draft-ietf-uta-mta-sts-10.txt   draft-ietf-uta-mta-sts-11.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: April 1, 2018 B. Ramakrishnan Expires: May 12, 2018 B. Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
A. Brotman A. Brotman
Comcast, Inc Comcast, Inc
J. Jones J. Jones
Microsoft, Inc Microsoft, Inc
September 28, 2017 November 8, 2017
SMTP MTA Strict Transport Security (MTA-STS) SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-10 draft-ietf-uta-mta-sts-11
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 April 1, 2018. This Internet-Draft will expire on May 12, 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.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . 8 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
3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 9 3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 10
4. Policy Application . . . . . . . . . . . . . . . . . . . . . 10 4. Policy Application . . . . . . . . . . . . . . . . . . . . . 10
4.1. Policy Application Control Flow . . . . . . . . . . . . . 11 4.1. Policy Application Control Flow . . . . . . . . . . . . . 11
5. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11 5. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11
6. Operational Considerations . . . . . . . . . . . . . . . . . 12 6. Interoperability Considerations . . . . . . . . . . . . . . . 12
6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 12 6.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 12 6.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 12
6.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 13 7. Operational Considerations . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13
7.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 13 7.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13
7.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 13 7.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14
7.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 14 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 14
8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 15 8.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15
8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 15 8.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15
8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 16 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 16 9.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16
8.5. Compromise of the Web PKI System . . . . . . . . . . . . 17 9.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 16
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17 9.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 17
10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 17 9.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 18
11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 18 9.5. Compromise of the Web PKI System . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 21 11. Appendix 1: MTA-STS example record & policy . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 22 12. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Normative References . . . . . . . . . . . . . . . . . . 22
13.2. Informative References . . . . . . . . . . . . . . . . . 23
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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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policies specifying: policies specifying:
o whether MTAs sending mail to this domain can expect PKIX- o whether MTAs sending mail to this domain can expect PKIX-
authenticated TLS support authenticated TLS support
o what a conforming client should do with messages when TLS cannot o what a conforming client should do with messages when TLS cannot
be successfully negotiated be successfully negotiated
1.1. Terminology 1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" 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 ordinarily be "example.com", but "alice@example.com" this would ordinarily be "example.com", but
this may be overridden by explicit routing rules (as described in this may be overridden by explicit routing rules (as described in
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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-resistant encrypted transmission into authenticated, downgrade-resistant encrypted
transmission. 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 9, "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.
The primary motivation of MTA-STS is to provide a mechanism for The primary motivation of MTA-STS is to provide a mechanism for
domains to upgrade their transport security even when deploying domains to upgrade their transport security even when deploying
DNSSEC is undesirable or impractical. However, MTA-STS is designed DNSSEC is undesirable or impractical. However, MTA-STS is designed
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sts-policy-max-age-value sts-policy-max-age-value
sts-policy-max-age-field = %s"max_age" sts-policy-max-age-field = %s"max_age"
sts-policy-max-age-value = 1*10(DIGIT) sts-policy-max-age-value = 1*10(DIGIT)
sts-policy-extension = sts-policy-ext-name field-delim ; additional sts-policy-extension = sts-policy-ext-name field-delim ; additional
sts-policy-ext-value ; extension sts-policy-ext-value ; extension
; fields ; fields
sts-policy-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".") sts-policy-ext-name = (ALPHA / DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding
; "=", ";", SP, ; "=", ";", SP,
; and control ; and control
; chars ; 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 but containing additional key/value pairs colons for TXT records) and but containing additional key/value pairs
not specified in this document, in which case unknown fields SHALL be not specified in this document, in which case unknown fields SHALL be
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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) [RFC6960], certificate revocation
lists (CRLs), or some other mechanism. lists (CRLs), or some other mechanism.
Policies fetched via HTTPS are only valid if the HTTP response code Policies fetched via HTTPS are only valid if the HTTP response code
is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP
caching (as specified in [RFC7234]) MUST NOT be used. 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
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policies which specify mode values of "report" or "none" MUST NOT be policies which specify mode values of "report" or "none" MUST NOT be
interpreted as delivery failures, as described in Section 4, "Policy interpreted as delivery failures, as described in Section 4, "Policy
Application". Application".
3.5. 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 subject alternative name certificate MUST have a CN-ID ([RFC6125]) or subject alternative name
(SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's
certificate MAY also be checked for revocation via OCSP [RFC2560], certificate MAY also be checked for revocation via OCSP [RFC6960],
CRLs [RFC6818], 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.com". 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.com", we are required to implement "wildcard-to- matching "*.example.com", we are required to implement "wildcard-to-
wildcard" matching. wildcard" matching.
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1. "enforce": In this mode, sending MTAs MUST NOT deliver the 1. "enforce": In this mode, sending MTAs MUST NOT deliver the
message to hosts which fail MX matching or certificate message to hosts which fail MX matching or certificate
validation. validation.
2. "report": In this mode, sending MTAs which also implement the 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).
3. "none": In this mode, sending MTAs should treat the policy domain 3. "none": In this mode, sending MTAs should treat the policy domain
as though it does not have any active policy; see Section 6.3, as though it does not have any active policy; see Section 7.3,
"Removing MTA-STS", for use of this mode value. "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.
4.1. Policy Application Control Flow 4.1. Policy Application Control Flow
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o HTTPS policy fetch failures when a valid TXT record is present. 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 o Policy fetch failures of any kind when a valid policy exists in
the policy cache, except if that policy's mode is "none". the policy cache, except if that policy's mode is "none".
o Delivery attempts in which a contacted MX does not support o Delivery attempts in which a contacted MX does not support
STARTTLS or does not present a certificate which validates STARTTLS or does not present a certificate which validates
according to the applied policy, except if that policy's mode is according to the applied policy, except if that policy's mode is
"none". "none".
6. Operational Considerations 6. Interoperability Considerations
6.1. Policy Updates 6.1. SNI Support
To ensure that the server sends the right certificate chain, the SMTP
client MUST have support for the TLS SNI extension [RFC6066]. When
connecting to a HTTP server to retrieve the MTA-STS policy, the SNI
extension MUST contain the name of the policy host (e.g. "mta-
sts.example.com"). When connecting to an SMTP server, the SNI
extension MUST contain the MX hostname.
HTTP servers used to deliver MTA-STS policies MUST have support for
the TLS SNI extension and MAY rely on SNI to determine which
certificate chain to present to the client. In either case, HTTP
servers MUST respond with a certificate chain that matches the policy
hostname or abort the TLS handshake if unable to do so.
SMTP servers MUST have support for the TLS SNI extension and MAY rely
on SNI to determine which certificate chain to present to the client.
If the client sends no SNI extension or sends an SNI extension for an
unsupported server name, the server MUST simply send a fallback
certificate chain of its choice. The reason for not enforcing strict
matching of the requested SNI hostname is that MTA-STS TLS clients
may be typically willing to accept multiple server names but can only
send one name in the SNI extension. The server's fallback
certificate may match a different name that is acceptable to the
client, e.g., the original next-hop domain.
6.2. Minimum TLS Version Support
MTAs supporting MTA-STS MUST have support for TLS version 1.2
[RFC5246] or higher. The general TLS usage guidance in [RFC7525]
SHOULD be followed.
7. Operational Considerations
7.1. 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.
In other words, a sender who is unable to successfully deliver a In other words, a sender who is unable to successfully deliver a
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. Policy Delegation 7.2. Policy Delegation
Domain owners commonly delegate SMTP hosting to a different Domain owners commonly delegate SMTP hosting to a different
organization, such as an ISP or a Web host. In such a case, they may organization, such as an ISP or a Web host. In such a case, they may
wish to also delegate the MTA-STS policy to the same organization wish to also delegate the MTA-STS policy to the same organization
which can be accomplished with two changes. which can be accomplished with two changes.
First, the Policy Domain must point the "_mta-sts" record, via CNAME, First, the Policy Domain must point the "_mta-sts" record, via CNAME,
to the "_mta-sts" record maintained by the hosting organization. to the "_mta-sts" record maintained by the hosting organization.
This allows the hosting organization to control update signaling. This allows the hosting organization to control update signaling.
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DNS: DNS:
_mta-sts.user.com. IN CNAME _mta-sts.provider.com. _mta-sts.user.com. IN CNAME _mta-sts.provider.com.
Policy: Policy:
> GET /.well-known/mta-sts.txt > GET /.well-known/mta-sts.txt
> Host: mta-sts.user.com > Host: mta-sts.user.com
< HTTP/1.1 200 OK # Response proxies content from https://mta-sts.provider.com < HTTP/1.1 200 OK # Response proxies content from https://mta-sts.provider.com
6.3. Removing MTA-STS Note that while sending MTAs MUST NOT use HTTP caching when fetching
policies via HTTPS, such caching may nonetheless be useful to a
reverse proxy configured as described in this section. An HTTPS
policy endpoint expecting to be proxied for multiple hosted domains--
as with a large mail hosting provider or similar--may wish to
indicate an HTTP Cache-Control "max-age" response directive (as
specified in [RFC7234]) of 60 seconds as a reasonable value to save
reverse proxies an unnecessarily high-rate of proxied policy
fetching.
7.3. Removing MTA-STS
In order to facilitate clean opt-out of MTA-STS by implementing In order to facilitate clean opt-out of MTA-STS by implementing
policy domains, and to distinguish clearly between failures which policy domains, and to distinguish clearly between failures which
indicate attacks and those which indicate such opt-outs, MTA-STS indicate attacks and those which indicate such opt-outs, MTA-STS
implements the "none" mode, which allows validated policies to implements the "none" mode, which allows validated policies to
indicate authoritatively that the policy domain wishes to no longer indicate authoritatively that the policy domain wishes to no longer
implement MTA-STS and may, in the future, remove the MTA-STS TXT and implement MTA-STS and may, in the future, remove the MTA-STS TXT and
policy endpoints entirely. policy endpoints entirely.
A suggested workflow to implement such an opt out is as follows: A suggested workflow to implement such an opt out is as follows:
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"max_age" (e.g. one day). "max_age" (e.g. one day).
2. Publish a new TXT record to trigger fetching of the new policy. 2. Publish a new TXT record to trigger fetching of the new policy.
3. When all previously served policies have expired--normally this 3. When all previously served policies have expired--normally this
is the time the previously published policy was last served plus is the time the previously published policy was last served plus
that policy's "max_age", but note that older policies may have that policy's "max_age", but note that older policies may have
been served with a greater "max_age", allowing overlapping policy been served with a greater "max_age", allowing overlapping policy
caches--safely remove the TXT record and HTTPS endpoint. caches--safely remove the TXT record and HTTPS endpoint.
7. IANA Considerations 8. IANA Considerations
7.1. Well-Known URIs Registry 8.1. 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 8.2. MTA-STS TXT Record Fields
IANA is requested to create a new registry titled "MTA-STS TXT Record IANA is requested to create a new registry titled "MTA-STS TXT Record
Fields". The initial entries in the registry are: Fields". The initial entries in the registry are:
+------------+--------------------+------------------------+ +------------+--------------------+------------------------+
| Field Name | Description | Reference | | Field Name | Description | Reference |
+------------+--------------------+------------------------+ +------------+--------------------+------------------------+
| v | Record version | Section 3.1 of RFC XXX | | v | Record version | Section 3.1 of RFC XXX |
| id | Policy instance ID | Section 3.1 of RFC XXX | | id | Policy instance ID | Section 3.1 of RFC XXX |
+------------+--------------------+------------------------+ +------------+--------------------+------------------------+
New fields are added to this registry using IANA's "Expert Review" New fields are added to this registry using IANA's "Expert Review"
policy. policy.
7.3. MTA-STS Policy Fields 8.3. MTA-STS Policy Fields
IANA is requested to create a new registry titled "MTA-STS Policy IANA is requested to create a new registry titled "MTA-STS Policy
Fields". The initial entries in the registry are: Fields". The initial entries in the registry are:
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
| Field Name | Description | Reference | | Field Name | Description | Reference |
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
| version | Policy version | Section 3.2 of RFC XXX | | version | Policy version | Section 3.2 of RFC XXX |
| mode | Enforcement behavior | Section 3.2 of RFC XXX | | mode | Enforcement behavior | Section 3.2 of RFC XXX |
| max_age | Policy lifetime | Section 3.2 of RFC XXX | | max_age | Policy lifetime | Section 3.2 of RFC XXX |
| mx | MX identities | Section 3.2 of RFC XXX | | mx | MX identities | Section 3.2 of RFC XXX |
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
New fields are added to this registry using IANA's "Expert Review" New fields are added to this registry using IANA's "Expert Review"
policy. policy.
8. Security Considerations 9. Security Considerations
SMTP MTA Strict Transport Security attempts to protect against an SMTP MTA Strict Transport Security attempts to protect against an
active attacker who wishes to intercept or tamper with mail between active attacker who wishes to intercept or tamper with mail between
hosts who support STARTTLS. There are two classes of attacks hosts who support STARTTLS. There are two classes of attacks
considered: considered:
o Foiling TLS negotiation, for example by deleting the "250 o Foiling TLS negotiation, for example by deleting the "250
STARTTLS" response from a server or altering TLS session STARTTLS" response from a server or altering TLS session
negotiation. This would result in the SMTP session occurring over negotiation. This would result in the SMTP session occurring over
plaintext, despite both parties supporting TLS. plaintext, despite both parties supporting TLS.
skipping to change at page 15, line 11 skipping to change at page 16, line 14
for the recipient domain, or by redirecting client connections for the recipient domain, or by redirecting client connections
intended for the legitimate recipient server (for example, by intended for the legitimate recipient server (for example, by
altering BGP routing tables). altering BGP routing tables).
MTA-STS can thwart such attacks only if the sender is able to MTA-STS can thwart such attacks only if the sender is able to
previously obtain and cache a policy for the recipient domain, and previously obtain and cache a policy for the recipient domain, and
only if the attacker is unable to obtain a valid certificate that only if the attacker is unable to obtain a valid certificate that
complies with that policy. Below, we consider specific attacks on complies with that policy. Below, we consider specific attacks on
this model. this model.
8.1. Obtaining a Signed Certificate 9.1. Obtaining a Signed Certificate
SMTP MTA-STS relies on certificate validation via PKIX based TLS SMTP MTA-STS relies on certificate validation via PKIX based TLS
identity checking [RFC6125]. Attackers who are able to obtain a identity checking [RFC6125]. Attackers who are able to obtain a
valid certificate for the targeted recipient mail service (e.g. by valid certificate for the targeted recipient mail service (e.g. by
compromising a certificate authority) are thus able to circumvent STS compromising a certificate authority) are thus able to circumvent STS
authentication. authentication.
8.2. Preventing Policy Discovery 9.2. Preventing Policy Discovery
Since MTA-STS uses DNS TXT records for policy discovery, an attacker Since MTA-STS uses DNS TXT records for policy discovery, an attacker
who is able to block DNS responses can suppress the discovery of an who is able to block DNS responses can suppress the discovery of an
MTA-STS Policy, making the Policy Domain appear not to have an MTA- MTA-STS Policy, making the Policy Domain appear not to have an MTA-
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--
skipping to change at page 15, line 51 skipping to change at page 17, line 6
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.
Additionally, MTAs should alert administrators to repeated policy Additionally, MTAs should alert administrators to repeated policy
refresh failures long before cached policies expire (through warning refresh failures long before cached policies expire (through warning
logs or similar applicable mechanisms), allowing administrators to logs or similar applicable mechanisms), allowing administrators to
detect such a persistent attack on policy refresh. (However, they detect such a persistent attack on policy refresh. (However, they
should not implement such alerts if the cached policy has a "none" should not implement such alerts if the cached policy has a "none"
mode, to allow clean MTA-STS removal, as described in Section 6.3.) mode, to allow clean MTA-STS removal, as described in Section 7.3.)
Resistance to downgrade attacks of this nature--due to the ability to 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 9.3. 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
records. With MTA-STS, the attacker can additionally advertise a records. With MTA-STS, the attacker can additionally advertise a
new, long-"max_age" MTA-STS policy with "mx" constraints that new, long-"max_age" MTA-STS policy with "mx" constraints that
validate the malicious MX record, causing senders to cache the policy validate the malicious MX record, causing senders to cache the policy
and refuse to deliver messages once the victim has resecured the MX and refuse to deliver messages once the victim has resecured the MX
records. records.
skipping to change at page 16, line 44 skipping to change at page 18, line 5
In these cases, there is a risk that untrusted users would be able to 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 serve custom content at the "mta-sts" host, including serving an
illegitimate MTA-STS policy. We believe this attack is rendered more illegitimate MTA-STS policy. We believe this attack is rendered more
difficult by the need for the attacker to also serve the "_mta-sts" difficult by the need for the attacker to also serve the "_mta-sts"
TXT record on the same domain--something not, to our knowledge, TXT record on the same domain--something not, to our knowledge,
widely provided to untrusted users. This attack is additionally widely provided to untrusted users. This attack is additionally
mitigated by the aforementioned ability for a victim domain to update mitigated by the aforementioned ability for a victim domain to update
an invalid policy at any future date. an invalid policy at any future date.
8.4. Weak Policy Constraints 9.4. Weak Policy Constraints
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 9.5. Compromise of the Web PKI System
A host of risks apply to the PKI system used for certificate A host of risks apply to the PKI system used for certificate
authentication, both of the "mta-sts" HTTPS host's certificate and authentication, both of the "mta-sts" HTTPS host's certificate and
the SMTP servers' certificates. These risks are broadly applicable the SMTP servers' certificates. These risks are broadly applicable
within the Web PKI ecosystem and are not specific to MTA-STS; within the Web PKI ecosystem and are not specific to MTA-STS;
nonetheless, they deserve some consideration in this context. nonetheless, they deserve some consideration in this context.
Broadly speaking, attackers may compromise the system by obtaining Broadly speaking, attackers may compromise the system by obtaining
certificates under fraudulent circumstances (i.e. by impersonating certificates under fraudulent circumstances (i.e. by impersonating
the legitimate owner of the victim domain), by compromising a the legitimate owner of the victim domain), by compromising a
Certificate Authority or Delegate Authority's private keys, by Certificate Authority or Delegate Authority's private keys, by
obtaining a legitimate certificate issued to the victim domain, and obtaining a legitimate certificate issued to the victim domain, and
similar. similar.
One approach commonly employed by Web browsers to help mitigate One approach commonly employed by Web browsers to help mitigate
against some of these attacks is to allow for revocation of against some of these attacks is to allow for revocation of
compromised or fraudulent certificates via OCSP [RFC2560] or CRLs compromised or fraudulent certificates via OCSP [RFC6960] or CRLs
[RFC6818]. Such mechanisms themselves represent tradeoffs and are [RFC6818]. Such mechanisms themselves represent tradeoffs and are
not universally implemented; we nonetheless recommend implementors of not universally implemented; we nonetheless recommend implementors of
MTA-STS to implement revocation mechanisms which are most applicable MTA-STS to implement revocation mechanisms which are most applicable
to their implementations. to their implementations.
9. Contributors 10. 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
skipping to change at page 17, line 43 skipping to change at page 19, line 4
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 11. Appendix 1: MTA-STS example record & policy
The owner of "example.com" wishes to begin using MTA-STS with a The owner of "example.com" wishes to begin using MTA-STS with a
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 <https://mta- MTA-STS Policy file served as the response body at [1]
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 12. 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 implementers 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.
skipping to change at page 19, line 19 skipping to change at page 20, line 24
return true return true
} }
} }
return false 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 isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
return true return true
} }
} }
} }
skipping to change at page 21, line 4 skipping to change at page 22, line 6
if policy { if policy {
cachePolicy(domain, policy) cachePolicy(domain, policy)
} else { } else {
policy = tryGetCachedPolicy(domain) policy = tryGetCachedPolicy(domain)
} }
if policy { if policy {
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
12.1. Normative References 13. References
13.1. 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, Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
DOI 10.17487/RFC2119, March 1997, <https://www.rfc- RFC2119, March 1997,
editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3492>. <http://www.rfc-editor.org/info/rfc3492>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008, <https://www.rfc-editor.org/info/
rfc5246>.
[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,
<https://www.rfc-editor.org/info/rfc5280>. <http://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, <https://www.rfc- DOI 10.17487/RFC5321, October 2008,
editor.org/info/rfc5321>. <http://www.rfc-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, Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10
DOI 10.17487/RFC5785, April 2010, <https://www.rfc- .17487/RFC5785, April 2010,
editor.org/info/rfc5785>. <http://www.rfc-editor.org/info/rfc5785>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI 10
.17487/RFC6066, January 2011, <https://www.rfc-editor.org/
info/rfc6066>.
[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, <https://www.rfc-editor.org/info/rfc6125>. 2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
DOI 10.17487/RFC7231, June 2014, <https://www.rfc- 10.17487/RFC7231, June 2014, <https://www.rfc-editor.org/
editor.org/info/rfc7231>. info/rfc7231>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC
RFC 7405, DOI 10.17487/RFC7405, December 2014, 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>. <http://www.rfc-editor.org/info/rfc7405>.
12.2. Informative References [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. 13.2. Informative References
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 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>. February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements", RFC
RFC 4033, DOI 10.17487/RFC4033, March 2005, 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>. <http://www.rfc-editor.org/info/rfc4033>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
DOI 10.17487/RFC5322, October 2008, <https://www.rfc- 10.17487/RFC5322, October 2008,
editor.org/info/rfc5322>. <http://www.rfc-editor.org/info/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in [RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/
DOI 10.17487/RFC5891, August 2010, <https://www.rfc- RFC5891, August 2010,
editor.org/info/rfc5891>. <http://www.rfc-editor.org/info/rfc5891>.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key [RFC6818] Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
2013, <https://www.rfc-editor.org/info/rfc6818>. 2013, <https://www.rfc-editor.org/info/rfc6818>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013, <https://www
.rfc-editor.org/info/rfc6960>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www
<https://www.rfc-editor.org/info/rfc7234>. .rfc-editor.org/info/rfc7234>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10
DOI 10.17487/RFC7672, October 2015, <https://www.rfc- .17487/RFC7672, October 2015,
editor.org/info/rfc7672>. <http://www.rfc-editor.org/info/rfc7672>.
13.3. URIs
[1] https://mta-sts.example.com/.well-known/mta-sts.txt:
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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