diff --git a/DKIM-how-to.md b/DKIM-how-to.md index e7d396d..4ad38e5 100644 --- a/DKIM-how-to.md +++ b/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 - -Signing in Postfix - -## Implementing DKIM in Postfix with SpamAssassin +## Implementing DKIM with OpenDKIM for Postfix with SpamAssassin **Specifics for this setup** * Linux Debian 9.8 (Stretch) * SpamAssassin version 3.4.2 (running on Perl version 5.28.1) @@ -30,4 +26,86 @@ Signing in Postfix **Assumptions** * DNSSEC is used * Mail server is operational -* Software packages are already installed \ No newline at end of file +* 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.