Using DMARC
draft-crocker-dmarc-bcp-01

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.

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This Internet-Draft will expire on October 13, 2013.

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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Development
• 3. Barriers to Adoption (or, How I Learned to Stop Worrying and Love DMARC)
• 4. Planning for DMARC adoption
• 4.1. Decide what you need to do
• 4.2. Picking Alignment and SP Parameter Values
• 4.3. Incremental Roll-Out, Sending
• 4.4. Incremental Roll-Out, Receiving
• 4.5. [Original bullets]
• 5. DNS Configuration
• 6. Receiver Processing
• 7. Report Generation
• 7.1. DMARC aggregate XML report naming and report metadata
• 7.2. Minimum requirements for DMARC XML aggregate report records
• 7.3. Use and Reporting of Local Policy Overrides
• 7.4. Minimum requirements for DMARC forensic reports
• 7.5. Additional concerns
• 8. Report Receipt and Analysis
• 8.1. Report Receipt
• 8.2. Report Analysis
• 8.3. Report Processing and Analysis Services
• 8.4. Additional concerns
• 9. Miscellaneous
• 10. Interaction Issues
• 11. DMARC Commentary
• 12. Privacy Concerns.
• 13. Security Considerations
• 14. References
• 14.1. References - Normative
• 14.2. References - Informative
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

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

An IETF mailing list for discussing DMARC issues has been established: dmarc@ietf.org.

• Editor's Note:

  The current version incorporates new sections of relatively raw text from the original DMARC team and edited merely to convert to xml2rfc format. It has not yet received editing for content. The version is being issued as an Internet Draft in order to create an archived snapshot, and possibly to whet more appetites for adding content... /Dave

2. Development

{This section is meant for any any software development practices, relevant to the use of DMARC. -ed}

• Core Functionality
• Tools Support

3. Barriers to Adoption (or, How I Learned to Stop Worrying and Love DMARC)

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

• 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 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.
  
  
• 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.
  
  
• 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.

  
  
• 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 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

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 is intended to protect transactional mail.[*] If you don't send transactional mail, you are not in the primary use case for DMARC, and you probably do not want to set up domains with p=reject. 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.

• ISSUE: this is a major assertion and usage constraint. It might be worth a separate discussion in the document, to give guidance on different styles of 'users' and how DMARC is, or is not, appropriate to them. (/ed)

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.
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.

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 rollout 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)

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

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]

• Malformed DMARC policy TXT records in DNS.
  • Can these be used for anything?
  • Should a policy that does not conform to the spec be ignored?
  • Is there any benefit in attempting to infer meaning (monitor = none, non = ??, 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?
• DMARC records for many domain names.

6. Receiver Processing

• 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 forensic) providing the spammers insight that they did not already have?
  • ***POTENTIALLY PROBLEMATIC QUESTION TO FOLLOW*** Should I only send reports to domains that I trust?

7. Report Generation

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.

  <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.

<?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>

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.

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”.

  <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 RFC 5322.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 RFC 5321.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.

Below are three examples of real DMARC XML records for a domain with a DMARC reject policy in place.

<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. 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>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 2. 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.

<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 3. 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”.

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.

• <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.

<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> 
            <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 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>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> 
      </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> 

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.

7.4. Minimum requirements for DMARC forensic reports

DMARC forensic 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. Forensic 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 forensic 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.
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

• 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."
• Minimum requirements for forensic reports (soft-fail reports).
  • What is the bare minimum required to produce a forensic report.
  • Whether to request Failure Reports (ruf=)

• Utility of Local Policy Overrides.

8. Report Receipt and Analysis

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.

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?
   • 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.

• 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?
• 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?
• What is the daily volume that passed DMARC aligned SPF and DKIM? Passed only one but not the other?
• What is the daily volume of unauthenticated messages, those that failed both DMARC aligned SPF and DKIM?
• Are any previously unknown sources of email passing either or both of DMARC aligned SPF or DKIM? If so why?
• 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.

• 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>.
• 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.
• 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 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

Do DMARC.org members think it is appropriate to point out here that there are vendor and 3rd party resources that can help with report receipt, processing, and analysis? Of course we would not mention specifics here, but could point to the dmarc.org/resources. I feel it is appropriate to point out that a build vs buy decision is possible.

8.4. Additional concerns

• When can I expect to receive my first aggregate report?
• DMARC failure/forensic 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.

• Performing remote destination checking
  • What happens if I don't?

• Identifier alignment
  • {This has been cited, but not yet explained, as a potential BCP issue. -ed}
• 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

Some issues come into play when DMARC is used in conjunction with one or more other services.

• Sender using both DMARC and ADSP.

11. DMARC Commentary

This section explains a number of choices made for DMARC.

• Rationale for choosing ZIP for the aggregate reports.

12. Privacy Concerns.

• IP Addresses in reports

13. Security Considerations

In this section we describe several security considerations related to the implementation of the DMARC protocol.

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.
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.

[Original listing of issues]

• 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

14.2. References - Informative

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. 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