E-mail Authentication for Internationalized Mail
The information below is for an old version of the document.
This is an older version of an Internet-Draft that was ultimately published as RFC 8616.
|Author||John R. Levine|
|Last updated||2019-02-08 (Latest revision 2019-01-02)|
|RFC stream||Internet Engineering Task Force (IETF)|
I18NDIR Last Call review (of -03) Ready with Issues
SECDIR Last Call review (of -03) Has Issues
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|Additional resources||Mailing list discussion|
|Stream||WG state||In WG Last Call|
|Document shepherd||Kurt Andersen|
|IESG||IESG state||I-D Exists|
|Responsible AD||Alexey Melnikov|
|Send notices to||Kurt Andersen <firstname.lastname@example.org>|
Network Working Group J. Levine Internet-Draft Taughannock Networks Updates: 6376, 7208, 7489 (if approved) December 19, 2018 Intended status: Standards Track Expires: June 22, 2019 E-mail Authentication for Internationalized Mail draft-ietf-dmarc-eaiauth-00 Abstract SPF, DKIM, and DMARC 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. The Authentication-Results header reports the result of authentication checks made with SPF, DKIM, DMARC, and other schemes. This specification clarifies when to use which form of domain names when using SPF, DKIM, and DMARC and when creating Authentication- Results headers. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on June 22, 2019. Copyright Notice Copyright (c) 2018 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 Levine Expires June 22, 2019 [Page 1] Internet-Draft EAI Authentication December 2018 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. General principles . . . . . . . . . . . . . . . . . . . . . 3 4. SPF and internationalized mail . . . . . . . . . . . . . . . 3 5. DKIM and internationalized mail . . . . . . . . . . . . . . . 4 6. DMARC and internationalized mail . . . . . . . . . . . . . . 4 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 9. Normative References . . . . . . . . . . . . . . . . . . . . 5 Appendix A. Change history . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction SPF, DKIM, and DMARC 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 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 unencoded message bodies, in SMTP sessions, and in the DNS. Internationalized mail [RFC6530] allows U-labels in SMTP sessions [RFC6531] and in message headers [RFC6532]. Every U-label is equivalent to an A-label, so in principle the choice of label format should not cause any 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 characters in the local parts of mailbox names, which were historically only ASCII. Levine Expires June 22, 2019 [Page 2] Internet-Draft EAI Authentication December 2018 2. Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" when written in upper case in in this document are to be interpreted as described in [RFC2119] and [RFC8174]. The term IDN, for Internationalized Domain Name, refers to a doman 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 now be U-labels, and mailbox local parts can be UTF-8. Header 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 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 argument MUST be represented as an A-label. An IDN in MAIL FROM can be either U-labels or an 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. (Note that unlike U-labels, there is no way to rewrite non-ASCII local parts into ASCII.) Levine Expires June 22, 2019 [Page 3] Internet-Draft EAI Authentication December 2018 5. DKIM and internationalized mail DKIM [RFC6376] specifies a message header 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 internationalized messages so that non- ASCII UTF-8 characters need not be quoted. The ABNF for dkim-safe- char in internationalized messages is replaced by the following: dkim-safe-char = %x21-3A / %x3C / %x3E-7E / %x80-FF ; '!' - ':', '<', '>' - '~', non-ASCII Section 3.5 of [RFC6376] states that IDNs in the d=, i=, and s= tags of a DKIM-Signature header MUST be encoded as A-labels. This rule is relaxed only for headers in internationalized messages [RFC6532] so IDNs SHOULD be represented as U-labels but MAY be A-labels. This provides improved consistency with other headers. The set of allowable characters in the local-part of an i= tag is extended as described in [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. DKIM key records, described in section 3.6.1, 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 a RFC5322.From header. 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. Sections 6.7 and 7.1 are 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. Levine Expires June 22, 2019 [Page 4] Internet-Draft EAI Authentication December 2018 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. 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>. [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>. Levine Expires June 22, 2019 [Page 5] Internet-Draft EAI Authentication December 2018 [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 00 First WG version Author's Address John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 Email: email@example.com URI: http://jl.ly Levine Expires June 22, 2019 [Page 6]