draft-ietf-uta-mta-sts-08.txt   draft-ietf-uta-mta-sts-09.txt 
Using TLS in Applications D. Margolis Using TLS in Applications D. Margolis
Internet-Draft M. Risher Internet-Draft M. Risher
Intended status: Standards Track Google, Inc Intended status: Standards Track Google, Inc
Expires: February 16, 2018 B. Ramakrishnan Expires: March 9, 2018 B. Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
A. Brotman A. Brotman
Comcast, Inc Comcast, Inc
J. Jones J. Jones
Microsoft, Inc Microsoft, Inc
August 15, 2017 September 5, 2017
SMTP MTA Strict Transport Security (MTA-STS) SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-08 draft-ietf-uta-mta-sts-09
Abstract Abstract
SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a
mechanism enabling mail service providers to declare their ability to mechanism enabling mail service providers to declare their ability to
receive Transport Layer Security (TLS) secure SMTP connections, and receive Transport Layer Security (TLS) secure SMTP connections, and
to specify whether sending SMTP servers should refuse to deliver to to specify whether sending SMTP servers should refuse to deliver to
MX hosts that do not offer TLS with a trusted server certificate. MX hosts that do not offer TLS with a trusted server certificate.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 16, 2018. This Internet-Draft will expire on March 9, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3 2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3
3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4 3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4
3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4 3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4
3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5 3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5
3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 7 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8
3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9
4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 9 3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 9
4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 9 4. Policy Application . . . . . . . . . . . . . . . . . . . . . 10
5. Policy Application . . . . . . . . . . . . . . . . . . . . . 10 4.1. Policy Application Control Flow . . . . . . . . . . . . . 11
5.1. Policy Application Control Flow . . . . . . . . . . . . . 10 5. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11
6. Operational Considerations . . . . . . . . . . . . . . . . . 11 6. Operational Considerations . . . . . . . . . . . . . . . . . 12
6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 11 6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6.2. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 12
7.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 11 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 12 7.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 13
7.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 12 7.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 13
8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 13 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 13 8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 14
8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 14 8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 14
8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 14 8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 15
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15 8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 16
10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 15 8.5. Compromise of the Web PKI System . . . . . . . . . . . . 16
11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 15 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . 18 11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 17
12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
hosts to negotiate the use of a TLS channel for encrypted mail hosts to negotiate the use of a TLS channel for encrypted mail
transmission. transmission.
While this opportunistic encryption protocol by itself provides a While this opportunistic encryption protocol by itself provides a
high barrier against passive man-in-the-middle traffic interception, high barrier against passive man-in-the-middle traffic interception,
any attacker who can delete parts of the SMTP session (such as the any attacker who can delete parts of the SMTP session (such as the
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SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119]. document, are to be interpreted as described in [RFC2119].
We also define the following terms for further use in this document: We also define the following terms for further use in this document:
o MTA-STS Policy: A commitment by the Policy Domain to support PKIX o MTA-STS Policy: A commitment by the Policy Domain to support PKIX
authenticated TLS for the specified MX hosts. authenticated TLS for the specified MX hosts.
o Policy Domain: The domain for which an MTA-STS Policy is defined. o Policy Domain: The domain for which an MTA-STS Policy is defined.
This is the next-hop domain; when sending mail to This is the next-hop domain; when sending mail to
"alice@example.com" this would ordinarly be "example.com", but "alice@example.com" this would ordinarily be "example.com", but
this may be overriden by explicit routing rules (as described in this may be overridden by explicit routing rules (as described in
Section 3.4, "Policy Selection for Smart Hosts and Subdomains"). Section 3.4, "Policy Selection for Smart Hosts and Subdomains").
2. Related Technologies 2. Related Technologies
The DANE TLSA record [RFC7672] is similar, in that DANE is also The DANE TLSA record [RFC7672] is similar, in that DANE is also
designed to upgrade unauthenticated encryption or plaintext designed to upgrade unauthenticated encryption or plaintext
transmission into authenticated, downgrade-resistent encrypted transmission into authenticated, downgrade-resistant encrypted
tarnsmission. DANE requires DNSSEC [RFC4033] for authentication; the transmission. DANE requires DNSSEC [RFC4033] for authentication; the
mechanism described here instead relies on certificate authorities mechanism described here instead relies on certificate authorities
(CAs) and does not require DNSSEC, at a cost of risking malicious (CAs) and does not require DNSSEC, at a cost of risking malicious
downgrades. For a thorough discussion of this trade-off, see downgrades. For a thorough discussion of this trade-off, see
Section 8, "Security Considerations". Section 8, "Security Considerations".
In addition, MTA-STS provides an optional report-only mode, enabling In addition, MTA-STS provides an optional report-only mode, enabling
soft deployments to detect policy failures; partial deployments can soft deployments to detect policy failures; partial deployments can
be achieved in DANE by deploying TLSA records only for some of a be achieved in DANE by deploying TLSA records only for some of a
domain's MXs, but such a mechanism is not possible for the per-domain domain's MXs, but such a mechanism is not possible for the per-domain
policies used by MTA-STS. policies used by MTA-STS.
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updated by comparing to the "id" of a previously seen policy. updated by comparing to the "id" of a previously seen policy.
There is no implied ordering of "id" fields between revisions. There is no implied ordering of "id" fields between revisions.
An example TXT record is as below: An example TXT record is as below:
"_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"" "_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;""
The formal definition of the "_mta-sts" TXT record, defined using The formal definition of the "_mta-sts" TXT record, defined using
[RFC7405], is as follows: [RFC7405], is as follows:
sts-text-record = sts-version field-delim sts-id sts-text-record = sts-version 1*(field-delim sts-field) [field-delim]
*(field-delim sts-extension) [field-delim]
sts-field = sts-id / ; Note that sts-id record
sts-extension ; is required.
field-delim = *WSP ";" *WSP field-delim = *WSP ";" *WSP
sts-version = %s"v=STSv1" sts-version = %s"v=STSv1"
sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=... sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=...
sts-extension = sts-ext-name "=" sts-ext-value ; name=value sts-extension = sts-ext-name "=" sts-ext-value ; name=value
sts-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".") sts-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")
sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding "=",
; "=", ";", SP, and ; ";", SP, and control
; control chars ; chars
If multiple TXT records for "_mta-sts" are returned by the resolver, If multiple TXT records for "_mta-sts" are returned by the resolver,
records which do not begin with "v=STSv1;" are discarded. If the records which do not begin with "v=STSv1;" are discarded. If the
number of resulting records is not one, senders MUST assume the number of resulting records is not one, senders MUST assume the
recipient domain does not implement MTA-STS and skip the remaining recipient domain does not implement MTA-STS and skip the remaining
steps of policy discovery. steps of policy discovery. If the resulting TXT record contains
multiple strings, then the record MUST be treated as if those strings
are concatenated together without adding spaces.
3.2. MTA-STS Policies 3.2. MTA-STS Policies
The policy itself is a set of key/value pairs served via the HTTPS The policy itself is a set of key/value pairs (similar to [RFC5322]
GET method from the fixed [RFC5785] "well-known" path of ".well- header fields) served via the HTTPS GET method from the fixed
known/mta-sts.txt" served by the "mta-sts" host at the Policy Domain; [RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by
the [RFC2616] "Content-Type" header MUST be "text/plain". Thus for the "mta-sts" host at the Policy Domain. Thus for "example.com" the
"example.com" the path is "https://mta-sts.example.com/.well-known/ path is "https://mta-sts.example.com/.well-known/mta-sts.txt".
mta-sts.txt".
The [RFC7231] "Content-Type" media type for this resource MUST be
"text/plain". When fetching a policy, senders SHOULD validate that
the media type is "text/plain" to guard against cases where
webservers allow untrusted users to host non-text content (typically,
HTML or images) at a user-defined path. Additional "Content-Type"
parameters are ignored.
This resource contains the following line-separated key/value pairs: This resource contains the following line-separated key/value pairs:
o "version": (plain-text, required). Currently only "STSv1" is o "version": (plain-text). Currently only "STSv1" is supported.
supported.
o "mode": (plain-text, required). Either "enforce" or "report", o "mode": (plain-text). One of "enforce", "report", or "none",
indicating the expected behavior of a sending MTA in the case of a indicating the expected behavior of a sending MTA in the case of a
policy validation failure. policy validation failure.
o "max_age": Max lifetime of the policy (plain-text non-negative o "max_age": Max lifetime of the policy (plain-text non-negative
integer seconds, required). Well-behaved clients SHOULD cache a integer seconds, maximum value of 31557600). Well-behaved clients
policy for up to this value from last policy fetch time. To SHOULD cache a policy for up to this value from last policy fetch
mitigate the risks of attacks at policy refresh time, it is time. To mitigate the risks of attacks at policy refresh time, it
expected that this value typically be in the range of weeks or is expected that this value typically be in the range of weeks or
greater. greater.
o "mx": MX identity patterns (list of plain-text strings, required). o "mx": MX identity patterns (list of plain-text strings). One or
One or more patterns matching a Common Name ([RFC6125]) or Subject more patterns matching a Common Name ([RFC6125]) or Subject
Alternative Name ([RFC5280]) DNS-ID present in the X.509 Alternative Name ([RFC5280]) DNS-ID present in the X.509
certificate presented by any MX receiving mail for this domain. certificate presented by any MX receiving mail for this domain.
For example: "mx: mail.example.com mx: .example.net" indicates For example: "mx: mail.example.com mx: .example.net" indicates
that mail for this domain might be handled by any MX with a that mail for this domain might be handled by any MX with a
certificate valid for a host at "mail.example.com" or certificate valid for a host at "mail.example.com" or
"example.net". Valid patterns can be either fully specified names "example.net". Valid patterns can be either fully specified names
("example.com") or suffixes (".example.net") matching the right- ("example.com") or suffixes (".example.net") matching the right-
hand parts of a server's identity; the latter case are hand parts of a server's identity; the latter case are
distinguished by a leading period. If there are more than one MX distinguished by a leading period. If there are more than one MX
specified by the policy, they MUST be on separate lines within the specified by the policy, they MUST be on separate lines within the
policy file. In the case of Internationalized Domain Names policy file. In the case of Internationalized Domain Names
([RFC5891]), the MX MUST specify the Punycode-encoded A-label ([RFC5891]), the MX MUST specify the Punycode-encoded A-label
[RFC3492] and not the Unicode-encoded U-label. The full semantics [RFC3492] and not the Unicode-encoded U-label. The full semantics
of certificate validation are described in Section 4.1, "MX of certificate validation are described in Section 3.5, "MX
Certificate Validation." Certificate Validation."
An example policy is as below: An example policy is as below:
version: STSv1 version: STSv1
mode: enforce mode: enforce
mx: mail.example.com mx: mail.example.com
mx: .example.net mx: .example.net
mx: backupmx.example.com mx: backupmx.example.com
max_age: 123456 max_age: 123456
The formal definition of the policy resource, defined using The formal definition of the policy resource, defined using
[RFC7405], is as follows: [RFC7405], is as follows:
sts-policy-record = sts-policy-version CRLF sts-policy-record = *WSP sts-policy-field *WSP
sts-policy-mode CRLF *(CRLF *WSP sts-policy-field *WSP)
1*(sts-policy-mx CRLF) [CRLF]
sts-policy-max-age
field-delim = ":" *WSP sts-policy-field = sts-policy-version / ; required once
sts-policy-mode / ; required once
sts-policy-max-age / ; required once
0*(sts-policy-mx *WSP CRLF) / ; required at
; least once
; except when mode
; is "none"
sts-policy-extension ; other fields
sts-policy-version = sts-policy-version-field field-delim field-delim = ":" *WSP
sts-policy-version-value
sts-policy-version-field = %s"version" sts-policy-version = sts-policy-version-field field-delim
sts-policy-version-value
sts-policy-version-value = %s"STSv1" sts-policy-version-field = %s"version"
sts-policy-mode = sts-policy-mode-field field-delim sts-policy-version-value = %s"STSv1"
sts-policy-mode-value
sts-policy-mode-field = %s"mode" sts-policy-mode = sts-policy-mode-field field-delim
sts-policy-mode-value
sts-policy-model-value = %s"report" / %s"enforce" sts-policy-mode-field = %s"mode"
sts-policy-mx = sts-policy-mx-field field-delim sts-policy-model-value = %s"report" / %s"enforce" / %s"none"
sts-policy-mx-value
sts-policy-mx-field = %s"mx" sts-policy-mx = sts-policy-mx-field field-delim
sts-policy-mx-value
sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".") sts-policy-mx-field = %s"mx"
sts-policy-max-age = sts-policy-max-age-field field-delim sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-max-age-value
sts-policy-max-age-field = %s"max_age" sts-policy-max-age = sts-policy-max-age-field field-delim
sts-policy-max-age-value
sts-policy-max-age-value = 1*10(DIGIT) sts-policy-max-age-field = %s"max_age"
sts-policy-max-age-value = 1*10(DIGIT)
sts-policy-extension = sts-policy-ext-name field-delim ; additional
sts-policy-ext-value ; extension
; fields
sts-policy-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding
; "=", ";", SP,
; and control
; chars
Parsers MUST accept TXT records and policy files which are Parsers MUST accept TXT records and policy files which are
syntactically valid (i.e. valid key/value pairs separated by semi- syntactically valid (i.e. valid key/value pairs separated by semi-
colons for TXT records) and implementing a superset of this colons for TXT records) and but containing additional key/value pairs
specification, in which case unknown fields SHALL be ignored. If any not specified in this document, in which case unknown fields SHALL be
field other than "mx" is duplicated, the first entry will be honored, ignored. If any non-repeated field--i.e. all fields excepting "mx"--
the rest should be ignored. For the "mx" field, all valid entries is duplicated, all entries except for the first SHALL be ignored. If
will be utilized when enforcing the stated policy. any field is not specified, the policy SHALL be treated as invalid.
3.3. HTTPS Policy Fetching 3.3. HTTPS Policy Fetching
When fetching a new policy or updating a policy, the HTTPS endpoint When fetching a new policy or updating a policy, the HTTPS endpoint
MUST present a X.509 certificate which is valid for the "mta-sts" MUST present a X.509 certificate which is valid for the "mta-sts"
host (as described below), chain to a root CA that is trusted by the host (e.g. "mta-sts.example.com") as described below, chain to a
sending MTA, and be non-expired. It is expected that sending MTAs root CA that is trusted by the sending MTA, and be non-expired. It
use a set of trusted CAs similar to those in widely deployed Web is expected that sending MTAs use a set of trusted CAs similar to
browsers and operating systems. those in widely deployed Web browsers and operating systems.
The certificate is valid for the "mta-sts" host with respect to the The certificate is valid for the "mta-sts" host with respect to the
rules described in [RFC6125], with the following application-specific rules described in [RFC6125], with the following application-specific
considerations: considerations:
o Matching is performed only against the DNS-ID and CN-ID o Matching is performed only against the DNS-ID and CN-ID
identifiers. identifiers.
o DNS domain names in server certificates MAY contain the wildcard o DNS domain names in server certificates MAY contain the wildcard
character '*' as the complete left-most label within the character '*' as the complete left-most label within the
identifier. identifier.
The certificate MAY be checked for revocation via the Online The certificate MAY be checked for revocation via the Online
Certificate Status Protocol (OCSP) [RFC2560], certificate revocation Certificate Status Protocol (OCSP) [RFC2560], certificate revocation
lists (CRLs), or some other mechanism. lists (CRLs), or some other mechanism.
HTTP 3xx redirects MUST NOT be followed. Policies fetched via HTTPS are only valid if the HTTP response code
is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP
caching (as specified in [RFC7234]) MUST NOT be used.
Senders may wish to rate-limit the frequency of attempts to fetch the Senders may wish to rate-limit the frequency of attempts to fetch the
HTTPS endpoint even if a valid TXT record for the recipient domain HTTPS endpoint even if a valid TXT record for the recipient domain
exists. In the case that the HTTPS GET fails, we suggest exists. In the case that the HTTPS GET fails, we suggest
implementions may limit further attempts to a period of five minutes implementions may limit further attempts to a period of five minutes
or longer per version ID, to avoid overwhelming resource-constrained or longer per version ID, to avoid overwhelming resource-constrained
recipients with cascading failures. recipients with cascading failures.
Senders MAY impose a timeout on the HTTPS GET and/or a limit on the Senders MAY impose a timeout on the HTTPS GET and/or a limit on the
maximum size of the response body to avoid long delays or resource maximum size of the response body to avoid long delays or resource
exhaustion during attempted policy updates. A suggested timeout is exhaustion during attempted policy updates. A suggested timeout is
one minute, and a suggested maximum policy size 64 kilobytes; policy one minute, and a suggested maximum policy size 64 kilobytes; policy
hosts SHOULD respond to requests with a complete policy body within hosts SHOULD respond to requests with a complete policy body within
that timeout and size limit. that timeout and size limit.
If a valid TXT record is found but no policy can be fetched via HTTPS If a valid TXT record is found but no policy can be fetched via HTTPS
(for any reason), and there is no valid (non-expired) previously- (for any reason), and there is no valid (non-expired) previously-
cached policy, senders MUST continue with delivery as though the cached policy, senders MUST continue with delivery as though the
domain has not implemented MTA-STS. Senders who implement TLSRPT domain has not implemented MTA-STS.
(TODO: add ref) should, however, report this failure to the recipient
domain if the domain implements TLSRPT as well.
Conversely, if no "live" policy can be discovered via DNS or fetched Conversely, if no "live" policy can be discovered via DNS or fetched
via HTTPS, but a valid (non-expired) policy exists in the sender's via HTTPS, but a valid (non-expired) policy exists in the sender's
cache, the sender MUST apply that cached policy. cache, the sender MUST apply that cached policy.
3.4. Policy Selection for Smart Hosts and Subdomains 3.4. Policy Selection for Smart Hosts and Subdomains
When sending mail via a "smart host"--an intermediate SMTP relay When sending mail via a "smart host"--an intermediate SMTP relay
rather than the message recipient's server--compliant senders MUST rather than the message recipient's server--compliant senders MUST
treat the smart host domain as the policy domain for the purposes of treat the smart host domain as the policy domain for the purposes of
policy discovery and application. policy discovery and application.
When sending mail to a mailbox at a subdomain, compliant senders MUST When sending mail to a mailbox at a subdomain, compliant senders MUST
NOT attempt to fetch a policy from the parent zone. Thus for mail NOT attempt to fetch a policy from the parent zone. Thus for mail
sent to "user@mail.example.com", the policy can be fetched only from sent to "user@mail.example.com", the policy can be fetched only from
"mail.example.com", not "example.com". "mail.example.com", not "example.com".
4. Policy Validation #Policy Validation
When sending to an MX at a domain for which the sender has a valid When sending to an MX at a domain for which the sender has a valid
and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST
validate: validate:
1. That the recipient MX supports STARTTLS and offers a valid PKIX- 1. That the recipient MX supports STARTTLS and offers a valid PKIX-
based TLS certificate. based TLS certificate.
2. That at least one of the policy's "mx" patterns matches at least 2. That at least one of the policy's "mx" patterns matches at least
one of the identities presented in the MX's X.509 certificate, as one of the identities presented in the MX's X.509 certificate, as
described in "MX Certificate Validation". described in "MX Certificate Validation".
This section does not dictate the behavior of sending MTAs when This section does not dictate the behavior of sending MTAs when
policies fail to validate; in particular, validation failures of policies fail to validate; in particular, validation failures of
policies which specify "report" mode MUST NOT be interpreted as policies which specify mode values of "report" or "none" MUST NOT be
delivery failures, as described in Section 5, "Policy Application". interpreted as delivery failures, as described in Section 4, "Policy
Application".
4.1. MX Certificate Validation 3.5. MX Certificate Validation
The certificate presented by the receiving MX MUST chain to a root CA The certificate presented by the receiving MX MUST chain to a root CA
that is trusted by the sending MTA and be non-expired. The that is trusted by the sending MTA and be non-expired. The
certificate MUST have a CN-ID ([RFC6125]) or SAN ([RFC5280]) with a certificate MUST have a CN-ID ([RFC6125]) or subject alternative name
DNS-ID matching the "mx" pattern. The MX's certificate MAY also be (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's
checked for revocation via OCSP [RFC2560], certificate revocation certificate MAY also be checked for revocation via OCSP [RFC2560],
lists (CRLs), or some other mechanism. CRLs [RFC6818], or some other mechanism.
Because the "mx" patterns are not hostnames, however, matching is not Because the "mx" patterns are not hostnames, however, matching is not
identical to other common cases of X.509 certificate authentication identical to other common cases of X.509 certificate authentication
(as described, for example, in [RFC6125]). Consider the example (as described, for example, in [RFC6125]). Consider the example
policy given above, with an "mx" pattern containing ".example.net". policy given above, with an "mx" pattern containing ".example.com".
In this case, if the MX server's X.509 certificate contains a SAN In this case, if the MX server's X.509 certificate contains a SAN
matching "*.example.net", we are required to implement "wildcard-to- matching "*.example.com", we are required to implement "wildcard-to-
wildcard" matching. wildcard" matching.
To simplify this case, we impose the following constraints on To simplify this case, we impose the following constraints on
wildcard certificates, identical to those in [RFC7672] section 3.2.3 wildcard certificates, identical to those in [RFC7672] section 3.2.3
and [@!RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN- and [@?RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN-
IDs, but must be the entire first label of the identifier (that is, IDs, but must be the entire first label of the identifier (that is,
"*.example.com", not "mail*.example.com"). Senders who are comparing "*.example.com", not "mail*.example.com"). Senders who are comparing
a "suffix" MX pattern with a wildcard identifier should thus strip a "suffix" MX pattern with a wildcard identifier should thus strip
the wildcard and ensure that the two sides match label-by-label, the wildcard and ensure that the two sides match label-by-label,
until all labels of the shorter side (if unequal length) are until all labels of the shorter side (if unequal length) are
consumed. consumed.
Note that a wildcard _must_ match a label; an "mx" pattern of
".example.com" thus does not match a SAN of "example.com", nor does a
SAN of "*.example.com" match an "mx" of "example.com".
A simple pseudocode implementation of this algorithm is presented in A simple pseudocode implementation of this algorithm is presented in
the Appendix. the Appendix.
5. Policy Application 4. Policy Application
When sending to an MX at a domain for which the sender has a valid, When sending to an MX at a domain for which the sender has a valid,
non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies
the result of a policy validation failure one of two ways, depending the result of a policy validation failure one of two ways, depending
on the value of the policy "mode" field: on the value of the policy "mode" field:
1. "report": In this mode, sending MTAs which also implement the 1. "enforce": In this mode, sending MTAs MUST NOT deliver the
message to hosts which fail MX matching or certificate
validation.
2. "report": In this mode, sending MTAs which also implement the
TLSRPT specification (TODO: add ref) merely send a report TLSRPT specification (TODO: add ref) merely send a report
indicating policy application failures (so long as TLSRPT is also indicating policy application failures (so long as TLSRPT is also
implemented by the recipient domain). implemented by the recipient domain).
2. "enforce": In this mode, sending MTAs MUST NOT deliver the 3. "none": In this mode, sending MTAs should treat the policy domain
message to hosts which fail MX matching or certificate as though it does not have any active policy; see Section 6.2,
validation. "Removing MTA-STS", for use of this mode value.
When a message fails to deliver due to an "enforce" policy, a When a message fails to deliver due to an "enforce" policy, a
compliant MTA MUST NOT permanently fail to deliver messages before compliant MTA MUST NOT permanently fail to deliver messages before
checking for the presence of an updated policy at the Policy Domain. checking for the presence of an updated policy at the Policy Domain.
(In all cases, MTAs SHOULD treat such failures as transient errors (In all cases, MTAs SHOULD treat such failures as transient errors
and retry delivery later.) This allows implementing domains to and retry delivery later.) This allows implementing domains to
update long-lived policies on the fly. update long-lived policies on the fly.
Finally, in both "enforce" and "report" modes, failures to deliver in 4.1. Policy Application Control Flow
compliance with the applied policy result in failure reports to the
policy domain, as described in the TLSRPT specification (TODO: add
ref).
5.1. Policy Application Control Flow
An example control flow for a compliant sender consists of the An example control flow for a compliant sender consists of the
following steps: following steps:
1. Check for a cached policy whose time-since-fetch has not exceeded 1. Check for a cached policy whose time-since-fetch has not exceeded
its "max_age". If none exists, attempt to fetch a new policy its "max_age". If none exists, attempt to fetch a new policy
(perhaps asynchronously, so as not to block message delivery). (perhaps asynchronously, so as not to block message delivery).
Optionally, sending MTAs may unconditionally check for a new Optionally, sending MTAs may unconditionally check for a new
policy at this step. policy at this step.
2. For each candidate MX, in order of MX priority, attempt to 2. For each candidate MX, in order of MX priority, attempt to
deliver the message, enforcing STARTTLS and, assuming a policy is deliver the message, enforcing STARTTLS and, assuming a policy is
present, PKIX certificate validation as described in Section 4.1, present, PKIX certificate validation as described in Section 3.5,
"MX Certificate Validation." "MX Certificate Validation."
3. A message delivery MUST NOT be permanently failed until the 3. A message delivery MUST NOT be permanently failed until the
sender has first checked for the presence of a new policy (as sender has first checked for the presence of a new policy (as
indicated by the "id" field in the "_mta-sts" TXT record). If a indicated by the "id" field in the "_mta-sts" TXT record). If a
new policy is not found, existing rules for the case of temporary new policy is not found, existing rules for the case of temporary
message delivery failures apply (as discussed in [RFC5321] message delivery failures apply (as discussed in [RFC5321]
section 4.5.4.1). section 4.5.4.1).
5. Reporting Failures
MTA-STS is intended to be used along with TLSRPT (TODO: add ref) in
order to ensure implementing domains can detect cases of both benign
and malicious failures, and to ensure that failures that indicate an
active attack are discoverable. As such, senders who also implement
TLSRPT SHOULD treat the following events as reportable failures:
o HTTPS policy fetch failures when a valid TXT record is present.
o Policy fetch failures of any kind when a valid policy exists in
the policy cache, except if that policy's mode is "none".
o Delivery attempts in which a contacted MX does not support
STARTTLS or does not present a certificate which validates
according to the applied policy, except if that policy's mode is
"none".
6. Operational Considerations 6. Operational Considerations
6.1. Policy Updates 6.1. Policy Updates
Updating the policy requires that the owner make changes in two Updating the policy requires that the owner make changes in two
places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and
at the corresponding HTTPS endpoint. As a result, recipients should at the corresponding HTTPS endpoint. As a result, recipients should
expect a policy will continue to be used by senders until both the expect a policy will continue to be used by senders until both the
HTTPS and TXT endpoints are updated and the TXT record's TTL has HTTPS and TXT endpoints are updated and the TXT record's TTL has
passed. passed.
skipping to change at page 11, line 42 skipping to change at page 12, line 27
message while applying a cache of the recipient's now-outdated policy message while applying a cache of the recipient's now-outdated policy
may be unable to discover that a new policy exists until the DNS TTL may be unable to discover that a new policy exists until the DNS TTL
has passed. Recipients should therefore ensure that old policies has passed. Recipients should therefore ensure that old policies
continue to work for message delivery during this period of time, or continue to work for message delivery during this period of time, or
risk message delays. risk message delays.
Recipients should also prefer to update the HTTPS policy body before Recipients should also prefer to update the HTTPS policy body before
updating the TXT record; this ordering avoids the risk that senders, updating the TXT record; this ordering avoids the risk that senders,
seeing a new TXT record, mistakenly cache the old policy from HTTPS. seeing a new TXT record, mistakenly cache the old policy from HTTPS.
6.2. Removing MTA-STS
In order to facilitate clean opt-out of MTA-STS by implementing
policy domains, and to distinguish clearly between failures which
indicate attacks and those which indicate such opt-outs, MTA-STS
implements the "none" mode, which allows validated policies to
indicate authoritatively that the policy domain wishes to no longer
implement MTA-STS and may, in the future, remove the MTA-STS TXT and
policy endpoints entirely.
A suggested workflow to implement such an opt out is as follows:
1. Publish a new policy with "mode" equal to "none" and a small
"max_age" (e.g. one day).
2. Publish a new TXT record to trigger fetching of the new policy.
3. When all previously served policies have expired--normally this
is the time the previously published policy was last served plus
that policy's "max_age", but note that older policies may have
been served with a greater "max_age", allowing overlapping policy
caches--safely remove the TXT record and HTTPS endpoint.
7. IANA Considerations 7. IANA Considerations
7.1. Well-Known URIs Registry 7.1. Well-Known URIs Registry
A new .well-known URI will be registered in the Well-Known URIs A new .well-known URI will be registered in the Well-Known URIs
registry as described below: registry as described below:
URI Suffix: mta-sts.txt Change Controller: IETF URI Suffix: mta-sts.txt Change Controller: IETF
7.2. MTA-STS TXT Record Fields 7.2. MTA-STS TXT Record Fields
skipping to change at page 13, line 37 skipping to change at page 14, line 47
STS Policy. The sender policy cache is designed to resist this STS Policy. The sender policy cache is designed to resist this
attack by decreasing the frequency of policy discovery and thus attack by decreasing the frequency of policy discovery and thus
reducing the window of vulnerability; it is nonetheless a risk that reducing the window of vulnerability; it is nonetheless a risk that
attackers who can predict or induce policy discovery--for example, by attackers who can predict or induce policy discovery--for example, by
inducing a victim sending domain to send mail to a never-before- inducing a victim sending domain to send mail to a never-before-
contacted recipient while carrying out a man-in-the-middle attack-- contacted recipient while carrying out a man-in-the-middle attack--
may be able to foil policy discovery and effectively downgrade the may be able to foil policy discovery and effectively downgrade the
security of the message delivery. security of the message delivery.
Since this attack depends upon intercepting initial policy discovery, Since this attack depends upon intercepting initial policy discovery,
we strongly recommend implementors to prefer policy "max_age" values we strongly recommend implementers to prefer policy "max_age" values
to be as long as is practical. to be as long as is practical.
Because this attack is also possible upon refresh of a cached policy, Because this attack is also possible upon refresh of a cached policy,
we suggest implementors do not wait until a cached policy has expired we suggest implementers do not wait until a cached policy has expired
before checking for an update; if senders attempt to refresh the before checking for an update; if senders attempt to refresh the
cache regularly (for instance, by checking their cached version cache regularly (for instance, by checking their cached version
string against the TXT record on each successful send, or in a string against the TXT record on each successful send, or in a
background task that runs daily or weekly), an attacker would have to background task that runs daily or weekly), an attacker would have to
foil policy discovery consistently over the lifetime of a cached foil policy discovery consistently over the lifetime of a cached
policy to prevent a successful refresh. policy to prevent a successful refresh.
Resistence to downgrade attacks of this nature--due to the ability to Additionally, MTAs should alert administrators to repeated policy
refresh failures long before cached policies expire (through warning
logs or similar applicable mechanisms), allowing administrators to
detect such a persistent attack on policy refresh. (However, they
should not implement such alerts if the cached policy has a "none"
mode, to allow clean MTA-STS removal, as described in Section 6.2.)
Resistance to downgrade attacks of this nature--due to the ability to
authoritatively determine "lack of a record" even for non- authoritatively determine "lack of a record" even for non-
participating recipients--is a feature of DANE, due to its use of participating recipients--is a feature of DANE, due to its use of
DNSSEC for policy discovery. DNSSEC for policy discovery.
8.3. Denial of Service 8.3. Denial of Service
We additionally consider the Denial of Service risk posed by an We additionally consider the Denial of Service risk posed by an
attacker who can modify the DNS records for a victim domain. Absent attacker who can modify the DNS records for a victim domain. Absent
MTA-STS, such an attacker can cause a sending MTA to cache invalid MX MTA-STS, such an attacker can cause a sending MTA to cache invalid MX
records, but only for however long the sending resolver caches those records, but only for however long the sending resolver caches those
skipping to change at page 15, line 7 skipping to change at page 16, line 21
Even if an attacker cannot modify a served policy, the potential Even if an attacker cannot modify a served policy, the potential
exists for configurations that allow attackers on the same domain to exists for configurations that allow attackers on the same domain to
receive mail for that domain. For example, an easy configuration receive mail for that domain. For example, an easy configuration
option when authoring an MTA-STS Policy for "example.com" is to set option when authoring an MTA-STS Policy for "example.com" is to set
the "mx" equal to ".example.com"; recipient domains must consider in the "mx" equal to ".example.com"; recipient domains must consider in
this case the risk that any user possessing a valid hostname and CA- this case the risk that any user possessing a valid hostname and CA-
signed certificate (for example, "dhcp-123.example.com") will, from signed certificate (for example, "dhcp-123.example.com") will, from
the perspective of MTA-STS Policy validation, be a valid MX host for the perspective of MTA-STS Policy validation, be a valid MX host for
that domain. that domain.
8.5. Compromise of the Web PKI System
A host of risks apply to the PKI system used for certificate
authentication, both of the "mta-sts" HTTPS host's certificate and
the SMTP servers' certificates. These risks are broadly applicable
within the Web PKI ecosystem and are not specific to MTA-STS;
nonetheless, they deserve some consideration in this context.
Broadly speaking, attackers may compromise the system by obtaining
certificates under fraudulent circumstances (i.e. by impersonating
the legitimate owner of the victim domain), by compromising a
Certificate Authority or Delegate Authority's private keys, by
obtaining a legitimate certificate issued to the victim domain, and
similar.
One approach commonly employed by Web browsers to help mitigate
against some of these attacks is to allow for revocation of
compromised or fraudulent certificates via OCSP [RFC2560] or CRLs
[RFC6818]. Such mechanisms themselves represent tradeoffs and are
not universally implemented; we nonetheless recommend implementors of
MTA-STS to implement revocation mechanisms which are most applicable
to their implementations.
9. Contributors 9. Contributors
Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)
Wei Chuang Google, Inc weihaw (at) google (dot com) Wei Chuang Google, Inc weihaw (at) google (dot com)
Brandon Long Google, Inc blong (at) google (dot com) Brandon Long Google, Inc blong (at) google (dot com)
Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com)
Klaus Umbach 1&1 Mail & Media Development & Technology GmbH Klaus Umbach 1&1 Mail & Media Development & Technology GmbH
klaus.umbach (at) 1und1 (dot de) klaus.umbach (at) 1und1 (dot de)
Markus Laber 1&1 Mail & Media Development & Technology GmbH Markus Laber 1&1 Mail & Media Development & Technology GmbH
markus.laber (at) 1und1 (dot de) markus.laber (at) 1und1 (dot de)
10. Appendix 1: MTA-STS example record & policy 10. Appendix 1: MTA-STS example record & policy
skipping to change at page 15, line 36 skipping to change at page 17, line 25
policy that will solicit reports from senders without affecting how policy that will solicit reports from senders without affecting how
the messages are processed, in order to verify the identity of MXs the messages are processed, in order to verify the identity of MXs
that handle mail for "example.com", confirm that TLS is correctly that handle mail for "example.com", confirm that TLS is correctly
used, and ensure that certificates presented by the recipient MX used, and ensure that certificates presented by the recipient MX
validate. validate.
MTA-STS policy indicator TXT RR: MTA-STS policy indicator TXT RR:
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
MTA-STS Policy file served as the response body at [1] MTA-STS Policy file served as the response body at <https://mta-
sts.example.com/.well-known/mta-sts.txt:>
version: STSv1 version: STSv1
mode: report mode: report
mx: mx1.example.com mx: mx1.example.com
mx: mx2.example.com mx: mx2.example.com
mx: mx.backup-example.com mx: mx.backup-example.com
max_age: 12345678 max_age: 12345678
11. Appendix 2: Message delivery pseudocode 11. Appendix 2: Message delivery pseudocode
Below is pseudocode demonstrating the logic of a compliant sending Below is pseudocode demonstrating the logic of a compliant sending
MTA. MTA.
While this pseudocode implementation suggests synchronous policy While this pseudocode implementation suggests synchronous policy
retrieval in the delivery path, in a working implementation that may retrieval in the delivery path, in a working implementation that may
be undesirable, and we expect some implementors to instead prefer a be undesirable, and we expect some implementers to instead prefer a
background fetch that does not block delivery if no cached policy is background fetch that does not block delivery if no cached policy is
present. present.
func isEnforce(policy) { func isEnforce(policy) {
// Return true if the policy mode is "enforce". // Return true if the policy mode is "enforce".
} }
func isNonExpired(policy) { func isNonExpired(policy) {
// Return true if the policy is not expired. // Return true if the policy is not expired.
} }
func tryStartTls(connection) { func tryStartTls(connection) {
// Attempt to open an SMTP connection with STARTTLS with the MX. // Attempt to open an SMTP connection with STARTTLS with the MX.
} }
func isWildcardMatch(pat, host) {
// Literal matches are true.
if pat == host {
return true
}
// Leading '.' matches a wildcard against the first part, i.e.
// .example.com matches x.example.com but not x.y.example.com.
if pat[0] == '.' {
parts = SplitN(host, '.', 2) // Split on the first '.'.
if len(parts) > 1 && parts[1] == pat[1:] {
return true
}
}
return false
}
func certMatches(connection, policy) { func certMatches(connection, policy) {
// Assume a handy function to return CN and DNS-ID SANs. // Assume a handy function to return CN and DNS-ID SANs.
for san in getDnsIdSansAndCnFromCert(connection) { for san in getDnsIdSansAndCnFromCert(connection) {
for mx in policy.mx { for mx in policy.mx {
// Return if the server certificate from "connection" matches the "mx" host. // Return if the server certificate from "connection" matches the "mx" host.
if san[0] == '*' { if san[0] == '*' {
// Invalid wildcard! // Invalid wildcard!
if san[1] != '.' continue if san[1] != '.' continue
san = san[1:] san = san[1:]
} }
if san[0] == '.' && HasSuffix(mx, san) { if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
return true
}
if mx[0] == '.' && HasSuffix(san, mx) {
return true
}
if mx == san {
return true return true
} }
} }
} }
return false return false
} }
func tryDeliverMail(connection, message) { func tryDeliverMail(connection, message) {
// Attempt to deliver "message" via "connection". // Attempt to deliver "message" via "connection".
} }
skipping to change at page 18, line 19 skipping to change at page 20, line 19
return tryWithPolicy(message, domain, policy) return tryWithPolicy(message, domain, policy)
} }
// Try to deliver the message normally (i.e. without MTA-STS). // Try to deliver the message normally (i.e. without MTA-STS).
} }
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc2119>. editor.org/info/rfc2119>.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, DOI 10
.17487/RFC2560, June 1999,
<http://www.rfc-editor.org/info/rfc2560>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/
RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<http://www.rfc-editor.org/info/rfc3492>. <https://www.rfc-editor.org/info/rfc3492>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc5321>. editor.org/info/rfc5321>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10 Uniform Resource Identifiers (URIs)", RFC 5785,
.17487/RFC5785, April 2010, DOI 10.17487/RFC5785, April 2010, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc5785>. editor.org/info/rfc5785>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/
RFC5891, August 2010,
<http://www.rfc-editor.org/info/rfc5891>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
7405, DOI 10.17487/RFC7405, December 2014, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
<http://www.rfc-editor.org/info/rfc7405>. DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
editor.org/info/rfc7231>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
Opportunistic DNS-Based Authentication of Named Entities RFC 7405, DOI 10.17487/RFC7405, December 2014,
(DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10 <https://www.rfc-editor.org/info/rfc7405>.
.17487/RFC7672, October 2015,
<http://www.rfc-editor.org/info/rfc7672>.
12.2. URIs 12.2. Informative References
[1] https://mta-sts.example.com/.well-known/mta-sts.txt: [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560,
DOI 10.17487/RFC2560, June 1999, <https://www.rfc-
editor.org/info/rfc2560>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, <https://www.rfc-
editor.org/info/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010, <https://www.rfc-
editor.org/info/rfc5891>.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
2013, <https://www.rfc-editor.org/info/rfc6818>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, <https://www.rfc-
editor.org/info/rfc7672>.
Authors' Addresses Authors' Addresses
Daniel Margolis Daniel Margolis
Google, Inc Google, Inc
Email: dmargolis (at) google.com Email: dmargolis (at) google.com
Mark Risher Mark Risher
Google, Inc Google, Inc
Email: risher (at) google (dot com) Email: risher (at) google (dot com)
Binu Ramakrishnan Binu Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
Email: rbinu (at) yahoo-inc (dot com) Email: rbinu (at) yahoo-inc (dot com)
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