DMARC Working Group F. Martin, Ed.
Internet-Draft LinkedIn
Intended status: Informational E. Lear, Ed.
Expires: September 21, October 30, 2015 Cisco Systems GmbH
T. Draegen, Ed.
Eudaemon
Eudaemonic Development LLC
E. Zwicky, Ed.
Yahoo
March 20,
April 28, 2015
Interoperability Issues Between DMARC and Indirect Email Flows
draft-ietf-dmarc-interoperability-01
draft-ietf-dmarc-interoperability-02
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 do not flow directly from third party sources, are modified in transit,
or are forwarded enroute the author's administrative domain
to their the final destination. recipients. 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
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This Internet-Draft will expire on September 21, October 30, 2015.
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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 6
3.1. Message Handling System . . . . . . . . . . . . . . . . . 5 6
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 8
3.1.3. Message Delivery Agents . . . . . . . . . . . . . . . 8
3.2. Mediators . . . . . . . . . . . . . . . . . . . . . . . . 8 9
3.2.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 8 9
3.2.2. ReSenders . . . . . . . . . . . . . . . . . . . . . . 9 10
3.2.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . 10
3.2.4. Gateways . . . . . . . . . . . . . . . . . . . . . . 10 11
3.2.5. Boundary Filters . . . . . . . . . . . . . . . . . . 11
3.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 11 12
4. Possible Solutions to Mitigations of Interoperability Issues . . . . . . . . 12
4.1. Identifier Alignment . . . . . . Mitigations in Current Use . . . . . . . . . . . . 12
4.2. Message Modification . . . 13
4.1.1. Mitigations for Senders . . . . . . . . . . . . . . . 13
4.3. Message Forwarding . . . .
4.1.1.1. Identifier Alignment . . . . . . . . . . . . . . 13
4.1.1.2. Message Modification . 14
4.3.1. Original-Authentication-Results . . . . . . . . . . . 14
4.4. Message Handling Services . . 13
4.1.2. Mitigations for Receivers . . . . . . . . . . . . . . 14
4.4.1. Message Transfer Agents .
4.1.2.1. Identifier Alignment . . . . . . . . . . . . . . 14
4.4.1.1. Encoding . . .
4.1.2.2. Policy Override . . . . . . . . . . . . . . . . . 14
4.4.1.2. Filters
4.1.2.3. Email Address Internationalization . . . . . . . 14
4.1.3. Mitigations for ReSenders . . . . . . . . . . . . . . 14
4.4.1.3. Email Address Internationalization . .
4.1.3.1. Changes to the RFC5322.From . . . . . 15
4.5. Mediators . . . . . . 14
4.1.3.2. Avoiding Message Modification . . . . . . . . . . 15
4.1.3.3. Mailing Lists . . . . . . . . 15
4.5.1. Mailing Lists . . . . . . . . . . 15
4.2. Proposed and In-Progress Mitigations . . . . . . . . . . 15
4.6. 16
4.2.1. Getting More Radical: Requiring New Communication
Paths Between MUA and the Message Store MUAs . . . . . . . . . . . . 16 . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 17
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 17 18
8.2. Informative References . . . . . . . . . . . . . . . . . 18 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 20
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
Cases in which mail email does not go flow directly from the author's
administrative domain to the recipients are known collectively referred to
in this document as indirect email flows.
The next section describes interoperability issues between DMARC Due to existing and
increasing adoption of DMARC, the impact of DMARC-based email
rejection policies on both direct and indirect email flows. These flows can be
significant.
Several known causes of interoperability 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 presented,
followed by a description of components within the Internet Mail
Architecture [RFC5598]. [RFC5598] where interoperability issues can arise.
Lastly, known and 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
Interoperability issues between DMARC
introduces interoperability issues or problems, and indirect email flows arise
when conformance to the DMARC specification leads an implementation
to reject a message apply DMARC based policy to messages that is are both compliant with
the architecture as specified in [RFC5598] and would have been viewed as legitimate in the eyes of
by the intended recipient. Therefore, we can already conclude that DMARC
poses no interoperability problems when To be clear, this document does not
address emails considered legitimate messages properly
validate through its specified processes. by the intended recipient but
which are not conformant to other email specifications. The rest of
this section
delves into how legitimate messages may get rejected. describes several conceptual causes of interoperability
issues.
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 relies on DKIM [RFC6376] and SPF [RFC7208]) takes a
different approach. Therefore, the [RFC7208] to perform message
source validation. The DMARC [RFC7489] mechanism
attempts refers to source
domains that are validated by DKIM or SPF as Authenticated
Identifiers. DMARC requires an Authenticated Identifier to be
relevant to predictably specify the domain of found in the originator that
will be used for its purposes (reporting/message disposition). message's RFC5322.From header
field [RFC5322]. This
step relevancy is referred to as Identifier
Alignment.
Identifier Alignment can be strict, where the domains exactly match
each others, or relaxed where the domains are part of the same
Organizational Domain. There are, in general, the same
interoperability issues between strict and relaxed alignment, however
in strict mode the possible solutions are more constrained when
possible. This Document mainly implies relaxed 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 of a valid
signature must be part of the same Organizational Domain as the
domain in the RFC5322.From header field [RFC5322], and the signature must be valid. [RFC5322].
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 implementations that cannot process multiple DKIM signatures
it
may lead erroneously apply DMARC based policy to perfectly valid messages being flagged as not
authentic. otherwise legitimate
messages.
SPF can provides two Authenticated Identifiers the first one is
RFC7208.HELO [RFC7208] based on RFC5321.HELO/EHLO two different
SPF identities: RFC7208.HELO [RFC7208] 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
[RFC7208]. DMARC often uses only use the RFC7208.MAILFROM identifier. Again, identifier for an
alignment. The SPF identifier to align
in DMARC, the validated domain in RFC7208.MAILFROM must be part
of the same Organizational Domain as the domain in the RFC5322.From
header field.
Even field to be aligned. It is important to note that 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. RFC7208.MAILFROM,
furthermore, RFC7208.MAILFROM definition is different from
RFC5321.MailFrom [RFC5321] definition.
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 email 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 RFC7208.MAILFROM, must be aligned with that of the
RFC5322.From header field. field:
o If the RFC5321.MailFrom is not empty and 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
email sending infrastructure. If On another hand, if the forwarder
uses its own domain in the
RFC5321.MailFrom and/or RFC5321.HELO/EHLO, RFC5321.MailFrom, SPF passes but the
alignment with the RFC5322.From header field fails.
o If the RFC5321.MailFrom is empty, the RFC5321.Helo of the
forwarder will likely be in different organizational domain of the
RFC5322.From. SPF may pass 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 signatures, and
many message forwarding systems modify email content. Mailing list
processors are the most common example of such systems, but other
forwarding systems also make modifications. Although 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. seldom found in
emails in part because of its security challenges.
DKIM has two canonicalizations: canonicalizations to use for headers and body
separately: 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 is more computing
intensive and may not have been preferred in the early deployment of DKIM.
DKIM as this may have been more significant than today.
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
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 establishing alignment
with 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.
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 modify the message encoding, for instance by converting
8-bit mail MIME sections to quoted-printable 7-bit sections. This
modification 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 This correction is outside the scope of DKIM
canonicalization and will invalidate DKIM signatures. This
correction is also outside the scope of this document in providing
solutions for non RFC compliant emails.
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 may result in an invalid domain in the
RFC5322.From header field. impossibility of
finding a domain with a DMARC policy associated with it. If the
receiving MTA pays attention to the validity and reputation of
domains, this may present its own set of delivery problems. For
instance an MTA may refuse emails with no valid or emailable domain
in the RFC5322.From as to avoid simple workarounds against DMARC.
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 only 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
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 Recipient(s) 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.
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
These changes are common for many mailing lists and receivers are
used to them. Furthermore MUA expects certain mailing list behavior
in presenting emails to the end users
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.
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 company's
domains by rewriting messages to use the domain of the acquiring
company. Since the To: RFC5322.To header field 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.
Issues may arise if SPF and DKIM is evaluated after the filter
modifications.
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.
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 Mitigations of 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 of
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. Mitigations in Current Use
At many places where DMARC is already deployed, mitigations are in
use. These mitigations vary in their effectiveness and side effects,
but have the advantage that they are currently available.
4.1.1. Mitigations for Senders
4.1.1.1. 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
RFC5321.HELO/EHLO to RFC5322.From, particularly when sending
bounce messages. Adjusting dynamically the RFC5321.HELO based on
the RFC5322.From may not be possible for some MTA software.
o MTAs may choose to DKIM sign bounces with an aligned domain 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.
Handling DKIM keys with a third party has its security challenges.
o ReSenders Senders who are sending on behalf of users in other Administrative
Domains may choose to change use an RFC5322.From to one under the
ReSender's control, avoiding alignment issues with sender's
control. The new From can be either a forwarding address in a
domain controlled by the original.
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 Sender, or a placeholder address, with
the control of original user's address in a RFC5322.Reply-to header field.
However this may affect what the domain owner.
4.2. recipient expects in its MUA.
4.1.1.2. 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 In order to add email headers instead minimize the chances of modifying
existing headers or bodies.
o Forwarders transport conversions, Senders
can minimize convert the circumstances in which they choose message to
fix messages, preferring preserving non-compliant headers a suitable MIME content-transfer
encoding such as quoted-printable or base64 before signing
([RFC6376] Section 5.3).
4.1.2. Mitigations for Receivers
4.1.2.1. Identifier Alignment
o Receivers should update DKIM handling libraries to
creating ensure that
they process all valid DKIM failures. signatures and check them for
alignment.
4.1.2.2. Policy Override
o Forwarders Receivers can choose amalgamate data from their user base to reject messages with suspect or harmful
content instead of modifying them.
o If message modification is required, the RFC5322.From identify
forwarders and use such list for a DMARC local policy override.
This process may be
changed.
Proposed and in-progress work-arounds easier for large receivers, where there is
data and fixes resources to message
modification issues: create such lists, than for small receivers.
4.1.2.3. Email Address Internationalization
o DKIM with constrained transformations,
[I-D.kucherawy-dkim-list-canon]
4.3. Message Forwarding
Forwarding messages without modification is referred During the transition from email systems that do not allow EAI
(SMTPUTF8) to as
"transparent forwarding", and email systems that allow it, [RFC6854] allows the
MUA or MSA to use the group syntax for the RFC5322.From header
field to avoid a known MTA to reject the message (if RFC5322 is
implemented strictly). While this will alleviate the EAI problem,
it will allow a way simple DMARC workaround since the message may no
longer have a single usable domain in the RFC5322.From header
field, making DMARC inapplicable. DMARC implementations that pay
attention to preserve the validity of
DKIM signatures.
Currently used work-arounds and fixes the domains in the RFC5322.From may
then have to message forwarding issues:
o Senders should use DKIM signing choose between EAI transitioning compliance
(accepting the email without a domain in RFC5322.From) and
security (rejecting RFC-compliant email for lack of a domain in
the RFC5322.From). The real solution is to allow transparent forwarding upgrade MTAs to
succeed.
o
support EAI (SMTPUTF8), avoiding the transition issue.
4.1.3. Mitigations for ReSenders may choose
4.1.3.1. Changes to change the RFC5322.From
Many ReSender issues can be avoided by using an RFC5322.From to one under
the ReSender's control, avoiding instead of the initial RFC5322.From. This
will correct identifier alignment issues with and allow arbitrary message
modification, for instance. When ReSenders change the RFC5322.From,
it is desirable to preserve the information about the original
initiator of the original. message. The Original-From [RFC5703] (or X-Original-From) X-
Original-From) header field is used for this purpose in various
contexts (X- header fields name are discouraged by [RFC6648]).
Note that
However, handling of 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.
4.4.1.3. Email Address Internationalization
During the transition from email systems that do X-Original-From) is not allow EAI
(SMTPUTF8)
defined anywhere. It is not currently used consistently or displayed
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 user, but in any situation where it is implemented strictly). Allowing the group
syntax used, it is at the appreciation of the postmaster, that will always a new
unauthenticated identifier available for exploitation.
4.1.3.2. Avoiding Message Modification
o Forwarders can choose the solution best to add email header fields instead of
modifying existing headers or bodies, for its user, but really instance to avoid DMARC not
finding indicate a single useable domain in the RFC5322.From header field,
message may be spam.
o Forwarders can minimize the
real solution is circumstances in which they choose to upgrade your MTAs,
fix messages, preferring to support EAI (SMTPUTF8). In
that case a sending SMTPUTF8 MTA does not need preserve non-compliant headers to require a downgrade
of the message
creating DKIM failures.
o Forwarders can choose to ASCII identifiers. Encouraging, by rejection reject messages with suspect or
reputation scoring, the presence harmful
content instead of a domain in the RFC5322.From
header field is easier.
4.5. Mediators
4.5.1. modifying them.
4.1.3.3. Mailing Lists
[RFC6377] provides some guidance on using DKIM with Mailing lists.
Here are some other remediations remediation 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 modifications may to some extent
defeats
defeat the purpose of DMARC itself. It may make it difficult to
ensure that users of all email clients can easily reply to the
author, the list, or all using the email client features provided
for that purpose. Use of "Reply-To" RFC5322.Reply-To header field can
alleviate this problem depending
if on whether the mailing list is
configured to reply-to-list, reply-to-
author reply-to-author or reply-to-fixed-
address, however it is important to note that this header field
can take multiple email addresses. When altering the RFC5322.From
there are two possibilities, to change it to put the mailing list
email address, or reply-to-fixed-address. to change it to add a suffix like ".invalid" to
the domain of the email address present there. The later
modification may create issues because it is an invalid domain
name, and some MTAs may take particular attention to the validity
of email addresses in RFC5322.From and the reputation of the
domains present there.
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 MLM to "wrap"
the message in a MIME message/rfc822 part and to send as a message. the
Mailing List email address. Many email clients (as of August
2014) have difficulty reading such messages.
o Finally a A, now, less common mitigation policy, is to configure the MLM to
not modify the message so that the DKIM signature remains valid.
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. [RFC3463] specifies Enhanced
Mail System Status Codes which help differentiate between various
bounces. Correctly interpreting Extended SMTP error messages is
useful in this case ([RFC7372]). in particular codes defined in [RFC7372] and
in DMARC.
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.
4.2. Proposed and In-Progress Mitigations
The following mitigations are based on Internet Drafts which have not
yet received broad consensus. They are described here to offer
exploratory path for solutions. These solutions should not be used
in a production environment.
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.
o DKIM with constrained transformations,
[I-D.kucherawy-dkim-list-canon] is proposed to allow message
modification.
o DKIM with recorded transformations, [I-D.kucherawy-dkim-transform]
is proposed to indicate what limited transformations were done to
the message so that a receiver could reverse them and confirm the
validity of the orignal DKIM signature.
o [I-D.kucherawy-original-authres] is intended as a way to pass
along Original Authentication Results to "downstream" receivers.
It is not widely implemented and relies on a trust relationship
between the forwarder and the other receivers.
o [I-D.levine-dkim-conditional] could be used to have the sender add
a limited DKIM signature, that signs only a very limited set of
header fields and not the body of the message. This DKIM
signature would come with the condition that a subsequent known
domain fully DKIM sign the message. For instance a Mailing List
could transform the message, add its DKIM signature and there
would be a valid DKIM signature aligned with the RFC5322.From that
would satisfy DMARC while limiting the possibilities of replay
attack.
4.2.1. Getting More Radical: Requiring New Communication Paths Between
MUA and the Message Store
MUAs
In practice a number of operators are using strict alignement alignment mode in
DMARC in order to avoid receiving new and innovative forms of
unwanted and unauthentic mail email 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. some form of transitive trust.
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 Draegen and Franck Martin contributed to the
IETF DMARC Working Group's wiki page listing all known
interoperability issues with DMARC and indirect mail email flows.
Tim Draegen created the first draft of this document from these
contributions and by carefully hamfistedly 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.
[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-dkim-transform]
Kucherawy, M., "Recognized Transformations of Messages
Bearing DomainKeys Identified Mail (DKIM) Signatures",
draft-kucherawy-dkim-transform-00 (work in progress),
April 2015.
[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
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
Eudaemonic Development LLC
PO Box 19443
Asheville, NC 28815
USA
Email: tim@eudaemon.net tim@eudev.net
Elizabeth Zwicky (editor)
Yahoo
Sunnyvale, CA
USA
Email: zwicky@yahoo-inc.com