DMARC Working Group F. Martin, Ed.
Internet-Draft LinkedIn
Intended status: Informational E. Lear, Ed.
Expires: September 21, 2015 Cisco Systems GmbH
T. Draegen, Ed.
Eudaemon
E. Zwicky, Ed.
Yahoo
March 20, 2015
Interoperability Issues Between DMARC and Indirect Email Flows
draft-ietf-dmarc-interoperability-01
Abstract
DMARC introduces a mechanism for expressing domain-level policies and
preferences for email message validation, disposition, and reporting.
The DMARC mechanism can encounter interoperability issues when
messages originate from third party sources, are modified in transit,
or are forwarded enroute to their final destination. Collectively
these email flows are referred to as indirect email flows. This
document describes interoperability issues between DMARC and indirect
email flows. Possible methods for addressing interoperability issues
are presented.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 21, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document Conventions . . . . . . . . . . . . . . . . . . 3
2. Causes of Interoperability Issues . . . . . . . . . . . . . . 3
2.1. Identifier Alignment . . . . . . . . . . . . . . . . . . 4
2.2. Message Forwarding . . . . . . . . . . . . . . . . . . . 5
2.3. Message Modification . . . . . . . . . . . . . . . . . . 5
3. Internet Mail Architecture, DMARC, and Indirect Email Flows . 5
3.1. Message Handling System . . . . . . . . . . . . . . . . . 5
3.1.1. Message Submission Agents . . . . . . . . . . . . . . 6
3.1.2. Message Transfer Agents . . . . . . . . . . . . . . . 7
3.1.2.1. Message Encoding . . . . . . . . . . . . . . . . 7
3.1.2.2. Header Standardization . . . . . . . . . . . . . 7
3.1.2.3. Email Address Internationalization . . . . . . . 7
3.1.3. Message Delivery Agents . . . . . . . . . . . . . . . 8
3.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.2. ReSenders . . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . 10
3.2.4. Gateways . . . . . . . . . . . . . . . . . . . . . . 10
3.2.5. Boundary Filters . . . . . . . . . . . . . . . . . . 11
3.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 11
4. Possible Solutions to Interoperability Issues . . . . . . . . 12
4.1. Identifier Alignment . . . . . . . . . . . . . . . . . . 12
4.2. Message Modification . . . . . . . . . . . . . . . . . . 13
4.3. Message Forwarding . . . . . . . . . . . . . . . . . . . 14
4.3.1. Original-Authentication-Results . . . . . . . . . . . 14
4.4. Message Handling Services . . . . . . . . . . . . . . . . 14
4.4.1. Message Transfer Agents . . . . . . . . . . . . . . . 14
4.4.1.1. Encoding . . . . . . . . . . . . . . . . . . . . 14
4.4.1.2. Filters . . . . . . . . . . . . . . . . . . . . . 14
4.4.1.3. Email Address Internationalization . . . . . . . 15
4.5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 15
4.5.1. Mailing Lists . . . . . . . . . . . . . . . . . . . . 15
4.6. Getting More Radical: Requiring New Communication Paths
Between MUA and the Message Store . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
DMARC [RFC7489] introduces a mechanism for expressing domain-level
policies and preferences for message validation, disposition, and
reporting. DMARC is used to combat exact-domain phishing, to gain
visibility into email infrastructure, and to provide email egress
controls. Due to wide adoption, the impact of DMARC-based email
rejection policies on both direct and indirect email flows can be
significant.
The DMARC mechanism can encounter several different types of
interoperability issues due to third-party message sourcing, message
transformation or rerouting. These cases in which mail does not go
directly from the author's administrative domain to the recipients
are known collectively as indirect email flows.
The next section describes interoperability issues between DMARC and
indirect email flows. These issues are first described in the
context of configuration behavior that DMARC requires from underlying
authentication technology, and then described as they appear in
context of the Internet Mail Architecture [RFC5598].
Lastly, possible methods for addressing interoperability issues are
presented. There are often multiple ways to address any given
interoperability issue. While this document strives to be
comprehensive in its review, it should not be treated as complete.
1.1. Document Conventions
Notation regarding structured fields is taken from [RFC5598].
Organizational Domain and Authenticated Identifiers are specified in
DMARC [RFC7489].
2. Causes of Interoperability Issues
What do we mean by "interoperability issues"? We say that DMARC
introduces interoperability issues or problems, when conformance to
the DMARC specification leads an implementation to reject a message
that is both compliant with the architecture as specified in
[RFC5598] and would have been viewed as legitimate in the eyes of the
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intended recipient. Therefore, we can already conclude that DMARC
poses no interoperability problems when legitimate messages properly
validate through its specified processes. The rest of this section
delves into how legitimate messages may get rejected.
2.1. Identifier Alignment
A fundamental aspect of message source validation is understanding
what defines the source that is validated. Each of the underlying
mechanisms that DMARC uses (DKIM [RFC6376] and SPF [RFC7208]) takes a
different approach. Therefore, the DMARC [RFC7489] mechanism
attempts to predictably specify the domain of the originator that
will be used for its purposes (reporting/message disposition). This
step is referred to as Identifier Alignment.
DKIM provides a cryptographic means for a domain to be associated
with a particular message. DKIM does not make any constraints on
what domains may or must present this association. However, for a
DKIM identifier to align in DMARC, the signing domain must be part of
the same Organizational Domain as the domain in the RFC5322.From
header field [RFC5322], and the signature must be valid.
In addition, DKIM allows for the possibility of multiple valid
signatures. The DMARC mechanism will process Authenticated
Identifiers that are based on DKIM signatures until an aligned
Authenticated Identifier is found (if any). However, operational
experience has shown that some implementations have difficulty
processing multiple signatures. The impact on DMARC processing is
clear: if an implementation cannot process multiple DKIM signatures
it may lead to perfectly valid messages being flagged as not
authentic.
SPF provides two Authenticated Identifiers the first one is
RFC7208.HELO [RFC7208] based on RFC5321.HELO/EHLO and the second one
is RFC7208.MAILFROM [RFC7208] based on the RFC5321.MailFrom
[RFC5321] domain or, if the RFC5321.MailFrom address is absent (as in
the case of "bounces"), on the domain found in the HELO/EHLO SMTP
command. Local policies, as well as DMARC often only use the
RFC7208.MAILFROM identifier. Again, for an SPF identifier to align
in DMARC, the validated domain must be part of the same
Organizational Domain as the domain in the RFC5322.From header field.
Even when an SPF record exists for the domain in RFC5322.From, SPF
will not authenticate it unless it is also the domain SPF checks.
While aligning RFC5322.From and RFC5321.MailFrom is usually possible,
it can be difficult to change the domain in the HELO/EHLO used for
bounces to the domain in the RFC5322.From header field, especially
when several mail streams share the same sending IP address.
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2.2. Message Forwarding
Message forwarding is a generic concept encapsulating a variety of
behaviors. Section 3 describes forwarding behavior as it relates to
the components of the Internet Mail Architecture.
All of these behaviors involve mail being retransmitted by a new SMTP
server. As discussed above, for SPF to cause a DMARC pass, the
domain of the RFC5321.MailFrom or RFC5321.HELO/EHLO must be aligned
with that of the RFC5322.From header field. If the forwarder keeps
the RFC5321.MailFrom, the SPF validation will fail altogether unless
the forwarder is an authorized part of the originator's mail sending
infrastructure. If the forwarder uses its own domain in the
RFC5321.MailFrom and/or RFC5321.HELO/EHLO, SPF passes but the
alignment with the RFC5322.From header field fails. In either case,
SPF cannot produce a DMARC pass, and DKIM will be required to get
DMARC to pass.
2.3. Message Modification
Modification of email content invalidates most DKIM signatures. For
instance while DKIM provides a length flag so that content can be
appended (See Section 8.2 of RFC6376 [RFC6376] for additional
security considerations), in practice, particularly with MIME-encoded
[RFC2045] messages, a mailing list processor will do more than append
(See Section 5.3 of [RFC5598] for details). Even forwarding systems
make content modifications. Furthermore, the use of the length flag
is by no means universal.
DKIM has two canonicalizations: simple and relaxed. The latter
allows some modest in transit modifications that do not change the
interpretation of the content of the email. The relaxed
canonicalization used to be computing intensive and may not have been
preferred in the early deployment of DKIM.
3. Internet Mail Architecture, DMARC, and Indirect Email Flows
This section describes components within the Internet Mail
Architecture [RFC5598] where interoperability issues between DMARC
and indirect email flows can be found.
3.1. Message Handling System
Section 4 of [RFC5598] describes six basic components that make up
the Message Handling System (MHS):
o Message
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o Message User Agent (MUA)
o Message Submission Agent (MSA)
o Message Transfer Agent (MTA)
o Message Delivery Agent (MDA)
o Message Store (MS)
Of these components MSA, MTA, and MDA are discussed in relation to
interoperability with DMARC.
A Mediator is a special class of MUA that is given special
consideration in this section due to the unique issues Mediators face
when attempting to interoperate with DMARC.
3.1.1. Message Submission Agents
An MSA accepts messages submitted by a Message User Agent (MUA) and
enforces the policies of the hosting ADministrative Management Domain
(ADMD) and the requirements of Internet standards.
MSAs are split into two sub-components:
o Author-focused MSA functions (aMSA)
o MHS-focused MSA functions (hMSA)
MSA interoperability issues with DMARC begin when an aMSA accepts a
message where the RFC5322.From header field contains a domain that is
outside of the ADMD of the MSA. The ADMD will almost certainly not
be capable of sending email that yields Authenticated Identifiers
aligned with the domain found in the RFC5322.From header field.
Examples of this issue include "forward-to-friend" functionality
commonly found on news/article websites or "send-as" functionality
present on some MUAs.
When an hMSA takes responsibility for transit of a message containing
a domain in the RFC5322.From header field that is outside of the
hMSA's ADMD, the hMSA faces DMARC interoperability issues if the
domain publishes a DMARC policy of "quarantine" or "reject". These
issues are marked by an inherent difficulty in modifying the domain
present in a message's RFC5322.From header field. Examples of this
issue include:
o Pseudo-open relays - a residential ISP that allows its customers
to relay any domains through its infrastructure.
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o Embedded devices - cable/dsl modems, firewalls, wireless access
points that send email using hardcoded domains.
o Email service providers - ESPs that service customers that are
using domains that publish a DMARC "reject" policy.
o Calendaring software - an invited member of an event modifies the
event causing calendaring software to emit an update that appears
to come from the creator of the event.
3.1.2. Message Transfer Agents
MTAs relay a message until the message reaches a destination MDA.
3.1.2.1. Message Encoding
An MTA may change the message encoding, for instance by converting
8-bit mail sections to quoted-printable 7-bit sections. This is
outside the scope of DKIM canonicalization and will invalidate DKIM
signatures that include message content.
3.1.2.2. Header Standardization
An MTA may standardize headers, usually in order to make non-RFC
compliant headers properly compliant. For instance, some common MTAs
will correct comprehensible but non-compliant date formats to
compliant ones. Again, this is outside the scope of DKIM
canonicalization and will invalidate DKIM signatures.
3.1.2.3. Email Address Internationalization
A DMARC interoperability issue arises in the context of Email Address
Internationalization [RFC6530]. [RFC6854] allows group syntax in the
RFC5322.From header field during the transition period to SMTPUTF8.
If an EAI/SMTPUTF8-aware MTA needs to transmit a message to a non-
aware MTA, the EAI/SMTPUTF8-aware system may transform the
RFC5322.From header field of the message to include group syntax to
allow the non-aware MTA to receive the email.
This transformation will modify the original content of the message
and may invalidate any DKIM signatures if the transformation is not
done by the MSA or MUA. In addition, group syntax will remove the
ability for the DMARC mechanism to find an Organizational Domain that
aligns with any authenticated domain identifier from SPF or DKIM.
In addition, the group syntax will result in an invalid domain in the
RFC5322.From header field. If the receiving MTA pays attention to
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the validity and reputation of domains, this may present its own set
of delivery problems.
3.1.3. Message Delivery Agents
The MDA transfers a message from the MHS to a mailbox. Like the MSA,
the MDA consists of two sub-components:
o MHS-focused MDA functions (hMDA)
o Recipient-focused MDA functions (rMDA)
Both the hMDA and the rMDA can redirect a message to an alternative
address. DMARC interoperability issues related to redirecting of
messages are described in Section 3.2.
SIEVE [RFC5228] functionality often lives in the rMDA sub-component
and can cause DMARC interoperability issues. The SIEVE 'addheader'
and 'deleteheader' filtering actions can modify messages and
invalidate DKIM signatures, removing DKIM-supplied Authenticated
Identifiers as inputs to the DMARC mechanism. There are also SIEVE
extensions that modify the body. SIEVE may become an issue when the
email is reintroduced in the transport infrastructure.
3.2. Mediators
See [RFC5598] for a complete definition of Mediators.
Mediators forward messages through a re-posting process. Mediators
share some functionality with basic MTA relaying, but have greater
flexibility in both addressing and content modifications.
DMARC interoperability issues are prevalent within the context of
Mediators, which are often used precisely for their ability to modify
messages.
3.2.1. Alias
An Alias is a simple re-addressing facility that provides one or more
new Internet Mail addresses, rather than a single, internal one. A
message continues through the transfer service for delivery to one or
more alternative addresses.
Aliases can be implemented by mailbox-level forwarding (e.g. through
"dot-forwarding") or SIEVE-level forwarding (through the SIEVE
'redirect' action) or other methods. When an Alias preserves message
content and does not make significant header changes, DKIM signatures
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may remain valid. However, Aliases often extend the delivery path
beyond SPF's ability to grant authorization.
Examples of Aliasing include:
o Forwarding email between freemail providers to try different
interfaces while maintaining an original email address.
o Consolidating many email addresses into a single acccount to
centralize processing.
o Services that provides "activity based", "role based" , "vanity"
or "temporary" email addresses such as universities and
professional associations. For instance professional or alumni
institutions may offer to their members an alias for the duration
of their membership but may not want to deal with the long term
storage of emails.
In most cases, the aMSA providing Alias services has no
administrative relationship to the ADMD of the final recipient, so
solutions to Alias-related DMARC failure should not assume such a
relationship.
3.2.2. ReSenders
ReSenders "splice" a message's addressing information to connect the
Author of the original message with the Recipient of the new message.
The new Recipient sees the message as being from the original Author,
even if the Mediator adds commentary.
ReSenders introduce DMARC interoperability issues as content
modification invalidates DKIM signatures. SPF's ability to grant
authorization via alignment is removed as the new Recipient receives
the message from the Mediator.
Without an ability to produce Authenticated Identifiers relevant to
the Author's RFC5322.From header field domain using either DKIM or
SPF, the new Recipient has almost no chance of successfully applying
the DMARC mechanism.
Examples of ReSenders include MUA-level forwarding by resending a
message to a new recipient or by forwarding a message "inline" to a
new recipient (this does not include forwarding a message "as an
attachment"). An additional example comes in the form of calendaring
software that allows a meeting attendee (not the meeting organizer)
to modify the content of an invite causing the invitations to appear
to be reissued from the meeting organizer.
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3.2.3. Mailing Lists
A Mailing List receives messages as an explicit addressee and then
re-posts them to a list of subscribed members. The Mailing List
performs a task that can be viewed as an elaboration of the ReSender.
Mailing Lists share the same DMARC interoperability issues as
ReSenders (Section 3.2.2), and very commonly modify headers or
message content in ways that will cause DKIM to fail, including:
o prepending the RFC5322.Subject header field with a tag, to allow
the receiver to identify visually the mailing list.
o adding a footer to the email body to contain administrative
instructions.
o removing some MIME-parts from the email or converting the message
to text only.
o PGP-encrypting the body to the receiver's key.
o enforcing community standards by rewriting banned words.
o allowing moderators to add arbitrary commentary to messages.
Any such modifications would invalidate a DKIM signature.
Mailing Lists may also have the following DMARC interoperability
issues:
o Subscribed members may not receive email from members that post
using domains that publish a DMARC "p=reject" policy.
o Mailing Lists may interpret DMARC-related email rejection as an
inability to deliver email to the recipients that are checking and
enforcing DMARC policy. This processing may cause subscribed
members to be suspended or removed from the Mailing List.
[RFC3463] specifies Enhanced Mail System Status Codes which help
differentiate between various bounces. DMARC even defines
specific codes to be used.
3.2.4. Gateways
A Gateway performs the basic routing and transfer work of message
relaying, but it also is permitted to modify content, structure,
address, or attributes as needed to send the message into a messaging
environment that operates under different standards or potentially
incompatible policies.
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Gateways share the same DMARC interoperability issues as ReSenders
(Section 3.2.2).
Gateways may share also the same DMARC interoperability issues as
MTAs (Section 3.1.2).
Gateway-level forwarding can introduce DMARC interoperability issues
if the Gateway is configured to rewrite the message to map between
recipient domains. For example, an acquisition may lead the
acquiring company to decide to decommission the acquired companies
domains by rewriting messages to use the domain of the acquiring
company. Since the To: header is usually DKIM-signed, this kind of
rewriting will also cause DKIM signatures to fail.
3.2.5. Boundary Filters
To enforce security boundaries, organizations can subject messages to
analysis for conformance with their safety policies. A filter might
alter the content to render it safe, such as by removing content
deemed unacceptable.
Boundary Filters share the same DMARC interoperability issues as
ReSenders.
Examples of Boundary Filters include:
o Anti-spam: To keep its reputation, an MTA that transfers a message
may remove harmful content from messages that are likely to be
unwanted by the next MTA and/or add text in the body to indicate
the message has been scanned. Any such modifications would
invalidate a DKIM signature.
o Any service that reformulates the RFC5322.body for any other
reason, for instance adding an organizational disclaimer.
o Secondary MX services. In this case, however, it is inappropriate
for a primary MX server to perform an SPF check against its own
secondaries. Rather, the secondary MX should perform this
function.
3.3. Combinations
The causes of indirect email flows can be combined. For example, a
university student may subscribe to a mailing list (using his
university email address) while this university email address is
configured to forward all emails to a freemail provider where a more
permanent email address for this student exists.
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Within an organization the message may pass through various MTAs
(Section 3.1.2), each of which performs a different function
(authentication, filtering, distribution, etc.)
4. Possible Solutions to Interoperability Issues
Solutions to interoperability issues between DMARC and indirect email
flows vary widely in their scope and implications. They range from
improvements to underlying processors, such as proper handling
multiple DKIM signatures, to more radical approaches to the messaging
architecture. This section describes possible ways to address
interoperability issues.
Mail systems are diverse and widely deployed and are expected to
continue to work with old systems. For instance, Qmail is still used
and the base code has not been updated since 1998. Ezmlm, a once
popular mailing list manager, is still deployed and has not been
updated since 1997, although a new version, Ezmlm-idx exists. In
this constrained environment, some solutions may be time-consuming
and/or disruptive to implement.
DMARC provides for receivers to make decisions about identity
alignment acceptability based on information outside the DMARC
headers and communicate those decisions as "overrides" to the sender.
This facility can be used to ease some interoperability issues,
although care is needed to ensure that this does not create loopholes
that abusers can use arbitrarily.
4.1. Identifier Alignment
Currently used work-arounds and fixes to identifier alignment issues:
o MTAs handling multiple domains may choose to change
RFC5321.MailFrom to align with RFC5322.From to improve SPF
usability for DMARC.
o MTAs handling multiple domains may also choose to align HELO/EHLO
to RFC5322.From, particularly when sending bounce messages.
o MTAs may choose to DKIM sign bounces to allow DKIM-based DMARC
pass.
o MTAs handling multiple domains may require DMARC-using senders to
provide DKIM keys and use DKIM to avoid SPF alignment issues.
o ReSenders may choose to change RFC5322.From to one under the
ReSender's control, avoiding alignment issues with the original.
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o Receivers should update DKIM handling libraries to ensure that
they process all valid DKIM signatures and check them for
alignment.
Proposed and in-progress work-arounds and fixes to identifier
alignment issues:
o Third party authorization, [RFC6541], [I-D.otis-tpa-label] and
[I-D.kucherawy-dkim-delegate] provide ways to extend identifier
alignment under the control of the domain owner.
4.2. Message Modification
Message modification invalidates DKIM signatures and complicates a
receiver's ability to generate Authenticated Identifiers from a
message. Avoiding message modification wherever possible is
therefore desirable.
Currently used work-arounds and fixes to message modification issues:
o Senders can maximize survivability of DKIM signatures by limiting
the header fields they sign, using relaxed canonicalization and by
using length to allow appended signatures.
o Senders can also maximize survivability by starting with RFC-
compliant headers and common body formats.
o Forwarders can choose to add email headers instead of modifying
existing headers or bodies.
o Forwarders can minimize the circumstances in which they choose to
fix messages, preferring preserving non-compliant headers to
creating DKIM failures.
o Forwarders can choose to reject messages with suspect or harmful
content instead of modifying them.
o If message modification is required, the RFC5322.From may be
changed.
Proposed and in-progress work-arounds and fixes to message
modification issues:
o DKIM with constrained transformations,
[I-D.kucherawy-dkim-list-canon]
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4.3. Message Forwarding
Forwarding messages without modification is referred to as
"transparent forwarding", and is a way to preserve the validity of
DKIM signatures.
Currently used work-arounds and fixes to message forwarding issues:
o Senders should use DKIM signing to allow transparent forwarding to
succeed.
o ReSenders may choose to change RFC5322.From to one under the
ReSender's control, avoiding alignment issues with the original.
The Original-From [RFC5703] (or X-Original-From) header is used in
various contexts (X- header fields name are discouraged by
[RFC6648]).
Note that Original-From (or X-Original-From) is merely adding
complexity to the 'who was the author of this message' assessment,
very possibly creating yet-another security hole.
4.3.1. Original-Authentication-Results
[I-D.kucherawy-original-authres] has been mentioned in early DMARC
drafts as a way to pass along Original Authentication Results to
"downstream" receivers.
4.4. Message Handling Services
4.4.1. Message Transfer Agents
4.4.1.1. Encoding
There are few reasons to modify the encoding of the message,
compatibility issues between international character sets are few
nowadays. More mail systems supports 8bitMIME, therefore the need
for transport encoding changes are rarer. By default no modification
of the message should be done when simply forwarding the message.
4.4.1.2. Filters
Filters should not add to or modify the body of the message, but
either should reject the message or add new email headers (not under
DKIM) to indicate the result of the filter.
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4.4.1.3. Email Address Internationalization
During the transition from email systems that do not allow EAI
(SMTPUTF8) to email system that allows it, [RFC6854] allows using the
group syntax for the RFC5322.From header field rather than rejecting
the message (if RFC5322 is implemented strictly). Allowing the group
syntax is at the appreciation of the postmaster, that will always
choose the solution best for its user, but really to avoid DMARC not
finding a single useable domain in the RFC5322.From header field, the
real solution is to upgrade your MTAs, to support EAI (SMTPUTF8). In
that case a sending SMTPUTF8 MTA does not need to require a downgrade
of the message to ASCII identifiers. Encouraging, by rejection or
reputation scoring, the presence of a domain in the RFC5322.From
header field is easier.
4.5. Mediators
4.5.1. Mailing Lists
[RFC6377] provides some guidance on using DKIM with Mailing lists.
Here are some other remediations techniques:
o One common mitigation policy is to configure the Mailing List
Manager (MLM) to alter the RFC5322.From header field to use the
domain of the MLM. Since most list subscribers prefer to know the
identity of the author of the original message, typically this
information may be provided in the display name part of the
RFC5322.From header field. This display name needs to be
carefully crafted as to not collide with the original display name
of the author, nor contain something that looks like an email
address or domain name. These modification may to some extent
defeats the purpose of DMARC itself. It may make it difficult to
ensure that users of all email clients can easily reply to author,
list, or all using the email client features provided for that
purpose. Use of "Reply-To" can alleviate this problem depending
if the mailing list is configured to reply-to-list, reply-to-
author or reply-to-fixed-address.
o Another common mitigation policy is to configure the MLM to "wrap"
the message in a MIME message/rfc822 part. This completely
bypasses the DMARC policy in clients that allow reading the part
as a message. Many email clients (as of August 2014) have
difficulty reading such messages.
o Finally a less common mitigation policy, is to configure the MLM
to not modify the message so that the DKIM signature remains
valid.
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o To alleviate unsubscribes to the mailing list due to the messages
bouncing because of DMARC, the MLM needs to not act on bounces due
to Message Authentication issues. Correctly interpreting Extended
SMTP error messages is useful in this case ([RFC7372]).
All these techniques may provide some specific challenges in MUAs and
different operational usages for end users (like rewriting filters to
sort emails in folders). There will be some time before all
implications are understood and alleviated.
4.6. Getting More Radical: Requiring New Communication Paths Between
MUA and the Message Store
In practice a number of operators are using strict alignement mode in
DMARC in order to avoid receiving new and innovative forms of
unwanted and unauthentic mail through systems purporting to be
mailing list handlers. The receiving ADMD has no knowledge of which
lists the user has subscribed to and which they have not. One avenue
of exploration would be for the user to authorize mailing lists as
proxies for authentication, at which point the receiving ADMD would
be vesting some trust in the mailing list service. The creators of
DKIM foresaw precisely this possibility at the time by not tightly
binding any semantics to the RFC5322.From header field. Some
experimental work has taken place in this area, as mentioned above.
Additional work might examine a new communication path to the user to
authorize third party signatures.
5. IANA Considerations
This document contains no actions for IANA. [RFC Editor: Please
delete this section prior to publication.]
6. Security Considerations
This document is an analysis of DMARC's impact on indirect email
flows. It describes the possibility of accidental denial-of-service
that can be created by rejections of messages by DMARC-aware Mail
Receivers. However, it introduces no new security issues to Internet
messaging.
7. Acknowledgments
Miles Fidelman, John Levine, David Crocker, Stephen J. Turnbull,
Rolf E. Sonneveld, Tim Dragen and Franck Martin contributed to the
IETF DMARC Working Group's wiki page listing all known
interoperability issues with DMARC and indirect mail flows.
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Tim Draegen created the first draft of this document from these
contributions and by carefully mapping contributions into the
language of [RFC5598].
8. References
8.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
3463, January 2003.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[RFC5703] Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
Tests, Iteration, Extraction, Replacement, and Enclosure",
RFC 5703, October 2009.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
September 2011.
[RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
Mailing Lists", BCP 167, RFC 6377, September 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[RFC6541] Kucherawy, M., "DomainKeys Identified Mail (DKIM)
Authorized Third-Party Signatures", RFC 6541, February
2012.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
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[RFC6854] Leiba, B., "Update to Internet Message Format to Allow
Group Syntax in the "From:" and "Sender:" Header Fields",
RFC 6854, March 2013.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
April 2014.
[RFC7372] Kucherawy, M., "Email Authentication Status Codes", RFC
7372, September 2014.
8.2. Informative References
[I-D.kucherawy-dkim-delegate]
Kucherawy, M. and D. Crocker, "Delegating DKIM Signing
Authority", draft-kucherawy-dkim-delegate-01 (work in
progress), June 2014.
[I-D.kucherawy-dkim-list-canon]
Kucherawy, M., "A List-safe Canonicalization for
DomainKeys Identified Mail (DKIM)", draft-kucherawy-dkim-
list-canon-00 (work in progress), June 2014.
[I-D.kucherawy-original-authres]
Chew, M. and M. Kucherawy, "Original-Authentication-
Results Header Field", draft-kucherawy-original-authres-00
(work in progress), February 2012.
[I-D.levine-dkim-conditional]
Levine, J., "DKIM Conditional Signatures", draft-levine-
dkim-conditional-00 (work in progress), June 2014.
[I-D.otis-tpa-label]
Otis, D. and D. Black, "Third-Party Authorization Label",
draft-otis-tpa-label-00 (work in progress), May 2014.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message
Authentication, Reporting, and Conformance (DMARC)", RFC
7489, March 2015.
Authors' Addresses
Franck Martin (editor)
LinkedIn
Mountain View, CA
USA
Email: fmartin@linkedin.com
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Eliot Lear (editor)
Cisco Systems GmbH
Richtistrasse 7
Wallisellen, ZH CH-8304
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com
Tim Draegen (editor)
Eudaemon
NC
USA
Email: tim@eudaemon.net
Elizabeth Zwicky (editor)
Yahoo
Sunnyvale, CA
USA
Email: zwicky@yahoo-inc.com
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