Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376, 7208, 7489 (if approved) February 22, 2019
Intended status: Standards Track
Expires: August 26, 2019

E-mail Authentication for Internationalized Mail


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. The Authentication-Results header reports the result of authentication checks made with SPF, DKIM, DMARC, and other schemes. This specification updates the SPF, DKIM, and DMARC specifications to clarify which form of internationalized domain names to use in those specifications, and when creating Authentication-Results headers.

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This Internet-Draft will expire on August 26, 2019.

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Table of Contents

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 in unencoded message bodies, in SMTP sessions, and in the DNS. Internationalized mail allows U-labels in SMTP sessions and in message headers.

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.

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] and [RFC8174]. 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 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 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 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.)

5. DKIM and internationalized mail

DKIM 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 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.

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 the usual ASCII conversion.

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 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.

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.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, February 2012.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized Email", RFC 6531, DOI 10.17487/RFC6531, February 2012.
[RFC6532] Yang, A., Steele, S. and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2012.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

Appendix A. Change history

01 to 02
update references
00 to 01
Relaxed canon, Typos
First WG version

Author's Address

John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: URI: