SMTP MTA Strict Transport Security (MTA-STS)
RFC 8461
| Document | Type |
RFC - Proposed Standard
(September 2018; No errata)
Was draft-ietf-uta-mta-sts (uta WG)
|
|
|---|---|---|---|
| Last updated | 2018-09-26 | ||
| Replaces | draft-brotman-mta-sts | ||
| Stream | IETF | ||
| Formats | plain text pdf htmlized bibtex | ||
| 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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