SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)
RFC 7672

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Internet Engineering Task Force (IETF)                       V. Dukhovni
Request for Comments: 7672                                     Two Sigma
Category: Standards Track                                    W. Hardaker
ISSN: 2070-1721                                                  Parsons
                                                            October 2015

   SMTP Security via Opportunistic DNS-Based Authentication of Named
             Entities (DANE) Transport Layer Security (TLS)

Abstract

   This memo describes a downgrade-resistant protocol for SMTP transport
   security between Message Transfer Agents (MTAs), based on the DNS-
   Based Authentication of Named Entities (DANE) TLSA DNS record.
   Adoption of this protocol enables an incremental transition of the
   Internet email backbone to one using encrypted and authenticated
   Transport Layer Security (TLS).

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7672.

Copyright Notice

   Copyright (c) 2015 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
   (http://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.

Dukhovni & Hardaker          Standards Track                    [Page 1]
RFC 7672        SMTP Security via Opportunistic DANE TLS    October 2015

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
      1.2. Background .................................................6
      1.3. SMTP Channel Security ......................................6
           1.3.1. STARTTLS Downgrade Attack ...........................7
           1.3.2. Insecure Server Name without DNSSEC .................7
           1.3.3. Sender Policy Does Not Scale ........................8
           1.3.4. Too Many Certification Authorities ..................9
   2. Identifying Applicable TLSA Records .............................9
      2.1. DNS Considerations .........................................9
           2.1.1. DNS Errors, "Bogus" Responses, and
                  "Indeterminate" Responses ...........................9
           2.1.2. DNS Error Handling .................................11
           2.1.3. Stub Resolver Considerations .......................12
      2.2. TLS Discovery .............................................13
           2.2.1. MX Resolution ......................................14
           2.2.2. Non-MX Destinations ................................16
           2.2.3. TLSA Record Lookup .................................18
   3. DANE Authentication ............................................20
      3.1. TLSA Certificate Usages ...................................20
           3.1.1. Certificate Usage DANE-EE(3) .......................21
           3.1.2. Certificate Usage DANE-TA(2) .......................22
           3.1.3. Certificate Usages PKIX-TA(0) and PKIX-EE(1) .......23
      3.2. Certificate Matching ......................................24
           3.2.1. DANE-EE(3) Name Checks .............................24
           3.2.2. DANE-TA(2) Name Checks .............................24
           3.2.3. Reference Identifier Matching ......................25
   4. Server Key Management ..........................................26
   5. Digest Algorithm Agility .......................................27
   6. Mandatory TLS Security .........................................27
   7. Note on DANE for Message User Agents ...........................28
   8. Interoperability Considerations ................................28
      8.1. SNI Support ...............................................28
      8.2. Anonymous TLS Cipher Suites ...............................29
   9. Operational Considerations .....................................29
      9.1. Client Operational Considerations .........................29
      9.2. Publisher Operational Considerations ......................30