SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-02

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Document Type Active Internet-Draft (uta WG)
Last updated 2016-12-15
Replaces draft-brotman-mta-sts
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Using TLS in Applications                                    D. Margolis
Internet-Draft                                                 M. Risher
Intended status: Standards Track                             Google, Inc
Expires: June 18, 2017                                   B. Ramakrishnan
                                                             Yahoo!, Inc
                                                              A. Brotman
                                                            Comcast, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                       December 15, 2016

              SMTP MTA Strict Transport Security (MTA-STS)
                       draft-ietf-uta-mta-sts-02

Abstract

   SMTP Mail Transfer Agent Strict Transport Security (SMTP STS) is a
   mechanism enabling mail service providers to declare their ability to
   receive TLS-secured connections and an expected validity of
   certificates presented by their MX hosts, 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 18, 2017.

Copyright Notice

   Copyright (c) 2016 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

Margolis, et al.          Expires June 18, 2017                 [Page 1]
Internet-Draft                   MTA-STS                   December 2016

   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   3
   3.  Policy Discovery  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  MTA-STS TXT Records . . . . . . . . . . . . . . . . . . .   4
     3.2.  MTA-STS Policies  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  HTTPS Policy Fetching . . . . . . . . . . . . . . . . . .   6
     3.4.  Policy Selection for Smart Hosts  . . . . . . . . . . . .   6
   4.  Policy Validation . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  MX Matching . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  MX Certificate Validation . . . . . . . . . . . . . . . .   7
   5.  Policy Application  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  MX Preference . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Policy Application Control Flow . . . . . . . . . . . . .   8
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
     6.1.  Policy Updates  . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Appendix 1: Domain Owner STS example record . . . . . . . . .  11
     10.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . .  11
   11. Appendix 2: Message delivery pseudocode . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to negotiate the use of a TLS channel for secure mail
   transmission.

   While such _opportunistic_ encryption protocols provide a high
   barrier against passive man-in-the-middle traffic interception, any
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