--- 1/draft-ietf-uta-smtp-require-tls-06.txt 2019-01-22 17:19:16.710514552 -0800 +++ 2/draft-ietf-uta-smtp-require-tls-07.txt 2019-01-22 17:19:16.746515430 -0800 @@ -1,18 +1,18 @@ Internet Engineering Task Force J. Fenton Internet-Draft Altmode Networks -Intended status: Standards Track December 4, 2018 -Expires: June 7, 2019 +Intended status: Standards Track January 22, 2019 +Expires: July 26, 2019 SMTP Require TLS Option - draft-ietf-uta-smtp-require-tls-06 + draft-ietf-uta-smtp-require-tls-07 Abstract The SMTP STARTTLS option, used in negotiating transport-level encryption of SMTP connections, is not as useful from a security standpoint as it might be because of its opportunistic nature; message delivery is, by default, prioritized over security. This document describes an SMTP service extension, REQUIRETLS, and message header field, RequireTLS. If the REQUIRETLS option or RequireTLS message header field is used when sending a message, it asserts a @@ -30,73 +30,77 @@ 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 7, 2019. + This Internet-Draft will expire on July 26, 2019. Copyright Notice - Copyright (c) 2018 IETF Trust and the persons identified as the + 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 4 - 3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 4 - 4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 5 - 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 5 - 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 5 - 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 5 - 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 6 - 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 7 - 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7 - 5. Non-delivery message handling . . . . . . . . . . . . . . . . 7 - 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 - 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10 - 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 - 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 - 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 - 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 - 10.1. Changes since -05 Draft . . . . . . . . . . . . . . . . 11 - 10.2. Changes since -04 Draft . . . . . . . . . . . . . . . . 11 - 10.3. Changes since -03 Draft . . . . . . . . . . . . . . . . 11 - 10.4. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 - 10.5. Changes since -01 Draft . . . . . . . . . . . . . . . . 12 - 10.6. Changes since -00 Draft . . . . . . . . . . . . . . . . 12 - 10.7. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 - 10.8. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 - 10.9. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 - 10.10. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13 - 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 - 11.2. Informative References . . . . . . . . . . . . . . . . . 15 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 + 3. The RequireTLS Header Field . . . . . . . . . . . . . . . . . 5 + 4. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 6 + 4.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 6 + 4.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 6 + 4.2.1. Sending with TLS Required . . . . . . . . . . . . . . 6 + 4.2.2. Sending with TLS Optional . . . . . . . . . . . . . . 7 + 4.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 8 + 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 8 + 5. Non-delivery message handling . . . . . . . . . . . . . . . . 8 + 6. Mailing list considerations . . . . . . . . . . . . . . . . . 9 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 + 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 11 + 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 11 + 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 11 + 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 + 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 12 + 10.1. Changes since -06 Draft . . . . . . . . . . . . . . . . 12 + 10.2. Changes since -05 Draft . . . . . . . . . . . . . . . . 12 + 10.3. Changes since -04 Draft . . . . . . . . . . . . . . . . 12 + 10.4. Changes since -03 Draft . . . . . . . . . . . . . . . . 13 + 10.5. Changes since -02 Draft . . . . . . . . . . . . . . . . 13 + 10.6. Changes since -01 Draft . . . . . . . . . . . . . . . . 13 + 10.7. Changes since -00 Draft . . . . . . . . . . . . . . . . 13 + 10.8. Changes since fenton-03 Draft . . . . . . . . . . . . . 13 + 10.9. Changes Since -02 Draft . . . . . . . . . . . . . . . . 14 + 10.10. Changes Since -01 Draft . . . . . . . . . . . . . . . . 14 + 10.11. Changes Since -00 Draft . . . . . . . . . . . . . . . . 14 + 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 + 11.1. Normative References . . . . . . . . . . . . . . . . . . 14 + 11.2. Informative References . . . . . . . . . . . . . . . . . 16 + Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17 + A.1. REQUIRETLS SMTP Option . . . . . . . . . . . . . . . . . 17 + A.2. RequireTLS Header Field . . . . . . . . . . . . . . . . . 18 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 1. Introduction The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a means by which an SMTP server and client can establish a Transport Layer Security (TLS) protected session for the transmission of email messages. By default, TLS is used only upon mutual agreement (successful negotiation) of STARTTLS between the client and server; if this is not possible, the message is sent without transport encryption. Furthermore, it is common practice for the client to @@ -134,38 +138,62 @@ to transit through one or more MTAs that do not support REQUIRETLS. 1.1. Requirements Language 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 formal syntax uses the Augmented Backus-Naur Form (ABNF) + [RFC5234] including the core rules defined in Appendix B of that + document. + 2. The REQUIRETLS Service Extension 1. The textual name of the extension is "Require TLS". 2. The EHLO keyword value associated with this extension is "REQUIRETLS". - 3. One MAIL FROM option is defined by this extension. + 3. No additional SMTP verbs are defined by this extension. - 4. One new SMTP status code is defined by this extension to convey + 4. One optional parameter ("REQUIRETLS") is added to the MAIL FROM + command by this extension. No value is associated with this + parameter. + + 5. The maximum length of a MAIL FROM command line is increased by 11 + octets by the possible addition of a space and the REQUIRETLS + keyword. + + 6. One new SMTP status code is defined by this extension to convey an error condition resulting from failure of the client to send to a server not also supporting the REQUIRETLS extension. + 7. The REQUIRETLS extension is valid for message relay [RFC5321], + submission [RFC6409], and the Local Mail Transfer Protocol (LMTP) + [RFC2033] + + 8. The ABNF syntax for the MAIL FROM parameter is as follows: + + requiretls-param = "REQUIRETLS" + ; where requiretls-param is an instance of an + ; esmtp-param used in Mail-parameters in + ; RFC 5321 Section 4.1.2. There is no esmtp-value + ; associated with requiretls-param. + In order to specify REQUIRETLS treatment for a given message, the REQUIRETLS option is specified on the MAIL FROM command when that message is transmitted. This option MUST only be specified in the - context of an SMTP session meeting the security requirements that - have been specified: + context of an SMTP session meeting the security requirements of + REQUIRETLS: o The session itself MUST employ TLS transmission. o If the SMTP server to which the message is being transmitted is identified through an MX record lookup, its name MUST be validated via a DNSSEC signature on the recipient domain's MX record, or the MX hostname MUST be validated by an MTA-STS policy as described in Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033 [RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035]. @@ -175,102 +203,114 @@ specified in RFC 7672 [RFC7672]. For trust chains, the choice of trusted (root) certificates is at the discretion of the SMTP client. o Following the negotiation of STARTTLS, the SMTP server MUST advertise in the subsequent EHLO response that it supports REQUIRETLS. 3. The RequireTLS Header Field - One new message header field, RequireTLS, is defined by this - specification. It is used for messages requesting that recipient TLS - policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored. + One new message header field [RFC5322], RequireTLS, is defined by + this specification. It is used for messages for which the originator + requests that recipient TLS policy (including MTA-STS [RFC8461] and + DANE [RFC7672]) be ignored. This might be done, for example, to + report a misconfigured mail server, such as an expired TLS + certificate. The RequireTLS header field has a single REQUIRED parameter: o NO - The SMTP client SHOULD attempt to send the message regardless of its ability to negotiate STARTTLS with the SMTP server, ignoring policy-based mechanisms (including MTA-STS and DANE), if any, asserted by the recipient domain. Nevertheless, the client SHOULD negotiate STARTTLS with the server if available. More than one instance of the RequireTLS header field MUST NOT appear in a given message. + The ABNF syntax for the RequireTLS header field is as follows: + + requiretls-field = "RequireTLS:" [FWS] "No" CRLF + ; where requiretls-field in an instance of an + ; optional-field defined in RFC 5322 Section + ; 3.6.8. + FWS = + CRLF = + 4. REQUIRETLS Semantics 4.1. REQUIRETLS Receipt Requirements Upon receipt of the REQUIRETLS option on a MAIL FROM command during - the receipt of a message, an SMTP server MUST tag that message as - needing REQUIRETLS handling. + the receipt of a message for which the return-path is not empty + (indicating a bounce message), an SMTP server MUST tag that message + as needing REQUIRETLS handling. Upon receipt of a message not specifying the REQUIRETLS option on its MAIL FROM command but containing the RequireTLS header field in its message header, an SMTP server implementing this specification MUST tag that message with the option specified in the RequireTLS header field. If the REQUIRETLS MAIL FROM parameter is specified, the RequireTLS header field MUST be ignored but MAY be included in onward relay of the message. The manner in which the above tagging takes place is implementation- dependent. If the message is being locally aliased and redistributed to multiple addresses, all instances of the message MUST be tagged in the same manner. 4.2. REQUIRETLS Sender Requirements 4.2.1. Sending with TLS Required - When sending a message tagged as requiring TLS, the sending (client) - MTA MUST: + When sending a message tagged as requiring TLS for which the MAIL + FROM return-path is not empty (an empty MAIL FROM return-path + indicating a bounce message), the sending (client) MTA MUST: 1. Look up the SMTP server to which the message is to be sent as described in [RFC5321] Section 5.1. 2. If the server lookup is accomplished via the recipient domain's MX record (the usual case) and is not accompanied by a valid DNSSEC signature, the client MUST also validate the SMTP server - name using MTA-STS as described in Section 4.1 of RFC 8461 - [RFC8461]. + name using MTA-STS as described in RFC 8461 [RFC8461] + Section 4.1. 3. Open an SMTP session with the peer SMTP server using the EHLO verb. 4. Establish a TLS-protected SMTP session with its peer SMTP server and authenticate the server's certificate as specified in [RFC6125] or [RFC7672] as applicable. 5. Ensure that the response to the subsequent EHLO following establishment of the TLS protection advertises the REQUIRETLS capability. - 6. The SMTP client SHOULD also require that meaningfully secure - cipher algorithms and key lengths be negotiated with the server. - The choices of key lengths and algorithms change over time, so a - specific requirement is not presented here. + The SMTP client SHOULD follow the recommendations in [RFC7525] or its + successor with respect to negotiation of the TLS session. If any of the above steps fail, the client MUST issue a QUIT to the server and repeat steps 2-5 with each host on the recipient domain's list of MX hosts in an attempt to find a mail path that meets the sender's requirements. The client MAY send other, unprotected, messages to that server if it has any prior to issuing the QUIT. If there are no more MX hosts, the client MUST NOT transmit the message to the domain. Following such a failure, the SMTP client MUST send a non-delivery notification to the reverse-path of the failed message as described in section 3.6 of [RFC5321]. The following status codes [RFC5248] SHOULD be used: - o REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed + o REQUIRETLS not supported by server: 5.7.YYY REQUIRETLS needed o Unable to establish TLS-protected SMTP session: 5.7.10 Encryption needed Refer to Section 5 for further requirements regarding non-delivery messages. If all REQUIRETLS requirements have been met, transmit the message, issuing the REQUIRETLS option on the MAIL FROM command with the required option(s), if any. @@ -337,21 +377,21 @@ The path from the origination of an error bounce message back to the MAIL FROM address may not share the same REQUIRETLS support as the forward path. Therefore, users requiring TLS are advised to make sure that they are capable of receiving mail using REQUIRETLS as well. Otherwise, such non-delivery messages will be lost. If a REQUIRETLS message is bounced, the server MUST behave as if RET=HDRS was present as described in [RFC3461]. If both RET=FULL and REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS - bounce message MUST use an empty MAIL FROM return-path as required by + bounce message uses an empty MAIL FROM return-path as required by [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS parameter SHOULD NOT cause a bounce message to be discarded even if the next-hop relay does not advertise REQUIRETLS. Senders of messages requiring TLS are advised to consider the possibility that bounce messages will be lost as a result of REQUIRETLS return path failure, and that some information could be leaked if a bounce message is not able to be transmitted with REQUIRETLS. @@ -414,27 +454,25 @@ Header field name: RequireTLS Applicable protocol: mail Status: standard Author/change controller: IETF Specification document: (this document) This section is to be updated for publication by the RFC Editor. 8. Security Considerations - The purpose of REQUIRETLS is to improve communications security for - email by giving the originator of a message an expectation that it - will be transmitted in an encrypted form "over the wire". When used, - REQUIRETLS changes the traditional behavior of email transmission, - which favors delivery over the ability to send email messages using - transport-layer security, to one in which requested security takes - precedence over delivery and domain-level policy. + The purpose of REQUIRETLS is to give the originator of a message + control over the security of email they send, either by conveying an + expectation that it will be transmitted in an encrypted form "over + the wire" or explicitly that transport encryption is not required if + it cannot be successfully negotiated. The following considerations apply to the REQUIRETLS service extension but not the RequireTLS header field, since messages specifying the header field are less concerned with transport security. 8.1. Passive attacks REQUIRETLS is generally effective against passive attackers who are merely trying to eavesdrop on an SMTP exchange between an SMTP client @@ -487,101 +525,121 @@ The author would like to acknowledge many helpful suggestions on the ietf-smtp and uta mailing lists, in particular those of Viktor Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. 10. Revision History To be removed by RFC Editor upon publication as an RFC. -10.1. Changes since -05 Draft +10.1. Changes since -06 Draft + + Various changes in response to AD review: + + o Reference RFC 7525 for TLS negotiation recommendations. + + o Make reference to requested 5.7.YYY error code consistent. + + o Clarify applicability to LMTP and submission. + + o Provide ABNF for syntax of SMTP option and header field and + examples in Appendix A. + + o Correct use of normative language in Section 5. + + o Clarify case where REQUIRETLS option is used on bounce messages. + + o Improve Security Requirements wording to be incusive of both SMTP + option and header field. + +10.2. Changes since -05 Draft Corrected IANA Permanent Message Header Fields Registry request. -10.2. Changes since -04 Draft +10.3. Changes since -04 Draft Require validation of SMTP server hostname via DNSSEC or MTA-STS policy when TLS is required. -10.3. Changes since -03 Draft +10.4. Changes since -03 Draft Working Group Last Call changes, including: o Correct reference for SMTP DANE o Clarify that RequireTLS: NO applies to both MTA-STS and DANE policies o Correct newly-defined status codes o Update MTA-STS references to RFC -10.4. Changes since -02 Draft +10.5. Changes since -02 Draft o More complete documentation for IANA registration requests. o Changed bounce handling to use RET parameters of RFC 3461, along with slightly more liberal transmission of bounces even if REQUIRETLS can't be negotiated. -10.5. Changes since -01 Draft +10.6. Changes since -01 Draft o Converted DEEP references to RFC 8314. o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. o Editorial corrections, notably making the header field name consistent (RequireTLS rather than Require-TLS). -10.6. Changes since -00 Draft +10.7. Changes since -00 Draft o Created new header field, Require-TLS, for use by "NO" option. o Removed "NO" option from SMTP service extension. o Recommend DEEP requirements for delivery of messages requiring TLS. o Assorted copy edits -10.7. Changes since fenton-03 Draft +10.8. Changes since fenton-03 Draft o Wording improvements from Rolf Sonneveld review 22 July 2017 o A few copy edits o Conversion from individual to UTA WG draft -10.8. Changes Since -02 Draft +10.9. Changes Since -02 Draft o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per suggestion on UTA WG mailing list. o Additional guidance on bounce messages -10.9. Changes Since -01 Draft +10.10. Changes Since -01 Draft o Specified retries when multiple MX hosts exist for a given domain. o Clarified generation of non-delivery messages o Specified requirements for application of REQUIRETLS to mail forwarders and mailing lists. o Clarified DNSSEC requirements to include MX lookup only. o Corrected terminology regarding message retrieval vs. delivery. o Changed category to standards track. -10.10. Changes Since -00 Draft +10.11. Changes Since -00 Draft o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM parameter to better associate REQUIRETLS requirements with transmission of individual messages. o Addition of an option to require DNSSEC lookup of the remote mail server, since this affects the common name of the certificate that is presented. o Clarified the wording to more clearly state that TLS sessions must @@ -629,36 +687,57 @@ [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005, . [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, . + [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", STD 68, RFC 5234, + DOI 10.17487/RFC5234, January 2008, + . + [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail System Status Codes", BCP 138, RFC 5248, DOI 10.17487/RFC5248, June 2008, . [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, . + [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, + DOI 10.17487/RFC5322, October 2008, + . + [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, . + [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, + "Recommendations for Secure Use of Transport Layer + Security (TLS) and Datagram Transport Layer Security + (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May + 2015, . + + [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via + Opportunistic DNS-Based Authentication of Named Entities + (DANE) Transport Layer Security (TLS)", RFC 7672, + DOI 10.17487/RFC7672, October 2015, + . + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, . [RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., @@ -671,38 +750,105 @@ Transfer Protocol (SMTP) Enhanced Status Codes Registry", 2008, . 11.2. Informative References [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, . + [RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033, + DOI 10.17487/RFC2033, October 1996, + . + [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007, . [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010, . - [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via - Opportunistic DNS-Based Authentication of Named Entities - (DANE) Transport Layer Security (TLS)", RFC 7672, - DOI 10.17487/RFC7672, October 2015, - . + [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", + STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, + . + +Appendix A. Examples + + This section is informative. + +A.1. REQUIRETLS SMTP Option + + The RequireTLS SMTP option is used to express the intent of the + sender that the associated message be relayed using TLS. In the + following example, lines beginning with C: are transmitted from the + SMTP client to the server, and lines beginning with S: are + transmitted in the opposite direction. + + S: 220 mail.example.net ESMTP + C: EHLO mail.example.org + S: 250-mail.example.net Hello example.org [192.0.2.1] + S: 250-SIZE 52428800 + S: 250-8BITMIME + S: 250-PIPELINING + S: 250-STARTTLS + S: 250 HELP + C: STARTTLS + S: TLS go ahead + + (at this point TLS negotiation takes place. The remainder of this + session occurs within TLS.) + + S: 220 mail.example.net ESMTP + C: EHLO mail.example.org + S: 250-mail.example.net Hello example.org [192.0.2.1] + S: 250-SIZE 52428800 + S: 250-8BITMIME + S: 250-PIPELINING + S: 250-REQUIRETLS + S: 250 HELP + C: MAIL FROM: REQUIRETLS + S: 250 OK + C: RCPT TO: + S: 250 Accepted + C: DATA + S: 354 Enter message, ending with "." on a line by itself + + (message follows) + C: . + S: 250 OK + C: QUIT + +A.2. RequireTLS Header Field + + The RequireTLS header field is used when the sender of the message + wants to override the default policy of the recipient domain to + require TLS. It might be used, for example, to allow problems with + the recipient domain's TLS certificate to be reported: + + From: Roger Reporter + To: Andy Admin + Subject: Certificate problem? + RequireTLS: NO + Date: Fri, 18 Jan 2019 10:26:55 -0800 + Message-ID: <5c421a6f79c0e_d153ff8286d45c468473@mail.example.org> + + Andy, there seems to be a problem with the TLS certificate + on your mail server. Are you aware of this? + + Roger Author's Address Jim Fenton Altmode Networks Los Altos, California 94024 USA Email: fenton@bluepopcorn.net