Internet DRAFT - draft-ietf-dmarc-eaiauth
draft-ietf-dmarc-eaiauth
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376, 7208, 7489 (if approved) April 11, 2019
Intended status: Standards Track
Expires: October 13, 2019
E-mail Authentication for Internationalized Mail
draft-ietf-dmarc-eaiauth-06
Abstract
SPF (RFC7208), DKIM (RFC6376), and DMARC (RFC7489) enable a domain
owner to publish e-mail authentication and policy information in the
DNS. In internationalized e-mail, domain names can occur both as
U-labels and A-labels. This specification updates the SPF, DKIM, and
DMARC specifications to clarify which form of internationalized
domain names to use in those specifications.
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 October 13, 2019.
Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General principles . . . . . . . . . . . . . . . . . . . . . 3
4. SPF and internationalized mail . . . . . . . . . . . . . . . 3
5. DKIM and internationalized mail . . . . . . . . . . . . . . . 4
6. DMARC and internationalized mail . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Change history . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
SPF [RFC7208], DKIM [RFC6376], and DMARC [RFC7489] enable a domain
owner to publish e-mail authentication and policy information in the
DNS. SPF primarily publishes information about what host addresses
are authorized to send mail for a domain. DKIM places cryptographic
signatures on e-mail messages, with the validation keys published in
the DNS. DMARC publishes policy information related to the domain in
the From: header field of e-mail messages.
In conventional e-mail, all domain names are ASCII in all contexts so
there is no question about the representation of the domain names.
All internationalized domain names are represented as A-labels
[RFC5890] in message header fields, in SMTP sessions, and in the DNS.
Internationalized mail [RFC6530], (generally called EAI for E-mail
Address Internationalization), allows U-labels in SMTP sessions
[RFC6531] and in message header fields [RFC6532].
Every U-label is equivalent to an A-label, so in principle the choice
of label format will not cause ambiguities. But in practice,
consistent use of label formats will make it more likely that mail
senders' and receivers' code interoperates.
Internationalized mail also allows UTF-8 encoded Unicode characters
in the local parts of mailbox names, which were historically only
ASCII.
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2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term IDN, for Internationalized Domain Name, refers to a domain
name containing either U-labels or A-labels.
Since DMARC is not currently a standards track protocol, this
specification offers advice rather than requirements for DMARC.
3. General principles
In headers in EAI mail messages, domain names that were restricted to
ASCII can be U-labels, and mailbox local parts can be UTF-8. Header
field names and other text intended primarily to be interpreted by
computers rather than read by people remains ASCII.
Strings stored in DNS records remain ASCII since there is no way to
tell whether a client retrieving a DNS record expects an EAI or an
ASCII result. When a domain name found in a mail header field
includes U-labels, those labels are translated to A-labels before
being looked up in the DNS, as described in [RFC5891].
4. SPF and internationalized mail
SPF [RFC7208] uses two identities from the SMTP session, the host
name in the EHLO command, and the domain in the address in the MAIL
FROM command. Since the EHLO command precedes the server response
that tells whether the server supports the SMTPUTF8 extension, an IDN
host name MUST be represented as A-labels. An IDN in MAIL FROM can
be either U-labels or A-labels.
All U-labels MUST be converted to A-labels before being used for an
SPF validation. This includes both the original DNS lookup,
described in Section 3 of [RFC7208] and the macro expansion of
domain-spec described in section 7. Section 4.3 of [RFC7208] states
that all IDNs in an SPF DNS record MUST be A-labels; this rule is
unchanged since any SPF record can be used to authorize either EAI or
conventional mail.
SPF macros %{s} and %{l} expand the local-part of the sender's
mailbox. If the local-part contains non-ASCII characters, terms that
include %{s} or %{l} do not match anything, because non-ASCII local
parts cannot be used as the DNS labels the macros are intended to
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match. Since these macros are rarely used, this is unlikely to be an
issue in practice.
5. DKIM and internationalized mail
DKIM [RFC6376] specifies a mail header field that contains a
cryptographic message signature and a DNS record that contains the
validation key.
Section 2.11 of [RFC6376] defines dkim-quoted-printable. Its
definition is modified in messages with internationalized header
fields so that non-ASCII UTF-8 characters need not be quoted. The
ABNF for dkim-safe-char in those messages is replaced by the
following, adding non-ASCII UTF-8 characters from [RFC3629]:
dkim-safe-char = %x21-3A / %x3C / %x3E-7E /
UTF8-2 / UTF8-3 / UTF8-4
; '!' - ':', '<', '>' - '~', non-ASCII
UTF8-2 = <Defined in Section 4 of RFC 3629>
UTF8-3 = <Defined in Section 4 of RFC 3629>
UTF8-4 = <Defined in Section 4 of RFC 3629>
Section 3.5 of [RFC6376] states that IDNs in the d=, i=, and s= tags
of a DKIM-Signature header field MUST be encoded as A-labels. This
rule is relaxed only for internationalized messages header fields
[RFC6532] so IDNs SHOULD be represented as U-labels. This provides
improved consistency with other header fields. (A-labels remain
valid to allow a transition from older software.) The set of
allowable characters in the local-part of an i= tag is extended in
the same fashion as local parts of e-mail addresses as described in
section 3.2 of [RFC6532]. When computing or verifying the hash in a
DKIM signature as described in section 3.7, the hash MUST use the
domain name in the format it occurs in the header field.
Section 3.4.2 of [RFC6376] describes relaxed header canonicalization.
Its first step converts all header field names from upper case to
lower case. Field names are restricted to printable ASCII (see
[RFC5322] section 3.6.8) so this case conversion remains ASCII case
conversion.
DKIM key records, described in section 3.6.1, do not contain domain
names, so there is no change to their specification.
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6. DMARC and internationalized mail
DMARC [RFC7489] defines a policy language that domain owners can
specify for the domain of the address in a RFC5322.From header field.
Section 6.6.1 specifies, somewhat imprecisely, how IDNs in the
RFC5322.From address domain are to be handled. That section is
updated to say that all U-labels in the domain are converted to
A-labels before further processing. Section 7.1 is similarly updated
to say that all U-labels in domains being handled are converted to
A-labels before further processing.
DMARC policy records, described in sections 6.3 and 7.1, can contain
e-mail addresses in the rua and ruf tags. Since a policy record can
be used for both internationalized and conventional mail, those
addresses still have to be conventional addresses, not
internationalized addresses.
7. IANA Considerations
This document makes no request of IANA.
8. Security Considerations
E-mail is subject to a vast range of threats and abuses. This
document attempts to slightly mitigate some of them but does not, as
far as the author knows, add any new ones. The updates to SPF, DKIM,
and DMARC are intended to allow the respective specifications work as
reliably on internationalized mail as they do on ASCII mail, so that
applications that use them, such as some kinds of spam and phish
filtering, can work more reliably on internationalized mail.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
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[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Appendix A. Change history
05 to 06 more editorial nits
04 to 05 editorial nits
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03 to 04 remove dangling A-R reference, add more i18nish and
security goodness
02 to 03 minor edits per Alexey
01 to 02 update references
00 to 01 Relaxed canon, Typos
00 First WG version
Author's Address
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Email: standards@taugh.com
URI: http://jl.ly
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