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Internet Engineering Task Force (IETF)                         J. LevineRequest for Comments: 8616                          Taughannock NetworksUpdates:6376,7208,7489                                      June 2019Category: Standards TrackISSN: 2070-1721Email Authentication for Internationalized MailAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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.Levine                       Standards Track                    [Page 1]

RFC 8616                   EAI Authentication                  June 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .23.  General Principles  . . . . . . . . . . . . . . . . . . . . .34.  SPF and Internationalized Mail  . . . . . . . . . . . . . . .35.  DKIM and Internationalized Mail . . . . . . . . . . . . . . .36.  DMARC and Internationalized Mail  . . . . . . . . . . . . . .47.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .58.  Security Considerations . . . . . . . . . . . . . . . . . . .59.  Normative References  . . . . . . . . . . . . . . . . . . . .5   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .61.  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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.Levine                       Standards Track                    [Page 2]

RFC 8616                   EAI Authentication                  June 2019   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 inSection 3 of [RFC7208], and   those used in the macro expansion of domain-spec, described inSection 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.Levine                       Standards Track                    [Page 3]

RFC 8616                   EAI Authentication                  June 2019Section 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 inSection 4 of RFC 3629>   UTF8-3                = <Defined inSection 4 of RFC 3629>   UTF8-4                = <Defined inSection 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 inSection 3.2 of [RFC6532].  When computing or verifying the hash in a   DKIM signature as described inSection 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 inSection 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 anRFC5322.From header   field.Section 6.6.1 of [RFC7489] specifies, somewhat imprecisely, how IDNs   in theRFC5322.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] isLevine                       Standards Track                    [Page 4]

RFC 8616                   EAI Authentication                  June 2019   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 Sections6.3 and7.1 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>.Levine                       Standards Track                    [Page 5]

RFC 8616                   EAI Authentication                  June 2019   [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 inRFC2119 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.lyLevine                       Standards Track                    [Page 6]