Cleartext Considered Obsolete: Use of TLS for Email Submission and Access
draft-ietf-uta-email-deep-08

Document Type Active Internet-Draft (uta WG)
Last updated 2017-07-21
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Network Working Group                                           K. Moore
Internet-Draft                                                  Windrock
Updates: 1939, 2595, 3464, 3501, 5068,                         C. Newman
         6186, 6409 (if approved)                                 Oracle
Intended status: Standards Track                           July 21, 2017
Expires: January 22, 2018

   Cleartext Considered Obsolete: Use of TLS for Email Submission and
                                 Access
                      draft-ietf-uta-email-deep-08

Abstract

   This specification outlines current recommendations for use of
   Transport Layer Security (TLS) to provide confidentiality of email
   traffic between a mail user agent (MUA) and a mail submission or mail
   access server.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 January 22, 2018.

Copyright Notice

   Copyright (c) 2017 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

Moore & Newman          Expires January 22, 2018                [Page 1]
Internet-Draft   Use of TLS for Email Submission/Access        July 2017

   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology Used in This Document . . . . . .   3
   3.  Implicit TLS  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Implicit TLS for POP  . . . . . . . . . . . . . . . . . .   5
     3.2.  Implicit TLS for IMAP . . . . . . . . . . . . . . . . . .   5
     3.3.  Implicit TLS for SMTP Submission  . . . . . . . . . . . .   5
     3.4.  Implicit TLS Connection Closure for POP, IMAP and SMTP
           Submission  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Recommendations for Use of TLS by Mail Access
       Services and Message Submission Services  . . . . . . . . . .   6
     4.1.  Deprecation of Services Using Cleartext and TLS Versions
           < 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Mail Server Use of Client Certificate Authentication  . .   8
     4.3.  Recording TLS Cipher Suite in Received Header . . . . . .   9
     4.4.  TLS Server Certificate Requirements . . . . . . . . . . .   9
     4.5.  Recommended DNS records for mail protocol servers . . . .  10
       4.5.1.  MX records  . . . . . . . . . . . . . . . . . . . . .  10
       4.5.2.  SRV records . . . . . . . . . . . . . . . . . . . . .  10
       4.5.3.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  10
       4.5.4.  TLSA records  . . . . . . . . . . . . . . . . . . . .  10
     4.6.  Changes to Internet Facing Servers  . . . . . . . . . . .  10
   5.  Recommendations for use of TLS by Mail User Agents  . . . . .  10
     5.1.  Use of SRV records in Establishing Configuration  . . . .  12
     5.2.  Minimum Confidentiality Level . . . . . . . . . . . . . .  13
     5.3.  Certificiate Validation . . . . . . . . . . . . . . . . .  14
     5.4.  Certificate Pinning . . . . . . . . . . . . . . . . . . .  14
     5.5.  Client Certificate Authentication . . . . . . . . . . . .  15
   6.  Considerations related to Anti-Virus/Anti-Spam Software and
       Services  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  POP3S Port Registration Update  . . . . . . . . . . . . .  15
     7.2.  IMAPS Port Registration Update  . . . . . . . . . . . . .  16
     7.3.  Submissions Port Registration . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
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