Using TLS in Applications D. Margolis
Internet-Draft M. Risher
Intended status: Standards Track Google, Inc
Expires: June 18, 2017 B. Ramakrishnan
Yahoo!, Inc
A. Brotman
Comcast, Inc
J. Jones
Microsoft, Inc
December 15, 2016
SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-02
Abstract
SMTP Mail Transfer Agent Strict Transport Security (SMTP STS) is a
mechanism enabling mail service providers to declare their ability to
receive TLS-secured connections and an expected validity of
certificates presented by their MX hosts, 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 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 June 18, 2017.
Copyright Notice
Copyright (c) 2016 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
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3
3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4
3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4
3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5
3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 6
3.4. Policy Selection for Smart Hosts . . . . . . . . . . . . 6
4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 6
4.1. MX Matching . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. MX Certificate Validation . . . . . . . . . . . . . . . . 7
5. Policy Application . . . . . . . . . . . . . . . . . . . . . 7
5.1. MX Preference . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Policy Application Control Flow . . . . . . . . . . . . . 8
6. Operational Considerations . . . . . . . . . . . . . . . . . 8
6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Appendix 1: Domain Owner STS example record . . . . . . . . . 11
10.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . 11
11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
hosts to negotiate the use of a TLS channel for secure mail
transmission.
While such _opportunistic_ encryption protocols provide a high
barrier against passive man-in-the-middle traffic interception, any
attacker who can delete parts of the SMTP session (such as the "250
STARTTLS" response) or who can redirect the entire SMTP session
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(perhaps by overwriting the resolved MX record of the delivery
domain) can perform downgrade or interception attacks.
This document defines a mechanism for recipient domains to publish
policies specifying:
o whether MTAs sending mail to this domain can expect TLS support
o expected validity of server certificates presented by the domain's
MX hosts
o what a conforming client should do with messages when TLS cannot
be successfully negotiated
1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
We also define the following terms for further use in this document:
o STS Policy: A committment by the Policy Domain to support PKIX
authenticated TLS for the specified MX hosts.
o Policy Domain: The domain for which an STS Policy is defined.
(For example, when sending mail to "alice@example.com", the policy
domain is "example.com".)
o Policy Authentication: Authentication of the STS policy retrieved
for a recipient domain by the sender.
2. Related Technologies
The DANE TLSA record [RFC7672] is similar, in that DANE is also
designed to upgrade opportunistic, unauthenticated encryption into
required, authenticated encryption. DANE requires DNSSEC [RFC4033]
for authentication; the mechanism described here instead relies on
certificate authorities (CAs) and does not require DNSSEC. For a
thorough discussion of this trade-off, see the section _Security_
_Considerations_.
In addition, SMTP STS provides an optional report-only mode, enabling
soft deployments to detect policy failures.
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3. Policy Discovery
SMTP STS policies are distributed via HTTPS from a "well-known"
[RFC5785] path served within the Policy Domain, and their presence
and current version are indicated by a TXT record at the Policy
Domain. These TXT records additionally contain a policy "id" field,
allowing sending MTAs to check the currency of a cached policy
without performing an HTTPS request.
To discover if a recipient domain implements MTA-STS, a sender need
only resolve a single TXT record. To see if an updated policy is
available for a domain for which the sender has a previously cached
policy, the sender need only check the TXT record's version "id"
against the cached value.
3.1. MTA-STS TXT Records
The MTA-STS TXT record is a TXT record with the name "_mta-sts" at
the Policy Domain. For the domain "example.com", this record would
be "_mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII,
semicolon-separated key/value pairs containing the following fields:
o "v": (plain-text, required). Currently only "STSv1" is supported.
o "id": (plain-text, required). A short string used to track policy
updates. This string MUST uniquely identify a given instance of a
policy, such that senders can determine when the policy has been
updated by comparing to the "id" of a previously seen policy.
There is no implied ordering of "id" fields between revisions.
An example TXT record is as below:
"_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;""
The formal definition of the "_mta-sts" TXT record, defined using
[RFC5234], is as follows:
sts-text-record = sts-version *WSP %x3B *WSP sts-id [%x3B]
sts-version = "v" *WSP "=" *WSP %x53 %x54 ; "STSv1"
%x53 %x76 %x31
sts-id = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT)
If multiple TXT records for "_mta-sts" are returned by the resolver,
records which do not begin with "v=STSv1;" are discarded. If the
number of resulting records is not one, senders MUST assume the
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recipient domain does not implement MTA STS and skip the remaining
steps of policy discovery.
3.2. MTA-STS Policies
The policy itself is a JSON [RFC4627] object served via the HTTPS GET
method from the fixed [RFC5785] "well-known" path of ".well-known/
mta-sts.json" served by the "mta-sts" host at the Policy Domain.
Thus for "example.com" the path is "https://mta-sts.example.com
/.well-known/mta-sts.json".
This JSON object contains the following key/value pairs:
o "version": (plain-text, required). Currently only "STSv1" is
supported.
o "mode": (plain-text, required). Either "enforce" or "report",
indicating the expected behavior of a sending MTA in the case of a
policy validation failure.
o "max_age": Max lifetime of the policy (plain-text non-negative
integer seconds, required). Well-behaved clients SHOULD cache a
policy for up to this value from last policy fetch time. To
mitigate the risks of attacks at policy refresh time, it is
expected that this value typically be in the range of weeks or
greater.
o "mx": MX patterns (list of plain-text MX match strings, required).
One or more patterns matching the expected MX for this domain.
For example, "["*.example.com", "*.example.net"]" indicates that
mail for this domain might be handled by any MX with a hostname at
"example.com" or "example.net". Valid patterns can be either
hostname literals (e.g. "mx1.example.com") or wildcard matches, so
long as the wildcard occupies the full left-most label in the
pattern. (Thus "*.example.com" is valid but "mx*.example.com" is
not.)
An example JSON policy is as below:
{
"version": "STSv1",
"mode": "enforce",
"mx": ["*.mail.example.com"],
"max_age": 123456
}
A lenient parser SHOULD accept TXT records and policy files which are
syntactically valid (i.e. valid key-value pairs separated by semi-
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colons for TXT records and valid JSON for policy files) and
implementing a superset of this specification, in which case unknown
fields SHALL be ignored.
3.3. HTTPS Policy Fetching
When fetching a new policy or updating a policy, the HTTPS endpoint
MUST present a TLS certificate which is valid for the "mta-sts" host
(as described in [RFC6125]), chain to a root CA that is trusted by
the sending MTA, and be non-expired. It is expected that sending
MTAs use a set of trusted CAs similar to those in widely deployed Web
browsers and operating systems.
HTTP 3xx redirects MUST NOT be followed.
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
exists. In the case that the HTTPS GET fails, we suggest
implementions may limit further attempts to a period of five minutes
or longer per version ID, to avoid overwhelming resource-constrained
recipients with cascading failures.
Senders MAY impose a timeout on the HTTPS GET to avoid long delays
imposed by attempted policy updates. A suggested timeout is one
minute; policy hosts SHOULD respond to requests with a complete
policy body within that timeout.
3.4. Policy Selection for Smart Hosts
When sending mail via a "smart host"--an intermediate SMTP relay
rather than the message recipient's server--compliant senders MUST
treat the smart host domain as the policy domain for the purposes of
policy discovery and application.
4. Policy Validation
When sending to an MX at a domain for which the sender has a valid
and non-expired SMTP MTA-STS policy, a sending MTA honoring SMTP STS
MUST validate:
1. That the recipient MX matches the "mx" pattern from the recipient
domain's policy.
2. That the recipient MX supports STARTTLS and offers a valid PKIX
based TLS certificate.
This section does not dictate the behavior of sending MTAs when
policies fail to validate; in particular, validation failures of
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policies which specify "report" mode MUST NOT be interpreted as
delivery failures, as described in the section _Policy_
_Application_.
4.1. MX Matching
When delivering mail for the Policy Domain to a recipient MX host,
the sender validates the MX match against the "mx" pattern from the
applied policy. The semantics for these patterns are those found in
section 6.4 of [RFC6125].
Patterns may contain a wildcard character "*" which matches any
single domain name component or component fragment, though only as
the leftmost component in a pattern. For example, "*.example.com" is
a valid pattern, but "foo.*.example.com" is not. Given the pattern
"*.example.com", "mx1.example.com" is a valid MX host, but
"1234.dhcp.example.com" is not.
4.2. MX Certificate Validation
The certificate presented by the receiving MX MUST be valid for the
MX hostname and chain to a root CA that is trusted by the sending
MTA. The certificate MUST have a CN or SAN matching the MX hostname
(as described in [RFC6125]) and be non-expired.
In the case of an "implicit" MX record (as specified in [RFC2821])
where no MX RR exists for the recipient domain but there is an A RR,
the MX hostname is assumed to be that of the A RR and should be
validated as such.
5. Policy Application
When sending to an MX at a domain for which the sender has a valid,
non-expired STS policy, a sending MTA honoring SMTP STS applies the
result of a policy validation one of two ways, depending on the value
of the policy "mode" field:
1. "report": In this mode, sending MTAs merely send a report (as
described in the TLSRPT specification (TODO: add ref)) indicating
policy application failures.
2. "enforce": In this mode, sending MTAs treat STS policy failures
as a mail delivery error, and MUST NOT deliver the message to
this host.
When a message fails to deliver due to an "enforce" policy, a
compliant MTA MUST check for the presence of an updated policy at the
Policy Domain before permanently failing to deliver the message.
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This allows implementing domains to update long-lived policies on the
fly.
Finally, in both "enforce" and "report" modes, failures to deliver in
compliance with the applied policy result in failure reports to the
policy domain, as described in the TLSRPT specification (TODO: add
ref).
5.1. MX Preference
When applying a policy, sending MTAs SHOULD select recipient MXs by
first eliminating any MXs at lower priority than the current host (if
in the MX candidate set), then eliminating any non-matching (as
specified by the STS Policy) MX hosts from the candidate MX set, and
then attempting delivery to matching hosts as indicated by their MX
priority, until delivery succeeds or the MX candidate set is empty.
5.2. Policy Application Control Flow
An example control flow for a compliant sender consists of the
following steps:
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.
(Optionally, sending MTAs may unconditionally check for a new
policy at this step.)
2. Filter candidate MXs against the current policy.
3. If no candidate MXs are valid and the policy mode is "enforce",
temporarily fail the message. (Otherwise, generate a failure
report but deliver as though MTA STS were not implemented.)
4. For each candidate MX, in order of MX priority, attempt to
deliver the message, enforcing STARTTLS and the MX host's PKIX
certificate validation.
5. Upon message retries, a message MAY be permanently failed
following first checking for the presence of a new policy (as
indicated by the "id" field in the "_mta-sts" TXT record).
6. Operational Considerations
6.1. Policy Updates
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
at the corresponding HTTPS endpoint. In the case where the HTTPS
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endpoint has been updated but the TXT record has not yet been,
senders will not know there is a new policy released and may thus
continue to use old, previously cached versions. Recipients should
thus 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
passed.
7. IANA Considerations
A new .well-known URI will be registered in the Well-Known URIs
registry as described below:
URI Suffix: mta-sts.json Change Controller: IETF
8. Security Considerations
SMTP Strict Transport Security attempts to protect against an active
attacker who wishes to intercept or tamper with mail between hosts
who support STARTTLS. There are two classes of attacks considered:
1. Foiling TLS negotiation, for example by deleting the "250
STARTTLS" response from a server or altering TLS session
negotiation. This would result in the SMTP session occurring
over plaintext, despite both parties supporting TLS.
2. Impersonating the destination mail server, whereby the sender
might deliver the message to an impostor, who could then monitor
and/or modify messages despite opportunistic TLS. This
impersonation could be accomplished by spoofing the DNS MX record
for the recipient domain, or by redirecting client connections
intended for the legitimate recipient server (for example, by
altering BGP routing tables).
SMTP Strict Transport Security relies on certificate validation via
PKIX based TLS identity checking [RFC6125]. Attackers who are able
to obtain a valid certificate for the targeted recipient mail service
(e.g. by compromising a certificate authority) are thus able to
circumvent STS authentication.
Since we use DNS TXT records for policy discovery, an attacker who is
able to block DNS responses can suppress the discovery of an STS
Policy, making the Policy Domain appear not to have an STS Policy.
The sender policy cache is designed to resist this attack.
We additionally consider the Denial of Service risk posed by an
attacker who can modify the DNS records for a victim domain. Absent
SMTP STS, such an attacker can cause a sending MTA to cache invalid
MX records for a long TTL. With SMTP STS, the attacker can
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additionally advertise a new, long-"max_age" SMTP STS policy with
"mx" constraints that validate the malicious MX record, causing
senders to cache the policy and refuse to deliver messages once the
victim has resecured the MX records.
This attack is mitigated in part by the ability of a victim domain to
(at any time) publish a new policy updating the cached, malicious
policy, though this does require the victim domain to both obtain a
valid CA-signed certificate and to understand and properly configure
SMTP STS.
Similarly, we consider the possibilty of domains that deliberately
allow untrusted users to serve untrusted content on user-specified
subdomains. In some cases (e.g. the service Tumblr.com) this takes
the form of providing HTTPS hosting of user-registered subdomains; in
other cases (e.g. dynamic DNS providers) this takes the form of
allowing untrusted users to register custom DNS records at the
provider's domain.
In these cases, there is a risk that untrusted users would be able to
serve custom content at the "mta-sts" host, including serving an
illegitimate SMTP STS policy. We believe this attack is rendered
more difficult by the need for the attacker to both inject malicious
(but temporarily working) MX records and also serve the "_mta-sts"
TXT record on the same domain--something not, to our knowledge,
widely provided to untrusted users. This attack is additionally
mitigated by the aforementioned ability for a victim domain to update
an invalid policy at any future date.
Even if an attacker cannot modify a served policy, the potential
exists for configurations that allow attackers on the same domain to
receive mail for that domain. For example, an easy configuration
option when authoring an STS Policy for "example.com" is to set the
"mx" equal to "*.example.com"; recipient domains must consider in
this case the risk that any user possessing a valid hostname and CA-
signed certificate (for example, "dhcp-123.example.com") will, from
the perspective of STS Policy validation, be a valid MX host for that
domain.
9. Contributors
Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)
Wei Chuang Google, Inc weihaw (at) google (dot com)
Brandon Long Google, Inc blong (at) google (dot com)
Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com)
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Klaus Umbach 1&1 Mail & Media Development & Technology GmbH
klaus.umbach (at) 1und1 (dot de)
Markus Laber 1&1 Mail & Media Development & Technology GmbH
markus.laber (at) 1und1 (dot de)
10. Appendix 1: Domain Owner STS example record
10.1. Example 1
The owner of "example.com" wishes to begin using STS with a policy
that will solicit reports from receivers without affecting how the
messages are processed, in order to verify the identity of MXs that
handle mail for "example.com", confirm that TLS is correctly used,
and ensure that certificates presented by the recipient MX validate.
STS policy indicator TXT RR:
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
STS Policy JSON served as the response body at [1]
{
"version": "STSv1",
"mode": "report",
"mx": ["mx1.example.com", "mx2.example.com"],
"max_age": 123456
}
11. Appendix 2: Message delivery pseudocode
Below is pseudocode demonstrating the logic of a complaint sending
MTA. This implements the "two-pass" approach, first attempting
delivery with a newly fetched policy (if present) before falling back
to a cached policy (if present).
func isEnforce(policy) {
// Return true if the policy mode is "enforce".
}
func isNonExpired(policy) {
// Return true if the policy is not expired.
}
func tryStartTls(mx) {
// Attempt to open an SMTP connection with STARTTLS with the MX.
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}
func certMatches(connection, mx) {
// Return if the server certificate from "connection" matches the "mx" host.
}
func tryDeliverMail(connection, message) {
// Attempt to deliver "message" via "connection".
}
func getMxsForPolicy(domain, policy) {
// Sort the MXs by priority, filtering out those which are invalid according
// to "policy".
}
func tryGetNewPolicy(domain) {
// Check for an MTA STS TXT record for "domain" in DNS, and return the
// indicated policy (or a local cache of the unvalidated policy).
}
func cachePolicy(domain, policy) {
// Store "policy" as the cached policy for "domain".
}
func tryGetCachedPolicy(domain, policy) {
// Return a cached policy for "domain".
}
func reportError(error) {
// Report an error via TLSRPT.
}
func tryMxAccordingTo(message, mx, policy) {
connection := connect(mx)
if !connection {
return false // Can't connect to the MX so it's not an STS error.
}
status := !(tryStartTls(mx, &connection) && certMatches(connection, mx))
status = true
if !tryStartTls(mx, &connection) {
status = false
reportError(E_NO_VALID_TLS)
} else if certMatches(connection, mx) {
status = false
reportError(E_CERT_MISMATCH)
}
if status || !isEnforce(policy) {
return tryDeliverMail(connection, message)
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}
return false
}
func tryWithPolicy(message, domain, policy) {
mxes := getMxesForPolicy(domain, policy)
if mxs is empty {
reportError(E_NO_VALID_MXES)
}
for mx in mxes {
if tryMxAccordingTo(message, mx, policy) {
return true
}
}
return false
}
func handleMessage(message) {
domain := ... // domain part after '@' from recipient
oldPolicy := tryGetCachedPolicy(domain)
newPolicy := tryGetNewPolicy(domain)
if newPolicy {
cachePolicy(domain, newPolicy)
oldPolicy = newPolicy
}
if oldPolicy {
return tryWithPolicy(message, oldPolicy)
}
// There is no policy or there's a new policy that did not work.
// Try to deliver the message normally (i.e. without STS).
}
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
2821, DOI 10.17487/RFC2821, April 2001,
<http://www.rfc-editor.org/info/rfc2821>.
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[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>.
[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>.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, DOI 10
.17487/RFC4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10
.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[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,
<http://www.rfc-editor.org/info/rfc7672>.
12.2. URIs
[1] https://mta-sts.example.com/.well-known/mta-sts.json:
Authors' Addresses
Daniel Margolis
Google, Inc
Email: dmargolis (at) google.com
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Mark Risher
Google, Inc
Email: risher (at) google (dot com)
Binu Ramakrishnan
Yahoo!, Inc
Email: rbinu (at) yahoo-inc (dot com)
Alexander Brotman
Comcast, Inc
Email: alexander_brotman (at) cable.comcast (dot com)
Janet Jones
Microsoft, Inc
Email: janet.jones (at) microsoft (dot com)
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