7Internet Engineering Task Force (IETF) J. Levine
8Request for Comments: 8616 Taughannock Networks
9Updates: 6376, 7208, 7489 June 2019
10Category: Standards Track
14 Email Authentication for Internationalized Mail
18 Sender Policy Framework (SPF) (RFC 7208), DomainKeys Identified Mail
19 (DKIM) (RFC 6376), and Domain-based Message Authentication,
20 Reporting, and Conformance (DMARC) (RFC 7489) enable a domain owner
21 to publish email authentication and policy information in the DNS.
22 In internationalized email, domain names can occur both as U-labels
23 and A-labels. This specification updates the SPF, DKIM, and DMARC
24 specifications to clarify which form of internationalized domain
25 names to use in those specifications.
29 This is an Internet Standards Track document.
31 This document is a product of the Internet Engineering Task Force
32 (IETF). It represents the consensus of the IETF community. It has
33 received public review and has been approved for publication by the
34 Internet Engineering Steering Group (IESG). Further information on
35 Internet Standards is available in Section 2 of RFC 7841.
37 Information about the current status of this document, any errata,
38 and how to provide feedback on it may be obtained at
39 https://www.rfc-editor.org/info/rfc8616.
43 Copyright (c) 2019 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (https://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
58Levine Standards Track [Page 1]
60RFC 8616 EAI Authentication June 2019
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
66 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
67 3. General Principles . . . . . . . . . . . . . . . . . . . . . 3
68 4. SPF and Internationalized Mail . . . . . . . . . . . . . . . 3
69 5. DKIM and Internationalized Mail . . . . . . . . . . . . . . . 3
70 6. DMARC and Internationalized Mail . . . . . . . . . . . . . . 4
71 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
72 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
73 9. Normative References . . . . . . . . . . . . . . . . . . . . 5
74 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
78 SPF [RFC7208], DKIM [RFC6376], and DMARC [RFC7489] enable a domain
79 owner to publish email authentication and policy information in the
80 DNS. SPF primarily publishes information about what host addresses
81 are authorized to send mail for a domain. DKIM places cryptographic
82 signatures on email messages, with the validation keys published in
83 the DNS. DMARC publishes policy information related to the domain in
84 the From: header field of email messages.
86 In conventional email, all domain names are ASCII in all contexts, so
87 there is no question about the representation of the domain names.
88 All internationalized domain names are represented as A-labels
89 [RFC5890] in message header fields, SMTP sessions, and the DNS.
91 Internationalized mail [RFC6530] (generally called "EAI" for Email
92 Address Internationalization) allows U-labels in SMTP sessions
93 [RFC6531] and message header fields [RFC6532].
95 Every U-label is equivalent to an A-label, so in principle, the
96 choice of label format will not cause ambiguities. But in practice,
97 consistent use of label formats will make it more likely that code
98 for mail senders and receivers interoperates.
100 Internationalized mail also allows UTF-8-encoded Unicode characters
101 in the local parts of mailbox names, which were historically only
106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
108 "OPTIONAL" in this document are to be interpreted as described in
109 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
110 capitals, as shown here.
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116RFC 8616 EAI Authentication June 2019
119 The term "IDN", for Internationalized Domain Name, refers to a domain
120 name containing either U-labels or A-labels.
122 Since DMARC is not currently a Standards Track protocol, this
123 specification offers advice rather than requirements for DMARC.
127 In headers in EAI mail messages, domain names that were restricted to
128 ASCII can be U-labels, and mailbox local parts can be UTF-8. Header
129 field names and other text intended primarily to be interpreted by
130 computers rather than read by people remains ASCII.
132 Strings stored in DNS records remain ASCII since there is no way to
133 tell whether a client retrieving a DNS record expects an EAI or an
134 ASCII result. When a domain name found in a mail header field
135 includes U-labels, those labels are translated to A-labels before
136 being looked up in the DNS, as described in [RFC5891].
1384. SPF and Internationalized Mail
140 SPF [RFC7208] uses two identities from the SMTP session: the host
141 name in the EHLO command and the domain in the address in the MAIL
142 FROM command. Since the EHLO command precedes the server response
143 that tells whether the server supports the SMTPUTF8 extension, an IDN
144 host name MUST be represented as A-labels. An IDN in MAIL FROM can
145 be either U-labels or A-labels.
147 All U-labels MUST be converted to A-labels before being used for an
148 SPF validation. This includes both the labels in the name used for
149 the original DNS lookup, described in Section 3 of [RFC7208], and
150 those used in the macro expansion of domain-spec, described in
151 Section 7. Section 4.3 of [RFC7208] states that all IDNs in an SPF
152 DNS record MUST be A-labels; this rule is unchanged since any SPF
153 record can be used to authorize either EAI or conventional mail.
155 SPF macros %{s} and %{l} expand the local part of the sender's
156 mailbox. If the local part contains non-ASCII characters, terms that
157 include %{s} or %{l} do not match anything, because non-ASCII local
158 parts cannot be used as the DNS labels the macros are intended to
159 match. Since these macros are rarely used, this is unlikely to be an
1625. DKIM and Internationalized Mail
164 DKIM [RFC6376] specifies a mail header field that contains a
165 cryptographic message signature and a DNS record that contains the
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172RFC 8616 EAI Authentication June 2019
175 Section 2.11 of [RFC6376] defines dkim-quoted-printable. Its
176 definition is modified in messages with internationalized header
177 fields so that non-ASCII UTF-8 characters need not be quoted. The
178 ABNF [RFC5234] for dkim-safe-char in those messages is replaced by
179 the following, adding non-ASCII UTF-8 characters from [RFC3629]:
181 dkim-safe-char = %x21-3A / %x3C / %x3E-7E /
182 UTF8-2 / UTF8-3 / UTF8-4
183 ; '!' - ':', '<', '>' - '~', non-ASCII
185 UTF8-2 = <Defined in Section 4 of RFC 3629>
187 UTF8-3 = <Defined in Section 4 of RFC 3629>
189 UTF8-4 = <Defined in Section 4 of RFC 3629>
191 Section 3.5 of [RFC6376] states that IDNs in the d=, i=, and s= tags
192 of a DKIM-Signature header field MUST be encoded as A-labels. This
193 rule is relaxed only for internationalized message header fields
194 [RFC6532], so IDNs SHOULD be represented as U-labels. This provides
195 improved consistency with other header fields. (A-labels remain
196 valid to allow a transition from older software.) The set of
197 allowable characters in the local part of an i= tag is extended in
198 the same fashion as local parts of email addresses as described in
199 Section 3.2 of [RFC6532]. When computing or verifying the hash in a
200 DKIM signature as described in Section 3.7 of [RFC6376], the hash
201 MUST use the domain name in the format it occurs in the header field.
203 Section 3.4.2 of [RFC6376] describes relaxed header canonicalization.
204 Its first step converts all header field names from uppercase to
205 lowercase. Field names are restricted to printable ASCII (see
206 [RFC5322], Section 3.6.8), so this case conversion remains ASCII case
209 DKIM key records, described in Section 3.6.1 of [RFC6376], do not
210 contain domain names, so there is no change to their specification.
2126. DMARC and Internationalized Mail
214 DMARC [RFC7489] defines a policy language that domain owners can
215 specify for the domain of the address in an RFC5322.From header
218 Section 6.6.1 of [RFC7489] specifies, somewhat imprecisely, how IDNs
219 in the RFC5322.From address domain are to be handled. That section
220 is updated to say that all U-labels in the domain are converted to
221 A-labels before further processing. Section 7.1 of [RFC7489] is
226Levine Standards Track [Page 4]
228RFC 8616 EAI Authentication June 2019
231 similarly updated to say that all U-labels in domains being handled
232 are converted to A-labels before further processing.
235 can contain email addresses in the "rua" and "ruf" tags. Since a
236 policy record can be used for both internationalized and conventional
237 mail, those addresses still have to be conventional addresses, not
238 internationalized addresses.
2407. IANA Considerations
242 This document has no IANA actions.
2448. Security Considerations
246 Email is subject to a vast range of threats and abuses. This
247 document attempts to slightly mitigate some of them but does not, as
248 far as the author knows, add any new ones. The updates to SPF, DKIM,
249 and DMARC are intended to allow the respective specifications to work
250 as reliably on internationalized mail as they do on ASCII mail, so
251 that applications that use them, such as some kinds of mail filters
252 that catch spam and phish, can work more reliably on
253 internationalized mail.
2559. Normative References
257 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
258 Requirement Levels", BCP 14, RFC 2119,
259 DOI 10.17487/RFC2119, March 1997,
260 <https://www.rfc-editor.org/info/rfc2119>.
262 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
263 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
264 2003, <https://www.rfc-editor.org/info/rfc3629>.
266 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
267 Specifications: ABNF", STD 68, RFC 5234,
268 DOI 10.17487/RFC5234, January 2008,
269 <https://www.rfc-editor.org/info/rfc5234>.
271 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
272 DOI 10.17487/RFC5322, October 2008,
273 <https://www.rfc-editor.org/info/rfc5322>.
275 [RFC5890] Klensin, J., "Internationalized Domain Names for
276 Applications (IDNA): Definitions and Document Framework",
277 RFC 5890, DOI 10.17487/RFC5890, August 2010,
278 <https://www.rfc-editor.org/info/rfc5890>.
282Levine Standards Track [Page 5]
284RFC 8616 EAI Authentication June 2019
287 [RFC5891] Klensin, J., "Internationalized Domain Names in
288 Applications (IDNA): Protocol", RFC 5891,
289 DOI 10.17487/RFC5891, August 2010,
290 <https://www.rfc-editor.org/info/rfc5891>.
292 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
293 "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
294 RFC 6376, DOI 10.17487/RFC6376, September 2011,
295 <https://www.rfc-editor.org/info/rfc6376>.
297 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
298 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
299 February 2012, <https://www.rfc-editor.org/info/rfc6530>.
301 [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
302 Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
303 <https://www.rfc-editor.org/info/rfc6531>.
305 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
306 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
307 2012, <https://www.rfc-editor.org/info/rfc6532>.
309 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
310 Authorizing Use of Domains in Email, Version 1", RFC 7208,
311 DOI 10.17487/RFC7208, April 2014,
312 <https://www.rfc-editor.org/info/rfc7208>.
314 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
315 Message Authentication, Reporting, and Conformance
316 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
317 <https://www.rfc-editor.org/info/rfc7489>.
319 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
320 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
321 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
328 Trumansburg, NY 14886
329 United States of America
331 Email: standards@taugh.com
338Levine Standards Track [Page 6]