Internet DRAFT - draft-ietf-dmarc-arc-protocol
draft-ietf-dmarc-arc-protocol
DMARC Working Group K. Andersen
Internet-Draft LinkedIn
Intended status: Experimental B. Long, Ed.
Expires: June 21, 2019 Google
S. Blank, Ed.
Valimail
M. Kucherawy, Ed.
TDP
December 18, 2018
Authenticated Received Chain (ARC) Protocol
draft-ietf-dmarc-arc-protocol-23
Abstract
The Authenticated Received Chain (ARC) protocol 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.
ARC allows Internet Mail Handlers to attach assertions of message
authentication assessment to individual messages. As messages
traverse ARC-enabled Internet Mail Handlers, additional ARC
assertions can be attached to messages to form ordered sets of ARC
assertions that represent the authentication assessment at each step
of message handling paths.
ARC-enabled Internet Mail Handlers can process sets of ARC assertions
to inform message disposition decisions, to identify Internet Mail
Handlers that might break existing authentication mechanisms, and to
convey original authentication assessments across trust boundaries.
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 June 21, 2019.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Concepts . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Custody . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Chain of Custody . . . . . . . . . . . . . . . . . . . . 5
2.4. Validation of Chain of Custody . . . . . . . . . . . . . 5
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 6
3.1. ARC Set . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Authenticated Received Chain (ARC) . . . . . . . . . . . 7
3.3. Internet Mail Handlers / Intermediaries . . . . . . . . . 7
3.4. Authentication Assessment . . . . . . . . . . . . . . . . 7
3.5. Signing vs Sealing . . . . . . . . . . . . . . . . . . . 7
3.6. Sealer . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Validator . . . . . . . . . . . . . . . . . . . . . . . . 8
3.8. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . 8
3.9. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . 8
4. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
4.1. ARC Header Fields . . . . . . . . . . . . . . . . . . . . 8
4.1.1. ARC-Authentication-Results (AAR) . . . . . . . . . . 9
4.1.2. ARC-Message-Signature (AMS) . . . . . . . . . . . . . 9
4.1.3. ARC-Seal (AS) . . . . . . . . . . . . . . . . . . . . 10
4.1.4. Internationalized Email (EAI) . . . . . . . . . . . . 11
4.2. ARC Set . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1. Instance Tags . . . . . . . . . . . . . . . . . . . . 12
4.3. Authenticated Received Chain . . . . . . . . . . . . . . 12
4.4. Chain Validation Status . . . . . . . . . . . . . . . . . 13
5. Protocol Actions . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Sealer Actions . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Header Fields To Include In ARC-Seal Signatures . . . 15
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5.1.2. Marking and Sealing "cv=fail" (Invalid) Chains . . . 15
5.1.3. Only One Authenticated Received Chain Per Message . . 15
5.1.4. Broad Ability to Seal . . . . . . . . . . . . . . . . 16
5.1.5. Sealing is Always Safe . . . . . . . . . . . . . . . 16
5.2. Validator Actions . . . . . . . . . . . . . . . . . . . . 16
5.2.1. All Failures Are Permanent . . . . . . . . . . . . . 18
5.2.2. Responding to ARC Validation Failures During the SMTP
Transaction . . . . . . . . . . . . . . . . . . . . . 18
6. Communication of Validation Results . . . . . . . . . . . . . 18
7. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Communicate Authentication Assessment Across Trust
Boundaries . . . . . . . . . . . . . . . . . . . . . . . 19
7.1.1. Message Scanning Services . . . . . . . . . . . . . . 19
7.1.2. Multi-tier MTA Processing . . . . . . . . . . . . . . 19
7.1.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . 20
7.2. Inform Message Disposition Decisions . . . . . . . . . . 20
7.2.1. DMARC Local Policy Overrides . . . . . . . . . . . . 20
7.2.2. DMARC Reporting . . . . . . . . . . . . . . . . . . . 21
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9.1. Increased Header Field Size . . . . . . . . . . . . . . . 22
9.2. DNS Operations . . . . . . . . . . . . . . . . . . . . . 22
9.3. Message Content Suspicion . . . . . . . . . . . . . . . . 23
9.4. Message Sealer Suspicion . . . . . . . . . . . . . . . . 23
9.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10.1. Email Authentication Results Names Registry Update . . . 24
10.2. Email Authentication Methods Registry Update . . . . . . 24
10.3. Definitions of the ARC header fields . . . . . . . . . . 24
10.4. New Enhanced Status Code - ARC Validation . . . . . . . 25
11. Experimental Considerations . . . . . . . . . . . . . . . . . 25
11.1. Success Consideration . . . . . . . . . . . . . . . . . 25
11.2. Failure Considerations . . . . . . . . . . . . . . . . . 26
11.3. Open Questions . . . . . . . . . . . . . . . . . . . . . 26
11.3.1. Value of the ARC-Seal (AS) Header Field . . . . . . 26
11.3.2. Usage and/or signals from multiple selectors and/or
domains in ARC sets . . . . . . . . . . . . . . . . 26
11.3.3. DNS Overhead . . . . . . . . . . . . . . . . . . . . 26
11.3.4. What Trace Information is Valuable . . . . . . . . . 27
12. Implementation Status . . . . . . . . . . . . . . . . . . . . 27
12.1. GMail test reflector and incoming validation . . . . . . 28
12.2. AOL test reflector and internal tagging . . . . . . . . 28
12.3. dkimpy . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.4. OpenARC . . . . . . . . . . . . . . . . . . . . . . . . 29
12.5. Mailman 3.x patch . . . . . . . . . . . . . . . . . . . 29
12.6. Copernica/MailerQ web-based validation . . . . . . . . . 30
12.7. Rspamd . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.8. PERL MAIL::DKIM module . . . . . . . . . . . . . . . . . 31
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12.9. PERL Mail::Milter::Authentication module . . . . . . . . 31
12.10. Sympa List Manager . . . . . . . . . . . . . . . . . . . 32
12.11. Oracle Messaging Server . . . . . . . . . . . . . . . . 32
12.12. MessageSystems Momentum and PowerMTA platforms . . . . . 32
12.13. Exim . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.14. Halon MTA . . . . . . . . . . . . . . . . . . . . . . . 33
12.15. IIJ . . . . . . . . . . . . . . . . . . . . . . . . . . 33
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.1. Normative References . . . . . . . . . . . . . . . . . . 33
13.2. Informative References . . . . . . . . . . . . . . . . . 35
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Appendix A. Design Requirements . . . . . . . . . . . . . . . . 36
A.1. Primary Design Criteria . . . . . . . . . . . . . . . . . 36
A.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Example Usage . . . . . . . . . . . . . . . . . . . 37
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 39
Appendix D. Comments and Feedback . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
The utility of widely deployed email authentication technologies such
as Sender Policy Framework (SPF) [RFC7208] and DomainKeys Identified
Mail (DKIM) [RFC6376] is impacted by the processing of Internet Mail
by intermediate handlers. This impact is thoroughly documented in
the defining documents for SPF and DKIM and further discussed in
[RFC6377] and [RFC7960].
DMARC [RFC7489] also relies upon SPF and DKIM authentication
mechanisms. Failures of authentication caused by the actions of
intermediate handlers can cause legitimate mail to be incorrectly
rejected or misdirected.
Authenticated Received Chain (ARC) creates a mechanism for individual
Internet Mail Handlers to add their authentication assessment to a
message's ordered set of handling results. ARC encapsulates the
authentication assessment in a DKIM signature derivative to grant
other handlers the ability to verify the authenticity of the
individual assessment assertion as well as the aggregate set and
sequence of results.
Ordered sets of authentication assessments can be used by ARC-enabled
Internet Mail Handlers to inform message handling disposition, to
identify where alteration of message content might have occurred, and
to provide additional trace information for use in understanding
message handling paths.
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2. General Concepts
ARC is loosely based on concepts from evidence collection. Evidence
is usually collected, labeled, stored, and transported in specific
ways to preserve the state of evidence and to document all processing
steps.
2.1. Evidence
In ARC's situation, the "evidence" is a message's authentication
assessment at any point along the delivery path between origination
and final delivery. Determination of message authentication can be
affected when intermediate handlers modify message content (header
fields and/or body content), route messages through unforeseen paths,
or change envelope information.
The authentication assessment for a message is determined upon
receipt of a message and documented in the Authentication-Results
header field(s). ARC extends this mechanism to survive transit
through intermediary ADMDs.
Because the first-hand determination of an authentication assessment
can never be reproduced by other handlers, the assertion of the
authentication assessment is more akin to testimony by a verifiable
party than hard evidence which can be independently evaluated.
2.2. Custody
"Custody" refers to when an Internet Mail Handler processes a
message. When a handler takes custody of a message, the handler
becomes a custodian and attaches their own evidence (authentication
assessment upon receipt) to the message if they are ARC-enabled.
Evidence is added in such a way so that future handlers can verify
the authenticity of both evidence and custody.
2.3. Chain of Custody
The "chain of custody" of ARC is the entire set of evidence and
custody that travels with a message.
2.4. Validation of Chain of Custody
Any ARC-enabled Internet Mail Handler can validate the entire set of
custody and the authentication assessments asserted by each party to
yield a valid Chain of Custody. If the evidence-supplying custodians
can be trusted, then the validated Chain of Custody describes the
(possibly changing) authentication assessment as the message traveled
through various custodians.
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Even though a message's authentication assessment might have changed,
the validated chain of custody can be used to determine if the
changes (and the custodians responsible for the changes) can be
tolerated.
3. Terminology and Definitions
This section defines terms used in the rest of the document.
Readers should to be familiar with the contents, core concepts, and
definitions found in [RFC5598]. The potential roles of transit
services in the delivery of email are directly relevant.
Language, syntax (including some ABNF constructs), and concepts are
imported from DKIM [RFC6376]. Specific references to DKIM are made
throughout this document. The following terms are imported from
[RFC5598]:
o ADministrative Management Domain (ADMD), Section 2.3
o Message Transfer Agents (MTA), Section 4.3.2
o Message Submission Agent (MSA), Section 4.3.1
o Message Delivery Agent (MDA), Section 4.3.3
Syntax descriptions use Augmented BNF (ABNF) [RFC5234] and [RFC7405].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
3.1. ARC Set
Section 4.1 introduces three (3) ARC header fields which are added to
a message by an ARC-enabled internet mail handler. Together, these
three header fields compose a single "ARC Set". An ARC Set provides
the means for an Internet Mail Handler to attach an authentication
assessment to a message in a manner that can be verified by future
handlers. A single message can contain multiple ARC Sets.
In general concept terms, an ARC Set represents Evidence and Custody.
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3.2. Authenticated Received Chain (ARC)
The sequence of ARC Sets attached to a message at a given time is
called the Authenticated Received Chain. An Authenticated Received
Chain is the record of individual authentication assessments as a
message traverses through ARC-participating ADMDs.
The first attachment of an ARC Set to a message causes an
Authenticated Received Chain to be created. Additional attachments
of ARC Sets cause the Authenticated Received Chain to be extended.
In General concept terms, an Authenticated Received Chain represents
Chain of Custody.
3.3. Internet Mail Handlers / Intermediaries
Internet Mail Handlers process and deliver messages across the
Internet and include MSAs, MTAs, MDAs, gateways, and mailing lists as
defined in [RFC5598].
Throughout this document the term "intermediaries" refers to the both
regular MTAs as well as delivery/reposting agents such as mailing
lists covered within the scope of [RFC5598]'s transit services.
"Intermediaries" and "Internet Mail Handlers" are used synonymously
throughout this document.
3.4. Authentication Assessment
The Authentication Assessment which is affixed to a message as part
of each ARC Set consists of the "authres-payload" [I-D-7601bis]. For
the integrity of an ARC Set, the Authentication Assessment only needs
to be properly encapsulated within the ARC Set as defined below
Section 4.1. The accuracy or syntax of the authres-payload field
does not affect the validity of the ARC chain itself.
3.5. Signing vs Sealing
Signing is the process of affixing a digital signature to a message
as a header field, such as when a DKIM-Signature (as in [RFC6376]
section 2.1), or an AMS or AS is added. Sealing is when an ADMD
affixes a complete and valid ARC Set to a message creating or
continuing an Authenticated Received Chain.
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3.6. Sealer
A Sealer is an Internet Mail Handler that attaches a complete and
valid ARC Set to a message.
In general concept terms, a Sealer adds its testimony (assertion of
authentication assessment) and proof of custody to the Chain of
Custody.
3.7. Validator
A Validator is an ARC-enabled Internet Mail Handler that evaluates an
Authenticated Received Chain for validity and content. The process
of evaluation of the individual ARC Sets that compose an
Authenticated Received Chain is described in Section 5.2.
In general concept terms, a Validator inspects the Chain of Custody
to determine the content and validity of individual Evidence supplied
by custodians.
3.8. Imported ABNF Tokens
The following ABNF tokens are imported:
o tag-list ([RFC6376] section 3.2)
o authres-payload ([I-D-7601bis] section 2.2)
o cfws ([RFC5322] section 3.2.2)
3.9. Common ABNF Tokens
The following ABNF tokens are used elsewhere in this document:
position = 1*2DIGIT ; 1 - 50
instance = [CFWS] %s"i" [CFWS] "="
[CFWS] position
chain-status = ("none" / "fail" / "pass")
seal-cv-tag = %s"cv" [CFWS] "="
[CFWS] chain-status
4. Protocol Elements
4.1. ARC Header Fields
ARC introduces three new header fields. Syntax for new header fields
adapts existing specifications. This document only describes where
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ARC-specific changes in syntax and semantics differ from existing
specifications.
4.1.1. ARC-Authentication-Results (AAR)
The ARC-Authentication-Results (AAR) header field records the message
authentication assessment as processed by an ARC-participating ADMD
at message arrival time.
In general concept terms, the AAR header field is where Evidence is
recorded by a custodian.
The AAR header field is similar in syntax and semantics to an
Authentication-Results field [I-D-7601bis], with two (2) differences:
o the name of the header field itself;
o the presence of the "instance tag". Additional information on the
"instance tag" can be found in Section 4.2.1.
The formal ABNF for the AAR header field is:
arc-info = instance [CFWS] ";" authres-payload
arc-authres-header = "ARC-Authentication-Results:" [CFWS] arc-info
Because there is only one AAR allowed per ARC set, the AAR MUST
contain the combined authres-payload with all of the authentication
results from within the participating ADMD, regardless of how many
Authentication-Results header fields are attached to the message.
4.1.2. ARC-Message-Signature (AMS)
The ARC-Message-Signature (AMS) header field allows an ARC-
participating ADMD to convey some responsibility (custodianship) for
a message and possible message modifications to future ARC-
participating custodians.
In general concept terms, the AMS header field identifies a
custodian.
The AMS header field has the same syntax and semantics as the DKIM-
Signature field [RFC6376], with three (3) differences:
o the name of the header field itself;
o no version tag ("v") is defined for the AMS header field. As
required for undefined tags (in [RFC6376]), if seen, a version tag
MUST be ignored;
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o the "i" (AUID) tag is not imported from DKIM; instead, this tag is
replaced by the "instance tag" as defined in Section 4.2.1;
ARC places no requirements on the selectors and/or domains used for
the AMS header field signatures.
The formal ABNF for the AMS header field is:
arc-ams-info = instance [CFWS] ";" tag-list
arc-message-signature = "ARC-Message-Signature:" [CFWS] arc-ams-info
To reduce the chances of accidental invalidation of AMS signatures:
o AMS header fields are added by ARC-participating ADMDs as messages
exit the ADMD. AMS header fields SHOULD be attached so that any
modifications made by the ADMD are included in the signature of
the AMS header field.
o Authentication-Results header fields MUST NOT be included in AMS
signatures as they are likely to be deleted by downstream ADMDs
(per [I-D-7601bis] Section 5).
o ARC-related header fields (ARC-Authentication-Results, ARC-
Message-Signature, ARC-Seal) MUST NOT be included in the list of
header fields covered by the signature of the AMS header field.
To preserve the ability to verify the integrity of a message, the
signature of the AMS header field SHOULD include any DKIM-Signature
header fields already present in the message.
4.1.3. ARC-Seal (AS)
The ARC-Seal (AS) header field permits ARC-participating ADMDs to
verify the integrity of AAR header fields and corresponding AMS
header fields.
In general concept terms, the AS header field is how custodians bind
their authentication assessments (testimonial) into a Chain of
Custody so that Validators can inspect individual evidence and
custodians.
The AS header field is similar in syntax and semantics to DKIM-
Signatures [RFC6376], with the following differences:
o the "i" (AUID) tag is not imported from DKIM; instead, this tag is
replaced by the "instance tag" as defined in Section 4.2.1;
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o the signature of the AS header field does not cover the body of
the message and therefore there is no 'bh' tag. The signature of
the AS header field only covers specific header fields as defined
in Section 5.1.1;
o no body canonicalization is performed as the AS signature does not
cover the body of a message;
o only "relaxed" header field canonicalization ([RFC6376] section
3.4.2) is used;
o the only supported tags are "i" (from Section 4.2.1 of this
document), and "a", "b", "d, "s", "t" from [RFC6376] Section 3.5.
Note especially that the DKIM "h" tag is NOT allowed and if found,
MUST result in a cv status of "fail" (for more information see
Section 5.1.1);
o an additional tag, "cv" ("seal-cv-tag" in the ARC-Seal ABNF
definition) is used to communicate Chain Validation Status to
subsequent ADMDs.
ARC places no requirements on the selectors and/or domains used for
the AS header field signatures.
The formal ABNF for the AS header field is:
arc-as-info = instance [CFWS] ";" tag-list
arc-seal = "ARC-Seal:" [CFWS] arc-as-info
4.1.4. Internationalized Email (EAI)
In internationalized messages [RFC6532] many header fields can
contain UTF-8 as well as ASCII text. The changes for EAI are all
inherited from DKIM as updated by [draft-levine-eaiauth] and
Authentication-Results as updated in [I-D-7601bis], but are called
out here for emphasis.
In all ARC header fields, the d= s= tags can contain U-labels. In
all tags, non-ASCII characters need not be quoted in dkim-quoted-
printable.
The AAR header allows UTF-8 in the same places that A-R does, as
described in [I-D-7601bis].
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4.2. ARC Set
An "ARC Set" is a single collection of three ARC header fields (AAR,
AMS, and AS). ARC header fields of an ARC Set share the same
"instance" value.
By adding all ARC header fields to a message, an ARC Sealer adds an
ARC Set to a message. A description of how Sealers add an ARC Set to
a message is found in Section 5.1.
4.2.1. Instance Tags
Instance tags describe which ARC header fields belong to an ARC Set.
Each ARC header field of an ARC Set shares the same instance tag
value.
Instance tag values are integers that begin at 1 and are incremented
by each addition of an ARC Set. Through the incremental values of
instance tags, an ARC Validator can determine the order in which ARC
Sets were added to a message.
Instance tag values can range from 1-50 (inclusive).
_INFORMATIONAL:_ The upper limit of 50 was picked based on some
initial observations reported by early working group members. The
value was chosen so as to balance the risk of excessive header field
growth Section 9.1 against expert opinion regarding the probability
of long-tail but non-looping multiple-intermediary mail flows.
Longer ARC chains will also impose load on validators and DNS to
support additional verification steps. Observed quantities of
"Received" header fields was also considered in establishing this as
an experimental initial value.
Valid ARC Sets MUST have exactly one instance of each ARC header
field (AAR, AMS, and AS) for a given instance value and signing
algorithm.
For handling multiple signing algorithms, see [ARC-MULTI].
4.3. Authenticated Received Chain
An Authenticated Received Chain is an ordered collection of ARC Sets.
As ARC Sets are enumerated sets of ARC header fields, an
Authenticated Received Chain represents the output of message
authentication assessments along the handling path of ARC-enabled
processors.
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Authentication Assessments determined at each step of the ARC-enabled
handling path is present in an Authenticated Received Chain in the
form of AAR header fields. The ability to verify the identity of
message handlers and the integrity of message content is provided by
AMS header fields. AS header fields allow messages handlers to
validate the assertions, order and sequence of the Authenticated
Received Chain itself.
In general concept terms, an Authenticated Received Chain represents
a message's Chain of Custody. Validators can consult a message's
Chain of Custody to gain insight regarding each custodian of a
message and the Evidence collected by each custodian.
4.4. Chain Validation Status
The state of the Authenticated Received Chain at a specific
processing step is called the "Chain Validation Status". Chain
Validation Status information is communicated in several ways:
o the AS header field in the "cv" tag, and
o as part of Authentication-Results and AAR header field(s).
Chain Validation Status has one of three possible values:
o none: There was no Authenticated Received Chain on the message
when it arrived for validation. Typically, this occurs when a
message is received directly from a message's original Message
Transfer Agent (MTA) or Message Submission Agent (MSA), or from an
upstream Internet Mail Handler that is not participating in ARC
handling.
o fail: The message contains an Authenticated Received Chain whose
validation failed.
o pass: The message contains an Authenticated Received Chain whose
validation succeeded.
5. Protocol Actions
ARC-enabled Internet Mail Handlers generally act as both ARC
Validators (when receiving messages) and ARC Sealers (when sending
messages onward, not originated locally).
An Authenticated Received Chain with a Chain Validation Status of
"pass" (or "none") allows Internet Mail Handlers to ascertain:
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o all ARC-participating ADMDs that claim responsibility for handling
(and possibly modifying) the message in transit;
o the authentication assessments of the message as determined by
each ADMD (from AAR header fields).
With this information, Internet Mail Handlers MAY inform local policy
decisions regarding disposition of messages that experience
authentication failure due to intermediate processing.
5.1. Sealer Actions
To "seal" a message, an ARC Sealer adds an ARC Set (the three ARC
header fields AAR, AMS, and AS) to a message. All ARC header fields
in an ARC Set share the same instance tag value.
To perform Sealing (aka to build and attach a new ARC Set), the
following actions must be taken by an ARC Sealer when presented with
a message:
1. All message modifications (including adding DKIM-Signature header
field(s)) MUST be performed before Sealing.
2. If the message already contains an Authenticated Received Chain
with the most recent AS reporting "cv=fail", then there is no
need to proceed and the algorithm stops here.
3. Calculate the instance value: if the message already contains an
Authenticated Received Chain, the instance value is 1 more than
the highest instance number found in the Authenticated Received
Chain. If no Authenticated Received Chain exists, the instance
value is 1.
4. Using the calculated instance value, generate and attach a
complete ARC set to the message as follows:
1. Generate and attach an ARC-Authentication-Results header
field as defined in Section 4.1.1.
2. Generate and attach an ARC-Message-Signature header field as
defined in Section 4.1.2.
3. Generate and attach an ARC-Seal header field using the AS
definition found in Section 4.1.3, the prescribed headers
defined in Section 5.1.1, and the Chain Validation Status as
determined during ARC Validation.
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5.1.1. Header Fields To Include In ARC-Seal Signatures
The ARC-Seal is generated in a manner similar to how DKIM-Signatures
are added to messages ([RFC6376], section 3.7), with explicit
requirements on the header fields and ordering of those fields.
The signature of an AS header field signs a canonicalized form of the
ARC Set header field values. The ARC set header field values are
supplied to the hash function in increasing instance order, starting
at 1, and include the ARC Set being added at the time of Sealing the
message.
Within an ARC Set, header fields are supplied to the hash function in
the following order:
1. ARC-Authentication-Results
2. ARC-Message-Signature
3. ARC-Seal
Note that when an Authenticated Received Chain has failed validation,
the signing scope for the ARC-Seal is modified as specified in
Section 5.1.2.
5.1.2. Marking and Sealing "cv=fail" (Invalid) Chains
In the case of a failed Authenticated Received Chain, the header
fields included in the signature scope of the AS header field b=
value MUST only include the ARC Set header fields created by the MTA
which detected the malformed chain, as if this newest ARC Set was the
only set present.
_INFORMATIONAL_: This approach is mandated to handle the case of a
malformed or otherwise invalid Authenticated Received Chain. There
is no way to generate a deterministic set of AS header fields
(Section 5.1.1) in most cases of invalid chains.
5.1.3. Only One Authenticated Received Chain Per Message
A message can have only one Authenticated Received Chain on it at a
time. Once broken, the chain cannot be continued, as the chain of
custody is no longer valid and responsibility for the message has
been lost. For further discussion of this topic and the design
restriction which prevents chain continuation or re-establishment,
see [ARC-USAGE].
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5.1.4. Broad Ability to Seal
ARC is not solely intended for perimeter MTAs. Any Internet Mail
Handler MAY seal a message by adding a complete ARC set, whether or
not they have modified or are aware of having modified the message.
For additional information, see Section 7.1.
5.1.5. Sealing is Always Safe
The utility of an Authenticated Received Chain is limited to very
specific cases. Authenticated Received Chains are designed to
provide additional information to an Internet Mail Handler when
evaluating messages for delivery in the context of authentication
failures. Specifically:
o Properly adding an ARC Set to a message does not damage or
invalidate an existing Authenticated Received Chain.
o Sealing an Authenticated Received Chain when a message has not
been modified does not negatively affect the chain.
o Validating a message exposes no new threat vectors (see
Section 9).
o An ADMD may choose to Seal all inbound messages whether or not a
message has been modified or will be retransmitted.
5.2. Validator Actions
A validator performs the following steps, in sequence, to process an
Authenticated Received Chain. Canonicalization, hash functions, and
signature validation methods are imported from [RFC6376] section 5.
1. Collect all ARC Sets currently attached to the message.
* If there are none, the Chain Validation Status is "none" and
the algorithm stops here.
* The maximum number of ARC Sets that can be attached to a
message is 50. If more than the maximum number exist the
Chain Validation Status is "fail" and the algorithm stops
here.
* In the following algorithm, the maximum discovered ARC
instance value is referred to as "N".
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2. If the Chain Validation Status of the highest instance value ARC
Set is "fail", then the Chain Validation status is "fail" and the
algorithm stops here.
3. Validate the structure of the Authenticated Received Chain. A
valid ARC has the following conditions:
1. Each ARC Set MUST contain exactly one each of the three ARC
header fields (AAR, AMS, and AS).
2. The instance values of the ARC Sets MUST form a continuous
sequence from 1..N with no gaps or repetition.
3. The "cv" value for all ARC-Seal header fields MUST NOT be
"fail". For ARC Sets with instance values > 1, the values
MUST be "pass". For the ARC Set with instance value = 1, the
value MUST be "none".
* If any of these conditions are not met, the Chain Validation
Status is "fail" and the algorithm stops here.
4. Validate the AMS with the greatest instance value (most recent).
If validation fails, then the Chain Validation Status is "fail"
and the algorithm stops here.
5. _OPTIONAL:_ Determine the "oldest-pass" value from the ARC Set by
validating each prior AMS beginning with the N-1 and proceeding
in decreasing order to the AMS with the instance value of 1:
1. If an AMS fails to validate (for instance value "M"), then
set the oldest-pass value to the lowest AMS instance value
which passed (M+1) and go to the next step (there is no need
to check any other (older) AMS header fields). This does not
affect the validity of the Authenticated Received Chain.
2. If all AMS header fields verify, set the oldest-pass value to
zero (0).
6. Validate each AS beginning with the greatest instance value and
proceeding in decreasing order to the AS with the instance value
of 1. If any AS fails to validate, the Chain Validation Status
is "fail" and the algorithm stops here.
7. If the algorithm reaches this step, then the Chain Validation
Status is "pass", and the algorithm is complete.
The end result of this Validation algorithm SHOULD be included within
the Authentication-Results header field for the ADMD.
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As with a DKIM signature ([RFC6376] section 6.3) which fails
verification, a message with an Authenticated Received Chain with a
Chain Validation status of "fail" MUST be treated the same as a
message with no Authenticated Received Chain.
_INFORMATIONAL_: Recipients of an invalid or failing Authenticated
Received Chain can use that information as part of a wider handling
context. ARC adoption cannot be assumed by intermediaries; many
intermediaries will continue to modify messages without adding ARC
Seals.
5.2.1. All Failures Are Permanent
Authenticated Received Chains represent the traversal of messages
through one or more intermediaries. All errors, including DNS
failures, become unrecoverable and are considered permanent.
Any error validating an Authenticated Received Chain results in a
Chain Validation Status of "fail". For further discussion of this
topic and the design restriction which prevents chain continuation or
re-establishment, see [ARC-USAGE].
5.2.2. Responding to ARC Validation Failures During the SMTP
Transaction
If an ARC Validator determines that the incoming message fails ARC
validation, the Validator MAY signal the breakage through the
extended SMTP response code 5.7.29 "ARC validation failure" and
corresponding SMTP basic response code. Because ARC failures are
likely only to be detected in the context of other underlying
authentication mechanism failures, validators MAY use the more
general 5.7.26 "Multiple authentication checks failed" instead of the
ARC-specific code.
6. Communication of Validation Results
Chain Validation Status (described in Section 4.4) is communicated
via Authentication-Results (and AAR) header fields using the auth
method "arc". This auth method is described in Section 10.1.
If necessary data is available, the ptypes and properties defined in
Section 10.2 SHOULD be recorded in an Authentication-Results header
field:
o smtp.remote-ip - The address of the connection-initiating SMTP
server, from which the message is being relayed.
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o header.oldest-pass - The instance number of the oldest AMS that
still validates, or 0 if all pass.
7. Use Cases
This section explores several messaging handling use cases that are
addressed by ARC.
7.1. Communicate Authentication Assessment Across Trust Boundaries
When an intermediary ADMD adds an ARC Set to a message's
Authenticated Received Chain (or creates the initial ARC Set), the
ADMD communicates its authentication assessment to the next ARC-
participating ADMD in the message handling path.
If ARC-enabled ADMDs are trusted, Authenticated Received Chains can
be used to bridge administrative boundaries.
7.1.1. Message Scanning Services
Message services are available to perform anti-spam, anti-malware,
and anti-phishing scanning. Such services typically remove malicious
content, replace HTTP links in messages with sanitized links, and/or
attach footers to messages advertising the abilities of the message
scanning service. These modifications almost always break signature-
based authentication (such as DKIM).
Scanning services typically require clients to point MX records of an
Internet domain to the scanning service. Messages destined for the
Internet domain are initially delivered to the scanning service.
Once scanning is performed, messages are then routed to the client's
own mail handling infrastructure. Re-routing messages in this way
almost always breaks path-based authentication (such as SPF).
Message scanning services can attach Authenticated Received Chains to
messages to communicate authentication assessment into client ADMDs.
Clients can then benefit from the message scanning service while
processing messages as if the client's infrastructure were the
original destination of the Internet domain's MX record.
7.1.2. Multi-tier MTA Processing
Large message processing infrastructure is often divided into several
processing tiers that can break authentication information between
tiers. For example, a large site may maintain a cluster of MTAs
dedicated to connection handling and enforcement of IP-based
reputation filtering. A secondary cluster of MTAs may be dedicated
and optimized for content-based processing of messages.
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Authenticated Received Chains can be used to communicate
authentication assessment between processing tiers.
7.1.3. Mailing Lists
Mailing lists take delivery of messages and re-post them to
subscribers. A full description of authentication-related mailing
list issues can be found in [RFC7960] Section 3.2.3.
Mailing list services can implement ARC to convey the authentication
assessment of posted messages sent to the list's subscriber base.
The ADMDs of the mailing list subscribers can then use the
Authenticated Received Chain to determine the authentication
assessment of the original message before mailing list handling.
7.2. Inform Message Disposition Decisions
Intermediaries often break authentication through content
modification, interfere with path-based authentication (such as SPF),
and strip authentication results (if an MTA removes Authentication-
Results header fields).
Authenticated Received Chains allow ARC Validators to:
1. identify ARC-enabled ADMDs that break authentication while
processing messages;
2. gain extended visibility into the authentication-preserving
abilities of ADMDs that relay messages into ARC-enabled ADMDs.
Through the collection of ARC-related data, an ADMD can identify
handling paths that have broken authentication.
An Authenticated Received Chain allows an Internet Mail Handler to
potentially base decisions of message disposition on authentication
assessments provided by different ADMDs.
7.2.1. DMARC Local Policy Overrides
DMARC introduces a policy model where Domain Owners can request email
receivers to reject or quarantine messages that fail DMARC alignment.
Interoperability issues between DMARC and indirect email flows are
documented in [RFC7960].
Authenticated Received Chains allow DMARC processors to consider
authentication assessments provided by other ADMDs. As a matter of
local policy, a DMARC processor MAY choose to accept the
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authentication assessments provided by an Authenticated Received
Chain when determining if a message is DMARC compliant.
When an Authenticated Received Chain is used to determine message
disposition, the DMARC processor can communicate this local policy
decision to Domain Owners as described in Section 7.2.2.
7.2.2. DMARC Reporting
DMARC-enabled receivers indicate when ARC Validation influences
DMARC-related local policy decisions. When an ARC-enabled handler
generates a DMARC report, it MAY indicate the influence of ARC on
their local policy decision(s) by adding a reason of "local_policy"
with a comment string (per [RFC7489] Appendix C) containing a list of
data discovered during ARC Validation, which at a minimum includes:
o the Chain Validation Status,
o the domain and selector for each AS,
o the originating IP address from the first ARC Set:
EXAMPLE:
<policy_evaluated>
<disposition>none</disposition>
<dkim>fail</dkim>
<spf>fail</spf>
<reason>
<type>local_policy</type>
<comment>arc=pass as[2].d=d2.example as[2].s=s2
as[1].d=d1.example as[1].s=s3
remote-ip[1]=2001:DB8::1A</comment>
</reason>
</policy_evaluated>
In the above example DMARC XML reporting fragment, data relating to
specific validated ARC Sets are enumerated using array syntax (eg,
"as[2]" means AS header field with instance value of 2). d2.example
is the Sealing domain for ARC Set #2 (i=2) and d1.example is the
Sealing domain for ARC Set #1 (i=1).
Depending on the reporting practices of intermediate message
handlers, Domain Owners may receive multiple DMARC reports for a
single message. Receivers of DMARC reports should be aware of this
behaviour and make the necessary accommodations.
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8. Privacy Considerations
The Authenticated Received Chain provides a verifiable record of the
handlers for a message. This record may include Personally
Identifiable Information such as IP address(es) and domain names.
Such information is also included in existing non-ARC related header
fields such as the "Received" header fields.
9. Security Considerations
The Security Considerations of [RFC6376] and [I-D-7601bis] apply
directly to this specification.
As with other domain authentication technologies (such as SPF, DKIM,
and DMARC), ARC makes no claims about the semantic content of
messages.
A received message with an ARC chain provides evidence (at instance
N) that: The sealing domain (ARC-Seal d=) processed a message with
this body, determined the authentication assessment reported in the
ARC-Authentication-Results, and the ARC chain 1..N-1 (with the
validation status as reported in the cv field).
9.1. Increased Header Field Size
Inclusion of Authenticated Received Chains into messages may cause
issues for older or constrained MTAs due to increased total header
field size. Large header field blocks, in general, may cause
failures to deliver or other outage scenarios for such MTAs. ARC
itself would not cause problems.
9.2. DNS Operations
The validation of an Authenticated Received Chain composed of N ARC
Sets can require up to 2*N DNS queries (not including any DNS
redirection mechanisms which can increase the total number of
queries). This leads to two considerations:
1. An attacker can send a message to an ARC participant with a
concocted sequence of ARC Sets bearing the domains of intended
victims, and all of them will be queried by the participant until
a failure is discovered. DNS caching and the difficulty of
forging the signature values should limit the extent of this load
to domains under control of the attacker. Query traffic pattern
analysis may expose information about downstream validating ADMD
infrastructure.
2. DKIM only performs one DNS query per signature, while ARC can
introduce many (per chain). Absent caching, slow DNS responses
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can cause SMTP timeouts; and backlogged delivery queues on
Validating systems. This could be exploited as a DoS attack.
9.3. Message Content Suspicion
Recipients are cautioned to treat messages bearing Authenticated
Received Chains with the same suspicion applied to all other
messages. This includes appropriate content scanning and other
checks for potentially malicious content.
ARC authenticates the identity of some email handling actors. It
does not make any assessment of their trustworthiness.
Just as passing message authentication is not an indication of
message safety, forwarding that information through the mechanism of
ARC is also not an indication of message safety. Even if all ARC-
enabled ADMDs are trusted, ADMDs may have become compromised, may
miss unsafe content, or may not properly authenticate messages.
9.4. Message Sealer Suspicion
Recipients are cautioned to treat every Sealer of the ARC Chain with
suspicion. Just as with a validated DKIM signature, responsibility
for message handling is attributed to the Sealing domain, but whether
or not that Sealer is a malicious actor is out of scope of the
authentication mechanism. Since ARC aids message delivery in the
event of an authentication failure, ARC Sealers should be treated
with suspicion, so that a malicious actor cannot Seal spam or other
fraudulent messages to aid their delivery, too.
9.5. Replay Attacks
Since ARC inherits heavily from DKIM, it has similar attack vectors.
In particular, the Replay Attack described in [RFC6376] section 8.6
is potentially amplified by ARC's chained statuses. In an ARC replay
attack, a malicious actor would take an intact and passing ARC Chain,
and then resend it to many recipients without making any
modifications that invalidate the latest AMS or AS. The impact to a
receiver would be more DNS lookups and signature evaluations. This
scope of this attack can be limited by caching DNS queries and
following the same signing scope guidance from [RFC6376] section
5.4.1.
10. IANA Considerations
[[ *Note to the RFC Editors:* "dkim - header - s" is defined in
[I-D-7601bis]. Please adjust the list below as appropriate. ]]
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This draft introduces three new headers fields and updates the Email
Authentication Parameters registry with one new authentication method
and several status codes.
10.1. Email Authentication Results Names Registry Update
This draft adds one Auth Method with three Codes to the IANA "Email
Authentication Result Names" registry:
o Auth Method : arc
Code: "none", "pass", "fail"
Specification: this document 2.2
Status: active
10.2. Email Authentication Methods Registry Update
This draft adds several new items to the Email Authentication Methods
registry, most recently defined in [I-D-7601bis]:
o Method: arc
Definition: this document section 6
ptype: smtp
Property: remote-ip
Value: IP address (v4 or v6) of originating SMTP connection
Status: active
Version: 1
o Method: arc
Definition: this document section 6
ptype: header
Property: oldest-pass
Value: The instance id of the oldest validating AMS, or 0 if they
all pass (see Section 5.2)
Status: active
Version: 1
10.3. Definitions of the ARC header fields
This specification adds three new header fields to the "Permanent
Message Header Field Registry", as follows:
o Header field name: ARC-Seal
Applicable protocol: mail
Status: Experimental
Author/Change controller: IETF
Specification document(s): this document
Related information: [RFC6376]
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o Header field name: ARC-Message-Signature
Applicable protocol: mail
Status: Experimental
Author/Change controller: IETF
Specification document(s): this document
Related information: [RFC6376]
o Header field name: ARC-Authentication-Results
Applicable protocol: mail
Status: Experimental
Author/Change controller: IETF
Specification document(s): this document
Related information: [I-D-7601bis]
10.4. New Enhanced Status Code - ARC Validation
The following value should be added to the [ENHANCED-STATUS]
registry, as follows:
o Code: X.7.29
Sample Text: ARC validation failure
Associated basic status code: 550
Description: This status code may be returned when a message fails
ARC validation
Reference: this document
Submitter: K. Andersen
Change controller: IESG
11. Experimental Considerations
The ARC protocol is designed to address common interoperability
issues introduced by intermediate message handlers. Interoperability
issues are described in [RFC6377] and [RFC7960].
As the ARC protocol is implemented by Internet Mail Handlers over
time, the following should be evaluated in order to determine the
success of the protocol in accomplishing the intended benefits.
11.1. Success Consideration
In an attempt to deliver legitimate messages that users desire, many
receivers use heuristic-based methods to identify messages that
arrive via indirect delivery paths.
ARC will be a success if the presence of Authenticated Received
Chains allows for improved decision making when processing legitimate
messages, specifically resulting in equal or better delivery rates
than achieve through the use of heuristic approaches.
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11.2. Failure Considerations
ARC should function without introducing significant new vectors for
abuse (see Section 9). If unforeseen vectors are enabled by ARC,
then this protocol will be a failure. Note that weaknesses inherent
in the mail protocols ARC is built upon (such as DKIM replay attacks
and other known issues) are not new vectors which can be attributed
to this specification.
11.3. Open Questions
The following open questions are academic and have no clear answer at
the time of the development of the protocol. However, additional
deployments should be able to gather the necessary data to answer
some or all of them.
11.3.1. Value of the ARC-Seal (AS) Header Field
Data should be collected to show if the ARC-Seal (AS) provides value
beyond the ARC Message Signature (AMS) for either making delivery
decisions or catching malicious actors trying to craft or replay
malicious chains.
11.3.2. Usage and/or signals from multiple selectors and/or domains in
ARC sets
Any selectors and/or (sub)domains (under the control of the sealing
ADMD) may be used for ARC header field signatures.
While implementers may choose to use various selectors and/or domains
for ARC set header fields, no compelling argument for or against such
usage has been made within the working group. As such we have chosen
to allow maximum freedom for the experimental definition of this
protocol.
Wider deployment experience and higher volumes of traffic may show
whether this is useful.
11.3.3. DNS Overhead
Longer Authenticated Received Chains will require more queries to
retrieve the keys for validating the chain. While this is not
believed to be a security issue (see Section 9.2), it is unclear how
much overhead will truly be added. This is similar to some of the
initial processing and query load concerns which were debated at the
time of the DKIM specification development.
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Data should be collected to better understand usable length and
distribution of lengths found in valid Authenticated Received Chains
along with the DNS impact of processing Authenticated Received
Chains.
An effective operational maximum will have to be developed through
deployment experience in the field.
11.3.4. What Trace Information is Valuable
There are several edge cases where the information in the AAR can
make the difference between message delivery or rejection. For
example, if there is a well known mailing list that seals with ARC
but doesn't do its own initial DMARC enforcement, an Internet Mail
Handler with this knowledge could make a delivery decision based upon
the authentication information it sees in the corresponding AAR
header field.
Certain trace information in the AAR is useful/necessary in the
construction of DMARC reports.
Further, certain receivers believe the entire set of trace
information would be valuable to feed into machine learning systems
to identify fraud and/or provide other signals related to message
delivery.
At this point, however, it is unclear what trace information will be
valuable for all receivers, regardless of size.
Data should be collected on what trace information receivers are
using that provides useful signals that affect deliverability, and
what portions of the trace data are left untouched or provide no
useful information.
Since many such systems are intentionally proprietary or confidential
to prevent gaming by abusers, it may not be viable to reliably answer
this particular question. The evolving nature of attacks can also
shift the landscape of "useful" information over time.
12. Implementation Status
[[ Note to the RFC Editor: Please remove this section before
publication along with the reference to [RFC7942]. ]]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
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assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
This information is known to be correct as of the eighth
interoperability test event which was held on 2018-03-17 at IETF101.
For a few of the implementations, later status information was
available as of August 2018.
12.1. GMail test reflector and incoming validation
Organization: Google
Description: Internal production implementation with both debug
analysis and validating + sealing pass-through function
Status of Operation: Production - Incoming Validation
Coverage: Full spec implemented as of this document
Licensing: Internal only
Implementation Notes:
o Full functionality was demonstrated during the interop testing on
2018-03-17 and 2018-10-12. All traffic going into GSuite, Google
Groups, or GMail mailboxes will have ARC validation and sealing.
Contact Info: arc-discuss@dmarc.org [1]
12.2. AOL test reflector and internal tagging
Organization: AOL
Description: Internal prototype implementation with both debug
analysis and validating + sealing pass-through function
Status of Operation: Beta
Coverage: ARC Chain validity status checking is operational, but only
applied to email addresses enrolled in the test program. This system
conforms to [ARC-DRAFT-05]
Licensing: Proprietary - Internal only
Implementation Notes:
o 2017-07-15: Full functionality verified during the interop
testing.
o 2018-06: Partially retired but still accessible by special request
due to the in process evolution of the AOL mail infrastructure to
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the integrated OATH environment. The implementation was based on
the Apache James DKIM code base.
o 2018-10: No longer available due to infrastucture changes at AOL/
Yahoo!/Oath.
Contact Info: arc-discuss@dmarc.org [2]
12.3. dkimpy
Organization: dkimpy developers/Scott Kitterman
Description: Python DKIM package
Status of Operation: Production
Coverage: Full spec implemented as of this document
o 2017-07-15: The internal test suite is incomplete, but the command
line developmental version of validator was demonstrated to
interoperate with the Google and AOL implementations during the
interop on 2017-07-15 and the released version passes the tests in
[ARC-TEST] arc_test_suite [3] with both python and python3.
o 2018-10: Re-validated in the interop
Licensing: Open/Other (same as dkimpy package = BCD version 2)
Contact Info: https://launchpad.net/dkimpy
12.4. OpenARC
Organization: TDP/Murray Kucherawy
Description: Implementation of milter functionality related to the
OpenDKIM and OpenDMARC packages
Status of Operation: Beta
Coverage: Built to support this document
Licensing: Open/Other (same as OpenDKIM and OpenDMARC packages)
Implementation Notes:
o 2018-10: Validated with one bug discovered during interop
o 2018-11: Known issues have been resolved with release 1.0.0-Beta2
Contact Info: arc-discuss@dmarc.org [4], openarc-users@openarc.org
[5]
12.5. Mailman 3.x patch
Organization: Mailman development team
Description: Integrated ARC capabilities within the Mailman 3.2
package
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Status of Operation: Patch submitted
Coverage: Based on OpenARC
Licensing: Same as mailman package - GPL
Implementation Notes:
o Appears to work properly in at least one beta deployment, but
waiting on acceptance of the pull request into the mainline of
mailman development
o Discussions continuing with Mailman team to get this integrated
Contact Info: https://www.gnu.org/software/mailman/contact.html
12.6. Copernica/MailerQ web-based validation
Organization: Copernica
Description: Web-based validation of ARC-signed messages
Status of Operation: Beta
Coverage: Built to support [ARC-DRAFT-05]
Licensing: On-line usage only
Implementation Notes:
o Released 2016-10-24
o Requires full message content to be pasted into a web form found
at http://arc.mailerq.com/ (warning - https is not supported).
o An additional instance of an ARC signature can be added if one is
willing to paste a private key into an unsecured web form.
o 2017-07-15: Testing shows that results match the other
implementations listed in this section.
o 2018-10: not tested during interop
Contact Info: https://www.copernica.com/
12.7. Rspamd
Organization: Rspamd community
Description: ARC signing and verification module
Status of Operation: Production, though deployment usage is unknown
Coverage: Built to support [ARC-DRAFT-14]
Licensing: Open source
Implementation Notes:
o 2017-06-12: Released with version 1.6.0
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o 2017-07-15: Testing during the interop showed that the validation
functionality interoperated with the Google, AOL, dkimpy and
MailerQ implementations
o 2018-10: Re-validated during the interop
Contact Info: https://rspamd.com/doc/modules/arc.html and
https://github.com/vstakhov/rspamd
12.8. PERL MAIL::DKIM module
Organization: FastMail
Description: Email domain authentication (sign and/or verify) module,
previously included SPF / DKIM / DMARC, now has ARC added
Status of Operation: Production, deployment usage unknown
Coverage: Built to support [ARC-DRAFT-10]
Licensing: Open Source
Implementation Notes:
o 2017-12-15: v0.50 released with full test set passing for ARC
o 2018-10: Revalidated during the interop and used for the creation
of the Appendix B example
Contact Info: http://search.cpan.org/~mbradshaw/Mail-DKIM-0.50/
12.9. PERL Mail::Milter::Authentication module
Organization: FastMail
Description: Email domain authentication milter, uses MAIL::DKIM (see
above)
Status of Operation: Initial validation completed during IETF99
hackathon with some follow-on work during the week
Coverage: Built to support [ARC-DRAFT-14]
Licensing: Open Source
Implementation Notes:
o 2017-07-15: Validation functionality which interoperates with
Gmail, AOL, dkimpy was demonstrated; later in the week of IETF99,
the signing functionality was reported to be working
o 2017-07-20: ARC functionality has not yet been pushed back to the
github repo but should be showing up soon
o 2018-10: Revalidated during the interop
Contact Info: https://github.com/fastmail/authentication_milter
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12.10. Sympa List Manager
Organization: Sympa Dev Community
Description: Beta released Status of Operation: Beta released
Coverage: Built to support this document, based on Mail::DKIM module
Licensing: open source
Implementation Notes:
o 2018-01-05: Tracked as https://github.com/sympa-community/sympa/
issues/153
o 2018-12-08: Sympa 6.2.37 beta 3 incorporates ARC support,
scheduled for stable release 6.2.38 on 2018-12-21
Contact Info: https://github.com/sympa-community
12.11. Oracle Messaging Server
Organization: Oracle
Description:
Status of Operation: Initial development work during IETF99
hackathon. Framework code is complete, crypto functionality requires
integration with libsodium
Coverage: Work in progress
Licensing: Unknown
Implementation Notes:
o 2018-03: Protocol handling components are completed, but crypto is
not yet functional.
Contact Info: Chris Newman, Oracle
12.12. MessageSystems Momentum and PowerMTA platforms
Organization: MessageSystems/SparkPost
Description: OpenARC integration into the LUA-enabled Momentum
processing space
Status of Operation: Beta
Coverage: Same as OpenARC
Licensing: Unknown
Implementation Notes:
o 2018-10: Beta version in private evaluation, not tested during
interop.
Contact Info: TBD
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12.13. Exim
Organization: Exim developers
Status of Operation: Operational; requires specific enabling for
compile.
Coverage: Full spec implemented as of [ARC-DRAFT-13]
Licensing: GPL
Contact Info: exim-users@exim.org
Implementation notes:
o Implemented as of Exim 4.91
12.14. Halon MTA
Organization: Halon
Status of Operation: Operational as of May 2018
Coverage: Full spec implemented as of this document Licensing:
Commercial, trial version available for download
Contact Info: https://halon.io
Implementation notes:
o GPL'd library with ARC capabilities: https://github.com/halon/
libdkimpp
o 2018-10: Validated during interop
12.15. IIJ
Organization: Internet Initiative Japan (IIJ) Status of Operation:
Operational as of October 2018
Coverage: Full spec implemented as of this document
Licensing: Internal
Contact Info: https://www.iij.ad.jp/en/
Implementation notes:
o 2018-10: Internal MTA implementation validated during the ARC
interop
13. References
13.1. Normative References
[draft-levine-eaiauth]
Levine, J., "E-mail Authentication for Internationalized
Mail", August 2018, <https://tools.ietf.org/html/
draft-levine-appsarea-eaiauth-03>.
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[I-D-7601bis]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", February 2018,
<https://datatracker.ietf.org/doc/
draft-ietf-dmarc-rfc7601bis/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[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,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
September 2011, <https://www.rfc-editor.org/info/rfc6377>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[ARC-DRAFT-05]
Andersen, K., "Authenticated Received Chain (ARC) Protocol
(I-D-05)", n.d., <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-protocol-05>.
[ARC-DRAFT-10]
Andersen, K., "Authenticated Received Chain (ARC) Protocol
(I-D-10)", n.d., <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-protocol-10>.
[ARC-DRAFT-13]
Andersen, K., "Authenticated Received Chain (ARC) Protocol
(I-D-13)", n.d., <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-protocol-13>.
[ARC-DRAFT-14]
Andersen, K., "Authenticated Received Chain (ARC) Protocol
(I-D-14)", n.d., <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-protocol-14>.
[ARC-MULTI]
Andersen, K., "Using Multiple Signing Algorithms with
ARC", June 2018, <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-multi-02>.
[ARC-TEST]
Blank, S., "ARC Test Suite", January 2017,
<https://github.com/Valimail/arc_test_suite>.
[ARC-USAGE]
Jones, S., Adams, T., Rae-Grant, J., and K. Andersen,
"Recommended Usage of the ARC Headers", April 2018,
<https://tools.ietf.org/html/
draft-ietf-dmarc-arc-usage-05>.
[ENHANCED-STATUS]
"IANA SMTP Enhanced Status Codes", n.d.,
<http://www.iana.org/assignments/smtp-enhanced-status-
codes/smtp-enhanced-status-codes.xhtml>.
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[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
E., Ed., and K. Andersen, Ed., "Interoperability Issues
between Domain-based Message Authentication, Reporting,
and Conformance (DMARC) and Indirect Email Flows",
RFC 7960, DOI 10.17487/RFC7960, September 2016,
<https://www.rfc-editor.org/info/rfc7960>.
13.3. URIs
[1] mailto:arc-discuss@dmarc.org
[2] mailto:arc-discuss@dmarc.org
[3] https://github.com/Valimail/arc_test_suite
[4] mailto:arc-discuss@dmarc.org
[5] mailto:openarc-users@openarc.org
[6] mailto:dmarc@ietf.org
[7] mailto:arc-discuss@dmarc.org
[8] mailto:arc-interop@dmarc.org
[9] https://arc-spec.org
Appendix A. Design Requirements
The specification of the ARC framework is driven by the following
high-level goals, security considerations, and practical operational
requirements.
A.1. Primary Design Criteria
o Provide a verifiable "chain of custody" for email messages;
o Not require changes for originators of email;
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o Support the verification of the ARC header field set by each hop
in the handling chain;
o Work at Internet scale; and
o Provide a trustable mechanism for the communication of
Authentication-Results across trust boundaries.
A.2. Out of Scope
ARC is not a trust framework. Users of the ARC header fields are
cautioned against making unsubstantiated conclusions when
encountering a "broken" ARC sequence.
Appendix B. Example Usage
The following message is an example of one which has passed through
several intermediary handlers, some of which have modified the
message and others which have not:
Return-Path: <jqd@d1.example>
Received: from example.org (example.org [208.69.40.157])
by gmail.example with ESMTP id d200mr22663000ykb.93.1421363207
for <fmartin@example.com>; Thu, 14 Jan 2015 15:02:40 -0800 (PST)
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
(envelope-from jqd@d1.example)
Received: from [2001:DB8::1A] (w-x-y-z.dsl.static.isp.example [w.x.y.z])
(authenticated bits=0)
by segv.d1.example with ESMTP id t0FN4a8O084569;
Thu, 14 Jan 2015 15:00:01 -0800 (PST)
(envelope-from jqd@d1.example)
Received: from mail-ob0-f188.google.example
(mail-ob0-f188.google.example [208.69.40.157]) by
clochette.example.org with ESMTP id d200mr22663000ykb.93.1421363268
for <fmartin@example.org>; Thu, 14 Jan 2015 15:03:15 -0800 (PST)
ARC-Seal: i=3; a=rsa-sha256; cv=pass; d=clochette.example.org; s=
clochette; t=12345; b=CU87XzXlNlk5X/yW4l73UvPUcP9ivwYWxyBWcVrRs7
+HPx3K05nJhny2fvymbReAmOA9GTH/y+k9kEc59hAKVg==
ARC-Message-Signature: i=3; a=rsa-sha256; c=relaxed/relaxed; d=
clochette.example.org; h=message-id:date:from:to:subject; s=
clochette; t=12345; bh=KWSe46TZKCcDbH4klJPo+tjk5LWJnVRlP5pvjXFZY
LQ=; b=o71vwyLsK+Wm4cOSlirXoRwzEvi0vqIjd/2/GkYFYlSd/GGfKzkAgPqxf
K7ccBMP7Zjb/mpeggswHjEMS8x5NQ==
ARC-Authentication-Results: i=3; clochette.example.org; spf=fail
smtp.from=jqd@d1.example; dkim=fail (512-bit key)
header.i=@d1.example; dmarc=fail; arc=pass (as.2.gmail.example=pass,
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ams.2.gmail.example=pass, as.1.lists.example.org=pass,
ams.1.lists.example.org=fail (message has been altered))
Authentication-Results: clochette.example.org; spf=fail
smtp.from=jqd@d1.example; dkim=fail (512-bit key)
header.i=@d1.example; dmarc=fail; arc=pass (as.2.gmail.example=pass,
ams.2.gmail.example=pass, as.1.lists.example.org=pass,
ams.1.lists.example.org=fail (message has been altered))
ARC-Seal: i=2; a=rsa-sha256; cv=pass; d=gmail.example; s=20120806; t=
12345; b=Zpukh/kJL4Q7Kv391FKwTepgS56dgHIcdhhJZjsalhqkFIQQAJ4T9BE
8jjLXWpRNuh81yqnT1/jHn086RwezGw==
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=
gmail.example; h=message-id:date:from:to:subject; s=20120806; t=
12345; bh=KWSe46TZKCcDbH4klJPo+tjk5LWJnVRlP5pvjXFZYLQ=; b=CVoG44
cVZvoSs2mMig2wwqPaJ4OZS5XGMCegWqQs1wvRZJS894tJM0xO1RJLgCPsBOxdA5
9WSqI9s9DfyKDfWg==
ARC-Authentication-Results: i=2; gmail.example; spf=fail
smtp.from=jqd@d1.example; dkim=fail (512-bit key)
header.i=@example.org; dmarc=fail; arc=pass
(as.1.lists.example.org=pass, ams.1.lists.example.org=pass)
ARC-Seal: i=1; a=rsa-sha256; cv=none; d=lists.example.org; s=dk-lists;
t=12345; b=TlCCKzgk3TrAa+G77gYYO8Fxk4q/Ml0biqduZJeOYh6+0zhwQ8u/
lHxLi21pxu347isLSuNtvIagIvAQna9a5A==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=
lists.example.org; h=message-id:date:from:to:subject; s=
dk-lists; t=12345; bh=KWSe46TZKCcDbH4klJPo+tjk5LWJnVRlP5pvjXFZYL
Q=; b=DsoD3n3hiwlrN1ma8IZQFgZx8EDO7Wah3hUjIEsYKuShRKYB4LwGUiKD5Y
yHgcIwGHhSc/4+ewYqHMWDnuFxiQ==
ARC-Authentication-Results: i=1; lists.example.org; spf=pass
smtp.mfrom=jqd@d1.example; dkim=pass (512-bit key)
header.i=@d1.example; dmarc=pass
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; d=d1.example; h=
message-id:date:from:to:subject; s=origin2015; bh=bIxxaeIQvmOBdT
AitYfSNFgzPP4=; b=qKjd5fYibKXWWIcMKCgRYuo1vJ2fD+IAQPjX+uamXIGY2Q
0HjQ+Lq3/yHzG3JHJp6780/nKQPOWt2UDJQrJkEA==
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@dmarc.example
Subject: [List 2] Example 1
Hey gang,
This is a test message.
--J.
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Appendix C. Acknowledgements
This draft originated with the work of OAR-Dev Group.
The authors thank all of the OAR-Dev and the subsequent DMARC-WG
group for the ongoing help and though-provoking 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, Gene Shuman, Scott Kitterman, Bron Gondwana.
Grateful appreciation is extended to the people who provided feedback
through the discuss mailing list.
Appendix D. Comments and Feedback
Please address all comments, discussions, and questions to
dmarc@ietf.org [6]. Earlier discussions can be found at arc-
discuss@dmarc.org [7]. Interop discussions planning can be found at
arc-interop@dmarc.org [8].
Some introductory material for less technical people can be found at
https://arc-spec.org [9].
Authors' Addresses
Kurt Andersen
LinkedIn
1000 West Maude Ave
Sunnyvale, California 94085
USA
Email: kurt+ietf@drkurt.com
Brandon Long (editor)
Google
Email: blong@google.com
Seth Blank (editor)
Valimail
Email: seth@valimail.com
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Murray Kucherawy (editor)
TDP
Email: superuser@gmail.com
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