Continuing my review of: draft-ietf-dmarc-arc-protocol-16
NB: These are comments, not demands. Use however is helpful...
4. Protocol Elements
I keep thinking that it would help to have some summary text, possibly
with a figure, that shows the role of individual header fields and sets
of them.
My first inclination is to suggest putting it here, but perhaps it would
actually be better to have it at the /end/ of this section, after each
component has been defined.
(I'd offer some candidate text/figure, but I am not sure I have a solid
enough sense of the details.)
4.1. ARC Headers
Headers -> Header Fields
(I feel compelled to constantly apologize that RFC 733 made the term be
'header field' rather than 'header'...)
ARC introduces three new header fields. Syntax for new header fields
borrows heavily from existing specifications. This document only
borrows heavily from -> adapts
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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.
I'll note my continuing concern for 'authentication state'...
In General Concept terms, the AAR header field is where Evidence is
recorded by a Custodian.
The arguments against the chain of custody model and terminology have
swayed me. The operation of ARC is not strict enough or reliable enough
to justify the model or terms, which means that using them sets
expectations to high.
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 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.
In General Concept terms, the AMS header field identifies a
Custodian.
The text after this provides some technical details about differences,
but in terms of utility/purpose, how does this compare to doing a DKIM
signature by this ADMD? It's worth providing some comment about this.
The AMS header field is similar in syntax and semantics to a DKIM-
Signature field [RFC6376], with three (3) differences:
is similar in -> has the same
to a -> as the
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 presence of the "instance tag". Additional information on the
"instance tag" can be found in Section 4.2.1. The instance tag
replaces the DKIM "AUID" tag;
o when building the header field list to be signed, ARC-related
headers MUST be submitted to the hash function in increasing
instance order.
header -> fields
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:
There are cases where invalidation is /wanted/???
Perhaps:
To minimize the problem of AMS signature invalidation:
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.
Arguably, including it/them does NOT alter integrity validation. i
suspect /ever/. At the least, be explicit about /what/ integrity is
being maintain.
That is, the dkim signature provides a specific kind of integrity. If
it validates, that integrity is proved. If it doesn't, it isn't.
covering it by ARC doesn't affect either outcome.
4.1.3. ARC-Seal (AS)
The ARC-Seal (AS) header field is the mechanism by which ARC-
is a mechanism by which -> permits
participating ADMDs can verify the integrity of AAR header fields and
can -> to
corresponding AMS header fields.
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:
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o the presence of the "instance tag". Additional information on the
"instance tag" can be found in 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 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 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.
what does this mean? how is it relevant?
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 5.1.
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
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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 with a
safety margin multiple added on top to support the vast majority of
all intermediary mail flows.
Rather than citing a wg process, document the technical, administrative
and/or operation concerns, benefits, etc. that justify the choice.
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].
As a rule, RFCs should not refer to parallel activities, since the
reference is soon to become stale and wrong.
If additional signing algorithms are anticipated -- and of course they
should be -- then define a means for extending what is permitted, noting
that the initial algorithm is provided to ensure basic, initial
interoperability.
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
o as part of Authentication-Results and AAR headers.
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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).
generally? that statement is worth expanding. When are they only one?
When are they only the other? Is it possible to do neither and still be
an Arc-enabled IM handler? What is the benefit of doing only one?
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.
Does it include adding Received fields? Given how common the action,
it's worth citing it explicitly.
2. Calculate the instance value: if the message contains an
contains -> already contains
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:
4-6 are subordinate to 3. They should be sub-numbered. Alternatively
(and probably better) is to get rid of 3 and mildly modify 4, 5 and 6 to
have the 'generate and attach' verbiage directly...
4. An ARC-Authentication-Results header field as defined in
Section 4.1.1.
5. An ARC-Message-Signature header field as defined in
Section 4.1.2.
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6. An ARC-Seal header field using the AS definition found in
Section 4.1.3, the method described in 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
delete 'specific'.
form of the ARC Set header values. The ARC set header values are
supplied to the hash function in increasing instance order, starting
'the hash function'. Not clear to be that which hash function will be
clear to the reader, especially since there is more than one in ARC,
isn't there -- one for arc signature and one for arc seal?
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).
Not sure how the above sentence is supposed to be used, especially in
the middle of the detailed procedural specification.
Note that when an Authenticated Received Chain has failed validation,
the signing scope for the ARC-Seal is modified (see Section 5.1.2).
This sounds like it should be more than a terse, offhand 'note' and it
seems to imply that the reader should already be aware of the point.
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.
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].
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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 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.
5.1.6. 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.
This paragraph should not be buried deep here. It should probably be at
the beginning of 5, or a similar 'introductory' place.
I'm starting to wonder about whether the term should be 'seal' or 'ARC
seal'. Here's my concern: 'signing' is a well-established, generic
term. 'Sealing' is not. Sealing applies only to ARC; the term is being
created here. There is less likelihood of confusion about the term if
it is always formally referred to as 'ARC Sealing', because that will
always make clear the context for its use.
BTW, do a complete pass over the doc, looking for 'header' and check
whether it should actually be 'field' or 'header field'...
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
break this into a list of sub-steps/-conditions
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".
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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 be non-
"non-failing" is not a listed value. I think an acceptable for of what
you want to say:
Each ARC-Seal MUST NOT have a "cv" value of "fail".
failing. For instance values > 1, the value must be "pass".
For instance value = 1, the value must be "none".
must -> MUST
* 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:
why do this?
i gather some of the following are subordinate to 5?
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.
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.
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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.
fail"ing"?
I think you mean DKIM signature validation failure.
_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 incoming message fails
authentication checks (potentially including ARC validation), 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).
Given this information, handlers can inform local policy decisions
regarding disposition of messages that experience authentication
failure due to intermediate processing.
5.3 isn't really a results specification. It seems more like a useful
summary of the tool's value proposition.
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6. Communication of Validation Results
Chain Validation Status (described in Section 4.4) is communicated
via Authentication-Results (and AAR) headers 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.client-ip - The connecting client IP address from which the
message is received.
this seems such a large privacy concern, I question allowing it here.
(This highlights the difference between passing information inside an
enterprise, vs. over the open Internet, across administrations.)
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.
1. Don't capitalize GC
2. Each of these summary statements is better put much earlier.
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.
to the next ARC-participating ADMD...
If ARC-enabled ADMDs are trusted, Authenticated Received Chains can
be used to bridge administrative boundaries.
This use case really isn't a use case, IMO. Rather, it is a basic
observation about the nature and purpose of ARC.
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).
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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.
A message scanning service has a tight relationship with the receiving
ADMD. In fact they are arguably /part/ of the receiving ADMD, in terms
of trust among actors. Hence I don't understand they they need ARC.
They do all of the assessment work. Why can't they just attach a normal
auth-results field?
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.
Again, this is inside an enterprise's operation - a single ADMD - where
the trust of actors should be quite high. Why is ARC needed here, rather
than normal Auth-Results?
7.1.3. Mailing Lists
Mailing lists resend posted messages to subscribers. A full
re-send posted messages -> take delivery of messages and re-post them
'send' is not precise enough here.
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.
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.
This seems a highly redundant paragraph.
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).
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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
may -> MAY
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 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
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:
a local policy comment /where/? according to what specification for
such comments?
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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.
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. Large header blocks, in general, may cause failures to deliver
header /field/?
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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. 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
ARC authenticates the identity of some email handling actors. It
does not make any assessment of their trustworthiness.
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.
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.
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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 directive suggests a problem in clarity about document
relationship. This happens often in IETF specifications, where
documents mutually cross reference, rather than establishing a clear
hierarchical relationship. Very rarely, documents really are co-equal.
I think this is /not/ such a case.
My view is that rfc760bis has higher referential precedence and should
therefore be the place for the cited definition. The ARC document
should inherit that definition.
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] 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: [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]
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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
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].
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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.
+1
11.2. Failure Considerations
ARC should function without introducing significant new vectors for
abuse (see Section 9). If unforseen vectors are enabled by ARC, then
unforeseen
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.
/d
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
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