draft-ietf-uta-mta-sts-03.txt   draft-ietf-uta-mta-sts-04.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: August 19, 2017 B. Ramakrishnan Expires: October 5, 2017 B. Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
A. Brotman A. Brotman
Comcast, Inc Comcast, Inc
J. Jones J. Jones
Microsoft, Inc Microsoft, Inc
February 15, 2017 April 3, 2017
SMTP MTA Strict Transport Security (MTA-STS) SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-03 draft-ietf-uta-mta-sts-04
Abstract Abstract
SMTP Mail Transfer Agent Strict Transport Security (SMTP 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 TLS-secured connections and an expected validity of receive Transport Layer Security (TLS) secure SMTP connections, and
certificates presented by their MX hosts, and to specify whether to specify whether sending SMTP servers should refuse to deliver to
sending SMTP servers should refuse to deliver to MX hosts that do not MX hosts that do not offer TLS with a trusted server certificate.
offer TLS with a trusted server certificate.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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
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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 August 19, 2017. This Internet-Draft will expire on October 5, 2017.
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
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publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 21 skipping to change at page 2, line 20
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 . . . . . . . . . . . . . . . . . . 6 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 6
3.4. Policy Selection for Smart Hosts . . . . . . . . . . . . 6 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 7
4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 6 4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 7
4.1. MX Matching . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 7
4.2. MX Certificate Validation . . . . . . . . . . . . . . . . 7 5. Policy Application . . . . . . . . . . . . . . . . . . . . . 8
5. Policy Application . . . . . . . . . . . . . . . . . . . . . 7 5.1. Policy Application Control Flow . . . . . . . . . . . . . 8
5.1. MX Preference . . . . . . . . . . . . . . . . . . . . . . 8 6. Operational Considerations . . . . . . . . . . . . . . . . . 9
5.2. Policy Application Control Flow . . . . . . . . . . . . . 8 6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 9
6. Operational Considerations . . . . . . . . . . . . . . . . . 8
6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 10
10. Appendix 1: Domain Owner STS example record . . . . . . . . . 11 8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 10
10.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . 11 8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 11
11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 11 8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 13 10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 12
12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 14 11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . 15
12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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 secure mail hosts to negotiate the use of a TLS channel for encrypted mail
transmission. transmission.
While such _opportunistic_ encryption protocols provide a high While this opportunistic encryption protocol by itself provides a
barrier against passive man-in-the-middle traffic interception, any high barrier against passive man-in-the-middle traffic interception,
attacker who can delete parts of the SMTP session (such as the "250 any attacker who can delete parts of the SMTP session (such as the
STARTTLS" response) or who can redirect the entire SMTP session "250 STARTTLS" response) or who can redirect the entire SMTP session
(perhaps by overwriting the resolved MX record of the delivery (perhaps by overwriting the resolved MX record of the delivery
domain) can perform downgrade or interception attacks. domain) can perform downgrade or interception attacks.
This document defines a mechanism for recipient domains to publish This document defines a mechanism for recipient domains to publish
policies specifying: policies specifying:
o whether MTAs sending mail to this domain can expect TLS support o whether MTAs sending mail to this domain can expect PKIX-
authenticated 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 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 keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
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 STS Policy: A committment 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 STS Policy is defined. o Policy Domain: The domain for which an MTA-STS Policy is defined.
(For example, when sending mail to "alice@example.com", the policy This is the next-hop domain; when sending mail to
domain is "example.com".) "alice@example.com" this would ordinarly be "example.com", but
this may be overriden by explicit routing rules (as described in
o Policy Authentication: Authentication of the STS policy retrieved "Policy Selection for Smart Hosts").
for a recipient domain by the sender.
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 opportunistic, unauthenticated encryption into designed to upgrade unauthenticated encryption or plaintext
required, authenticated encryption. DANE requires DNSSEC [RFC4033] transmission into authenticated, downgrade-resistent encrypted
for authentication; the mechanism described here instead relies on tarnsmission. DANE requires DNSSEC [RFC4033] for authentication; the
certificate authorities (CAs) and does not require DNSSEC. For a mechanism described here instead relies on certificate authorities
thorough discussion of this trade-off, see the section _Security_ (CAs) and does not require DNSSEC, at a cost of risking malicious
_Considerations_. downgrades. For a thorough discussion of this trade-off, see the
section "Security Considerations".
In addition, SMTP 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. soft deployments to detect policy failures; partial deployments can
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
policies used by MTA-STS.
The primary motivation of MTA-STS is to provide a mechanism for
domains to upgrade their transport security even when deploying
DNSSEC is undesirable or impractical. However, MTA-STS is designed
not to interfere with DANE deployments when the two overlap; in
particular, senders who implement MTA-STS validation MUST NOT allow a
"valid" or "report-only" MTA-STS validation to override a failing
DANE validation.
3. Policy Discovery 3. Policy Discovery
SMTP STS policies are distributed via HTTPS from a "well-known" MTA-STS policies are distributed via HTTPS from a "well-known"
[RFC5785] path served within the Policy Domain, and their presence [RFC5785] path served within the Policy Domain, and their presence
and current version are indicated by a TXT record at the Policy and current version are indicated by a TXT record at the Policy
Domain. These TXT records additionally contain a policy "id" field, Domain. These TXT records additionally contain a policy "id" field,
allowing sending MTAs to check the currency of a cached policy allowing sending MTAs to check the currency of a cached policy
without performing an HTTPS request. without performing an HTTPS request.
To discover if a recipient domain implements MTA-STS, a sender need 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 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 available for a domain for which the sender has a previously cached
policy, the sender need only check the TXT record's version "id" policy, the sender need only check the TXT record's version "id"
against the cached value. against the cached value.
3.1. MTA-STS TXT Records 3.1. MTA-STS TXT Records
The MTA-STS TXT record is a TXT record with the name "mta-sts" at the The MTA-STS TXT record is a TXT record with the name "_mta-sts" at
Policy Domain. For the domain "example.com", this record would be the Policy Domain. For the domain "example.com", this record would
"mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII, be "_mta-sts.example.com". MTA-STS TXT records MUST be US-ASCII,
semicolon-separated key/value pairs containing the following fields: semicolon-separated key/value pairs containing the following fields:
o "v": (plain-text, required). Currently only "STSv1" is supported. o "v": (plain-text, required). Currently only "STSv1" is supported.
o "id": (plain-text, required). A short string used to track policy o "id": (plain-text, required). A short string used to track policy
updates. This string MUST uniquely identify a given instance of a updates. This string MUST uniquely identify a given instance of a
policy, such that senders can determine when the policy has been policy, such that senders can determine when the policy has been
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
[RFC5234], is as follows: [RFC5234], is as follows:
sts-text-record = sts-version *WSP %x3B *WSP sts-id [%x3B] sts-text-record = sts-version *WSP %x3B *WSP sts-id [%x3B]
sts-version = "v" *WSP "=" *WSP %x53 %x54 ; "STSv1" sts-version = "v" *WSP "=" *WSP %x53 %x54 ; "STSv1"
%x53 %x76 %x31 %x53 %x76 %x31
sts-id = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT) sts-id = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT)
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.
3.2. MTA-STS Policies 3.2. MTA-STS Policies
The policy itself is a JSON [RFC4627] object served via the HTTPS GET The policy itself is a JSON [RFC4627] object served via the HTTPS GET
method from the fixed [RFC5785] "well-known" path of ".well-known/ method from the fixed [RFC5785] "well-known" path of ".well-known/
mta-sts.json" served by the "mta-sts" host at the Policy Domain. 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 Thus for "example.com" the path is "https://mta-sts.example.com
/.well-known/mta-sts.json". /.well-known/mta-sts.json".
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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, required). Well-behaved clients SHOULD cache a
policy for up to this value from last policy fetch time. To policy for up to this value from last policy fetch time. To
mitigate the risks of attacks at policy refresh time, it is mitigate the risks of attacks at policy refresh time, it is
expected that this value typically be in the range of weeks or expected that this value typically be in the range of weeks or
greater. greater.
o "mx": MX patterns (list of plain-text MX match strings, required). o "mx": MX identity patterns (list of plain-text strings, required).
One or more patterns matching the expected MX for this domain. One or more patterns matching a Common Name ([RFC6125]) or Subject
For example, "["*.example.com", "*.example.net"]" indicates that Alternative Name ([RFC5280]) DNS-ID present in the X.509
mail for this domain might be handled by any MX with a hostname at certificate presented by any MX receiving mail for this domain.
"example.com" or "example.net". Valid patterns can be either For example, "["mail.example.com", ".example.net"]" indicates that
hostname literals (e.g. "mx1.example.com") or wildcard matches, so mail for this domain might be handled by any MX with a certificate
long as the wildcard occupies the full left-most label in the valid for a host at "example.com" or "example.net". Valid
pattern. (Thus "*.example.com" is valid but "mx*.example.com" is patterns can be either fully specified names ("example.com") or
not.) suffixes (".example.net") matching the right-hand parts of a
server's identity; the latter case are distinguished by a leading
period. In the case of Internationalized Domain Names
([RFC5891]), the MX MUST specify the Punycode-encoded A-label
[RFC3492] and not the Unicode-encoded U-label. The full semantics
of certificate validation are described in "MX Certificate
Validation."
An example JSON policy is as below: An example JSON policy is as below:
{ {
"version": "STSv1", "version": "STSv1",
"mode": "enforce", "mode": "enforce",
"mx": ["*.mail.example.com"], "mx": [".mail.example.com"],
"max_age": 123456 "max_age": 123456
} }
A lenient parser SHOULD accept TXT records and policy files which are Parsers SHOULD 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 valid JSON for policy files) and colons for TXT records and valid JSON for policy files) and
implementing a superset of this specification, in which case unknown implementing a superset of this specification, in which case unknown
fields SHALL be ignored. fields SHALL be ignored.
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 TLS certificate which is valid for the "mta-sts" host MUST present a X.509 certificate which is valid for the "mta-sts"
(as described in [RFC6125]), chain to a root CA that is trusted by host (as described in [RFC6125]), chain to a root CA that is trusted
the sending MTA, and be non-expired. It is expected that sending 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 MTAs use a set of trusted CAs similar to those in widely deployed Web
browsers and operating systems. browsers and operating systems.
HTTP 3xx redirects MUST NOT be followed. HTTP 3xx redirects MUST NOT be followed.
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 to avoid long delays Senders MAY impose a timeout on the HTTPS GET to avoid long delays
imposed by attempted policy updates. A suggested timeout is one imposed by attempted policy updates. A suggested timeout is one
minute; policy hosts SHOULD respond to requests with a complete minute; policy hosts SHOULD respond to requests with a complete
policy body within that timeout. policy body within that timeout.
3.4. Policy Selection for Smart Hosts If a valid TXT record is found but no policy can be fetched via
HTTPS, and there is no valid (non-expired) previously-cached policy,
senders MUST treat the recipient domain as one that has not
implemented MTA-STS.
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
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
"mail.example.com", not "example.com".
4. Policy Validation 4. 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 SMTP MTA-STS policy, a sending MTA honoring SMTP STS and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST
MUST validate: 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 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
one of the identities presented in the MX's X.509 certificate, as
descriped 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 "report" mode MUST NOT be interpreted as
delivery failures, as described in the section _Policy_ delivery failures, as described in the section "Policy Application".
_Application_.
4.1. MX Matching
When delivering mail for the Policy Domain to a recipient MX host, 4.1. MX Certificate Validation
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 The certificate presented by the receiving MX MUST chain to a root CA
single domain name component or component fragment, though only as that is trusted by the sending MTA and be non-expired. The
the leftmost component in a pattern. For example, "*.example.com" is certificate MUST have a CN-ID ([RFC6125]) or SAN ([RFC5280]) with a
a valid pattern, but "foo.*.example.com" is not. Given the pattern DNS-ID matching the "mx" pattern.
"*.example.com", "mx1.example.com" is a valid MX host, but
"1234.dhcp.example.com" is not.
4.2. MX Certificate Validation Because the "mx" patterns are not hostnames, however, matching is not
identical to other common cases of X.509 certificate authentication
(as described, for example, in [RFC6125]). Consider the example
policy given above, with an "mx" pattern containing ".example.net".
In this case, if the MX server's X.509 certificate contains a SAN
matching "*.example.net", we are required to implement "wildcard-to-
wildcard" matching.
The certificate presented by the receiving MX MUST be valid for the To simplify this case, we impose the following constraints on
MX hostname and chain to a root CA that is trusted by the sending wildcard certificates, identical to those in [RFC7672] section 3.2.3:
MTA. The certificate MUST have a CN or SAN matching the MX hostname wildcards are valid in DNS-IDs or CN-IDs, but must be the entire
(as described in [RFC6125]) and be non-expired. first label of the identifier (that is, "*.example.com", not
"mail*.example.com"). Senders who are comparing a "suffix" MX
pattern with a wildcard identifier should thus strip the wildcard and
ensure that the two sides match label-by-label, until all labels of
the shorter side (if unequal length) are consumed.
In the case of an "implicit" MX record (as specified in [RFC2821]) A simple pseudocode implementation of this algorithm is presented in
where no MX RR exists for the recipient domain but there is an A RR, the Appendix.
the MX hostname is assumed to be that of the A RR and should be
validated as such.
5. Policy Application 5. 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 STS policy, a sending MTA honoring SMTP STS applies the non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies
result of a policy validation one of two ways, depending on the value the result of a policy validation failure one of two ways, depending
of the policy "mode" field: on the value of the policy "mode" field:
1. "report": In this mode, sending MTAs merely send a report (as 1. "report": In this mode, sending MTAs merely send a report (as
described in the TLSRPT specification (TODO: add ref)) indicating described in the TLSRPT specification (TODO: add ref)) indicating
policy application failures. policy application failures.
2. "enforce": In this mode, sending MTAs treat STS policy failures 2. "enforce": In this mode, sending MTAs MUST NOT deliver the
as a mail delivery error, and MUST NOT deliver the message to message to hosts which fail MX matching or certificate
this host. validation.
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 check for the presence of an updated policy at the compliant MTA MUST NOT permanently fail to deliver messages before
Policy Domain before permanently failing to deliver the message. checking for the presence of an updated policy at the Policy Domain.
(In all cases, MTAs SHOULD treat such failures as transient errors
This allows implementing domains to update long-lived policies on the and retry delivery later.) This allows implementing domains to
fly. update long-lived policies on the fly.
Finally, in both "enforce" and "report" modes, failures to deliver in Finally, in both "enforce" and "report" modes, failures to deliver in
compliance with the applied policy result in failure reports to the compliance with the applied policy result in failure reports to the
policy domain, as described in the TLSRPT specification (TODO: add policy domain, as described in the TLSRPT specification (TODO: add
ref). ref).
5.1. MX Preference 5.1. Policy Application Control Flow
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 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
(Optionally, sending MTAs may unconditionally check for a new (perhaps asynchronously, so as not to block message delivery).
policy at this step.) 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 2. For each candidate MX, in order of MX priority, attempt to
deliver the message, enforcing STARTTLS and the MX host's PKIX deliver the message, enforcing STARTTLS and, assuming a policy is
certificate validation. present, PKIX certificate validation, and certificate validation
as described in "MX Certificate Validation."
5. Upon message retries, a message MAY be permanently failed 3. Upon message retries, a message MAY be permanently failed
following first checking for the presence of a new policy (as following first checking for the presence of a new policy (as
indicated by the "id" field in the "mta-sts" TXT record). indicated by the "id" field in the "_mta-sts" TXT record).
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. In the case where the HTTPS at the corresponding HTTPS endpoint. As a result, recipients should
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 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 the 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
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
has passed. Recipients should therefore ensure that old policies
continue to work for message delivery during this period of time, or
risk message delays.
7. IANA Considerations 7. IANA Considerations
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.json Change Controller: IETF URI Suffix: mta-sts.json Change Controller: IETF
8. Security Considerations 8. Security Considerations
SMTP Strict Transport Security attempts to protect against an active SMTP MTA Strict Transport Security attempts to protect against an
attacker who wishes to intercept or tamper with mail between hosts active attacker who wishes to intercept or tamper with mail between
who support STARTTLS. There are two classes of attacks considered: hosts who support STARTTLS. There are two classes of attacks
considered:
1. 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 negotiation. This would result in the SMTP session occurring over
over plaintext, despite both parties supporting TLS. plaintext, despite both parties supporting TLS.
2. Impersonating the destination mail server, whereby the sender o Impersonating the destination mail server, whereby the sender
might deliver the message to an impostor, who could then monitor might deliver the message to an impostor, who could then monitor
and/or modify messages despite opportunistic TLS. This and/or modify messages despite opportunistic TLS. This
impersonation could be accomplished by spoofing the DNS MX record impersonation could be accomplished by spoofing the DNS MX record
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).
SMTP Strict Transport Security relies on certificate validation via MTA-STS can thwart such attacks only if the sender is able to
PKIX based TLS identity checking [RFC6125]. Attackers who are able previously obtain and cache a policy for the recipient domain, and
to obtain a valid certificate for the targeted recipient mail service only if the attacker is unable to obtain a valid certificate that
(e.g. by compromising a certificate authority) are thus able to complies with that policy. Below, we consider specific attacks on
circumvent STS authentication. this model.
Since we use DNS TXT records for policy discovery, an attacker who is 8.1. Obtaining a Signed Certificate
able to block DNS responses can suppress the discovery of an STS
Policy, making the Policy Domain appear not to have an STS Policy. SMTP MTA-STS relies on certificate validation via PKIX based TLS
The sender policy cache is designed to resist this attack. 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.
8.2. Preventing Policy Discovery
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
MTA-STS Policy, making the Policy Domain appear not to have an MTA-
STS Policy. The sender policy cache is designed to resist this
attack by decreasing the frequency of policy discovery and thus
reducing the window of vulnerability; it is nonetheless a risk that
attackers who can predict or induce policy discovery--for example, by
inducing a victim sending domain to send mail to a never-before-
contacted recipient while carrying out a man-in-the-middle attack--
may be able to foil policy discovery and effectively downgrade the
security of the message delivery.
Since this attack depends upon intercepting initial policy discovery,
we strongly recommend implementors to prefer policy "max_age" values
to be as long as is practical.
Because this attack is also possible upon refresh of a cached policy,
we suggest implementors do not wait until a cached policy has expired
before checking for an update; if senders attempt to refresh the
cache regularly (for instance, by checking their cached version
string against the TXT record on each successful send, or in a
background task that runs daily or weekly), an attacker would have to
foil policy discovery consistently over the lifetime of a cached
policy to prevent a successful refresh.
Resistence to downgrade attacks of this nature--due to the ability to
authoritatively determine "lack of a record" even for non-
participating recipients--is a feature of DANE, due to its use of
DNSSEC for policy discovery.
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
SMTP STS, such an attacker can cause a sending MTA to cache invalid MTA-STS, such an attacker can cause a sending MTA to cache invalid MX
MX records for a long TTL. With SMTP STS, the attacker can records, but only for however long the sending resolver caches those
additionally advertise a new, long-"max_age" SMTP STS policy with records. With MTA-STS, the attacker can additionally advertise a
"mx" constraints that validate the malicious MX record, causing new, long-"max_age" MTA-STS policy with "mx" constraints that
senders to cache the policy and refuse to deliver messages once the validate the malicious MX record, causing senders to cache the policy
victim has resecured the MX records. 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 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 (at any time) publish a new policy updating the cached, malicious
policy, though this does require the victim domain to both obtain a policy, though this does require the victim domain to both obtain a
valid CA-signed certificate and to understand and properly configure valid CA-signed certificate and to understand and properly configure
SMTP STS. MTA-STS.
Similarly, we consider the possibilty of domains that deliberately Similarly, we consider the possibility of domains that deliberately
allow untrusted users to serve untrusted content on user-specified allow untrusted users to serve untrusted content on user-specified
subdomains. In some cases (e.g. the service Tumblr.com) this takes subdomains. In some cases (e.g. the service Tumblr.com) this takes
the form of providing HTTPS hosting of user-registered subdomains; in the form of providing HTTPS hosting of user-registered subdomains; in
other cases (e.g. dynamic DNS providers) this takes the form of other cases (e.g. dynamic DNS providers) this takes the form of
allowing untrusted users to register custom DNS records at the allowing untrusted users to register custom DNS records at the
provider's domain. provider's domain.
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 SMTP STS policy. We believe this attack is rendered illegitimate MTA-STS policy. We believe this attack is rendered more
more difficult by the need for the attacker to both inject malicious difficult by the need for the attacker to also serve the "_mta-sts"
(but temporarily working) MX records and also serve the "mta-sts" TXT TXT record on the same domain--something not, to our knowledge,
record on the same domain--something not, to our knowledge, widely widely provided to untrusted users. This attack is additionally
provided to untrusted users. This attack is additionally mitigated mitigated by the aforementioned ability for a victim domain to update
by the aforementioned ability for a victim domain to update an an invalid policy at any future date.
invalid policy at any future date.
8.4. 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 STS Policy for "example.com" is to set the option when authoring an MTA-STS Policy for "example.com" is to set
"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 STS Policy validation, be a valid MX host for that the perspective of MTA-STS Policy validation, be a valid MX host for
domain. that domain.
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)
skipping to change at page 11, line 4 skipping to change at page 12, line 17
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: Domain Owner STS example record 10. Appendix 1: MTA-STS example record & policy
10.1. Example 1
The owner of "example.com" wishes to begin using STS with a policy The owner of "example.com" wishes to begin using MTA-STS with a
that will solicit reports from receivers without affecting how the policy that will solicit reports from senders without affecting how
messages are processed, in order to verify the identity of MXs that the messages are processed, in order to verify the identity of MXs
handle mail for "example.com", confirm that TLS is correctly used, that handle mail for "example.com", confirm that TLS is correctly
and ensure that certificates presented by the recipient MX validate. used, and ensure that certificates presented by the recipient MX
validate.
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;"
STS Policy JSON served as the response body at [1] MTA-STS Policy JSON served as the response body at [1]
{ {
"version": "STSv1", "version": "STSv1",
"mode": "report", "mode": "report",
"mx": ["mx1.example.com", "mx2.example.com"], "mx": ["mx1.example.com", "mx2.example.com"],
"max_age": 123456 "max_age": 12345678
} }
11. Appendix 2: Message delivery pseudocode 11. Appendix 2: Message delivery pseudocode
Below is pseudocode demonstrating the logic of a complaint sending Below is pseudocode demonstrating the logic of a compliant sending
MTA. This implements the "two-pass" approach, first attempting MTA.
delivery with a newly fetched policy (if present) before falling back
to a cached policy (if present). While this pseudocode implementation suggests synchronous policy
retrieval in the delivery path, in a working implementation that may
be undesirable, and we expect some implementors to instead prefer a
background fetch that does not block delivery if no cached policy is
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(mx) { func tryStartTls(mx) {
skipping to change at page 12, line 4 skipping to change at page 13, line 21
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(mx) { func tryStartTls(mx) {
// Attempt to open an SMTP connection with STARTTLS with the MX. // Attempt to open an SMTP connection with STARTTLS with the MX.
} }
func certMatches(connection, mx) { func certMatches(connection, mx) {
// Return if the server certificate from "connection" matches the "mx" host. // For simplicity, we are not checking CNs here.
for san in getSansFromCert(connection) {
// Return if the server certificate from "connection" matches the "mx" host.
if san[0] == '*' {
// Invalid wildcard!
if san[1] != '.' return false
san = san[1:]
}
if san[0] == '.' && HasSuffix(mx, san) {
return true
}
if mx[0] == '.' && HasSuffix(san, mx) {
return true
}
if mx == san {
return true
}
}
return false
} }
func tryDeliverMail(connection, message) { func tryDeliverMail(connection, message) {
// Attempt to deliver "message" via "connection". // 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) { func tryGetNewPolicy(domain) {
// Check for an MTA STS TXT record for "domain" in DNS, and return the // Check for an MTA-STS TXT record for "domain" in DNS, and return the
// indicated policy (or a local cache of the unvalidated policy). // indicated policy.
} }
func cachePolicy(domain, policy) { func cachePolicy(domain, policy) {
// Store "policy" as the cached policy for "domain". // Store "policy" as the cached policy for "domain".
} }
func tryGetCachedPolicy(domain, policy) { func tryGetCachedPolicy(domain) {
// Return a cached policy for "domain". // Return a cached policy for "domain".
} }
func reportError(error) { func reportError(error) {
// Report an error via TLSRPT. // Report an error via TLSRPT.
} }
func tryMxAccordingTo(message, mx, policy) { func tryMxAccordingTo(message, mx, policy) {
connection := connect(mx) connection := connect(mx)
if !connection { if !connection {
return false // Can't connect to the MX so it's not an STS error. return false // Can't connect to the MX so it's not an MTA-STS error.
} }
status := !(tryStartTls(mx, &connection) && certMatches(connection, mx)) secure := true
status = true
if !tryStartTls(mx, &connection) { if !tryStartTls(mx, &connection) {
status = false secure = false
reportError(E_NO_VALID_TLS) reportError(E_NO_VALID_TLS)
} else if certMatches(connection, mx) { } else if !certMatches(connection, mx) {
status = false secure = false
reportError(E_CERT_MISMATCH) reportError(E_CERT_MISMATCH)
} }
if status || !isEnforce(policy) { if secure || !isEnforce(policy) {
return tryDeliverMail(connection, message) return tryDeliverMail(connection, message)
} }
return false return false
} }
func tryWithPolicy(message, domain, policy) { func tryWithPolicy(message, domain, policy) {
mxes := getMxesForPolicy(domain, policy)
if mxs is empty {
reportError(E_NO_VALID_MXES)
}
for mx in mxes { for mx in mxes {
if tryMxAccordingTo(message, mx, policy) { if tryMxAccordingTo(message, mx, policy) {
return true return true
} }
} }
return false return false
} }
func handleMessage(message) { func handleMessage(message) {
domain := ... // domain part after '@' from recipient domain := ... // domain part after '@' from recipient
oldPolicy := tryGetCachedPolicy(domain) policy := tryGetNewPolicy(domain)
newPolicy := tryGetNewPolicy(domain) if policy {
if newPolicy { cachePolicy(domain, policy)
cachePolicy(domain, newPolicy) } else {
oldPolicy = newPolicy policy = tryGetCachedPolicy(domain)
} }
if oldPolicy { if policy {
return tryWithPolicy(message, oldPolicy) return tryWithPolicy(message, policy)
} }
// There is no policy or there's a new policy that did not work. // Try to deliver the message normally (i.e. without MTA-STS).
// Try to deliver the message normally (i.e. without 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, DOI 10.17487/
RFC2119, March 1997, RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <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>.
[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, <http://www.rfc-editor.org/info/rfc3207>. February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<http://www.rfc-editor.org/info/rfc3492>.
[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", RFC Rose, "DNS Security Introduction and Requirements", RFC
4033, DOI 10.17487/RFC4033, March 2005, 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>. <http://www.rfc-editor.org/info/rfc4033>.
[RFC4627] Crockford, D., "The application/json Media Type for [RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, DOI 10 JavaScript Object Notation (JSON)", RFC 4627, DOI 10
.17487/RFC4627, July 2006, .17487/RFC4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>. <http://www.rfc-editor.org/info/rfc4627>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008, RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[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, DOI 10
.17487/RFC5785, April 2010, .17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>. <http://www.rfc-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, <http://www.rfc-editor.org/info/rfc6125>.
[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, DOI 10 (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10
 End of changes. 84 change blocks. 
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