Internet DRAFT - draft-crocker-dmarc-bcp
draft-crocker-dmarc-bcp
Network Working Group D. Crocker, Ed.
Internet-Draft Brandenburg InternetWorking
Intended status: Best Current Practice November 05, 2013
Expires: May 09, 2014
Using DMARC
draft-crocker-dmarc-bcp-03
Abstract
Email abuse often relies on unauthorized use of a domain name, such
as one belonging to a well-known company (brand). SPF and DKIM
provide basic domain name authentication methods for email. A recent
industry effort built an additional authentication-based layer,
called Domain-based Message Authentication, Reporting & Conformance
(DMARC). It allows a sender to indicate that their emails are
protected by SPF and/or DKIM, and tells a receiver what to do if
neither of those authentication methods passes; it also provides a
reporting mechanism, from receivers back to domain owners. Such
capabilities over the public Internet are unusual and their use is
not yet well-understood. This document formulates a set of best
practices for the use of DMARC.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on May 09, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Development {T. Draegen} . . . . . . . . . . . . . . . . . . 4
2.1. Developing DMARC Components . . . . . . . . . . . . . . . 4
2.2. Developing DMARC-compliant Systems . . . . . . . . . . . 5
2.3. Sending DMARC-compliant Email . . . . . . . . . . . . . . 6
2.4. Original bullets: . . . . . . . . . . . . . . . . . . . . 6
3. Barriers to Adoption (or, How I Learned to Stop
Worrying and Love DMARC) {A. Popowycz} . . . . . . . . . . . 6
4. Planning for DMARC adoption {E. Zwicky} . . . . . . . . . . . 8
4.1. Decide what you need to do . . . . . . . . . . . . . . . 8
4.2. Picking Alignment and SP Parameter Values . . . . . . . . 10
4.3. Incremental Roll-Out, Sending . . . . . . . . . . . . . . 10
4.4. Incremental Roll-Out, Receiving . . . . . . . . . . . . . 11
4.5. [Original bullets] . . . . . . . . . . . . . . . . . . . 11
5. DNS Configuration {M. Hammer} . . . . . . . . . . . . . . . . 11
6. Receiver Processing {S.Solanki} . . . . . . . . . . . . . . . 12
6.1. Preparing for DMARC processing . . . . . . . . . . . . . 12
6.2. Implementing DMARC at receiver . . . . . . . . . . . . . 13
6.3. Rolling out DMARC . . . . . . . . . . . . . . . . . . . . 13
6.4. Post roll-out . . . . . . . . . . . . . . . . . . . . . . 14
6.5. Original Bullets . . . . . . . . . . . . . . . . . . . . 14
7. Report Generation {M. Jones} . . . . . . . . . . . . . . . . 15
7.1. DMARC aggregate XML report naming and report metadata . . 15
7.2. Minimum requirements for DMARC XML aggregate
report records . . . . . . . . . . . . . . . . . . . . . 17
7.3. Use and Reporting of Local Policy Overrides . . . . . . . 21
7.4. Minimum requirements for DMARC failure reports . . . . . 24
7.5. Additional concerns . . . . . . . . . . . . . . . . . . . 25
8. Report Receipt and Analysis {M. Jones} . . . . . . . . . . . 25
8.1. Report Receipt . . . . . . . . . . . . . . . . . . . . . 25
8.2. Report Analysis . . . . . . . . . . . . . . . . . . . . . 27
8.3. Report Processing and Analysis Services . . . 29
8.4. Additional concerns . . . . . . . . . . . . . . . . . . . 29
9. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . 29
10. Interaction Issues . . . . . . . . . . . . . . . . . . . . . 29
11. DMARC Commentary -- What were they thinking of...? . . . . . 30
12. Privacy Considerations. {T. Adams} . . . . . . . . . . . . . 32
13. Security Considerations . . . . . . . . . . . . . . . . . . . 33
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14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. References - Normative . . . . . . . . . . . . . . . . . 34
14.2. References - Informative . . . . . . . . . . . . . . . . 34
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 35
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
v-03 submission: This version is merely meant to kee the draft
alive. /d
*** EDITOR'S NOTE *** The current version incorporates new
sections of relatively raw text from the original DMARC.org
team and has been edited merely to convert to xml2rfc
format. This version has not yet received editing for
content, or any of the wider, public review, discussion and
revision that is intended. This version is being issued as
an Internet Draft in order to create an archived snapshot,
including noting the authors of specific sub-sections. It
is also hoped this version whets more appetites for adding
content... /Dave
Email abuse often relies on unauthorized use of a domain name, such
as one belonging to a well-known company (brand). SPF [RFC4408] and
DKIM [RFC6376] provide basic domain name authentication methods for
email. A recent industry effort (DMARC.org) built an additional
authentication-based layer, called Domain-based Message
Authentication, Reporting & Conformance. [DMARC] allows a sender to
indicate that their emails are protected by SPF and/or DKIM, and
tells a receiver what to do if neither of those authentication
methods passes; it also provides a reporting mechanism, from
receivers back to domain owners. Such capabilities over the public
Internet are unusual and their use is not yet well-understood. This
document formulates a set of best practices for the use of DMARC.
Discussion is divided along basic lines of activity:
* Development
* DNS Configuration
* Report Generation
* Receiver Processing
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An IETF mailing list for discussing DMARC issues has been
established: dmarc@ietf.org.
2. Development {T. Draegen}
This section addresses issues that can occur when developing systems
related to DMARC. Additional resources for developing DMARC can be
found at DMARC.ORG.
2.1. Developing DMARC Components
DMARC builds on a number of existing technologies, and introduces new
components only when necessary. This section helps to identify which
components are readily available and where new development may be
necessary. Combined, the components discussed here form an end-to-
end DMARC solution.
2.1.1. Runtime DMARC processing - Sender side
DMARC requires very little new functionality for senders of email.
SPF and DKIM are well developed technologies that can be implemented
and deployed in isolation, and are supported by existing communities.
DMARC uses TXT-based DNS records that are common. Support for
underscores in DNS labels is important to consider while developing
DNS tools that support DMARC, as DMARC records are located by
prepending "_dmarc." to a domain and performing a TXT query.
The ability to configure the domain used for rfc5322.MailFrom
addresses and for DKIM d= tags is important from a development
perspective. Users of DMARC often require flexibility in specifying
the domains used in both, as DMARC introduces the concept of
Identifier Alignment to tie existing authentication technologies to
email's rfc5322.From header domain.
2.1.2. Runtime DMARC processing - Receiver side
Because DMARC leverages existing authentication technology,
developers can often integrate this functionality into existing
systems without needing to develop entirely new modules. Developers
need to take note that DMARC introduces a linkage between email's
rfc5322.From header domain and the domains authenticated by SPF and
DKIM, and so Authenticated Domains must be made available from SPF
and DKIM processing.
Receiver side processing of DMARC results can also be performed
during the SMTP conversation, which may cause developers to re-factor
SPF and DKIM processing code that otherwise might occur after a
message has been accepted. Performing the DMARC check during the
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SMTP conversation allows receivers to enforce "reject" policies
immediately.
Receiver side DMARC development also introduces the concept of
Organization Domain, which should be developed in a modular way so
that future enhancements/development in the area of Organization
Domain discovery can be easily integrated.
The use of Authentication-Results headers is highly recommended to
ease down-stream processing by receivers.
The collection of feedback statistics for use in generating DMARC
aggregate likely presents the most opportunity for new development.
Collection, harvesting, and storage of such statistics should be
properly assessed and scoped. The size of collected datasets can
become quite large or high-volume email sites.
2.1.3. Feedback processing - Generator
Receivers that generate DMARC feedback should be aware of several
facets of DMARC-related development.
For feedback that is generated once a day, feedback should span an
entire 24 hour UTC-based cycle.
Monitoring and controls around the generation of DMARC feedback
should be constructed to allow operators the ability to discover
abuse of report generating code. Abusers will construct DMARC
records for large numbers of domains that send unwanted email.
Consider developing hooks so that intelligence decisions can be made
as to when DMARC data collection occurs to limit wasted data
collection and storage resources.
See Section 7. "Report Generation" for additional details.
2.1.4. Feedback processing - Consumer
Report consumers should be able to control the flow of incoming data
to disallow abuse of reporting addresses.
See Section 8. "Report Receipt and Analysis" for additional details.
2.2. Developing DMARC-compliant Systems
Developers of DMARC-compliant systems should consider DMARC
functionality from the perspective of the day to day operator, and
ensure their ease of use and deployment scenarios are addressed. The
configuration of domains, the maintenance of SPF records and DKIM
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signing keys, and the ability to quickly diagnose and correct
problems are essential when creating robust DMARC-compliant systems.
Integration of existing functionality to build a DMARC-compliant
system can be helped by leveraging interoperability tests for various
integrated components.
2.3. Sending DMARC-compliant Email
Large portions of this document are relevant to sending DMARC-
compliant email.
2.4. Original bullets:
o Core Functionality
o Tools Support
3. Barriers to Adoption (or, How I Learned to Stop Worrying and Love
DMARC) {A. Popowycz}
Within this section are discussed various hurdles, either real or
feared, that might need to be overcome to incorporate DMARC into
successful operational service. This includes issues that are
applicable from either a sending perspective, as a receiver, or both.
When considering DMARC, one must consider the operational and
technical maturity that would support a successful implementation
o I don't know where all my email is sent from!
One of the first questions that is often raised pertains to
having an accurate inventory of all sources that may send email
for a given domain as well as having processes to keep such an
inventory up-to-date. This is critical as any policy expressed
may have implications to mail delivery on that base domain.
In addition, it is all too easy to have any source send email
on your domain (which is the underlying problem leading to
phishing and other similar email threats).
Such an inventory must consider sources that include organic or
on-site mail capabilities as well as outsourced mail delivery.
Also on this list are special purpose emails that are often
sent as part broader capability, usually in an outsourcing or
'cloud' capacity
In actuality, the deployment of DMARC as a sender can actually
help allay such concerns. Through both aggregate and message
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level feedback, DMARC provides a more regular and reliable
reporting capability which can augment an established inventory
effort or serve as a core capability in setting such an
inventory management regime. Starting with a 'none' policy
(p=none) expressed in a published policy, one can achieve that
visibility without having any other implications to mail
handling behavior.
o If I publish a DMARC policy, I lose flexibility in email delivery.
While DMARC is an important new technology to help address
phishing, it is not a strategy of itself. Part of the
deployment of DMARC is the consideration of segmentation of
various email sources, usually through the use of subdomains.
Whereas it is common to send email using a second-level domain
(e.g. blah.com), segmenting different email sources and
providers (e.g. source1.blah.com, source2.blah.com) can enhance
email visibility and diagnostic capability while providing a
way out of the 'one-size-fits-all' concern. Furthermore, this
allows for a more manageable and incremental approach to
deploying DMARC allowing more aggressive policies to be
deployed in areas where there is higher confidence in email
authentication practices.
o I use third-party senders for my emails.
While the use of third party vendors for email delivery
requires some additional consideration, it is not a major
hurdle for adoption. Having good Service Level Agreements
(SLAs) with your providers that touch upon any changes that
might impact email authentication should be explicit if at all
possible to include such items as changes to email hosts and
addresses (usually impacting SPF) or DKIM key rotations.
Providing visibility to vendors of their results via regular
aggregate or message level reporting can help tremendously if
mail rejection due to 'false-negatives'.[*] Applying subdomain
segregation as noted previously can also be helpful for
reporting purposes when more than one email provider is used.
[*]In this context, a false positive is the identification
of a valid email failing authentication and having a DMARC
policy applied when it should not have.
o Some of my email is auto-forwarded.
The email auto-forwarding scenario has been considered in the
deployment of DMARC. It is for that reason that a DMARC policy
is only applied if both SPF and DMARC authentication fails. In
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the auto-forwarding scenario, one can expect SPF to fail at the
final destination, while DKIM is more resilient in that regard.
But there are those circumstances that in the process of auto-
forwarding emails are modified (including changes to message
contents through automatic footer insertion such as Anti-Virus
stamps).
Any time an email is handled, the chance for non-delivery
increases. From a sender's point of view though, the email did
reach the mailbox as expressed by the individual who supplied
the address in the first place.
Additional concerns:
Reasons for not publishing a DMARC record (yet).
Concern for incremental deployment.
Going through mailing lists.
Variability in receiver processing; creates unpredictability
overall.
4. Planning for DMARC adoption {E. Zwicky}
Distinct from lower-level issues in the specific steps for deploying
and using DMARC is the approach taken in overall planning for its
integration and use within the operational email service.
4.1. Decide what you need to do
DMARC provides protection against some forms of email domain name
abuse. Its restricted policy form (p=quarantine or p=reject) is not
intended for all use cases, and is beneficial only in the presence of
an actual spoofing problem. You may, however, want to protect
mailboxes by checking inbound mail, and you may want information
about how your domains are showing up at recipients by publishing a
p=none record.
So, first you need to determine what domains you own, and sort them
into piles.
Sending domains:
1. Domains that do not send mail. These should be protected with
p=reject, and you can do this quite rapidly -- you might want to
set them to p=none briefly to verify that you really don't send
mail from them, but otherwise you can move directly to p=reject.
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2. Domains that send only transactional mail. These should also be
protected at p=reject, but you will need to follow normal roll-
out procedures.
3. Domains that send mail, but never send transactional mail. (For
instance, if you are a mailbox provider, the customer domains do
not send transactional mail.) These can be set to p=none if you
want reporting, or left without records.
4. Domains that send both transactional mail and mail for
individuals -- for many companies, their main corporate domains
are in this situation. They both send transactional mail and
contain users who do things like send mail to mailing lists.
If you have mixed domains that contain both transactional mail and
mail from individuals that join mailing lists, you have four main
choices, in order from best protection of your brand to least:
1. Make a declaration that these domains officially represent your
brand and you do not, as a matter of policy, support forwarding
mail from them or joining mailing lists from them. When you have
enforced this to the extent appropriate to your environment, roll
out p=reject.
2. Move the individuals to another domain, ideally an entirely
separate one (so if your original domain is example.com, you
might move the individuals to example-employees.com, letting you
protect all of example.com at p=reject).
3. Move the transactional mail to another domain, usually a
subdomain of the original domain (official.example.com, for
instance).
4. Decide not to protect the mail with DMARC directly and use a
p=none record to get reporting.
Receiving domains:
1. Domains that receive transactional mail (for instance, domains
where you handle customer inquiries). If you think forged mail
from official transactional domains will be a problem here, then
you should implement inbound DMARC; in most cases, however, you
derive no large benefit from having it on, and may lose mail from
misconfigured domains, and might as well leave it off.
2. Domains that receive mail for individuals. These domains should
implement inbound DMARC to mitigate phishing problems.
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4.2. Picking Alignment and SP Parameter Values
Once you know what domains you are trying to protect, you can pick
values for alignment and for sp.
If you have a large pile of sending domains like
shipping.example.com, ordering.example.com, todays-
promotions.example.com, you probably want to make a single, relaxed
alignment record for example.com. The same is true for any domain in
which you know people frequently create new sending domains with
little coordination.
If you have a small number of sending domains, you probably want to
make an entry for each with strict alignment and sp=reject. You will
also want a sp=reject entry on the parent domain. So, for instance,
if example.com sends transactional mail only from
important.example.com and marketing mail from marketing.example.com,
while the employees hang out on example.com, all three domains should
be set to strict alignment and sp=reject. In theory you could in
omit the records for important.example.com and marketing.example.com
in this situation. In practice, you will almost certainly work your
way up to p=reject on different schedules for them, so at least
during roll-out you will need separate records.
4.3. Incremental Roll-Out, Sending
The procedure for incrementally rolling out DMARC on an individual
Sending domain is discussed elsewhere.
Most senders have multiple domains, however, and will have to pick
which ones to work on first. Start with some combination of
1. Your most attacked domains.
2. The easiest domains to secure.
If you can't easily identify domains in either of these categories,
turn on DMARC with p=none for a wide assortment of domains, and see
if that clarifies matters. (Often it reveals domains that are in
both categories at once; domains used for sending some particular
piece of signed, transactional mail, where much more forged mail than
good mail is sent.) You can use the p=none reporting on a lower
domain to substitute for turning on p=none on a subdomain (so, for
instance, if you have example.com at p=none and notice that
alerts.example.com sends out only DKIM-signed mail, you can move
alerts.example.com directly to p=quarantine, and senders who are
familiar with DMARC and their sending environment may choose to move
it directly to p=reject)
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4.4. Incremental Roll-Out, Receiving
In order to have fully implemented DMARC on the receiving side, you
must both act on mail and report on mail. In general, you will have
to pick one of these to do first; do you turn on reporting, and
report all mail as exempted for local reasons, or do you turn on
actions first? Both of these are acceptable compromises, as long as
you are rejecting rather than silently discarding mail when p=reject.
However, you should not spend very long in either state.
4.5. [Original bullets]
How to implement DMARC on a corporate email domain.
Choosing the domain name for alignment.
Choosing Strict vs. Relaxed.
Incremental Roll-Out.
5. DNS Configuration {M. Hammer}
Malformed Policies: DMARC policies published in DNS which are
malformed should be ignored by validators with regard to policy
assertions.
Reporting malformed policies: If a malformed record is identified
by a validating/reporting entity and that entity wishes to attempt
reporting a problem with the record, a report may be sent to the
address(s) indicated in the RUA field. As an alternative, a
report may be sent to the technical contact indicated in the WHOIS
record for the domain.
Managing DMARC records: Implementers publishing quarantine or
reject policies should take care to ensure the integrity of their
mail streams when rotating DKIM keys. For owners/administrators
that manage large numbers of domains, it is recommended to
automate management and configuration of DMARC records as well as
underlying DKIM and SPF records to ensure consistency and correct
deployment of records.
Publishing DMARC reject policies for non mailing domains: For
domains which never send mail, it is appropriate to publish a
DMARC reject policy as a way to prevent abuse.
[Original listing of issues]
o Malformed DMARC policy TXT records in DNS.
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Can these be used for anything?
Should a policy that does not conform to the spec be ignored?
Should a receiver try to infer meaning in the case where the
syntax of a DMARC TXT record isn't correct? For example, if
"p=monitor" (or p="non", etc.) should the receiver assume the
domain owner wants to only "monitor" the domain (requesting no
action other than reports) and apply processing as if it was
"p=none"? What if there are what appear to be obvious spelling
mistakes (e.g. "p=rejec")?
How lenient can one safely be as regards spacing, quotes,
separators and slashes?
Is there anyway I can report the malformation to the domain
owner?
o DMARC records for many domain names.
6. Receiver Processing {S.Solanki}
6.1. Preparing for DMARC processing
DMARC is a policy enforcement and reporting standard that builds on
SPF and DKIM to provide more deterministic outcomes of
authentication-failed messages. DMARC implementation helps receivers
to more easily identify email from senders as legitimate, and helps
keep spam and phishing messages from reaching the inbox.
The receiver could use the following data points to better understand
their incoming email traffic:
o Total # of phishing messages and campaigns
o # of highly phished domains
o Total # of senders publishing DMARC records
o Total # of messages likely to be processed for DMARC evaluation
The next step is to scope the engineering work to implement DMARC
processing. This includes but not limited to
o Support for DKIM and SPF evaluation
o Processing DMARC record policy
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o Aggregate and message level reporting
o Evaluating for Privacy and performance implications
o Integration with existing authentication mechanisms from
processing to user interface
It is recommended to participate in open DMARC forums, for pursuing
general questions or learning from experience of receivers who have
already previously implemented DMARC
6.2. Implementing DMARC at receiver
During implementation the goal is to be compliant with the
specification. In particular the "must have" attributes have to be
implemented to be considered compliant. The optional attributes are
left to receiver to determine implementation.
While implementing the specification consider leveraging or building
the following technologies
o DNS service to do real-time lookup
o Cache to store recently obtained DMARC records
o Dedicated reporting infrastructure to send regular aggregate and
message level reporting
o Instrumentation to track the flow of messages through DMARC
processing pipeline
o Configuration knobs to tune sampling rate and DMARC authentication
priority
o Trusted sender visual identification in the emails that pass DMARC
authentication
6.3. Rolling out DMARC
A phased approach is best recommended for DMARC roll-out at receiver
side. Start with a small percentage of your email traffic being
subject to DMARC processing.
Reach out to a select group of trusted senders who you know have
already published DMARC records. Work closely with them over a
period of weeks to determine if the implementation is working as
expected.
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Things to observe include but not limited to
Messages not getting DMARC verdict as expected (either false
positives or false negatives)
Reports not being sent in expected format or with missing
information
Once any potential issues are fixed, consider ramping up the traffic
to a larger portion of the incoming traffic. This will identify
issues normally caught at scale such as excessing reporting queues or
caching performance. This is also a good time to monitor feedback on
dmarc-dev forums where senders usually post observations or
complaints about DMARC issues.
Once satisfied with the feedback you are ready to roll it out full
scale to the site and announce to the world.
6.4. Post roll-out
Post roll-out give sufficient time to evaluate if the processing
pipeline is working as expected.
These include monitoring the generated reports in your
instrumentation infrastructure. Specifically the "reject" and
"quarantine" verdicts will give a direct idea of how many phishing
messages were filtered
This is a good time to share a summary of your experience
implementing DMARC and its benefits to you as the receiver over time.
Feedback regarding quality of documentation and any items missing or
incorrect should be informed to the community via dmarc-dev for
future versions of the document.
6.5. Original Bullets
o Rapid adoption of DMARC by apparent spammers.
Thousands upon thousands of DMARC records published by abusive
domains -- probably wildcarding, but it works out the same
Using data that is several weeks old, we have a handful of
domains that are accepting aggregate reports for over 20
thousand subdomains each
Are the reports (either aggregate or failure) providing the
spammers insight that they did not already have?
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***POTENTIALLY PROBLEMATIC QUESTION TO FOLLOW*** Should I only
send reports to domains that I trust?
7. Report Generation {M. Jones}
7.1. DMARC aggregate XML report naming and report metadata
The first step most recipients of DMARC aggregate XML reports will
take, after accepting the compressed files via email, will be to
uncompress the file and run a script or program to parse and store
the data contained within the reports. In order for this to work
properly, the filenames and report metadata must work in a standard
way from all DMARC compliant report generators.
The correct compression mechanism and filename convention is
described in section 12.2.1 [DMARC] for emailed reports. It is
important to ensure that the uncompressed filename still adheres to
the specified naming convention. Cases have been noted were upon
uncompressing a file, the new filename is something entirely
different than what is specified in section 12.2.1.
In the report metadata there are several fields that contain
important information for a report recipient. Here is a brief
description of each field and some notes on usage and/or problems
encountered with each.
<report_metadata>
<org_name> This is generally the domain responsible for
producing the data in the report (i.e. the report
generator). In the example below it is
'google.com'. The report itself contains data
about messages addressed to users at gmail.com and
many other domains hosted by Google.
<email> The email address where a report recipient can
alert the report generator to problems related to
the DMARC aggregate report. This can be a mailing
list address or contain multiple email addresses.
<extra_contact_info> A place to point out additional
information or resources provided by the report
generator. This could be the URL of a website with
additional help, a phone number, etc.
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<report_id> A unique report identifier. This should be
unique across all reports by the report generator
over time, including reports generated in parallel
across multiple MTA's.
<date_range> Date range of messages included in the report.
The specification says "specified in seconds since
epoch". Note that the begin timestamp for your
first report should not be the value 0, but should
instead be the begin date/time for the interval.
<begin> UNIX timestamp in UTC.
<end> UNIX timestamp in UTC.
Below is a sample of report metadata and policy discovery sections of
DMARC XML report with a the file name:
google.com!facebook.com!1364169600!1364255999.xml.
<?xml version="1.0" encoding="UTF-8" ?>
<feedback>
<report_metadata>
<org_name>google.com</org_name>
<email>noreply-dmarc-support@google.com</email>
<extra_contact_info>http://support.google.com/a/bin/answer.py?answer=2466580</extra_contact_info>
<report_id>303460054821571539</report_id>
<date_range>
<begin>1364169600</begin>
<end>1364255999</end>
</date_range>
</report_metadata>
<policy_published>
<domain>facebook.com</domain>
<adkim>r</adkim>
<aspf>r</aspf>
<p>reject</p>
<sp>reject</sp>
<pct>100</pct>
</policy_published>
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7.2. Minimum requirements for DMARC XML aggregate report records
The data contained in a DMARC aggregate report, at minimum, allows a
report recipient to:
1. Identify sources sending email purporting to be "From" its domain
and sub-domains.
2. Determine the aligned DMARC results of SPF and DKIM checks.
3. Determine the DMARC disposition of the message.
4. Determine the identities used to check the underlying
authentication mechanisms.
5. Determine the results of the underlying authentication mechanism
checks.
To make this possible, each record in a DMARC report must contain a
minimum set of data. The fields below are defined in Appendix C.
DMARC XML Schema and are critical to producing a complete aggregate
report. Some notes on usage of these fields are included to help
guide you in your deployment.
<source_ip> IP address (IPv4 or IPv6) of the connecting SMTP
host.
<count> The aggregated count of messages represented by this
record. The values of all fields contained within the
<record> must be identical to be part of the aggregate count
in this record.
<policy_evaluated>
<disposition> The <disposition> field in DMARC aggregate
data is intended to convey the final DMARC
disposition of the message. This is the result of
the domain's policy combined with the DKIM and SPF
aligned policy results. The <disposition> field is
NOT intended to convey the ultimate placement of
the message by the receiving MTA, if for example
the report generator decides not to take the DMARC
action based on a local policy (Section 7.3). The
allowable results are "none", "quarantine", or
"reject".
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<spf> The DMARC aligned result for SPF. This must be
either "pass" or "fail".
<dkim> The DMARC aligned result for DKIM. This must be
either "pass" or "fail".
<identifiers>
<header_from> This is the rfc5322.From domain in the
message(s) for this record. Note that this is the
domain following the @ in the original address,
stripped of all surrounding non-domain commentary.
<auth_results>
<dkim>
<domain> This is the DKIM signing domain (d=) in
the DKIM signature that the receiver
chose to validate and report.
<result> This is the result of the DKIM
authentication check. This is NOT the
DMARC aligned result. The allowable
results are specified in the DMARC XML
Schema and refer to RFC 5451. Note
that the correct value to report when
no signature is present is "none" as
opposed to "neutral" or NULL.
<spf>
<domain> This is the domain used for the SPF
check. Usually it will be the
rfc5321.MailFrom domain. In cases of a
NULL MAILFROM it may be the EHLO
domain, depending on receiver
implementation of SPF.
<result> This is the result of the SPF
authentication check. This is NOT the
DMARC aligned result. The allowable
results are specified in the DMARC XML
Schema. Note that the correct way to
report that no record exists is "none"
as opposed to a NULL value.
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Below are three examples of real DMARC XML records for a domain with
a DMARC reject policy in place.
This record indicates a single message matching this set of data
points. The DMARC disposition for this message was "reject" based on
DMARC aligned results for SPF and DKIM of "fail" and the domain's
reject policy. The empty DKIM domain field and DKIM authentication
result of "none" indicates that there was no DKIM signature. The
result of the SPF authentication check was "neutral".
<record>
<row>
<source_ip>91.98.85.94</source_ip>
<count>1</count>
<policy_evaluated>
<disposition>reject</disposition>
<dkim>fail</dkim>
<spf>fail</spf>
</policy_evaluated>
</row>
<identifiers>
<header_from>blog.facebook.com</header_from>
</identifiers>
<auth_results>
<dkim>
<domain></domain>
<result>none</result>
</dkim>
<spf>
<domain>blog.facebook.com</domain>
<result>neutral</result>
</spf>
</auth_results>
</record>
Example - 1 Msg / Disp Reject / No DKIM
This record indicates that 3 messages were represented by this set of
data points. The disposition for these messages was "none" because
the DMARC aligned result for DKIM was "pass". The DMARC aligned
result for SPF was "fail". The messages passed the DKIM
authentication check and failed the SPF authentication check.
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<record>
<row>
<source_ip>141.161.2.153</source_ip>
<count>3</count>
<policy_evaluated>
<disposition>none</disposition>
<dkim>pass</dkim>
<spf>fail</spf>
</policy_evaluated>
</row>
<identifiers>
<header_from>facebook.com</header_from>
</identifiers>
<auth_results>
<dkim>
<domain>facebook.com</domain>
<result>pass</result>
</dkim>
<spf>
<domain>facebook.com</domain>
<result>fail</result>
</spf>
</auth_results>
</record>
Example - 3 Msgs / Disp None / DKIM-SPF Mix
This record indicates a single message matching this set of data
points. The DMARC disposition for this message was "reject" based on
DMARC aligned results for SPF and DKIM of "fail" and the domain's
reject policy. There was no DKIM signature on this message, as in
Example 1. The SPF authentication result was "pass" with a MAILFROM
domain of "classifiedads.com". The SPF domain is not aligned with
the header From domain, causing the DMARC aligned SPF result to be
"fail".
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<record>
<row>
<source_ip>65.61.105.5</source_ip>
<count>1</count>
<policy_evaluated>
<disposition>reject</disposition>
<dkim>fail</dkim>
<spf>fail</spf>
</policy_evaluated>
</row>
<identifiers>
<header_from>facebook.com</header_from>
</identifiers>
<auth_results>
<dkim>
<domain></domain>
<result>none</result>
</dkim>
<spf>
<domain>classifiedads.com</domain>
<result>pass</result>
</spf>
</auth_results>
</record>
Example - 1 msg / Disp Reject / No DKIM
7.3. Use and Reporting of Local Policy Overrides
The DMARC specification allows for a DMARC compliant receiver to take
an action that is different than the DMARC disposition for the
message (see Section B.3.1, SMTP-time Processing [DMARC]). Reasons
that a receiver may choose to do so include overriding a reject
policy if the message source is a known forwarder or mailing list
that breaks DKIM and SPF. If a message source has a high reputation
the receiver may choose to accept and/or analyze a message with local
rules despite a DMARC disposition of "reject". While ultimately an
email receiver's local policy and the final placement of a message
cannot be standardized by DMARC, the DMARC related reporting of such
can.
The reporting of a "PolicyOverrideReason" is specified in Appendix C
[DMARC]. A <reason> tag is included in the <policy_evaluated>
section of the <record> with two sub-fields, <type> and <comment>.
Below are the DMARC XML tags and fields involved with a brief
explanation of each and two real examples of DMARC records with a
PolicyOverrideReason.
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<policy_evaluated>
<disposition> See Section 7.2.
<spf> See Section 7.2.
<dkim> See Section 7.2.
<reason> Present ONLY if the report generator means to tell
the report recipient that they did not follow the
DMARC disposition.
<type> If a <reason> is included in the record
the <type> must be present. The
purpose of this field is to explain why
the DMARC policy was overridden.
Allowable values are "forwarded",
"sampled_out", "trusted_forwarder",
"mailing_list", "local_policy", and
"other". DMARC does NOT attempt to
standardize the meaning of each of
these types nor the methodology by
which a report generator determines the
<type> to report.
<comment> Optional. A report generator may
include extra explanatory text here.
Example 1. In this example the DMARC disposition is reject, based on
the domain's DMARC reject policy and DMARC aligned SPF and DKIM
results of "fail". But a <reason><type> of "mailing_list" is
reported by the report generator. The report recipient can interpret
this to mean that the domain's reject policy was correctly applied,
but the receiver chose to override the reject action because the
message source is a known mailing list which cause SPF and DKIM to
break.
<record>
<row>
<source_ip>132.205.122.20</source_ip>
<count>19</count>
<policy_evaluated>
<disposition>reject</disposition>
<dkim>fail</dkim>
<spf>fail</spf>
<reason>
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<type>mailing_list</type>
<comment></comment>
</reason>
</policy_evaluated>
</row>
<identifiers>
<header_from>star.c10r.facebook.com</header_from>
</identifiers>
<auth_results>
<dkim>
<domain>facebookmail.com</domain>
<result>neutral</result>
<human_result></human_result>
</dkim>
<spf>
<domain>star.c10r.facebook.com</domain>
<result>neutral</result>
</spf>
</auth_results>
</record>
Example 2. In this example the DMARC disposition is "none" because
the DMARC aligned DKIM result is "pass", thus the domain's DMARC
reject policy is not applicable. Yet the report generator still
chose to report a policy override <reason><type> of "forwarded" in
the record. This is perfectly acceptable, even though the policy
override reason did not impact the treatment of the message as far as
DMARC is concerned.
<record>
<row>
<source_ip>195.23.141.86</source_ip>
<count>2</count>
<policy_evaluated>
<disposition>none</disposition>
<dkim>pass</dkim>
<spf>fail</spf>
<reason>
<type>forwarded</type>
<comment></comment>
</reason>
</policy_evaluated>
</row>
<identifiers>
<header_from>support.facebook.com</header_from>
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</identifiers>
<auth_results>
<dkim>
<domain>support.facebook.com</domain>
<result>pass</result>
<human_result></human_result>
</dkim>
<spf>
<domain>support.facebook.com</domain>
</spf>
</auth_results>
</record>
7.4. Minimum requirements for DMARC failure reports
DMARC failure reports serve two primary purposes. First is to allow
a domain owner to investigate in more detail, legitimate sources of
their email that are failing one or both modes of authentication.
For example, from aggregate data one might know that a domain's 3rd
party email is failing DKIM some percentage of the time yet not know
which messages are affected. Failure reports could show a consistent
From address and Subject from the source that are unsigned,
indicating a specific campaign not being signed by DKIM. Second is
to allow a domain owner to understand and mitigate specific threat
campaigns abusing their domain. Examples include early
identification of phishing URLs in current campaigns for quick
takedown or identification of Subjects used in current campaigns
which can be used by customer service call center personnel to handle
customer calls.
The data contained in a DMARC failure report, at minimum, allows a
report recipient to:
1. Identify the source sending each failed message purporting to be
"From" its domain or sub-domain.
2. Determine the aligned DMARC result of the SPF and DKIM checks.
3. Identify the domain used to check SPF. If this is different than
the MailFrom domain or the MailFrom domain is NULL and the
receiver checks EHLO, that identifier must be indicated in the
failure report.
4. Identify the full DKIM signature checked (if the message was DKIM
signed).
5. Identify the results of the SPF and DKIM authentication checks.
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6. Identify the URLs in the body of the message.
7. Identify the full rfc5322.From email address in the message.
This should include the display name portion of the From.
8. Identify the Subject of the message.
Details on the format of failure reports are found in sections 8.4.
and 8.4.1 [DMARC].
7.5. Additional concerns
o Minimum requirements for aggregate report XML documents.
What is the bare minimum (in terms of data gleaned from a given
email) I need to produce a valid report?
What sections of the XML need to be repeated when there is a
new aggregation point (email from a different IP, for example)
Essentially this could be considered "XML for people who
thought they could figure out how to read an XSD but reality
crashed that party."
o Minimum requirements for failure reports (soft-fail reports).
What is the bare minimum required to produce a failure report.
Whether to request Failure Reports (ruf=)
o Utility of Local Policy Overrides.
8. Report Receipt and Analysis {M. Jones}
8.1. Report Receipt
8.1.1. Your first DMARC aggregate reports
Upon successful publication of a DMARC record for your domain you
will soon begin to receive DMARC data. Current report generator
practice is to aggregate DMARC data daily. Therefore you should
expect to receive your first aggregate data in the range of 12 - 48
hours after you first publish the record. This can vary depending on
the time of day your record was published. DMARC aggregate reports
should use UTC day boundaries for the reporting intervals. There is
also some time lag while your record propagates through DNS is
discoverable by DMARC compliant receivers.
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The aggregate data files will arrive as gzipped email attachments.
While the DMARC specification allows for multiple types of URIs to
indicate your preference for aggregate data delivery, current report
generator practice is to deliver reports via email using the mailto:
URI specified in the rua tag. Note that if you list multiple email
addresses in your rua tag, you should list them in the order of the
highest priority address first, as there have been report generators
who do not send to all addresses. In these cases the report
generator does send reports to at least the first address listed.
Upon receipt of these files you will need to uncompress them for
further processing or manual inspection.
8.1.2. Your first DMARC failure reports
Upon successful publication of a DMARC record for your domain you
will also begin to receive DMARC failure data for individual message
failures at the email address specified in your DMARC record's ruf
tag. You will likely receive your first reports within the first 24
hours of your record being published. There are several factors that
will affect the timing and source of your first failure reports.
First, the current practice is that only a small number of DMARC
receivers are providing failure reports, as it is optional for a
DMARC receiver to provide this data. Therefore you should not expect
failure reports from all of the same report generators that send you
aggregate reports. Next, failure reports are not aggregated and the
current practice among report generators is to generate a report near
real time when they receive the failed message.
The failure reports you will receive are specified in section 8.4.
and 8.4.1 [DMARC].. These reports are intended to be machine readable
and it is recommended that you automate the process of parsing and
storing the data contained in the reports. The volume of these
reports you will receive can be highly variable and it is not limited
by the amount of email that you send. Attacks using your domain can
happen at any time and their nature is largely outside of your
control. Therefore it is also recommended that your report
processing infrastructure be able to rate limit or sample the inbound
reports in a way that does not negatively impact the sending
infrastructure of report generators.
8.1.3. What if I do not receive any DMARC reports?
If you have published DMARC records and waited 24 to 48 hours yet
still received no data you should check the following:
1. Check the rua and ruf addresses in your DMARC record.
Are they correct without typographical errors?
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Did you include the "mailto:" prefix?
2. Is the domain in your rua and ruf address the same as the domain
of your DMARC record?
If not, did you follow the process to verify external
destinations for data? See section 6.2 [DMARC]. for details.
3. Check your email receiving infrastructure and mail logs for
indications of problems receiving email.
Are you accepting email with potentially large attachments?
Do you need to whitelist any IP addresses? Or are you
checking DMARC on the incoming messages which may be failing?
8.2. Report Analysis
8.2.1. Data contained in DMARC reports
For detailed descriptions on the meaning of different data fields in
DMARC aggregate XML files please see the descriptions in subsections
.1-.3 of Section 7. For a description of the data contained in a
failure report (see Section 7.4). In order to move on to the
analysis of DMARC data, it is important to understand what the report
generator is telling you in each field.
8.2.2. What should I look for first?
There are many ways you can approach the analysis of DMARC data for
your domain. The approach you take will likely depend on what you
already know about your domain's email sending and abuse of your
domain. It is recommended that in general you start with aggregate
data. Here are some suggestions on initial things to look for.
o Identify the number of sources (i.e. IP addresses) sending email
using your From domain. How many are there and how does that
compare to your knowledge of your organization's email sending
infrastructure?
o What is the daily volume of email messages (i.e. sum of all
counts) sent using your From domain? What is the volume from your
known sources vs. those you did not know about?
o What is the daily volume that passed DMARC aligned SPF and DKIM?
Passed only one but not the other?
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o What is the daily volume of unauthenticated messages, those that
failed both DMARC aligned SPF and DKIM?
o Are any previously unknown sources of email passing either or both
of DMARC aligned SPF or DKIM? If so why?
o What are all of these previously unknown sources of email using
your From domain?
As you analyze data and answer some of these questions, it will lead
to deeper investigation. At some point you will reach a limit to
what you can learn from the aggregate data and likely need to look
further using failure reports if they are available. You may want to
search for examples of failures coming from a particular IP address
to understand what kind of messages are being sent. With the From,
Subject, and URLs in failure reports you may be able to identify
specific phishing or spam campaigns using your domain.
Once you have a good understanding of the current state of your
domain's email you can use DMARC data to begin remediating problems
and tracking the ongoing progress of your changes. Depending on your
domain's email characteristics your ultimate goal may be to publish
DMARC reject policies or to simply continue collecting intelligence
about your email.
8.2.3. Investigating Anomalies in DMARC data
When analyzing DMARC data you will likely come across results that
you want to verify or understand better. In some cases this is
possible via further analysis of additional DMARC aggregate data
fields. In other cases it is helpful if you have failure reports to
analyze. A few examples of common things to explore are noted below.
o Some number of records will indicate that the authentication check
(either SPF or DKIM) passed, yet the DMARC aligned value is a
failure. If you want to verify that these are all in fact due to
misaligned identifiers you can query your data store to show a
count of all such cases where the <spf><domain> or <dkim><domain>
does not match or have a sub-domain match to the <header_from>.
o If your domain has a reject policy, you may want to check how many
records have a <dkim> and <spf> value of "fail" in the
<policy_evaluated> section, but do not have a <disposition> of
"reject". If that occurs it indicates a problem with the
evaluation of your DMARC record.
o If a message that failed both DKIM and SPF is still delivered and
your domain has a reject policy (you may know this if it is
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reported to you via customer complaint or from your own testing),
before assuming receiver error you should check for local policy
overrides (Section 7.3). Try to identify records in DMARC data
where the <disposition> is correctly designated as "reject", but a
<reason><type> is indicated for the policy override. This is not
an error on the part of the receiver, but is allowable per the
DMARC specification.
8.3. Report Processing and Analysis Services
Is it appropriate to point out here that there are vendor and 3rd
party resources that can help with report receipt, processing, and
analysis? Of course specifics would not be mentioned here, but the
document could point to the dmarc.org/resources page. This could aid
in making build vs. buy decisions.
8.4. Additional concerns
When can I expect to receive my first aggregate report?
DMARC failure reports are not being received.
Distinguishing mis-processing of DMARC versus local policy
variations.
9. Miscellaneous
This section is meant as a catch-all, for items that haven't yet been
assigned to their appropriate section.
o Performing remote destination checking
What happens if I don't?
o Identifier alignment
{This has been cited, but not yet explained, as a potential BCP
issue. -ed}
o Owner vs. Operator
{It is possible that there are distinctive practices for domain
name owners, versus agents that operate DNS or email services
on behalf of the name owner. -ed}
10. Interaction Issues
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Some issues come into play when DMARC is used in conjunction with one
or more other services.
o Sender using both DMARC and ADSP.
11. DMARC Commentary -- What were they thinking of...?
This section explains a number of choices made for DMARC.
Rationale for choosing ZIP for the aggregate reports: {P. Midgen}
primary consideration was increasing the amount of data that could
be presented in a single message and avoiding report truncation
(e.g. many folks limit inbound message size to 25MB). text
compresses well in general, so we thought compression would make
sense. // rather than compress at the transport layer (e.g. send a
chunked/compressed stream over http) we thought email attachments
were kinda braindead given the existing message-based feedback
infrastructure (e.g. ndr, arf, etc.). // zip is everywhere, we
needed to make no special consideration using it, and (i believe,
but this needs checking) it is also a registered content-type
(golden arrow of rationale leads back to "zip is everywhere").
Why doesn't DMARC solve its problem with mailing lists/gatewats? {P.
Midgen}
This question typically refers to a set of well-known issues where
messages transiting mailing list managers (MLMs), relays, or
forwarders fail DKIM or SPF checks.
As a first principle, solving common operational issues in
underlying authentication mechanisms is out of scope for DMARC.
The recommendation is for senders to use both SPF and DKIM, since
we know from research conducted during draft development that the
likelihood of simultaneous SPF & DKIM failure, while possible, is
far less common than individual failures.
Senders are encouraged to use both SPF & DKIM to ensure robust
operation of DMARC and to pave the way for future technologies
that will benefit from broad adoption of email authentication,
such as domain reputation.
This question also refers to the issue of expanding the set of
alignable authenticated identities under DMARC. At the moment
DMARC looks at the rfc5321.MailFrom (a/k/a envelope sender or
return-path) for SPF, and the d= field in the message's DKIM-
Signature block(s), and attempts to align them with the
rfc5322.From domain since this in the majority of cases is
displayed to the message recipient by the MUA.
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The import of that consideration can be argued, but we universally
felt it was important because it is such a common practice and
because we are able in majority of cases representing a high
volume of mail to authenticate and align identities with
rfc5322.From.
There is no standardized practice for authenticating against
the Sender or other identity sources, it is out of scope for
DMARC to do so and until there exists a reliable and reasonably
well-adopted mechanism that does so, DMARC will rely on the
envelope and body From identities.
As a final statement on this issue, it is worth reiterating the
role of local policy in the determination of message disposition.
In a sense SPF and DKIM serve to inform local policy mechanisms on
the disposition of authenticated mail. DMARC carries that
tradition forward, but at the end of the day it is a matter of
local policy whether to act on the suggestions of authentication
and policy mechanisms or simply mark the message and move it
further down the delivery pipeline.
Who is the target audience for the DMARC specification? {P. Midgen}
DMARC is intended for submission to become an IETF standard; in a
sense this means the intended audience is the entire internet and
email community. That said, it isn't for everyone. We believe
DMARC does four important things:
1. Provides a method for uniform evaluation of the authentication
results generated by standard and well-deployed email
authentication mechanisms against a common set of identities.
2. Provides a reporting mechanism allowing senders to understand
the performance of their email authentication practices, and
get an idea of the IPs sending email on behalf of their
domains.
3. Provides a deterministic policy mechanism allowing the sender
to tell the receiver how to dispose of their email in the
event the policy conditions aren't met.
4. It achieves this sender/receiver communication via DNS, at
Internet scale.
We figured most folks would find 1, 2 and 4 appealing; everyone
likes information and even more so if it is obtained easily and at
low risk. Number 3 is more tricky: under what circumstances
should a sender publish anything other than p=none? This depends
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on a huge number of variables and there is no if-then-else type of
guideline. Here are some of the things to consider:
1. Are your domains used fraudulently? DMARC will help curb
fraudulent use of your organizational domain and subdomains.
It will not stop homograph/cousin domain abuse (e.g.
fac3book.com), and it will not stop domain-padding (e.g.
facebook.superbaddudes.junkyjunkjunkorama1234.com) abuse.
2. Are you an ISP, MBP, or EDU? Your users tend to move around,
have multiple accounts, forward messages and do other things
that increase the possibility for a DMARC false positive.
You, even more so than everyone else, will want to start with
a monitor-only policy so you can model the what-ifs.
3. Do you send mostly transactional, point-to-point mail (e.g.
account statements, password resets, etc.)? These are good
candidates for DMARC since our observation has been that these
tend to be low risk of failure.
12. Privacy Considerations. {T. Adams}
NOTE: Capitalized terms related to privacy used in this section
are consistent with the "Guidelines on the Protection of Privacy
and Transborder Flows of Personal Data" published in 1980 (and
subsequently reviewed in 2011) by the Organisation for Economic
Co-operation and Development (OECD).
Each of the two DMARC feedback reports have different characteristics
related to privacy based on their design. Aggregate Reports (RUA)
are sent by a receiver at periodic intervals (e.g. daily) and contain
summary data regarding the number and type of email messages that are
processed in relation to the domain owner's DMARC policy. As such,
they do not contain Personal Data and are considered to have no
impact on privacy.
The one identified exception to this is when an individual (a.k.a. "a
Natural Person with legal rights") is operating his or her own email
server. In this case, the IP address of the sending server reported
in the RUA may identify the individual and thus be classified as
Personal Data in some jurisdictions.
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Failure Reports (RUF), on the other hand, are sent by an email
receiver each time an email message being processed fails the DMARC
authentication checks. The RUF may include Personal Data as well as
Sensitive Data, depending on the applicable regulatory jurisdiction.
When supporting RUF reports, an email receiver should consider
whether or not to include the full content of the unauthenticated
email, as well as what email header fields to redact.
As part of this consideration, the email receiver will need to
understand its roll in the flow of Personal Data. For example, the
domain owner can be viewed as the Data Controller for email sent
under its aegis as is the case for email sent from the servers of a
company by its employees or transactional systems. In this case, the
email receiver is the Data Processor acting on behalf of the Data
Controller, and as such the legal responsibility for protection of
the data rests with the domain owner. Thus, legitimate mail that
inadvertently fails the DMARC authorization checks (e.g. if the email
was handled by mailing list management software acting as an
Intermediary Processor) is clearly handled by this case.
The primary area of debate about privacy and RUF reporting revolves
around the protections offered to the individual (i.e. the "bad
actor") who sends fraudulent email purporting to be from another
domain. Specifically, should the bad actor be protected by privacy
regulations within the applicable jurisdiction? Within the context
of current regulatory guidance, it is possible to view the email
receiver as the Data Processor on behalf of the bad actor, who is
filling the role of Data Controller. It is an open question only
insofar as what to do about this bad actor who is clearly not
participating in the data flow in good faith.
Given that there is significant value in quickly identifying bad
actors, and taking appropriate enforcement action, it is worth
exploring how to clarify that existing data sharing regulations allow
RUF reports. In any case, both the email receiver (as the report
generator) and the domain owner (as the report recipient) should
ensure that the appropriate operational policies are in place to
protect the Personal Data being handled as dictated by their part of
the data flow.
IP Addresses in reports
13. Security Considerations
In this section we describe several security considerations related
to the implementation of the DMARC protocol.
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1. The authors see DNS as a potential abuse target. According to
the DMARC specification, mail receivers read the DMARC policy
from the corresponding DNS txt record. Theoretically a wrong or
malicious implementation might result in multiple DNS queries per
message, resulting in a DoS attack on DNS. Such an attack is
unlikely to happen; not only is it expensive, the same result can
be achieved by simpler means. To avoid causing accidental DNS
DoS attacks, implementers consider using a DNS cache. {O.
Gavrylyako}
2. The authors see the volume of aggregated reports generated by
other hosts as potential for abuse. Sending a large volume of
reports could constitute a DoS attack on the sender domain. Such
an attack is also expensive and more theoretical than practical.
Nevertheless, to protect against this type of abuse, one should
publish a _reports._dmarc DNS txt record to restrict malicious
domains from redirecting their aggregate reports to an abuse
target. Another related potential risk is excessively large
aggregate reports that could be damaging to the recipient, though
the same abuse affect can be achieved without the DMARC protocol.
The majority of mail recipients enforce message size limits. {O.
Gavrylyako}
[Original listing of issues]
o DNS Abuse.
Most probably don't see this, but adding potentially multiple
new DNS lookups per email multiplies rapidly
DNS Cache can only help for so long
Agg reports are (probably) created from some other host
If Org Domain policy lookups are required, you may get two
lookups every time there is a single lookup
14. References
14.1. References - Normative
[DMARC] Kucherawy, M., Ed., "Domain-based Message Authentication,
Reporting and Conformance (DMARC)", URL https://
datatracker.ietf.org/doc/draft-kucherawy-dmarc-base/,
March 2013.
14.2. References - Informative
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[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", RFC
4408, April 2006.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 6376, September
2011.
Appendix A. Acknowledgements
DMARC and the version of this document submitted to the IETF were the
result of lengthy efforts by an informal industry consortium:
DMARC.org [1]. Participating companies included: Agari, American
Greetings, AOL, Bank of America, Cloudmark, Comcast, Facebook,
Fidelity Investments, Google, JPMorgan Chase & Company, LinkedIn,
Microsoft, Netease, Paypal, ReturnPath, Trusted Domain Project, and
Yahoo!. Although the number of contributors and supporters are too
numerous to mention, notable individual contributions were made by J.
Trent Adams, Michael Adkins, Monica Chew, Dave Crocker, Tim Draegen,
Murray Kucherawy, Steve Jones, Franck Martin, Brett McDowell, and
Paul Midgen. The contributors would also like to recognize the
invaluable input and guidance that was provided by J.D. Falk.
Author's Address
Dave Crocker (editor)
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
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