SMTP MTA Strict Transport Security (MTA-STS)
RFC 8461

Document Type RFC - Proposed Standard (September 2018; No errata)
Last updated 2018-09-26
Replaces draft-brotman-mta-sts
Stream IETF
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Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Leif Johansson
Shepherd write-up Show (last changed 2018-03-06)
IESG IESG state RFC 8461 (Proposed Standard)
Consensus Boilerplate Yes
Telechat date
Responsible AD Alexey Melnikov
Send notices to Leif Johansson <leifj@sunet.se>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
Internet Engineering Task Force (IETF)                       D. Margolis
Request for Comments: 8461                                     M. Risher
Category: Standards Track                                   Google, Inc.
ISSN: 2070-1721                                          B. Ramakrishnan
                                                              Oath, Inc.
                                                              A. Brotman
                                                           Comcast, Inc.
                                                                J. Jones
                                                         Microsoft, Inc.
                                                          September 2018

              SMTP MTA Strict Transport Security (MTA-STS)

Abstract

   SMTP MTA Strict Transport Security (MTA-STS) is a mechanism enabling
   mail service providers (SPs) to declare their ability to receive
   Transport Layer Security (TLS) secure SMTP connections and to specify
   whether sending SMTP servers should refuse to deliver to MX hosts
   that do not offer TLS with a trusted server certificate.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8461.

Margolis, et al.             Standards Track                    [Page 1]
RFC 8461                         MTA-STS                  September 2018

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
   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.

Margolis, et al.             Standards Track                    [Page 2]
RFC 8461                         MTA-STS                  September 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   5
   3.  Policy Discovery  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  MTA-STS TXT Records . . . . . . . . . . . . . . . . . . .   6
     3.2.  MTA-STS Policies  . . . . . . . . . . . . . . . . . . . .   7
     3.3.  HTTPS Policy Fetching . . . . . . . . . . . . . . . . . .  10
     3.4.  Policy Selection for Smart Hosts and Subdomains . . . . .  11
   4.  Policy Validation . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  MX Host Validation  . . . . . . . . . . . . . . . . . . .  12
     4.2.  Recipient MTA Certificate Validation  . . . . . . . . . .  12
   5.  Policy Application  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Policy Application Control Flow . . . . . . . . . . . . .  13
   6.  Reporting Failures  . . . . . . . . . . . . . . . . . . . . .  13
   7.  Interoperability Considerations . . . . . . . . . . . . . . .  14
     7.1.  SNI Support . . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Minimum TLS Version Support . . . . . . . . . . . . . . .  14
   8.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
     8.1.  Policy Updates  . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  Policy Delegation . . . . . . . . . . . . . . . . . . . .  15
     8.3.  Removing MTA-STS  . . . . . . . . . . . . . . . . . . . .  16
     8.4.  Preserving MX Candidate Traversal . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Well-Known URIs Registry  . . . . . . . . . . . . . . . .  17
     9.2.  MTA-STS TXT Record Fields . . . . . . . . . . . . . . . .  17
     9.3.  MTA-STS Policy Fields . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
     10.1.  Obtaining a Signed Certificate . . . . . . . . . . . . .  18
     10.2.  Preventing Policy Discovery  . . . . . . . . . . . . . .  19
     10.3.  Denial of Service  . . . . . . . . . . . . . . . . . . .  19
     10.4.  Weak Policy Constraints  . . . . . . . . . . . . . . . .  20
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