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

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 © 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.

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.

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.

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 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>.
   
   [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