Email Authentication for Internationalized Mail
RFC 8616

Internet Engineering Task Force (IETF)                         J. Levine
Request for Comments: 8616                          Taughannock Networks
Updates: 6376, 7208, 7489                                      June 2019
Category: Standards Track
ISSN: 2070-1721

            Email Authentication for Internationalized Mail

Abstract

   Sender Policy Framework (SPF) (RFC 7208), DomainKeys Identified Mail
   (DKIM) (RFC 6376), and Domain-based Message Authentication,
   Reporting, and Conformance (DMARC) (RFC 7489) enable a domain owner
   to publish email authentication and policy information in the DNS.
   In internationalized email, 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8616.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  General Principles  . . . . . . . . . . . . . . . . . . . . .   3
   4.  SPF and Internationalized Mail  . . . . . . . . . . . . . . .   3
   5.  DKIM and Internationalized Mail . . . . . . . . . . . . . . .   3
   6.  DMARC and Internationalized Mail  . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   SPF [RFC7208], DKIM [RFC6376], and DMARC [RFC7489] enable a domain
   owner to publish email 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 email messages, with the validation keys published in
   the DNS.  DMARC publishes policy information related to the domain in
   the From: header field of email messages.

   In conventional email, 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, SMTP sessions, and the DNS.

   Internationalized mail [RFC6530] (generally called "EAI" for Email
   Address Internationalization) allows U-labels in SMTP sessions
   [RFC6531] and 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 code
   for mail senders and receivers interoperates.

   Internationalized mail also allows UTF-8-encoded Unicode characters
   in the local parts of mailbox names, which were historically only
   ASCII.

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.

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   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 labels in the name used for
   the original DNS lookup, described in Section 3 of [RFC7208], and
   those used in 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
   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.

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   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 [RFC5234] 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 message 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 email 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 of [RFC6376], 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 uppercase to
   lowercase.  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 of [RFC6376], 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 an RFC5322.From header
   field.

   Section 6.6.1 of [RFC7489] 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 of [RFC7489] is

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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 Sections 6.3 and 7.1 of [RFC7489],
   can contain email 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 has no IANA actions.

8.  Security Considerations

   Email 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 to work
   as reliably on internationalized mail as they do on ASCII mail, so
   that applications that use them, such as some kinds of mail filters
   that catch spam and phish, 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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [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>.

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   [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>.

Author's Address

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886
   United States of America

   Email: standards@taugh.com
   URI:   http://jl.ly

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