SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-05

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DANE                                                         V. Dukhovni
Internet-Draft                                              Unaffiliated
Intended status: Standards Track                             W. Hardaker
Expires: August 13, 2014                                         Parsons
                                                        February 9, 2014

                SMTP security via opportunistic DANE TLS
                   draft-ietf-dane-smtp-with-dane-05

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 August 13, 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   5
       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   5
       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   6
       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   7
       1.3.4.  Too many certificate authorities  . . . . . . . . . .   7
   2.  Hardening (pre-DANE) Opportunistic TLS  . . . . . . . . . . .   8
     2.1.  DNS errors, bogus and indeterminate responses . . . . . .   8
     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  11
       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  13
       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  14
       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  16
     2.3.  DANE authentication . . . . . . . . . . . . . . . . . . .  17
       2.3.1.  TLSA certificate usages . . . . . . . . . . . . . . .  18
       2.3.2.  Certificate matching  . . . . . . . . . . . . . . . .  20
       2.3.3.  Digest algorithm agility  . . . . . . . . . . . . . .  23
   3.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  25
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  25
     4.1.  Client Operational Considerations . . . . . . . . . . . .  25
     4.2.  Publisher Operational Considerations  . . . . . . . . . .  25
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  26
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  27
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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 with MTA to MTA SMTP the use of TLS is generally
   opportunistic.

   We note that the SMTP protocol is also used between Message User
   Agents (MUAs) and Message Submission Agents (MSAs).  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

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   requirements 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.

   However, until MUAs begin to adopt the dynamic configuration
   mechanisms of [RFC6186] they are adequately served by more
   traditional static TLS security policies.  This document will not
   discuss the MUA use case further, leaving specification of DANE TLS
   for MUAs to future documents that focus specifically on SMTP security
   between MUAs and MSAs.  The rest of this memo will focus on securing
   MTA to MTA SMTP connections.

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:

   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.

   MX hostname:  The RRDATA of an MX record consists of a 16 bit
      preference followed by a Mail Exchange domain name (see [RFC1035],

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      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.

   SMTP server:  An SMTP server whose name appears in an MX record for a
      particular domain.  Used to refer specifically to the host and
      SMTP service itself, not its DNS 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
      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) adds 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 Certificate Authority (CA) PKI suffers from an

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   over-abundance of trusted certificate authorities 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 man-in-the-middle (MITM)
   attacks.

1.3.  SMTP channel security

   With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
   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 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, there is no URI scheme for
   email addresses to designate whether communication with the SMTP
   server should be conducted via a cleartext or a TLS-encrypted
   channel.  Indeed no such URI scheme 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 a man in the middle 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 bypass 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 man in the middle (MITM) attacks
   unless it verifies that the server's certificate binds its public key
   to its name.  However, with SMTP server names are obtained indirectly
   via MX records.  Without DNSSEC, the MX lookup is vulnerable 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 is not possible without DNSSEC.

   One might try to harden the use of TLS with SMTP against DNS attacks
   by requiring each SMTP server to possess a trusted certificate for
   the envelope recipient domain rather than the MX hostname.
   Unfortunately, this is impractical, as email for many domains is
   handled by third parties that are not in a position to obtain
   certificates for all the domains they serve.  Deployment of the
   Server Name Indication (SNI) extension to TLS (see [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
   authentication identity, 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

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   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 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.  In such a case there is not an "additional" trusted
   authority, rather the root trust anchor is replaced with a more
   specific trust anchor for each of the domains in question.  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 specifically selected peer domains.  This requires
   MTAs to be configured with appropriate subject names or certificate
   content digests to expect in the presented host 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 use X.509 certificates that are self-
   signed or issued by a private CA.

1.3.4.  Too many certificate 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 certificate 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 sites to sites employing an unknown certificate authority.

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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.  Hardening (pre-DANE) Opportunistic TLS

   Neither email addresses nor MX hostnames (or submission SRV records)
   signal a requirement for either secure or cleartext transport.
   Therefore, SMTP transport security is of necessity generally
   opportunistic (barring manually configured exceptions).

   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, and 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 non-compliant 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.

2.1.  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.  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.

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   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 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.

   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.

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   There is an important non-failure condition we need to highlight in
   addition to the obvious case of the DNS client obtaining a non-empty
   "secure" or "insecure" RRset of the requested type.  Namely, it is
   not an error when either "secure" or "insecure" 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 can
   not 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.

   When a stub resolver receives a response containing a CNAME alias, it
   will generally restart 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 recursive CNAME
   expansion a query fails, the stub resolver's lookup of the original
   requested records will result in a failure status being returned.  If
   at any stage of recursive expansion the response is "insecure", then
   it and all subsequent results (in particular, the final result) MUST
   be considered "insecure" regardless of whether the other responses
   received were deemed "secure".  If at any stage of recursive
   expansion the validation status is "bogus" or "indeterminate" or
   associated with another DNS lookup error, the resolution of the
   requested records MUST be considered to have failed.

   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

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   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 selected as the delivery
   destination, a set of DNS lookups must be performed to discover 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.

2.2.  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

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      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
      optionally retry in cleartext when a TLS handshake fails.

   Any lookup error:  Lookup errors, including "bogus" and
      "indeterminate", as explained in Section 2.1 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.

   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.

   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.  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.

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   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 an 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.  Clearly, if DANE TLS security is to
   apply to message delivery via any of the SMTP servers, the MX records
   must be obtained securely via a DNSSEC validated MX 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), DANE TLS does not apply, and delivery proceeds via pre-
   DANE opportunistic TLS.  Otherwise (assuming no DNS errors or "bogus"
   /"indeterminate" responses), 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-

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   DANE opportunistic TLS.  To avoid downgrade attacks, any errors
   during TLSA lookups MUST, as explained in Section 2.1, 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 the
      hostname of its sole SMTP server.

   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 each stage of the chain of CNAME queries
   required to reach the final result.

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   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" (there may be additional queries
   if the initial response is a CNAME alias), 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.

   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).  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).  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

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   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.  The
   SMTP client MUST use the appropriate number in the "_<port>" prefix
   in place of "_25".  If, for example, the candidate base domain is
   "mail.example.com", and the SMTP connection is to port 25, the TLSA
   RRset is obtained via a DNSSEC query of the form:

   _25._tcp.mail.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 should proceed via pre-DANE opportunistic TLS.

   TLSA record publishers may leverage CNAMEs to reference a single
   authoritative TLSA RRset specifying a common certificate 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.mail.example.com
   is a CNAME, the base domain remains mail.example.com and is still the
   name used 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

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   certificate.  Such shared end entity records should be avoided unless
   the servers in question are interchangeable.

   For example, given the DNSSEC validated records below:

     example.com.                IN MX 0 mail.example.com.
     example.com.                IN MX 0 mail2.example.com.
     _25._tcp.mail.example.com.  IN CNAME tlsa211._dane.example.com.
     _25._tcp.mail2.example.com. IN CNAME tlsa211._dane.example.com.
     tlsa211._dane.example.com.  IN  TLSA 2 1 1 e3b0c44298fc1c14....

   The SMTP servers mail.example.com and mail2.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.  Each SMTP server's certificate subject name (or
   one of the subject alternative names) is expected to match either the
   corresponding MX hostname or else "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 SHOULD 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".

2.3.  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, therefore authentication via this protocol is expected to
   be less prone to connection failure caused by incompatible
   configuration of the client and server.

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2.3.1.  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
   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.

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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.

2.3.1.1.  Certificate usage DANE-EE(3)

   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.

   Authentication via certificate usage DANE-EE(3) TLSA records involves
   simply checking that the server's leaf certificate matches the TLSA
   record.  Other than extracting the relevant certificate elements for
   comparison, no other use is made of the certificate content.
   Authentication via certificate usage DANE-EE(3) TLSA records involves
   no certificate authority signature checks.  It also involves no
   server name checks, and thus does not impose any new requirements on
   the names contained in the server certificate (SNI is not required
   when the TLSA record matches the server's default certificate).

   Two TLSA records MUST be published before updating a server's public
   key, 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, the server may be reconfigured to use the
   new private key and associated public key certificate.  Once the
   server is using the new key, the TLSA RR that matches the retired key
   can be removed from DNS, leaving only the RR that matches the new
   key.

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

2.3.1.2.  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 Certificate 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

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   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.

   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 Certificate Authority known to the SMTP
   client, authentication is likely to fail unless the TA is included in
   the TLS server certificate message.

   TLSA Publishers should publish either "DANE-TA(2) SPKI(1) Full(0)" or
   "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA parameters.  As with leaf
   certificate rollover discussed in Section 2.3.1.1, two such TLSA RRs
   need to be published to facilitate TA certificate rollover.

2.3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)

   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.  Even
   with a full set of public CAs, SMTP clients cannot (without relying
   on DNSSEC for secure MX records and DANE for STARTTLS support
   signalling) perform [RFC6125] server identity verification or prevent
   STARTTLS downgrade attacks.  The use of trusted public 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 client treatment of TLSA RRs with certificate usages "PKIX-
   TA(0)" or "PKIX-EE(1)" is undefined.  For example, clients MAY (will
   likely) treat such TLSA records as unusable.

2.3.2.  Certificate matching

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

   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

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   reached the correct server.  In all cases the SMTP client MUST accept
   the TLSA base domain as a valid DNS name in the server certificate.

   TLSA records for MX hostnames:  If the TLSA base domain was obtained
      indirectly via an MX lookup (including any CNAME-expanded name of
      an MX hostname), then the original next-hop domain used in the MX
      lookup MUST be accepted in the peer certificate.  The CNAME-
      expanded original next-hop domain MUST also be accepted if
      different from the initial query name.

   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 also be accepted.

   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 inter-
   operability 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

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   "mx15.example.com" or "mx20.example.com" then name checks MUST accept
   the respective TLSA base domains "mx15.example.com" and
   "mxbackup.example.net".

   The SMTP client MUST NOT perform certificate usage name checks with
   certificate usage DANE-EE(3), since with usage DANE-EE(3) the server
   is authenticated directly by matching the TLSA RRset to its
   certificate or public key without resorting to any issuing authority.
   The certificate content is ignored except to match the certificate or
   public key (ASN.1 DER encoding or its digest) with the TLSA RRset.

   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 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 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 its default certificate chain.  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
   default certificate may match a different name acceptable to the
   client, e.g., the original next-hop domain.

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   An SMTP client employing pre-DANE opportunistic TLS MAY include some
   anonymous TLS cipher suites in its TLS HELLO in addition to at least
   one non-anonymous cipher suite (since servers often do support any of
   the anonymous ones).  Therefore, an SMTP server MUST either select
   some suitable non-anonymous cipher suite offered by the client, or if
   it selects an anonymous cipher suite, it MUST NOT fail to complete
   the handshake merely because an anonymous cipher suite was chosen.

   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.

2.3.3.  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 compatibilty 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 BETTER stronger than digest algorithm WORSE.
   Suppose further that a server's TLSA RRset contains some records with
   BETTER 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 WORSE with some usage and selector it also appears with
   algorithm BETTER with the same usage and selector.  In that case our
   client can safely ignore TLSA records with the weaker algorithm
   WORSE, because it suffices to check the records with the stronger
   algorithm BETTER.

   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.

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   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.

   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.

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   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.

3.  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
   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.

4.  Operational Considerations

4.1.  Client Operational Considerations

   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.  Unless
   local policy specifies that opportunistic DANE TLS is not to be used
   for a particular destination, client MUST NOT deliver mail via a
   server whose certificate chain fails to match at least one TLSA
   record when usable TLSA records are available.

4.2.  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.

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   TLSA Publishers must follow the digest agility guidelines in
   Section 2.3.3 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".

5.  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.

6.  IANA considerations

   This specification requires no support from IANA.

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7.  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.

8.  References

8.1.  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.

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   [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.

   [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.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 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.

8.2.  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-smtp]
              Finch, T., "Secure SMTP using DNS-Based Authentication of
              Named Entities (DANE) TLSA records.", draft-ietf-dane-
              smtp-01 (work in progress), February 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.

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   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July
              2009.

   [RFC6394]  Barnes, R., "Use Cases and Requirements for DNS-Based
              Authentication of Named Entities (DANE)", RFC 6394,
              October 2011.

   [RFC6895]  Eastlake, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, April 2013.

Authors' Addresses

   Viktor Dukhovni
   Unaffiliated

   Email: ietf-dane@dukhovni.org

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

   Email: ietf@hardakers.net

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