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RFC 7672

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

Pages: 34
Proposed Standard
Errata
Part 1 of 2 – Pages 1 to 19
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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.
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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
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1. Introduction

This memo specifies a new connection security model for Message Transfer Agents (MTAs). This model is motivated by key features of inter-domain SMTP delivery, 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.
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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.
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   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.
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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.
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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.
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   One might try to harden TLS for SMTP against DNS attacks by using the
   envelope recipient domain as a reference identifier and by requiring
   each SMTP server to possess a trusted certificate for the envelope
   recipient domain rather than the MX hostname.  Unfortunately, this is
   impractical, as email for many domains is handled by third parties
   that are not in a position to obtain certificates for all the domains
   they serve.  Deployment of the Server Name Indication (SNI) extension
   to TLS (see 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.
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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.
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   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.
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   There is an important non-failure condition we need to highlight in
   addition to the obvious case of the DNS client obtaining a non-empty
   "secure" or "insecure" RRset of the requested type.  Namely, it is
   not an error when either "secure" or "insecure" nonexistence is
   determined for the requested data.  When a DNSSEC response with a
   validation status that is either "secure" or "insecure" reports
   either no records of the requested type or nonexistence of the query
   domain, the response is not a DNS error condition.  The DNS client
   has not been left without an answer; it has learned that records of
   the requested type do not exist.

   Security-aware stub resolvers will, of course, also signal DNS lookup
   errors in other cases, for example, when processing a "SERVFAIL"
   [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
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   client MUST treat that server as unreachable and MUST NOT deliver the
   message via that server.  If no servers are reachable, delivery is
   delayed.

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

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

   If the SMTP client needs to determine the security status of the DNS
   zone containing the initial CNAME record, it will need to issue a
   separate query of type "CNAME" that returns only the initial CNAME
   record.  Specifically, as discussed in 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.
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   An "insecure" TLSA RRset or DNSSEC-authenticated denial of existence
   of the TLSA records:
      A connection to the MTA SHOULD be made using (pre-DANE)
      opportunistic TLS; this includes using cleartext delivery when the
      remote SMTP server does not appear to support TLS.  The MTA MAY
      retry in cleartext when delivery via TLS fails during the
      handshake or even during data transfer.

   Any lookup error:  Lookup errors, including "bogus" and
      "indeterminate" as explained in 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
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   security.  Domains that want secure inbound mail delivery need to
   ensure that all their SMTP servers and MX records are configured
   accordingly.

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

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

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

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

   Non-CNAME:  The answer is not a CNAME alias.  If the address RRset is
      "secure", TLSA lookups are performed as described in 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.
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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
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   domain remains mx.example.com, and this is still the reference
   identifier used together with the next-hop domain in peer certificate
   name checks.

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

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

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

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

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

   As noted previously (in 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".


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