutoRestartRate         10/1h
+    ExternalIgnoreList      refile:/etc/opendkim/trusted_hosts
+    InternalHosts           refile:/etc/opendkim/trusted_hosts
+    KeyTable                refile:/etc/opendkim/key_table
+    SigningTable            refile:/etc/opendkim/signing_table
+    PidFile                 /var/run/opendkim/opendkim.pid
+    SignatureAlgorithm      rsa-sha256
+    UserID                  opendkim:opendkim
+    Socket                  inet:12301@localhost
+
+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.
+
+> -----BEGIN RSA PRIVATE KEY-----  
+> MIICXAIBAAKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP  
+> +8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2c  
+> L+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB  
+> AoGASy+V+/Efbxogw0DmRgoLb4+pTU87+d7XJC2YxVN3V9tdq6vxSRslPr8QCuZs  
+> Ievp2XN0K7qE2BbbYbhq5nHDjwzPJ7vCZzN3JI8eOC9gKP++Te6AAcDjP+G3LND4  
+> Np2AWsn6JwGeM0QYI5Ehrxrw5HlqNb620N6wOEyd/7s4Px0CQQDVT3LhDzUOkbAW  
+> J/jUHdV4WYozRcBGFFJvH85ASJAbK9OSrF3tfJZj9e78xP4Z5EZ8jp9iKgajt5zl  
+> fYtAYjZfAkEAyl/gascX17nxO3rH/8hr8dPS6hY0KYTKXCZfuvYaSG7AZ3oaSQSc  
+> mz3Rz67cm14DZNc01aBE7PwiRjq9TsQo6wJAQkppijXeqENwdMJBWzJWWAuDnoGL  
+> ynugTraUs3eZiUgqfUeh/R8d4bzZY6aYzVUa7rSoJaqn25NBaDSG5SBggwJBAKrp  
+> VepXwjcafjSxeP74ENHnBxVTMzJtR0mTzv1iosfRYQUDBffswSYKi4tOLlm4iD09  
+> 0w0nkY5jUb7mFMLUv4kCQDjgGWNO8AeAohYF47fmYXeMMvS29rKtdLiR7D41WtOo  
+> +zM7YTQa9kGRihEK+iT8v1x7ZX3mt0WZ5eoupeGauio=  
+> -----END RSA PRIVATE KEY-----
+
+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:
+
+> selector201906._domainkey.example.nl example.nl:selector201906:/etc/opendkim/keys/example.nl/selector201906.private
+
+Create the file **/etc/opendkim/signing_table** and add the following line:
+
+> *@example.nl selector201906._domainkey.example.nl
+
+This concludes the configuration of OpenDKIM. Start OpenDKIM and check your logfiles for possible errors.
+
+### Publish the DNS record
+
+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.