<pre>Internet Engineering Task Force (IETF)                       V. Dukhovni
Request for Comments: 7672                                     Two Sigma
Category: Standards Track                                    W. Hardaker
ISSN: 2070-1721                                                  Parsons
                                                            October 2015


   <span class="h1">SMTP Security via Opportunistic DNS-Based Authentication of Named</span>
             <span class="h1">Entities (DANE) Transport Layer Security (TLS)</span>

Abstract

   This memo describes a downgrade-resistant protocol for SMTP transport
   security between Message Transfer Agents (MTAs), based on the DNS-
   Based Authentication of Named Entities (DANE) TLSA DNS record.
   Adoption of this protocol enables an incremental transition of the
   Internet email backbone to one using encrypted and authenticated
   Transport Layer Security (TLS).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7672">http://www.rfc-editor.org/info/rfc7672</a>.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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<span class="grey"><a href="./rfc7672">RFC 7672</a>        SMTP Security via Opportunistic DANE TLS    October 2015</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Terminology ................................................<a href="#page-4">4</a>
      <a href="#section-1.2">1.2</a>. Background .................................................<a href="#page-6">6</a>
      <a href="#section-1.3">1.3</a>. SMTP Channel Security ......................................<a href="#page-6">6</a>
           <a href="#section-1.3.1">1.3.1</a>. STARTTLS Downgrade Attack ...........................<a href="#page-7">7</a>
           <a href="#section-1.3.2">1.3.2</a>. Insecure Server Name without DNSSEC .................<a href="#page-7">7</a>
           <a href="#section-1.3.3">1.3.3</a>. Sender Policy Does Not Scale ........................<a href="#page-8">8</a>
           <a href="#section-1.3.4">1.3.4</a>. Too Many Certification Authorities ..................<a href="#page-9">9</a>
   <a href="#section-2">2</a>. Identifying Applicable TLSA Records .............................<a href="#page-9">9</a>
      <a href="#section-2.1">2.1</a>. DNS Considerations .........................................<a href="#page-9">9</a>
           2.1.1. DNS Errors, "Bogus" Responses, and
                  "Indeterminate" Responses ...........................<a href="#page-9">9</a>
           <a href="#section-2.1.2">2.1.2</a>. DNS Error Handling .................................<a href="#page-11">11</a>
           <a href="#section-2.1.3">2.1.3</a>. Stub Resolver Considerations .......................<a href="#page-12">12</a>
      <a href="#section-2.2">2.2</a>. TLS Discovery .............................................<a href="#page-13">13</a>
           <a href="#section-2.2.1">2.2.1</a>. MX Resolution ......................................<a href="#page-14">14</a>
           <a href="#section-2.2.2">2.2.2</a>. Non-MX Destinations ................................<a href="#page-16">16</a>
           <a href="#section-2.2.3">2.2.3</a>. TLSA Record Lookup .................................<a href="#page-18">18</a>
   <a href="#section-3">3</a>. DANE Authentication ............................................<a href="#page-20">20</a>
      <a href="#section-3.1">3.1</a>. TLSA Certificate Usages ...................................<a href="#page-20">20</a>
           <a href="#section-3.1.1">3.1.1</a>. Certificate Usage DANE-EE(3) .......................<a href="#page-21">21</a>
           <a href="#section-3.1.2">3.1.2</a>. Certificate Usage DANE-TA(2) .......................<a href="#page-22">22</a>
           <a href="#section-3.1.3">3.1.3</a>. Certificate Usages PKIX-TA(0) and PKIX-EE(1) .......<a href="#page-23">23</a>
      <a href="#section-3.2">3.2</a>. Certificate Matching ......................................<a href="#page-24">24</a>
           <a href="#section-3.2.1">3.2.1</a>. DANE-EE(3) Name Checks .............................<a href="#page-24">24</a>
           <a href="#section-3.2.2">3.2.2</a>. DANE-TA(2) Name Checks .............................<a href="#page-24">24</a>
           <a href="#section-3.2.3">3.2.3</a>. Reference Identifier Matching ......................<a href="#page-25">25</a>
   <a href="#section-4">4</a>. Server Key Management ..........................................<a href="#page-26">26</a>
   <a href="#section-5">5</a>. Digest Algorithm Agility .......................................<a href="#page-27">27</a>
   <a href="#section-6">6</a>. Mandatory TLS Security .........................................<a href="#page-27">27</a>
   <a href="#section-7">7</a>. Note on DANE for Message User Agents ...........................<a href="#page-28">28</a>
   <a href="#section-8">8</a>. Interoperability Considerations ................................<a href="#page-28">28</a>
      <a href="#section-8.1">8.1</a>. SNI Support ...............................................<a href="#page-28">28</a>
      <a href="#section-8.2">8.2</a>. Anonymous TLS Cipher Suites ...............................<a href="#page-29">29</a>
   <a href="#section-9">9</a>. Operational Considerations .....................................<a href="#page-29">29</a>
      <a href="#section-9.1">9.1</a>. Client Operational Considerations .........................<a href="#page-29">29</a>
      <a href="#section-9.2">9.2</a>. Publisher Operational Considerations ......................<a href="#page-30">30</a>
   <a href="#section-10">10</a>. Security Considerations .......................................<a href="#page-30">30</a>
   <a href="#section-11">11</a>. References ....................................................<a href="#page-31">31</a>
      <a href="#section-11.1">11.1</a>. Normative References .....................................<a href="#page-31">31</a>
      <a href="#section-11.2">11.2</a>. Informative References ...................................<a href="#page-33">33</a>
   Acknowledgements ..................................................<a href="#page-34">34</a>
   Authors' Addresses ................................................<a href="#page-34">34</a>






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<span class="grey"><a href="./rfc7672">RFC 7672</a>        SMTP Security via Opportunistic DANE TLS    October 2015</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This memo specifies a new connection security model for Message
   Transfer Agents (MTAs).  This model is motivated by key features of
   inter-domain SMTP delivery, principally, the fact that the
   destination server is selected indirectly via DNS Mail Exchange (MX)
   records and that neither email addresses nor MX hostnames signal a
   requirement for either secure or cleartext transport.  Therefore,
   aside from a few manually configured exceptions, SMTP transport
   security is, by necessity, opportunistic (for a definition of
   "Opportunistic Security", see [<a href="./rfc7435" title="&quot;Opportunistic Security: Some Protection Most of the Time&quot;">RFC7435</a>]).

   This specification uses the presence of DANE TLSA records to securely
   signal TLS support and to publish the means by which SMTP clients can
   successfully authenticate legitimate SMTP servers.  This becomes
   "opportunistic DANE TLS" and is resistant to downgrade and
   man-in-the-middle (MITM) attacks.  It enables an incremental
   transition of the email backbone to authenticated TLS delivery, with
   increased global protection as adoption increases.

   With opportunistic DANE TLS, traffic from SMTP clients to domains
   that publish "usable" DANE TLSA records in accordance with this memo
   is authenticated and encrypted.  Traffic from legacy clients or to
   domains that do not publish TLSA records will continue to be sent in
   the same manner as before, via manually configured security,
   (pre-DANE) opportunistic TLS, or just cleartext SMTP.

   Problems with the existing use of TLS in MTA-to-MTA SMTP that
   motivate this specification are described in <a href="#section-1.3">Section 1.3</a>.  The
   specification itself follows, in Sections <a href="#section-2">2</a> and <a href="#section-3">3</a>, which describe,
   respectively, how to locate and use DANE TLSA records with SMTP.  In
   <a href="#section-6">Section 6</a>, we discuss the application of DANE TLS to destinations for
   which channel integrity and confidentiality are mandatory.  In
   <a href="#section-7">Section 7</a>, we briefly comment on the potential applicability of this
   specification to Message User Agents.
















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<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

   The following terms or concepts are used throughout this document:

   Man-in-the-middle (MITM) attack:  Active modification of network
      traffic by an adversary able to thereby compromise the
      confidentiality or integrity of the data.

   Downgrade attack:  (From [<a href="./rfc4949" title="&quot;Internet Security Glossary, Version 2&quot;">RFC4949</a>].)  A type of MITM attack in which
      the attacker can cause two parties, at the time they negotiate a
      security association, to agree on a lower level of protection than
      the highest level that could have been supported by both of them.

   Downgrade-resistant:  A protocol is "downgrade-resistant" if it
      employs effective countermeasures against downgrade attacks.

   "Secure", "bogus", "insecure", "indeterminate":  DNSSEC validation
      results, as defined in <a href="./rfc4035#section-4.3">Section&nbsp;4.3 of [RFC4035]</a>.

   Validating security-aware stub resolver and non-validating
   security-aware stub resolver:
      Capabilities of the stub resolver in use, as defined in [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>];
      note that this specification requires the use of a security-aware
      stub resolver.

   (Pre-DANE) opportunistic TLS:  Best-effort use of TLS that is
      generally vulnerable to DNS forgery and STARTTLS downgrade
      attacks.  When a TLS-encrypted communication channel is not
      available, message transmission takes place in the clear.  MX
      record indirection generally precludes authentication even when
      TLS is available.

   Opportunistic DANE TLS:  Best-effort use of TLS that is resistant to
      downgrade attacks for destinations with DNSSEC-validated TLSA
      records.  When opportunistic DANE TLS is determined to be
      unavailable, clients should fall back to pre-DANE opportunistic
      TLS.  Opportunistic DANE TLS requires support for DNSSEC, DANE,
      and STARTTLS on the client side, and STARTTLS plus a DNSSEC
      published TLSA record on the server side.







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   Reference identifier:  (Special case of [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>] definition.)  One
      of the domain names associated by the SMTP client with the
      destination SMTP server for performing name checks on the server
      certificate.  When name checks are applicable, at least one of the
      reference identifiers MUST match an [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>] DNS-ID (or, if none
      are present, the [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>] CN-ID) of the server certificate (see
      <a href="#section-3.2.3">Section 3.2.3</a>).

   MX hostname:  The RRDATA of an MX record consists of a 16 bit
      preference followed by a Mail Exchange domain name (see <a href="./rfc1035#section-3.3.9">[RFC1035],
      Section&nbsp;3.3.9</a>).  We will use the term "MX hostname" to refer to
      the latter, that is, the DNS domain name found after the
      preference value in an MX record.  Thus, an "MX hostname" is
      specifically a reference to a DNS domain name rather than any host
      that bears that name.

   Delayed delivery:  Email delivery is a multi-hop store-and-forward
      process.  When an MTA is unable to forward a message that may
      become deliverable later, the message is queued and delivery is
      retried periodically.  Some MTAs may be configured with a fallback
      next-hop destination that handles messages that the MTA would
      otherwise queue and retry.  When a fallback next-hop destination
      is configured, messages that would otherwise have to be delayed
      may be sent to the fallback next-hop destination instead.  The
      fallback destination may itself be subject to opportunistic or
      mandatory DANE TLS (<a href="#section-6">Section 6</a>) as though it were the original
      message destination.

   Original next-hop destination:  The logical destination for mail
      delivery.  By default, this is the domain portion of the recipient
      address, but MTAs may be configured to forward mail for some or
      all recipients via designated relays.  The original next-hop
      destination is, respectively, either the recipient domain or the
      associated configured relay.

   MTA:  Message Transfer Agent (<a href="./rfc5598#section-4.3.2">[RFC5598], Section&nbsp;4.3.2</a>).

   MSA:  Message Submission Agent (<a href="./rfc5598#section-4.3.1">[RFC5598], Section&nbsp;4.3.1</a>).

   MUA:  Message User Agent (<a href="./rfc5598#section-4.2.1">[RFC5598], Section&nbsp;4.2.1</a>).

   RR:  A DNS resource record as defined in <a href="./rfc1034#section-3.6">[RFC1034], Section&nbsp;3.6</a>.

   RRset:  An RRset (<a href="./rfc2181#section-5">[RFC2181], Section&nbsp;5</a>) is a group of DNS resource
      records that share the same label, class, and type.






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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Background</span>

   The Domain Name System Security Extensions (DNSSEC) add data origin
   authentication, data integrity, and data nonexistence proofs to the
   Domain Name System (DNS).  DNSSEC is defined in [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>], [<a href="./rfc4034" title="&quot;Resource Records for the DNS Security Extensions&quot;">RFC4034</a>],
   and [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>].

   As described in the introduction of [<a href="./rfc6698" title="&quot;The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA&quot;">RFC6698</a>], TLS authentication via
   the existing public Certification Authority (CA) PKI suffers from an
   overabundance of trusted parties capable of issuing certificates for
   any domain of their choice.  DANE leverages the DNSSEC infrastructure
   to publish public keys and certificates for use with the Transport
   Layer Security (TLS) [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>] protocol via the "TLSA" DNS record
   type.  With DNSSEC, each domain can only vouch for the keys of its
   delegated sub-domains.

   The TLS protocol enables secure TCP communication.  In the context of
   this memo, channel security is assumed to be provided by TLS.  Used
   without authentication, TLS provides only privacy protection against
   eavesdropping attacks.  Otherwise, TLS also provides data origin
   authentication to guard against MITM attacks.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  SMTP Channel Security</span>

   With HTTPS, TLS employs X.509 certificates [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>] issued by one of
   the many CAs bundled with popular web browsers to allow users to
   authenticate their "secure" websites.  Before we specify a new DANE
   TLS security model for SMTP, we will explain why a new security model
   is needed.  In the process, we will explain why the familiar HTTPS
   security model is inadequate to protect inter-domain SMTP traffic.

   The subsections below outline four key problems with applying
   traditional Web PKI [<a href="./rfc7435" title="&quot;Opportunistic Security: Some Protection Most of the Time&quot;">RFC7435</a>] to SMTP; these problems are addressed
   by this specification.  Since an SMTP channel security policy is not
   explicitly specified in either the recipient address or the MX
   record, a new signaling mechanism is required to indicate when
   channel security is possible and should be used.  The publication of
   TLSA records allows server operators to securely signal to SMTP
   clients that TLS is available and should be used.  DANE TLSA makes it
   possible to simultaneously discover which destination domains support
   secure delivery via TLS and how to verify the authenticity of the
   associated SMTP services, providing a path forward to ubiquitous SMTP
   channel security.








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<span class="h4"><a class="selflink" id="section-1.3.1" href="#section-1.3.1">1.3.1</a>.  STARTTLS Downgrade Attack</span>

   SMTP [<a href="./rfc5321" title="&quot;Simple Mail Transfer Protocol&quot;">RFC5321</a>] is a single-hop protocol in a multi-hop store-and-
   forward email delivery process.  An SMTP envelope recipient address
   does not correspond to a specific transport-layer endpoint address;
   rather, at each relay hop, the transport-layer endpoint is the
   next-hop relay, while the envelope recipient address typically
   remains the same.  Unlike HTTP and its corresponding secured version,
   HTTPS, where the use of TLS is signaled via the URI scheme, email
   recipient addresses do not directly signal transport security policy.
   Indeed, no such signaling could work well with SMTP, since TLS
   encryption of SMTP protects email traffic on a hop-by-hop basis while
   email addresses could only express end-to-end policy.

   With no mechanism available to signal transport security policy, SMTP
   relays employ a best-effort "opportunistic" security model for TLS.
   A single SMTP server TCP listening endpoint can serve both TLS and
   non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
   command [<a href="./rfc3207" title="&quot;SMTP Service Extension for Secure SMTP over Transport Layer Security&quot;">RFC3207</a>].  The server signals TLS support to the client over
   a cleartext SMTP connection, and, if the client also supports TLS, it
   may negotiate a TLS-encrypted channel to use for email transmission.
   The server's indication of TLS support can be easily suppressed by an
   MITM attacker.  Thus, pre-DANE SMTP TLS security can be subverted by
   simply downgrading a connection to cleartext.  No TLS security
   feature can prevent this.  The attacker can simply disable TLS.

<span class="h4"><a class="selflink" id="section-1.3.2" href="#section-1.3.2">1.3.2</a>.  Insecure Server Name without DNSSEC</span>

   With SMTP, DNS MX records abstract the next-hop transport endpoint
   and allow administrators to specify a set of target servers to which
   SMTP traffic should be directed for a given domain.

   A TLS client is vulnerable to MITM attacks unless it verifies that
   the server's certificate binds the public key to a name that matches
   one of the client's reference identifiers.  A natural choice of
   reference identifier is the server's domain name.  However, with
   SMTP, server names are not directly encoded in the recipient address;
   instead, they are obtained indirectly via MX records.  Without
   DNSSEC, the MX lookup is vulnerable to MITM and DNS cache poisoning
   attacks.  Active attackers can forge DNS replies with fake MX records
   and can redirect email to servers with names of their choice.
   Therefore, secure verification of SMTP TLS certificates matching the
   server name is not possible without DNSSEC.








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   One might try to harden TLS for SMTP against DNS attacks by using the
   envelope recipient domain as a reference identifier and by requiring
   each SMTP server to possess a trusted certificate for the envelope
   recipient domain rather than the MX hostname.  Unfortunately, this is
   impractical, as email for many domains is handled by third parties
   that are not in a position to obtain certificates for all the domains
   they serve.  Deployment of the Server Name Indication (SNI) extension
   to TLS (see <a href="./rfc6066#section-3">Section&nbsp;3 of [RFC6066]</a>) is no panacea, since SNI key
   management is operationally challenging except when the email service
   provider is also the domain's registrar and its certificate issuer;
   this is rarely the case for email.

   Since the recipient domain name cannot be used as the SMTP server
   reference identifier, and neither can the MX hostname without DNSSEC,
   large-scale deployment of authenticated TLS for SMTP requires that
   the DNS be secure.

   Since SMTP security depends critically on DNSSEC, it is important to
   point out that SMTP with DANE is consequently the most conservative
   possible trust model.  It trusts only what must be trusted and no
   more.  Adding any other trusted actors to the mix can only reduce
   SMTP security.  A sender may choose to further harden DNSSEC for
   selected high-value receiving domains by configuring explicit trust
   anchors for those domains instead of relying on the chain of trust
   from the root domain.  However, detailed discussion of DNSSEC
   security practices is out of scope for this document.

<span class="h4"><a class="selflink" id="section-1.3.3" href="#section-1.3.3">1.3.3</a>.  Sender Policy Does Not Scale</span>

   Sending systems are in some cases explicitly configured to use TLS
   for mail sent to selected peer domains, but this requires configuring
   sending MTAs with appropriate subject names or certificate content
   digests from their peer domains.  Due to the resulting administrative
   burden, such statically configured SMTP secure channels are used
   rarely (generally only between domains that make bilateral
   arrangements with their business partners).  Internet email, on the
   other hand, requires regularly contacting new domains for which
   security configurations cannot be established in advance.

   The abstraction of the SMTP transport endpoint via DNS MX records,
   often across organizational boundaries, limits the use of public CA
   PKI with SMTP to a small set of sender-configured peer domains.  With
   little opportunity to use TLS authentication, sending MTAs are rarely
   configured with a comprehensive list of trusted CAs.  SMTP services
   that support STARTTLS often deploy X.509 certificates that are
   self-signed or issued by a private CA.





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<span class="h4"><a class="selflink" id="section-1.3.4" href="#section-1.3.4">1.3.4</a>.  Too Many Certification Authorities</span>

   Even if it were generally possible to determine a secure server name,
   the SMTP client would still need to verify that the server's
   certificate chain is issued by a trusted CA (a trust anchor).  MTAs
   are not interactive applications where a human operator can make a
   decision (wisely or otherwise) to selectively disable TLS security
   policy when certificate chain verification fails.  With no user to
   "click OK", the MTA's list of public CA trust anchors would need to
   be comprehensive in order to avoid bouncing mail addressed to sites
   that employ unknown CAs.

   On the other hand, each trusted CA can issue certificates for any
   domain.  If even one of the configured CAs is compromised or operated
   by an adversary, it can subvert TLS security for all destinations.
   Any set of CAs is simultaneously both overly inclusive and not
   inclusive enough.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Identifying Applicable TLSA Records</span>

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  DNS Considerations</span>

<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>.  DNS Errors, "Bogus" Responses, and "Indeterminate" Responses</span>

   An SMTP client that implements opportunistic DANE TLS per this
   specification depends critically on the integrity of DNSSEC lookups,
   as discussed in <a href="#section-1.3.2">Section 1.3.2</a>.  This section lists the DNS resolver
   requirements needed to avoid downgrade attacks when using
   opportunistic DANE TLS.

   A DNS lookup may signal an error or return a definitive answer.  A
   security-aware resolver MUST be used for this specification.
   Security-aware resolvers will indicate the security status of a DNS
   RRset with one of four possible values defined in <a href="./rfc4035#section-4.3">Section&nbsp;4.3 of
   [RFC4035]</a>: "secure", "insecure", "bogus", and "indeterminate".  In
   [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>], the meaning of the "indeterminate" security status is:

      An RRset for which the resolver is not able to determine whether
      the RRset should be signed, as the resolver is not able to obtain
      the necessary DNSSEC RRs.  This can occur when the security-aware
      resolver is not able to contact security-aware name servers for
      the relevant zones.

   Note that the "indeterminate" security status has a conflicting
   definition in <a href="./rfc4033#section-5">Section&nbsp;5 of [RFC4033]</a>:

      There is no trust anchor that would indicate that a specific
      portion of the tree is secure.



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   In this document, the term "indeterminate" will be used exclusively
   in the [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>] sense.  Therefore, obtaining "indeterminate" lookup
   results is a (transient) failure condition, namely, the inability to
   locate the relevant DNS records.  DNS records that would be
   classified "indeterminate" in the sense of [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>] are simply
   classified as "insecure".

   We do not need to distinguish between zones that lack a suitable
   ancestor trust anchor, and delegations (ultimately) from a trust
   anchor that designate a child zone as being "insecure".  All
   "insecure" RRsets MUST be handled identically: in either case,
   non-validated data for the query domain is all that is and can be
   available, and authentication using the data is impossible.  As the
   DNS root zone has been signed, we expect that validating resolvers
   used by Internet-facing MTAs will be configured with trust anchor
   data for the root zone and that therefore domains with no ancestor
   trust anchor will not be possible in most deployments.

   As noted in <a href="./rfc4035#section-4.3">Section&nbsp;4.3 of [RFC4035]</a>, a security-aware DNS resolver
   MUST be able to determine whether a given non-error DNS response is
   "secure", "insecure", "bogus", or "indeterminate".  It is expected
   that most security-aware stub resolvers will not signal an
   "indeterminate" security status (in the sense of [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>]) to the
   application and will instead signal a "bogus" or error result.  If a
   resolver does signal an [<a href="./rfc4035" title="&quot;Protocol Modifications for the DNS Security Extensions&quot;">RFC4035</a>] "indeterminate" security status,
   this MUST be treated by the SMTP client as though a "bogus" or error
   result had been returned.

   An MTA using a non-validating security-aware stub resolver MAY use
   the stub resolver's ability, if available, to signal DNSSEC
   validation status based on information the stub resolver has learned
   from an upstream validating recursive resolver.  Security-oblivious
   stub resolvers [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>] MUST NOT be used.  In accordance with
   <a href="./rfc4035#section-4.9.3">Section&nbsp;4.9.3 of [RFC4035]</a>:

      ... a security-aware stub resolver MUST NOT place any reliance on
      signature validation allegedly performed on its behalf, except
      when the security-aware stub resolver obtained the data in
      question from a trusted security-aware recursive name server via a
      secure channel.

   To avoid much repetition in the text below, we will pause to explain
   the handling of "bogus" or "indeterminate" DNSSEC query responses.
   These are not necessarily the result of a malicious actor; they can,
   for example, occur when network packets are corrupted or lost in
   transit.  Therefore, "bogus" or "indeterminate" replies are equated
   in this memo with lookup failure.




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   There is an important non-failure condition we need to highlight in
   addition to the obvious case of the DNS client obtaining a non-empty
   "secure" or "insecure" RRset of the requested type.  Namely, it is
   not an error when either "secure" or "insecure" nonexistence is
   determined for the requested data.  When a DNSSEC response with a
   validation status that is either "secure" or "insecure" reports
   either no records of the requested type or nonexistence of the query
   domain, the response is not a DNS error condition.  The DNS client
   has not been left without an answer; it has learned that records of
   the requested type do not exist.

   Security-aware stub resolvers will, of course, also signal DNS lookup
   errors in other cases, for example, when processing a "SERVFAIL"
   [<a href="./rfc2136" title="&quot;Dynamic Updates in the Domain Name System (DNS UPDATE)&quot;">RFC2136</a>] response code (RCODE) [<a href="./rfc1035" title="&quot;Domain names - implementation and specification&quot;">RFC1035</a>], which will not have an
   associated DNSSEC status.  All lookup errors are treated the same way
   by this specification, regardless of whether they are from a "bogus"
   or "indeterminate" DNSSEC status or from a more generic DNS error:
   the information that was requested cannot be obtained by the
   security-aware resolver at this time.  Thus, a lookup error is either
   a failure to obtain the relevant RRset if it exists or a failure to
   determine that no such RRset exists when it does not.

   In contrast to a "bogus" response or an "indeterminate" response, an
   "insecure" DNSSEC response is not an error; rather, as explained
   above, it indicates that the target DNS zone is either delegated as
   an "insecure" child of a "secure" parent zone or not a descendant of
   any of the configured DNSSEC trust anchors in use by the SMTP client.
   "Insecure" results will leave the SMTP client with degraded channel
   security but do not stand in the way of message delivery.  See
   <a href="#section-2.2">Section 2.2</a> for further details.

<span class="h4"><a class="selflink" id="section-2.1.2" href="#section-2.1.2">2.1.2</a>.  DNS Error Handling</span>

   When a DNS lookup failure (an error, "bogus", or "indeterminate", as
   defined above) prevents an SMTP client from determining which SMTP
   server or servers it should connect to, message delivery MUST be
   delayed.  This naturally includes, for example, the case when a
   "bogus" or "indeterminate" response is encountered during MX
   resolution.  When multiple MX hostnames are obtained from a
   successful MX lookup but a later DNS lookup failure prevents network
   address resolution for a given MX hostname, delivery may proceed via
   any remaining MX hosts.

   When a particular SMTP server is securely identified as the delivery
   destination, a set of DNS lookups (<a href="#section-2.2">Section 2.2</a>) MUST be performed to
   locate any related TLSA records.  If any DNS queries used to locate
   TLSA records fail (due to "bogus" or "indeterminate" records,
   timeouts, malformed replies, SERVFAIL responses, etc.), then the SMTP



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   client MUST treat that server as unreachable and MUST NOT deliver the
   message via that server.  If no servers are reachable, delivery is
   delayed.

   In the text that follows, we will only describe what happens when all
   relevant DNS queries succeed.  If any DNS failure occurs, the SMTP
   client MUST behave as described in this section, by "skipping" the
   SMTP server or destination that is problematic.  Queries for
   candidate TLSA records are explicitly part of "all relevant DNS
   queries", and SMTP clients MUST NOT continue to connect to an SMTP
   server or destination whose TLSA record lookup fails.

<span class="h4"><a class="selflink" id="section-2.1.3" href="#section-2.1.3">2.1.3</a>.  Stub Resolver Considerations</span>

   A note about DNAME aliases: a query for a domain name whose ancestor
   domain is a DNAME alias returns the DNAME RR for the ancestor domain
   along with a CNAME that maps the query domain to the corresponding
   sub-domain of the target domain of the DNAME alias [<a href="./rfc6672" title="&quot;DNAME Redirection in the DNS&quot;">RFC6672</a>].
   Therefore, whenever we speak of CNAME aliases, we implicitly allow
   for the possibility that the alias in question is the result of an
   ancestor domain DNAME record.  Consequently, no explicit support for
   DNAME records is needed in SMTP software; it is sufficient to process
   the resulting CNAME aliases.  DNAME records only require special
   processing in the validating stub resolver library that checks the
   integrity of the combined DNAME + CNAME reply.  When DNSSEC
   validation is handled by a local caching resolver rather than the MTA
   itself, even that part of the DNAME support logic is outside the MTA.

   When a stub resolver returns a response containing a CNAME alias that
   does not also contain the corresponding query results for the target
   of the alias, the SMTP client will need to repeat the query at the
   target of the alias and should do so recursively up to some
   configured or implementation-dependent recursion limit.  If at any
   stage of CNAME expansion an error is detected, the lookup of the
   original requested records MUST be considered to have failed.

   Whether a chain of CNAME records was returned in a single stub
   resolver response or via explicit recursion by the SMTP client, if at
   any stage of recursive expansion an "insecure" CNAME record is
   encountered, then it and all subsequent results (in particular, the
   final result) MUST be considered "insecure", regardless of whether or
   not any earlier CNAME records leading to the "insecure" record were
   "secure".

   Note that a security-aware non-validating stub resolver may return to
   the SMTP client an "insecure" reply received from a validating
   recursive resolver that contains a CNAME record along with additional
   answers recursively obtained starting at the target of the CNAME.  In



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   this case, the only possible conclusion is that some record in the
   set of records returned is "insecure", and it is, in fact, possible
   that the initial CNAME record and a subset of the subsequent records
   are "secure".

   If the SMTP client needs to determine the security status of the DNS
   zone containing the initial CNAME record, it will need to issue a
   separate query of type "CNAME" that returns only the initial CNAME
   record.  Specifically, as discussed in <a href="#section-2.2.2">Section 2.2.2</a>, when "insecure"
   A or AAAA records are found for an SMTP server via a CNAME alias, the
   SMTP client will need to perform an additional CNAME query in order
   to determine whether or not the DNS zone in which the alias is
   published is DNSSEC signed.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  TLS Discovery</span>

   As noted previously (in <a href="#section-1.3.1">Section 1.3.1</a>), opportunistic TLS with SMTP
   servers that advertise TLS support via STARTTLS is subject to an MITM
   downgrade attack.  Also, some SMTP servers that are not, in fact, TLS
   capable erroneously advertise STARTTLS by default, and clients need
   to be prepared to retry cleartext delivery after STARTTLS fails.  In
   contrast, DNSSEC-validated TLSA records MUST NOT be published for
   servers that do not support TLS.  Clients can safely interpret their
   presence as a commitment by the server operator to implement TLS and
   STARTTLS.

   This memo defines four actions to be taken after the search for a
   TLSA record returns "secure" usable results, "secure" unusable
   results, "insecure" or no results, or an error signal.  The term
   "usable" in this context is in the sense of <a href="./rfc6698#section-4.1">Section&nbsp;4.1 of [RFC6698]</a>.
   Specifically, if the DNS lookup for a TLSA record returns:

   A "secure" TLSA RRset with at least one usable record:  Any
      connection to the MTA MUST employ TLS encryption and MUST
      authenticate the SMTP server using the techniques discussed in the
      rest of this document.  Failure to establish an authenticated TLS
      connection MUST result in falling back to the next SMTP server or
      delayed delivery.

   A "secure" non-empty TLSA RRset where all the records are unusable:
      Any connection to the MTA MUST be made via TLS, but authentication
      is not required.  Failure to establish an encrypted TLS connection
      MUST result in falling back to the next SMTP server or delayed
      delivery.







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   An "insecure" TLSA RRset or DNSSEC-authenticated denial of existence
   of the TLSA records:
      A connection to the MTA SHOULD be made using (pre-DANE)
      opportunistic TLS; this includes using cleartext delivery when the
      remote SMTP server does not appear to support TLS.  The MTA MAY
      retry in cleartext when delivery via TLS fails during the
      handshake or even during data transfer.

   Any lookup error:  Lookup errors, including "bogus" and
      "indeterminate" as explained in <a href="#section-2.1.1">Section 2.1.1</a>, MUST result in
      falling back to the next SMTP server or delayed delivery.

   An SMTP client MAY be configured to mandate DANE-verified delivery
   for some destinations.  With mandatory DANE TLS (<a href="#section-6">Section 6</a>), delivery
   proceeds only when "secure" TLSA records are used to establish an
   encrypted and authenticated TLS channel with the SMTP server.

   When the original next-hop destination is an address literal rather
   than a DNS domain, DANE TLS does not apply.  Delivery proceeds using
   any relevant security policy configured by the MTA administrator.
   Similarly, when an MX RRset incorrectly lists a network address in
   lieu of an MX hostname, if an MTA chooses to connect to the network
   address in the nonconformant MX record, DANE TLSA does not apply for
   such a connection.

   In the subsections that follow, we explain how to locate the SMTP
   servers and the associated TLSA records for a given next-hop
   destination domain.  We also explain which name or names are to be
   used in identity checks of the SMTP server certificate.

<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>.  MX Resolution</span>

   In this section, we consider next-hop domains that are subject to MX
   resolution and have MX records.  The TLSA records and the associated
   base domain are derived separately for each MX hostname that is used
   to attempt message delivery.  DANE TLS can authenticate message
   delivery to the intended next-hop domain only when the MX records are
   obtained securely via a DNSSEC-validated lookup.

   MX records MUST be sorted by preference; an MX hostname with a worse
   (numerically higher) MX preference that has TLSA records MUST NOT
   preempt an MX hostname with a better (numerically lower) preference
   that has no TLSA records.  In other words, prevention of delivery
   loops by obeying MX preferences MUST take precedence over channel
   security considerations.  Even with two equal-preference MX records,
   an MTA is not obligated to choose the MX hostname that offers more





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   security.  Domains that want secure inbound mail delivery need to
   ensure that all their SMTP servers and MX records are configured
   accordingly.

   In the language of <a href="./rfc5321#section-5.1">[RFC5321], Section&nbsp;5.1</a>, the original next-hop
   domain is the "initial name".  If the MX lookup of the initial name
   results in a CNAME alias, the MTA replaces the initial name with the
   resulting name and performs a new lookup with the new name.  MTAs
   typically support recursion in CNAME expansion, so this replacement
   is performed repeatedly (up to the MTA's recursion limit) until the
   ultimate non-CNAME domain is found.

   If the MX RRset (or any CNAME leading to it) is "insecure" (see
   <a href="#section-2.1.1">Section 2.1.1</a>) and DANE TLS for the given destination is mandatory
   (<a href="#section-6">Section 6</a>), delivery MUST be delayed.  If the MX RRset is "insecure"
   and DANE TLS is not mandatory, the SMTP client is free to use
   pre-DANE opportunistic TLS (possibly even cleartext).

   Since the protocol in this memo is an Opportunistic Security protocol
   [<a href="./rfc7435" title="&quot;Opportunistic Security: Some Protection Most of the Time&quot;">RFC7435</a>], the SMTP client MAY elect to use DANE TLS (as described in
   <a href="#section-2.2.2">Section 2.2.2</a> below), even with MX hosts obtained via an "insecure"
   MX RRset.  For example, when a hosting provider has a signed DNS zone
   and publishes TLSA records for its SMTP servers, hosted domains that
   are not signed may still benefit from the provider's TLSA records.
   Deliveries via the provider's SMTP servers will not be subject to
   active attacks when sending SMTP clients elect to use the provider's
   TLSA records (active attacks that tamper with the "insecure" MX RRset
   are of course still possible in this case).

   When the MX records are not (DNSSEC) signed, an active attacker can
   redirect SMTP clients to MX hosts of his choice.  Such redirection is
   tamper-evident when SMTP servers found via "insecure" MX records are
   recorded as the next-hop relay in the MTA delivery logs in their
   original (rather than CNAME-expanded) form.  Sending MTAs SHOULD log
   unexpanded MX hostnames when these result from "insecure" MX lookups.
   Any successful authentication via an insecurely determined MX host
   MUST NOT be misrepresented in the mail logs as secure delivery to the
   intended next-hop domain.

   In the absence of DNS lookup errors (<a href="#section-2.1.1">Section 2.1.1</a>), if the MX RRset
   is not "insecure", then it is "secure", and the SMTP client MUST
   treat each MX hostname as described in <a href="#section-2.2.2">Section 2.2.2</a>.  When, for a
   given MX hostname, no TLSA records are found or only "insecure" TLSA
   records are found, DANE TLSA is not applicable with the SMTP server
   in question, and delivery proceeds to that host as with pre-DANE
   opportunistic TLS.  To avoid downgrade attacks, any errors during
   TLSA lookups MUST, as explained in <a href="#section-2.1.2">Section 2.1.2</a>, cause the SMTP
   server in question to be treated as unreachable.



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<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a>.  Non-MX Destinations</span>

   This section describes the algorithm used to locate the TLSA records
   and associated TLSA base domain for an input domain that is not
   subject to MX resolution, that represents a hostname from a "secure"
   MX RRset, or that lacks MX records.  Such domains include:

   o  Any host that is configured by the sending MTA administrator as
      the next-hop relay for some or all domains and that is not subject
      to MX resolution.

   o  A domain that has MX records.  When a domain has MX records, we
      treat each MX host listed in those MX records as though it were a
      non-MX destination -- that is, in the same way as we would treat
      an administrator-configured relay that handles mail for that
      domain.  (Unlike administrator-specified relays, MTAs are not
      required to support CNAME expansion of next-hop names found via MX
      lookups.)

   o  A next-hop destination domain subject to MX resolution that has no
      MX records.  In this case, the domain's name is implicitly also
      its sole SMTP server name.

   Note that DNS queries with type TLSA are mishandled by load-balancing
   nameservers that serve the MX hostnames of some large email
   providers.  The DNS zones served by these nameservers are not signed
   and contain no TLSA records.  These nameservers SHOULD provide
   "insecure" negative replies that indicate the nonexistence of the
   TLSA records, but instead they fail by not responding at all or by
   responding with a DNS RCODE [<a href="./rfc1035" title="&quot;Domain names - implementation and specification&quot;">RFC1035</a>] other than NXDOMAIN, e.g.,
   SERVFAIL or NOTIMP [<a href="./rfc2136" title="&quot;Dynamic Updates in the Domain Name System (DNS UPDATE)&quot;">RFC2136</a>].

   To avoid problems delivering mail to domains whose SMTP servers are
   served by these problematic nameservers, the SMTP client MUST perform
   any A and/or AAAA queries for the destination before attempting to
   locate the associated TLSA records.  This lookup is needed in any
   case to determine (1) whether or not the destination domain is
   reachable and (2) the DNSSEC validation status of the chain of CNAME
   queries required to reach the ultimate address records.

   If no address records are found, the destination is unreachable.  If
   address records are found but the DNSSEC validation status of the
   first query response is "insecure" (see <a href="#section-2.1.3">Section 2.1.3</a>), the SMTP
   client SHOULD NOT proceed to search for any associated TLSA records.
   In the case of these problematic domains, TLSA queries would lead to
   DNS lookup errors and would cause messages to be consistently delayed
   and ultimately returned to the sender.  We don't expect to find any




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   "secure" TLSA records associated with a TLSA base domain that lies in
   an unsigned DNS zone.  Therefore, skipping TLSA lookups in this case
   will also reduce latency, with no detrimental impact on security.

   If the A and/or AAAA lookup of the initial name yields a CNAME, we
   replace it with the resulting name as if it were the initial name and
   perform a lookup again using the new name.  This replacement is
   performed recursively (up to the MTA's recursion limit).

   We consider the following cases for handling a DNS response for an
   A or AAAA DNS lookup:

   Not found:  When the DNS queries for A and/or AAAA records yield
      neither a list of addresses nor a CNAME (or CNAME expansion is not
      supported), the destination is unreachable.

   Non-CNAME:  The answer is not a CNAME alias.  If the address RRset is
      "secure", TLSA lookups are performed as described in <a href="#section-2.2.3">Section 2.2.3</a>
      with the initial name as the candidate TLSA base domain.  If no
      "secure" TLSA records are found, DANE TLS is not applicable and
      mail delivery proceeds with pre-DANE opportunistic TLS (which,
      being best-effort, degrades to cleartext delivery when STARTTLS is
      not available or the TLS handshake fails).

   Insecure CNAME:  The input domain is a CNAME alias, but the ultimate
      network address RRset is "insecure" (see <a href="#section-2.1.1">Section 2.1.1</a>).  If the
      initial CNAME response is also "insecure", DANE TLS does not
      apply.  Otherwise, this case is treated just like the non-CNAME
      case above, where a search is performed for a TLSA record with the
      original input domain as the candidate TLSA base domain.

   Secure CNAME:  The input domain is a CNAME alias, and the ultimate
      network address RRset is "secure" (see <a href="#section-2.1.1">Section 2.1.1</a>).  Two
      candidate TLSA base domains are tried: the fully CNAME-expanded
      initial name and, failing that, the initial name itself.

   In summary, if it is possible to securely obtain the full,
   CNAME-expanded, DNSSEC-validated address records for the input
   domain, then that name is the preferred TLSA base domain.  Otherwise,
   the unexpanded input domain is the candidate TLSA base domain.  When
   no "secure" TLSA records are found at either the CNAME-expanded or
   unexpanded domain, then DANE TLS does not apply for mail delivery via
   the input domain in question.  And, as always, errors, "bogus"
   results, or "indeterminate" results for any query in the process MUST
   result in delaying or abandoning delivery.






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<span class="h4"><a class="selflink" id="section-2.2.3" href="#section-2.2.3">2.2.3</a>.  TLSA Record Lookup</span>

   When the SMTP server's hostname is not a CNAME or DNAME alias, the
   list of associated candidate TLSA base domains (see below) consists
   of just the server hostname.

   When the hostname is an alias with a "secure" (at every stage) full
   expansion, the list of candidate TLSA base domains (see below) is a
   pair of domains: the fully expanded server hostname first, and the
   unexpanded server hostname second.

   Each candidate TLSA base domain (alias-expanded or original) is in
   turn prefixed with service labels of the form "_&lt;port&gt;._tcp".  The
   resulting domain name is used to issue a DNSSEC query with the query
   type set to TLSA (<a href="./rfc6698#section-7.1">[RFC6698], Section&nbsp;7.1</a>).

   The first of these candidate domains to yield a "secure" TLSA RRset
   becomes the actual TLSA base domain.

   For SMTP, the destination TCP port is typically 25, but this may be
   different with custom routes specified by the MTA administrator, in
   which case the SMTP client MUST use the appropriate number in the
   "_&lt;port&gt;" prefix in place of "_25".  If, for example, the candidate
   base domain is "mx.example.com" and the SMTP connection is to port
   25, the TLSA RRset is obtained via a DNSSEC query of the form:

      _25._tcp.mx.example.com. IN TLSA ?

   The query response may be a CNAME or the actual TLSA RRset.  If the
   response is a CNAME, the SMTP client (through the use of its
   security-aware stub resolver) restarts the TLSA query at the target
   domain, following CNAMEs as appropriate, and keeps track of whether
   or not the entire chain is "secure".  If any "insecure" records are
   encountered or the TLSA records don't exist, the next candidate TLSA
   base domain is tried instead.

   If the ultimate response is a "secure" TLSA RRset, then the candidate
   TLSA base domain will be the actual TLSA base domain, and the TLSA
   RRset will constitute the TLSA records for the destination.  If none
   of the candidate TLSA base domains yield "secure" TLSA records, then
   the SMTP client is free to use pre-DANE opportunistic TLS (possibly
   even cleartext).

   TLSA record publishers may leverage CNAMEs to reference a single
   authoritative TLSA RRset specifying a common CA or a common
   end-entity certificate to be used with multiple TLS services.  Such
   CNAME expansion does not change the SMTP client's notion of the TLSA
   base domain; thus, when _25._tcp.mx.example.com is a CNAME, the base



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   domain remains mx.example.com, and this is still the reference
   identifier used together with the next-hop domain in peer certificate
   name checks.

   Note that shared end-entity certificate associations expose the
   publishing domain to substitution attacks, where an MITM attacker can
   reroute traffic to a different server that shares the same end-entity
   certificate.  Such shared end-entity TLSA records SHOULD be avoided
   unless the servers in question are functionally equivalent or employ
   mutually incompatible protocols (an active attacker gains nothing by
   diverting client traffic from one such server to another).

   A better example, employing a shared trust anchor rather than shared
   end-entity certificates, is illustrated by the DNSSEC-validated
   records below:

      example.com.                IN MX 0 mx1.example.com.
      example.com.                IN MX 0 mx2.example.com.
      _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
      _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
      tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c149a...

   The SMTP servers mx1.example.com and mx2.example.com will be expected
   to have certificates issued under a common trust anchor, but each MX
   hostname's TLSA base domain remains unchanged despite the above CNAME
   records.  Correspondingly, each SMTP server will be associated with a
   pair of reference identifiers consisting of its hostname plus the
   next-hop domain "example.com".

   If, during TLSA resolution (including possible CNAME indirection), at
   least one "secure" TLSA record is found (even if not usable because
   it is unsupported by the implementation or support is
   administratively disabled), then the corresponding host has signaled
   its commitment to implement TLS.  The SMTP client MUST NOT deliver
   mail via the corresponding host unless a TLS session is negotiated
   via STARTTLS.  This is required to avoid MITM STARTTLS downgrade
   attacks.

   As noted previously (in <a href="#section-2.2.2">Section 2.2.2</a>), when no "secure" TLSA records
   are found at the fully CNAME-expanded name, the original unexpanded
   name MUST be tried instead.  This supports customers of hosting
   providers where the provider's zone cannot be validated with DNSSEC
   but the customer has shared appropriate key material with the hosting
   provider to enable TLS via SNI.  Intermediate names that arise during
   CNAME expansion that are neither the original name nor the final name
   are never candidate TLSA base domains, even if "secure".





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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  DANE Authentication</span>

   This section describes which TLSA records are applicable to SMTP
   opportunistic DANE TLS and how to apply such records to authenticate
   the SMTP server.  With opportunistic DANE TLS, both the TLS support
   implied by the presence of DANE TLSA records and the verification
   parameters necessary to authenticate the TLS peer are obtained
   together.  In contrast to protocols where channel security policy is
   set exclusively by the client, authentication via this protocol is
   expected to be less prone to connection failure caused by
   incompatible configuration of the client and server.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  TLSA Certificate Usages</span>

   The DANE TLSA specification [<a href="./rfc6698" title="&quot;The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA&quot;">RFC6698</a>] defines multiple TLSA RR types
   via combinations of three numeric parameters.  The numeric values of
   these parameters were later given symbolic names in [<a href="./rfc7218" title="&quot;Adding Acronyms to Simplify Conversations about DNS-Based Authentication of Named Entities (DANE)&quot;">RFC7218</a>].  The
   rest of the TLSA record is the "certificate association data field",
   which specifies the full or digest value of a certificate or
   public key.

   Since opportunistic DANE TLS will be used by non-interactive MTAs,
   with no user to "click OK" when authentication fails, reliability of
   peer authentication is paramount.  Server operators are advised to
   publish TLSA records that are least likely to fail authentication due
   to interoperability or operational problems.  Because DANE TLS relies
   on coordinated changes to DNS and SMTP server settings, the best
   choice of records to publish will depend on site-specific practices.

   The certificate usage element of a TLSA record plays a critical role
   in determining how the corresponding certificate association data
   field is used to authenticate a server's certificate chain.
   Sections <a href="#section-3.1.1">3.1.1</a> and <a href="#section-3.1.2">3.1.2</a> explain the process for certificate usages
   DANE-EE(3) and DANE-TA(2), respectively.  <a href="#section-3.1.3">Section 3.1.3</a> briefly
   explains why certificate usages PKIX-TA(0) and PKIX-EE(1) are not
   applicable with opportunistic DANE TLS.

   In summary, we RECOMMEND the use of "DANE-EE(3) SPKI(1) SHA2-256(1)",
   with "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records as a second
   choice, depending on site needs.  See Sections <a href="#section-3.1.1">3.1.1</a> and <a href="#section-3.1.2">3.1.2</a> for
   more details.  Other combinations of TLSA parameters either (1) are
   explicitly unsupported or (2) offer little to recommend them over
   these two.








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<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>.  Certificate Usage DANE-EE(3)</span>

   Authentication via certificate usage DANE-EE(3) TLSA records involves
   simply checking that the server's leaf certificate matches the TLSA
   record.  In particular, the binding of the server public key to its
   name is based entirely on the TLSA record association.  The server
   MUST be considered authenticated even if none of the names in the
   certificate match the client's reference identity for the server.

   The expiration date of the server certificate MUST be ignored: the
   validity period of the TLSA record key binding is determined by the
   validity interval of the TLSA record DNSSEC signature.

   With DANE-EE(3), servers need not employ SNI (they may ignore the
   client's SNI message) even when the server is known under independent
   names that would otherwise require separate certificates.  It is
   instead sufficient for the TLSA RRsets for all the domains in
   question to match the server's default certificate.  Of course, with
   SMTP servers it is simpler still to publish the same MX hostname for
   all the hosted domains.

   For domains where it is practical to make coordinated changes in DNS
   TLSA records during SMTP server key rotation, it is often best to
   publish end-entity DANE-EE(3) certificate associations.  DANE-EE(3)
   certificates don't suddenly stop working when leaf or intermediate
   certificates expire, nor do they fail when the server operator
   neglects to configure all the required issuer certificates in the
   server certificate chain.

   TLSA records published for SMTP servers SHOULD, in most cases, be
   "DANE-EE(3) SPKI(1) SHA2-256(1)" records.  Since all DANE
   implementations are required to support SHA2-256, this record type
   works for all clients and need not change across certificate renewals
   with the same key.

















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<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>.  Certificate Usage DANE-TA(2)</span>

   Some domains may prefer to avoid the operational complexity of
   publishing unique TLSA RRs for each TLS service.  If the domain
   employs a common issuing CA to create certificates for multiple TLS
   services, it may be simpler to publish the issuing authority as a
   trust anchor (TA) for the certificate chains of all relevant
   services.  The TLSA query domain (TLSA base domain with port and
   protocol prefix labels) for each service issued by the same TA may
   then be set to a CNAME alias that points to a common TLSA RRset that
   matches the TA.  For example:

      example.com.                IN MX 0 mx1.example.com.
      example.com.                IN MX 0 mx2.example.com.
      _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
      _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
      tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c14....

   With usage DANE-TA(2), the server certificates will need to have
   names that match one of the client's reference identifiers (see
   [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>]).  The server MAY employ SNI to select the appropriate
   certificate to present to the client.

   SMTP servers that rely on certificate usage DANE-TA(2) TLSA records
   for TLS authentication MUST include the TA certificate as part of the
   certificate chain presented in the TLS handshake server certificate
   message even when it is a self-signed root certificate.  Many SMTP
   servers are not configured with a comprehensive list of trust
   anchors, nor are they expected to be at any point in the future.
   Some MTAs will ignore all locally trusted certificates when
   processing usage DANE-TA(2) TLSA records.  Thus, even when the TA
   happens to be a public CA known to the SMTP client, authentication is
   likely to fail unless the TA certificate is included in the TLS
   server certificate message.

   With some SMTP server software, it is not possible to configure the
   server to include self-signed (root) CA certificates in the server
   certificate chain.  Such servers either MUST publish DANE-TA(2)
   records for an intermediate certificate or MUST instead use
   DANE-EE(3) TLSA records.

   TLSA records with a matching type of Full(0) are discouraged.  While
   these potentially obviate the need to transmit the TA certificate in
   the TLS server certificate message, client implementations may not be
   able to augment the server certificate chain with the data obtained
   from DNS, especially when the TLSA record supplies a bare key
   (selector SPKI(1)).  Since the server will need to transmit the TA
   certificate in any case, server operators SHOULD publish TLSA records



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   with a matching type other than Full(0) and avoid potential
   interoperability issues with large TLSA records containing full
   certificates or keys.

   TLSA Publishers employing DANE-TA(2) records SHOULD publish records
   with a selector of Cert(0).  Such TLSA records are associated with
   the whole trust anchor certificate, not just with the trust anchor
   public key.  In particular, the SMTP client SHOULD then apply any
   relevant constraints from the trust anchor certificate, such as, for
   example, path length constraints.

   While a selector of SPKI(1) may also be employed, the resulting TLSA
   record will not specify the full trust anchor certificate content,
   and elements of the trust anchor certificate other than the public
   key become mutable.  This may, for example, allow a subsidiary CA to
   issue a chain that violates the trust anchor's path length or name
   constraints.

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>.  Certificate Usages PKIX-TA(0) and PKIX-EE(1)</span>

   Note that this section applies to MTA-to-MTA SMTP, which is normally
   on port 25 -- that is, to servers that are the SMTP servers for one
   or more destination domains.  Other uses of SMTP, such as in
   MUA-to-MSA submission on ports 587 or 465, are out of scope for this
   document.  Where those other uses also employ TLS opportunistically
   and/or depend on DNSSEC as a result of DNS-based discovery of service
   location, the relevant specifications should, as appropriate, arrive
   at similar conclusions.

   As noted in Sections <a href="#section-1.3.1">1.3.1</a> and <a href="#section-1.3.2">1.3.2</a>, sending MTAs cannot, without
   relying on DNSSEC for "secure" MX records and DANE for STARTTLS
   support signaling, perform server identity verification or prevent
   STARTTLS downgrade attacks.  The use of PKIX CAs offers no added
   security, since an attacker capable of compromising DNSSEC is free to
   replace any PKIX-TA(0) or PKIX-EE(1) TLSA records with records
   bearing any convenient non-PKIX certificate usage.  Finally, as
   explained in <a href="#section-1.3.4">Section 1.3.4</a>, there is no list of trusted CAs agreed
   upon by all MTAs and no user to "click OK" when a server's CA is not
   trusted by a client.

   Therefore, TLSA records for the port 25 SMTP service used by client
   MTAs SHOULD NOT include TLSA RRs with certificate usage PKIX-TA(0) or
   PKIX-EE(1).  SMTP client MTAs cannot be expected to be configured
   with a suitably complete set of trusted public CAs.  Lacking a
   complete set of public CAs, MTA clients would not be able to verify
   the certificates of SMTP servers whose issuing root CAs are not
   trusted by the client.




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   Opportunistic DANE TLS needs to interoperate without bilateral
   coordination of security settings between client and server systems.
   Therefore, parameter choices that are fragile in the absence of
   bilateral coordination are unsupported.  Nothing is lost; since the
   PKIX certificate usages cannot aid SMTP TLS security, they can only
   impede SMTP TLS interoperability.

   SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
   or PKIX-EE(1) is undefined.  As with any other unsupported
   certificate usage, SMTP clients MAY treat such records as "unusable".

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Certificate Matching</span>

   When at least one usable "secure" TLSA record is found, the SMTP
   client MUST use TLSA records to authenticate the SMTP server.
   Messages MUST NOT be delivered via the SMTP server if authentication
   fails; otherwise, the SMTP client is vulnerable to MITM attacks.

<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>.  DANE-EE(3) Name Checks</span>

   The SMTP client MUST NOT perform certificate name checks with
   certificate usage DANE-EE(3) (<a href="#section-3.1.1">Section 3.1.1</a>).

<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>.  DANE-TA(2) Name Checks</span>

   To match a server via a TLSA record with certificate usage
   DANE-TA(2), the client MUST perform name checks to ensure that it has
   reached the correct server.  In all DANE-TA(2) cases, the SMTP client
   MUST employ the TLSA base domain as the primary reference identifier
   for matching the server certificate.

   TLSA records for MX hostnames:  If the TLSA base domain was obtained
      indirectly via a "secure" MX lookup (including any CNAME-expanded
      name of an MX hostname), then the original next-hop domain used in
      the MX lookup MUST be included as a second reference identifier.
      The CNAME-expanded original next-hop domain MUST be included as a
      third reference identifier if different from the original next-hop
      domain.  When the client MTA is employing DANE TLS security
      despite "insecure" MX redirection, the MX hostname is the only
      reference identifier.

   TLSA records for non-MX hostnames:  If MX records were not used
      (e.g., if none exist) and the TLSA base domain is the
      CNAME-expanded original next-hop domain, then the original
      next-hop domain MUST be included as a second reference identifier.






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   Accepting certificates with the original next-hop domain in addition
   to the MX hostname allows a domain with multiple MX hostnames to
   field a single certificate bearing a single domain name (i.e., the
   email domain) across all the SMTP servers.  This also aids
   interoperability with pre-DANE SMTP clients that are configured to
   look for the email domain name in server certificates -- for example,
   with "secure" DNS records as shown below:

      exchange.example.org.            IN CNAME mail.example.org.
      mail.example.org.                IN CNAME example.com.
      example.com.                     IN MX    10 mx10.example.com.
      example.com.                     IN MX    15 mx15.example.com.
      example.com.                     IN MX    20 mx20.example.com.
      ;
      mx10.example.com.                IN A     192.0.2.10
      _25._tcp.mx10.example.com.       IN TLSA  2 0 1 ...
      ;
      mx15.example.com.                IN CNAME mxbackup.example.com.
      mxbackup.example.com.            IN A     192.0.2.15
      ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
      _25._tcp.mx15.example.com.       IN TLSA  2 0 1 ...
      ;
      mx20.example.com.                IN CNAME mxbackup.example.net.
      mxbackup.example.net.            IN A     198.51.100.20
      _25._tcp.mxbackup.example.net.   IN TLSA  2 0 1 ...

   Certificate name checks for delivery of mail to exchange.example.org
   via any of the associated SMTP servers MUST accept at least the names
   "exchange.example.org" and "example.com", which are, respectively,
   the original and fully expanded next-hop domain.  When the SMTP
   server is mx10.example.com, name checks MUST accept the TLSA base
   domain "mx10.example.com".  If, despite the fact that MX hostnames
   are required to not be aliases, the MTA supports delivery via
   "mx15.example.com" or "mx20.example.com", then name checks MUST
   accept the respective TLSA base domains "mx15.example.com" and
   "mxbackup.example.net".

<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>.  Reference Identifier Matching</span>

   When name checks are applicable (certificate usage DANE-TA(2)), if
   the server certificate contains a Subject Alternative Name extension
   [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>] with at least one DNS-ID [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>], then only the DNS-IDs
   are matched against the client's reference identifiers.  The CN-ID
   [<a href="./rfc6125" title="&quot;Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)&quot;">RFC6125</a>] is only considered when no DNS-IDs are present.  The server
   certificate is considered matched when one of its presented
   identifiers [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>] matches any of the client's reference
   identifiers.




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   Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
   The wildcard character must be the entire first label of the DNS-ID
   or CN-ID.  Thus, "*.example.com" is valid, while "smtp*.example.com"
   and "*smtp.example.com" are not.  SMTP clients MUST support wildcards
   that match the first label of the reference identifier, with the
   remaining labels matching verbatim.  For example, the DNS-ID
   "*.example.com" matches the reference identifier "mx1.example.com".
   SMTP clients MAY, subject to local policy, allow wildcards to match
   multiple reference identifier labels, but servers cannot expect broad
   support for such a policy.  Therefore, any wildcards in server
   certificates SHOULD match exactly one label in either the TLSA base
   domain or the next-hop domain.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Server Key Management</span>

   Two TLSA records MUST be published before employing a new EE or TA
   public key or certificate: one matching the currently deployed key
   and the other matching the new key scheduled to replace it.  Once
   sufficient time has elapsed for all DNS caches to expire the previous
   TLSA RRset and related signature RRsets, servers may be configured to
   use the new EE private key and associated public key certificate or
   may employ certificates signed by the new trust anchor.

   Once the new public key or certificate is in use, the TLSA RR that
   matches the retired key can be removed from DNS, leaving only RRs
   that match keys or certificates in active use.

   As described in <a href="#section-3.1.2">Section 3.1.2</a>, when server certificates are validated
   via a DANE-TA(2) trust anchor and CNAME records are employed to store
   the TA association data at a single location, the responsibility of
   updating the TLSA RRset shifts to the operator of the trust anchor.
   Before a new trust anchor is used to sign any new server
   certificates, its certificate (digest) is added to the relevant TLSA
   RRset.  After enough time elapses for the original TLSA RRset to age
   out of DNS caches, the new trust anchor can start issuing new server
   certificates.  Once all certificates issued under the previous trust
   anchor have expired, its associated RRs can be removed from the TLSA
   RRset.

   In the DANE-TA(2) key management model, server operators do not
   generally need to update DNS TLSA records after initially creating a
   CNAME record that references the centrally operated DANE-TA(2) RRset.
   If a particular server's key is compromised, its TLSA CNAME SHOULD be
   replaced with a DANE-EE(3) association until the certificate for the
   compromised key expires, at which point it can return to using a
   CNAME record.  If the central trust anchor is compromised, all





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   servers need to be issued new keys by a new TA, and an updated
   DANE-TA(2) TLSA RRset needs to be published containing just the
   new TA.

   SMTP servers cannot expect broad Certificate Revocation List (CRL) or
   Online Certificate Status Protocol (OCSP) support from SMTP clients.
   As outlined above, with DANE, compromised server or trust anchor keys
   can be "revoked" by removing them from the DNS without the need for
   client-side support for OCSP or CRLs.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Digest Algorithm Agility</span>

   While [<a href="./rfc6698" title="&quot;The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA&quot;">RFC6698</a>] specifies multiple digest algorithms, it does not
   specify a protocol by which the SMTP client and TLSA record publisher
   can agree on the strongest shared algorithm.  Such a protocol would
   allow the client and server to avoid exposure to deprecated weaker
   algorithms that are published for compatibility with less capable
   clients.  When stronger algorithms are an option, deprecated
   algorithms SHOULD be avoided.  Such a protocol is specified in
   [<a href="./rfc7671" title="&quot;The DNS-Based Authentication of Named Entities (DANE) Protocol: Updates and Operational Guidance&quot;">RFC7671</a>].  SMTP clients and servers that implement this
   specification MUST comply with the requirements outlined in <a href="./rfc7671#section-9">Section&nbsp;9
   of [RFC7671]</a>.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Mandatory TLS Security</span>

   An MTA implementing this protocol may require a stronger security
   assurance when sending email to selected destinations.  The sending
   organization may need to send sensitive email and/or may have
   regulatory obligations to protect its content.  This protocol is not
   in conflict with such a requirement and, in fact, can often simplify
   authenticated delivery to such destinations.

   Specifically, with domains that publish DANE TLSA records for their
   MX hostnames, a sending MTA can be configured to use the receiving
   domain's DANE TLSA records to authenticate the corresponding SMTP
   server.  Authentication via DANE TLSA records is easier to manage, as
   changes in the receiver's expected certificate properties are made on
   the receiver end and don't require manually communicated
   configuration changes.  With mandatory DANE TLS, when no usable TLSA
   records are found, message delivery is delayed.  Thus, mail is only
   sent when an authenticated TLS channel is established to the remote
   SMTP server.

   Administrators of mail servers that employ mandatory DANE TLS need to
   carefully monitor their mail logs and queues.  If a partner domain
   unwittingly misconfigures its TLSA records, disables DNSSEC, or
   misconfigures SMTP server certificate chains, mail will be delayed
   and may bounce if the issue is not resolved in a timely manner.



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<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Note on DANE for Message User Agents</span>

   We note that SMTP is also used between Message User Agents (MUAs) and
   Message Submission Agents (MSAs) [<a href="./rfc6409" title="&quot;Message Submission for Mail&quot;">RFC6409</a>].  In [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>], a protocol
   is specified that enables an MUA to dynamically locate the MSA based
   on the user's email address.  SMTP connection security considerations
   for MUAs implementing [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>] are largely analogous to connection
   security requirements for MTAs, and this specification could be
   applied largely verbatim with DNS MX records replaced by
   corresponding DNS Service (SRV) records [<a href="./rfc7673" title="&quot;Using DNS-Based Authentication of Named Entities (DANE) TLSA Records with SRV Records&quot;">RFC7673</a>].

   However, until MUAs begin to adopt the dynamic configuration
   mechanisms of [<a href="./rfc6186" title="&quot;Use of SRV Records for Locating Email Submission/Access Services&quot;">RFC6186</a>], they are adequately served by more
   traditional static TLS security policies.  Specification of DANE TLS
   for MUA-to-MSA SMTP is left to future documents that focus
   specifically on SMTP security between MUAs and MSAs.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Interoperability Considerations</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  SNI Support</span>

   To ensure that the server sends the right certificate chain, the SMTP
   client MUST send the TLS SNI extension containing the TLSA base
   domain.  This precludes the use of the Secure Socket Layer (SSL)
   HELLO that is SSL 2.0 compatible by the SMTP client.

   Each SMTP server MUST present a certificate chain (see <a href="./rfc5246#section-7.4.2">[RFC5246],
   Section&nbsp;7.4.2</a>) that matches at least one of the TLSA records.  The
   server MAY rely on SNI to determine which certificate chain to
   present to the client.  Clients that don't send SNI information may
   not see the expected certificate chain.

   If the server's TLSA records match the server's default certificate
   chain, the server need not support SNI.  In either case, the server
   need not include the SNI extension in its TLS HELLO, as simply
   returning a matching certificate chain is sufficient.  Servers
   MUST NOT enforce the use of SNI by clients, as the client may be
   using unauthenticated opportunistic TLS and may not expect any
   particular certificate from the server.  If the client sends no SNI
   extension or sends an SNI extension for an unsupported domain, the
   server MUST simply send some fallback certificate chain of its
   choice.  The reason for not enforcing strict matching of the
   requested SNI hostname is that DANE TLS clients are 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 acceptable to the client, e.g., the original next-hop domain.





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<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Anonymous TLS Cipher Suites</span>

   Since many SMTP servers either do not support or do not enable any
   anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD
   offer to negotiate a typical set of non-anonymous cipher suites
   required for interoperability with such servers.  An SMTP client
   employing pre-DANE opportunistic TLS MAY also include one or more
   anonymous TLS cipher suites in its TLS HELLO.  SMTP servers that need
   to interoperate with opportunistic TLS clients SHOULD be prepared to
   interoperate with such clients by either always selecting a mutually
   supported non-anonymous cipher suite or correctly handling client
   connections that negotiate anonymous cipher suites.

   Note that while SMTP server operators are under no obligation to
   enable anonymous cipher suites, no security is gained by sending
   certificates to clients that will ignore them.  Indeed, support for
   anonymous cipher suites in the server makes audit trails more
   informative.  Log entries that record connections that employed an
   anonymous cipher suite record the fact that the clients did not care
   to authenticate the server.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Operational Considerations</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Client Operational Considerations</span>

   An operational error on the sending or receiving side that cannot be
   corrected in a timely manner may, at times, lead to consistent
   failure to deliver time-sensitive email.  The sending MTA
   administrator may have to choose between allowing email to queue
   until the error is resolved and disabling opportunistic or mandatory
   DANE TLS (<a href="#section-6">Section 6</a>) for one or more destinations.  The choice to
   disable DANE TLS security should not be made lightly.  Every
   reasonable effort should be made to determine that problems with mail
   delivery are the result of an operational error and not an attack.  A
   fallback strategy may be to configure explicit out-of-band TLS
   security settings if supported by the sending MTA.

   SMTP clients may deploy opportunistic DANE TLS incrementally by
   enabling it only for selected sites or may occasionally need to
   disable opportunistic DANE TLS for peers that fail to interoperate
   due to misconfiguration or software defects on either end.  Some
   implementations MAY support DANE TLS in an "audit only" mode in which
   failure to achieve the requisite security level is logged as a
   warning and delivery proceeds at a reduced security level.  Unless
   local policy specifies "audit only" or specifies that opportunistic
   DANE TLS is not to be used for a particular destination, an SMTP





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   client MUST NOT deliver mail via a server whose certificate chain
   fails to match at least one TLSA record when usable TLSA records are
   found for that server.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Publisher Operational Considerations</span>

   Some MTAs enable STARTTLS selectively.  For example, they might only
   support STARTTLS with clients that have previously demonstrated
   "proper MTA behavior", e.g., by retrying the delivery of deferred
   messages (greylisting).  If such an MTA publishes DANE TLSA records,
   sending MTAs that implement this specification will not attempt the
   initial cleartext SMTP transaction needed to establish the "proper
   MTA behavior", because they cannot establish the required channel
   security.  Server operators MUST NOT implement selective STARTTLS if
   they also want to support DANE TLSA.

   TLSA Publishers MUST follow the guidelines in <a href="./rfc7671#section-8">Section&nbsp;8 of [RFC7671]</a>.

   TLSA Publishers SHOULD follow the TLSA publication size guidance
   found in <a href="./rfc7671#section-10.1">Section&nbsp;10.1 of [RFC7671]</a>.

   TLSA Publishers SHOULD follow the TLSA record TTL and signature
   lifetime recommendations found in <a href="./rfc7671#section-13">Section&nbsp;13 of [RFC7671]</a>.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   This protocol leverages DANE TLSA records to implement MITM-resistant
   Opportunistic Security [<a href="./rfc7435" title="&quot;Opportunistic Security: Some Protection Most of the Time&quot;">RFC7435</a>] for SMTP.  For destination domains
   that sign their MX records and publish signed TLSA records for their
   MX hostnames, this protocol allows sending MTAs to securely discover
   both the availability of TLS and how to authenticate the destination.

   This protocol does not aim to secure all SMTP traffic, as that is not
   practical until DNSSEC and DANE adoption are universal.  The
   incremental deployment provided by following this specification is a
   best possible path for securing SMTP.  This protocol coexists and
   interoperates with the existing insecure Internet email backbone.

   The protocol does not preclude existing non-opportunistic SMTP TLS
   security arrangements, which can continue to be used as before via
   manual configuration with negotiated out-of-band key and TLS
   configuration exchanges.

   Opportunistic SMTP TLS depends critically on DNSSEC for downgrade
   resistance and secure resolution of the destination name.  If DNSSEC
   is compromised, it is not possible to fall back on the public CA PKI
   to prevent MITM attacks.  A successful breach of DNSSEC enables the
   attacker to publish TLSA usage 3 certificate associations and thereby



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   bypass any security benefit the legitimate domain owner might hope to
   gain by publishing usage 0 or usage 1 TLSA RRs.  Given the lack of
   public CA PKI support in existing MTA deployments, avoiding
   certificate usages 0 and 1 simplifies implementation and deployment
   with no adverse security consequences.

   Implementations must strictly follow Sections <a href="#section-2.1.2">2.1.2</a>, <a href="#section-2.1.3">2.1.3</a>, <a href="#section-2.2">2.2</a>,
   2.2.1, 2.2.2, 2.2.3, 3.2, and 9.1 of this specification; these
   sections indicate when it is appropriate to initiate a
   non-authenticated connection or cleartext connection to an SMTP
   server.  Specifically, in order to prevent downgrade attacks on this
   protocol, implementations must not initiate a connection when this
   specification indicates that a particular SMTP server must be
   considered unreachable.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-RFC1034">RFC1034</a>]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, <a href="./rfc1034">RFC 1034</a>, DOI 10.17487/RFC1034, November 1987,
              &lt;<a href="http://www.rfc-editor.org/info/rfc1034">http://www.rfc-editor.org/info/rfc1034</a>&gt;.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3207">RFC3207</a>]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", <a href="./rfc3207">RFC 3207</a>, DOI 10.17487/RFC3207,
              February 2002, &lt;<a href="http://www.rfc-editor.org/info/rfc3207">http://www.rfc-editor.org/info/rfc3207</a>&gt;.

   [<a id="ref-RFC4033">RFC4033</a>]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              <a href="./rfc4033">RFC 4033</a>, DOI 10.17487/RFC4033, March 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4033">http://www.rfc-editor.org/info/rfc4033</a>&gt;.

   [<a id="ref-RFC4034">RFC4034</a>]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              <a href="./rfc4034">RFC 4034</a>, DOI 10.17487/RFC4034, March 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4034">http://www.rfc-editor.org/info/rfc4034</a>&gt;.

   [<a id="ref-RFC4035">RFC4035</a>]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", <a href="./rfc4035">RFC 4035</a>, DOI 10.17487/RFC4035, March 2005,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4035">http://www.rfc-editor.org/info/rfc4035</a>&gt;.





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   [<a id="ref-RFC5246">RFC5246</a>]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>,
              DOI 10.17487/RFC5246, August 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5246">http://www.rfc-editor.org/info/rfc5246</a>&gt;.

   [<a id="ref-RFC5280">RFC5280</a>]  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", <a href="./rfc5280">RFC 5280</a>, DOI 10.17487/RFC5280, May 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5280">http://www.rfc-editor.org/info/rfc5280</a>&gt;.

   [<a id="ref-RFC5321">RFC5321</a>]  Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc5321">RFC 5321</a>,
              DOI 10.17487/RFC5321, October 2008,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5321">http://www.rfc-editor.org/info/rfc5321</a>&gt;.

   [<a id="ref-RFC5598">RFC5598</a>]  Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>,
              DOI 10.17487/RFC5598, July 2009,
              &lt;<a href="http://www.rfc-editor.org/info/rfc5598">http://www.rfc-editor.org/info/rfc5598</a>&gt;.

   [<a id="ref-RFC6066">RFC6066</a>]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", <a href="./rfc6066">RFC 6066</a>,
              DOI 10.17487/RFC6066, January 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6066">http://www.rfc-editor.org/info/rfc6066</a>&gt;.

   [<a id="ref-RFC6125">RFC6125</a>]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", <a href="./rfc6125">RFC 6125</a>, DOI 10.17487/RFC6125,
              March 2011, &lt;<a href="http://www.rfc-editor.org/info/rfc6125">http://www.rfc-editor.org/info/rfc6125</a>&gt;.

   [<a id="ref-RFC6186">RFC6186</a>]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", <a href="./rfc6186">RFC 6186</a>,
              DOI 10.17487/RFC6186, March 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6186">http://www.rfc-editor.org/info/rfc6186</a>&gt;.

   [<a id="ref-RFC6672">RFC6672</a>]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", <a href="./rfc6672">RFC 6672</a>, DOI 10.17487/RFC6672, June 2012,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6672">http://www.rfc-editor.org/info/rfc6672</a>&gt;.

   [<a id="ref-RFC6698">RFC6698</a>]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", <a href="./rfc6698">RFC 6698</a>, DOI 10.17487/RFC6698,
              August 2012, &lt;<a href="http://www.rfc-editor.org/info/rfc6698">http://www.rfc-editor.org/info/rfc6698</a>&gt;.







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   [<a id="ref-RFC7218">RFC7218</a>]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", <a href="./rfc7218">RFC 7218</a>, DOI 10.17487/RFC7218,
              April 2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7218">http://www.rfc-editor.org/info/rfc7218</a>&gt;.

   [<a id="ref-RFC7671">RFC7671</a>]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", <a href="./rfc7671">RFC 7671</a>, DOI 10.17487/RFC7671,
              October 2015, &lt;<a href="http://www.rfc-editor.org/info/rfc7671">http://www.rfc-editor.org/info/rfc7671</a>&gt;.

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-RFC1035">RFC1035</a>]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, <a href="./rfc1035">RFC 1035</a>, DOI 10.17487/RFC1035,
              November 1987, &lt;<a href="http://www.rfc-editor.org/info/rfc1035">http://www.rfc-editor.org/info/rfc1035</a>&gt;.

   [<a id="ref-RFC2136">RFC2136</a>]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              <a href="./rfc2136">RFC 2136</a>, DOI 10.17487/RFC2136, April 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2136">http://www.rfc-editor.org/info/rfc2136</a>&gt;.

   [<a id="ref-RFC2181">RFC2181</a>]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", <a href="./rfc2181">RFC 2181</a>, DOI 10.17487/RFC2181, July 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2181">http://www.rfc-editor.org/info/rfc2181</a>&gt;.

   [<a id="ref-RFC4949">RFC4949</a>]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, <a href="./rfc4949">RFC 4949</a>, DOI 10.17487/RFC4949, August 2007,
              &lt;<a href="http://www.rfc-editor.org/info/rfc4949">http://www.rfc-editor.org/info/rfc4949</a>&gt;.

   [<a id="ref-RFC6409">RFC6409</a>]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, <a href="./rfc6409">RFC 6409</a>, DOI 10.17487/RFC6409, November 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6409">http://www.rfc-editor.org/info/rfc6409</a>&gt;.

   [<a id="ref-RFC7435">RFC7435</a>]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", <a href="./rfc7435">RFC 7435</a>, DOI 10.17487/RFC7435,
              December 2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7435">http://www.rfc-editor.org/info/rfc7435</a>&gt;.

   [<a id="ref-RFC7673">RFC7673</a>]  Finch, T., Miller, M., and P. Saint-Andre, "Using
              DNS-Based Authentication of Named Entities (DANE) TLSA
              Records with SRV Records", <a href="./rfc7673">RFC 7673</a>, DOI 10.17487/RFC7673,
              October 2015, &lt;<a href="http://www.rfc-editor.org/info/rfc7673">http://www.rfc-editor.org/info/rfc7673</a>&gt;.










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Acknowledgements

   The authors would like to extend great thanks to Tony Finch, who
   started the original version of a DANE SMTP document.  His work is
   greatly appreciated and has been incorporated into this document.
   The authors would like to additionally thank Phil Pennock for his
   comments and advice on this document.

   Acknowledgements from Viktor: Thanks to Paul Hoffman, who motivated
   me to begin work on this memo and provided feedback on early draft
   versions of this document.  Thanks to Patrick Koetter, Perry Metzger,
   and Nico Williams for valuable review comments.  Thanks also to
   Wietse Venema, who created Postfix, and whose advice and feedback
   were essential to the development of the Postfix DANE implementation.

Authors' Addresses

   Viktor Dukhovni
   Two Sigma

   Email: ietf-dane@dukhovni.org


   Wes Hardaker
   Parsons
   P.O. Box 382
   Davis, CA  95617
   United States

   Email: ietf@hardakers.net





















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