GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc8616

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.

Levine Standards Track [Page 1] RFC 8616 EAI Authentication June 2019

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.

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 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.

Levine Standards Track [Page 3] RFC 8616 EAI Authentication June 2019

 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

Levine 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 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>.

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 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

Levine Standards Track [Page 6]

/data/webs/external/dokuwiki/data/pages/rfc/rfc8616.txt · Last modified: 2019/07/01 01:18 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki