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140 lines
9.2 KiB
Markdown
140 lines
9.2 KiB
Markdown
# Introduction
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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.
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# What is DKIM?
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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.
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# Why use DKIM?
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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.
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# Tips, tricks and notices for implementation
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* 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.
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* Make sure you to change your DKIM keys regularly. A rotation scheme of 6 months is recommended.
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* Parked domains should be explicitly configured to not use e-mail. For DKIM this is done with an empty policy: "v=DKIM1; p=".
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# Implementing DKIM with OpenDKIM for Postfix with SpamAssassin
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**Specifics for this setup**
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* Linux Debian 9.8 (Stretch)
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* SpamAssassin version 3.4.2 (running on Perl version 5.28.1)
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* Postfix 3.4.5
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* BIND 9.10.3-P4-Debian
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* OpenDKIM v2.11.0
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**Assumptions**
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* DNSSEC is used
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* Mail server is operational
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* Software packages are already installed
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## Outbound e-mail traffic
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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.
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### Set up OpenDKIM and created key pair for your domain
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Make sure the file ***/etc/opendkim.conf** has a least the following configuration options.
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UMask 002
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Canonicalization relaxed/simple
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Mode sv
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AutoRestart Yes
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AutoRestartRate 10/1h
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ExternalIgnoreList refile:/etc/opendkim/trusted_hosts
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InternalHosts refile:/etc/opendkim/trusted_hosts
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KeyTable refile:/etc/opendkim/key_table
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SigningTable refile:/etc/opendkim/signing_table
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PidFile /var/run/opendkim/opendkim.pid
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SignatureAlgorithm rsa-sha256
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UserID opendkim:opendkim
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Socket inet:12301@localhost
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Create the the file **/etc/opendkim/trusted_hosts** and make sure it contains the following:
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127.0.0.1
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localhost
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example.nl
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mail.example.nl
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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.
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`opendkim-genkey -s selectorYYYYMM -d example.nl`
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There are now 2 files in **/etc/opendkim/keys/example.nl** (the key pair):
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* selector201906.txt: this file contains DNS complete DKIM DNS record including the public key.
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> selector201906._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP+8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2cL+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB" ; ----- DKIM key selector201906 for example.nl
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* selector201906.private: this file contains the private key which is going to be used by Postfix to sign all outbound e-mails.
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> -----BEGIN RSA PRIVATE KEY-----
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> MIICXAIBAAKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP
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> +8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2c
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> L+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB
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> AoGASy+V+/Efbxogw0DmRgoLb4+pTU87+d7XJC2YxVN3V9tdq6vxSRslPr8QCuZs
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> Ievp2XN0K7qE2BbbYbhq5nHDjwzPJ7vCZzN3JI8eOC9gKP++Te6AAcDjP+G3LND4
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> Np2AWsn6JwGeM0QYI5Ehrxrw5HlqNb620N6wOEyd/7s4Px0CQQDVT3LhDzUOkbAW
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> J/jUHdV4WYozRcBGFFJvH85ASJAbK9OSrF3tfJZj9e78xP4Z5EZ8jp9iKgajt5zl
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> fYtAYjZfAkEAyl/gascX17nxO3rH/8hr8dPS6hY0KYTKXCZfuvYaSG7AZ3oaSQSc
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> mz3Rz67cm14DZNc01aBE7PwiRjq9TsQo6wJAQkppijXeqENwdMJBWzJWWAuDnoGL
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> ynugTraUs3eZiUgqfUeh/R8d4bzZY6aYzVUa7rSoJaqn25NBaDSG5SBggwJBAKrp
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> VepXwjcafjSxeP74ENHnBxVTMzJtR0mTzv1iosfRYQUDBffswSYKi4tOLlm4iD09
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> 0w0nkY5jUb7mFMLUv4kCQDjgGWNO8AeAohYF47fmYXeMMvS29rKtdLiR7D41WtOo
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> +zM7YTQa9kGRihEK+iT8v1x7ZX3mt0WZ5eoupeGauio=
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> -----END RSA PRIVATE KEY-----
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Now make sure that the private key can only be read by the user opendkim by executing the following command:
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`chown opendkim:opendkim selector201906.private`
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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:
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> selector201906._domainkey.example.nl example.nl:selector201906:/etc/opendkim/keys/example.nl/selector201906.private
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Create the file **/etc/opendkim/signing_table** and add the following line:
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> *@example.nl selector201906._domainkey.example.nl
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Start OpenDKIM and check your logfiles for possible errors.
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#### Key rotation
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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].
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### Publish the DNS record
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Make sure to add the following lines to you domain's zone file:
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> selector201906._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCooJQftNOg3wOqVW5wOpr1PhhzgeP1IE9dTOtpUOCENP+z1HwP+8fFp9aGo/EKHoDQRhDUxXlVfocmRjb0lyjHD5ax16BBKLAd8+AgHZt1er8fmm2cL+7nurv0vU5YBG9LGUklD9qO/zJrIz+Lp+YO7D2rt0qYAgGzUOLJBWLBNQIDAQAB"
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> _adsp._domainkey IN TXT "dkim=all"
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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.
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### Configure Postfix
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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**:
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milter_protocol = 6
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milter_default_action = accept
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smtpd_milters = inet:localhost:12301
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non_smtpd_milters = inet:localhost:12301
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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.
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## Inbound e-mail
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### Configuring SpamAssassin
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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**.
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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.
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With SpamAssassin this can be configured by adding the following scoring configuration parameters to **/etc/spamassassin/local.cf**:
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```
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score DKIM_ADSP_ALL 5.0
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# No valid author signature, domain signs all mail
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score DKIM_ADSP_DISCARD 5.0
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# No valid author signature, domain signs all mail and suggests discarding the rest
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score DKIM_ADSP_NXDOMAIN 5.0
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# No valid author signature and domain not in DNS
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```
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This means that incoming e-mail is instantly classificied as spam if there is not a valid DKIM signature in the mail header and:
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* the sending domain's DKIM ADSP record states that all e-mail should be signed and all unsigned mails should be discarded (DISCARD).
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* the sending domain's DKIM ADSP record states that all e-mail should be signed (ALL).
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* the sending domain used in the "From"-header (a.k.a. RFC5322.From, Header From, Message From) does not exist.
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