email-toolbox-wiki/DMARC-how-to.md

DMARC how-to

This how-to is created by the Dutch Internet Standards Platform (the organization behind internet.nl) and is meant to provide practical information and guidance on implementing DMARC.

Table of contents

• What is DMARC?
• Why use DMARC?
• Tips, tricks and notices for implementation
• Overview of DMARC configuration tags
• Implementing DMARC with OpenDMARC for Postfix with SpamAssassin
  • Outbound e-mail traffic
    • Setting up a DMARC record
  • Inbound e-mail traffic
    • Set up OpenDMARC
    • Integrate with Postfix
    • Set up reporting
    • Configuring SpamAssassin
• Special thanks

Table of contents generated with markdown-toc

What is DMARC?

DMARC is short for Domain based Message Authentication, Reporting and Conformance and is described in RFC 7489. With DMARC the owner of a domain can, by means of a DNS record, publish a policy that states how to handle e-mail (deliver, quarantine, reject) which is not properly authenticated using SPF and/or DKIM. Because DMARC depends on the security of DNS, the use of DNSSEC is highly recommended.

At the same time DMARC also provides the means for receiving reports which allows a domain's administrator to detect whether their domainname is used for phishing or spam.

Why use DMARC?

Before DMARC, organizations already took several measures to determine the authenticity of an e-mail (like SPF and DKIM) to reduce the received amount of SPAM to a minimum. This is basically a good thing, but if these measures fail to choose whether or not an email is SPAM with a high level of certainty, the choice is redirected to the addressee (receiving party). This methodology is prone to abuse, since users are generally not equiped with the knowledge and/or means to classify incoming emails.

DMARC addresses this problem and enables the owner of a domain to take explicit responsiblity with regard to the actions taken by the sending party when the validity of an incoming email cannot be determined.

Tips, tricks and notices for implementation

• Interoperabily issues: https://tools.ietf.org/html/rfc7960
• DMARC does not require both DKIM or SPF. But implementation of both is strongly advised.
• DMARC is about aligning the domain in the DKIM header (the "d=" value) and SPF header (a.k.a. RFC5321.MailFrom, Return-Path, Envelope Sender, Envelope From) with the organizational domain in the message From header (a.k.a. RFC5322.From, Header From, Message From) which is visible to the user.
  • If these values do not align this could mean for example, that an attacker placed a valid DKIM signature header in an email with a "d=" value that points to a domain the attacker controls, allowing DKIM to pass while still spoofing the From address to the user.
• Parked domain: “DMARC p=reject”. Make sure to include rua and ruf addresses, since this allows monitoring of possible abuse attempts. Implement additional records (SPF, DKIM, NullMX) if possible, see also our Parked domain how-to.
• RFC 7489 states that the tags dmarc-version ("v=") and dmarc-request ("p=") should be on the first and second position of the DMARC record. The order of the other tags does not matter: "components other than dmarc-version and dmarc-request may appear in any order".
• The verified erratum 5440 of RFC 7489 states that a semicolon should be included in the DMARC version tag. Correct: "v=DMARC1;". Incorrect: "v=DMARC1".
• When using office 365, the forwarding of calendar appointments from a DMARC projected domain fails. This is a known issue. Read more on the Office 365 UserVoice forum and don't forget to submit your vote!
  • There is a workaround: Forward the appointment as an "iCalendar file" or as an attachment.
• When processing incoming mail we advise to favor a DMARC policy over an SPF policy. Do not configure SPF rejection to go into effect early in handling, but take full advantage of the enhancements DMARC is offering. A message might still pass based on DKIM.
  • At the same time, be aware that some operaters still allow a hard fail (-all) to go into effect early in handling and skip DMARC operations.
• When using a different domain for the rua and/or ruf address, make sure that the DMARC authorization records (example.nl._report._dmarc.differentdomain.nl) are properly DNSSEC signed. A regularly occuring mistake is the presence of "proof of non-existence" (NSEC3) for the ancestor domain (_report._dmarc.differentdomain.nl). If this happens then resolvers that use qname minimization (like the resolver used by Internet.nl) think that example.nl._report._dmarc.differentdomain.nl does not exists since _report._dmarc.differentdomain.nl does not exists. Therefore the resolver can't get the DMARC authorization record which makes DMARC fail.
  • Check your DNSSEC implementation on DNSViz. Enter "example.nl._report._dmarc.differentdomain.nl".
  • You can also manually check for this error. dig example.nl._report._dmarc.differentdomain.nl +dnssec results in a NOERROR response, while dig _report._dmarc.differentdomain.nl +dnssec results in a NXDOMAIN response.

Overview of DMARC configuration tags

The DMARC policy is published by means of a DNS TXT record. A DMARC record can contain several configuration tags. The table below will list all configuration tags and explain their purpose.

DMARC configuration tag	Required?	Value(s)	Explanation
v	mandatory	DMARC1;
p	mandatory	none quarantine reject	None: don't do anything if DMARC verification fails (used for testing) quarantine: treat mail that fails DMARC check as suspicious reject: reject mail that fail DMARC check
rua	optional	rua@example.nl	This field contains the email address used to send aggregate reports to
ruf	optional	ruf@example.nl	This field contains the email address used to send forensic reports to
fo	mandatory	0 1 s d	Reporting options for failure reports. Generates a report if: - both SPF and DKIM tests fail (0) - either SPF or DKIM test fail (1) - SPF test fails (s) - DKIM test fails (d)
adkim	optional	s r	Controls how strict the result of DKIM alignment checks should be intepreted. Strict or relaxed.
aspf	optional	s r	Controls how strict the result of SPF alignment checks should be intepreted. Strict or relaxed.
pct	optional	0..100	Determine percentage of mail from your domain to have the DMARC verificaton done by other mail providers. Default is 100.
rf	optional	afrf aodef	Preferred reporting format for forensic reports. Default is afrf.
ri	optional	60..86400	Interval (in seconds) at which you want to receive DMARC reports. The DMARC RFC specifies that organizations should be able to send reports at least once every day (86400 seconds) and also states a minimum interval of one hour (60 seconds). Default is 86400.
sp	optional	none quarantine reject	Subdomain policy. If not set the policy defined under "p=" will apply to all your subdomains.

Be aware that implementing a DMARC record without a rua configuration is possible, this is not advised because the DMARC XML files that are received by implementing a rua email address can help with implementing DKIM or SPF to meet the DMARC requirements.

Implementing DMARC with OpenDMARC for Postfix with SpamAssassin

Specifics for this setup

• Linux Debian 9.8 (Stretch)
• Postfix 3.4.5
• BIND 9.10.3-P4-Debian
• OpenDMARC v1.3.2

Assumptions

• DNSSEC is used
• Mail server is operational
• Software packages are already installed

Outbound e-mail traffic

DMARC for outbound e-mail traffic can be accomplished by publishing a DMARC policy as a TXT record in a domain name's DNS zone.

Setting up a DMARC record

Depending on your preferences and needs, you can determine the value of the configuration tags. The values below seem like a good starting point when setting up DMARC.

_dmarc IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@example.nl; ruf=mailto:dmarc@example.nl; fo=1; adkim=r; aspf=r; pct=100; rf=afrf; ri=86400; sp=quarantine"

Because this specific setup uses SpamAssassin for classifying e-mail to be SPAM or legitimate (HAM), the DMARC policy used is quarantine. This is done to prevent OpenDMARC from blocking the e-mail and, as a result, not enabling SpamAssassin to do its job.

Inbound e-mail traffic

Ideally incoming e-mail is processed by making a single decision based on a collective evaluation of all relevant e-mail standards (SPF, DKIM, DMARC). Although this would be the most elegant way of processing incoming e-mail, most e-mail servers process e-mail standards in a sequential order. This should be taken into consideration when configuring your own e-mail environment; depending on a domain owner's preferences it is also possible to implement a "single decision" e-mail environment.

Thereafter, it is stated in the DMARC RFC that some receiver architectures might implement SPF in advance of any DMARC operations. This means that a "-" prefix on a sender's SPF mechanism, such as "-all", could cause that rejection to go into effect early in handling, causing message rejection before any DMARC processing takes place. While operators choosing to use "-all" should be aware of this, we advise to favor a DMARC policy over an SPF policy. As also stated in the DMARC RFC, the final diposition of a message is always a matter of local policy. With this in mind we feel that operators should not configure SPF rejection to go into effect early in handling, and thus disregarding DMARC. At the cost of processing the entire message body, we advise to always evaluate all relevant standards before coming to a decision. If SPF fails, DKIM might still pass resulting in a satisfying DMARC evaluation.

DMARC for inbound e-mail traffic can be accomplished by setting up OpenDMARC and integrate it with Postfix.

Set up OpenDMARC

Make sure the file /etc/opendmarc.conf has a least the following configuration parameters.

AuthservID mail.example.nl
PidFile /var/run/opendmarc/opendmarc.pid
RejectFailures false
Syslog true
TrustedAuthservIDs mail.example.nl,mail2.example.nl,localhost,127.0.0.1
UMask 0002
UserID opendmarc:opendmarc
IgnoreAuthenticatedClients true
IgnoreHosts /etc/opendmarc/ignore.hosts
HistoryFile /var/run/opendmarc/opendmarc.dat
Socket inet:54321@localhost

For more information about these configuration parameters, take a look at its man page.

Make sure the file /etc/opendmarc/ignore.hosts contains all hosts that you trust. The e-mail coming from these hosts will not be checked by OpenDMARC:

127.0.0.1
localhost

Make sure the default file /etc/default/opendmarc contains:

RUNDIR=/var/run/opendmarc
SOCKET=inet:54321@localhost
USER=opendmarc
GROUP=opendmarc
PIDFILE=$RUNDIR/opendmarc.pid

Integrate with Postfix

Now we need to tell Postfix to use OpenDMARC as a mail filter in order to use its functionality. This is done by making sure that /etc/postfix/main.cf contains the configuration parameters as listed below. Notice that the DKIM check (localhost:12301) is done before DMARC (localhost:54321) since DMARC relies on the DKIM results.

smtpd_milters = inet:localhost:12301,inet:localhost:54321
non_smtpd_milters = inet:localhost:12301,inet:localhost:54321

Set up reporting

In order to become DMARC compliant, we need to start sending aggregate DMARC reports. Let's start by creating an OpenDMARC database using the provided MySQL script (/usr/share/doc/opendmarc/schema.mysql) in the OpenDMARC package.

In the example below the user root is used to create the new "opendmarc" database. If you would also like to create a user with access to the database, then uncomment the last two lines in the MySQL script and make sure to change the default 'changeme' password into something strong.

mysql -u root -p < /usr/share/doc/opendmarc/schema.mysql

After creation of the database, you need to create an executable Bash script in order to be able to send out reports. Create the file /etc/opendmarc/report_script and use the following content:

#!/bin/bash
 
DB_SERVER='database.example.nl'
DB_USER='opendmarc'
DB_PASS='somethingstrong'
DB_NAME='opendmarc'
WORK_DIR='/var/run/opendmarc'
REPORT_EMAIL='dmarc@example.nl'
REPORT_ORG='Example Company (example.nl)'
 
mv ${WORK_DIR}/opendmarc.dat ${WORK_DIR}/opendmarc_import.dat -f
cat /dev/null > ${WORK_DIR}/opendmarc.dat
 
/usr/sbin/opendmarc-import --dbhost=${DB_SERVER} --dbuser=${DB_USER} --dbpasswd=${DB_PASS} --dbname=${DB_NAME} --verbose < ${WORK_DIR}/opendmarc_import.dat
/usr/sbin/opendmarc-reports --dbhost=${DB_SERVER} --dbuser=${DB_USER} --dbpasswd=${DB_PASS} --dbname=${DB_NAME} --verbose --interval=86400 --report-email $REPORT_EMAIL --report-org $REPORT_ORG

Make sure the script is executable

chmod +x /etc/opendmarc/report_script

and run it every day under the user opendmarc by adding the following to /etc/crontab:

1 0 * * * opendmarc /etc/opendmarc/report_script

Configuring SpamAssassin

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 performed. The tests performed can be configured by adding specific configuration parameters 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 DMARC 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 DMARC validation did fail.

With SpamAssassin this can be configured by adding the following scoring configuration parameters to /etc/spamassassin/local.cf:

#dmarc fail
header CUST_DMARC_FAIL Authentication-Results =~ /mail\.example\.nl; dmarc=fail/
score CUST_DMARC_FAIL 5.0

#dmarc pass
header CUST_DMARC_PASS Authentication-Results =~ /mail\.example\.nl; dmarc=pass/
score CUST_DMARC_PASS -1.0

This means that when the "Authentication-Results" header of your e-mail contains "mail.example.nl; dmarc=fail" 5 points will be added to the score; instantly classifying this e-mail as SPAM. On the other hand, if the "Authentication-Results" header of your e-mail contains "mail.example.nl; dmarc=pass" -1 points will be added to the score; classifying this e-mail as legitimate.

Special thanks

Our infinite gratitude goes out to the following people for their support in building this how-to for DANE.

Alwin de Bruin