DMARC Working Group S. Jones, Ed.
Intended status: Informational K. Andersen
Expires: April 24, 2020 LinkedIn
October 22, 2019

Recommended Usage of the Authenticated Received Chain (ARC)


The Authentication Received Chain (ARC) provides an authenticated “chain of custody” for a message, allowing each entity that handles the message to see what entities handled it before, and to see what the message’s authentication assessment was at each step in the handling. But the specification does not indicate how the entities handling these messages should interpret or utilize ARC results in making decisions about message disposition. This document will provide guidance in these areas.

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 April 24, 2020.

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Table of Contents

1. Introduction

[ARC] is intended to be used by Internet Mail Handlers who forward or resend messages, with or without alterations, such that they will no longer pass the SPF [RFC7208], DKIM [RFC6376], and/or DMARC [RFC7489] mechanisms when evaluated by subsequent message handlers or the final recipient. In such cases ARC may provide useful information about the message before the forwarding and/or alterations took place, and recipients may choose to use this information to influence delivery decisions.

2. Overview

2.1. How does ARC work?

Consider a message sent to a mailing list. Assume that the message author’s domain publishes an SPF record, signs messages with a DKIM signature that includes the RFC5322.Subject header and the message body, and publishes a DMARC policy of “p=reject”. Finally, assume that the final recipient(s) of the message implement SPF, DKIM and DMARC authentication checks on incoming messages.

This message is received by the ADMD hosting the Mailing List Manager (MLM) software. Upon receipt from the message author’s ADMD, the results from any DKIM, DMARC, and SPF checks would be recorded in an Authentication-Results header. Then as part of normal list operation the following changes are made to the message:

After these alterations have been made, the message is sent to list members.

A list member’s ADMD receiving the message will typically strip out any existing Authentication-Results headers. It will then perform an SPF check using the domain in the RFC5321.MailFrom address field, and would find that the sending host is in the list of authorized senders for the MLM’s domain. However under DMARC, since this domain does not match the domain in the RFC5322.From address field, the DMARC SPF result is “fail.”

The DKIM signature from the domain in the RFC5322.From address field - the message author’s domain - will fail to verify, because the RFC5322.Subject header and the message body were altered by the MLM. Therefore the DMARC DKIM result is also “fail,” even if there is a valid DKIM signature attached by the MLM’s ADMD using its domain.

Since neither SPF or DKIM yield a “pass” under DMARC’s alignment rules, the DMARC result for this message is “fail.” Therefore under the DMARC policy published by the message author’s domain, the list member’s ADMD should reject the message.

If the MLM implemented ARC, it would record the results of its email authentication checks when receiving the message from the author’s ADMD in the Authentication-Results header, then perform the alterations described above. It would then “seal” the message under ARC, which includes the following steps.

It would record the contents of the Authentication-Results header(s) in a newly created ARC-Authentication-Results header. It would create an ARC-Message-Signature header, which includes a cryptographic signature of the message itself very similar to a DKIM signature, but excluding any ARC headers. Then it would create an ARC-Seal header, which includes a cryptographic signature of all ARC headers present in the message. The MLM’s ADMD would then send the ARC “sealed” message to the list members.

When the message reaches a list member’s ADMD, the SPF and DKIM results will still not pass the DMARC check. However if the receiving ADMD implements ARC, it can check for and validate the ARC chain in the message, and verify that the contents of the ARC-Authentication-Results header were conveyed intact from the MLM’s ADMD. At that point the final recipient’s ADMD might choose to use those authentication results in the decision whether or not to deliver the message, even though it failed to pass conventional SPF, DKIM, and DMARC checks.

2.2. What new headers are introduced by ARC?

The following new headers are defined in [ARC] Section 4.1, “ARC Header Fields”:

Each time a message passes through an ARC Intermediary, an ARC Set consisting of these three headers will be attached to the message. More information about ARC Sets can be found in [ARC] Section 4.2, “ARC Set.” The entire collection of ARC Sets in a message is commonly referred to as the ARC Chain.

2.3. Does ARC support Internationalized Email (EAI)?

Changes to support EAI are inherited from DKIM [RFC6376] as updated by [draft-levine-eaiauth], and Authentication-Results as updated in [I-D-7601bis]. For more details, please refer to [ARC] Section 4.1.4, “Internationalized Email (EAI).”

2.4. Does ARC support multiple digital signature algorithms?

Originally ARC only supported a single signing algorithm, but the DCRUP working group is expanding the set of supported algorithms available to DKIM [RFC6376] and derived protocols. [ARC-MULTI] is adapting this work to extend [ARC] to support multiple algorithms.

3. Guidance for Receivers/Validators

3.1. What is the significance of an intact ARC chain?

An intact ARC chain conveys authentication results like SPF and DKIM as observed by the first ARC participant. In cases where the message no longer produces passing results for DKIM, SPF, or DMARC but an intact ARC chain is present, the message receiver may choose to use the contents of the first ARC-Authentication-Results header field in determining how to handle the message.

3.2. What exactly is an “intact” ARC chain?

Note that not all ADMDs will implement ARC, and receivers will see messages where one or more non-participating ADMDs handled a message before, after, or in between participating ADMDs.

An intact ARC chain is one where the ARC headers that are present can be validated, and in particular the ARC-Message-Signature header from the last ARC participant can still be validated. This shows that the portions of the message covered by that signature were not altered. If any non-participating ADMDs handled the message since the last ARC intermediary but did not alter the message in a way that invalidated the most recent ARC-Message-Signature present, the chain would still be considered intact by the next ARC-enabled ADMD.

Message receivers may make local policy decisions about whether to use the contents of the ARC-Authentication-Results header field in cases where a message no longer passes DKIM, DMARC, and/or SPF checks. Whether an ARC chain is intact can be used to inform that local policy decision.

So for example one message receiver may decide that, for messages with an intact ARC chain where a DMARC evaluation does not pass, but the ARC-Authentication-Results header field from the first ARC participant indicates a DKIM pass was reported that matches the domain in the RFC5322.From header field, it may override a DMARC “p=reject” policy. Another message receiver may decide to do so only for a limited number of ARC-enabled ADMDs. A third message receiver may choose not to take ARC information into account at all.

3.3. What is the significance of an invalid (“broken”) ARC chain?

An ARC chain is broken if the signatures in the ARC-Seal header fields cannot be verified, or if the most recent AMS can not be verified. For example if a non-ARC-enabled ADMD delivers a message with ARC header sets to the validating ADMD, but modified the message such that those ARC and DKIM signatures already in the message were invalidated.

In case of a broken ARC chain, the message should be treated the same as if there was no ARC chain at all. For example, a message that fails under DMARC and has an invalid ARC chain would be subject to that DMARC policy, which may cause it to be quarantined or rejected.

Email transit can produce broken signatures for a wide variety of benign reasons. This includes possibly breaking one or more ARC signatures. Therefore, receivers need to be wary of ascribing motive to such breakage, although patterns of common behaviour may provide some basis for adjusting local policy decisions.

3.4. What error code(s) should be returned if an invalid ARC chain is detected during an SMTP transaction?

According to [ARC] Section 5.2.2, a Validator MAY signal the breakage during the SMTP transaction by returning the extended SMTP response code 5.7.29 “ARC validation failure” and corresponding SMTP basic response code. Since ARC failures are likely the be detected due to other, underlying authentication failures, Validators may also choose to return the more general 5.7.26 “Multiple authentication checks failed instead of the ARC-specific code.

3.5. What does the absence of an ARC chain in a message mean?

The absence of an ARC chain means nothing. ARC is intended to allow a participating message handler to preserve certain authentication results when a message is being forwarded and/or modified such that the final recipient can evaluate this information. If they are absent, there is nothing extra that ARC requires the final recipient to do.

3.6. What reasonable conclusions can you draw based upon seeing lots of mail with ARC chains?

With sufficient history, ARC can be used to augment DMARC authentication policy (i.e. a message could fail DMARC, but validated ARC information and therefore could be considered as validly authenticated as reported by the first ARC participant).

If the validator does content analysis and reputation tracking, the ARC participants in a message can be credited or discredited for good or bad content. By analyzing different ARC chains involved in “bad” messages, a validator might identify malicious participating intermediaries.

With a valid chain and good reputations for all ARC participants, receivers may choose to apply a “local policy override” to the DMARC policy assertion for the domain authentication evaluation, depending on the ARC-Authentication-Results header field value. Normal content analysis should never be skipped.

3.7. What if none of the intermediaries have been seen previously?

This has no impact on the operation of ARC, as ARC is not a reputation system. ARC conveys the results of other authentication mechanisms such that the participating message handlers can be positively identified. Final message recipients may or may not choose to examine these results when messages fail other authentication checks. They are more likely to override, say, a failing DMARC result in the presence of an intact ARC chain where the participating ARC message handlers have been observed to not convey “bad” content in the past, and the initial ARC participant indicates the message they received had passed authentication checks.

3.8. What about ARC chains where some intermediaries are known and others are not?

Validators may choose to build reputation models for ARC message handlers they have observed. Generally speaking it is more feasible to accrue positive reputation to intermediaries when they consistently send messages that are evaluated positively in terms of content and ARC chains. When messages are received with ARC chains that are not intact, it is very difficult to identify which intermediaries may have manipulated the message or injected bad content.

3.9. What should message handlers do when they detect malicious content in messages where ARC is present?

Message handlers should do what they normally do when they detect malicious content in a message - hopefully that means quarantining or discarding the message. ARC information should never make malicious content acceptable.

In such cases it is difficult to determine where the malicious content may have been injected. What ARC can do in such cases is verify that a given intermediary or message handler did in fact handle the message as indicated in the header fields. In such cases a message recipient who maintains a reputation system about email senders may wish to incorporate this information as an additional factor in the score for the intermediaries and sender in question. However reputation systems are very complex, and usually unique to those organizations operating them, and therefore beyond the scope of this document.

3.10. What feedback does a sender or domain owner get about ARC when it is applied to their messages?

ARC itself does not include any mechanism for feedback or reporting. It does however recommend that message receiving systems that use ARC to augment their delivery decisions, who use DMARC and decide to deliver a message because of ARC information, should include a notation to that effect in their normal DMARC reports. These notations would be easily identifiable by report processors, so that senders and domain owners can see where ARC is being used to augment the deliverability of their messages.

3.11. What prevents a malicious actor from removing the ARC header fields, altering the content, and creating a new ARC chain?

ARC does not prevent a malicious actor from doing this. Nor does it prevent a malicious actor from removing all but the first ADMD’s ARC header fields and altering the message, eliminating intervening participants from the ARC chain. Or similar variations.

A valid ARC chain does not provide any automatic benefit. With an intact ARC chain, the final message recipient may choose to use the contents of the ARC-Authentication-Results header field in determining how to handle the message. The decision to use the ARC-Authentication-Results header field is dependent on evaluation of those ARC intermediaries.

In the first case, the bad actor has succeeded in manipulating the message but they have attached a verifiable signature identifying themselves. While not an ideal situation, it is something they are already able to do without ARC involved, but now a signature linked to the domain responsible for the manipulation is present.

Additionally in the second case it is possible some negative reputational impact might accrue to the first ARC participant left in place until more messages reveal the pattern of activity by the bad actor. But again, a bad actor can similarly manipulate a sequence of RFC5322.Received header fields today without ARC, but with ARC that bad actor has verifiably identified themselves.

4. Guidance for Intermediaries

4.1. What is an Intermediary under ARC?

In the context of ARC, an Intermediary is typically an Administrative Management Domain [RFC5598] that is receiving a message, potentially manipulating or altering it, and then passing it on to another ADMD for delivery. Common examples of Intermediaries are mailing lists, alumni or professional email address providers that forward messages such as universities or professional organizations, et cetera.

4.2. What are the minimum requirements for an ARC Intermediary?

A participating ARC intermediary must validate the ARC chain on a message it receives, if one is present. It then attaches its own ARC seal and signature, including an indication if the chain failed to validate upon receipt.

4.2.1. More specifically a participating ARC intermediary must do the following:

  1. Validate that the ARC chain, if one is already present in the message, is intact and well-formed. ([ARC] Section 5.2, “Validator Actions”)
  2. Record the ARC status in an Authentication-Results header ([RFC7601])
  3. Generate a new ARC set and add it to the message. ([ARC] Section 5.1, “Sealer Actions”)

4.3. Should every MTA be an ARC participant?

Generally speaking, ARC is designed to operate at the ADMD level. When a message is first received by an ADMD, the traditional authentication results should be captured and preserved - this could be the common case of creating an Authentication-Results header field. But when it is determined that the message is being sent on outside of that ADMD, that is when the ADMD should add itself to the ARC chain - before sending the message outside of the ADMD.

Some organizations may operate multiple ADMDs, with more or less independence between them. While they should make a determination based on their specific circumstances, it may be useful and appropriate to have multiple ADMDs be ARC participants.

4.4. What should an intermediary do in the case of an invalid or “broken” ARC chain?

In general terms, a participating ARC intermediary will note that an ARC chain was present and invalid, or broken, when it attaches its own ARC seal and signature. However the fact that the ARC chain was invalid should have no impact on whether and how the message is delivered.

4.5. What should I do in the case where there is no ARC chain present in a message?

A participating ARC intermediary receiving a message with no ARC chain, and which will be delivered outside its ADMD, should start an ARC chain according to the ARC specification. This will include capturing the normal email authentication results for the intermediary (SPF, DKIM, DMARC, etc), which will be conveyed as part of the ARC chain.

4.6. How could ARC affect my reputation as an intermediary?

Message receivers often operate reputation systems, which build a behavioral profile of various message handlers and intermediaries. The presence or absence of ARC is yet another data point that may be used as an input to such reputation systems. Messages deemed to have good content may provide a positive signal for the intermediaries that handled it, while messages with bad content may provide a negative signal for the those intermediaries. Intact and valid ARC elements may amplify or attenuate such signals, depending on the circumstances.

Reputation systems are complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.

4.7. What can I do to influence my reputation as an intermediary?

Today it is extremely simple for a malicious actor to construct a message that includes your identity as an intermediary, even though you never handled the message. It is possible that an intermediary implementing ARC on all traffic it handles might receive some reputational benefit by making it easier to detect when their involvement in conveying bad traffic has been “forged.”

As mentioned previously reputation systems are very complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.

5. Guidance for Originators

5.1. Where can I find out more information?

Please visit the web site, or join the arc-discuss mailing list at

To discuss the [ARC] specification itself, please join the DMARC working group at

5.2. How/where can I test interoperabililty for my implementation?

There have been numerous interoperability tests during the development of the [ARC] specification. These tests are usually announced on both the arc-discuss mailing list at, and the DMARC working group at https://datatracker/ Join whichever body is most appropriate for you and/or your organization to receive future announcements.

5.3. How can ARC impact my email?

Prior to ARC, certain DMARC policies on a domain would cause messages using those domains in the RFC5322.From field, and which pass through certain kinds of intermediaries (mailing lists, forwarding services), to fail authentication checks at the message receiver. As a result these messages might not be delivered to the intended recipient.

ARC seeks to provide these so-called “indirect mailflows” with a means to preserve email authentication results as recorded by participating intermediaries. Message receivers may accept validated ARC information to supplement the information that DMARC provides, potentially deciding to deliver the message even though a DMARC check did not pass.

The net result for domain owners and senders is that ARC may allow messages routed through participating ARC intermediaries to be delivered, even though those messages would not have been delivered in the absence of ARC.

5.4. How can ARC impact my reputation as a message sender?

Message receivers often operate reputation systems, which build a behavioral profile of various message senders (and perhaps intermediaries). The presence or absence of ARC is yet another data point that may be used as an input to such reputation systems. Messages deemed to have good content may provide a positive signal for the sending domain and the intermediaries that handled it, while messages with bad content may provide a negative signal for the sending domain and the intermediaries that handled it. Intact and valid ARC elements may amplify or attenuate such signals, depending on the circumstances.

Reputation systems are complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.

5.5. Can I tell intermediaries not to use ARC?

At present there is no way for a message sender to request that intermediaries not employ ARC.

6. Considerations

6.1. IANA Considerations

This document has no actions for IANA.

6.2. Security Considerations

This document does not have security considerations aside from those raised in the main content.

7. References

7.1. Normative References

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009.
[RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377, September 2011.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7601, DOI 10.17487/RFC7601, August 2015.

7.2. Informative References

[ARC] Andersen, K., Long, B., Blank, S. and M. Kucherawy, "Authenticated Received Chain (ARC) Protocol", December 2018.
[ARC-MULTI] Andersen, K., Blank, S. and J. Levine, "Using Multiple Signing Algorithms with ARC", June 2018.
[draft-levine-eaiauth] Levine, J., "E-mail Authentication for Internationalized Mail", August 2018.
[ENHANCED-STATUS] "IANA SMTP Enhanced Status Codes", n.d..
[I-D-7601bis] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", February 2018.
[OAR] Chew, M. and M. Kucherawy, "Original-Authentication-Results Header Field", February 2012.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "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.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.
[RFC7960] Martin, F., Lear, E., Draegen. Ed., T., Zwicky, E. and K. Andersen, "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows", RFC 7960, DOI 10.17487/RFC7960, September 2016.

Appendix A. Glossary

Administrative Management Domain as used in [RFC5598] and similar references refers to a single entity operating one or more computers within one or more domain names under said entity’s control. One example might be a small company with a single server, handling email for that company’s domain. Another example might be a large university, operating many servers that fulfill different roles, all handling email for several different domains representing parts of the university.
ARC is an acronym: Authentication Results Chain - see also [ARC]
An [RFC5322] message header field formed in compliance with the ARC specification. It includes certain content from all prior ARC participants, if there are any.
ARC-Message-Signature (also abbreviated as “AMS”)
An [RFC5322] message header field formed in compliance with the [ARC] specification. It includes certain content about the message as it was received and manipulated by the intermediary who inserted it.
ARC-Authentication-Results (also abbreviated as “AAR”)
An [RFC5322] message header field formed in compliance with the [ARC] specification. It includes certain content about the message as it was received by the intermediary.
Authentication Results Chain (ARC)
A system that allows a Message Receiver to identify Intermediaries or Message Handlers who have conveyed a particular message. For more information see the Abstract of this document, or refer to [ARC].
Domain Naming System Block List (DNSBL)
This is a system widely used in email filtering services whereby information about the past activity of a set of hosts or domains indicates that messages should not be accepted from them, or at least should be subject to greater scrutiny before being accepted. Common examples would be SpamCop,, SORBS, etc.
Email Service Provider (ESP)
An Email Service Provider is typically a vendor or partner firm that sends mail on behalf of another company. They may use email addresses in Internet domains belonging to the client or partner firm in various [RFC5321] fields or [RFC5322] message header fields of the messages they send on their behalf.
In the context of [ARC], an Intermediary is typically an Administrative Management Domain (per [RFC5598]) that is receiving a message, potentially manipulating or altering it, and then passing it on to another ADMD for delivery. Also see [RFC7960] for more information and discussion. Common examples of Intermediaries are mailing lists, alumni or professional email address providers like universities or professional organizations, et cetera.
Mail/Message Transfer Agent (MTA)
This refers to software that sends and receives email messsages across a network with other MTAs. Often run on dedicated servers, common examples are Exim, Microsoft Exchange, Postfix, and Sendmail.
A group of messages that share features in common. Typical examples would be all messages sent by a given Message Sender to a Message Receiver, related to a particular announcement, a given mailing list, et cetera.
Malicious Actor
A Malicious Actor is a party, often an Intermediary, that will take actions that seek to exploit or defraud the ultimate recipient of the message, or subvert the network controls and infrastructure of the Message Receiver. Typical examples would be a spammer who forges content or attributes of a message in order to evade anti-spam measures, or an entity that adds an attachment containing a virus to a message.
Message Handler
A Message Handler is another name for an Intermediary.
Message Receiver
In the transmission of an email message from one ADMD to another, this is the organization receiving the message on behalf of the intended recipient or end user. The Message Receiver may do this because the intended recipient is an employee or member of the organization, or because the end user utilizes email services provided by the Message Receiver (Comcast, GMail, Yahoo, QQ, et cetera).
Message Sender
In the transmission of an email message from one ADMD to another, this is the organization sending the message on behalf of the Originator or end user.
This refers to the author of a given email message. In different contexts it may refer to the end-user writing the message, or the ADMD providing email services to that end-user.
In the larger context of email hygiene - blocking spam and malicious messages - reputation generally refers to a wide variety of techniques and mechanisms whereby a message receiver uses the past actions of a sending host or domain to influence the handling of messages received from them in the future. One of the classic examples would be a Spamhaus-style DNSBL, where individual IP addresses will be blocked from sending messages because they’ve been identified as being bad actors. Very large message receivers may build and maintain their own reputation systems of this kind, whereas other organizations might choose to use commercial products or free services.
Reputation Service Provider
A Reputation Service Provider would be a source of reputation information about a message sender. In this context, the DNSBL services offered by Spamhaus would allow them to be referred to as an RPS. Many spam and virus filtering vendors incorporate similar functionality into their services.
Request For Comment (RFC)
RFCs are memoranda that “contain technical and organizational notes about the Internet.” Created and managed by the Internet Engineering Task Force (IETF), they are de facto standards for various methods of communicating or collaborating over the Internet.
RFC5321 - Simple Mail Transfer Protocol
This document describes the protocol used to transfer email messages between Message Transfer Agents (MTA) over a network. Link: [RFC5321]
RFC5322 - Internet Message Format
This document describes the format of Internet email messages, including both the header fields within the message and various types of content within the message body. Link: [RFC5322]
A Message Receiver that attempts to validate the ARC chain in a message.

Appendix B. References

Appendix C. Acknowledgements

This draft is based on the work of OAR-Dev Group.

The authors thank the entire OAR-Dev group for the ongoing help, innumerable diagrams and discussions from all the participants, especially: Alex Brotman, Brandon Long, Dave Crocker, Elizabeth Zwicky, Franck Martin, Greg Colburn, J. Trent Adams, John Rae-Grant, Mike Hammer, Mike Jones, Steve Jones, Terry Zink, Tim Draegen.

This document was influenced by questions posed in the mailing list, and the authors thank all the list participants for their input.

Appendix D. Comments and Feedback

Please address all comments, discussions, and questions about this document, or about [ARC] itself, to the dmarc working group at

Readers looking for general information about ARC may refer to the website, or to the mailing list at

Authors' Addresses

Steven M Jones (editor) 2419 McGee Avenue Berkeley, California 94703 USA EMail:
Kurt Andersen LinkedIn 2029 Stierlin Ct. Mountain View, California 94043 USA EMail: