SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-13
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DANE V. Dukhovni
Internet-Draft Two Sigma
Intended status: Standards Track W. Hardaker
Expires: April 29, 2015 Parsons
October 26, 2014
SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-13
Abstract
This memo describes a downgrade-resistant protocol for SMTP transport
security between Mail 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 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
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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 April 29, 2015.
Copyright Notice
Copyright (c) 2014 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
Dukhovni & Hardaker Expires April 29, 2015 [Page 1]
Internet-Draft SMTP security via opportunistic DANE TLS October 2014
the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6
1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6
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 . . . . . . . . . 8
2. Identifying applicable TLSA records . . . . . . . . . . . . . 9
2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 9
2.1.1. DNS errors, bogus 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 . . . . . . . . . . . . . . . . . 15
2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17
3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19
3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19
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 . . . . . . . . . . . . . . . . . . 26
6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 27
7. Note on DANE for Message User Agents . . . . . . . . . . . . 27
8. Interoperability considerations . . . . . . . . . . . . . . . 28
8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 28
8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 28
9. Operational Considerations . . . . . . . . . . . . . . . . . 29
9.1. Client Operational Considerations . . . . . . . . . . . . 29
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