DMARC A. Brotman (ed) Internet-Draft Comcast, Inc. Obsoletes: 7489 (if approved) 21 February 2021 Intended status: Standards Track Expires: 25 August 2021 DMARC Aggregate Reporting draft-ietf-dmarc-aggregate-reporting-01 Abstract DMARC allows for domain holders to request aggregate reports from receivers. This report is an XML document, and contains extensible elements that allow for other types of data to be specified later. The aggregate reports can be submitted to the domain holder's specified destination as supported by the receiver. This document (along with others) obsoletes RFC7489. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 25 August 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Brotman (ed) Expires 25 August 2021 [Page 1] Internet-Draft DMARC Aggregate Reporting February 2021 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 2. DMARC Feedback . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Verifying External Destinations . . . . . . . . . . . . . 3 2.2. Aggregate Reports . . . . . . . . . . . . . . . . . . . . 5 2.3. Extensions . . . . . . . . . . . . . . . . . . . . . . . 7 2.3.1. Transport . . . . . . . . . . . . . . . . . . . . . . 8 3. Extensible Reporting . . . . . . . . . . . . . . . . . . . . 11 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11 5.1. Data Exposure Considerations . . . . . . . . . . . . . . 11 5.2. Report Recipients . . . . . . . . . . . . . . . . . . . . 12 5.3. Data Contained Within Reports (Tkt64) . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Appendix A. DMARC XML Schema . . . . . . . . . . . . . . . . 13 8. Appendix B. Sample Report . . . . . . . . . . . . . . . . . 18 9. Normative References . . . . . . . . . . . . . . . . . . . . 19 10. Informative References . . . . . . . . . . . . . . . . . . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction A key component of DMARC is the ability for domain holders to request that receivers provide various types of reports. These reports allow domain holders to have insight into which IP addresses are sending on their behalf, and some insight into whether or not the volume may be legitimate. These reports expose information relating to the DMARC policy, as well as the outcome of SPF [RFC7208] & DKIM [RFC6376] validation. 1.1. Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. Brotman (ed) Expires 25 August 2021 [Page 2] Internet-Draft DMARC Aggregate Reporting February 2021 2. DMARC Feedback Providing Domain Owners with visibility into how Mail Receivers implement and enforce the DMARC mechanism in the form of feedback is critical to establishing and maintaining accurate authentication deployments. When Domain Owners can see what effect their policies and practices are having, they are better willing and able to use quarantine and reject policies. 2.1. Verifying External Destinations It is possible to specify destinations for the different reports that are outside the authority of the Domain Owner making the request. This allows domains that do not operate mail servers to request reports and have them go someplace that is able to receive and process them. Without checks, this would allow a bad actor to publish a DMARC policy record that requests that reports be sent to a victim address, and then send a large volume of mail that will fail both DKIM and SPF checks to a wide variety of destinations; the victim will in turn be flooded with unwanted reports. Therefore, a verification mechanism is included. When a Mail Receiver discovers a DMARC policy in the DNS, and the Organizational Domain at which that record was discovered is not identical to the Organizational Domain of the host part of the authority component of a [URI] specified in the "rua" or "ruf" tag, the following verification steps are to be taken: 1. Extract the host portion of the authority component of the URI. Call this the "destination host", as it refers to a Report Receiver. 2. Prepend the string "_report._dmarc". 3. Prepend the domain name from which the policy was retrieved, after conversion to an A-label if needed. 4. Query the DNS for a TXT record at the constructed name. If the result of this request is a temporary DNS error of some kind (e.g., a timeout), the Mail Receiver MAY elect to temporarily fail the delivery so the verification test can be repeated later. Brotman (ed) Expires 25 August 2021 [Page 3] Internet-Draft DMARC Aggregate Reporting February 2021 5. For each record returned, parse the result as a series of "tag=value" pairs, i.e., the same overall format as the policy record (see Section 6.4). In particular, the "v=DMARC1" tag is mandatory and MUST appear first in the list. Discard any that do not pass this test. 6. If the result includes no TXT resource records that pass basic parsing, a positive determination of the external reporting relationship cannot be made; stop. 7. If at least one TXT resource record remains in the set after parsing, then the external reporting arrangement was authorized by the Report Receiver. 8. If a "rua" or "ruf" tag is thus discovered, replace the corresponding value extracted from the domain's DMARC policy record with the one found in this record. This permits the Report Receiver to override the report destination. However, to prevent loops or indirect abuse, the overriding URI MUST use the same destination host from the first step. For example, if a DMARC policy query for "blue.example.com" contained "rua=mailto:reports@red.example.net", the host extracted from the latter ("red.example.net") does not match "blue.example.com", so this procedure is enacted. A TXT query for "blue.example.com._report._dmarc.red.example.net" is issued. If a single reply comes back containing a tag of "v=DMARC1", then the relationship between the two is confirmed. Moreover, "red.example.net" has the opportunity to override the report destination requested by "blue.example.com" if needed. Where the above algorithm fails to confirm that the external reporting was authorized by the Report Receiver, the URI MUST be ignored by the Mail Receiver generating the report. Further, if the confirming record includes a URI whose host is again different than the domain publishing that override, the Mail Receiver generating the report MUST NOT generate a report to either the original or the override URI. A Report Receiver publishes such a record in its DNS if it wishes to receive reports for other domains. A Report Receiver that is willing to receive reports for any domain can use a wildcard DNS record. For example, a TXT resource record at "*._report._dmarc.example.com" containing at least "v=DMARC1" confirms that example.com is willing to receive DMARC reports for any domain. Brotman (ed) Expires 25 August 2021 [Page 4] Internet-Draft DMARC Aggregate Reporting February 2021 If the Report Receiver is overcome by volume, it can simply remove the confirming DNS record. However, due to positive caching, the change could take as long as the time-to-live (TTL) on the record to go into effect. A Mail Receiver might decide not to enact this procedure if, for example, it relies on a local list of domains for which external reporting addresses are permitted. 2.2. Aggregate Reports The DMARC aggregate feedback report is designed to provide Domain Owners with precise insight into: * authentication results, * corrective action that needs to be taken by Domain Owners, and * the effect of Domain Owner DMARC policy on email streams processed by Mail Receivers. Aggregate DMARC feedback provides visibility into real-world email streams that Domain Owners need to make informed decisions regarding the publication of DMARC policy. When Domain Owners know what legitimate mail they are sending, what the authentication results are on that mail, and what forged mail receivers are getting, they can make better decisions about the policies they need and the steps they need to take to enable those policies. When Domain Owners set policies appropriately and understand their effects, Mail Receivers can act on them confidently. Visibility comes in the form of daily (or more frequent) Mail Receiver-originated feedback reports that contain aggregate data on message streams relevant to the Domain Owner. This information includes data about messages that passed DMARC authentication as well as those that did not. The format for these reports is defined in Appendix A. The report SHOULD include the following data: * The DMARC policy discovered and applied, if any * The selected message disposition * The identifier evaluated by SPF and the SPF result, if any * The identifier evaluated by DKIM and the DKIM result, if any Brotman (ed) Expires 25 August 2021 [Page 5] Internet-Draft DMARC Aggregate Reporting February 2021 * For both DKIM and SPF, an indication of whether the identifier was in alignment * A separate report should be generated for each 5322.From subdomain, regardless of which policy domain was used during receipt of messages * Sending and receiving domains * The policy requested by the Domain Owner and the policy actually applied (if different) * The number of successful authentications * The counts of messages based on all messages received, even if their delivery is ultimately blocked by other filtering agents ProposedAddition: [[ DMARC Aggregate Reports MUST contain two primary sections; one consisting of descriptive information and the other a set of IP- focused row-based data. Each report MUST contain data for only one Author Domain. A single report SHOULD contain data for one policy configuration. If multiple configurations were observed during a single reporting period, a reporting entity MAY choose to send multiple reports, otherwise the reporting entity SHOULD note only the final configuration observed during the period. See below for further information. The informative section MUST contain two sub-sections. One will be the metadata section which MUST contain the fields related to "org_name", "email", "report_id", and "date_range". Optional fields MAY include "extra_contact_info" and an "error" field. The "date_range" section which will note "begin" and "end" values as epoch timestamps. The other sub-section will be the "policy_published", and record the policy configuration observed by the receiving system. Mandatory fields are "domain", "p", "sp", "pct". Optional fields are "fo", "adkim", "aspf". Within the data section, the report will contain row(s) of data stating which IPs were seen to have delivered messages for the Author Domain to the receiving system. For each IP that is being reported, there will be a "record" element, which will then have each of a "row", "identifiers", and "auth_results" sub-element. Within the "row" element, there MUST be "source_ip" and "count". There MUST also exist a "policy_evaluated", with subelements of "disposition", "dkim", and "spf". There MAY be an element for "reason", meant to include any notes the reporter might want to include as to why the Brotman (ed) Expires 25 August 2021 [Page 6] Internet-Draft DMARC Aggregate Reporting February 2021 "disposition" policy does not match the "policy_published", such as a Local Policy override (possible values listed in Appendex A). The "dkim" and "spf" elements MUST be the evaluated values as they relate to DMARC, not the values the receiver may have used when overriding the policy. Within the "identifiers" element, there MUST exist the data that was used to apply policy for the given IP. In most cases, this will be a "header_from" element, which will contain the 5322.From domain from the message. There MUST be an "auth_results" element within the 'record' element. This will contain the data related to authenticating the messages associated with this sending IP. The "dkim" subelement is optional as not all messages are signed, while there MUST be at least one "spf" subelement. These elements MUST have a "domain" that was used during validation, as well as "result". Optionally, the "dkim" element MAY include a "selector" element that was observed during validation. For the "spf" element, the "result" element MUST contain a lower-case string where the value is one of none/neutral/pass/fail/softfail/temperror/permerror. The "dkim" result MUST contain a lower-case string where the value is one of none/pass/fail/policy/neutral/temperror/permerror. 2.3. Extensions There MAY be an optional section for extensions within the "feedback" element. The absence or existence of this section SHOULD NOT create an error when processing reports. This will be covered in a separate section. ]] Note that Domain Owners or their agents may change the published DMARC policy for a domain or subdomain at any time. From a Mail Receiver's perspective, this will occur during a reporting period and may be noticed during that period, at the end of that period when reports are generated, or during a subsequent reporting period, all depending on the Mail Receiver's implementation. Under these conditions, it is possible that a Mail Receiver could do any of the following: * generate for such a reporting period a single aggregate report that includes message dispositions based on the old policy, or a mix of the two policies, even though the report only contains a single "policy_published" element; * generate multiple reports for the same period, one for each published policy occurring during the reporting period; Brotman (ed) Expires 25 August 2021 [Page 7] Internet-Draft DMARC Aggregate Reporting February 2021 * generate a report whose end time occurs when the updated policy was detected, regardless of any requested report interval. Such policy changes are expected to be infrequent for any given domain, whereas more stringent policy monitoring requirements on the Mail Receiver would produce a very large burden at Internet scale. Therefore, it is the responsibility of report consumers and Domain Owners to be aware of this situation and allow for such mixed reports during the propagation of the new policy to Mail Receivers. Aggregate reports are most useful when they all cover a common time period. By contrast, correlation of these reports from multiple generators when they cover incongruent time periods is difficult or impossible. Report generators SHOULD, wherever possible, adhere to hour boundaries for the reporting period they are using. For example, starting a per-day report at 00:00; starting per-hour reports at 00:00, 01:00, 02:00; etc. Report generators using a 24-hour report period are strongly encouraged to begin that period at 00:00 UTC, regardless of local timezone or time of report production, in order to facilitate correlation. A Mail Receiver discovers reporting requests when it looks up a DMARC policy record that corresponds to an RFC5322.From domain on received mail. The presence of the "rua" tag specifies where to send feedback. 2.3.1. Transport Where the URI specified in a "rua" tag does not specify otherwise, a Mail Receiver generating a feedback report SHOULD employ a secure transport mechanism. The Mail Receiver, after preparing a report, MUST evaluate the provided reporting URIs in the order given. Any reporting URI that includes a size limitation exceeded by the generated report (after compression and after any encoding required by the particular transport mechanism) MUST NOT be used. An attempt MUST be made to deliver an aggregate report to every remaining URI, up to the Receiver's limits on supported URIs. If transport is not possible because the services advertised by the published URIs are not able to accept reports (e.g., the URI refers to a service that is unreachable, or all provided URIs specify size limits exceeded by the generated record), the Mail Receiver SHOULD send a short report (see Section 7.2.2) indicating that a report is available but could not be sent. The Mail Receiver MAY cache that data and try again later, or MAY discard data that could not be sent. Brotman (ed) Expires 25 August 2021 [Page 8] Internet-Draft DMARC Aggregate Reporting February 2021 2.3.1.1. Email The message generated by the Mail Receiver MUST be a [MAIL] message formatted per [MIME]. The aggregate report itself MUST be included in one of the parts of the message. A human-readable portion MAY be included as a MIME part (such as a text/plain part). The aggregate data MUST be an XML file that SHOULD be subjected to GZIP compression. Declining to apply compression can cause the report to be too large for a receiver to process (a commonly observed receiver limit is ten megabytes); doing the compression increases the chances of acceptance of the report at some compute cost. The aggregate data SHOULD be present using the media type "application/ gzip" if compressed (see [GZIP]), and "text/xml" otherwise. The filename is typically constructed using the following ABNF: filename = receiver "!" policy-domain "!" begin-timestamp "!" end-timestamp [ "!" unique-id ] "." extension unique-id = 1*(ALPHA / DIGIT) receiver = domain ; imported from [MAIL] policy-domain = domain begin-timestamp = 1*DIGIT ; seconds since 00:00:00 UTC January 1, 1970 ; indicating start of the time range contained ; in the report end-timestamp = 1*DIGIT ; seconds since 00:00:00 UTC January 1, 1970 ; indicating end of the time range contained ; in the report extension = "xml" / "xml.gz" The extension MUST be "xml" for a plain XML file, or "xml.gz" for an XML file compressed using GZIP. "unique-id" allows an optional unique ID generated by the Mail Receiver to distinguish among multiple reports generated simultaneously by different sources within the same Domain Owner. For example, this is a possible filename for the gzip file of a report to the Domain Owner "example.com" from the Mail Receiver "mail.receiver.example": Brotman (ed) Expires 25 August 2021 [Page 9] Internet-Draft DMARC Aggregate Reporting February 2021 mail.receiver.example!example.com!1013662812!1013749130.gz No specific MIME message structure is required. It is presumed that the aggregate reporting address will be equipped to extract MIME parts with the prescribed media type and filename and ignore the rest. Email streams carrying DMARC feedback data MUST conform to the DMARC mechanism, thereby resulting in an aligned "pass" (see Section 3.1). This practice minimizes the risk of report consumers processing fraudulent reports. The RFC5322.Subject field for individual report submissions SHOULD conform to the following ABNF: dmarc-subject = %x52.65.70.6f.72.74 1*FWS ; "Report" %x44.6f.6d.61.69.6e.3a 1*FWS ; "Domain:" domain-name 1*FWS ; from RFC 6376 %x53.75.62.6d.69.74.74.65.72.3a ; "Submitter:" 1*FWS domain-name 1*FWS %x52.65.70.6f.72.74.2d.49.44.3a ; "Report-ID:" msg-id ; from RFC 5322 The first domain-name indicates the DNS domain name about which the report was generated. The second domain-name indicates the DNS domain name representing the Mail Receiver generating the report. The purpose of the Report-ID: portion of the field is to enable the Domain Owner to identify and ignore duplicate reports that might be sent by a Mail Receiver. For instance, this is a possible Subject field for a report to the Domain Owner "example.com" from the Mail Receiver "mail.receiver.example". It is line-wrapped as allowed by [MAIL]: Subject: Report Domain: example.com Submitter: mail.receiver.example Report-ID: <2002.02.15.1> This transport mechanism potentially encounters a problem when feedback data size exceeds maximum allowable attachment sizes for either the generator or the consumer. See Section 7.2.2 for further discussion. 2.3.1.2. Other Methods The specification as written allows for the addition of other registered URI schemes to be supported in later versions. Brotman (ed) Expires 25 August 2021 [Page 10] Internet-Draft DMARC Aggregate Reporting February 2021 3. Extensible Reporting A DMARC report should allow for some extensibility, as defined by future documents that utilize DMARC as a foundation. These extensions MUST be properly formatted XML and meant to exist within the structure of a DMARC report. They MUST NOT alter the existing DMARC structure, but instead exist self-contained within an "" element. This element MUST be a child of the "" element. ... ... A DMARC report receiver SHOULD NOT generate a processing error when this "" element is absent or empty. Furthermore, if a processor is unable to handle an extension in a report, it SHOULD ignore the data, and continue to the next extension. 4. IANA Considerations TBD 5. Privacy Considerations This section will discuss exposure related to DMARC aggregate reporting. 5.1. Data Exposure Considerations Aggregate reports are limited in scope to DMARC policy and disposition results, to information pertaining to the underlying authentication mechanisms, and to the identifiers involved in DMARC validation. Aggregate report may expose sender and recipient identifiers, specifically the RFC5322.From addresses. Domain Owners requesting reports will receive information about mail claiming to be from them, which includes mail that was not, in fact, from them. Information about the final destination of mail where it might otherwise be obscured by intermediate systems will therefore be exposed. Brotman (ed) Expires 25 August 2021 [Page 11] Internet-Draft DMARC Aggregate Reporting February 2021 When message-forwarding arrangements exist, Domain Owners requesting reports will also receive information about mail forwarded to domains that were not originally part of their messages' recipient lists. This means that destination domains previously unknown to the Domain Owner may now become visible. Disclosure of information about the messages is being requested by the entity generating the email in the first place, i.e., the Domain Owner and not the Mail Receiver, so this may not fit squarely within existing privacy policy provisions. For some providers, aggregate reporting is viewed as a function similar to complaint reporting about spamming or phishing and are treated similarly under the privacy policy. Report generators (i.e., Mail Receivers) are encouraged to review their reporting limitations under such policies before enabling DMARC reporting. 5.2. Report Recipients A DMARC record can specify that reports should be sent to an intermediary operating on behalf of the Domain Owner. This is done when the Domain Owner contracts with an entity to monitor mail streams for abuse and performance issues. Receipt by third parties of such data may or may not be permitted by the Mail Receiver's privacy policy, terms of use, or other similar governing document. Domain Owners and Mail Receivers should both review and understand if their own internal policies constrain the use and transmission of DMARC reporting. Some potential exists for report recipients to perform traffic analysis, making it possible to obtain metadata about the Receiver's traffic. In addition to verifying compliance with policies, Receivers need to consider that before sending reports to a third party. 5.3. Data Contained Within Reports (Tkt64) Aggregate feedback reports contain aggregated data relating to messages purportedly originating from the Domain Owner. The data does not contain any identifying characteristics about individual users. No personal information such as individual email addresses, IP addresses of individuals, or the content of any messages, is included in reports. Brotman (ed) Expires 25 August 2021 [Page 12] Internet-Draft DMARC Aggregate Reporting February 2021 Mail Receivers should have no concerns in sending reports as they do not contain personal information. In all cases, the data within the reports relates to the domain-level authentication information provided by mail servers sending messages on behalf of the Domain Owner. This information is necessary to assist Domain Owners in implementing and maintaining DMARC. Domain Owners should have no concerns in receiving reports as they do not contain personal information. The reports only contain aggregated data related to the domain-level authentication details of messages claiming to originate from their domain. This information is essential for the proper implementation and operation of DMARC. Domain Owners who are unable to receive reports for organizational reasons, can choose to exclusively direct the reports to an external processor. 6. Security Considerations TBD 7. Appendix A. DMARC XML Schema Brotman (ed) Expires 25 August 2021 [Page 13] Internet-Draft DMARC Aggregate Reporting February 2021 Brotman (ed) Expires 25 August 2021 [Page 14] Internet-Draft DMARC Aggregate Reporting February 2021 Brotman (ed) Expires 25 August 2021 [Page 15] Internet-Draft DMARC Aggregate Reporting February 2021 Brotman (ed) Expires 25 August 2021 [Page 17] Internet-Draft DMARC Aggregate Reporting February 2021 8. Appendix B. Sample Report Brotman (ed) Expires 25 August 2021 [Page 18] Internet-Draft DMARC Aggregate Reporting February 2021 Sample Reporter report_sender@example-reporter.com ... 3v98abbp8ya9n3va8yr8oa3ya 161212415 161221511 example.com

quarantine

none 100 192.168.4.4 123 quarantine pass fail example.com example.com pass abc123 example.com> fail 9. Normative References Brotman (ed) Expires 25 August 2021 [Page 19] Internet-Draft DMARC Aggregate Reporting February 2021 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, . [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014, . 10. Informative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Author's Address Alex Brotman Comcast, Inc. Email: alex_brotman@comcast.com Brotman (ed) Expires 25 August 2021 [Page 20]