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DANE                                                         V. Dukhovni
Internet-Draft                                                 Two Sigma
Intended status: Standards Track                             W. Hardaker
Expires: November 30, 2015                                       Parsons
                                                            May 29, 2015


                SMTP security via opportunistic DANE TLS
                   draft-ietf-dane-smtp-with-dane-19

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 30, 2015.

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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   6
       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   6
       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   7
       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   8
       1.3.4.  Too many certification authorities  . . . . . . . . .   8
   2.  Identifying applicable TLSA records . . . . . . . . . . . . .   8
     2.1.  DNS considerations  . . . . . . . . . . . . . . . . . . .   9
       2.1.1.  DNS errors, bogus and indeterminate responses . . . .   9
       2.1.2.  DNS error handling  . . . . . . . . . . . . . . . . .  11
       2.1.3.  Stub resolver considerations  . . . . . . . . . . . .  11
     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  12
       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  14
       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  15
       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  17
   3.  DANE authentication . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  TLSA certificate usages . . . . . . . . . . . . . . . . .  19
       3.1.1.  Certificate usage DANE-EE(3)  . . . . . . . . . . . .  20
       3.1.2.  Certificate usage DANE-TA(2)  . . . . . . . . . . . .  21
       3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)  . . . .  22
     3.2.  Certificate matching  . . . . . . . . . . . . . . . . . .  23
       3.2.1.  DANE-EE(3) name checks  . . . . . . . . . . . . . . .  23
       3.2.2.  DANE-TA(2) name checks  . . . . . . . . . . . . . . .  23
       3.2.3.  Reference identifier matching . . . . . . . . . . . .  24
   4.  Server key management . . . . . . . . . . . . . . . . . . . .  25
   5.  Digest algorithm agility  . . . . . . . . . . . . . . . . . .  26
   6.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  26
   7.  Note on DANE for Message User Agents  . . . . . . . . . . . .  27
   8.  Interoperability considerations . . . . . . . . . . . . . . .  27
     8.1.  SNI support . . . . . . . . . . . . . . . . . . . . . . .  27
     8.2.  Anonymous TLS cipher suites . . . . . . . . . . . . . . .  28
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  28
     9.1.  Client Operational Considerations . . . . . . . . . . . .  28
     9.2.  Publisher Operational Considerations  . . . . . . . . . .  29
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  29
   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  30
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     13.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32



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1.  Introduction

   This memo specifies a new connection security model for Message
   Transfer Agents (MTAs).  This model is motivated by key features of
   inter-domain SMTP delivery, in particular the fact that the
   destination server is selected indirectly via DNS Mail Exchange (MX)
   records and that neither email addresses nor MX hostnames signal a
   requirement for either secure or cleartext transport.  Therefore,
   aside from a few manually configured exceptions, SMTP transport
   security is of necessity opportunistic (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 existing use of TLS in MTA to MTA SMTP that motivate
   this specification are described in Section 1.3.  The specification
   itself follows in Section 2 and Section 3 which describe respectively
   how to locate and use DANE TLSA records with SMTP.  In Section 6, we
   discuss application of DANE TLS to destinations for which channel
   integrity and confidentiality are mandatory.  In Section 7 we briefly
   comment on potential applicability of this specification to Message
   User Agents.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   The following terms or concepts are used through the document:

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



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   Downgrade attack:  (From [RFC4949]).  A type of man-in-the-middle
      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 counter-measures 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, resistant to
      downgrade attacks for destinations with DNSSEC-validated TLSA
      records.  When opportunistic DANE TLS is determined to be
      unavailable, clients should fall back to opportunistic TLS.
      Opportunistic DANE TLS requires support for DNSSEC, DANE and
      STARTTLS on the client side and STARTTLS plus a DNSSEC published
      TLSA record on the server side.

   reference identifier:  (Special case of [RFC6125] definition).  One
      of the domain names associated by the SMTP client with the
      destination SMTP server for performing name checks on the server
      certificate.  When name checks are applicable, at least one of the
      reference identifiers MUST match an [RFC6125] DNS-ID (or if none
      are present the [RFC6125] CN-ID) of the server certificate (see
      Section 3.2.3).

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



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   delayed delivery:  Email delivery is a multi-hop store & 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 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.

1.2.  Background

   The Domain Name System Security Extensions (DNSSEC) add data origin
   authentication, data integrity and data non-existence proofs to the
   Domain Name System (DNS).  DNSSEC is defined in [RFC4033], [RFC4034]
   and [RFC4035].

   As described in the introduction of [RFC6698], TLS authentication via
   the existing public Certification Authority (CA) PKI suffers from an
   over-abundance of trusted parties capable of issuing certificates for
   any domain of their choice.  DANE leverages the DNSSEC infrastructure
   to publish 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



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   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, Transport Layer Security (TLS) employs X.509 certificates
   [RFC5280] issued by one of the many Certification Authorities (CAs)
   bundled with popular web browsers to allow users to authenticate
   their "secure" websites.  Before we specify a new DANE TLS security
   model for SMTP, we will explain why a new security model is needed.
   In the process, we will explain why the familiar HTTPS security model
   is inadequate to protect inter-domain SMTP traffic.

   The subsections below outline four key problems with applying
   traditional Web PKI to SMTP that are addressed by this specification.
   Since SMTP channel security policy is not explicitly specified in
   either the recipient address or the MX record, a new signaling
   mechanism is required to indicate when channel security is possible
   and should be used.  The publication of TLSA records allows server
   operators to securely signal to SMTP clients that TLS is available
   and should be used.  DANE TLSA makes it possible to simultaneously
   discover which destination domains support secure delivery via TLS
   and how to verify the authenticity of the associated SMTP services,
   providing a path forward to ubiquitous SMTP channel security.

1.3.1.  STARTTLS downgrade attack

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

   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



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

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

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

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

   Since SMTP security depends critically on DNSSEC, it is important to
   point out that consequently SMTP with DANE is the most conservative
   possible trust model.  It trusts only what must be trusted and no
   more.  Adding any other trusted actors to the mix can only reduce
   SMTP security.  A sender may choose to further harden DNSSEC for
   selected high-value receiving domains by configuring explicit trust



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

1.3.4.  Too many certification authorities

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

   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







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2.1.  DNS considerations

2.1.1.  DNS errors, bogus and indeterminate responses

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

   A DNS lookup may signal an error or return a definitive answer.  A
   security-aware resolver MUST be used for this specification.
   Security-aware resolvers will indicate the security status of a DNS
   RRSet with one of four possible values defined in Section 4.3 of
   [RFC4035]: "secure", "insecure", "bogus" and "indeterminate".  In
   [RFC4035] the meaning of the "indeterminate" security status is:

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

   Note, the "indeterminate" security status has a conflicting
   definition in section 5 of [RFC4033].

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

   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
   unvalidated 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.





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   As noted in section 4.3 of [RFC4035], a security-aware DNS resolver
   MUST be able to determine whether a given non-error DNS response is
   "secure", "insecure", "bogus" or "indeterminate".  It is expected
   that most security-aware stub resolvers will not signal an
   "indeterminate" security status (in the sense of RFC4035) to the
   application, and will signal a "bogus" or error result instead.  If a
   resolver does signal an RFC4035 "indeterminate" security status, this
   MUST be treated by the SMTP client as though a "bogus" or error
   result had been returned.

   An MTA making use of a non-validating security-aware stub resolver
   MAY use the stub resolver's ability, if available, to signal DNSSEC
   validation status based on information the stub resolver has learned
   from an upstream validating recursive resolver.  Security-Oblivious
   stub-resolvers ([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.

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

   Security-aware stub resolvers will, of course, also signal DNS lookup
   errors in other cases, for example when processing a "ServFail"
   RCODE, which will not have an associated DNSSEC status.  All lookup
   errors are treated the same way by this specification, regardless of
   whether they are from a "bogus" or "indeterminate" DNSSEC status or
   from a more generic DNS error: the information that was requested
   cannot be obtained by the security-aware resolver at this time.  A



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   lookup error is thus a failure to obtain the relevant RRSet if it
   exists, or to determine that no such RRSet exists when it does not.

   In contrast to a "bogus" or an "indeterminate" response, an
   "insecure" DNSSEC response is not an error, rather, as explained
   above, it indicates that the target DNS zone is either delegated as
   an "insecure" child of a "secure" parent zone, or is 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 Section 2.2 for further details.

2.1.2.  DNS error handling

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

   When a particular SMTP server is securely identified as the delivery
   destination, a set of DNS lookups (Section 2.2) MUST be performed to
   locate any related TLSA records.  If any DNS queries used to locate
   TLSA records fail (be it due to "bogus" or "indeterminate" records,
   timeouts, malformed replies, ServFails, etc.), then the SMTP client
   MUST treat that server as unreachable and MUST NOT deliver the
   message via that server.  If no servers are reachable, delivery is
   delayed.

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

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



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   for the possibility that the alias in question is the result of an
   ancestor domain DNAME record.  Consequently, no explicit support for
   DNAME records is needed in SMTP software; it is sufficient to process
   the resulting CNAME aliases.  DNAME records only require special
   processing in the validating stub-resolver library that checks the
   integrity of the combined DNAME + CNAME reply.  When DNSSEC
   validation is handled by a local caching resolver, rather than the
   MTA itself, even that part of the DNAME support logic is outside the
   MTA.

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

   Whether a chain of CNAME records was returned in a single stub
   resolver response or via explicit recursion by the SMTP client, if at
   any stage of recursive expansion an "insecure" CNAME record is
   encountered, then it and all subsequent results (in particular, the
   final result) MUST be considered "insecure" regardless of whether any
   earlier CNAME records leading to the "insecure" record were "secure".

   Note that a security-aware non-validating stub resolver may return to
   the SMTP client an "insecure" reply received from a validating
   recursive resolver that contains a CNAME record along with additional
   answers recursively obtained starting at the target of the CNAME.  In
   this case, the only possible conclusion is that some record in the
   set of records returned is "insecure", and it is in fact possible
   that the initial CNAME record and a subset of the subsequent records
   are "secure".

   If the SMTP client needs to determine the security status of the DNS
   zone containing the initial CNAME record, it will need to issue a
   separate query of type "CNAME" that returns only the initial CNAME
   record.  Specifically, 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 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



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   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.

   An insecure TLSA RRSet or DNSSEC validated proof-of-non-existent TLSA
    records:
      A connection to the MTA SHOULD be made using (pre-DANE)
      opportunistic TLS, this includes using cleartext delivery when the
      remote SMTP server does not appear to support TLS.  The MTA MAY
      retry in cleartext when delivery via TLS fails either during the
      handshake or even during data transfer.

   Any lookup error:  Lookup errors, including "bogus" and
      "indeterminate", as explained in Section 2.1.1 MUST result in
      falling back to the next SMTP server or delayed delivery.

   An SMTP client MAY be configured to 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



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   address in the non-conformant MX record, DANE TLSA does not apply for
   such a connection.

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

2.2.1.  MX resolution

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

   MX records MUST be sorted by preference; an MX hostname with a worse
   (numerically higher) MX preference that has TLSA records MUST NOT
   preempt an MX hostname with a better (numerically lower) preference
   that has no TLSA records.  In other words, prevention of delivery
   loops by obeying MX preferences MUST take precedence over channel
   security considerations.  Even with two equal-preference MX records,
   an MTA is not obligated to choose the MX hostname that offers more
   security.  Domains that want secure inbound mail delivery need to
   ensure that all their SMTP servers and MX records are configured
   accordingly.

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

   If the MX RRSet (or any CNAME leading to it) is "insecure" (see
   Section 2.1.1) 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



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   provider's TLSA records.  Deliveries via the provider's SMTP servers
   will not be subject to active attacks when sending SMTP clients elect
   to make use of the provider's TLSA records (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 2.1.1, cause the SMTP
   server in question to be treated as unreachable.

2.2.2.  Non-MX destinations

   This section describes the algorithm used to locate the TLSA records
   and associated TLSA base domain for an input domain 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 configured by the sending MTA administrator as the next-
      hop relay for some or all domains, that is not subject to MX
      resolution.

   o  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.




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   Note that DNS queries with type TLSA are mishandled by load balancing
   nameservers that serve the MX hostnames of some large email
   providers.  The DNS zones served by these nameservers are not signed
   and contain no TLSA records, but queries for TLSA records fail,
   rather than returning the non-existence of the requested TLSA
   records.

   To avoid problems delivering mail to domains whose SMTP servers are
   served by the problem nameservers the SMTP client MUST perform any A
   and/or AAAA queries for the destination before attempting to locate
   the associated TLSA records.  This lookup is needed in any case to
   determine whether the destination domain is reachable and the DNSSEC
   validation status of the chain of CNAME queries required to reach the
   ultimate address records.

   If no address records are found, the destination is unreachable.  If
   address records are found, but the DNSSEC validation status of the
   first query response is "insecure" (see Section 2.1.3), the SMTP
   client SHOULD NOT proceed to search for any associated TLSA records.
   With the problem domains, TLSA queries will lead to DNS lookup errors
   and cause messages to be consistently delayed and ultimately returned
   to the sender.  We don't expect to find any "secure" TLSA records
   associated with a TLSA base domain that lies in an unsigned DNS zone.
   Therefore, skipping TLSA lookups in this case will also reduce
   latency with no detrimental impact on security.

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

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

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

   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



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      initial CNAME response is also "insecure", DANE TLS does not
      apply.  Otherwise, this case is treated just like the non-CNAME
      case above, where a search is performed for a TLSA record with the
      original input domain as the candidate TLSA base domain.

   Secure CNAME:   The input domain is a CNAME alias, and the ultimate
      network address RRSet is "secure" (see Section 2.1.1).  Two
      candidate TLSA base domains are tried: the fully CNAME-expanded
      initial name and, failing that, then the initial name itself.

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

2.2.3.  TLSA record lookup

   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 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 ?




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

   If the ultimate response is a "secure" TLSA RRSet, then the candidate
   TLSA base domain will be the actual TLSA base domain and the TLSA
   RRSet will constitute the TLSA records for the destination.  If none
   of the candidate TLSA base domains yield "secure" TLSA records then
   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 Certification Authority
   or a common end-entity certificate to be used with multiple TLS
   services.  Such CNAME expansion does not change the SMTP client's
   notion of the TLSA base domain; thus, when _25._tcp.mx.example.com is
   a CNAME, the base domain remains mx.example.com and this is still the
   reference identifier used together with the next-hop domain in peer
   certificate name checks.

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

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

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

   The SMTP servers mx1.example.com and mx2.example.com will be expected
   to have certificates issued under a common trust anchor, but each MX
   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".



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

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

3.  DANE authentication

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

3.1.  TLSA certificate usages

   The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
   via combinations of 3 numeric parameters.  The numeric values of
   these parameters were later given symbolic names in [RFC7218].  The
   rest of the TLSA record is the "certificate association data field",
   which specifies the full or digest value of a certificate or public
   key.

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





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   The certificate usage element of a TLSA record plays a critical role
   in determining how the corresponding certificate association data
   field is used to authenticate server's certificate chain.  The next
   two subsections explain the process for certificate usages DANE-EE(3)
   and DANE-TA(2).  The third subsection briefly explains why
   certificate usages PKIX-TA(0) and PKIX-EE(1) are not applicable with
   opportunistic DANE TLS.

   In summary, we RECOMMEND the use of "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 the following two subsections
   for more details.  Other combinations of TLSA parameters are either
   explicitly unsupported, or offer little to recommend them over these
   two.

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, and don't fail when the server operator neglects
   to configure all the required issuer certificates in the server
   certificate chain.

   TLSA records published for SMTP servers SHOULD, in most cases, be
   "DANE-EE(3) SPKI(1) SHA2-256(1)" records.  Since all DANE
   implementations are required to support SHA2-256, this record type




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   works for all clients and need not change across certificate renewals
   with the same key.

3.1.2.  Certificate usage DANE-TA(2)

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

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

   With usage DANE-TA(2) the server certificates will need to have names
   that match one of the client's reference identifiers (see [RFC6125]).
   The server MAY employ SNI to select the appropriate certificate to
   present to the client.

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

   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 matching type 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



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   from DNS, especially when the TLSA record supplies a bare key
   (selector SPKI(1)).  Since the server will need to transmit the TA
   certificate in any case, server operators SHOULD publish TLSA records
   with a 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, 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 Section 1.3.1 and Section 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 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



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   the certificates of SMTP servers whose issuing root CAs are not
   trusted by the client.

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

   SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
   or PKIX-EE(1) is undefined.  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); see Section 3.1.1 above.

3.2.2.  DANE-TA(2) name checks

   To match a server via a TLSA record with certificate usage DANE-
   TA(2), the client MUST perform name checks to ensure that it has
   reached the correct server.  In all DANE-TA(2) cases the SMTP client
   MUST 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 as a second reference
      identifier.  The CNAME-expanded original next-hop domain MUST be
      included as a third reference identifier if different from the
      original next-hop domain.  When the client MTA is employing DANE
      TLS security despite "insecure" MX redirection the MX hostname is
      the only reference identifier.

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



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




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

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
   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.



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   SMTP servers cannot expect broad CRL or 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 any deprecated
   weaker algorithms that are published for compatibility with less
   capable clients, but should be ignored when possible.  Such a
   protocol is specified in [I-D.ietf-dane-ops].  SMTP clients and
   servers that implement this specification MUST comply with the
   requirements outlined under "Digest Algorithm Agility" in
   [I-D.ietf-dane-ops].

6.  Mandatory TLS Security

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

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

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








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7.  Note on DANE for Message User Agents

   We note that the SMTP protocol is also used between Message User
   Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409].  In
   [RFC6186] a protocol is specified that enables an MUA to dynamically
   locate the MSA based on the user's email address.  SMTP connection
   security considerations for MUAs implementing [RFC6186] are largely
   analogous to connection security requirements for MTAs, and this
   specification could be applied largely verbatim with DNS MX records
   replaced by corresponding DNS Service (SRV) records
   [I-D.ietf-dane-srv].

   However, until MUAs begin to adopt the dynamic configuration
   mechanisms of [RFC6186] they are adequately served by more
   traditional static TLS security policies.  Specification of DANE TLS
   for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
   is left to future documents that focus specifically on SMTP security
   between MUAs and MSAs.

8.  Interoperability considerations

8.1.  SNI support

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



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8.2.  Anonymous TLS cipher suites

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

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

9.  Operational Considerations

9.1.  Client Operational Considerations

   An operational error on the sending or receiving side that cannot be
   corrected in a timely manner may, at times, lead to consistent
   failure to deliver time-sensitive email.  The sending MTA
   administrator may have to choose between letting email queue until
   the error is resolved and disabling opportunistic or mandatory DANE
   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 that opportunistic DANE TLS is
   not to be used for a particular destination, an SMTP client MUST NOT
   deliver mail via a server whose certificate chain fails to match at




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   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", for example 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 the "TLSA Publisher
   Requirements" section of [I-D.ietf-dane-ops].

   TLSA Publishers SHOULD follow the TLSA publication size guidance
   found in [I-D.ietf-dane-ops] under "DANE DNS Record Size Guidelines".

   TLSA Publishers SHOULD follow the TLSA record TTL and signature
   lifetime recommendations found in [I-D.ietf-dane-ops] under
   "Operational Considerations".

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



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   attacker to publish TLSA usage 3 certificate associations, and
   thereby bypass any security benefit the legitimate domain owner might
   hope to gain by publishing usage 0 or 1 TLSA RRs.  Given the lack of
   public CA PKI support in existing MTA deployments, avoiding
   certificate usages 0 and 1 simplifies implementation and deployment
   with no adverse security consequences.

   Implementations must strictly follow the portions of this
   specification that indicate when it is appropriate to initiate a non-
   authenticated connection or cleartext connection to a SMTP server.
   Specifically, in order to prevent downgrade attacks on this protocol,
   implementation must not initiate a connection when this specification
   indicates a particular SMTP server must be considered unreachable.

11.  IANA considerations

   This specification requires no support from IANA.

12.  Acknowledgements

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

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

13.  References

13.1.  Normative References

   [I-D.ietf-dane-ops]
              Dukhovni, V. and W. Hardaker, "Updates to and Operational
              Guidance for the DANE Protocol", draft-ietf-dane-ops-09
              (work in progress), May 2015.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

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

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, March 2011.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.




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   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, April 2014.

13.2.  Informative References

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", draft-ietf-dane-srv-14 (work in
              progress), April 2015.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, December 2014.

Authors' Addresses

   Viktor Dukhovni
   Two Sigma

   Email: ietf-dane@dukhovni.org


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

   Email: ietf@hardakers.net









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