Email Authentication for Internationalized Mail
draft-ietf-dmarc-eaiauth-06

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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 13, 2019.

Copyright Notice

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

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   the Trust Legal Provisions and are provided without warranty as
   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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