Internet DRAFT - draft-levine-appsarea-eaiauth
draft-levine-appsarea-eaiauth
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376, 7208, 7489 (if approved) July 31, 2018
Intended status: Standards Track
Expires: February 1, 2019
E-mail Authentication for Internationalized Mail
draft-levine-appsarea-eaiauth-05
Abstract
SPF, DKIM, and DMARC 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. The Authentication-Results header reports the result of
authentication checks made with SPF, DKIM, DMARC, and other schemes.
This specification clarifies when to use which form of domain names
when using SPF, DKIM, and DMARC and when creating Authentication-
Results headers.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 1, 2019.
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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 . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Change history . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
SPF, DKIM, and DMARC 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 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 unencoded message bodies, in SMTP sessions, and in the
DNS. Internationalized mail [RFC6530] allows U-labels in SMTP
sessions [RFC6531] and in message headers [RFC6532].
Every U-label is equivalent to an A-label, so in principle the choice
of label format should not cause any 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 arbitrary UTF-8 strings in the
local parts of mailbox names, which were historically arbitrary
ASCII.
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2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" when
written in upper case in in this document are to be interpreted as
described in [RFC2119].
The term IDN, for Internationalized Domain Name, refers to a doman
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 now be U-labels, and mailbox local parts can be UTF-8.
Header 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 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
argument MUST be represented as an A-label. An IDN in MAIL FROM can
be either U-labels or an 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.
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5. DKIM and internationalized mail
DKIM [RFC6376] specifies a message header 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 internationalized messages so that non-
ASCII UTF-8 characters need not be quoted. The ABNF for dkim-safe-
char in internationalized messages is replaced by the following:
dkim-safe-char = %x21-3A / %x3C / %x3E-7E / %x80-FF
; '!' - ':', '<', '>' - '~', non-ASCII
Section 3.5 of [RFC6376] states that IDNs in the d=, i=, and s= tags
of a DKIM-Signature header MUST be encoded as A-labels. This rule is
relaxed only for headers in internationalized messages [RFC6532] so
IDNs SHOULD be represented as U-labels but MAY be A-labels. This
provides improved consistency with other headers. The set of
allowable characters in the local-part of an i= tag is extended as
described in [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.
DKIM key records, described in section 3.6.1, do not contain domain
names, so there is no change to their specification.
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.
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. Sections 6.7 and 7.1 are
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.
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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.
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>.
[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>.
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[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>.
Appendix A. Change history
04 to 05 Fix dkim-quoted-printable.
03 to 04 Take out Authentication-Results, now handled elsewhere.
02 to 03 SPF local-part macros don't match non-ASCII. Add S/MIME
auth results.
Author's Address
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
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