draft-ietf-uta-smtp-require-tls-05.txt   draft-ietf-uta-smtp-require-tls-06.txt 
Internet Engineering Task Force J. Fenton Internet Engineering Task Force J. Fenton
Internet-Draft Altmode Networks Internet-Draft Altmode Networks
Intended status: Standards Track November 20, 2018 Intended status: Standards Track December 4, 2018
Expires: May 24, 2019 Expires: June 7, 2019
SMTP Require TLS Option SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-05 draft-ietf-uta-smtp-require-tls-06
Abstract Abstract
The SMTP STARTTLS option, used in negotiating transport-level The SMTP STARTTLS option, used in negotiating transport-level
encryption of SMTP connections, is not as useful from a security encryption of SMTP connections, is not as useful from a security
standpoint as it might be because of its opportunistic nature; standpoint as it might be because of its opportunistic nature;
message delivery is, by default, prioritized over security. This message delivery is, by default, prioritized over security. This
document describes an SMTP service extension, REQUIRETLS, and message document describes an SMTP service extension, REQUIRETLS, and message
header field, RequireTLS. If the REQUIRETLS option or RequireTLS header field, RequireTLS. If the REQUIRETLS option or RequireTLS
message header field is used when sending a message, it asserts a message header field is used when sending a message, it asserts a
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 24, 2019. This Internet-Draft will expire on June 7, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7 4.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 7
5. Non-delivery message handling . . . . . . . . . . . . . . . . 7 5. Non-delivery message handling . . . . . . . . . . . . . . . . 7
6. Mailing list considerations . . . . . . . . . . . . . . . . . 8 6. Mailing list considerations . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10 8.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 10
8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10 8.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 10
8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10 8.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11 10. Revision History . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Changes since -04 Draft . . . . . . . . . . . . . . . . 11 10.1. Changes since -05 Draft . . . . . . . . . . . . . . . . 11
10.2. Changes since -03 Draft . . . . . . . . . . . . . . . . 11 10.2. Changes since -04 Draft . . . . . . . . . . . . . . . . 11
10.3. Changes since -02 Draft . . . . . . . . . . . . . . . . 11 10.3. Changes since -03 Draft . . . . . . . . . . . . . . . . 11
10.4. Changes since -01 Draft . . . . . . . . . . . . . . . . 12 10.4. Changes since -02 Draft . . . . . . . . . . . . . . . . 11
10.5. Changes since -00 Draft . . . . . . . . . . . . . . . . 12 10.5. Changes since -01 Draft . . . . . . . . . . . . . . . . 12
10.6. Changes since fenton-03 Draft . . . . . . . . . . . . . 12 10.6. Changes since -00 Draft . . . . . . . . . . . . . . . . 12
10.7. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12 10.7. Changes since fenton-03 Draft . . . . . . . . . . . . . 12
10.8. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12 10.8. Changes Since -02 Draft . . . . . . . . . . . . . . . . 12
10.9. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13 10.9. Changes Since -01 Draft . . . . . . . . . . . . . . . . 12
10.10. Changes Since -00 Draft . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 15 11.2. Informative References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
means by which an SMTP server and client can establish a Transport means by which an SMTP server and client can establish a Transport
Layer Security (TLS) protected session for the transmission of email Layer Security (TLS) protected session for the transmission of email
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o If the SMTP server to which the message is being transmitted is o If the SMTP server to which the message is being transmitted is
identified through an MX record lookup, its name MUST be validated identified through an MX record lookup, its name MUST be validated
via a DNSSEC signature on the recipient domain's MX record, or the 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 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 Section 4.1 of RFC 8461 [RFC8461]. DNSSEC is defined in RFC 4033
[RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035]. [RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035].
o The certificate presented by the SMTP server MUST either verify o The certificate presented by the SMTP server MUST either verify
successfully in a trust chain leading to a certificate trusted by successfully in a trust chain leading to a certificate trusted by
the SMTP client or it MUST verify succesfully using DANE as the SMTP client or it MUST verify successfully using DANE as
specified in RFC 7672 [RFC7672]. For trust chains, the choice of specified in RFC 7672 [RFC7672]. For trust chains, the choice of
trusted (root) certificates is at the discretion of the SMTP trusted (root) certificates is at the discretion of the SMTP
client. client.
o Following the negotiation of STARTTLS, the SMTP server MUST o Following the negotiation of STARTTLS, the SMTP server MUST
advertise in the subsequent EHLO response that it supports advertise in the subsequent EHLO response that it supports
REQUIRETLS. REQUIRETLS.
3. The RequireTLS Header Field 3. The RequireTLS Header Field
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Reference: (this document) Reference: (this document)
Submitter: J. Fenton Submitter: J. Fenton
Change controller: IESG Change controller: IESG
If published as an RFC, this draft requests the addition of an entry If published as an RFC, this draft requests the addition of an entry
to the Permanent Message Header Field Names Registry to the Permanent Message Header Field Names Registry
[PermMessageHeaderFields]: [PermMessageHeaderFields]:
Header field name: RequireTLS Header field name: RequireTLS
Applicable protocol: mail Applicable protocol: mail
Status: provisional Status: standard
Author/change controller: IETF UTA Working Group Author/change controller: IETF
Specification document: (this document) Specification document: (this document)
This section is to be updated for publication by the RFC Editor. This section is to be updated for publication by the RFC Editor.
8. Security Considerations 8. Security Considerations
The purpose of REQUIRETLS is to improve communications security for The purpose of REQUIRETLS is to improve communications security for
email by giving the originator of a message an expectation that it email by giving the originator of a message an expectation that it
will be transmitted in an encrypted form "over the wire". When used, will be transmitted in an encrypted form "over the wire". When used,
REQUIRETLS changes the traditional behavior of email transmission, REQUIRETLS changes the traditional behavior of email transmission,
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The author would like to acknowledge many helpful suggestions on the The author would like to acknowledge many helpful suggestions on the
ietf-smtp and uta mailing lists, in particular those of Viktor ietf-smtp and uta mailing lists, in particular those of Viktor
Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock,
John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim. John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
10. Revision History 10. Revision History
To be removed by RFC Editor upon publication as an RFC. To be removed by RFC Editor upon publication as an RFC.
10.1. Changes since -04 Draft 10.1. Changes since -05 Draft
Corrected IANA Permanent Message Header Fields Registry request.
10.2. Changes since -04 Draft
Require validation of SMTP server hostname via DNSSEC or MTA-STS Require validation of SMTP server hostname via DNSSEC or MTA-STS
policy when TLS is required. policy when TLS is required.
10.2. Changes since -03 Draft 10.3. Changes since -03 Draft
Working Group Last Call changes, including: Working Group Last Call changes, including:
o Correct reference for SMTP DANE o Correct reference for SMTP DANE
o Clarify that RequireTLS: NO applies to both MTA-STS and DANE o Clarify that RequireTLS: NO applies to both MTA-STS and DANE
policies policies
o Correct newly-defined status codes o Correct newly-defined status codes
o Update MTA-STS references to RFC o Update MTA-STS references to RFC
10.3. Changes since -02 Draft 10.4. Changes since -02 Draft
o More complete documentation for IANA registration requests. o More complete documentation for IANA registration requests.
o Changed bounce handling to use RET parameters of RFC 3461, along o Changed bounce handling to use RET parameters of RFC 3461, along
with slightly more liberal transmission of bounces even if with slightly more liberal transmission of bounces even if
REQUIRETLS can't be negotiated. REQUIRETLS can't be negotiated.
10.4. Changes since -01 Draft 10.5. Changes since -01 Draft
o Converted DEEP references to RFC 8314. o Converted DEEP references to RFC 8314.
o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC. o Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.
o Editorial corrections, notably making the header field name o Editorial corrections, notably making the header field name
consistent (RequireTLS rather than Require-TLS). consistent (RequireTLS rather than Require-TLS).
10.5. Changes since -00 Draft 10.6. Changes since -00 Draft
o Created new header field, Require-TLS, for use by "NO" option. o Created new header field, Require-TLS, for use by "NO" option.
o Removed "NO" option from SMTP service extension. o Removed "NO" option from SMTP service extension.
o Recommend DEEP requirements for delivery of messages requiring o Recommend DEEP requirements for delivery of messages requiring
TLS. TLS.
o Assorted copy edits o Assorted copy edits
10.6. Changes since fenton-03 Draft 10.7. Changes since fenton-03 Draft
o Wording improvements from Rolf Sonneveld review 22 July 2017 o Wording improvements from Rolf Sonneveld review 22 July 2017
o A few copy edits o A few copy edits
o Conversion from individual to UTA WG draft o Conversion from individual to UTA WG draft
10.7. Changes Since -02 Draft 10.8. Changes Since -02 Draft
o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per o Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
suggestion on UTA WG mailing list. suggestion on UTA WG mailing list.
o Additional guidance on bounce messages o Additional guidance on bounce messages
10.8. Changes Since -01 Draft 10.9. Changes Since -01 Draft
o Specified retries when multiple MX hosts exist for a given domain. o Specified retries when multiple MX hosts exist for a given domain.
o Clarified generation of non-delivery messages o Clarified generation of non-delivery messages
o Specified requirements for application of REQUIRETLS to mail o Specified requirements for application of REQUIRETLS to mail
forwarders and mailing lists. forwarders and mailing lists.
o Clarified DNSSEC requirements to include MX lookup only. o Clarified DNSSEC requirements to include MX lookup only.
o Corrected terminology regarding message retrieval vs. delivery. o Corrected terminology regarding message retrieval vs. delivery.
o Changed category to standards track. o Changed category to standards track.
10.9. Changes Since -00 Draft 10.10. Changes Since -00 Draft
o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
parameter to better associate REQUIRETLS requirements with parameter to better associate REQUIRETLS requirements with
transmission of individual messages. transmission of individual messages.
o Addition of an option to require DNSSEC lookup of the remote mail o Addition of an option to require DNSSEC lookup of the remote mail
server, since this affects the common name of the certificate that server, since this affects the common name of the certificate that
is presented. is presented.
o Clarified the wording to more clearly state that TLS sessions must o Clarified the wording to more clearly state that TLS sessions must
 End of changes. 15 change blocks. 
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