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rfc:rfc7672

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

                                                          October 2015
 SMTP Security via Opportunistic DNS-Based Authentication of Named
           Entities (DANE) Transport Layer Security (TLS)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7672.

Copyright Notice

 Copyright (c) 2015 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
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Dukhovni & Hardaker Standards Track [Page 1] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................4
    1.2. Background .................................................6
    1.3. SMTP Channel Security ......................................6
         1.3.1. STARTTLS Downgrade Attack ...........................7
         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 ..................9
 2. Identifying Applicable TLSA Records .............................9
    2.1. DNS Considerations .........................................9
         2.1.1. DNS Errors, "Bogus" Responses, 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 ................................16
         2.2.3. TLSA Record Lookup .................................18
 3. DANE Authentication ............................................20
    3.1. TLSA Certificate Usages ...................................20
         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 .......................................27
 6. Mandatory TLS Security .........................................27
 7. Note on DANE for Message User Agents ...........................28
 8. Interoperability Considerations ................................28
    8.1. SNI Support ...............................................28
    8.2. Anonymous TLS Cipher Suites ...............................29
 9. Operational Considerations .....................................29
    9.1. Client Operational Considerations .........................29
    9.2. Publisher Operational Considerations ......................30
 10. Security Considerations .......................................30
 11. References ....................................................31
    11.1. Normative References .....................................31
    11.2. Informative References ...................................33
 Acknowledgements ..................................................34
 Authors' Addresses ................................................34

Dukhovni & Hardaker Standards Track [Page 2] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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, principally, the fact that the
 destination server is selected indirectly via DNS Mail Exchange (MX)
 records and that neither email addresses nor MX hostnames signal a
 requirement for either secure or cleartext transport.  Therefore,
 aside from a few manually configured exceptions, SMTP transport
 security is, by necessity, opportunistic (for a definition of
 "Opportunistic Security", see [RFC7435]).
 This specification uses the presence of DANE TLSA records to securely
 signal TLS support and to publish the means by which SMTP clients can
 successfully authenticate legitimate SMTP servers.  This becomes
 "opportunistic DANE TLS" and is resistant to downgrade and
 man-in-the-middle (MITM) attacks.  It enables an incremental
 transition of the email backbone to authenticated TLS delivery, with
 increased global protection as adoption increases.
 With opportunistic DANE TLS, traffic from SMTP clients to domains
 that publish "usable" DANE TLSA records in accordance with this memo
 is authenticated and encrypted.  Traffic from legacy clients or to
 domains that do not publish TLSA records will continue to be sent in
 the same manner as before, via manually configured security,
 (pre-DANE) opportunistic TLS, or just cleartext SMTP.
 Problems with the existing use of TLS in MTA-to-MTA SMTP that
 motivate this specification are described in Section 1.3.  The
 specification itself follows, in Sections 2 and 3, which describe,
 respectively, how to locate and use DANE TLSA records with SMTP.  In
 Section 6, we discuss the application of DANE TLS to destinations for
 which channel integrity and confidentiality are mandatory.  In
 Section 7, we briefly comment on the potential applicability of this
 specification to Message User Agents.

Dukhovni & Hardaker Standards Track [Page 3] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 throughout this document:
 Man-in-the-middle (MITM) attack:  Active modification of network
    traffic by an adversary able to thereby compromise the
    confidentiality or integrity of the data.
 Downgrade attack:  (From [RFC4949].)  A type of MITM attack in which
    the attacker can cause two parties, at the time they negotiate a
    security association, to agree on a lower level of protection than
    the highest level that could have been supported by both of them.
 Downgrade-resistant:  A protocol is "downgrade-resistant" if it
    employs effective countermeasures against downgrade attacks.
 "Secure", "bogus", "insecure", "indeterminate":  DNSSEC validation
    results, as defined in 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 that is resistant to
    downgrade attacks for destinations with DNSSEC-validated TLSA
    records.  When opportunistic DANE TLS is determined to be
    unavailable, clients should fall back to pre-DANE opportunistic
    TLS.  Opportunistic DANE TLS requires support for DNSSEC, DANE,
    and STARTTLS on the client side, and STARTTLS plus a DNSSEC
    published TLSA record on the server side.

Dukhovni & Hardaker Standards Track [Page 4] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 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-and-forward
    process.  When an MTA is unable to forward a message that may
    become deliverable later, the message is queued and delivery is
    retried periodically.  Some MTAs may be configured with a fallback
    next-hop destination that handles messages that the MTA would
    otherwise queue and retry.  When a fallback next-hop destination
    is configured, messages that would otherwise have to be delayed
    may be sent to the fallback next-hop destination instead.  The
    fallback destination may itself be subject to opportunistic or
    mandatory DANE TLS (Section 6) 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 as defined in [RFC1034], Section 3.6.
 RRset:  An RRset ([RFC2181], Section 5) is a group of DNS resource
    records that share the same label, class, and type.

Dukhovni & Hardaker Standards Track [Page 5] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

1.2. Background

 The Domain Name System Security Extensions (DNSSEC) add data origin
 authentication, data integrity, and data nonexistence proofs to the
 Domain Name System (DNS).  DNSSEC is defined in [RFC4033], [RFC4034],
 and [RFC4035].
 As described in the introduction of [RFC6698], TLS authentication via
 the existing public Certification Authority (CA) PKI suffers from an
 overabundance of trusted parties capable of issuing certificates for
 any domain of their choice.  DANE leverages the DNSSEC infrastructure
 to publish public keys and certificates for use with the Transport
 Layer Security (TLS) [RFC5246] protocol via the "TLSA" DNS record
 type.  With DNSSEC, each domain can only vouch for the keys of its
 delegated sub-domains.
 The TLS protocol enables secure TCP communication.  In the context of
 this memo, channel security is assumed to be provided by TLS.  Used
 without authentication, TLS provides only privacy protection against
 eavesdropping attacks.  Otherwise, TLS also provides data origin
 authentication to guard against MITM attacks.

1.3. SMTP Channel Security

 With HTTPS, TLS employs X.509 certificates [RFC5280] issued by one of
 the many CAs bundled with popular web browsers to allow users to
 authenticate their "secure" websites.  Before we specify a new DANE
 TLS security model for SMTP, we will explain why a new security model
 is needed.  In the process, we will explain why the familiar HTTPS
 security model is inadequate to protect inter-domain SMTP traffic.
 The subsections below outline four key problems with applying
 traditional Web PKI [RFC7435] to SMTP; these problems are addressed
 by this specification.  Since an SMTP channel security policy is not
 explicitly specified in either the recipient address or the MX
 record, a new signaling mechanism is required to indicate when
 channel security is possible and should be used.  The publication of
 TLSA records allows server operators to securely signal to SMTP
 clients that TLS is available and should be used.  DANE TLSA makes it
 possible to simultaneously discover which destination domains support
 secure delivery via TLS and how to verify the authenticity of the
 associated SMTP services, providing a path forward to ubiquitous SMTP
 channel security.

Dukhovni & Hardaker Standards Track [Page 6] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

1.3.1. STARTTLS Downgrade Attack

 SMTP [RFC5321] is a single-hop protocol in a multi-hop store-and-
 forward email delivery process.  An SMTP envelope recipient address
 does not correspond to a specific transport-layer endpoint address;
 rather, at each relay hop, the transport-layer endpoint is the
 next-hop relay, while the envelope recipient address typically
 remains the same.  Unlike HTTP and its corresponding secured version,
 HTTPS, where the use of TLS is signaled via the URI scheme, email
 recipient addresses do not directly signal transport security policy.
 Indeed, no such signaling could work well with SMTP, since TLS
 encryption of SMTP protects email traffic on a hop-by-hop basis while
 email addresses could only express end-to-end policy.
 With no mechanism available to signal transport security policy, SMTP
 relays employ a best-effort "opportunistic" security model for TLS.
 A single SMTP server TCP listening endpoint can serve both TLS and
 non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
 command [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 can prevent this.  The attacker can simply disable TLS.

1.3.2. Insecure Server Name without DNSSEC

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

Dukhovni & Hardaker Standards Track [Page 7] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 One might try to harden TLS for SMTP against DNS attacks by using the
 envelope recipient domain as a reference identifier and by requiring
 each SMTP server to possess a trusted certificate for the envelope
 recipient domain rather than the MX hostname.  Unfortunately, this is
 impractical, as email for many domains is handled by third parties
 that are not in a position to obtain certificates for all the domains
 they serve.  Deployment of the Server Name Indication (SNI) extension
 to TLS (see Section 3 of [RFC6066]) is no panacea, since SNI key
 management is operationally challenging except when the email service
 provider is also the domain's registrar and its certificate issuer;
 this is rarely the case for email.
 Since the recipient domain name cannot be used as the SMTP server
 reference identifier, and neither can the MX hostname without DNSSEC,
 large-scale deployment of authenticated TLS for SMTP requires that
 the DNS be secure.
 Since SMTP security depends critically on DNSSEC, it is important to
 point out that SMTP with DANE is consequently the most conservative
 possible trust model.  It trusts only what must be trusted and no
 more.  Adding any other trusted actors to the mix can only reduce
 SMTP security.  A sender may choose to further harden DNSSEC for
 selected high-value receiving domains by configuring explicit trust
 anchors for those domains instead of relying on the chain of trust
 from the root domain.  However, detailed discussion of DNSSEC
 security practices is out of scope for this document.

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, but this requires configuring
 sending MTAs with appropriate subject names or certificate content
 digests from their peer domains.  Due to the resulting administrative
 burden, such statically configured SMTP secure channels are used
 rarely (generally only between domains that make bilateral
 arrangements with their business partners).  Internet email, on the
 other hand, requires regularly contacting new domains for which
 security configurations cannot be established in advance.
 The abstraction of the SMTP transport endpoint via DNS MX records,
 often across organizational boundaries, limits the use of public CA
 PKI with SMTP to a small set of sender-configured peer domains.  With
 little opportunity to use TLS authentication, sending MTAs are rarely
 configured with a comprehensive list of trusted CAs.  SMTP services
 that support STARTTLS often deploy X.509 certificates that are
 self-signed or issued by a private CA.

Dukhovni & Hardaker Standards Track [Page 8] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 CA (a trust anchor).  MTAs
 are not interactive applications where a human operator can make a
 decision (wisely or otherwise) to selectively disable TLS security
 policy when certificate chain verification fails.  With no user to
 "click OK", the MTA's list of public CA trust anchors would need to
 be comprehensive in order to avoid bouncing mail addressed to sites
 that employ unknown CAs.
 On the other hand, each trusted CA can issue certificates for any
 domain.  If even one of the configured CAs is compromised or operated
 by an adversary, it can subvert TLS security for all destinations.
 Any set of CAs is simultaneously both overly inclusive and not
 inclusive enough.

2. Identifying Applicable TLSA Records

2.1. DNS Considerations

2.1.1. DNS Errors, "Bogus" Responses, 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 that 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.

Dukhovni & Hardaker Standards Track [Page 9] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 In this document, the term "indeterminate" will be used exclusively
 in the [RFC4035] sense.  Therefore, obtaining "indeterminate" lookup
 results is a (transient) failure condition, namely, the inability to
 locate the relevant DNS records.  DNS records that would be
 classified "indeterminate" in the sense of [RFC4035] are simply
 classified as "insecure".
 We do not need to distinguish between zones that lack a suitable
 ancestor trust anchor, and delegations (ultimately) from a trust
 anchor that designate a child zone as being "insecure".  All
 "insecure" RRsets MUST be handled identically: in either case,
 non-validated data for the query domain is all that is and can be
 available, and authentication using the data is impossible.  As the
 DNS root zone has been signed, we expect that validating resolvers
 used by Internet-facing MTAs will be configured with trust anchor
 data for the root zone and that therefore domains with no ancestor
 trust anchor will not be possible in most deployments.
 As noted in 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 instead signal a "bogus" or error result.  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 using a non-validating security-aware stub resolver MAY use
 the stub resolver's ability, if available, to signal DNSSEC
 validation status based on information the stub resolver has learned
 from an upstream validating recursive resolver.  Security-oblivious
 stub resolvers [RFC4033] 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.

Dukhovni & Hardaker Standards Track [Page 10] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 There is an important non-failure condition we need to highlight in
 addition to the obvious case of the DNS client obtaining a non-empty
 "secure" or "insecure" RRset of the requested type.  Namely, it is
 not an error when either "secure" or "insecure" nonexistence is
 determined for the requested data.  When a DNSSEC response with a
 validation status that is either "secure" or "insecure" reports
 either no records of the requested type or nonexistence of the query
 domain, the response is not a DNS error condition.  The DNS client
 has not been left without an answer; it has learned that records of
 the requested type do not exist.
 Security-aware stub resolvers will, of course, also signal DNS lookup
 errors in other cases, for example, when processing a "SERVFAIL"
 [RFC2136] response code (RCODE) [RFC1035], which will not have an
 associated DNSSEC status.  All lookup errors are treated the same way
 by this specification, regardless of whether they are from a "bogus"
 or "indeterminate" DNSSEC status or from a more generic DNS error:
 the information that was requested cannot be obtained by the
 security-aware resolver at this time.  Thus, a lookup error is either
 a failure to obtain the relevant RRset if it exists or a failure to
 determine that no such RRset exists when it does not.
 In contrast to a "bogus" response or an "indeterminate" response, an
 "insecure" DNSSEC response is not an error; rather, as explained
 above, it indicates that the target DNS zone is either delegated as
 an "insecure" child of a "secure" parent zone or not a descendant of
 any of the configured DNSSEC trust anchors in use by the SMTP client.
 "Insecure" results will leave the SMTP client with degraded channel
 security but do not stand in the way of message delivery.  See
 Section 2.2 for further details.

2.1.2. DNS Error Handling

 When a DNS lookup failure (an error, "bogus", or "indeterminate", as
 defined above) prevents an SMTP client from determining which SMTP
 server or servers it should connect to, message delivery MUST be
 delayed.  This naturally includes, for example, the case when a
 "bogus" or "indeterminate" response is encountered during MX
 resolution.  When multiple MX hostnames are obtained from a
 successful MX lookup but a later DNS lookup failure prevents network
 address resolution for a given MX hostname, delivery may proceed via
 any remaining MX hosts.
 When a particular SMTP server is securely identified as the delivery
 destination, a set of DNS lookups (Section 2.2) MUST be performed to
 locate any related TLSA records.  If any DNS queries used to locate
 TLSA records fail (due to "bogus" or "indeterminate" records,
 timeouts, malformed replies, SERVFAIL responses, etc.), then the SMTP

Dukhovni & Hardaker Standards Track [Page 11] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 client MUST treat that server as unreachable and MUST NOT deliver the
 message via that server.  If no servers are reachable, delivery is
 delayed.
 In the text that follows, we will only describe what happens when all
 relevant DNS queries succeed.  If any DNS failure occurs, the SMTP
 client MUST behave as described in this section, by "skipping" the
 SMTP server or destination that is problematic.  Queries for
 candidate TLSA records are explicitly part of "all relevant DNS
 queries", and SMTP clients MUST NOT continue to connect to an SMTP
 server or destination whose TLSA record lookup fails.

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
 of the alias, the SMTP client will need to repeat the query at the
 target of the alias and should do so recursively up to some
 configured or implementation-dependent recursion limit.  If at any
 stage of CNAME expansion an error is detected, the lookup of the
 original requested records MUST be considered to have failed.
 Whether a chain of CNAME records was returned in a single stub
 resolver response or via explicit recursion by the SMTP client, if at
 any stage of recursive expansion an "insecure" CNAME record is
 encountered, then it and all subsequent results (in particular, the
 final result) MUST be considered "insecure", regardless of whether or
 not any earlier CNAME records leading to the "insecure" record were
 "secure".
 Note that a security-aware non-validating stub resolver may return to
 the SMTP client an "insecure" reply received from a validating
 recursive resolver that contains a CNAME record along with additional
 answers recursively obtained starting at the target of the CNAME.  In

Dukhovni & Hardaker Standards Track [Page 12] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 this case, the only possible conclusion is that some record in the
 set of records returned is "insecure", and it is, in fact, possible
 that the initial CNAME record and a subset of the subsequent records
 are "secure".
 If the SMTP client needs to determine the security status of the DNS
 zone containing the initial CNAME record, it will need to issue a
 separate query of type "CNAME" that returns only the initial CNAME
 record.  Specifically, as discussed in Section 2.2.2, when "insecure"
 A or AAAA records are found for an SMTP server via a CNAME alias, the
 SMTP client will need to perform an additional CNAME query in order
 to determine whether or not the DNS zone in which the alias is
 published is DNSSEC signed.

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:  Any
    connection to the MTA MUST employ TLS encryption and MUST
    authenticate the SMTP server using the techniques discussed in the
    rest of this document.  Failure to establish an authenticated TLS
    connection MUST result in falling back to the next SMTP server or
    delayed delivery.
 A "secure" non-empty TLSA RRset where all the records are unusable:
    Any connection to the MTA MUST be made via TLS, but authentication
    is not required.  Failure to establish an encrypted TLS connection
    MUST result in falling back to the next SMTP server or delayed
    delivery.

Dukhovni & Hardaker Standards Track [Page 13] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 An "insecure" TLSA RRset or DNSSEC-authenticated denial of existence
 of the TLSA records:
    A connection to the MTA SHOULD be made using (pre-DANE)
    opportunistic TLS; this includes using cleartext delivery when the
    remote SMTP server does not appear to support TLS.  The MTA MAY
    retry in cleartext when delivery via TLS fails during the
    handshake or even during data transfer.
 Any lookup error:  Lookup errors, including "bogus" and
    "indeterminate" as explained in Section 2.1.1, MUST result in
    falling back to the next SMTP server or delayed delivery.
 An SMTP client MAY be configured to mandate DANE-verified delivery
 for some destinations.  With mandatory DANE TLS (Section 6), delivery
 proceeds only when "secure" TLSA records are used to establish an
 encrypted and authenticated TLS channel with the SMTP server.
 When the original next-hop destination is an address literal rather
 than a DNS domain, DANE TLS does not apply.  Delivery proceeds using
 any relevant security policy configured by the MTA administrator.
 Similarly, when an MX RRset incorrectly lists a network address in
 lieu of an MX hostname, if an MTA chooses to connect to the network
 address in the nonconformant MX record, DANE TLSA does not apply for
 such a connection.
 In the subsections that follow, we explain how to locate the SMTP
 servers and the associated TLSA records for a given next-hop
 destination domain.  We also explain which name or names are to be
 used in identity checks of the SMTP server certificate.

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

Dukhovni & Hardaker Standards Track [Page 14] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 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) and DANE TLS for the given destination is mandatory
 (Section 6), delivery MUST be delayed.  If the MX RRset is "insecure"
 and DANE TLS is not mandatory, the SMTP client is free to use
 pre-DANE opportunistic TLS (possibly even cleartext).
 Since the protocol in this memo is an Opportunistic Security protocol
 [RFC7435], the SMTP client MAY elect 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 use the provider's
 TLSA records (active attacks that tamper with the "insecure" MX RRset
 are of course still possible in this case).
 When the MX records are not (DNSSEC) signed, an active attacker can
 redirect SMTP clients to MX hosts of his choice.  Such redirection is
 tamper-evident when SMTP servers found via "insecure" MX records are
 recorded as the next-hop relay in the MTA delivery logs in their
 original (rather than CNAME-expanded) form.  Sending MTAs SHOULD log
 unexpanded MX hostnames when these result from "insecure" MX lookups.
 Any successful authentication via an insecurely determined MX host
 MUST NOT be misrepresented in the mail logs as secure delivery to the
 intended next-hop domain.
 In the absence of DNS lookup errors (Section 2.1.1), if the MX RRset
 is not "insecure", then it is "secure", and the SMTP client MUST
 treat each MX hostname 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 2.1.2, cause the SMTP
 server in question to be treated as unreachable.

Dukhovni & Hardaker Standards Track [Page 15] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 that is not
 subject to MX resolution, that represents a hostname from a "secure"
 MX RRset, or that lacks MX records.  Such domains include:
 o  Any host that is configured by the sending MTA administrator as
    the next-hop relay for some or all domains and that is not subject
    to MX resolution.
 o  A domain that has MX records.  When a domain has MX records, we
    treat each MX host listed in those MX records as though it were a
    non-MX destination -- that is, in the same way as we would treat
    an administrator-configured relay that handles mail for that
    domain.  (Unlike administrator-specified relays, MTAs are not
    required to support CNAME expansion of next-hop names found via MX
    lookups.)
 o  A next-hop destination domain subject to MX resolution that has no
    MX records.  In this case, the domain's name is implicitly also
    its sole SMTP server name.
 Note that DNS queries with type TLSA are mishandled by load-balancing
 nameservers that serve the MX hostnames of some large email
 providers.  The DNS zones served by these nameservers are not signed
 and contain no TLSA records.  These nameservers SHOULD provide
 "insecure" negative replies that indicate the nonexistence of the
 TLSA records, but instead they fail by not responding at all or by
 responding with a DNS RCODE [RFC1035] other than NXDOMAIN, e.g.,
 SERVFAIL or NOTIMP [RFC2136].
 To avoid problems delivering mail to domains whose SMTP servers are
 served by these problematic nameservers, the SMTP client MUST perform
 any A and/or AAAA queries for the destination before attempting to
 locate the associated TLSA records.  This lookup is needed in any
 case to determine (1) whether or not the destination domain is
 reachable and (2) the DNSSEC validation status of the chain of CNAME
 queries required to reach the ultimate address records.
 If no address records are found, the destination is unreachable.  If
 address records are found but the DNSSEC validation status of the
 first query response is "insecure" (see Section 2.1.3), the SMTP
 client SHOULD NOT proceed to search for any associated TLSA records.
 In the case of these problematic domains, TLSA queries would lead to
 DNS lookup errors and would cause messages to be consistently delayed
 and ultimately returned to the sender.  We don't expect to find any

Dukhovni & Hardaker Standards Track [Page 16] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 "secure" TLSA records associated with a TLSA base domain that lies in
 an unsigned DNS zone.  Therefore, skipping TLSA lookups in this case
 will also reduce latency, with no detrimental impact on security.
 If the A and/or AAAA lookup of the initial name yields a CNAME, we
 replace it with the resulting name as if it were the initial name and
 perform a lookup again using the new name.  This replacement is
 performed recursively (up to the MTA's recursion limit).
 We consider the following cases for handling a DNS response for an
 A or AAAA DNS lookup:
 Not found:  When the DNS queries for A and/or AAAA records yield
    neither a list of addresses nor a CNAME (or CNAME expansion is not
    supported), the destination is unreachable.
 Non-CNAME:  The answer is not a CNAME alias.  If the address RRset is
    "secure", TLSA lookups are performed as described in 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, the initial name itself.
 In summary, if it is possible to securely obtain the full,
 CNAME-expanded, DNSSEC-validated address records for the input
 domain, then that name is the preferred TLSA base domain.  Otherwise,
 the unexpanded input domain is the candidate TLSA base domain.  When
 no "secure" TLSA records are found at either the CNAME-expanded or
 unexpanded domain, then DANE TLS does not apply for mail delivery via
 the input domain in question.  And, as always, errors, "bogus"
 results, or "indeterminate" results for any query in the process MUST
 result in delaying or abandoning delivery.

Dukhovni & Hardaker Standards Track [Page 17] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

2.2.3. TLSA Record Lookup

 When the SMTP server's hostname is not a CNAME or DNAME alias, the
 list of associated candidate TLSA base domains (see below) consists
 of just the server hostname.
 When the hostname is an alias with a "secure" (at every stage) full
 expansion, the list of candidate TLSA base domains (see below) is a
 pair of domains: the fully expanded server hostname first, and the
 unexpanded server hostname second.
 Each candidate TLSA base domain (alias-expanded or original) is in
 turn prefixed with service labels of the form "_<port>._tcp".  The
 resulting domain name is used to issue a DNSSEC query with the query
 type set to TLSA ([RFC6698], Section 7.1).
 The first of these candidate domains to yield a "secure" TLSA RRset
 becomes the actual TLSA base domain.
 For SMTP, the destination TCP port is typically 25, but this may be
 different with custom routes specified by the MTA administrator, in
 which case the SMTP client MUST use the appropriate number in the
 "_<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 keeps track of whether
 or not the entire chain is "secure".  If any "insecure" records are
 encountered or the TLSA records don't exist, the next candidate TLSA
 base domain is tried instead.
 If the ultimate response is a "secure" TLSA RRset, then the candidate
 TLSA base domain will be the actual TLSA base domain, and the TLSA
 RRset will constitute the TLSA records for the destination.  If none
 of the candidate TLSA base domains yield "secure" TLSA records, then
 the SMTP client is free to use pre-DANE opportunistic TLS (possibly
 even cleartext).
 TLSA record publishers may leverage CNAMEs to reference a single
 authoritative TLSA RRset specifying a common CA or a common
 end-entity certificate to be used with multiple TLS services.  Such
 CNAME expansion does not change the SMTP client's notion of the TLSA
 base domain; thus, when _25._tcp.mx.example.com is a CNAME, the base

Dukhovni & Hardaker Standards Track [Page 18] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 domain remains mx.example.com, and this is still the reference
 identifier used together with the next-hop domain in peer certificate
 name checks.
 Note that shared end-entity certificate associations expose the
 publishing domain to substitution attacks, where an MITM attacker can
 reroute traffic to a different server that shares the same end-entity
 certificate.  Such shared end-entity TLSA records SHOULD be avoided
 unless the servers in question are functionally equivalent or employ
 mutually incompatible protocols (an active attacker gains nothing by
 diverting client traffic from one such server to another).
 A better example, employing a shared trust anchor rather than shared
 end-entity certificates, is illustrated by the DNSSEC-validated
 records below:
    example.com.                IN MX 0 mx1.example.com.
    example.com.                IN MX 0 mx2.example.com.
    _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
    _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
    tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c149a...
 The SMTP servers mx1.example.com and mx2.example.com will be expected
 to have certificates issued under a common trust anchor, but each MX
 hostname's TLSA base domain remains unchanged despite the above CNAME
 records.  Correspondingly, each SMTP server will be associated with a
 pair of reference identifiers consisting of its hostname plus the
 next-hop domain "example.com".
 If, during TLSA resolution (including possible CNAME indirection), at
 least one "secure" TLSA record is found (even if not usable because
 it is unsupported by the implementation or support is
 administratively disabled), then the corresponding host has signaled
 its commitment to implement TLS.  The SMTP client MUST NOT deliver
 mail via the corresponding host unless a TLS session is negotiated
 via STARTTLS.  This is required to avoid MITM STARTTLS downgrade
 attacks.
 As noted previously (in 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 name nor the final name
 are never candidate TLSA base domains, even if "secure".

Dukhovni & Hardaker Standards Track [Page 19] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 three 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.
 Since opportunistic DANE TLS will be used by non-interactive MTAs,
 with no user to "click OK" when authentication fails, reliability of
 peer authentication is paramount.  Server operators are advised to
 publish TLSA records that are least likely to fail authentication due
 to interoperability or operational problems.  Because DANE TLS relies
 on coordinated changes to DNS and SMTP server settings, the best
 choice of records to publish will depend on site-specific practices.
 The certificate usage element of a TLSA record plays a critical role
 in determining how the corresponding certificate association data
 field is used to authenticate a server's certificate chain.
 Sections 3.1.1 and 3.1.2 explain the process for certificate usages
 DANE-EE(3) and DANE-TA(2), respectively.  Section 3.1.3 briefly
 explains why certificate usages PKIX-TA(0) and PKIX-EE(1) are not
 applicable with opportunistic DANE TLS.
 In summary, we RECOMMEND the use of "DANE-EE(3) SPKI(1) SHA2-256(1)",
 with "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records as a second
 choice, depending on site needs.  See Sections 3.1.1 and 3.1.2 for
 more details.  Other combinations of TLSA parameters either (1) are
 explicitly unsupported or (2) offer little to recommend them over
 these two.

Dukhovni & Hardaker Standards Track [Page 20] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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.
 The expiration date of the server certificate MUST be ignored: the
 validity period of the TLSA record key binding is determined by the
 validity interval of the TLSA record DNSSEC signature.
 With DANE-EE(3), servers need not employ SNI (they may ignore the
 client's SNI message) even when the server is known under independent
 names that would otherwise require separate certificates.  It is
 instead sufficient for the TLSA RRsets for all the domains in
 question to match the server's default certificate.  Of course, with
 SMTP servers it is simpler still to publish the same MX hostname for
 all the hosted domains.
 For domains where it is practical to make coordinated changes in DNS
 TLSA records during SMTP server key rotation, it is often best to
 publish end-entity DANE-EE(3) certificate associations.  DANE-EE(3)
 certificates don't suddenly stop working when leaf or intermediate
 certificates expire, nor do they fail when the server operator
 neglects to configure all the required issuer certificates in the
 server certificate chain.
 TLSA records published for SMTP servers SHOULD, in most cases, be
 "DANE-EE(3) SPKI(1) SHA2-256(1)" records.  Since all DANE
 implementations are required to support SHA2-256, this record type
 works for all clients and need not change across certificate renewals
 with the same key.

Dukhovni & Hardaker Standards Track [Page 21] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 CA to create certificates for multiple TLS
 services, it may be simpler to publish the issuing authority as a
 trust anchor (TA) for the certificate chains of all relevant
 services.  The TLSA query domain (TLSA base domain with port and
 protocol prefix labels) for each service issued by the same TA may
 then be set to a CNAME alias that points to a common TLSA RRset that
 matches the TA.  For example:
    example.com.                IN MX 0 mx1.example.com.
    example.com.                IN MX 0 mx2.example.com.
    _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
    _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
    tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c14....
 With usage DANE-TA(2), the server certificates will need to have
 names that match one of the client's reference identifiers (see
 [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.  Many SMTP
 servers are not configured with a comprehensive list of trust
 anchors, nor are they expected to be at any point in the future.
 Some MTAs will ignore all locally trusted certificates when
 processing usage DANE-TA(2) TLSA records.  Thus, even when the TA
 happens to be a public CA known to the SMTP client, authentication is
 likely to fail unless the TA certificate is included in the TLS
 server certificate message.
 With some SMTP server software, it is not possible to configure the
 server to include self-signed (root) CA certificates in the server
 certificate chain.  Such servers either MUST publish DANE-TA(2)
 records for an intermediate certificate or MUST instead use
 DANE-EE(3) TLSA records.
 TLSA records with a matching type of Full(0) are discouraged.  While
 these potentially obviate the need to transmit the TA certificate in
 the TLS server certificate message, client implementations may not be
 able to augment the server certificate chain with the data obtained
 from DNS, especially when the TLSA record supplies a bare key
 (selector SPKI(1)).  Since the server will need to transmit the TA
 certificate in any case, server operators SHOULD publish TLSA records

Dukhovni & Hardaker Standards Track [Page 22] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 with a matching type other than Full(0) and avoid potential
 interoperability issues with large TLSA records containing full
 certificates or keys.
 TLSA Publishers employing DANE-TA(2) records SHOULD publish records
 with a selector of Cert(0).  Such TLSA records are associated with
 the whole trust anchor certificate, not just with the trust anchor
 public key.  In particular, the SMTP client SHOULD then apply any
 relevant constraints from the trust anchor certificate, such as, for
 example, path length constraints.
 While a selector of SPKI(1) may also be employed, the resulting TLSA
 record will not specify the full trust anchor certificate content,
 and elements of the trust anchor certificate other than the public
 key become mutable.  This may, for example, allow a subsidiary CA to
 issue a chain that violates the trust anchor's path length or name
 constraints.

3.1.3. Certificate Usages PKIX-TA(0) and PKIX-EE(1)

 Note that this section applies to MTA-to-MTA SMTP, which is normally
 on port 25 -- that is, to servers that are the SMTP servers for one
 or more destination domains.  Other uses of SMTP, such as in
 MUA-to-MSA submission on ports 587 or 465, are out of scope for this
 document.  Where those other uses also employ TLS opportunistically
 and/or depend on DNSSEC as a result of DNS-based discovery of service
 location, the relevant specifications should, as appropriate, arrive
 at similar conclusions.
 As noted in Sections 1.3.1 and 1.3.2, sending MTAs cannot, without
 relying on DNSSEC for "secure" MX records and DANE for STARTTLS
 support signaling, perform server identity verification or prevent
 STARTTLS downgrade attacks.  The use of PKIX CAs offers no added
 security, since an attacker capable of compromising DNSSEC is free to
 replace any PKIX-TA(0) or PKIX-EE(1) TLSA records with records
 bearing any convenient non-PKIX certificate usage.  Finally, as
 explained in Section 1.3.4, there is no list of trusted CAs agreed
 upon by all MTAs and no user to "click OK" when a server's CA is not
 trusted by a client.
 Therefore, TLSA records for the port 25 SMTP service used by client
 MTAs SHOULD NOT include TLSA RRs with certificate usage PKIX-TA(0) or
 PKIX-EE(1).  SMTP client MTAs cannot be expected to be configured
 with a suitably complete set of trusted public CAs.  Lacking a
 complete set of public CAs, MTA clients would not be able to verify
 the certificates of SMTP servers whose issuing root CAs are not
 trusted by the client.

Dukhovni & Hardaker Standards Track [Page 23] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 Opportunistic DANE TLS needs to interoperate without bilateral
 coordination of security settings between client and server systems.
 Therefore, parameter choices that are fragile in the absence of
 bilateral coordination are unsupported.  Nothing is lost; since the
 PKIX certificate usages cannot aid SMTP TLS security, they can only
 impede SMTP TLS interoperability.
 SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
 or PKIX-EE(1) is undefined.  As with any other unsupported
 certificate usage, SMTP clients MAY treat such records as "unusable".

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) (Section 3.1.1).

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 employ the TLSA base domain as the primary reference identifier
 for matching the server certificate.
 TLSA records for MX hostnames:  If the TLSA base domain was obtained
    indirectly via a "secure" MX lookup (including any CNAME-expanded
    name of an MX hostname), then the original next-hop domain used in
    the MX lookup MUST be included as a second reference identifier.
    The CNAME-expanded original next-hop domain MUST be included as a
    third reference identifier if different from the original next-hop
    domain.  When the client MTA is employing DANE TLS security
    despite "insecure" MX redirection, the MX hostname is the only
    reference identifier.
 TLSA records for non-MX hostnames:  If MX records were not used
    (e.g., if none exist) and the TLSA base domain is the
    CNAME-expanded original next-hop domain, then the original
    next-hop domain MUST be included as a second reference identifier.

Dukhovni & Hardaker Standards Track [Page 24] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 Accepting certificates with the original next-hop domain in addition
 to the MX hostname allows a domain with multiple MX hostnames to
 field a single certificate bearing a single domain name (i.e., the
 email domain) across all the SMTP servers.  This also aids
 interoperability with pre-DANE SMTP clients that are configured to
 look for the email domain name in server certificates -- for example,
 with "secure" DNS records as shown below:
    exchange.example.org.            IN CNAME mail.example.org.
    mail.example.org.                IN CNAME example.com.
    example.com.                     IN MX    10 mx10.example.com.
    example.com.                     IN MX    15 mx15.example.com.
    example.com.                     IN MX    20 mx20.example.com.
    ;
    mx10.example.com.                IN A     192.0.2.10
    _25._tcp.mx10.example.com.       IN TLSA  2 0 1 ...
    ;
    mx15.example.com.                IN CNAME mxbackup.example.com.
    mxbackup.example.com.            IN A     192.0.2.15
    ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
    _25._tcp.mx15.example.com.       IN TLSA  2 0 1 ...
    ;
    mx20.example.com.                IN CNAME mxbackup.example.net.
    mxbackup.example.net.            IN A     198.51.100.20
    _25._tcp.mxbackup.example.net.   IN TLSA  2 0 1 ...
 Certificate name checks for delivery of mail to exchange.example.org
 via any of the associated SMTP servers MUST accept at least the names
 "exchange.example.org" and "example.com", which are, respectively,
 the original and fully expanded next-hop domain.  When the SMTP
 server is mx10.example.com, name checks MUST accept the TLSA base
 domain "mx10.example.com".  If, despite the fact that MX hostnames
 are required to not be aliases, the MTA supports delivery via
 "mx15.example.com" or "mx20.example.com", then name checks MUST
 accept the respective TLSA base domains "mx15.example.com" and
 "mxbackup.example.net".

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.

Dukhovni & Hardaker Standards Track [Page 25] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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

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 a
 CNAME record.  If the central trust anchor is compromised, all

Dukhovni & Hardaker Standards Track [Page 26] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 servers need to be issued new keys by a new TA, and an updated
 DANE-TA(2) TLSA RRset needs to be published containing just the
 new TA.
 SMTP servers cannot expect broad Certificate Revocation List (CRL) or
 Online Certificate Status Protocol (OCSP) support from SMTP clients.
 As outlined above, with DANE, compromised server or trust anchor keys
 can be "revoked" by removing them from the DNS without the need for
 client-side support for OCSP or CRLs.

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 deprecated weaker
 algorithms that are published for compatibility with less capable
 clients.  When stronger algorithms are an option, deprecated
 algorithms SHOULD be avoided.  Such a protocol is specified in
 [RFC7671].  SMTP clients and servers that implement this
 specification MUST comply with the requirements outlined in Section 9
 of [RFC7671].

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
 domain's DANE TLSA records to authenticate the corresponding SMTP
 server.  Authentication via DANE TLSA records is easier to manage, as
 changes in the receiver's expected certificate properties are made on
 the receiver end and don't require manually communicated
 configuration changes.  With mandatory DANE TLS, when no usable TLSA
 records are found, message delivery is delayed.  Thus, mail is only
 sent when an authenticated TLS channel is established to the remote
 SMTP server.
 Administrators of mail servers that employ mandatory DANE TLS need to
 carefully monitor their mail logs and queues.  If a partner domain
 unwittingly misconfigures its TLSA records, disables DNSSEC, or
 misconfigures SMTP server certificate chains, mail will be delayed
 and may bounce if the issue is not resolved in a timely manner.

Dukhovni & Hardaker Standards Track [Page 27] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

7. Note on DANE for Message User Agents

 We note that SMTP 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 [RFC7673].
 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 MUA-to-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 Secure Socket Layer (SSL)
 HELLO that is SSL 2.0 compatible by the SMTP 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.

Dukhovni & Hardaker Standards Track [Page 28] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

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 also include one or more
 anonymous TLS cipher suites in its TLS HELLO.  SMTP servers that need
 to interoperate with opportunistic TLS clients SHOULD be prepared to
 interoperate with such clients by either always selecting a mutually
 supported non-anonymous cipher suite or correctly handling client
 connections that negotiate anonymous cipher suites.
 Note that while SMTP server operators are under no obligation to
 enable anonymous cipher suites, no security is gained by sending
 certificates to clients that will ignore them.  Indeed, support for
 anonymous cipher suites in the server makes audit trails more
 informative.  Log entries that record connections that employed an
 anonymous cipher suite record the fact that the clients did not care
 to authenticate the server.

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 allowing email to queue
 until the error is resolved and disabling opportunistic or mandatory
 DANE TLS (Section 6) for one or more destinations.  The choice to
 disable DANE TLS security should not be made lightly.  Every
 reasonable effort should be made to determine that problems with mail
 delivery are the result of an operational error and not an attack.  A
 fallback strategy may be to configure explicit out-of-band TLS
 security settings if supported by the sending MTA.
 SMTP clients may deploy opportunistic DANE TLS incrementally by
 enabling it only for selected sites or may occasionally need to
 disable opportunistic DANE TLS for peers that fail to interoperate
 due to misconfiguration or software defects on either end.  Some
 implementations MAY support DANE TLS in an "audit only" mode in which
 failure to achieve the requisite security level is logged as a
 warning and delivery proceeds at a reduced security level.  Unless
 local policy specifies "audit only" or specifies that opportunistic
 DANE TLS is not to be used for a particular destination, an SMTP

Dukhovni & Hardaker Standards Track [Page 29] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 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

 Some MTAs enable STARTTLS selectively.  For example, they might only
 support STARTTLS with clients that have previously demonstrated
 "proper MTA behavior", e.g., by retrying the delivery of deferred
 messages (greylisting).  If such an MTA publishes DANE TLSA records,
 sending MTAs that implement this specification will not attempt the
 initial cleartext SMTP transaction needed to establish the "proper
 MTA behavior", because they cannot establish the required channel
 security.  Server operators MUST NOT implement selective STARTTLS if
 they also want to support DANE TLSA.
 TLSA Publishers MUST follow the guidelines in Section 8 of [RFC7671].
 TLSA Publishers SHOULD follow the TLSA publication size guidance
 found in Section 10.1 of [RFC7671].
 TLSA Publishers SHOULD follow the TLSA record TTL and signature
 lifetime recommendations found in Section 13 of [RFC7671].

10. Security Considerations

 This protocol leverages DANE TLSA records to implement MITM-resistant
 Opportunistic Security [RFC7435] 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

Dukhovni & Hardaker Standards Track [Page 30] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 bypass any security benefit the legitimate domain owner might hope to
 gain by publishing usage 0 or usage 1 TLSA RRs.  Given the lack of
 public CA PKI support in existing MTA deployments, avoiding
 certificate usages 0 and 1 simplifies implementation and deployment
 with no adverse security consequences.
 Implementations must strictly follow Sections 2.1.2, 2.1.3, 2.2,
 2.2.1, 2.2.2, 2.2.3, 3.2, and 9.1 of this specification; these
 sections indicate when it is appropriate to initiate a
 non-authenticated connection or cleartext connection to an SMTP
 server.  Specifically, in order to prevent downgrade attacks on this
 protocol, implementations must not initiate a connection when this
 specification indicates that a particular SMTP server must be
 considered unreachable.

11. References

11.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
            <http://www.rfc-editor.org/info/rfc1034>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
            February 2002, <http://www.rfc-editor.org/info/rfc3207>.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, DOI 10.17487/RFC4033, March 2005,
            <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, DOI 10.17487/RFC4034, March 2005,
            <http://www.rfc-editor.org/info/rfc4034>.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
            <http://www.rfc-editor.org/info/rfc4035>.

Dukhovni & Hardaker Standards Track [Page 31] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
            <http://www.rfc-editor.org/info/rfc5280>.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <http://www.rfc-editor.org/info/rfc5321>.
 [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
            DOI 10.17487/RFC5598, July 2009,
            <http://www.rfc-editor.org/info/rfc5598>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [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, DOI 10.17487/RFC6125,
            March 2011, <http://www.rfc-editor.org/info/rfc6125>.
 [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
            Submission/Access Services", RFC 6186,
            DOI 10.17487/RFC6186, March 2011,
            <http://www.rfc-editor.org/info/rfc6186>.
 [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
            DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
            <http://www.rfc-editor.org/info/rfc6672>.
 [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
            of Named Entities (DANE) Transport Layer Security (TLS)
            Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
            August 2012, <http://www.rfc-editor.org/info/rfc6698>.

Dukhovni & Hardaker Standards Track [Page 32] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

 [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
            Conversations about DNS-Based Authentication of Named
            Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
            April 2014, <http://www.rfc-editor.org/info/rfc7218>.
 [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
            Authentication of Named Entities (DANE) Protocol: Updates
            and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
            October 2015, <http://www.rfc-editor.org/info/rfc7671>.

11.2. Informative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
            "Dynamic Updates in the Domain Name System (DNS UPDATE)",
            RFC 2136, DOI 10.17487/RFC2136, April 1997,
            <http://www.rfc-editor.org/info/rfc2136>.
 [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
            Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
            <http://www.rfc-editor.org/info/rfc2181>.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
            <http://www.rfc-editor.org/info/rfc4949>.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
            <http://www.rfc-editor.org/info/rfc6409>.
 [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
            Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
            December 2014, <http://www.rfc-editor.org/info/rfc7435>.
 [RFC7673]  Finch, T., Miller, M., and P. Saint-Andre, "Using
            DNS-Based Authentication of Named Entities (DANE) TLSA
            Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673,
            October 2015, <http://www.rfc-editor.org/info/rfc7673>.

Dukhovni & Hardaker Standards Track [Page 33] RFC 7672 SMTP Security via Opportunistic DANE TLS October 2015

Acknowledgements

 The authors would like to extend great thanks to Tony Finch, who
 started the original version of a DANE SMTP document.  His work is
 greatly appreciated and has been incorporated into this document.
 The authors would like to additionally thank Phil Pennock for his
 comments and advice on this document.
 Acknowledgements from Viktor: Thanks to Paul Hoffman, who motivated
 me to begin work on this memo and provided feedback on early draft
 versions of this document.  Thanks to Patrick Koetter, Perry Metzger,
 and Nico Williams for valuable review comments.  Thanks also to
 Wietse Venema, who created Postfix, and whose advice and feedback
 were essential to the development of the Postfix DANE implementation.

Authors' Addresses

 Viktor Dukhovni
 Two Sigma
 Email: ietf-dane@dukhovni.org
 Wes Hardaker
 Parsons
 P.O. Box 382
 Davis, CA  95617
 United States
 Email: ietf@hardakers.net

Dukhovni & Hardaker Standards Track [Page 34]

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