Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track July 3, 2016
Expires: January 4, 2017
E-mail Authentication for Internationalized Mail
draft-levine-appsarea-eaiauth-01
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. This specification clarifies when to use which form of
domain names when using SPF, DKIM, and DMARC.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. SPF and internationalized mail . . . . . . . . . . . . . . . 3
4. DKIM and internationalized mail . . . . . . . . . . . . . . . 3
5. DMARC and internationalized mail . . . . . . . . . . . . . . 3
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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.
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The term IDN, for Internationalized Domain Name, refers to either a
U-label or an A-label.
Since DMARC is not currently a standards track protocol, this
specification offers advice rather than requirements for DMARC.
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3. 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
domain name argument MUST be represented as an A-label. An IDN
domain name in MAIL FROM can be either a U-label or an A-label.
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
internationalized or conventional mail.
4. 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 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 MAY be represented either as A-labels or U-labels. This
provides improved consistency with other headers, particularly since
the local-part of the i= tag is likely to be UTF-8 rather than ASCII.
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. When retrieving or processing a DKIM key
record from the DNS, all U-labels must first be converted into
A-labels before creating the domain name to be looked up.
DKIM key records, described in section 3.6.1, do not contain domain
names, so there is no change to their specification.
5. 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
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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 section 6.3, 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
have to be conventional addresses, not internationalized addresses.
6. IANA Considerations
This document makes no request of IANA.
7. 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.
8. 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,
<http://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,
<http://www.rfc-editor.org/info/rfc5890>.
[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,
<http://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, <http://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,
<http://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, <http://www.rfc-editor.org/info/rfc6532>.
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[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<http://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,
<http://www.rfc-editor.org/info/rfc7489>.
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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