Authenticated Received Chain (ARC) Protocol
draft-ietf-dmarc-arc-protocol-15
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8617.
|
|
|---|---|---|---|
| Authors | Kurt Andersen , Brandon Long , Seth Blank , Murray Kucherawy , Tim Draegen | ||
| Last updated | 2018-06-24 | ||
| Replaces | draft-andersen-arc | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-18)
by Ines Robles
Almost ready
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 8617 (Experimental) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dmarc-arc-protocol-15
DMARC Working Group K. Andersen
Internet-Draft LinkedIn
Intended status: Experimental B. Long, Ed.
Expires: December 26, 2018 Google
S. Blank, Ed.
Valimail
M. Kucherawy, Ed.
TDP
T. Draegon, Ed.
dmarcian
June 24, 2018
Authenticated Received Chain (ARC) Protocol
draft-ietf-dmarc-arc-protocol-15
Abstract
The Authenticated Received Chain (ARC) protocol allows Internet Mail
Handlers to attach assertions of message authentication state 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 authentication
state along 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 state across trust boundaries.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 26, 2018.
Andersen, et al. Expires December 26, 2018 [Page 1]
Internet-Draft ARC-Protocol June 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Note to Reviewers in the DMARC WG . . . . . . . . . . . . 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. Sealer . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Validator . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . 7
3.6. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
4. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
4.1. ARC Headers . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. ARC-Authentication-Results (AAR) . . . . . . . . . . 8
4.1.2. ARC-Message-Signature (AMS) . . . . . . . . . . . . . 8
4.1.3. ARC-Seal (AS) . . . . . . . . . . . . . . . . . . . . 9
4.2. ARC Set . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2.1. Instance Tags . . . . . . . . . . . . . . . . . . . . 11
4.3. Authenticated Received Chain . . . . . . . . . . . . . . 11
4.4. Chain Validation Status . . . . . . . . . . . . . . . . . 11
5. Protocol Actions . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Sealer Actions . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Header Fields To Include In ARC-Seal Signatures . . . 13
5.1.2. Marking and Sealing "cv=fail" (Invalid) Chains . . . 13
5.1.3. Only One Authenticated Received Chain Per Message . . 14
5.1.4. Broad Ability to Seal . . . . . . . . . . . . . . . . 14
5.1.5. Sealing is Always Safe . . . . . . . . . . . . . . . 14
5.1.6. Signing vs Sealing . . . . . . . . . . . . . . . . . 14
Andersen, et al. Expires December 26, 2018 [Page 2]
Internet-Draft ARC-Protocol June 2018
5.2. Validator Actions . . . . . . . . . . . . . . . . . . . . 14
5.2.1. All Failures Are Permanent . . . . . . . . . . . . . 16
5.2.2. Responding to ARC Validation Failures During the SMTP
Transaction . . . . . . . . . . . . . . . . . . . . . 16
5.3. Result of Validation . . . . . . . . . . . . . . . . . . 16
6. Communication of Validation Results . . . . . . . . . . . . . 17
7. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Communicate Authentication Results Across Trust
Boundaries . . . . . . . . . . . . . . . . . . . . . . . 17
7.1.1. Message Scanning Services . . . . . . . . . . . . . . 18
7.1.2. Multi-tier MTA Processing . . . . . . . . . . . . . . 18
7.1.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . 18
7.2. Inform Message Disposition Decisions . . . . . . . . . . 19
7.2.1. DMARC Local Policy Overrides . . . . . . . . . . . . 19
7.2.2. DMARC Reporting . . . . . . . . . . . . . . . . . . . 19
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9.1. Increased Header Size . . . . . . . . . . . . . . . . . . 21
9.2. DNS Operations . . . . . . . . . . . . . . . . . . . . . 21
9.3. Message Content Suspicion . . . . . . . . . . . . . . . . 21
9.4. Message Sealer Suspicion . . . . . . . . . . . . . . . . 22
9.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10.1. Email Authentication Results Names Registry Update . . . 22
10.2. Email Authentication Methods Registry Update . . . . . . 22
10.3. Definitions of the ARC header fields . . . . . . . . . . 23
11. Experimental Considerations . . . . . . . . . . . . . . . . . 23
11.1. Success Consideration . . . . . . . . . . . . . . . . . 23
11.2. Failure Considerations . . . . . . . . . . . . . . . . . 24
11.3. Open Questions . . . . . . . . . . . . . . . . . . . . . 24
11.3.1. Value of the ARC-Seal (AS) Header . . . . . . . . . 24
11.3.2. DNS Overhead . . . . . . . . . . . . . . . . . . . . 24
11.3.3. What Trace Information is Valuable . . . . . . . . . 24
12. Implementation Status . . . . . . . . . . . . . . . . . . . . 25
12.1. GMail test reflector and incoming validation . . . . . . 26
12.2. AOL test reflector and internal tagging . . . . . . . . 26
12.3. dkimpy . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.4. OpenARC . . . . . . . . . . . . . . . . . . . . . . . . 27
12.5. Mailman 3.x patch . . . . . . . . . . . . . . . . . . . 27
12.6. Copernica/MailerQ web-based validation . . . . . . . . . 27
12.7. Rspamd . . . . . . . . . . . . . . . . . . . . . . . . . 28
12.8. PERL MAIL::DKIM module . . . . . . . . . . . . . . . . . 28
12.9. PERL Mail::Milter::Authentication module . . . . . . . . 28
12.10. Sympa List Manager . . . . . . . . . . . . . . . . . . . 29
12.11. Oracle Messaging Server . . . . . . . . . . . . . . . . 29
12.12. MessageSystems Momentum and PowerMTA platforms . . . . . 29
12.13. Exim . . . . . . . . . . . . . . . . . . . . . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
Andersen, et al. Expires December 26, 2018 [Page 3]
Internet-Draft ARC-Protocol June 2018
13.1. Normative References . . . . . . . . . . . . . . . . . . 30
13.2. Informative References . . . . . . . . . . . . . . . . . 31
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Appendix A. Appendix A - Design Requirements . . . . . . . . . . 32
A.1. Primary Design Criteria . . . . . . . . . . . . . . . . . 33
A.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 33
Appendix B. Appendix B - Example Usage . . . . . . . . . . . . . 33
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 33
Appendix D. Comments and Feedback . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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].
The utility of technologies that build upon SPF and DKIM (such as
DMARC [RFC7489]) is similarly impacted by intermediate handlers. The
disruption of authentication mechanisms for legitimate messages by
intermediate handlers can impact all aspects of Internet Mail -
message authors, message recipients, and even the intermediary
handler itself.
Authenticated Received Chain (ARC) creates a mechanism for individual
Internet Mail Handlers to add their authentication processing results
to a message's ordered set of processing results. ARC encapsulates
processing results in a DKIM signature derivative to grant other
handlers the ability to verify the authenticity of each individual
processing results as well as the aggregate set and sequence of
results.
Ordered sets of processing results 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.
1.1. Note to Reviewers in the DMARC WG
[[ Note: This section is editorial to the WG. Will be removed for
final version. ]]
Andersen, et al. Expires December 26, 2018 [Page 4]
Internet-Draft ARC-Protocol June 2018
This version of the draft has been extensively reorganized for
readability. No changes to the wire protocol or specification are
intended from [ARC-DRAFT-14].
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
state at any point along the delivery path between origination and
final delivery. Authentication state can change when intermediate
handlers modify message content, route messages through unforeseen
paths, or change envelope information.
2.2. Custody
"Custody" refers to when an ARC-enabled 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 state upon receipt) to the message. 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
evidence and custody 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
state as the message traveled through various Custodians.
Even though a message's authentication state 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.
Andersen, et al. Expires December 26, 2018 [Page 5]
Internet-Draft ARC-Protocol June 2018
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
intermediaries in the delivery of email is 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
Internet Mail Handlers process and deliver messages across the
Internet and include MSAs, MTAs, MDAs, gateways, and mailing lists.
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 Section 4.1 introduces three (3) ARC header fields.
Together, the 3 header fields compose a single "ARC Set". An ARC Set
provides the means for an Internet Mail Handler to attach
authentication state 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.
Andersen, et al. Expires December 26, 2018 [Page 6]
Internet-Draft ARC-Protocol June 2018
3.2. Authenticated Received Chain (ARC)
The complete sequence of ARC Sets attached to a message is called the
Authenticated Received Chain. An Authenticated Received Chain is a
recording of individual authentication states 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. 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 Evidence and proof of Custody
to the Chain of Custody.
3.4. 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 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.
3.5. 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)
3.6. Common ABNF Tokens
The following ABNF tokens are used elsewhere in this document:
Andersen, et al. Expires December 26, 2018 [Page 7]
Internet-Draft ARC-Protocol June 2018
position = 1*2DIGIT ; 1 - 50
instance = [CFWS] %s"i" [CFWS] "=" [CFWS] position [CFWS] ";"
chain-status = ("none" / "fail" / "pass")
seal-cv-tag = %s"cv" [CFWS] "=" [CFWS] chain-status
4. Protocol Elements
4.1. ARC Headers
ARC introduces three new header fields. Syntax for new header fields
borrows heavily from existing specifications. This document only
describes where 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 state 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 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 all authentication results from within the participating
ADMD, regardless of how many Authentication-Results headers 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.
Andersen, et al. Expires December 26, 2018 [Page 8]
Internet-Draft ARC-Protocol June 2018
In General Concept terms, the AMS header field identifies a
Custodian.
The AMS header field is similar in syntax and semantics to a 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. As required
for undefined tags (in [RFC6376]), if seen, a version tag MUST be
ignored;
o the presence of the "instance tag". Additional information on the
"instance tag" can be found in Section Section 4.2.1. The
instance tag replaces the DKIM "AUID" tag.
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 avoid unwanted 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 Section 5 of [I-D-7601bis]).
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 is the mechanism by which ARC-
participating ADMDs can verify the integrity of AAR header fields and
corresponding AMS header fields.
Andersen, et al. Expires December 26, 2018 [Page 9]
Internet-Draft ARC-Protocol June 2018
In General Concept terms, the AS header field is how Custodians bind
Evidence 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 presence of the "instance tag". Additional information on the
"instance tag" can be found in Section Section 4.2.1.
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 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 canonicalization ([RFC6376] section 3.4.2)
is used.
o the only supported tags are "i" (from Section Section 4.2.1 of
this document), and "a", "b", "d, "s", "t" from Section 3.5 of
[RFC6376]. Note especially that the DKIM "h" header 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
4.2. ARC Set
An "ARC Set" is a single collection of three ARC Headers (AAR, AMS,
and AS). ARC Headers of an ARC Set share the same "instance" value.
By adding all ARC Headers 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 Section 5.1.
Andersen, et al. Expires December 26, 2018 [Page 10]
Internet-Draft ARC-Protocol June 2018
4.2.1. Instance Tags
Instance tags describe which ARC Headers belong to an ARC Set. Each
ARC Header 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).
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.
_INFORMATIONAL:_ Initial development of ARC is only being done with a
single allowed signing algorithm, but parallel work in the DCRUP
working group is expanding that. 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 Headers, an Authenticated
Received Chain represents the output of message authentication state
along the handling path of ARC-enabled processors.
Results of message authentication processing along 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
Andersen, et al. Expires December 26, 2018 [Page 11]
Internet-Draft ARC-Protocol June 2018
o as part of Authentication-Results and AAR headers.
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 Sealers
(when sending messages) and ARC Validators (when receiving messages).
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-Signatures) MUST
be performed before Sealing.
2. Calculate the instance value: if the message 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.
3. Using the calculated instance value, generate and attach to the
message in the following order:
4. An ARC-Authentication-Results header field as defined in
Section Section 4.1.1.
Andersen, et al. Expires December 26, 2018 [Page 12]
Internet-Draft ARC-Protocol June 2018
5. An ARC-Message-Signature header field as defined in
Section Section 4.1.2.
6. An ARC-Seal header field using the AS definition found in
Section Section 4.1.3, the method described in
Section Section 5.1.1, and the Chain Validation Status as
determined during ARC validation.
5.1.1. Header Fields To Include In ARC-Seal Signatures
The signature of an AS header field signs a specific canonicalized
form of the ARC Set header values. The ARC set header 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
The ARC-Seal is generated in a manner similar to when DKIM-Signatures
are added to messages ([RFC6376], section 3.7).
Note that when an Authenticated Received Chain has failed validation,
the signing scope for the ARC-Seal is modified (see
Section 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 headers 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.
Andersen, et al. Expires December 26, 2018 [Page 13]
Internet-Draft ARC-Protocol June 2018
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 designed
restriction which prevents chain continuation or re-establishment,
see [ARC-USAGE].
5.1.4. Broad Ability to Seal
ARC is not solely intended for perimeter MTAs. Any mediator
([RFC5598], section 5) that modifies a message may Seal its own
changes. For additional information, see Section 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 Section 9).
o An ADMD may choose to Seal all inbound messages whether or not a
message has been modified or will be retransmitted.
5.1.6. Signing vs Sealing
Signing is the process of affixing a digital signature to a message
as a header, 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.
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.
Andersen, et al. Expires December 26, 2018 [Page 14]
Internet-Draft ARC-Protocol June 2018
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 ARC instance value is
referred to as "N".
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 be non-
failing. For instance values > 1, the value must be "pass".
For 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:
6. 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 headers). This does not affect the
validity of the Authenticated Received Chain.
7. If all AMS headers verify, set the oldest-pass value to zero (0).
8. 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.
Andersen, et al. Expires December 26, 2018 [Page 15]
Internet-Draft ARC-Protocol June 2018
9. 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 is added into the
Authentication-Results header for the ADMD.
As with a failing DKIM signature ([RFC6376] section 6.3), a message
with a failing Authenticated Received Chain 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
failed Chain Validation Status. 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 Authenticated Received Chain
has failed, the Validator MAY signal the breakage through the
extended SMTP response code 5.7.7 [RFC3463] "message integrity
failure" [ENHANCED-STATUS] and corresponding SMTP response code.
5.3. Result of Validation
An Authenticated Received Chain with a Chain Validation Status of
"pass" allows Internet Mail Handlers to ascertain:
o all ARC-participating ADMDs that claim responsibility for handling
(and possibly modifying) the message in transit;
o the authentication state of the message as perceived by each ADMD
(from Authentication-Results header fields).
Andersen, et al. Expires December 26, 2018 [Page 16]
Internet-Draft ARC-Protocol June 2018
Given this information, handlers can inform local policy decisions
regarding disposition of messages that experience authentication
failure due to intermediate processing.
6. Communication of Validation Results
Chain Validation Status (described in Section Section 4.4) is
communicated via Authentication-Results (and AAR) headers using the
auth method "arc". This auth method is described in
Section Section 10.1.
If necessary data is available, the ptypes and properties defined in
Section Section 10.2 SHOULD be recorded in an Authentication-Results
header field:
o smtp.client-ip - The connecting client IP address from which the
message is received.
o header.oldest-pass - The instance number of the oldest AMS that
still validates, or 0 if all pass.
Upon Sealing of a message, this Authentication-Results information
along with all other Authentications-Results added by the ADMD will
be recorded into the AAR as defined in section Section 4.1.1.
In General Concept terms, the information recorded in the ARC-
Authentication-Results header field is the Evidence that gets
attached to a message.
7. Use Cases
This section explores several messaging handling use cases that are
addressed by ARC.
7.1. Communicate Authentication Results 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 authentication state to the next ADMD in the
message handling path.
If ARC-enabled ADMDs are trusted, Authenticated Received Chains can
be used to bridge administrative boundaries.
Andersen, et al. Expires December 26, 2018 [Page 17]
Internet-Draft ARC-Protocol June 2018
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 results 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.
Authenticated Received Chains can be used to communicated
authentication state between processing tiers.
7.1.3. Mailing Lists
Mailing lists resend posted messages 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 original
authentication state 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 state of
the original message before mailing list handling.
Andersen, et al. Expires December 26, 2018 [Page 18]
Internet-Draft ARC-Protocol June 2018
7.2. Inform Message Disposition Decisions
ARC functionality allows Internet Mail Handlers to reliably identify
intermediary ADMDs and for ADMDs to expose authentication state that
can survive additional intermediary handling.
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 headers).
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
state 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 states provided by other ADMDs. As a matter of local
policy, a DMARC processor may choose to accept the authentication
state 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 Section 7.2.2.
7.2.2. DMARC Reporting
DMARC-enabled receivers indicate when ARC Validation influences
DMARC-related local policy decisions. DMARC reporting of ARC-
influenced decisions is accomplished by adding a local_policy comment
Andersen, et al. Expires December 26, 2018 [Page 19]
Internet-Draft ARC-Protocol June 2018
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 ams[2].d=d2.example ams[2].s=s1
as[2].d=d2.example as[2].s=s2 as[1].d=d1.example
as[1].s=s3 client-ip[1]=10.10.10.13</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,
"ams[2]" means AMS 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. DMARC report processors should be aware of this
behaviour and make the necessary accommodations.
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 and domain names. Such
information is also including in existing header fields such as the
"Received" header field.
9. Security Considerations
The Security Considerations of [RFC6376] and [I-D-7601bis] apply
directly to this specification.
Andersen, et al. Expires December 26, 2018 [Page 20]
Internet-Draft ARC-Protocol June 2018
As with other domain authentication technologies (such as SPF, DKIM,
and DMARC), ARC makes no claims about the semantic content of
messages.
9.1. Increased Header Size
Inclusion of Authenticated Received Chains into messages may cause
issues for older or constrained MTAs due to increased total header
size.
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. 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
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.
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.
Andersen, et al. Expires December 26, 2018 [Page 21]
Internet-Draft ARC-Protocol June 2018
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 signing domain, but whether
or not that signer 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 both here and
in [I-D-7601bis]. Please delete the overlap from whichever document
goes through the publication process after the other. ]]
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:
[I-D.ARC] Section 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]:
Andersen, et al. Expires December 26, 2018 [Page 22]
Internet-Draft ARC-Protocol June 2018
o Method: arc Definition: [I-D.ARC] ptype: smtp Property: client-ip
Value: IP address of originating SMTP connection Status: active
Version: 1
o Method: arc Definition: [I-D.ARC] ptype: header Property: oldest-
pass Value: The instance id of the oldest validating AMS, or 0 if
they all pass (see Sectionn 4) Status: active Version: 1
o Method: dkim Definition: [RFC6376] ptype: header Property: s
Value: value of signature "s" tag 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:
draft Author/Change controller: IETF Specification document(s):
[I-D.ARC] Related information: [RFC6376]
o Header field name: ARC-Message-Signature Applicable protocol: mail
Status: draft Author/Change controller: IETF Specification
document(s): [I-D.ARC] Related information: [RFC6376]
o Header field name: ARC-Authentication-Results Applicable protocol:
mail Status: standard Author/Change controller: IETF Specification
document(s): [I-D.ARC] Related information: [I-D-7601bis]
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 intermediary 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.
Andersen, et al. Expires December 26, 2018 [Page 23]
Internet-Draft ARC-Protocol June 2018
11.2. Failure Considerations
ARC should function without introducing significant new vectors for
abuse (see Section Section 9). If unforseen 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
deployment 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
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. 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 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.
Data should be collected to better understand usable length and
distribution of lengths found in valid Authenticated Received Chains
along with the the DNS impact of processing Authenticated Received
Chains.
An effective operational maximum will have to be developed through
deployment experience in the field.
11.3.3. 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
Andersen, et al. Expires December 26, 2018 [Page 24]
Internet-Draft ARC-Protocol June 2018
the authentication information it sees in the corresponding AAR
header.
Certain trace information in the AAR is useful/necessary in the
construction of DMARC reports.
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.
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 [RFC6982]. ]]
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 [RFC6982].
The description of implementations in this section is intended to
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 June 2018.
Andersen, et al. Expires December 26, 2018 [Page 25]
Internet-Draft ARC-Protocol June 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 [ARC-DRAFT-14] Licensing:
Proprietary - Internal only Implementation Notes:
o Full functionality was demonstrated during the interop testing on
2018-03-17.
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
the integrated OATH environment. The implementation was based on
the Apache James DKIM code base and may be contributed back to
that project in the future.
Contact Info: arc-discuss@dmarc.org [2]
12.3. dkimpy
Organization: dkimpy developers/Scott Kitterman Description: Python
DKIM package Status of Operation: Production Coverage:
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.
Licensing: Open/Other (same as dkimpy package = BCD version 2)
Contact Info: https://launchpad.net/dkimpy
Andersen, et al. Expires December 26, 2018 [Page 26]
Internet-Draft ARC-Protocol June 2018
12.4. OpenARC
Organization: TDP/Murray Kucherawy Description: Implemention of
milter functionality related to the OpenDKIM and OpenDMARC packages
Status of Operation: Beta Coverage: Built to support [ARC-DRAFT-14]
Licensing: Open/Other (same as OpenDKIM and OpenDMARC packages)
Implementation Notes:
o The build is FreeBSD oriented but some packages have been built
for easier deployment on RedHat-based Linux platforms.
o Some issues still exist when deploying in a chained milter
arrangement (such as OpenSPF -> OpenDKIM -> OpenDMARC -> OpenARC)
with coordination between the stages. When deployed in a
"sandwich" configuration around an MLM, there is no effective
mechanism to convey trust from the ingress (validator) to egress
(signer) instances. (_NOTE_: this is expected to resolved with a
new release of OpenDMARC expected in mid-2018.)
Contact Info: arc-discuss@dmarc.org [4]
12.5. Mailman 3.x patch
Organization: Mailman development team Description: Integrated ARC
capabilities within the Mailman 3.2 package 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
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.
Andersen, et al. Expires December 26, 2018 [Page 27]
Internet-Draft ARC-Protocol June 2018
o 2017-07-15: Testing shows that results match the other
implementations listed in this section.
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
o 2017-07-15: Testing during the interop showed that the validation
functionality interoperated with the Google, AOL, dkimpy and
MailerQ implementations
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
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: Intial
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
Andersen, et al. Expires December 26, 2018 [Page 28]
Internet-Draft ARC-Protocol June 2018
Contact Info: https://github.com/fastmail/authentication_milter
12.10. Sympa List Manager
Organization: Sympa Dev Community Description: Work in progress
Status of Operation: Work in progress Coverage: unknown Licensing:
open source Implementation Notes:
o 2018-01-05: Tracked as https://github.com/sympa-community/sympa/
issues/153
Contact Info: https://github.com/sympa-community
12.11. Oracle Messaging Server
Organization: Oracle Description: Status of Operation: Intial
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 Initial deployments for validation expected in mid-2018.
Contact Info: TBD
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 Exim 4.91
Andersen, et al. Expires December 26, 2018 [Page 29]
Internet-Draft ARC-Protocol June 2018
13. References
13.1. Normative References
[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>.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, DOI 10.17487/RFC3463, January 2003,
<https://www.rfc-editor.org/info/rfc3463>.
[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>.
[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>.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601,
DOI 10.17487/RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>.
Andersen, et al. Expires December 26, 2018 [Page 30]
Internet-Draft ARC-Protocol June 2018
[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", January 2018, <https://tools.ietf.org/html/
draft-ietf-dmarc-arc-multi-01>.
[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>.
Andersen, et al. Expires December 26, 2018 [Page 31]
Internet-Draft ARC-Protocol June 2018
[I-D-7601bis]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", February 2018,
<https://datatracker.ietf.org/doc/
draft-ietf-dmarc-rfc7601bis/>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<https://www.rfc-editor.org/info/rfc6982>.
[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>.
[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] https://trac.ietf.org/trac/dmarc/ticket/17
[6] mailto:dmarc@ietf.org
[7] mailto:arc-discuss@dmarc.org
[8] mailto:arc-interop@dmarc.org
[9] https://arc-spec.org
Appendix A. Appendix A - Design Requirements
[[ Note: This section is re-inserted for background information from
early versions of the spec. ]]
Andersen, et al. Expires December 26, 2018 [Page 32]
Internet-Draft ARC-Protocol June 2018
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;
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. Appendix B - Example Usage
[[ Note: The following examples were mocked up early in the
definition process for the spec. They no longer reflect the current
definition and need various updates which will be included in a
future draft. Issue 17 [5] ]]
[[ Note: Need input from the WG as to what sort of sequence of
examples would be considered useful - otherwise we'll just drop this
section entirely. ]]
<removed for now to reduce confusion>
Appendix C. Acknowledgements
This draft originated with the work of OAR-Dev Group.
The authors thank all of the OAR-Dev 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.
Andersen, et al. Expires December 26, 2018 [Page 33]
Internet-Draft ARC-Protocol June 2018
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: kurta@linkedin.com
Brandon Long (editor)
Google
Email: blong@google.com
Seth Blank (editor)
Valimail
Email: seth@valimail.com
Murray Kucherawy (editor)
TDP
Email: superuser@gmail.com
Tim Draegon (editor)
dmarcian
Email: tim@dmarcian.com
Andersen, et al. Expires December 26, 2018 [Page 34]