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+DANE                                                         V. Dukhovni
+Internet-Draft                                                 Two Sigma
+Intended status: Standards Track                             W. Hardaker
+Expires: February 3, 2015                                        Parsons
+                                                          August 2, 2014
+
+
+                SMTP security via opportunistic DANE TLS
+                   draft-ietf-dane-smtp-with-dane-11
+
+Abstract
+
+   This memo describes a downgrade-resistant protocol for SMTP transport
+   security between Mail 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 Internet-Draft is submitted in full conformance with the
+   provisions of BCP 78 and BCP 79.
+
+   Internet-Drafts are working documents of the Internet Engineering
+   Task Force (IETF).  Note that other groups may also distribute
+   working documents as Internet-Drafts.  The list of current Internet-
+   Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+   Internet-Drafts are draft documents valid for a maximum of six months
+   and may be updated, replaced, or obsoleted by other documents at any
+   time.  It is inappropriate to use Internet-Drafts as reference
+   material or to cite them other than as "work in progress."
+
+   This Internet-Draft will expire on February 3, 2015.
+
+Copyright Notice
+
+   Copyright (c) 2014 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (http://trustee.ietf.org/license-info) 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
+
+
+
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+   the Trust Legal Provisions and are provided without warranty as
+   described in the Simplified BSD License.
+
+Table of Contents
+
+   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
+     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
+     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
+     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   6
+       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   6
+       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   7
+       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   8
+       1.3.4.  Too many certification authorities  . . . . . . . . .   8
+   2.  Identifying applicable TLSA records . . . . . . . . . . . . .   9
+     2.1.  DNS considerations  . . . . . . . . . . . . . . . . . . .   9
+       2.1.1.  DNS errors, bogus and indeterminate responses . . . .   9
+       2.1.2.  DNS error handling  . . . . . . . . . . . . . . . . .  11
+       2.1.3.  Stub resolver considerations  . . . . . . . . . . . .  12
+     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  13
+       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  14
+       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  15
+       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  17
+   3.  DANE authentication . . . . . . . . . . . . . . . . . . . . .  19
+     3.1.  TLSA certificate usages . . . . . . . . . . . . . . . . .  19
+       3.1.1.  Certificate usage DANE-EE(3)  . . . . . . . . . . . .  21
+       3.1.2.  Certificate usage DANE-TA(2)  . . . . . . . . . . . .  22
+       3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)  . . . .  23
+     3.2.  Certificate matching  . . . . . . . . . . . . . . . . . .  24
+       3.2.1.  DANE-EE(3) name checks  . . . . . . . . . . . . . . .  24
+       3.2.2.  DANE-TA(2) name checks  . . . . . . . . . . . . . . .  24
+       3.2.3.  Reference identifier matching . . . . . . . . . . . .  25
+   4.  Server key management . . . . . . . . . . . . . . . . . . . .  26
+   5.  Digest algorithm agility  . . . . . . . . . . . . . . . . . .  26
+   6.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  28
+   7.  Note on DANE for Message User Agents  . . . . . . . . . . . .  29
+   8.  Interoperability considerations . . . . . . . . . . . . . . .  29
+     8.1.  SNI support . . . . . . . . . . . . . . . . . . . . . . .  29
+     8.2.  Anonymous TLS cipher suites . . . . . . . . . . . . . . .  30
+   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  30
+     9.1.  Client Operational Considerations . . . . . . . . . . . .  30
+     9.2.  Publisher Operational Considerations  . . . . . . . . . .  31
+   10. Security Considerations . . . . . . . . . . . . . . . . . . .  31
+   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  32
+   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
+   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
+     13.1.  Normative References . . . . . . . . . . . . . . . . . .  33
+     13.2.  Informative References . . . . . . . . . . . . . . . . .  34
+   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34
+
+
+
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+1.  Introduction
+
+   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, in particular 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 of necessity opportunistic.
+
+   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 existing use of TLS in MTA to MTA SMTP that motivate
+   this specification are described in Section 1.3.  The specification
+   itself follows in Section 2 and Section 3 which describe respectively
+   how to locate and use DANE TLSA records with SMTP.  In Section 6, we
+   discuss application of DANE TLS to destinations for which channel
+   integrity and confidentiality are mandatory.  In Section 7 we briefly
+   comment on potential applicability of this specification to Message
+   User Agents.
+
+1.1.  Terminology
+
+   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
+   [RFC2119].
+
+   The following terms or concepts are used through the document:
+
+   Man-in-the-middle or MITM attack:  Active modification of network
+      traffic by an adversary able to thereby compromise the
+      confidentiality or integrity of the data.
+
+
+
+
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+   secure, bogus, insecure, indeterminate:  DNSSEC validation results,
+      as defined in Section 4.3 of [RFC4035].
+
+   Validating Security-Aware Stub Resolver and     Non-Validating
+   Security-Aware Stub Resolver:
+      Capabilities of the stub resolver in use as defined in [RFC4033];
+      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, 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 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.
+
+   reference identifier:  (Special case of [RFC6125] 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 [RFC6125] DNS-ID (or if none
+      are present the [RFC6125] CN-ID) of the server certificate (see
+      Section 3.2.3).
+
+   MX hostname:  The RRDATA of an MX record consists of a 16 bit
+      preference followed by a Mail Exchange domain name (see [RFC1035],
+      Section 3.3.9).  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 & forward
+      process.  When an MTA is unable 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 is configured, messages
+      that would otherwise have to be delayed may be sent to the
+      fallback next-hop destination instead.  The fallback destination
+
+
+
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+      may itself be subject to opportunistic or mandatory DANE TLS 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 ([RFC5598], Section 4.3.2).
+
+   MSA:   Message Submission Agent ([RFC5598], Section 4.3.1).
+
+   MUA:   Message User Agent ([RFC5598], Section 4.2.1).
+
+   RR:   A DNS Resource Record
+
+   RRset:   A set of DNS Resource Records for a particular class, domain
+      and record type.
+
+1.2.  Background
+
+   The Domain Name System Security Extensions (DNSSEC) add data origin
+   authentication, data integrity and data non-existence proofs to the
+   Domain Name System (DNS).  DNSSEC is defined in [RFC4033], [RFC4034]
+   and [RFC4035].
+
+   As described in the introduction of [RFC6698], TLS authentication via
+   the existing public Certification Authority (CA) PKI suffers from an
+   over-abundance of trusted parties capable of issuing certificates for
+   any domain of their choice.  DANE leverages the DNSSEC infrastructure
+   to publish trusted public keys and certificates for use with the
+   Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA"
+   DNS record type.  With DNSSEC each domain can only vouch for the keys
+   of its directly 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.  With authentication, TLS also provides data
+   integrity protection to guard against MITM attacks.
+
+
+
+
+
+
+
+
+
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+1.3.  SMTP channel security
+
+   With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
+   [RFC5280] issued by one of the many Certificate Authorities (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 PKI to SMTP that are addressed by this specification.
+   Since 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.
+
+1.3.1.  STARTTLS downgrade attack
+
+   The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop
+   protocol in a multi-hop store & 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 the Hypertext
+   Transfer Protocol (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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+   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 ([RFC3207]).  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, such as the use of PKIX, can prevent this.  The
+   attacker can simply disable TLS.
+
+1.3.2.  Insecure server name without DNSSEC
+
+   With SMTP, DNS Mail Exchange (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 PKIX 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.
+
+   One might try to harden TLS for SMTP against DNS attacks by using the
+   envelope recipient domain as a reference identifier and 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 [RFC6066] Section 3) 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.
+
+
+
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+   Since SMTP security depends critically on DNSSEC, it is important to
+   point out that consequently SMTP with DANE is 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.
+
+1.3.3.  Sender policy does not scale
+
+   Sending systems are in some cases explicitly configured to use TLS
+   for mail sent to selected peer domains.  This requires sending MTAs
+   to be configured with appropriate subject names or certificate
+   content digests to expect in the presented server certificates.
+   Because of the heavy 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 organization 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.
+
+1.3.4.  Too many certification authorities
+
+   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 Certification Authority (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 Certification
+   Authorities.
+
+
+
+
+
+
+
+
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+   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.
+
+2.  Identifying applicable TLSA records
+
+2.1.  DNS considerations
+
+2.1.1.  DNS errors, bogus and indeterminate responses
+
+   An SMTP client that implements opportunistic DANE TLS per this
+   specification depends critically on the integrity of DNSSEC lookups,
+   as discussed in Section 1.3.2.  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 Section 4.3 of
+   [RFC4035]: "secure", "insecure", "bogus" and "indeterminate".  In
+   [RFC4035] 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, the "indeterminate" security status has a conflicting
+   definition in section 5 of [RFC4033].
+
+     There is no trust anchor that would indicate that a specific
+     portion of the tree is secure.
+
+   To avoid further confusion, the adjective "anchorless" will be used
+   below to refer to domains or RRsets that are "indeterminate" in the
+   [RFC4033] sense, and the term "indeterminate" will be used
+   exclusively in the sense of [RFC4035].
+
+   SMTP clients following this specification SHOULD NOT distinguish
+   between "insecure" and "anchorless" DNS responses.  Both "insecure"
+   and "anchorless" RRsets MUST be handled identically: in either case
+   unvalidated data for the query domain is all that is and can be
+   available, and authentication using the data is impossible.  In what
+   follows, the term "insecure" will also includes the case of
+
+
+
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+   "anchorless" domains that lie in a portion of the DNS tree for which
+   there is no applicable trust anchor.  With the DNS root zone 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 "anchorless" domains should be rare in practice.
+
+   As noted in section 4.3 of [RFC4035], 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 RFC4035) to the
+   application, and will signal a "bogus" or error result instead.  If a
+   resolver does signal an RFC4035 "indeterminate" security status, this
+   MUST be treated by the SMTP client as though a "bogus" or error
+   result had been returned.
+
+   An MTA making use of 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 MUST NOT be used.  In accordance with section 4.9.3 of
+   [RFC4035]:
+
+     ... 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.
+
+   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" non-existence 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 non-existence 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.
+
+
+
+
+
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+
+
+   Security-aware stub resolvers will, of course, also signal DNS lookup
+   errors in other cases, for example when processing a "ServFail"
+   RCODE, 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.  A
+   lookup error is thus a failure to obtain the relevant RRset if it
+   exists, or to determine that no such RRset exists when it does not.
+
+   In contrast to a "bogus" or an "indeterminate" response, an
+   "insecure" DNSSEC response is not an error, rather it indicates that
+   the target DNS zone is either securely opted out of DNSSEC validation
+   or is not connected with the DNSSEC trust anchors being used.
+   Insecure results will leave the SMTP client with degraded channel
+   security, but do not stand in the way of message delivery.  See
+   section Section 2.2 for further details.
+
+2.1.2.  DNS error handling
+
+   When a DNS lookup failure (error or "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 (Section 2.2) MUST be performed to
+   locate any related TLSA records.  If any DNS queries used to locate
+   TLSA records fail (be it due to "bogus" or "indeterminate" records,
+   timeouts, malformed replies, ServFails, etc.), then the SMTP 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 what 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 problem SMTP
+   server, or the problem destination.  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.
+
+
+
+
+
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+
+
+2.1.3.  Stub resolver considerations
+
+   SMTP clients that employ opportunistic DANE TLS to secure connections
+   to SMTP servers MUST NOT use Security-Oblivious stub-resolvers.
+
+   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 [RFC6672].
+   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 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
+   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 may need to issue a
+   separate query of type "CNAME" that returns only the initial CNAME
+   record.  In particular in Section 2.2.2 when insecure A or AAAA
+
+
+
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+
+
+   records are found for an SMTP server via a CNAME alias, it may be
+   necessary to perform an additional CNAME query to determine whether
+   the DNS zone in which the alias is published is signed.
+
+2.2.  TLS discovery
+
+   As noted previously (in Section 1.3.1), 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 Section 4.1 of [RFC6698].  Specifically,
+   if the DNS lookup for a TLSA record returns:
+
+   A secure TLSA RRset with at least one usable record:  A connection to
+      the MTA MUST be made using authenticated and encrypted TLS, 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:  A
+      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.
+
+   An insecure TLSA RRset or DNSSEC validated proof-of-non-existent 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 either during the
+      handshake or even during data transfer.
+
+   Any lookup error:  Lookup errors, including "bogus" and
+      "indeterminate", as explained in Section 2.1.1 MUST result in
+      falling back to the next SMTP server or delayed delivery.
+
+   An SMTP client MAY be configured to require DANE verified delivery
+   for some destinations.  We will call such a configuration "mandatory
+
+
+
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+
+
+   DANE TLS".  With mandatory DANE TLS, 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 non-conformat 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.
+
+2.2.1.  MX resolution
+
+   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
+   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 [RFC5321] Section 5.1, 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
+   Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via
+   pre-DANE opportunistic TLS.  That said, the protocol in this memo is
+
+
+
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+
+
+   an "opportunistic security" protocol, meaning that it strives to
+   communicate with each peer as securely as possible, while maintaining
+   broad interoperability.  Therefore, the SMTP client MAY proceed to
+   use DANE TLS (as described in Section 2.2.2 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 make use of the provider's TLSA records.
+
+   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.  When DANE TLS is mandatory (Section 6) for
+   a given destination, delivery MUST be delayed when the MX RRset is
+   not "secure".
+
+   Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is
+   "secure", and the SMTP client MUST treat each MX hostname as a
+   separate non-MX destination for opportunistic DANE TLS as described
+   in Section 2.2.2.  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
+   Section 2.1.1, cause the SMTP server in question to be treated as
+   unreachable.
+
+2.2.2.  Non-MX destinations
+
+   This section describes the algorithm used to locate the TLSA records
+   and associated TLSA base domain for an input domain not subject to MX
+   resolution.  Such domains include:
+
+   o  Each MX hostname used in a message delivery attempt for an
+      original next-hop destination domain subject to MX resolution.
+      Note, MTAs are not obligated to support CNAME expansion of MX
+      hostnames.
+
+   o  Any administrator configured relay hostname, not subject to MX
+      resolution.  This frequently involves configuration set by the MTA
+      administrator to handle some or all mail.
+
+
+
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+
+
+   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, but queries for TLSA records fail,
+   rather than returning the non-existence of the requested TLSA
+   records.
+
+   To avoid problems delivering mail to domains whose SMTP servers are
+   served by the problem 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 whether the destination domain is reachable and 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 Section 2.1.3), the SMTP
+   client SHOULD NOT proceed to search for any associated TLSA records.
+   With the problem domains, TLSA queries will lead to DNS lookup errors
+   and cause messages to be consistently delayed and ultimately returned
+   to the sender.  We don't expect to find any "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.
+
+
+
+
+
+
+
+
+
+
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+
+
+   Non-CNAME:   The answer is not a CNAME alias.  If the address RRset
+      is "secure", TLSA lookups are performed as described in
+      Section 2.2.3 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 Section 2.1.1).  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 Section 2.1.1).  Two
+      candidate TLSA base domains are tried: the fully CNAME-expanded
+      initial name and, failing that, then 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-MX 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 or
+   indeterminate results for any query in the process MUST result in
+   delaying or abandoning delivery.
+
+2.2.3.  TLSA record lookup
+
+   Each candidate TLSA base domain (the original or fully CNAME-expanded
+   name of a non-MX destination or a particular MX hostname of an MX
+   destination) is in turn prefixed with service labels of the form
+   "_<port>._tcp".  The resulting domain name is used to issue a DNSSEC
+   query with the query type set to TLSA ([RFC6698] Section 7.1).
+
+   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
+   "_<port>" 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 ?
+
+
+
+
+
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+
+
+   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 keeping track of whether
+   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
+   delivery MAY proceed via pre-DANE opportunistic TLS.  SMTP clients
+   MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades
+   or even to skip SMTP servers that fail authentication, but MUST NOT
+   misrepresent authentication success as either a secure connection to
+   the SMTP server or as a secure delivery to the intended next-hop
+   domain.
+
+   TLSA record publishers may leverage CNAMEs to reference a single
+   authoritative TLSA RRset specifying a common Certification Authority
+   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 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
+
+
+
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+
+
+   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 Section Section 2.2.2), 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, nor the
+   final name, are never candidate TLSA base domains, even if "secure".
+
+3.  DANE authentication
+
+   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.
+
+3.1.  TLSA certificate usages
+
+   The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
+   via combinations of 3 numeric parameters.  The numeric values of
+   these parameters were later given symbolic names in [RFC7218].  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.  The parameters are:
+
+
+
+
+
+
+
+
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+
+
+   The TLSA Certificate Usage field:  Section 2.1.1 of [RFC6698]
+      specifies four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and
+      DANE-EE(3).  There is an additional private-use value:
+      PrivCert(255).  All other values are reserved for use by future
+      specifications.
+
+   The selector field:  Section 2.1.2 of [RFC6698] specifies two values:
+      Cert(0) and SPKI(1).  There is an additional private-use value:
+      PrivSel(255).  All other values are reserved for use by future
+      specifications.
+
+   The matching type field:  Section 2.1.3 of [RFC6698] specifies three
+      values: Full(0), SHA2-256(1) and SHA2-512(2).  There is an
+      additional private-use value: PrivMatch(255).  All other values
+      are reserved for use by future specifications.
+
+   We may think of TLSA Certificate Usage values 0 through 3 as a
+   combination of two one-bit flags.  The low bit chooses between trust
+   anchor (TA) and end entity (EE) certificates.  The high bit chooses
+   between public PKI issued and domain-issued certificates.
+
+   The selector field specifies whether the TLSA RR matches the whole
+   certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1).  The
+   subjectPublicKeyInfo is an ASN.1 DER ([X.690]) encoding of the
+   certificate's algorithm id, any parameters and the public key data.
+
+   The matching type field specifies how the TLSA RR Certificate
+   Association Data field is to be compared with the certificate or
+   public key.  A value of Full(0) means an exact match: the full DER
+   encoding of the certificate or public key is given in the TLSA RR.  A
+   value of SHA2-256(1) means that the association data matches the
+   SHA2-256 digest of the certificate or public key, and likewise
+   SHA2-512(2) means a SHA2-512 digest is used.
+
+   Since opportunistic DANE TLS will be used by non-interactive MTAs,
+   with no user to "press 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.
+
+
+
+
+
+
+
+
+
+
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+
+
+   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 server's certificate chain.  The next
+   two subsections explain the process for certificate usages DANE-EE(3)
+   and DANE-TA(2).  The third subsection 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 either "DANE-EE(3) SPKI(1)
+   SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records
+   depending on site needs.  Other combinations of TLSA parameters are
+   either explicitly unsupported, or offer little to recommend them over
+   these two.
+
+   The mandatory to support digest algorithm in [RFC6698] is
+   SHA2-256(1).  When the server's TLSA RRset includes records with a
+   matching type indicating a digest record (i.e., a value other than
+   Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be
+   provided along with any other digest published, since some SMTP
+   clients may support only SHA2-256(1).  If at some point the SHA2-256
+   digest algorithm is tarnished by new cryptanalytic attacks,
+   publishers will need to include an appropriate stronger digest in
+   their TLSA records, initially along with, and ultimately in place of,
+   SHA2-256.
+
+3.1.1.  Certificate usage DANE-EE(3)
+
+   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.
+
+   Similarly, 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 (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.
+
+
+
+
+
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+
+
+   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, and don't 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.
+
+3.1.2.  Certificate usage DANE-TA(2)
+
+   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 Certification Authority 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 [RFC6125]).
+   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.  At this
+   time, many SMTP servers are not configured with a comprehensive list
+   of trust anchors, nor are they expected to 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 Certification Authority known to the SMTP
+   client, authentication is likely to fail unless the TA certificate is
+   included in the TLS server certificate message.
+
+
+
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+
+
+   TLSA records with selector 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
+   with a selector 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.
+
+3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)
+
+   As noted in the introduction, SMTP clients 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.
+
+   SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX-
+   TA(0) or PKIX-EE(1).  SMTP clients cannot be expected to be
+   configured with a suitably complete set of trusted public CAs.
+   Lacking a complete set of public CAs, clients would not be able to
+   verify the certificates of SMTP servers whose issuing root CAs are
+   not trusted by the client.
+
+   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.
+
+
+
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+
+
+   SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
+   or PKIX-EE(1) is undefined.  SMTP clients should generally treat such
+   TLSA records as unusable.
+
+3.2.  Certificate matching
+
+   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.
+
+3.2.1.  DANE-EE(3) name checks
+
+   The SMTP client MUST NOT perform certificate name checks with
+   certificate usage DANE-EE(3); see Section 3.1.1 above.
+
+3.2.2.  DANE-TA(2) name checks
+
+   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 include the TLSA base domain as one of the valid reference
+   identifiers 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 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.
+
+   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 below:
+
+
+
+
+
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+
+
+     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".
+
+3.2.3.  Reference identifier matching
+
+   When name checks are applicable (certificate usage DANE-TA(2)), if
+   the server certificate contains a Subject Alternative Name extension
+   ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS-
+   IDs are matched against the client's reference identifiers.  The CN-
+   ID ([RFC6125]) is only considered when no DNS-IDs are present.  The
+   server certificate is considered matched when one of its presented
+   identifiers ([RFC5280]) matches any of the client's reference
+   identifiers.
+
+   Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
+   The wildcard character must be 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
+
+
+
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+
+
+   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.
+
+4.  Server key management
+
+   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 Section 3.1.2, 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 CNAME
+   record.  If the central trust anchor is compromised, all servers need
+   to be issued new keys by a new TA, and a shared DANE-TA(2) TLSA RRset
+   needs to be published containing just the new TA.  SMTP servers
+   cannot expect broad SMTP client CRL or OCSP support.
+
+5.  Digest algorithm agility
+
+   While [RFC6698] 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 any deprecated
+
+
+
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+
+
+   weaker algorithms that are published for compatibility with less
+   capable clients, but should be ignored when possible.  We specify
+   such a protocol below.
+
+   Suppose that a DANE TLS client authenticating a TLS server considers
+   digest algorithm "BetterAlg" stronger than digest algorithm
+   "WorseAlg".  Suppose further that a server's TLSA RRset contains some
+   records with "BetterAlg" as the digest algorithm.  Finally, suppose
+   that for every raw public key or certificate object that is included
+   in the server's TLSA RRset in digest form, whenever that object
+   appears with algorithm "WorseAlg" with some usage and selector it
+   also appears with algorithm "BetterAlg" with the same usage and
+   selector.  In that case our client can safely ignore TLSA records
+   with the weaker algorithm "WorseAlg", because it suffices to check
+   the records with the stronger algorithm "BetterAlg".
+
+   Server operators MUST ensure that for any given usage and selector,
+   each object (certificate or public key), for which a digest
+   association exists in the TLSA RRset, is published with the SAME SET
+   of digest algorithms as all other objects that published with that
+   usage and selector.  In other words, for each usage and selector, the
+   records with non-zero matching types will correspond to on a cross-
+   product of a set of underlying objects and a fixed set of digest
+   algorithms that apply uniformly to all the objects.
+
+   To achieve digest algorithm agility, all published TLSA RRsets for
+   use with opportunistic DANE TLS for SMTP MUST conform to the above
+   requirements.  Then, for each combination of usage and selector, SMTP
+   clients can simply ignore all digest records except those that employ
+   the strongest digest algorithm.  The ordering of digest algorithms by
+   strength is not specified in advance, it is entirely up to the SMTP
+   client.  SMTP client implementations SHOULD make the digest algorithm
+   preference order configurable.  Only the future will tell which
+   algorithms might be weakened by new attacks and when.
+
+   Note, TLSA records with a matching type of Full(0), that publish the
+   full value of a certificate or public key object, play no role in
+   digest algorithm agility.  They neither trump the processing of
+   records that employ digests, nor are they ignored in the presence of
+   any records with a digest (i.e. non-zero) matching type.
+
+
+
+
+
+
+
+
+
+
+
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+
+
+   SMTP clients SHOULD use digest algorithm agility when processing the
+   DANE TLSA records of an SMTP server.  Algorithm agility is to be
+   applied after first discarding any unusable or malformed records
+   (unsupported digest algorithm, or incorrect digest length).  Thus,
+   for each usage and selector, the client SHOULD process only any
+   usable records with a matching type of Full(0) and the usable records
+   whose digest algorithm is believed to be the strongest among usable
+   records with the given usage and selector.
+
+   The main impact of this requirement is on key rotation, when the TLSA
+   RRset is pre-populated with digests of new certificates or public
+   keys, before these replace or augment their predecessors.  Were the
+   newly introduced RRs to include previously unused digest algorithms,
+   clients that employ this protocol could potentially ignore all the
+   digests corresponding to the current keys or certificates, causing
+   connectivity issues until the new keys or certificates are deployed.
+   Similarly, publishing new records with fewer digests could cause
+   problems for clients using cached TLSA RRsets that list both the old
+   and new objects once the new keys are deployed.
+
+   To avoid problems, server operators SHOULD apply the following
+   strategy:
+
+   o  When changing the set of objects published via the TLSA RRset
+      (e.g. during key rotation), DO NOT change the set of digest
+      algorithms used; change just the list of objects.
+
+   o  When changing the set of digest algorithms, change only the set of
+      algorithms, and generate a new RRset in which all the current
+      objects are re-published with the new set of digest algorithms.
+
+   After either of these two changes are made, the new TLSA RRset should
+   be left in place long enough that the older TLSA RRset can be flushed
+   from caches before making another change.
+
+6.  Mandatory TLS Security
+
+   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
+   domains's DANE TLSA records to authenticate the corresponding SMTP
+   server.  Authentication via DANE TLSA records is easier to manage, as
+
+
+
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+
+
+   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 their 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.
+
+7.  Note on DANE for Message User Agents
+
+   We note that the SMTP protocol is also used between Message User
+   Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409].  In
+   [RFC6186] 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 [RFC6186] 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
+   [I-D.ietf-dane-srv].
+
+   However, until MUAs begin to adopt the dynamic configuration
+   mechanisms of [RFC6186] they are adequately served by more
+   traditional static TLS security policies.  Specification of DANE TLS
+   for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
+   is left to future documents that focus specifically on SMTP security
+   between MUAs and MSAs.
+
+8.  Interoperability considerations
+
+8.1.  SNI support
+
+   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 backward compatible SSL 2.0
+   compatible SSL HELLO by the SMTP client.  The minimum SSL/TLS client
+   HELLO version for SMTP clients performing DANE authentication is SSL
+   3.0, but a client that offers SSL 3.0 MUST also offer at least TLS
+   1.0 and MUST include the SNI extension.  Servers that don't make use
+   of SNI MAY negotiate SSL 3.0 if offered by the client.
+
+   Each SMTP server MUST present a certificate chain (see [RFC5246]
+   Section 7.4.2) that matches at least one of the TLSA records.  The
+   server MAY rely on SNI to determine which certificate chain to
+
+
+
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+
+
+   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.
+
+8.2.  Anonymous TLS cipher suites
+
+   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 in addition 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 by 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.
+
+9.  Operational Considerations
+
+9.1.  Client Operational Considerations
+
+   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 letting email queue until
+   the error is resolved and disabling opportunistic or mandatory DANE
+
+
+
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+
+
+   TLS 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 that opportunistic DANE TLS is
+   not to be used for a particular destination, an SMTP 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.
+
+9.2.  Publisher Operational Considerations
+
+   SMTP servers that publish certificate usage DANE-TA(2) associations
+   MUST include the TA certificate in their TLS server certificate
+   chain, even when that TA certificate is a self-signed root
+   certificate.
+
+   TLSA Publishers MUST follow the digest agility guidelines in
+   Section 5 and MUST make sure that all objects published in digest
+   form for a particular usage and selector are published with the same
+   set of digest algorithms.
+
+   TLSA Publishers should follow the TLSA publication size guidance
+   found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".
+
+10.  Security Considerations
+
+   This protocol leverages DANE TLSA records to implement MITM resistant
+   opportunistic security ([I-D.dukhovni-opportunistic-security]) 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.
+
+
+
+
+
+
+
+
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+
+
+   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 bypass any security benefit the legitimate domain owner might
+   hope to gain by publishing usage 0 or 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 the portions of this
+   specification that indicate when it is appropriate to initiate a non-
+   authenticated connection or cleartext connection to a SMTP server.
+   Specifically, in order to prevent downgrade attacks on this protocol,
+   implementation must not initiate a connection when this specification
+   indicates a particular SMTP server must be considered unreachable.
+
+11.  IANA considerations
+
+   This specification requires no support from IANA.
+
+12.  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.
+
+   Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
+   to begin work on this memo and provided feedback on early drafts.
+   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.
+
+
+
+
+Dukhovni & Hardaker     Expires February 3, 2015               [Page 32]
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+
+
+13.  References
+
+13.1.  Normative References
+
+   [I-D.ietf-dane-ops]
+              Dukhovni, V. and W. Hardaker, "DANE TLSA implementation
+              and operational guidance", draft-ietf-dane-ops-00 (work in
+              progress), October 2013.
+
+   [RFC1035]  Mockapetris, P., "Domain names - implementation and
+              specification", STD 13, RFC 1035, November 1987.
+
+   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
+              Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
+              Transport Layer Security", RFC 3207, February 2002.
+
+   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "DNS Security Introduction and Requirements", RFC
+              4033, March 2005.
+
+   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "Resource Records for the DNS Security Extensions",
+              RFC 4034, March 2005.
+
+   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "Protocol Modifications for the DNS Security
+              Extensions", RFC 4035, March 2005.
+
+   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
+              (TLS) Protocol Version 1.2", RFC 5246, August 2008.
+
+   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
+              Housley, R., and W. Polk, "Internet X.509 Public Key
+              Infrastructure Certificate and Certificate Revocation List
+              (CRL) Profile", RFC 5280, May 2008.
+
+   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
+              October 2008.
+
+   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
+              Extension Definitions", RFC 6066, January 2011.
+
+
+
+
+
+
+
+
+Dukhovni & Hardaker     Expires February 3, 2015               [Page 33]
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+
+
+   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
+              Verification of Domain-Based Application Service Identity
+              within Internet Public Key Infrastructure Using X.509
+              (PKIX) Certificates in the Context of Transport Layer
+              Security (TLS)", RFC 6125, March 2011.
+
+   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
+              Submission/Access Services", RFC 6186, March 2011.
+
+   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
+              DNS", RFC 6672, June 2012.
+
+   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
+              of Named Entities (DANE) Transport Layer Security (TLS)
+              Protocol: TLSA", RFC 6698, August 2012.
+
+   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
+              Conversations about DNS-Based Authentication of Named
+              Entities (DANE)", RFC 7218, April 2014.
+
+   [X.690]    International Telecommunications Union, "Recommendation
+              ITU-T X.690 (2002) | ISO/IEC 8825-1:2002, Information
+              technology - ASN.1 encoding rules: Specification of Basic
+              Encoding Rules (BER), Canonical Encoding Rules (CER) and
+              Distinguished Encoding Rules (DER)", July 2002.
+
+13.2.  Informative References
+
+   [I-D.dukhovni-opportunistic-security]
+              Dukhovni, V., "Opportunistic Security: some protection
+              most of the time", draft-dukhovni-opportunistic-
+              security-01 (work in progress), July 2014.
+
+   [I-D.ietf-dane-srv]
+              Finch, T., "Using DNS-Based Authentication of Named
+              Entities (DANE) TLSA records with SRV and MX records.",
+              draft-ietf-dane-srv-02 (work in progress), February 2013.
+
+   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July
+              2009.
+
+   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
+              STD 72, RFC 6409, November 2011.
+
+Authors' Addresses
+
+
+
+
+
+
+Dukhovni & Hardaker     Expires February 3, 2015               [Page 34]
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+
+
+   Viktor Dukhovni
+   Two Sigma
+
+   Email: ietf-dane@dukhovni.org
+
+
+   Wes Hardaker
+   Parsons
+   P.O. Box 382
+   Davis, CA  95617
+   US
+
+   Email: ietf@hardakers.net
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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@@ -0,0 +1,1904 @@
+
+
+
+
+DANE                                                         V. Dukhovni
+Internet-Draft                                                 Two Sigma
+Intended status: Standards Track                             W. Hardaker
+Expires: November 26, 2014                                       Parsons
+                                                            May 25, 2014
+
+
+                SMTP security via opportunistic DANE TLS
+                   draft-ietf-dane-smtp-with-dane-10
+
+Abstract
+
+   This memo describes a downgrade-resistant protocol for SMTP transport
+   security between Mail 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 Internet-Draft is submitted in full conformance with the
+   provisions of BCP 78 and BCP 79.
+
+   Internet-Drafts are working documents of the Internet Engineering
+   Task Force (IETF).  Note that other groups may also distribute
+   working documents as Internet-Drafts.  The list of current Internet-
+   Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+   Internet-Drafts are draft documents valid for a maximum of six months
+   and may be updated, replaced, or obsoleted by other documents at any
+   time.  It is inappropriate to use Internet-Drafts as reference
+   material or to cite them other than as "work in progress."
+
+   This Internet-Draft will expire on November 26, 2014.
+
+Copyright Notice
+
+   Copyright (c) 2014 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (http://trustee.ietf.org/license-info) 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
+
+
+
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+
+
+   the Trust Legal Provisions and are provided without warranty as
+   described in the Simplified BSD License.
+
+Table of Contents
+
+   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
+     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
+     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
+     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   6
+       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   6
+       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   7
+       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   7
+       1.3.4.  Too many certification authorities  . . . . . . . . .   8
+   2.  Identifying applicable TLSA records . . . . . . . . . . . . .   8
+     2.1.  DNS considerations  . . . . . . . . . . . . . . . . . . .   8
+       2.1.1.  DNS errors, bogus and indeterminate responses . . . .   8
+       2.1.2.  DNS error handling  . . . . . . . . . . . . . . . . .  11
+       2.1.3.  Stub resolver considerations  . . . . . . . . . . . .  11
+     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  12
+       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  13
+       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  15
+       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  17
+   3.  DANE authentication . . . . . . . . . . . . . . . . . . . . .  19
+     3.1.  TLSA certificate usages . . . . . . . . . . . . . . . . .  19
+       3.1.1.  Certificate usage DANE-EE(3)  . . . . . . . . . . . .  20
+       3.1.2.  Certificate usage DANE-TA(2)  . . . . . . . . . . . .  21
+       3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)  . . . .  22
+     3.2.  Certificate matching  . . . . . . . . . . . . . . . . . .  23
+       3.2.1.  DANE-EE(3) name checks  . . . . . . . . . . . . . . .  23
+       3.2.2.  DANE-TA(2) name checks  . . . . . . . . . . . . . . .  23
+       3.2.3.  Reference identifier matching . . . . . . . . . . . .  24
+   4.  Server key management . . . . . . . . . . . . . . . . . . . .  25
+   5.  Digest algorithm agility  . . . . . . . . . . . . . . . . . .  26
+   6.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  27
+   7.  Note on DANE for Message User Agents  . . . . . . . . . . . .  28
+   8.  Interoperability considerations . . . . . . . . . . . . . . .  29
+     8.1.  SNI support . . . . . . . . . . . . . . . . . . . . . . .  29
+     8.2.  Anonymous TLS cipher suites . . . . . . . . . . . . . . .  29
+   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  30
+     9.1.  Client Operational Considerations . . . . . . . . . . . .  30
+     9.2.  Publisher Operational Considerations  . . . . . . . . . .  30
+   10. Security Considerations . . . . . . . . . . . . . . . . . . .  31
+   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  31
+   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
+   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
+     13.1.  Normative References . . . . . . . . . . . . . . . . . .  32
+     13.2.  Informative References . . . . . . . . . . . . . . . . .  33
+   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33
+
+
+
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+
+
+1.  Introduction
+
+   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, in particular 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 of necessity opportunistic.
+
+   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 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 existing use of TLS in MTA to MTA SMTP that motivate
+   this specification are described in Section 1.3.  The specification
+   itself follows in Section 2 and Section 3 which describe respectively
+   how to locate and use DANE TLSA records with SMTP.  In Section 6, we
+   discuss application of DANE TLS to destinations for which channel
+   integrity and confidentiality are mandatory.  In Section 7 we briefly
+   comment on potential applicability of this specification to Message
+   User Agents.
+
+1.1.  Terminology
+
+   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
+   [RFC2119].
+
+   The following terms or concepts are used through the document:
+
+   Man-in-the-middle or MITM attack:  Active modification of network
+      traffic by an adversary able to thereby compromise the
+      confidentiality or integrity of the data.
+
+
+
+
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+
+
+   secure, bogus, insecure, indeterminate:  DNSSEC validation results,
+      as defined in Section 4.3 of [RFC4035].
+
+   Validating Security-Aware Stub Resolver and     Non-Validating
+   Security-Aware Stub Resolver:
+      Capabilities of the stub resolver in use as defined in [RFC4033];
+      note that this specification requires the use of a Security-Aware
+      Stub Resolver; Security-Oblivious stub-resolvers MUST NOT be used.
+
+   opportunistic DANE TLS:  Best-effort use of TLS, 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 opportunistic TLS below.
+      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.
+
+   (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.
+
+   reference identifier:  (Special case of [RFC6125] 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 [RFC6125] DNS-ID (or if none
+      are present the [RFC6125] CN-ID) of the server certificate (see
+      Section 3.2.3).
+
+   MX hostname:  The RRDATA of an MX record consists of a 16 bit
+      preference followed by a Mail Exchange domain name (see [RFC1035],
+      Section 3.3.9).  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 & forward
+      process.  When an MTA is unable 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.  In these cases, messages that would otherwise
+      have to be delayed, may be sent to the fallback next-hop
+      destination instead.  The fallback destination may itself be
+
+
+
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+
+
+      subject to opportunistic or mandatory DANE TLS 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 ([RFC5598], Section 4.3.2).
+
+   MSA:   Message Submission Agent ([RFC5598], Section 4.3.1).
+
+   MUA:   Message User Agent ([RFC5598], Section 4.2.1).
+
+   RR:   A DNS Resource Record
+
+   RRset:   A set of DNS Resource Records for a particular class, domain
+      and record type.
+
+1.2.  Background
+
+   The Domain Name System Security Extensions (DNSSEC) add data origin
+   authentication, data integrity and data non-existence proofs to the
+   Domain Name System (DNS).  DNSSEC is defined in [RFC4033], [RFC4034]
+   and [RFC4035].
+
+   As described in the introduction of [RFC6698], TLS authentication via
+   the existing public Certification Authority (CA) PKI suffers from an
+   over-abundance of trusted parties capable of issuing certificates for
+   any domain of their choice.  DANE leverages the DNSSEC infrastructure
+   to publish trusted public keys and certificates for use with the
+   Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA"
+   DNS record type.  With DNSSEC each domain can only vouch for the keys
+   of its directly 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.  With authentication, TLS also provides data
+   integrity protection to guard against MITM attacks.
+
+
+
+
+
+
+
+
+
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+
+
+1.3.  SMTP channel security
+
+   With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
+   [RFC5280] issued by one of the many Certificate Authorities (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 PKI to SMTP that are addressed by this specification.
+   Since 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.
+
+1.3.1.  STARTTLS downgrade attack
+
+   The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop
+   protocol in a multi-hop store & forward email delivery process.  SMTP
+   envelope recipient addresses are not transport addresses and are
+   security-agnostic.  Unlike the Hypertext Transfer Protocol (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 ([RFC3207]).  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, such as the use of PKIX, can prevent this.  The
+   attacker can simply disable TLS.
+
+
+
+
+Dukhovni & Hardaker     Expires November 26, 2014               [Page 6]
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+
+
+1.3.2.  Insecure server name without DNSSEC
+
+   With SMTP, DNS Mail Exchange (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 PKIX 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 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.
+
+   One might try to harden TLS for SMTP against DNS attacks by using the
+   envelope recipient domain as a reference identifier and 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 [RFC6066] Section 3) 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 consequently SMTP with DANE is 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.  Detailed discussion of DNSSEC security
+   practices is out of scope for this document.
+
+1.3.3.  Sender policy does not scale
+
+
+
+
+
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+
+
+   Sending systems are in some cases explicitly configured to use TLS
+   for mail sent to selected peer domains.  This requires sending MTAs
+   to be configured with appropriate subject names or certificate
+   content digests to expect in the presented server certificates.
+   Because of the heavy 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 organization 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.
+
+1.3.4.  Too many certification authorities
+
+   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 Certification Authority (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 MTAs list of public CA trust
+   anchors would need to be comprehensive in order to avoid bouncing
+   mail addressed to sites that employ unknown Certification
+   Authorities.
+
+   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.
+
+2.  Identifying applicable TLSA records
+
+2.1.  DNS considerations
+
+2.1.1.  DNS errors, bogus and indeterminate responses
+
+
+
+
+
+
+
+
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+
+
+   An SMTP client that implements opportunistic DANE TLS per this
+   specification depends critically on the integrity of DNSSEC lookups,
+   as discussed in Section 1.3.  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 Section 4.3 of
+   [RFC4035]: "secure", "insecure", "bogus" and "indeterminate".  In
+   [RFC4035] 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, the "indeterminate" security status has a conflicting
+   definition in section 5 of [RFC4033].
+
+     There is no trust anchor that would indicate that a specific
+     portion of the tree is secure.
+
+   SMTP clients following this specification SHOULD NOT distinguish
+   between "insecure" and "indeterminate" in the [RFC4033] sense.  Both
+   "insecure" and RFC4033 "indeterminate" are handled identically: in
+   either case unvalidated data for the query domain is all that is and
+   can be available, and authentication using the data is impossible.
+   In what follows, when we say "insecure", we include also DNS results
+   for domains that lie in a portion of the DNS tree for which there is
+   no applicable trust anchor.  With the DNS root zone signed, we expect
+   that validating resolvers used by Internet-facing MTAs will be
+   configured with trust anchor data for the root zone.  Therefore,
+   RFC4033-style "indeterminate" domains should be rare in practice.
+   From here on, when we say "indeterminate", it is exclusively in the
+   sense of [RFC4035].
+
+   As noted in section 4.3 of [RFC4035], 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 RFC4035-sense to the
+   application, and will signal a "bogus" or error result instead.  If a
+   resolver does signal an RFC4035 "indeterminate" security status, this
+   MUST be treated by the SMTP client as though a "bogus" or error
+   result had been returned.
+
+
+
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+
+
+   An MTA making use of 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.  In accordance with
+   section 4.9.3 of [RFC4035]:
+
+     ... 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.
+
+   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" non-existence 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 non-existence 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"
+   RCODE, 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.  A
+   lookup error is thus a failure to obtain the relevant RRset if it
+   exists, or to determine that no such RRset exists when it does not.
+
+   In contrast to a "bogus" or an "indeterminate" response, an
+   "insecure" DNSSEC response is not an error, rather it indicates that
+   the target DNS zone is either securely opted out of DNSSEC validation
+   or is not connected with the DNSSEC trust anchors being used.
+   Insecure results will leave the SMTP client with degraded channel
+   security, but do not stand in the way of message delivery.  See
+   section Section 2.2 for further details.
+
+
+
+
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+
+
+2.1.2.  DNS error handling
+
+   When a DNS lookup failure (error or "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 (Section 2.2) MUST be performed to
+   locate any related TLSA records.  If any DNS queries used to locate
+   TLSA records fail (be it due to "bogus" or "indeterminate" records,
+   timeouts, malformed replies, ServFails, etc.), then the SMTP 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 what 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 problem SMTP
+   server, or the problem destination.  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.
+
+2.1.3.  Stub resolver considerations
+
+   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 [RFC6672].
+   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
+
+
+
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+
+
+   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 any
+   earlier CNAME records leading to the "insecure" record were "secure".
+
+   Note, 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 this
+   all that one can say is that some record in the set of records
+   returned is "insecure", but it is 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 may need to issue an a
+   separate query of type "CNAME" that returns only the initial CNAME
+   record.  In particular in Section 2.2.2 when insecure A or AAAA
+   records are found for an SMTP server via a CNAME alias, it may be
+   necessary to perform an additional CNAME query to determine whether
+   the DNS zone in which the alias is published is signed.
+
+2.2.  TLS discovery
+
+   As noted previously (in Section 1.3.1), 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 Section 4.1 of [RFC6698].  Specifically,
+   if the DNS lookup for a TLSA record returns:
+
+
+
+
+
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+
+
+   A secure TLSA RRset with at least one usable record:  A connection to
+      the MTA MUST be made using authenticated and encrypted TLS, 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:  A
+      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.
+
+   An insecure TLSA RRset or DNSSEC validated proof-of-non-existent 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 either during the
+      handshake or even during data transfer.
+
+   Any lookup error:  Lookup errors, including "bogus" and
+      "indeterminate", as explained in Section 2.1.1 MUST result in
+      falling back to the next SMTP server or delayed delivery.
+
+   An SMTP client MAY be configured to require DANE verified delivery
+   for some destinations.  We will call such a configuration "mandatory
+   DANE TLS".  With mandatory DANE TLS, 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 the MTA chooses to connect to the network
+   address 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.
+
+2.2.1.  MX resolution
+
+   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
+
+
+
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+
+
+   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
+   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 [RFC5321] Section 5.1, 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 until the ultimate non-CNAME domain is found.
+
+   If the MX RRset (or any CNAME leading to it) is "insecure" (see
+   Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via
+   pre-DANE opportunistic TLS.  That said, the protocol in this memo is
+   an "opportunistic security" protocol, meaning that it strives to
+   communicate with each peer as securely as possible, while maintaining
+   broad interoperability.  Therefore, the SMTP client MAY proceed to
+   use DANE TLS (as described in Section 2.2.2 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 make use of the provider's TLSA records.
+
+   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.  When DANE TLS is mandatory (Section 6) for
+   a given destination, delivery MUST be delayed when the MX RRset is
+   not "secure".
+
+
+
+
+
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+
+
+   Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is
+   "secure", and the SMTP client MUST treat each MX hostname as a
+   separate non-MX destination for opportunistic DANE TLS as described
+   in Section 2.2.2.  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
+   Section 2.1.1, cause the SMTP server in question to be treated as
+   unreachable.
+
+2.2.2.  Non-MX destinations
+
+   This section describes the algorithm used to locate the TLSA records
+   and associated TLSA base domain for an input domain not subject to MX
+   resolution.  Such domains include:
+
+   o  Each MX hostname used in a message delivery attempt for an
+      original next-hop destination domain subject to MX resolution.
+      Note, MTAs are not obligated to support CNAME expansion of MX
+      hostnames.
+
+   o  Any administrator configured relay hostname, not subject to MX
+      resolution.  This frequently involves configuration set by the MTA
+      administrator to handle some or all mail.
+
+   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, but queries for TLSA records fail,
+   rather than returning the non-existence of the requested TLSA
+   records.
+
+   To avoid problems delivering mail to domains whose SMTP servers are
+   served by the problem 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 whether the destination domain is reachable and 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 Section 2.1.3), the SMTP
+
+
+
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+
+
+   client SHOULD NOT proceed to search for any associated TLSA records.
+   With the problem domains, TLSA queries will lead to DNS lookup errors
+   and cause messages to be consistently delayed and ultimately returned
+   to the sender.  We don't expect to find any "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.
+
+   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
+      Section 2.2.3 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 Section 2.1.1).  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 Section 2.1.1).  Two
+      candidate TLSA base domains are tried: the fully CNAME-expanded
+      initial name and, failing that, then the initial name itself.
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+   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-MX 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 or
+   indeterminate results for any query in the process MUST result in
+   delaying or abandoning delivery.
+
+2.2.3.  TLSA record lookup
+
+   Each candidate TLSA base domain (the original or fully CNAME-expanded
+   name of a non-MX destination or a particular MX hostname of an MX
+   destination) is in turn prefixed with service labels of the form
+   "_<port>._tcp".  The resulting domain name is used to issue a DNSSEC
+   query with the query type set to TLSA ([RFC6698] Section 7.1).
+
+   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
+   "_<port>" 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 keeping track of whether
+   the entire chain is "secure".  If any "insecure" records are
+   encountered, or the TLSA records don't exist, the next candidate TLSA
+   base 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
+   delivery MAY proceed via pre-DANE opportunistic TLS.  SMTP clients
+   MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades
+   or even to skip SMTP servers that fail authentication, but MUST NOT
+   misrepresent authentication success as either a secure connection to
+   the SMTP server or as a secure delivery to the intended next-hop
+   domain.
+
+   TLSA record publishers may leverage CNAMEs to reference a single
+   authoritative TLSA RRset specifying a common Certification Authority
+
+
+
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+
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+
+
+   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 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, 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 records SHOULD be avoided unless
+   the servers in question are functionally equivalent (an active
+   attacker gains nothing by diverting client traffic from one such
+   server to another).
+
+   For example, given 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 tlsa211._dane.example.com.
+     _25._tcp.mx2.example.com.   IN CNAME tlsa211._dane.example.com.
+     tlsa211._dane.example.com.  IN TLSA 2 1 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 Section Section 2.2.2), 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, nor the
+   final name, are never candidate TLSA base domains, even if "secure".
+
+
+
+
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+
+
+3.  DANE authentication
+
+   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.
+
+3.1.  TLSA certificate usages
+
+   The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
+   via combinations of 3 numeric parameters.  The numeric values of
+   these parameters were later given symbolic names in
+   [I-D.ietf-dane-registry-acronyms].  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.  The parameters are:
+
+   The TLSA Certificate Usage field:  Section 2.1.1 of [RFC6698]
+      specifies 4 values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and DANE-
+      EE(3).  There is an additional private-use value: PrivCert(255).
+      All other values are reserved for use by future specifications.
+
+   The selector field:  Section 2.1.2 of [RFC6698] specifies 2 values:
+      Cert(0), SPKI(1).  There is an additional private-use value:
+      PrivSel(255).  All other values are reserved for use by future
+      specifications.
+
+   The matching type field:  Section 2.1.3 of [RFC6698] specifies 3
+      values: Full(0), SHA2-256(1), SHA2-512(2).  There is an additional
+      private-use value: PrivMatch(255).  All other values are reserved
+      for use by future specifications.
+
+   We may think of TLSA Certificate Usage values 0 through 3 as a
+   combination of two one-bit flags.  The low bit chooses between trust
+   anchor (TA) and end entity (EE) certificates.  The high bit chooses
+   between public PKI issued and domain-issued certificates.
+
+   The selector field specifies whether the TLSA RR matches the whole
+   certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1).  The
+   subjectPublicKeyInfo is an ASN.1 DER encoding of the certificate's
+   algorithm id, any parameters and the public key data.
+
+   The matching type field specifies how the TLSA RR Certificate
+   Association Data field is to be compared with the certificate or
+
+
+
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+
+
+   public key.  A value of Full(0) means an exact match: the full DER
+   encoding of the certificate or public key is given in the TLSA RR.  A
+   value of SHA2-256(1) means that the association data matches the
+   SHA2-256 digest of the certificate or public key, and likewise
+   SHA2-512(2) means a SHA2-512 digest is used.
+
+   Since opportunistic DANE TLS will be used by non-interactive MTAs,
+   with no user to "press 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 server's certificate chain.  The next
+   two subsections explain the process for certificate usages DANE-EE(3)
+   and DANE-TA(2).  The third subsection 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 either "DANE-EE(3) SPKI(1)
+   SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records
+   depending on site needs.  Other combinations of TLSA parameters are
+   either explicitly unsupported, or offer little to recommend them over
+   these two.
+
+   The mandatory to support digest algorithm in [RFC6698] is
+   SHA2-256(1).  When the server's TLSA RRset includes records with a
+   matching type indicating a digest record (i.e., a value other than
+   Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be
+   provided along with any other digest published, since some SMTP
+   clients may support only SHA2-256(1).  If at some point the SHA2-256
+   digest algorithm is tarnished by new cryptanalytic attacks,
+   publishers will need to include an appropriate stronger digest in
+   their TLSA records, initially along with, and ultimately in place of,
+   SHA2-256.
+
+3.1.1.  Certificate usage DANE-EE(3)
+
+   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.
+
+
+
+
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+
+
+   Similarly, 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 (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, and don't 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.
+
+3.1.2.  Certificate usage DANE-TA(2)
+
+   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 Certification Authority 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 tlsa211._dane.example.com.
+     _25._tcp.mx2.example.com.   IN CNAME tlsa211._dane.example.com.
+     tlsa211._dane.example.com.  IN TLSA 2 1 1 e3b0c44298fc1c14....
+
+
+
+
+
+
+
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+
+
+   With usage DANE-TA(2) the server certificates will need to have names
+   that match one of the client's reference identifiers (see [RFC6125]).
+   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.  At this
+   time, many SMTP servers are not configured with a comprehensive list
+   of trust anchors, nor are they expected to 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 Certification Authority known to the SMTP
+   client, authentication is likely to fail unless the TA certificate is
+   included in the TLS server certificate message.
+
+   TLSA records with selector 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
+   with a selector 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.
+
+3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)
+
+   As noted in the introduction, SMTP clients 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
+
+
+
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+
+
+   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.
+
+   SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX-
+   TA(0) or PKIX-EE(1).  SMTP clients cannot be expected to be
+   configured with a suitably complete set of trusted public CAs.
+   Lacking a complete set of public CAs, clients would not be able to
+   verify the certificates of SMTP servers whose issuing root CAs are
+   not trusted by the client.
+
+   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.  SMTP clients should generally treat such
+   TLSA records as unusable.
+
+3.2.  Certificate matching
+
+   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.
+
+3.2.1.  DANE-EE(3) name checks
+
+   The SMTP client MUST NOT perform certificate name checks with
+   certificate usage DANE-EE(3), see Section 3.1.1 above.
+
+3.2.2.  DANE-TA(2) name checks
+
+   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 include the TLSA base domain as one of the valid reference
+   identifiers 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 as a second reference
+      identifier.  The CNAME-expanded original next-hop domain MUST be
+      included as a third reference identifier if different from the
+
+
+
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+
+
+      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.
+
+   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 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".
+
+3.2.3.  Reference identifier matching
+
+
+
+
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+
+
+   When name checks are applicable (certificate usage DANE-TA(2)), if
+   the server certificate contains a Subject Alternative Name extension
+   ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS-
+   IDs are matched against the client's reference identifiers.  The CN-
+   ID ([RFC6125]) is only considered when no DNS-IDs are present.  The
+   server certificate is considered matched when one of its presented
+   identifiers ([RFC5280]) matches any of the client's reference
+   identifiers.
+
+   Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
+   The wildcard character must be 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.
+
+4.  Server key management
+
+   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 Section 3.1.2, 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.
+
+
+
+
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+
+
+   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 CNAME
+   record.  If the central trust anchor is compromised, all servers need
+   to be issued new keys by a new TA, and a shared DANE-TA(2) TLSA RRset
+   needs to be published containing just the new TA.  SMTP servers
+   cannot expect broad SMTP client CRL or OCSP support.
+
+5.  Digest algorithm agility
+
+   While [RFC6698] 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 any deprecated
+   weaker algorithms that are published for compatibility with less
+   capable clients, but should be ignored when possible.  We specify
+   such a protocol below.
+
+   Suppose that a DANE TLS client authenticating a TLS server considers
+   digest algorithm "BetterAlg" stronger than digest algorithm
+   "WorseAlg".  Suppose further that a server's TLSA RRset contains some
+   records with "BetterAlg" as the digest algorithm.  Finally, suppose
+   that for every raw public key or certificate object that is included
+   in the server's TLSA RRset in digest form, whenever that object
+   appears with algorithm "WorseAlg" with some usage and selector it
+   also appears with algorithm "BetterAlg" with the same usage and
+   selector.  In that case our client can safely ignore TLSA records
+   with the weaker algorithm "WorseAlg", because it suffices to check
+   the records with the stronger algorithm "BetterAlg".
+
+   Server operators MUST ensure that for any given usage and selector,
+   each object (certificate or public key), for which a digest
+   association exists in the TLSA RRset, is published with the SAME SET
+   of digest algorithms as all other objects that published with that
+   usage and selector.  In other words, for each usage and selector, the
+   records with non-zero matching types will correspond to on a cross-
+   product of a set of underlying objects and a fixed set of digest
+   algorithms that apply uniformly to all the objects.
+
+   To achieve digest algorithm agility, all published TLSA RRsets for
+   use with opportunistic DANE TLS for SMTP MUST conform to the above
+   requirements.  Then, for each combination of usage and selector, SMTP
+   clients can simply ignore all digest records except those that employ
+   the strongest digest algorithm.  The ordering of digest algorithms by
+   strength is not specified in advance, it is entirely up to the SMTP
+
+
+
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+
+
+   client.  SMTP client implementations SHOULD make the digest algorithm
+   preference order configurable.  Only the future will tell which
+   algorithms might be weakened by new attacks and when.
+
+   Note, TLSA records with a matching type of Full(0), that publish the
+   full value of a certificate or public key object, play no role in
+   digest algorithm agility.  They neither trump the processing of
+   records that employ digests, nor are they ignored in the presence of
+   any records with a digest (i.e. non-zero) matching type.
+
+   SMTP clients SHOULD use digest algorithm agility when processing the
+   DANE TLSA records of an SMTP server.  Algorithm agility is to be
+   applied after first discarding any unusable or malformed records
+   (unsupported digest algorithm, or incorrect digest length).  Thus,
+   for each usage and selector, the client SHOULD process only any
+   usable records with a matching type of Full(0) and the usable records
+   whose digest algorithm is believed to be the strongest among usable
+   records with the given usage and selector.
+
+   The main impact of this requirement is on key rotation, when the TLSA
+   RRset is pre-populated with digests of new certificates or public
+   keys, before these replace or augment their predecessors.  Were the
+   newly introduced RRs to include previously unused digest algorithms,
+   clients that employ this protocol could potentially ignore all the
+   digests corresponding to the current keys or certificates, causing
+   connectivity issues until the new keys or certificates are deployed.
+   Similarly, publishing new records with fewer digests could cause
+   problems for clients using cached TLSA RRsets that list both the old
+   and new objects once the new keys are deployed.
+
+   To avoid problems, server operators SHOULD apply the following
+   strategy:
+
+   o  When changing the set of objects published via the TLSA RRset
+      (e.g. during key rotation), DO NOT change the set of digest
+      algorithms used; change just the list of objects.
+
+   o  When changing the set of digest algorithms, change only the set of
+      algorithms, and generate a new RRset in which all the current
+      objects are re-published with the new set of digest algorithms.
+
+   After either of these two changes are made, the new TLSA RRset should
+   be left in place long enough that the older TLSA RRset can be flushed
+   from caches before making another change.
+
+6.  Mandatory TLS Security
+
+
+
+
+
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+
+
+   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
+   domains'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 their 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.
+
+7.  Note on DANE for Message User Agents
+
+   We note that the SMTP protocol is also used between Message User
+   Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409].  In
+   [RFC6186] 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 [RFC6186] 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
+   [I-D.ietf-dane-srv].
+
+   However, until MUAs begin to adopt the dynamic configuration
+   mechanisms of [RFC6186] they are adequately served by more
+   traditional static TLS security policies.  Specification of DANE TLS
+   for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
+   is left to future documents that focus specifically on SMTP security
+   between MUAs and MSAs.
+
+
+
+
+
+
+
+
+
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+
+
+8.  Interoperability considerations
+
+8.1.  SNI support
+
+   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 backward compatible SSL 2.0
+   compatible SSL HELLO by the SMTP client.  The minimum SSL/TLS client
+   HELLO version for SMTP clients performing DANE authentication is SSL
+   3.0, but a client that offers SSL 3.0 MUST also offer at least TLS
+   1.0 and MUST include the SNI extension.  Servers that don't make use
+   of SNI MAY negotiate SSL 3.0 if offered by the client.
+
+   Each SMTP server MUST present a certificate chain (see [RFC5246]
+   Section 7.4.2) 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.
+
+8.2.  Anonymous TLS cipher suites
+
+   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 in addition 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 by correctly
+   handling client connections that negotiate anonymous cipher suites.
+
+
+
+
+
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+
+
+   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.
+
+9.  Operational Considerations
+
+9.1.  Client Operational Considerations
+
+   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 letting email queue until
+   the error is resolved and disabling opportunistic or mandatory DANE
+   TLS 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 that opportunistic DANE TLS is
+   not to be used for a particular destination, an SMTP 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.
+
+9.2.  Publisher Operational Considerations
+
+   SMTP servers that publish certificate usage DANE-TA(2) associations
+   MUST include the TA certificate in their TLS server certificate
+   chain, even when that TA certificate is a self-signed root
+   certificate.
+
+   TLSA Publishers must follow the digest agility guidelines in
+   Section 5 and must make sure that all objects published in digest
+   form for a particular usage and selector are published with the same
+   set of digest algorithms.
+
+
+
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+
+
+   TLSA Publishers should follow the TLSA publication size guidance
+   found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".
+
+10.  Security Considerations
+
+   This protocol leverages DANE TLSA records to implement MITM resistant
+   opportunistic channel security 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 bypass any security benefit the legitimate domain owner might
+   hope to gain by publishing usage 0 or 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 the portions of this
+   specification that indicate when it is appropriate to initiate a non-
+   authenticated connection or cleartext connection to a SMTP server.
+   Specifically, in order to prevent downgrade attacks on this protocol,
+   implementation must not initiate a connection when this specification
+   indicates a particular SMTP server must be considered unreachable.
+
+11.  IANA considerations
+
+   This specification requires no support from IANA.
+
+12.  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
+
+
+
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+
+
+   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.
+
+   Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
+   to begin work on this memo and provided feedback on early drafts.
+   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.
+
+13.  References
+
+13.1.  Normative References
+
+   [I-D.ietf-dane-ops]
+              Dukhovni, V. and W. Hardaker, "DANE TLSA implementation
+              and operational guidance", draft-ietf-dane-ops-00 (work in
+              progress), October 2013.
+
+   [RFC1035]  Mockapetris, P., "Domain names - implementation and
+              specification", STD 13, RFC 1035, November 1987.
+
+   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
+              Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
+              Transport Layer Security", RFC 3207, February 2002.
+
+   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "DNS Security Introduction and Requirements", RFC
+              4033, March 2005.
+
+   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "Resource Records for the DNS Security Extensions",
+              RFC 4034, March 2005.
+
+   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
+              Rose, "Protocol Modifications for the DNS Security
+              Extensions", RFC 4035, March 2005.
+
+   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
+              (TLS) Protocol Version 1.2", RFC 5246, August 2008.
+
+   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
+              Housley, R., and W. Polk, "Internet X.509 Public Key
+              Infrastructure Certificate and Certificate Revocation List
+              (CRL) Profile", RFC 5280, May 2008.
+
+
+
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+
+
+   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
+              October 2008.
+
+   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
+              Extension Definitions", RFC 6066, January 2011.
+
+   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
+              Verification of Domain-Based Application Service Identity
+              within Internet Public Key Infrastructure Using X.509
+              (PKIX) Certificates in the Context of Transport Layer
+              Security (TLS)", RFC 6125, March 2011.
+
+   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
+              Submission/Access Services", RFC 6186, March 2011.
+
+   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
+              DNS", RFC 6672, June 2012.
+
+   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
+              of Named Entities (DANE) Transport Layer Security (TLS)
+              Protocol: TLSA", RFC 6698, August 2012.
+
+13.2.  Informative References
+
+   [I-D.ietf-dane-registry-acronyms]
+              Gudmundsson, O., "Adding acronyms to simplify DANE
+              conversations", draft-ietf-dane-registry-acronyms-01 (work
+              in progress), October 2013.
+
+   [I-D.ietf-dane-srv]
+              Finch, T., "Using DNS-Based Authentication of Named
+              Entities (DANE) TLSA records with SRV and MX records.",
+              draft-ietf-dane-srv-02 (work in progress), February 2013.
+
+   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July
+              2009.
+
+   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
+              STD 72, RFC 6409, November 2011.
+
+Authors' Addresses
+
+   Viktor Dukhovni
+   Two Sigma
+
+   Email: ietf-dane@dukhovni.org
+
+
+
+
+
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+
+
+   Wes Hardaker
+   Parsons
+   P.O. Box 382
+   Davis, CA  95617
+   US
+
+   Email: ietf@hardakers.net
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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