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http://git.exim.org/exim.git/commitdiff/22e6f2949abfd9a4f167948a5f936a51d3203e98
Commit: 22e6f2949abfd9a4f167948a5f936a51d3203e98
Parent: b4161d10ee4c4eb7fd61224d827cc89726e2d8f8
Author: Jeremy Harris <jgh146exb@???>
AuthorDate: Sun Aug 10 14:43:59 2014 +0100
Committer: Jeremy Harris <jgh146exb@???>
CommitDate: Sun Aug 10 14:43:59 2014 +0100
Copy latest SMTP-with-DANE - draft 11
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+
+
+
+
+DANE V. Dukhovni
+Internet-Draft Two Sigma
+Intended status: Standards Track W. Hardaker
+Expires: February 3, 2015 Parsons
+ August 2, 2014
+
+
+ SMTP security via opportunistic DANE TLS
+ draft-ietf-dane-smtp-with-dane-11
+
+Abstract
+
+ This memo describes a downgrade-resistant protocol for SMTP transport
+ security between Mail Transfer Agents (MTAs) based on the DNS-Based
+ Authentication of Named Entities (DANE) TLSA DNS record. Adoption of
+ this protocol enables an incremental transition of the Internet email
+ backbone to one using encrypted and authenticated Transport Layer
+ Security (TLS).
+
+Status of This Memo
+
+ This Internet-Draft is submitted in full conformance with the
+ provisions of BCP 78 and BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF). Note that other groups may also distribute
+ working documents as Internet-Drafts. The list of current Internet-
+ Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ This Internet-Draft will expire on February 3, 2015.
+
+Copyright Notice
+
+ Copyright (c) 2014 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+
+
+
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+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5
+ 1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6
+ 1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6
+ 1.3.2. Insecure server name without DNSSEC . . . . . . . . . 7
+ 1.3.3. Sender policy does not scale . . . . . . . . . . . . 8
+ 1.3.4. Too many certification authorities . . . . . . . . . 8
+ 2. Identifying applicable TLSA records . . . . . . . . . . . . . 9
+ 2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 9
+ 2.1.1. DNS errors, bogus and indeterminate responses . . . . 9
+ 2.1.2. DNS error handling . . . . . . . . . . . . . . . . . 11
+ 2.1.3. Stub resolver considerations . . . . . . . . . . . . 12
+ 2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 13
+ 2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 14
+ 2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 15
+ 2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17
+ 3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19
+ 3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19
+ 3.1.1. Certificate usage DANE-EE(3) . . . . . . . . . . . . 21
+ 3.1.2. Certificate usage DANE-TA(2) . . . . . . . . . . . . 22
+ 3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) . . . . 23
+ 3.2. Certificate matching . . . . . . . . . . . . . . . . . . 24
+ 3.2.1. DANE-EE(3) name checks . . . . . . . . . . . . . . . 24
+ 3.2.2. DANE-TA(2) name checks . . . . . . . . . . . . . . . 24
+ 3.2.3. Reference identifier matching . . . . . . . . . . . . 25
+ 4. Server key management . . . . . . . . . . . . . . . . . . . . 26
+ 5. Digest algorithm agility . . . . . . . . . . . . . . . . . . 26
+ 6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 28
+ 7. Note on DANE for Message User Agents . . . . . . . . . . . . 29
+ 8. Interoperability considerations . . . . . . . . . . . . . . . 29
+ 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 29
+ 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 30
+ 9. Operational Considerations . . . . . . . . . . . . . . . . . 30
+ 9.1. Client Operational Considerations . . . . . . . . . . . . 30
+ 9.2. Publisher Operational Considerations . . . . . . . . . . 31
+ 10. Security Considerations . . . . . . . . . . . . . . . . . . . 31
+ 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 32
+ 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
+ 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
+ 13.1. Normative References . . . . . . . . . . . . . . . . . . 33
+ 13.2. Informative References . . . . . . . . . . . . . . . . . 34
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
+
+
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+1. Introduction
+
+ This memo specifies a new connection security model for Message
+ Transfer Agents (MTAs). This model is motivated by key features of
+ inter-domain SMTP delivery, in particular the fact that the
+ destination server is selected indirectly via DNS Mail Exchange (MX)
+ records and that neither email addresses nor MX hostnames signal a
+ requirement for either secure or cleartext transport. Therefore,
+ aside from a few manually configured exceptions, SMTP transport
+ security is of necessity opportunistic.
+
+ This specification uses the presence of DANE TLSA records to securely
+ signal TLS support and to publish the means by which SMTP clients can
+ successfully authenticate legitimate SMTP servers. This becomes
+ "opportunistic DANE TLS" and is resistant to downgrade and man-in-
+ the-middle (MITM) attacks. It enables an incremental transition of
+ the email backbone to authenticated TLS delivery, with increased
+ global protection as adoption increases.
+
+ With opportunistic DANE TLS, traffic from SMTP clients to domains
+ that publish "usable" DANE TLSA records in accordance with this memo
+ is authenticated and encrypted. Traffic from legacy clients or to
+ domains that do not publish TLSA records will continue to be sent in
+ the same manner as before, via manually configured security, (pre-
+ DANE) opportunistic TLS or just cleartext SMTP.
+
+ Problems with existing use of TLS in MTA to MTA SMTP that motivate
+ this specification are described in Section 1.3. The specification
+ itself follows in Section 2 and Section 3 which describe respectively
+ how to locate and use DANE TLSA records with SMTP. In Section 6, we
+ discuss application of DANE TLS to destinations for which channel
+ integrity and confidentiality are mandatory. In Section 7 we briefly
+ comment on potential applicability of this specification to Message
+ User Agents.
+
+1.1. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+ "OPTIONAL" in this document are to be interpreted as described in
+ [RFC2119].
+
+ The following terms or concepts are used through the document:
+
+ Man-in-the-middle or MITM attack: Active modification of network
+ traffic by an adversary able to thereby compromise the
+ confidentiality or integrity of the data.
+
+
+
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+ secure, bogus, insecure, indeterminate: DNSSEC validation results,
+ as defined in Section 4.3 of [RFC4035].
+
+ Validating Security-Aware Stub Resolver and Non-Validating
+ Security-Aware Stub Resolver:
+ Capabilities of the stub resolver in use as defined in [RFC4033];
+ note that this specification requires the use of a Security-Aware
+ Stub Resolver.
+
+ (pre-DANE) opportunistic TLS: Best-effort use of TLS that is
+ generally vulnerable to DNS forgery and STARTTLS downgrade
+ attacks. When a TLS-encrypted communication channel is not
+ available, message transmission takes place in the clear. MX
+ record indirection generally precludes authentication even when
+ TLS is available.
+
+ opportunistic DANE TLS: Best-effort use of TLS, resistant to
+ downgrade attacks for destinations with DNSSEC-validated TLSA
+ records. When opportunistic DANE TLS is determined to be
+ unavailable, clients should fall back to opportunistic TLS.
+ Opportunistic DANE TLS requires support for DNSSEC, DANE and
+ STARTTLS on the client side and STARTTLS plus a DNSSEC published
+ TLSA record on the server side.
+
+ reference identifier: (Special case of [RFC6125] definition). One
+ of the domain names associated by the SMTP client with the
+ destination SMTP server for performing name checks on the server
+ certificate. When name checks are applicable, at least one of the
+ reference identifiers MUST match an [RFC6125] DNS-ID (or if none
+ are present the [RFC6125] CN-ID) of the server certificate (see
+ Section 3.2.3).
+
+ MX hostname: The RRDATA of an MX record consists of a 16 bit
+ preference followed by a Mail Exchange domain name (see [RFC1035],
+ Section 3.3.9). We will use the term "MX hostname" to refer to
+ the latter, that is, the DNS domain name found after the
+ preference value in an MX record. Thus an "MX hostname" is
+ specifically a reference to a DNS domain name, rather than any
+ host that bears that name.
+
+ delayed delivery: Email delivery is a multi-hop store & forward
+ process. When an MTA is unable forward a message that may become
+ deliverable later the message is queued and delivery is retried
+ periodically. Some MTAs may be configured with a fallback next-
+ hop destination that handles messages that the MTA would otherwise
+ queue and retry. When a fallback next-hop is configured, messages
+ that would otherwise have to be delayed may be sent to the
+ fallback next-hop destination instead. The fallback destination
+
+
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+ may itself be subject to opportunistic or mandatory DANE TLS as
+ though it were the original message destination.
+
+ original next hop destination: The logical destination for mail
+ delivery. By default this is the domain portion of the recipient
+ address, but MTAs may be configured to forward mail for some or
+ all recipients via designated relays. The original next hop
+ destination is, respectively, either the recipient domain or the
+ associated configured relay.
+
+ MTA: Message Transfer Agent ([RFC5598], Section 4.3.2).
+
+ MSA: Message Submission Agent ([RFC5598], Section 4.3.1).
+
+ MUA: Message User Agent ([RFC5598], Section 4.2.1).
+
+ RR: A DNS Resource Record
+
+ RRset: A set of DNS Resource Records for a particular class, domain
+ and record type.
+
+1.2. Background
+
+ The Domain Name System Security Extensions (DNSSEC) add data origin
+ authentication, data integrity and data non-existence proofs to the
+ Domain Name System (DNS). DNSSEC is defined in [RFC4033], [RFC4034]
+ and [RFC4035].
+
+ As described in the introduction of [RFC6698], TLS authentication via
+ the existing public Certification Authority (CA) PKI suffers from an
+ over-abundance of trusted parties capable of issuing certificates for
+ any domain of their choice. DANE leverages the DNSSEC infrastructure
+ to publish trusted public keys and certificates for use with the
+ Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA"
+ DNS record type. With DNSSEC each domain can only vouch for the keys
+ of its directly delegated sub-domains.
+
+ The TLS protocol enables secure TCP communication. In the context of
+ this memo, channel security is assumed to be provided by TLS. Used
+ without authentication, TLS provides only privacy protection against
+ eavesdropping attacks. With authentication, TLS also provides data
+ integrity protection to guard against MITM attacks.
+
+
+
+
+
+
+
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+1.3. SMTP channel security
+
+ With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
+ [RFC5280] issued by one of the many Certificate Authorities (CAs)
+ bundled with popular web browsers to allow users to authenticate
+ their "secure" websites. Before we specify a new DANE TLS security
+ model for SMTP, we will explain why a new security model is needed.
+ In the process, we will explain why the familiar HTTPS security model
+ is inadequate to protect inter-domain SMTP traffic.
+
+ The subsections below outline four key problems with applying
+ traditional PKI to SMTP that are addressed by this specification.
+ Since SMTP channel security policy is not explicitly specified in
+ either the recipient address or the MX record, a new signaling
+ mechanism is required to indicate when channel security is possible
+ and should be used. The publication of TLSA records allows server
+ operators to securely signal to SMTP clients that TLS is available
+ and should be used. DANE TLSA makes it possible to simultaneously
+ discover which destination domains support secure delivery via TLS
+ and how to verify the authenticity of the associated SMTP services,
+ providing a path forward to ubiquitous SMTP channel security.
+
+1.3.1. STARTTLS downgrade attack
+
+ The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop
+ protocol in a multi-hop store & forward email delivery process. An
+ SMTP envelope recipient address does not correspond to a specific
+ transport-layer endpoint address, rather at each relay hop the
+ transport-layer endpoint is the next-hop relay, while the envelope
+ recipient address typically remains the same. Unlike the Hypertext
+ Transfer Protocol (HTTP) and its corresponding secured version,
+ HTTPS, where the use of TLS is signaled via the URI scheme, email
+ recipient addresses do not directly signal transport security policy.
+ Indeed, no such signaling could work well with SMTP since TLS
+ encryption of SMTP protects email traffic on a hop-by-hop basis while
+ email addresses could only express end-to-end policy.
+
+
+
+
+
+
+
+
+
+
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+ With no mechanism available to signal transport security policy, SMTP
+ relays employ a best-effort "opportunistic" security model for TLS.
+ A single SMTP server TCP listening endpoint can serve both TLS and
+ non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
+ command ([RFC3207]). The server signals TLS support to the client
+ over a cleartext SMTP connection, and, if the client also supports
+ TLS, it may negotiate a TLS encrypted channel to use for email
+ transmission. The server's indication of TLS support can be easily
+ suppressed by an MITM attacker. Thus pre-DANE SMTP TLS security can
+ be subverted by simply downgrading a connection to cleartext. No TLS
+ security feature, such as the use of PKIX, can prevent this. The
+ attacker can simply disable TLS.
+
+1.3.2. Insecure server name without DNSSEC
+
+ With SMTP, DNS Mail Exchange (MX) records abstract the next-hop
+ transport endpoint and allow administrators to specify a set of
+ target servers to which SMTP traffic should be directed for a given
+ domain.
+
+ A PKIX TLS client is vulnerable to MITM attacks unless it verifies
+ that the server's certificate binds the public key to a name that
+ matches one of the client's reference identifiers. A natural choice
+ of reference identifier is the server's domain name. However, with
+ SMTP, server names are not directly encoded in the recipient address,
+ instead they are obtained indirectly via MX records. Without DNSSEC,
+ the MX lookup is vulnerable to MITM and DNS cache poisoning attacks.
+ Active attackers can forge DNS replies with fake MX records and can
+ redirect email to servers with names of their choice. Therefore,
+ secure verification of SMTP TLS certificates matching the server name
+ is not possible without DNSSEC.
+
+ One might try to harden TLS for SMTP against DNS attacks by using the
+ envelope recipient domain as a reference identifier and requiring
+ each SMTP server to possess a trusted certificate for the envelope
+ recipient domain rather than the MX hostname. Unfortunately, this is
+ impractical as email for many domains is handled by third parties
+ that are not in a position to obtain certificates for all the domains
+ they serve. Deployment of the Server Name Indication (SNI) extension
+ to TLS (see [RFC6066] Section 3) is no panacea, since SNI key
+ management is operationally challenging except when the email service
+ provider is also the domain's registrar and its certificate issuer;
+ this is rarely the case for email.
+
+ Since the recipient domain name cannot be used as the SMTP server
+ reference identifier, and neither can the MX hostname without DNSSEC,
+ large-scale deployment of authenticated TLS for SMTP requires that
+ the DNS be secure.
+
+
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+ Since SMTP security depends critically on DNSSEC, it is important to
+ point out that consequently SMTP with DANE is the most conservative
+ possible trust model. It trusts only what must be trusted and no
+ more. Adding any other trusted actors to the mix can only reduce
+ SMTP security. A sender may choose to further harden DNSSEC for
+ selected high-value receiving domains by configuring explicit trust
+ anchors for those domains instead of relying on the chain of trust
+ from the root domain. However, detailed discussion of DNSSEC
+ security practices is out of scope for this document.
+
+1.3.3. Sender policy does not scale
+
+ Sending systems are in some cases explicitly configured to use TLS
+ for mail sent to selected peer domains. This requires sending MTAs
+ to be configured with appropriate subject names or certificate
+ content digests to expect in the presented server certificates.
+ Because of the heavy administrative burden, such statically
+ configured SMTP secure channels are used rarely (generally only
+ between domains that make bilateral arrangements with their business
+ partners). Internet email, on the other hand, requires regularly
+ contacting new domains for which security configurations cannot be
+ established in advance.
+
+ The abstraction of the SMTP transport endpoint via DNS MX records,
+ often across organization boundaries, limits the use of public CA PKI
+ with SMTP to a small set of sender-configured peer domains. With
+ little opportunity to use TLS authentication, sending MTAs are rarely
+ configured with a comprehensive list of trusted CAs. SMTP services
+ that support STARTTLS often deploy X.509 certificates that are self-
+ signed or issued by a private CA.
+
+1.3.4. Too many certification authorities
+
+ Even if it were generally possible to determine a secure server name,
+ the SMTP client would still need to verify that the server's
+ certificate chain is issued by a trusted Certification Authority (a
+ trust anchor). MTAs are not interactive applications where a human
+ operator can make a decision (wisely or otherwise) to selectively
+ disable TLS security policy when certificate chain verification
+ fails. With no user to "click OK", the MTA's list of public CA trust
+ anchors would need to be comprehensive in order to avoid bouncing
+ mail addressed to sites that employ unknown Certification
+ Authorities.
+
+
+
+
+
+
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+ On the other hand, each trusted CA can issue certificates for any
+ domain. If even one of the configured CAs is compromised or operated
+ by an adversary, it can subvert TLS security for all destinations.
+ Any set of CAs is simultaneously both overly inclusive and not
+ inclusive enough.
+
+2. Identifying applicable TLSA records
+
+2.1. DNS considerations
+
+2.1.1. DNS errors, bogus and indeterminate responses
+
+ An SMTP client that implements opportunistic DANE TLS per this
+ specification depends critically on the integrity of DNSSEC lookups,
+ as discussed in Section 1.3.2. This section lists the DNS resolver
+ requirements needed to avoid downgrade attacks when using
+ opportunistic DANE TLS.
+
+ A DNS lookup may signal an error or return a definitive answer. A
+ security-aware resolver must be used for this specification.
+ Security-aware resolvers will indicate the security status of a DNS
+ RRset with one of four possible values defined in Section 4.3 of
+ [RFC4035]: "secure", "insecure", "bogus" and "indeterminate". In
+ [RFC4035] the meaning of the "indeterminate" security status is:
+
+ An RRset for which the resolver is not able to determine whether
+ the RRset should be signed, as the resolver is not able to obtain
+ the necessary DNSSEC RRs. This can occur when the security-aware
+ resolver is not able to contact security-aware name servers for
+ the relevant zones.
+
+ Note, the "indeterminate" security status has a conflicting
+ definition in section 5 of [RFC4033].
+
+ There is no trust anchor that would indicate that a specific
+ portion of the tree is secure.
+
+ To avoid further confusion, the adjective "anchorless" will be used
+ below to refer to domains or RRsets that are "indeterminate" in the
+ [RFC4033] sense, and the term "indeterminate" will be used
+ exclusively in the sense of [RFC4035].
+
+ SMTP clients following this specification SHOULD NOT distinguish
+ between "insecure" and "anchorless" DNS responses. Both "insecure"
+ and "anchorless" RRsets MUST be handled identically: in either case
+ unvalidated data for the query domain is all that is and can be
+ available, and authentication using the data is impossible. In what
+ follows, the term "insecure" will also includes the case of
+
+
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+ "anchorless" domains that lie in a portion of the DNS tree for which
+ there is no applicable trust anchor. With the DNS root zone signed,
+ we expect that validating resolvers used by Internet-facing MTAs will
+ be configured with trust anchor data for the root zone, and that
+ therefore "anchorless" domains should be rare in practice.
+
+ As noted in section 4.3 of [RFC4035], a security-aware DNS resolver
+ MUST be able to determine whether a given non-error DNS response is
+ "secure", "insecure", "bogus" or "indeterminate". It is expected
+ that most security-aware stub resolvers will not signal an
+ "indeterminate" security status (in the sense of RFC4035) to the
+ application, and will signal a "bogus" or error result instead. If a
+ resolver does signal an RFC4035 "indeterminate" security status, this
+ MUST be treated by the SMTP client as though a "bogus" or error
+ result had been returned.
+
+ An MTA making use of a non-validating security-aware stub resolver
+ MAY use the stub resolver's ability, if available, to signal DNSSEC
+ validation status based on information the stub resolver has learned
+ from an upstream validating recursive resolver. Security-Oblivious
+ stub-resolvers MUST NOT be used. In accordance with section 4.9.3 of
+ [RFC4035]:
+
+ ... a security-aware stub resolver MUST NOT place any reliance on
+ signature validation allegedly performed on its behalf, except
+ when the security-aware stub resolver obtained the data in question
+ from a trusted security-aware recursive name server via a secure
+ channel.
+
+ To avoid much repetition in the text below, we will pause to explain
+ the handling of "bogus" or "indeterminate" DNSSEC query responses.
+ These are not necessarily the result of a malicious actor; they can,
+ for example, occur when network packets are corrupted or lost in
+ transit. Therefore, "bogus" or "indeterminate" replies are equated
+ in this memo with lookup failure.
+
+ There is an important non-failure condition we need to highlight in
+ addition to the obvious case of the DNS client obtaining a non-empty
+ "secure" or "insecure" RRset of the requested type. Namely, it is
+ not an error when either "secure" or "insecure" non-existence is
+ determined for the requested data. When a DNSSEC response with a
+ validation status that is either "secure" or "insecure" reports
+ either no records of the requested type or non-existence of the query
+ domain, the response is not a DNS error condition. The DNS client
+ has not been left without an answer; it has learned that records of
+ the requested type do not exist.
+
+
+
+
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+ Security-aware stub resolvers will, of course, also signal DNS lookup
+ errors in other cases, for example when processing a "ServFail"
+ RCODE, which will not have an associated DNSSEC status. All lookup
+ errors are treated the same way by this specification, regardless of
+ whether they are from a "bogus" or "indeterminate" DNSSEC status or
+ from a more generic DNS error: the information that was requested
+ cannot be obtained by the security-aware resolver at this time. A
+ lookup error is thus a failure to obtain the relevant RRset if it
+ exists, or to determine that no such RRset exists when it does not.
+
+ In contrast to a "bogus" or an "indeterminate" response, an
+ "insecure" DNSSEC response is not an error, rather it indicates that
+ the target DNS zone is either securely opted out of DNSSEC validation
+ or is not connected with the DNSSEC trust anchors being used.
+ Insecure results will leave the SMTP client with degraded channel
+ security, but do not stand in the way of message delivery. See
+ section Section 2.2 for further details.
+
+2.1.2. DNS error handling
+
+ When a DNS lookup failure (error or "bogus" or "indeterminate" as
+ defined above) prevents an SMTP client from determining which SMTP
+ server or servers it should connect to, message delivery MUST be
+ delayed. This naturally includes, for example, the case when a
+ "bogus" or "indeterminate" response is encountered during MX
+ resolution. When multiple MX hostnames are obtained from a
+ successful MX lookup, but a later DNS lookup failure prevents network
+ address resolution for a given MX hostname, delivery may proceed via
+ any remaining MX hosts.
+
+ When a particular SMTP server is securely identified as the delivery
+ destination, a set of DNS lookups (Section 2.2) MUST be performed to
+ locate any related TLSA records. If any DNS queries used to locate
+ TLSA records fail (be it due to "bogus" or "indeterminate" records,
+ timeouts, malformed replies, ServFails, etc.), then the SMTP client
+ MUST treat that server as unreachable and MUST NOT deliver the
+ message via that server. If no servers are reachable, delivery is
+ delayed.
+
+ In what follows, we will only describe what happens when all relevant
+ DNS queries succeed. If any DNS failure occurs, the SMTP client MUST
+ behave as described in this section, by skipping the problem SMTP
+ server, or the problem destination. Queries for candidate TLSA
+ records are explicitly part of "all relevant DNS queries" and SMTP
+ clients MUST NOT continue to connect to an SMTP server or destination
+ whose TLSA record lookup fails.
+
+
+
+
+
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+2.1.3. Stub resolver considerations
+
+ SMTP clients that employ opportunistic DANE TLS to secure connections
+ to SMTP servers MUST NOT use Security-Oblivious stub-resolvers.
+
+ A note about DNAME aliases: a query for a domain name whose ancestor
+ domain is a DNAME alias returns the DNAME RR for the ancestor domain
+ along with a CNAME that maps the query domain to the corresponding
+ sub-domain of the target domain of the DNAME alias [RFC6672].
+ Therefore, whenever we speak of CNAME aliases, we implicitly allow
+ for the possibility that the alias in question is the result of an
+ ancestor domain DNAME record. Consequently, no explicit support for
+ DNAME records is needed in SMTP software; it is sufficient to process
+ the resulting CNAME aliases. DNAME records only require special
+ processing in the validating stub-resolver library that checks the
+ integrity of the combined DNAME + CNAME reply. When DNSSEC
+ validation is handled by a local caching resolver, rather than the
+ MTA itself, even that part of the DNAME support logic is outside the
+ MTA.
+
+ When a stub resolver returns a response containing a CNAME alias that
+ does not also contain the corresponding query results for the target
+ of the alias, the SMTP client will need to repeat the query at the
+ target of the alias, and should do so recursively up to some
+ configured or implementation-dependent recursion limit. If at any
+ stage of CNAME expansion an error is detected, the lookup of the
+ original requested records MUST be considered to have failed.
+
+ Whether a chain of CNAME records was returned in a single stub
+ resolver response or via explicit recursion by the SMTP client, if at
+ any stage of recursive expansion an "insecure" CNAME record is
+ encountered, then it and all subsequent results (in particular, the
+ final result) MUST be considered "insecure" regardless of whether any
+ earlier CNAME records leading to the "insecure" record were "secure".
+
+ Note that a security-aware non-validating stub resolver may return to
+ the SMTP client an "insecure" reply received from a validating
+ recursive resolver that contains a CNAME record along with additional
+ answers recursively obtained starting at the target of the CNAME. In
+ this case, the only possible conclusion is that some record in the
+ set of records returned is "insecure", and it is in fact possible
+ that the initial CNAME record and a subset of the subsequent records
+ are "secure".
+
+ If the SMTP client needs to determine the security status of the DNS
+ zone containing the initial CNAME record, it may need to issue a
+ separate query of type "CNAME" that returns only the initial CNAME
+ record. In particular in Section 2.2.2 when insecure A or AAAA
+
+
+
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+ records are found for an SMTP server via a CNAME alias, it may be
+ necessary to perform an additional CNAME query to determine whether
+ the DNS zone in which the alias is published is signed.
+
+2.2. TLS discovery
+
+ As noted previously (in Section 1.3.1), opportunistic TLS with SMTP
+ servers that advertise TLS support via STARTTLS is subject to an MITM
+ downgrade attack. Also some SMTP servers that are not, in fact, TLS
+ capable erroneously advertise STARTTLS by default and clients need to
+ be prepared to retry cleartext delivery after STARTTLS fails. In
+ contrast, DNSSEC validated TLSA records MUST NOT be published for
+ servers that do not support TLS. Clients can safely interpret their
+ presence as a commitment by the server operator to implement TLS and
+ STARTTLS.
+
+ This memo defines four actions to be taken after the search for a
+ TLSA record returns secure usable results, secure unusable results,
+ insecure or no results or an error signal. The term "usable" in this
+ context is in the sense of Section 4.1 of [RFC6698]. Specifically,
+ if the DNS lookup for a TLSA record returns:
+
+ A secure TLSA RRset with at least one usable record: A connection to
+ the MTA MUST be made using authenticated and encrypted TLS, using
+ the techniques discussed in the rest of this document. Failure to
+ establish an authenticated TLS connection MUST result in falling
+ back to the next SMTP server or delayed delivery.
+
+ A secure non-empty TLSA RRset where all the records are unusable: A
+ connection to the MTA MUST be made via TLS, but authentication is
+ not required. Failure to establish an encrypted TLS connection
+ MUST result in falling back to the next SMTP server or delayed
+ delivery.
+
+ An insecure TLSA RRset or DNSSEC validated proof-of-non-existent TLSA
+ records:
+ A connection to the MTA SHOULD be made using (pre-DANE)
+ opportunistic TLS, this includes using cleartext delivery when the
+ remote SMTP server does not appear to support TLS. The MTA MAY
+ retry in cleartext when delivery via TLS fails either during the
+ handshake or even during data transfer.
+
+ Any lookup error: Lookup errors, including "bogus" and
+ "indeterminate", as explained in Section 2.1.1 MUST result in
+ falling back to the next SMTP server or delayed delivery.
+
+ An SMTP client MAY be configured to require DANE verified delivery
+ for some destinations. We will call such a configuration "mandatory
+
+
+
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+ DANE TLS". With mandatory DANE TLS, delivery proceeds only when
+ "secure" TLSA records are used to establish an encrypted and
+ authenticated TLS channel with the SMTP server.
+
+ When the original next-hop destination is an address literal, rather
+ than a DNS domain, DANE TLS does not apply. Delivery proceeds using
+ any relevant security policy configured by the MTA administrator.
+ Similarly, when an MX RRset incorrectly lists a network address in
+ lieu of an MX hostname, if an MTA chooses to connect to the network
+ address in the non-conformat MX record, DANE TLSA does not apply for
+ such a connection.
+
+ In the subsections that follow we explain how to locate the SMTP
+ servers and the associated TLSA records for a given next-hop
+ destination domain. We also explain which name or names are to be
+ used in identity checks of the SMTP server certificate.
+
+2.2.1. MX resolution
+
+ In this section we consider next-hop domains that are subject to MX
+ resolution and have MX records. The TLSA records and the associated
+ base domain are derived separately for each MX hostname that is used
+ to attempt message delivery. DANE TLS can authenticate message
+ delivery to the intended next-hop domain only when the MX records are
+ obtained securely via a DNSSEC validated lookup.
+
+ MX records MUST be sorted by preference; an MX hostname with a worse
+ (numerically higher) MX preference that has TLSA records MUST NOT
+ preempt an MX hostname with a better (numerically lower) preference
+ that has no TLSA records. In other words, prevention of delivery
+ loops by obeying MX preferences MUST take precedence over channel
+ security considerations. Even with two equal-preference MX records,
+ an MTA is not obligated to choose the MX hostname that offers more
+ security. Domains that want secure inbound mail delivery need to
+ ensure that all their SMTP servers and MX records are configured
+ accordingly.
+
+ In the language of [RFC5321] Section 5.1, the original next-hop
+ domain is the "initial name". If the MX lookup of the initial name
+ results in a CNAME alias, the MTA replaces the initial name with the
+ resulting name and performs a new lookup with the new name. MTAs
+ typically support recursion in CNAME expansion, so this replacement
+ is performed repeatedly (up to the MTA's recursion limit) until the
+ ultimate non-CNAME domain is found.
+
+ If the MX RRset (or any CNAME leading to it) is "insecure" (see
+ Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via
+ pre-DANE opportunistic TLS. That said, the protocol in this memo is
+
+
+
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+ an "opportunistic security" protocol, meaning that it strives to
+ communicate with each peer as securely as possible, while maintaining
+ broad interoperability. Therefore, the SMTP client MAY proceed to
+ use DANE TLS (as described in Section 2.2.2 below) even with MX hosts
+ obtained via an "insecure" MX RRset. For example, when a hosting
+ provider has a signed DNS zone and publishes TLSA records for its
+ SMTP servers, hosted domains that are not signed may still benefit
+ from the provider's TLSA records. Deliveries via the provider's SMTP
+ servers will not be subject to active attacks when sending SMTP
+ clients elect to make use of the provider's TLSA records.
+
+ When the MX records are not (DNSSEC) signed, an active attacker can
+ redirect SMTP clients to MX hosts of his choice. Such redirection is
+ tamper-evident when SMTP servers found via "insecure" MX records are
+ recorded as the next-hop relay in the MTA delivery logs in their
+ original (rather than CNAME expanded) form. Sending MTAs SHOULD log
+ unexpanded MX hostnames when these result from insecure MX lookups.
+ Any successful authentication via an insecurely determined MX host
+ MUST NOT be misrepresented in the mail logs as secure delivery to the
+ intended next-hop domain. When DANE TLS is mandatory (Section 6) for
+ a given destination, delivery MUST be delayed when the MX RRset is
+ not "secure".
+
+ Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is
+ "secure", and the SMTP client MUST treat each MX hostname as a
+ separate non-MX destination for opportunistic DANE TLS as described
+ in Section 2.2.2. When, for a given MX hostname, no TLSA records are
+ found, or only "insecure" TLSA records are found, DANE TLSA is not
+ applicable with the SMTP server in question and delivery proceeds to
+ that host as with pre-DANE opportunistic TLS. To avoid downgrade
+ attacks, any errors during TLSA lookups MUST, as explained in
+ Section 2.1.1, cause the SMTP server in question to be treated as
+ unreachable.
+
+2.2.2. Non-MX destinations
+
+ This section describes the algorithm used to locate the TLSA records
+ and associated TLSA base domain for an input domain not subject to MX
+ resolution. Such domains include:
+
+ o Each MX hostname used in a message delivery attempt for an
+ original next-hop destination domain subject to MX resolution.
+ Note, MTAs are not obligated to support CNAME expansion of MX
+ hostnames.
+
+ o Any administrator configured relay hostname, not subject to MX
+ resolution. This frequently involves configuration set by the MTA
+ administrator to handle some or all mail.
+
+
+
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+ o A next-hop destination domain subject to MX resolution that has no
+ MX records. In this case the domain's name is implicitly also its
+ sole SMTP server name.
+
+ Note that DNS queries with type TLSA are mishandled by load balancing
+ nameservers that serve the MX hostnames of some large email
+ providers. The DNS zones served by these nameservers are not signed
+ and contain no TLSA records, but queries for TLSA records fail,
+ rather than returning the non-existence of the requested TLSA
+ records.
+
+ To avoid problems delivering mail to domains whose SMTP servers are
+ served by the problem nameservers the SMTP client MUST perform any A
+ and/or AAAA queries for the destination before attempting to locate
+ the associated TLSA records. This lookup is needed in any case to
+ determine whether the destination domain is reachable and the DNSSEC
+ validation status of the chain of CNAME queries required to reach the
+ ultimate address records.
+
+ If no address records are found, the destination is unreachable. If
+ address records are found, but the DNSSEC validation status of the
+ first query response is "insecure" (see Section 2.1.3), the SMTP
+ client SHOULD NOT proceed to search for any associated TLSA records.
+ With the problem domains, TLSA queries will lead to DNS lookup errors
+ and cause messages to be consistently delayed and ultimately returned
+ to the sender. We don't expect to find any "secure" TLSA records
+ associated with a TLSA base domain that lies in an unsigned DNS zone.
+ Therefore, skipping TLSA lookups in this case will also reduce
+ latency with no detrimental impact on security.
+
+ If the A and/or AAAA lookup of the "initial name" yields a CNAME, we
+ replace it with the resulting name as if it were the initial name and
+ perform a lookup again using the new name. This replacement is
+ performed recursively (up to the MTA's recursion limit).
+
+ We consider the following cases for handling a DNS response for an A
+ or AAAA DNS lookup:
+
+ Not found: When the DNS queries for A and/or AAAA records yield
+ neither a list of addresses nor a CNAME (or CNAME expansion is not
+ supported) the destination is unreachable.
+
+
+
+
+
+
+
+
+
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+ Non-CNAME: The answer is not a CNAME alias. If the address RRset
+ is "secure", TLSA lookups are performed as described in
+ Section 2.2.3 with the initial name as the candidate TLSA base
+ domain. If no "secure" TLSA records are found, DANE TLS is not
+ applicable and mail delivery proceeds with pre-DANE opportunistic
+ TLS (which, being best-effort, degrades to cleartext delivery when
+ STARTTLS is not available or the TLS handshake fails).
+
+ Insecure CNAME: The input domain is a CNAME alias, but the ultimate
+ network address RRset is "insecure" (see Section 2.1.1). If the
+ initial CNAME response is also "insecure", DANE TLS does not
+ apply. Otherwise, this case is treated just like the non-CNAME
+ case above, where a search is performed for a TLSA record with the
+ original input domain as the candidate TLSA base domain.
+
+ Secure CNAME: The input domain is a CNAME alias, and the ultimate
+ network address RRset is "secure" (see Section 2.1.1). Two
+ candidate TLSA base domains are tried: the fully CNAME-expanded
+ initial name and, failing that, then the initial name itself.
+
+ In summary, if it is possible to securely obtain the full, CNAME-
+ expanded, DNSSEC-validated address records for the input domain, then
+ that name is the preferred TLSA base domain. Otherwise, the
+ unexpanded input-MX domain is the candidate TLSA base domain. When
+ no "secure" TLSA records are found at either the CNAME-expanded or
+ unexpanded domain, then DANE TLS does not apply for mail delivery via
+ the input domain in question. And, as always, errors, bogus or
+ indeterminate results for any query in the process MUST result in
+ delaying or abandoning delivery.
+
+2.2.3. TLSA record lookup
+
+ Each candidate TLSA base domain (the original or fully CNAME-expanded
+ name of a non-MX destination or a particular MX hostname of an MX
+ destination) is in turn prefixed with service labels of the form
+ "_<port>._tcp". The resulting domain name is used to issue a DNSSEC
+ query with the query type set to TLSA ([RFC6698] Section 7.1).
+
+ For SMTP, the destination TCP port is typically 25, but this may be
+ different with custom routes specified by the MTA administrator in
+ which case the SMTP client MUST use the appropriate number in the
+ "_<port>" prefix in place of "_25". If, for example, the candidate
+ base domain is "mx.example.com", and the SMTP connection is to port
+ 25, the TLSA RRset is obtained via a DNSSEC query of the form:
+
+ _25._tcp.mx.example.com. IN TLSA ?
+
+
+
+
+
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+ The query response may be a CNAME, or the actual TLSA RRset. If the
+ response is a CNAME, the SMTP client (through the use of its
+ security-aware stub resolver) restarts the TLSA query at the target
+ domain, following CNAMEs as appropriate and keeping track of whether
+ the entire chain is "secure". If any "insecure" records are
+ encountered, or the TLSA records don't exist, the next candidate TLSA
+ base domain is tried instead.
+
+ If the ultimate response is a "secure" TLSA RRset, then the candidate
+ TLSA base domain will be the actual TLSA base domain and the TLSA
+ RRset will constitute the TLSA records for the destination. If none
+ of the candidate TLSA base domains yield "secure" TLSA records then
+ delivery MAY proceed via pre-DANE opportunistic TLS. SMTP clients
+ MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades
+ or even to skip SMTP servers that fail authentication, but MUST NOT
+ misrepresent authentication success as either a secure connection to
+ the SMTP server or as a secure delivery to the intended next-hop
+ domain.
+
+ TLSA record publishers may leverage CNAMEs to reference a single
+ authoritative TLSA RRset specifying a common Certification Authority
+ or a common end entity certificate to be used with multiple TLS
+ services. Such CNAME expansion does not change the SMTP client's
+ notion of the TLSA base domain; thus, when _25._tcp.mx.example.com is
+ a CNAME, the base domain remains mx.example.com and this is still the
+ reference identifier used together with the next-hop domain in peer
+ certificate name checks.
+
+ Note that shared end entity certificate associations expose the
+ publishing domain to substitution attacks, where an MITM attacker can
+ reroute traffic to a different server that shares the same end entity
+ certificate. Such shared end entity TLSA records SHOULD be avoided
+ unless the servers in question are functionally equivalent or employ
+ mutually incompatible protocols (an active attacker gains nothing by
+ diverting client traffic from one such server to another).
+
+ A better example, employing a shared trust anchor rather than shared
+ end-entity certificates, is illustrated by the DNSSEC validated
+ records below:
+
+ example.com. IN MX 0 mx1.example.com.
+ example.com. IN MX 0 mx2.example.com.
+ _25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com.
+ _25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com.
+ tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c149a...
+
+ The SMTP servers mx1.example.com and mx2.example.com will be expected
+ to have certificates issued under a common trust anchor, but each MX
+
+
+
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+ hostname's TLSA base domain remains unchanged despite the above CNAME
+ records. Correspondingly, each SMTP server will be associated with a
+ pair of reference identifiers consisting of its hostname plus the
+ next-hop domain "example.com".
+
+ If, during TLSA resolution (including possible CNAME indirection), at
+ least one "secure" TLSA record is found (even if not usable because
+ it is unsupported by the implementation or support is
+ administratively disabled), then the corresponding host has signaled
+ its commitment to implement TLS. The SMTP client MUST NOT deliver
+ mail via the corresponding host unless a TLS session is negotiated
+ via STARTTLS. This is required to avoid MITM STARTTLS downgrade
+ attacks.
+
+ As noted previously (in Section Section 2.2.2), when no "secure" TLSA
+ records are found at the fully CNAME-expanded name, the original
+ unexpanded name MUST be tried instead. This supports customers of
+ hosting providers where the provider's zone cannot be validated with
+ DNSSEC, but the customer has shared appropriate key material with the
+ hosting provider to enable TLS via SNI. Intermediate names that
+ arise during CNAME expansion that are neither the original, nor the
+ final name, are never candidate TLSA base domains, even if "secure".
+
+3. DANE authentication
+
+ This section describes which TLSA records are applicable to SMTP
+ opportunistic DANE TLS and how to apply such records to authenticate
+ the SMTP server. With opportunistic DANE TLS, both the TLS support
+ implied by the presence of DANE TLSA records and the verification
+ parameters necessary to authenticate the TLS peer are obtained
+ together. In contrast to protocols where channel security policy is
+ set exclusively by the client, authentication via this protocol is
+ expected to be less prone to connection failure caused by
+ incompatible configuration of the client and server.
+
+3.1. TLSA certificate usages
+
+ The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
+ via combinations of 3 numeric parameters. The numeric values of
+ these parameters were later given symbolic names in [RFC7218]. The
+ rest of the TLSA record is the "certificate association data field",
+ which specifies the full or digest value of a certificate or public
+ key. The parameters are:
+
+
+
+
+
+
+
+
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+ The TLSA Certificate Usage field: Section 2.1.1 of [RFC6698]
+ specifies four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and
+ DANE-EE(3). There is an additional private-use value:
+ PrivCert(255). All other values are reserved for use by future
+ specifications.
+
+ The selector field: Section 2.1.2 of [RFC6698] specifies two values:
+ Cert(0) and SPKI(1). There is an additional private-use value:
+ PrivSel(255). All other values are reserved for use by future
+ specifications.
+
+ The matching type field: Section 2.1.3 of [RFC6698] specifies three
+ values: Full(0), SHA2-256(1) and SHA2-512(2). There is an
+ additional private-use value: PrivMatch(255). All other values
+ are reserved for use by future specifications.
+
+ We may think of TLSA Certificate Usage values 0 through 3 as a
+ combination of two one-bit flags. The low bit chooses between trust
+ anchor (TA) and end entity (EE) certificates. The high bit chooses
+ between public PKI issued and domain-issued certificates.
+
+ The selector field specifies whether the TLSA RR matches the whole
+ certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1). The
+ subjectPublicKeyInfo is an ASN.1 DER ([X.690]) encoding of the
+ certificate's algorithm id, any parameters and the public key data.
+
+ The matching type field specifies how the TLSA RR Certificate
+ Association Data field is to be compared with the certificate or
+ public key. A value of Full(0) means an exact match: the full DER
+ encoding of the certificate or public key is given in the TLSA RR. A
+ value of SHA2-256(1) means that the association data matches the
+ SHA2-256 digest of the certificate or public key, and likewise
+ SHA2-512(2) means a SHA2-512 digest is used.
+
+ Since opportunistic DANE TLS will be used by non-interactive MTAs,
+ with no user to "press OK" when authentication fails, reliability of
+ peer authentication is paramount. Server operators are advised to
+ publish TLSA records that are least likely to fail authentication due
+ to interoperability or operational problems. Because DANE TLS relies
+ on coordinated changes to DNS and SMTP server settings, the best
+ choice of records to publish will depend on site-specific practices.
+
+
+
+
+
+
+
+
+
+
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+ The certificate usage element of a TLSA record plays a critical role
+ in determining how the corresponding certificate association data
+ field is used to authenticate server's certificate chain. The next
+ two subsections explain the process for certificate usages DANE-EE(3)
+ and DANE-TA(2). The third subsection briefly explains why
+ certificate usages PKIX-TA(0) and PKIX-EE(1) are not applicable with
+ opportunistic DANE TLS.
+
+ In summary, we recommend the use of either "DANE-EE(3) SPKI(1)
+ SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records
+ depending on site needs. Other combinations of TLSA parameters are
+ either explicitly unsupported, or offer little to recommend them over
+ these two.
+
+ The mandatory to support digest algorithm in [RFC6698] is
+ SHA2-256(1). When the server's TLSA RRset includes records with a
+ matching type indicating a digest record (i.e., a value other than
+ Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be
+ provided along with any other digest published, since some SMTP
+ clients may support only SHA2-256(1). If at some point the SHA2-256
+ digest algorithm is tarnished by new cryptanalytic attacks,
+ publishers will need to include an appropriate stronger digest in
+ their TLSA records, initially along with, and ultimately in place of,
+ SHA2-256.
+
+3.1.1. Certificate usage DANE-EE(3)
+
+ Authentication via certificate usage DANE-EE(3) TLSA records involves
+ simply checking that the server's leaf certificate matches the TLSA
+ record. In particular the binding of the server public key to its
+ name is based entirely on the TLSA record association. The server
+ MUST be considered authenticated even if none of the names in the
+ certificate match the client's reference identity for the server.
+
+ Similarly, the expiration date of the server certificate MUST be
+ ignored, the validity period of the TLSA record key binding is
+ determined by the validity interval of the TLSA record DNSSEC
+ signature.
+
+ With DANE-EE(3) servers need not employ SNI (may ignore the client's
+ SNI message) even when the server is known under independent names
+ that would otherwise require separate certificates. It is instead
+ sufficient for the TLSA RRsets for all the domains in question to
+ match the server's default certificate. Of course with SMTP servers
+ it is simpler still to publish the same MX hostname for all the
+ hosted domains.
+
+
+
+
+
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+ For domains where it is practical to make coordinated changes in DNS
+ TLSA records during SMTP server key rotation, it is often best to
+ publish end-entity DANE-EE(3) certificate associations. DANE-EE(3)
+ certificates don't suddenly stop working when leaf or intermediate
+ certificates expire, and don't fail when the server operator neglects
+ to configure all the required issuer certificates in the server
+ certificate chain.
+
+ TLSA records published for SMTP servers SHOULD, in most cases, be
+ "DANE-EE(3) SPKI(1) SHA2-256(1)" records. Since all DANE
+ implementations are required to support SHA2-256, this record type
+ works for all clients and need not change across certificate renewals
+ with the same key.
+
+3.1.2. Certificate usage DANE-TA(2)
+
+ Some domains may prefer to avoid the operational complexity of
+ publishing unique TLSA RRs for each TLS service. If the domain
+ employs a common issuing Certification Authority to create
+ certificates for multiple TLS services, it may be simpler to publish
+ the issuing authority as a trust anchor (TA) for the certificate
+ chains of all relevant services. The TLSA query domain (TLSA base
+ domain with port and protocol prefix labels) for each service issued
+ by the same TA may then be set to a CNAME alias that points to a
+ common TLSA RRset that matches the TA. For example:
+
+ example.com. IN MX 0 mx1.example.com.
+ example.com. IN MX 0 mx2.example.com.
+ _25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com.
+ _25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com.
+ tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c14....
+
+ With usage DANE-TA(2) the server certificates will need to have names
+ that match one of the client's reference identifiers (see [RFC6125]).
+ The server MAY employ SNI to select the appropriate certificate to
+ present to the client.
+
+ SMTP servers that rely on certificate usage DANE-TA(2) TLSA records
+ for TLS authentication MUST include the TA certificate as part of the
+ certificate chain presented in the TLS handshake server certificate
+ message even when it is a self-signed root certificate. At this
+ time, many SMTP servers are not configured with a comprehensive list
+ of trust anchors, nor are they expected to at any point in the
+ future. Some MTAs will ignore all locally trusted certificates when
+ processing usage DANE-TA(2) TLSA records. Thus even when the TA
+ happens to be a public Certification Authority known to the SMTP
+ client, authentication is likely to fail unless the TA certificate is
+ included in the TLS server certificate message.
+
+
+
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+
+ TLSA records with selector Full(0) are discouraged. While these
+ potentially obviate the need to transmit the TA certificate in the
+ TLS server certificate message, client implementations may not be
+ able to augment the server certificate chain with the data obtained
+ from DNS, especially when the TLSA record supplies a bare key
+ (selector SPKI(1)). Since the server will need to transmit the TA
+ certificate in any case, server operators SHOULD publish TLSA records
+ with a selector other than Full(0) and avoid potential
+ interoperability issues with large TLSA records containing full
+ certificates or keys.
+
+ TLSA Publishers employing DANE-TA(2) records SHOULD publish records
+ with a selector of Cert(0). Such TLSA records are associated with
+ the whole trust anchor certificate, not just with the trust anchor
+ public key. In particular, the SMTP client SHOULD then apply any
+ relevant constraints from the trust anchor certificate, such as, for
+ example, path length constraints.
+
+ While a selector of SPKI(1) may also be employed, the resulting TLSA
+ record will not specify the full trust anchor certificate content,
+ and elements of the trust anchor certificate other than the public
+ key become mutable. This may, for example, allow a subsidiary CA to
+ issue a chain that violates the trust anchor's path length or name
+ constraints.
+
+3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1)
+
+ As noted in the introduction, SMTP clients cannot, without relying on
+ DNSSEC for secure MX records and DANE for STARTTLS support signaling,
+ perform server identity verification or prevent STARTTLS downgrade
+ attacks. The use of PKIX CAs offers no added security since an
+ attacker capable of compromising DNSSEC is free to replace any PKIX-
+ TA(0) or PKIX-EE(1) TLSA records with records bearing any convenient
+ non-PKIX certificate usage.
+
+ SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX-
+ TA(0) or PKIX-EE(1). SMTP clients cannot be expected to be
+ configured with a suitably complete set of trusted public CAs.
+ Lacking a complete set of public CAs, clients would not be able to
+ verify the certificates of SMTP servers whose issuing root CAs are
+ not trusted by the client.
+
+ Opportunistic DANE TLS needs to interoperate without bilateral
+ coordination of security settings between client and server systems.
+ Therefore, parameter choices that are fragile in the absence of
+ bilateral coordination are unsupported. Nothing is lost since the
+ PKIX certificate usages cannot aid SMTP TLS security, they can only
+ impede SMTP TLS interoperability.
+
+
+
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+
+ SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
+ or PKIX-EE(1) is undefined. SMTP clients should generally treat such
+ TLSA records as unusable.
+
+3.2. Certificate matching
+
+ When at least one usable "secure" TLSA record is found, the SMTP
+ client MUST use TLSA records to authenticate the SMTP server.
+ Messages MUST NOT be delivered via the SMTP server if authentication
+ fails, otherwise the SMTP client is vulnerable to MITM attacks.
+
+3.2.1. DANE-EE(3) name checks
+
+ The SMTP client MUST NOT perform certificate name checks with
+ certificate usage DANE-EE(3); see Section 3.1.1 above.
+
+3.2.2. DANE-TA(2) name checks
+
+ To match a server via a TLSA record with certificate usage DANE-
+ TA(2), the client MUST perform name checks to ensure that it has
+ reached the correct server. In all DANE-TA(2) cases the SMTP client
+ MUST include the TLSA base domain as one of the valid reference
+ identifiers for matching the server certificate.
+
+ TLSA records for MX hostnames: If the TLSA base domain was obtained
+ indirectly via a "secure" MX lookup (including any CNAME-expanded
+ name of an MX hostname), then the original next-hop domain used in
+ the MX lookup MUST be included as as a second reference
+ identifier. The CNAME-expanded original next-hop domain MUST be
+ included as a third reference identifier if different from the
+ original next-hop domain. When the client MTA is employing DANE
+ TLS security despite "insecure" MX redirection the MX hostname is
+ the only reference identifier.
+
+ TLSA records for Non-MX hostnames: If MX records were not used
+ (e.g., if none exist) and the TLSA base domain is the CNAME-
+ expanded original next-hop domain, then the original next-hop
+ domain MUST be included as a second reference identifier.
+
+ Accepting certificates with the original next-hop domain in addition
+ to the MX hostname allows a domain with multiple MX hostnames to
+ field a single certificate bearing a single domain name (i.e., the
+ email domain) across all the SMTP servers. This also aids
+ interoperability with pre-DANE SMTP clients that are configured to
+ look for the email domain name in server certificates. For example,
+ with "secure" DNS records as below:
+
+
+
+
+
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+ exchange.example.org. IN CNAME mail.example.org.
+ mail.example.org. IN CNAME example.com.
+ example.com. IN MX 10 mx10.example.com.
+ example.com. IN MX 15 mx15.example.com.
+ example.com. IN MX 20 mx20.example.com.
+ ;
+ mx10.example.com. IN A 192.0.2.10
+ _25._tcp.mx10.example.com. IN TLSA 2 0 1 ...
+ ;
+ mx15.example.com. IN CNAME mxbackup.example.com.
+ mxbackup.example.com. IN A 192.0.2.15
+ ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
+ _25._tcp.mx15.example.com. IN TLSA 2 0 1 ...
+ ;
+ mx20.example.com. IN CNAME mxbackup.example.net.
+ mxbackup.example.net. IN A 198.51.100.20
+ _25._tcp.mxbackup.example.net. IN TLSA 2 0 1 ...
+
+ Certificate name checks for delivery of mail to exchange.example.org
+ via any of the associated SMTP servers MUST accept at least the names
+ "exchange.example.org" and "example.com", which are respectively the
+ original and fully expanded next-hop domain. When the SMTP server is
+ mx10.example.com, name checks MUST accept the TLSA base domain
+ "mx10.example.com". If, despite the fact that MX hostnames are
+ required to not be aliases, the MTA supports delivery via
+ "mx15.example.com" or "mx20.example.com" then name checks MUST accept
+ the respective TLSA base domains "mx15.example.com" and
+ "mxbackup.example.net".
+
+3.2.3. Reference identifier matching
+
+ When name checks are applicable (certificate usage DANE-TA(2)), if
+ the server certificate contains a Subject Alternative Name extension
+ ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS-
+ IDs are matched against the client's reference identifiers. The CN-
+ ID ([RFC6125]) is only considered when no DNS-IDs are present. The
+ server certificate is considered matched when one of its presented
+ identifiers ([RFC5280]) matches any of the client's reference
+ identifiers.
+
+ Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
+ The wildcard character must be entire first label of the DNS-ID or
+ CN-ID. Thus, "*.example.com" is valid, while "smtp*.example.com" and
+ "*smtp.example.com" are not. SMTP clients MUST support wildcards
+ that match the first label of the reference identifier, with the
+ remaining labels matching verbatim. For example, the DNS-ID
+ "*.example.com" matches the reference identifier "mx1.example.com".
+ SMTP clients MAY, subject to local policy allow wildcards to match
+
+
+
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+
+ multiple reference identifier labels, but servers cannot expect broad
+ support for such a policy. Therefore any wildcards in server
+ certificates SHOULD match exactly one label in either the TLSA base
+ domain or the next-hop domain.
+
+4. Server key management
+
+ Two TLSA records MUST be published before employing a new EE or TA
+ public key or certificate, one matching the currently deployed key
+ and the other matching the new key scheduled to replace it. Once
+ sufficient time has elapsed for all DNS caches to expire the previous
+ TLSA RRset and related signature RRsets, servers may be configured to
+ use the new EE private key and associated public key certificate or
+ may employ certificates signed by the new trust anchor.
+
+ Once the new public key or certificate is in use, the TLSA RR that
+ matches the retired key can be removed from DNS, leaving only RRs
+ that match keys or certificates in active use.
+
+ As described in Section 3.1.2, when server certificates are validated
+ via a DANE-TA(2) trust anchor, and CNAME records are employed to
+ store the TA association data at a single location, the
+ responsibility of updating the TLSA RRset shifts to the operator of
+ the trust anchor. Before a new trust anchor is used to sign any new
+ server certificates, its certificate (digest) is added to the
+ relevant TLSA RRset. After enough time elapses for the original TLSA
+ RRset to age out of DNS caches, the new trust anchor can start
+ issuing new server certificates. Once all certificates issued under
+ the previous trust anchor have expired, its associated RRs can be
+ removed from the TLSA RRset.
+
+ In the DANE-TA(2) key management model server operators do not
+ generally need to update DNS TLSA records after initially creating a
+ CNAME record that references the centrally operated DANE-TA(2) RRset.
+ If a particular server's key is compromised, its TLSA CNAME SHOULD be
+ replaced with a DANE-EE(3) association until the certificate for the
+ compromised key expires, at which point it can return to using CNAME
+ record. If the central trust anchor is compromised, all servers need
+ to be issued new keys by a new TA, and a shared DANE-TA(2) TLSA RRset
+ needs to be published containing just the new TA. SMTP servers
+ cannot expect broad SMTP client CRL or OCSP support.
+
+5. Digest algorithm agility
+
+ While [RFC6698] specifies multiple digest algorithms, it does not
+ specify a protocol by which the SMTP client and TLSA record publisher
+ can agree on the strongest shared algorithm. Such a protocol would
+ allow the client and server to avoid exposure to any deprecated
+
+
+
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+ weaker algorithms that are published for compatibility with less
+ capable clients, but should be ignored when possible. We specify
+ such a protocol below.
+
+ Suppose that a DANE TLS client authenticating a TLS server considers
+ digest algorithm "BetterAlg" stronger than digest algorithm
+ "WorseAlg". Suppose further that a server's TLSA RRset contains some
+ records with "BetterAlg" as the digest algorithm. Finally, suppose
+ that for every raw public key or certificate object that is included
+ in the server's TLSA RRset in digest form, whenever that object
+ appears with algorithm "WorseAlg" with some usage and selector it
+ also appears with algorithm "BetterAlg" with the same usage and
+ selector. In that case our client can safely ignore TLSA records
+ with the weaker algorithm "WorseAlg", because it suffices to check
+ the records with the stronger algorithm "BetterAlg".
+
+ Server operators MUST ensure that for any given usage and selector,
+ each object (certificate or public key), for which a digest
+ association exists in the TLSA RRset, is published with the SAME SET
+ of digest algorithms as all other objects that published with that
+ usage and selector. In other words, for each usage and selector, the
+ records with non-zero matching types will correspond to on a cross-
+ product of a set of underlying objects and a fixed set of digest
+ algorithms that apply uniformly to all the objects.
+
+ To achieve digest algorithm agility, all published TLSA RRsets for
+ use with opportunistic DANE TLS for SMTP MUST conform to the above
+ requirements. Then, for each combination of usage and selector, SMTP
+ clients can simply ignore all digest records except those that employ
+ the strongest digest algorithm. The ordering of digest algorithms by
+ strength is not specified in advance, it is entirely up to the SMTP
+ client. SMTP client implementations SHOULD make the digest algorithm
+ preference order configurable. Only the future will tell which
+ algorithms might be weakened by new attacks and when.
+
+ Note, TLSA records with a matching type of Full(0), that publish the
+ full value of a certificate or public key object, play no role in
+ digest algorithm agility. They neither trump the processing of
+ records that employ digests, nor are they ignored in the presence of
+ any records with a digest (i.e. non-zero) matching type.
+
+
+
+
+
+
+
+
+
+
+
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+ SMTP clients SHOULD use digest algorithm agility when processing the
+ DANE TLSA records of an SMTP server. Algorithm agility is to be
+ applied after first discarding any unusable or malformed records
+ (unsupported digest algorithm, or incorrect digest length). Thus,
+ for each usage and selector, the client SHOULD process only any
+ usable records with a matching type of Full(0) and the usable records
+ whose digest algorithm is believed to be the strongest among usable
+ records with the given usage and selector.
+
+ The main impact of this requirement is on key rotation, when the TLSA
+ RRset is pre-populated with digests of new certificates or public
+ keys, before these replace or augment their predecessors. Were the
+ newly introduced RRs to include previously unused digest algorithms,
+ clients that employ this protocol could potentially ignore all the
+ digests corresponding to the current keys or certificates, causing
+ connectivity issues until the new keys or certificates are deployed.
+ Similarly, publishing new records with fewer digests could cause
+ problems for clients using cached TLSA RRsets that list both the old
+ and new objects once the new keys are deployed.
+
+ To avoid problems, server operators SHOULD apply the following
+ strategy:
+
+ o When changing the set of objects published via the TLSA RRset
+ (e.g. during key rotation), DO NOT change the set of digest
+ algorithms used; change just the list of objects.
+
+ o When changing the set of digest algorithms, change only the set of
+ algorithms, and generate a new RRset in which all the current
+ objects are re-published with the new set of digest algorithms.
+
+ After either of these two changes are made, the new TLSA RRset should
+ be left in place long enough that the older TLSA RRset can be flushed
+ from caches before making another change.
+
+6. Mandatory TLS Security
+
+ An MTA implementing this protocol may require a stronger security
+ assurance when sending email to selected destinations. The sending
+ organization may need to send sensitive email and/or may have
+ regulatory obligations to protect its content. This protocol is not
+ in conflict with such a requirement, and in fact can often simplify
+ authenticated delivery to such destinations.
+
+ Specifically, with domains that publish DANE TLSA records for their
+ MX hostnames, a sending MTA can be configured to use the receiving
+ domains's DANE TLSA records to authenticate the corresponding SMTP
+ server. Authentication via DANE TLSA records is easier to manage, as
+
+
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+ changes in the receiver's expected certificate properties are made on
+ the receiver end and don't require manually communicated
+ configuration changes. With mandatory DANE TLS, when no usable TLSA
+ records are found, message delivery is delayed. Thus, mail is only
+ sent when an authenticated TLS channel is established to the remote
+ SMTP server.
+
+ Administrators of mail servers that employ mandatory DANE TLS, need
+ to carefully monitor their mail logs and queues. If a partner domain
+ unwittingly misconfigures their TLSA records, disables DNSSEC, or
+ misconfigures SMTP server certificate chains, mail will be delayed
+ and may bounce if the issue is not resolved in a timely manner.
+
+7. Note on DANE for Message User Agents
+
+ We note that the SMTP protocol is also used between Message User
+ Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409]. In
+ [RFC6186] a protocol is specified that enables an MUA to dynamically
+ locate the MSA based on the user's email address. SMTP connection
+ security considerations for MUAs implementing [RFC6186] are largely
+ analogous to connection security requirements for MTAs, and this
+ specification could be applied largely verbatim with DNS MX records
+ replaced by corresponding DNS Service (SRV) records
+ [I-D.ietf-dane-srv].
+
+ However, until MUAs begin to adopt the dynamic configuration
+ mechanisms of [RFC6186] they are adequately served by more
+ traditional static TLS security policies. Specification of DANE TLS
+ for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
+ is left to future documents that focus specifically on SMTP security
+ between MUAs and MSAs.
+
+8. Interoperability considerations
+
+8.1. SNI support
+
+ To ensure that the server sends the right certificate chain, the SMTP
+ client MUST send the TLS SNI extension containing the TLSA base
+ domain. This precludes the use of the backward compatible SSL 2.0
+ compatible SSL HELLO by the SMTP client. The minimum SSL/TLS client
+ HELLO version for SMTP clients performing DANE authentication is SSL
+ 3.0, but a client that offers SSL 3.0 MUST also offer at least TLS
+ 1.0 and MUST include the SNI extension. Servers that don't make use
+ of SNI MAY negotiate SSL 3.0 if offered by the client.
+
+ Each SMTP server MUST present a certificate chain (see [RFC5246]
+ Section 7.4.2) that matches at least one of the TLSA records. The
+ server MAY rely on SNI to determine which certificate chain to
+
+
+
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+ present to the client. Clients that don't send SNI information may
+ not see the expected certificate chain.
+
+ If the server's TLSA records match the server's default certificate
+ chain, the server need not support SNI. In either case, the server
+ need not include the SNI extension in its TLS HELLO as simply
+ returning a matching certificate chain is sufficient. Servers MUST
+ NOT enforce the use of SNI by clients, as the client may be using
+ unauthenticated opportunistic TLS and may not expect any particular
+ certificate from the server. If the client sends no SNI extension,
+ or sends an SNI extension for an unsupported domain, the server MUST
+ simply send some fallback certificate chain of its choice. The
+ reason for not enforcing strict matching of the requested SNI
+ hostname is that DANE TLS clients are typically willing to accept
+ multiple server names, but can only send one name in the SNI
+ extension. The server's fallback certificate may match a different
+ name acceptable to the client, e.g., the original next-hop domain.
+
+8.2. Anonymous TLS cipher suites
+
+ Since many SMTP servers either do not support or do not enable any
+ anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD
+ offer to negotiate a typical set of non-anonymous cipher suites
+ required for interoperability with such servers. An SMTP client
+ employing pre-DANE opportunistic TLS MAY in addition include one or
+ more anonymous TLS cipher suites in its TLS HELLO. SMTP servers,
+ that need to interoperate with opportunistic TLS clients SHOULD be
+ prepared to interoperate with such clients by either always selecting
+ a mutually supported non-anonymous cipher suite or by correctly
+ handling client connections that negotiate anonymous cipher suites.
+
+ Note that while SMTP server operators are under no obligation to
+ enable anonymous cipher suites, no security is gained by sending
+ certificates to clients that will ignore them. Indeed support for
+ anonymous cipher suites in the server makes audit trails more
+ informative. Log entries that record connections that employed an
+ anonymous cipher suite record the fact that the clients did not care
+ to authenticate the server.
+
+9. Operational Considerations
+
+9.1. Client Operational Considerations
+
+ An operational error on the sending or receiving side that cannot be
+ corrected in a timely manner may, at times, lead to consistent
+ failure to deliver time-sensitive email. The sending MTA
+ administrator may have to choose between letting email queue until
+ the error is resolved and disabling opportunistic or mandatory DANE
+
+
+
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+ TLS for one or more destinations. The choice to disable DANE TLS
+ security should not be made lightly. Every reasonable effort should
+ be made to determine that problems with mail delivery are the result
+ of an operational error, and not an attack. A fallback strategy may
+ be to configure explicit out-of-band TLS security settings if
+ supported by the sending MTA.
+
+ SMTP clients may deploy opportunistic DANE TLS incrementally by
+ enabling it only for selected sites, or may occasionally need to
+ disable opportunistic DANE TLS for peers that fail to interoperate
+ due to misconfiguration or software defects on either end. Some
+ implementations MAY support DANE TLS in an "audit only" mode in which
+ failure to achieve the requisite security level is logged as a
+ warning and delivery proceeds at a reduced security level. Unless
+ local policy specifies "audit only" or that opportunistic DANE TLS is
+ not to be used for a particular destination, an SMTP client MUST NOT
+ deliver mail via a server whose certificate chain fails to match at
+ least one TLSA record when usable TLSA records are found for that
+ server.
+
+9.2. Publisher Operational Considerations
+
+ SMTP servers that publish certificate usage DANE-TA(2) associations
+ MUST include the TA certificate in their TLS server certificate
+ chain, even when that TA certificate is a self-signed root
+ certificate.
+
+ TLSA Publishers MUST follow the digest agility guidelines in
+ Section 5 and MUST make sure that all objects published in digest
+ form for a particular usage and selector are published with the same
+ set of digest algorithms.
+
+ TLSA Publishers should follow the TLSA publication size guidance
+ found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".
+
+10. Security Considerations
+
+ This protocol leverages DANE TLSA records to implement MITM resistant
+ opportunistic security ([I-D.dukhovni-opportunistic-security]) for
+ SMTP. For destination domains that sign their MX records and publish
+ signed TLSA records for their MX hostnames, this protocol allows
+ sending MTAs to securely discover both the availability of TLS and
+ how to authenticate the destination.
+
+
+
+
+
+
+
+
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+ This protocol does not aim to secure all SMTP traffic, as that is not
+ practical until DNSSEC and DANE adoption are universal. The
+ incremental deployment provided by following this specification is a
+ best possible path for securing SMTP. This protocol coexists and
+ interoperates with the existing insecure Internet email backbone.
+
+ The protocol does not preclude existing non-opportunistic SMTP TLS
+ security arrangements, which can continue to be used as before via
+ manual configuration with negotiated out-of-band key and TLS
+ configuration exchanges.
+
+ Opportunistic SMTP TLS depends critically on DNSSEC for downgrade
+ resistance and secure resolution of the destination name. If DNSSEC
+ is compromised, it is not possible to fall back on the public CA PKI
+ to prevent MITM attacks. A successful breach of DNSSEC enables the
+ attacker to publish TLSA usage 3 certificate associations, and
+ thereby bypass any security benefit the legitimate domain owner might
+ hope to gain by publishing usage 0 or 1 TLSA RRs. Given the lack of
+ public CA PKI support in existing MTA deployments, avoiding
+ certificate usages 0 and 1 simplifies implementation and deployment
+ with no adverse security consequences.
+
+ Implementations must strictly follow the portions of this
+ specification that indicate when it is appropriate to initiate a non-
+ authenticated connection or cleartext connection to a SMTP server.
+ Specifically, in order to prevent downgrade attacks on this protocol,
+ implementation must not initiate a connection when this specification
+ indicates a particular SMTP server must be considered unreachable.
+
+11. IANA considerations
+
+ This specification requires no support from IANA.
+
+12. Acknowledgements
+
+ The authors would like to extend great thanks to Tony Finch, who
+ started the original version of a DANE SMTP document. His work is
+ greatly appreciated and has been incorporated into this document.
+ The authors would like to additionally thank Phil Pennock for his
+ comments and advice on this document.
+
+ Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
+ to begin work on this memo and provided feedback on early drafts.
+ Thanks to Patrick Koetter, Perry Metzger and Nico Williams for
+ valuable review comments. Thanks also to Wietse Venema who created
+ Postfix, and whose advice and feedback were essential to the
+ development of the Postfix DANE implementation.
+
+
+
+
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+
+13. References
+
+13.1. Normative References
+
+ [I-D.ietf-dane-ops]
+ Dukhovni, V. and W. Hardaker, "DANE TLSA implementation
+ and operational guidance", draft-ietf-dane-ops-00 (work in
+ progress), October 2013.
+
+ [RFC1035] Mockapetris, P., "Domain names - implementation and
+ specification", STD 13, RFC 1035, November 1987.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
+ Transport Layer Security", RFC 3207, February 2002.
+
+ [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "DNS Security Introduction and Requirements", RFC
+ 4033, March 2005.
+
+ [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "Resource Records for the DNS Security Extensions",
+ RFC 4034, March 2005.
+
+ [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "Protocol Modifications for the DNS Security
+ Extensions", RFC 4035, March 2005.
+
+ [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
+ (TLS) Protocol Version 1.2", RFC 5246, August 2008.
+
+ [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
+ Housley, R., and W. Polk, "Internet X.509 Public Key
+ Infrastructure Certificate and Certificate Revocation List
+ (CRL) Profile", RFC 5280, May 2008.
+
+ [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
+ October 2008.
+
+ [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
+ Extension Definitions", RFC 6066, January 2011.
+
+
+
+
+
+
+
+
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+
+
+ [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
+ Verification of Domain-Based Application Service Identity
+ within Internet Public Key Infrastructure Using X.509
+ (PKIX) Certificates in the Context of Transport Layer
+ Security (TLS)", RFC 6125, March 2011.
+
+ [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
+ Submission/Access Services", RFC 6186, March 2011.
+
+ [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
+ DNS", RFC 6672, June 2012.
+
+ [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
+ of Named Entities (DANE) Transport Layer Security (TLS)
+ Protocol: TLSA", RFC 6698, August 2012.
+
+ [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
+ Conversations about DNS-Based Authentication of Named
+ Entities (DANE)", RFC 7218, April 2014.
+
+ [X.690] International Telecommunications Union, "Recommendation
+ ITU-T X.690 (2002) | ISO/IEC 8825-1:2002, Information
+ technology - ASN.1 encoding rules: Specification of Basic
+ Encoding Rules (BER), Canonical Encoding Rules (CER) and
+ Distinguished Encoding Rules (DER)", July 2002.
+
+13.2. Informative References
+
+ [I-D.dukhovni-opportunistic-security]
+ Dukhovni, V., "Opportunistic Security: some protection
+ most of the time", draft-dukhovni-opportunistic-
+ security-01 (work in progress), July 2014.
+
+ [I-D.ietf-dane-srv]
+ Finch, T., "Using DNS-Based Authentication of Named
+ Entities (DANE) TLSA records with SRV and MX records.",
+ draft-ietf-dane-srv-02 (work in progress), February 2013.
+
+ [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
+ 2009.
+
+ [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
+ STD 72, RFC 6409, November 2011.
+
+Authors' Addresses
+
+
+
+
+
+
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+
+ Viktor Dukhovni
+ Two Sigma
+
+ Email: ietf-dane@???
+
+
+ Wes Hardaker
+ Parsons
+ P.O. Box 382
+ Davis, CA 95617
+ US
+
+ Email: ietf@???
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
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+
+
+
+
+DANE V. Dukhovni
+Internet-Draft Two Sigma
+Intended status: Standards Track W. Hardaker
+Expires: November 26, 2014 Parsons
+ May 25, 2014
+
+
+ SMTP security via opportunistic DANE TLS
+ draft-ietf-dane-smtp-with-dane-10
+
+Abstract
+
+ This memo describes a downgrade-resistant protocol for SMTP transport
+ security between Mail Transfer Agents (MTAs) based on the DNS-Based
+ Authentication of Named Entities (DANE) TLSA DNS record. Adoption of
+ this protocol enables an incremental transition of the Internet email
+ backbone to one using encrypted and authenticated Transport Layer
+ Security (TLS).
+
+Status of This Memo
+
+ This Internet-Draft is submitted in full conformance with the
+ provisions of BCP 78 and BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF). Note that other groups may also distribute
+ working documents as Internet-Drafts. The list of current Internet-
+ Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ This Internet-Draft will expire on November 26, 2014.
+
+Copyright Notice
+
+ Copyright (c) 2014 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+
+
+
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+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5
+ 1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6
+ 1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6
+ 1.3.2. Insecure server name without DNSSEC . . . . . . . . . 7
+ 1.3.3. Sender policy does not scale . . . . . . . . . . . . 7
+ 1.3.4. Too many certification authorities . . . . . . . . . 8
+ 2. Identifying applicable TLSA records . . . . . . . . . . . . . 8
+ 2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 8
+ 2.1.1. DNS errors, bogus and indeterminate responses . . . . 8
+ 2.1.2. DNS error handling . . . . . . . . . . . . . . . . . 11
+ 2.1.3. Stub resolver considerations . . . . . . . . . . . . 11
+ 2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 12
+ 2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 13
+ 2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 15
+ 2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17
+ 3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19
+ 3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19
+ 3.1.1. Certificate usage DANE-EE(3) . . . . . . . . . . . . 20
+ 3.1.2. Certificate usage DANE-TA(2) . . . . . . . . . . . . 21
+ 3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) . . . . 22
+ 3.2. Certificate matching . . . . . . . . . . . . . . . . . . 23
+ 3.2.1. DANE-EE(3) name checks . . . . . . . . . . . . . . . 23
+ 3.2.2. DANE-TA(2) name checks . . . . . . . . . . . . . . . 23
+ 3.2.3. Reference identifier matching . . . . . . . . . . . . 24
+ 4. Server key management . . . . . . . . . . . . . . . . . . . . 25
+ 5. Digest algorithm agility . . . . . . . . . . . . . . . . . . 26
+ 6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 27
+ 7. Note on DANE for Message User Agents . . . . . . . . . . . . 28
+ 8. Interoperability considerations . . . . . . . . . . . . . . . 29
+ 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 29
+ 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 29
+ 9. Operational Considerations . . . . . . . . . . . . . . . . . 30
+ 9.1. Client Operational Considerations . . . . . . . . . . . . 30
+ 9.2. Publisher Operational Considerations . . . . . . . . . . 30
+ 10. Security Considerations . . . . . . . . . . . . . . . . . . . 31
+ 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 31
+ 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
+ 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
+ 13.1. Normative References . . . . . . . . . . . . . . . . . . 32
+ 13.2. Informative References . . . . . . . . . . . . . . . . . 33
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
+
+
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+1. Introduction
+
+ This memo specifies a new connection security model for Message
+ Transfer Agents (MTAs). This model is motivated by key features of
+ inter-domain SMTP delivery, in particular the fact that the
+ destination server is selected indirectly via DNS Mail Exchange (MX)
+ records and that neither email addresses nor MX hostnames signal a
+ requirement for either secure or cleartext transport. Therefore,
+ aside from a few manually configured exceptions, SMTP transport
+ security is of necessity opportunistic.
+
+ This specification uses the presence of DANE TLSA records to securely
+ signal TLS support and to publish the means by which SMTP clients can
+ successfully authenticate legitimate SMTP servers. This becomes
+ "opportunistic DANE TLS" and is resistant to downgrade and MITM
+ attacks. It enables an incremental transition of the email backbone
+ to authenticated TLS delivery, with increased global protection as
+ adoption increases.
+
+ With opportunistic DANE TLS, traffic from SMTP clients to domains
+ that publish "usable" DANE TLSA records in accordance with this memo
+ is authenticated and encrypted. Traffic from legacy clients or to
+ domains that do not publish TLSA records will continue to be sent in
+ the same manner as before, via manually configured security, (pre-
+ DANE) opportunistic TLS or just cleartext SMTP.
+
+ Problems with existing use of TLS in MTA to MTA SMTP that motivate
+ this specification are described in Section 1.3. The specification
+ itself follows in Section 2 and Section 3 which describe respectively
+ how to locate and use DANE TLSA records with SMTP. In Section 6, we
+ discuss application of DANE TLS to destinations for which channel
+ integrity and confidentiality are mandatory. In Section 7 we briefly
+ comment on potential applicability of this specification to Message
+ User Agents.
+
+1.1. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+ "OPTIONAL" in this document are to be interpreted as described in
+ [RFC2119].
+
+ The following terms or concepts are used through the document:
+
+ Man-in-the-middle or MITM attack: Active modification of network
+ traffic by an adversary able to thereby compromise the
+ confidentiality or integrity of the data.
+
+
+
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+ secure, bogus, insecure, indeterminate: DNSSEC validation results,
+ as defined in Section 4.3 of [RFC4035].
+
+ Validating Security-Aware Stub Resolver and Non-Validating
+ Security-Aware Stub Resolver:
+ Capabilities of the stub resolver in use as defined in [RFC4033];
+ note that this specification requires the use of a Security-Aware
+ Stub Resolver; Security-Oblivious stub-resolvers MUST NOT be used.
+
+ opportunistic DANE TLS: Best-effort use of TLS, resistant to
+ downgrade attacks for destinations with DNSSEC-validated TLSA
+ records. When opportunistic DANE TLS is determined to be
+ unavailable, clients should fall back to opportunistic TLS below.
+ Opportunistic DANE TLS requires support for DNSSEC, DANE and
+ STARTTLS on the client side and STARTTLS plus a DNSSEC published
+ TLSA record on the server side.
+
+ (pre-DANE) opportunistic TLS: Best-effort use of TLS that is
+ generally vulnerable to DNS forgery and STARTTLS downgrade
+ attacks. When a TLS-encrypted communication channel is not
+ available, message transmission takes place in the clear. MX
+ record indirection generally precludes authentication even when
+ TLS is available.
+
+ reference identifier: (Special case of [RFC6125] definition). One
+ of the domain names associated by the SMTP client with the
+ destination SMTP server for performing name checks on the server
+ certificate. When name checks are applicable, at least one of the
+ reference identifiers MUST match an [RFC6125] DNS-ID (or if none
+ are present the [RFC6125] CN-ID) of the server certificate (see
+ Section 3.2.3).
+
+ MX hostname: The RRDATA of an MX record consists of a 16 bit
+ preference followed by a Mail Exchange domain name (see [RFC1035],
+ Section 3.3.9). We will use the term "MX hostname" to refer to
+ the latter, that is, the DNS domain name found after the
+ preference value in an MX record. Thus an "MX hostname" is
+ specifically a reference to a DNS domain name, rather than any
+ host that bears that name.
+
+ delayed delivery: Email delivery is a multi-hop store & forward
+ process. When an MTA is unable forward a message that may become
+ deliverable later, the message is queued and delivery is retried
+ periodically. Some MTAs may be configured with a fallback next-
+ hop destination that handles messages that the MTA would otherwise
+ queue and retry. In these cases, messages that would otherwise
+ have to be delayed, may be sent to the fallback next-hop
+ destination instead. The fallback destination may itself be
+
+
+
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+ subject to opportunistic or mandatory DANE TLS as though it were
+ the original message destination.
+
+ original next hop destination: The logical destination for mail
+ delivery. By default this is the domain portion of the recipient
+ address, but MTAs may be configured to forward mail for some or
+ all recipients via designated relays. The original next hop
+ destination is, respectively, either the recipient domain or the
+ associated configured relay.
+
+ MTA: Message Transfer Agent ([RFC5598], Section 4.3.2).
+
+ MSA: Message Submission Agent ([RFC5598], Section 4.3.1).
+
+ MUA: Message User Agent ([RFC5598], Section 4.2.1).
+
+ RR: A DNS Resource Record
+
+ RRset: A set of DNS Resource Records for a particular class, domain
+ and record type.
+
+1.2. Background
+
+ The Domain Name System Security Extensions (DNSSEC) add data origin
+ authentication, data integrity and data non-existence proofs to the
+ Domain Name System (DNS). DNSSEC is defined in [RFC4033], [RFC4034]
+ and [RFC4035].
+
+ As described in the introduction of [RFC6698], TLS authentication via
+ the existing public Certification Authority (CA) PKI suffers from an
+ over-abundance of trusted parties capable of issuing certificates for
+ any domain of their choice. DANE leverages the DNSSEC infrastructure
+ to publish trusted public keys and certificates for use with the
+ Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA"
+ DNS record type. With DNSSEC each domain can only vouch for the keys
+ of its directly delegated sub-domains.
+
+ The TLS protocol enables secure TCP communication. In the context of
+ this memo, channel security is assumed to be provided by TLS. Used
+ without authentication, TLS provides only privacy protection against
+ eavesdropping attacks. With authentication, TLS also provides data
+ integrity protection to guard against MITM attacks.
+
+
+
+
+
+
+
+
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+1.3. SMTP channel security
+
+ With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
+ [RFC5280] issued by one of the many Certificate Authorities (CAs)
+ bundled with popular web browsers to allow users to authenticate
+ their "secure" websites. Before we specify a new DANE TLS security
+ model for SMTP, we will explain why a new security model is needed.
+ In the process, we will explain why the familiar HTTPS security model
+ is inadequate to protect inter-domain SMTP traffic.
+
+ The subsections below outline four key problems with applying
+ traditional PKI to SMTP that are addressed by this specification.
+ Since SMTP channel security policy is not explicitly specified in
+ either the recipient address or the MX record, a new signaling
+ mechanism is required to indicate when channel security is possible
+ and should be used. The publication of TLSA records allows server
+ operators to securely signal to SMTP clients that TLS is available
+ and should be used. DANE TLSA makes it possible to simultaneously
+ discover which destination domains support secure delivery via TLS
+ and how to verify the authenticity of the associated SMTP services,
+ providing a path forward to ubiquitous SMTP channel security.
+
+1.3.1. STARTTLS downgrade attack
+
+ The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop
+ protocol in a multi-hop store & forward email delivery process. SMTP
+ envelope recipient addresses are not transport addresses and are
+ security-agnostic. Unlike the Hypertext Transfer Protocol (HTTP) and
+ its corresponding secured version, HTTPS, where the use of TLS is
+ signaled via the URI scheme, email recipient addresses do not
+ directly signal transport security policy. Indeed, no such signaling
+ could work well with SMTP since TLS encryption of SMTP protects email
+ traffic on a hop-by-hop basis while email addresses could only
+ express end-to-end policy.
+
+ With no mechanism available to signal transport security policy, SMTP
+ relays employ a best-effort "opportunistic" security model for TLS.
+ A single SMTP server TCP listening endpoint can serve both TLS and
+ non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
+ command ([RFC3207]). The server signals TLS support to the client
+ over a cleartext SMTP connection, and, if the client also supports
+ TLS, it may negotiate a TLS encrypted channel to use for email
+ transmission. The server's indication of TLS support can be easily
+ suppressed by an MITM attacker. Thus pre-DANE SMTP TLS security can
+ be subverted by simply downgrading a connection to cleartext. No TLS
+ security feature, such as the use of PKIX, can prevent this. The
+ attacker can simply disable TLS.
+
+
+
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+1.3.2. Insecure server name without DNSSEC
+
+ With SMTP, DNS Mail Exchange (MX) records abstract the next-hop
+ transport endpoint and allow administrators to specify a set of
+ target servers to which SMTP traffic should be directed for a given
+ domain.
+
+ A PKIX TLS client is vulnerable to MITM attacks unless it verifies
+ that the server's certificate binds the public key to a name that
+ matches one of the client's reference identifiers. A natural choice
+ of reference identifier is the server's domain name. However, with
+ SMTP, server names are obtained indirectly via MX records. Without
+ DNSSEC, the MX lookup is vulnerable to MITM and DNS cache poisoning
+ attacks. Active attackers can forge DNS replies with fake MX records
+ and can redirect email to servers with names of their choice.
+ Therefore, secure verification of SMTP TLS certificates matching the
+ server name is not possible without DNSSEC.
+
+ One might try to harden TLS for SMTP against DNS attacks by using the
+ envelope recipient domain as a reference identifier and requiring
+ each SMTP server to possess a trusted certificate for the envelope
+ recipient domain rather than the MX hostname. Unfortunately, this is
+ impractical as email for many domains is handled by third parties
+ that are not in a position to obtain certificates for all the domains
+ they serve. Deployment of the Server Name Indication (SNI) extension
+ to TLS (see [RFC6066] Section 3) is no panacea, since SNI key
+ management is operationally challenging except when the email service
+ provider is also the domain's registrar and its certificate issuer;
+ this is rarely the case for email.
+
+ Since the recipient domain name cannot be used as the SMTP server
+ reference identifier, and neither can the MX hostname without DNSSEC,
+ large-scale deployment of authenticated TLS for SMTP requires that
+ the DNS be secure.
+
+ Since SMTP security depends critically on DNSSEC, it is important to
+ point out that consequently SMTP with DANE is the most conservative
+ possible trust model. It trusts only what must be trusted and no
+ more. Adding any other trusted actors to the mix can only reduce
+ SMTP security. A sender may choose to further harden DNSSEC for
+ selected high-value receiving domains, by configuring explicit trust
+ anchors for those domains instead of relying on the chain of trust
+ from the root domain. Detailed discussion of DNSSEC security
+ practices is out of scope for this document.
+
+1.3.3. Sender policy does not scale
+
+
+
+
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+ Sending systems are in some cases explicitly configured to use TLS
+ for mail sent to selected peer domains. This requires sending MTAs
+ to be configured with appropriate subject names or certificate
+ content digests to expect in the presented server certificates.
+ Because of the heavy administrative burden, such statically
+ configured SMTP secure channels are used rarely (generally only
+ between domains that make bilateral arrangements with their business
+ partners). Internet email, on the other hand, requires regularly
+ contacting new domains for which security configurations cannot be
+ established in advance.
+
+ The abstraction of the SMTP transport endpoint via DNS MX records,
+ often across organization boundaries, limits the use of public CA PKI
+ with SMTP to a small set of sender-configured peer domains. With
+ little opportunity to use TLS authentication, sending MTAs are rarely
+ configured with a comprehensive list of trusted CAs. SMTP services
+ that support STARTTLS often deploy X.509 certificates that are self-
+ signed or issued by a private CA.
+
+1.3.4. Too many certification authorities
+
+ Even if it were generally possible to determine a secure server name,
+ the SMTP client would still need to verify that the server's
+ certificate chain is issued by a trusted Certification Authority (a
+ trust anchor). MTAs are not interactive applications where a human
+ operator can make a decision (wisely or otherwise) to selectively
+ disable TLS security policy when certificate chain verification
+ fails. With no user to "click OK", the MTAs list of public CA trust
+ anchors would need to be comprehensive in order to avoid bouncing
+ mail addressed to sites that employ unknown Certification
+ Authorities.
+
+ On the other hand, each trusted CA can issue certificates for any
+ domain. If even one of the configured CAs is compromised or operated
+ by an adversary, it can subvert TLS security for all destinations.
+ Any set of CAs is simultaneously both overly inclusive and not
+ inclusive enough.
+
+2. Identifying applicable TLSA records
+
+2.1. DNS considerations
+
+2.1.1. DNS errors, bogus and indeterminate responses
+
+
+
+
+
+
+
+
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+ An SMTP client that implements opportunistic DANE TLS per this
+ specification depends critically on the integrity of DNSSEC lookups,
+ as discussed in Section 1.3. This section lists the DNS resolver
+ requirements needed to avoid downgrade attacks when using
+ opportunistic DANE TLS.
+
+ A DNS lookup may signal an error or return a definitive answer. A
+ security-aware resolver must be used for this specification.
+ Security-aware resolvers will indicate the security status of a DNS
+ RRset with one of four possible values defined in Section 4.3 of
+ [RFC4035]: "secure", "insecure", "bogus" and "indeterminate". In
+ [RFC4035] the meaning of the "indeterminate" security status is:
+
+ An RRset for which the resolver is not able to determine whether
+ the RRset should be signed, as the resolver is not able to obtain
+ the necessary DNSSEC RRs. This can occur when the security-aware
+ resolver is not able to contact security-aware name servers for
+ the relevant zones.
+
+ Note, the "indeterminate" security status has a conflicting
+ definition in section 5 of [RFC4033].
+
+ There is no trust anchor that would indicate that a specific
+ portion of the tree is secure.
+
+ SMTP clients following this specification SHOULD NOT distinguish
+ between "insecure" and "indeterminate" in the [RFC4033] sense. Both
+ "insecure" and RFC4033 "indeterminate" are handled identically: in
+ either case unvalidated data for the query domain is all that is and
+ can be available, and authentication using the data is impossible.
+ In what follows, when we say "insecure", we include also DNS results
+ for domains that lie in a portion of the DNS tree for which there is
+ no applicable trust anchor. With the DNS root zone signed, we expect
+ that validating resolvers used by Internet-facing MTAs will be
+ configured with trust anchor data for the root zone. Therefore,
+ RFC4033-style "indeterminate" domains should be rare in practice.
+ From here on, when we say "indeterminate", it is exclusively in the
+ sense of [RFC4035].
+
+ As noted in section 4.3 of [RFC4035], a security-aware DNS resolver
+ MUST be able to determine whether a given non-error DNS response is
+ "secure", "insecure", "bogus" or "indeterminate". It is expected
+ that most security-aware stub resolvers will not signal an
+ "indeterminate" security status in the RFC4035-sense to the
+ application, and will signal a "bogus" or error result instead. If a
+ resolver does signal an RFC4035 "indeterminate" security status, this
+ MUST be treated by the SMTP client as though a "bogus" or error
+ result had been returned.
+
+
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+ An MTA making use of a non-validating security-aware stub resolver
+ MAY use the stub resolver's ability, if available, to signal DNSSEC
+ validation status based on information the stub resolver has learned
+ from an upstream validating recursive resolver. In accordance with
+ section 4.9.3 of [RFC4035]:
+
+ ... a security-aware stub resolver MUST NOT place any reliance on
+ signature validation allegedly performed on its behalf, except
+ when the security-aware stub resolver obtained the data in question
+ from a trusted security-aware recursive name server via a secure
+ channel.
+
+ To avoid much repetition in the text below, we will pause to explain
+ the handling of "bogus" or "indeterminate" DNSSEC query responses.
+ These are not necessarily the result of a malicious actor; they can,
+ for example, occur when network packets are corrupted or lost in
+ transit. Therefore, "bogus" or "indeterminate" replies are equated
+ in this memo with lookup failure.
+
+ There is an important non-failure condition we need to highlight in
+ addition to the obvious case of the DNS client obtaining a non-empty
+ "secure" or "insecure" RRset of the requested type. Namely, it is
+ not an error when either "secure" or "insecure" non-existence is
+ determined for the requested data. When a DNSSEC response with a
+ validation status that is either "secure" or "insecure" reports
+ either no records of the requested type or non-existence of the query
+ domain, the response is not a DNS error condition. The DNS client
+ has not been left without an answer; it has learned that records of
+ the requested type do not exist.
+
+ Security-aware stub resolvers will, of course, also signal DNS lookup
+ errors in other cases, for example when processing a "ServFail"
+ RCODE, which will not have an associated DNSSEC status. All lookup
+ errors are treated the same way by this specification, regardless of
+ whether they are from a "bogus" or "indeterminate" DNSSEC status or
+ from a more generic DNS error: the information that was requested
+ cannot be obtained by the security-aware resolver at this time. A
+ lookup error is thus a failure to obtain the relevant RRset if it
+ exists, or to determine that no such RRset exists when it does not.
+
+ In contrast to a "bogus" or an "indeterminate" response, an
+ "insecure" DNSSEC response is not an error, rather it indicates that
+ the target DNS zone is either securely opted out of DNSSEC validation
+ or is not connected with the DNSSEC trust anchors being used.
+ Insecure results will leave the SMTP client with degraded channel
+ security, but do not stand in the way of message delivery. See
+ section Section 2.2 for further details.
+
+
+
+
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+2.1.2. DNS error handling
+
+ When a DNS lookup failure (error or "bogus" or "indeterminate" as
+ defined above) prevents an SMTP client from determining which SMTP
+ server or servers it should connect to, message delivery MUST be
+ delayed. This naturally includes, for example, the case when a
+ "bogus" or "indeterminate" response is encountered during MX
+ resolution. When multiple MX hostnames are obtained from a
+ successful MX lookup, but a later DNS lookup failure prevents network
+ address resolution for a given MX hostname, delivery may proceed via
+ any remaining MX hosts.
+
+ When a particular SMTP server is securely identified as the delivery
+ destination, a set of DNS lookups (Section 2.2) MUST be performed to
+ locate any related TLSA records. If any DNS queries used to locate
+ TLSA records fail (be it due to "bogus" or "indeterminate" records,
+ timeouts, malformed replies, ServFails, etc.), then the SMTP client
+ MUST treat that server as unreachable and MUST NOT deliver the
+ message via that server. If no servers are reachable, delivery is
+ delayed.
+
+ In what follows, we will only describe what happens when all relevant
+ DNS queries succeed. If any DNS failure occurs, the SMTP client MUST
+ behave as described in this section, by skipping the problem SMTP
+ server, or the problem destination. Queries for candidate TLSA
+ records are explicitly part of "all relevant DNS queries" and SMTP
+ clients MUST NOT continue to connect to an SMTP server or destination
+ whose TLSA record lookup fails.
+
+2.1.3. Stub resolver considerations
+
+ A note about DNAME aliases: a query for a domain name whose ancestor
+ domain is a DNAME alias returns the DNAME RR for the ancestor domain,
+ along with a CNAME that maps the query domain to the corresponding
+ sub-domain of the target domain of the DNAME alias [RFC6672].
+ Therefore, whenever we speak of CNAME aliases, we implicitly allow
+ for the possibility that the alias in question is the result of an
+ ancestor domain DNAME record. Consequently, no explicit support for
+ DNAME records is needed in SMTP software, it is sufficient to process
+ the resulting CNAME aliases. DNAME records only require special
+ processing in the validating stub-resolver library that checks the
+ integrity of the combined DNAME + CNAME reply. When DNSSEC
+ validation is handled by a local caching resolver, rather than the
+ MTA itself, even that part of the DNAME support logic is outside the
+ MTA.
+
+ When a stub resolver returns a response containing a CNAME alias that
+ does not also contain the corresponding query results for the target
+
+
+
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+ of the alias, the SMTP client will need to repeat the query at the
+ target of the alias, and should do so recursively up to some
+ configured or implementation-dependent recursion limit. If at any
+ stage of CNAME expansion an error is detected, the lookup of the
+ original requested records MUST be considered to have failed.
+
+ Whether a chain of CNAME records was returned in a single stub
+ resolver response or via explicit recursion by the SMTP client, if at
+ any stage of recursive expansion an "insecure" CNAME record is
+ encountered, then it and all subsequent results (in particular, the
+ final result) MUST be considered "insecure" regardless of whether any
+ earlier CNAME records leading to the "insecure" record were "secure".
+
+ Note, a security-aware non-validating stub resolver may return to the
+ SMTP client an "insecure" reply received from a validating recursive
+ resolver that contains a CNAME record along with additional answers
+ recursively obtained starting at the target of the CNAME. In this
+ all that one can say is that some record in the set of records
+ returned is "insecure", but it is possible that the initial CNAME
+ record and a subset of the subsequent records are "secure".
+
+ If the SMTP client needs to determine the security status of the DNS
+ zone containing the initial CNAME record, it may need to issue an a
+ separate query of type "CNAME" that returns only the initial CNAME
+ record. In particular in Section 2.2.2 when insecure A or AAAA
+ records are found for an SMTP server via a CNAME alias, it may be
+ necessary to perform an additional CNAME query to determine whether
+ the DNS zone in which the alias is published is signed.
+
+2.2. TLS discovery
+
+ As noted previously (in Section 1.3.1), opportunistic TLS with SMTP
+ servers that advertise TLS support via STARTTLS is subject to an MITM
+ downgrade attack. Also some SMTP servers that are not, in fact, TLS
+ capable erroneously advertise STARTTLS by default and clients need to
+ be prepared to retry cleartext delivery after STARTTLS fails. In
+ contrast, DNSSEC validated TLSA records MUST NOT be published for
+ servers that do not support TLS. Clients can safely interpret their
+ presence as a commitment by the server operator to implement TLS and
+ STARTTLS.
+
+ This memo defines four actions to be taken after the search for a
+ TLSA record returns secure usable results, secure unusable results,
+ insecure or no results or an error signal. The term "usable" in this
+ context is in the sense of Section 4.1 of [RFC6698]. Specifically,
+ if the DNS lookup for a TLSA record returns:
+
+
+
+
+
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+ A secure TLSA RRset with at least one usable record: A connection to
+ the MTA MUST be made using authenticated and encrypted TLS, using
+ the techniques discussed in the rest of this document. Failure to
+ establish an authenticated TLS connection MUST result in falling
+ back to the next SMTP server or delayed delivery.
+
+ A Secure non-empty TLSA RRset where all the records are unusable: A
+ connection to the MTA MUST be made via TLS, but authentication is
+ not required. Failure to establish an encrypted TLS connection
+ MUST result in falling back to the next SMTP server or delayed
+ delivery.
+
+ An insecure TLSA RRset or DNSSEC validated proof-of-non-existent TLSA
+ records:
+ A connection to the MTA SHOULD be made using (pre-DANE)
+ opportunistic TLS, this includes using cleartext delivery when the
+ remote SMTP server does not appear to support TLS. The MTA MAY
+ retry in cleartext when delivery via TLS fails either during the
+ handshake or even during data transfer.
+
+ Any lookup error: Lookup errors, including "bogus" and
+ "indeterminate", as explained in Section 2.1.1 MUST result in
+ falling back to the next SMTP server or delayed delivery.
+
+ An SMTP client MAY be configured to require DANE verified delivery
+ for some destinations. We will call such a configuration "mandatory
+ DANE TLS". With mandatory DANE TLS, delivery proceeds only when
+ "secure" TLSA records are used to establish an encrypted and
+ authenticated TLS channel with the SMTP server.
+
+ When the original next-hop destination is an address literal, rather
+ than a DNS domain, DANE TLS does not apply. Delivery proceeds using
+ any relevant security policy configured by the MTA administrator.
+ Similarly, when an MX RRset incorrectly lists a network address in
+ lieu of an MX hostname, if the MTA chooses to connect to the network
+ address DANE TLSA does not apply for such a connection.
+
+ In the subsections that follow we explain how to locate the SMTP
+ servers and the associated TLSA records for a given next-hop
+ destination domain. We also explain which name or names are to be
+ used in identity checks of the SMTP server certificate.
+
+2.2.1. MX resolution
+
+ In this section we consider next-hop domains that are subject to MX
+ resolution and have MX records. The TLSA records and the associated
+ base domain are derived separately for each MX hostname that is used
+ to attempt message delivery. DANE TLS can authenticate message
+
+
+
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+ delivery to the intended next-hop domain only when the MX records are
+ obtained securely via a DNSSEC validated lookup.
+
+ MX records MUST be sorted by preference; an MX hostname with a worse
+ (numerically higher) MX preference that has TLSA records MUST NOT
+ preempt an MX hostname with a better (numerically lower) preference
+ that has no TLSA records. In other words, prevention of delivery
+ loops by obeying MX preferences MUST take precedence over channel
+ security considerations. Even with two equal-preference MX records,
+ an MTA is not obligated to choose the MX hostname that offers more
+ security. Domains that want secure inbound mail delivery need to
+ ensure that all their SMTP servers and MX records are configured
+ accordingly.
+
+ In the language of [RFC5321] Section 5.1, the original next-hop
+ domain is the "initial name". If the MX lookup of the initial name
+ results in a CNAME alias, the MTA replaces the initial name with the
+ resulting name and performs a new lookup with the new name. MTAs
+ typically support recursion in CNAME expansion, so this replacement
+ is performed repeatedly until the ultimate non-CNAME domain is found.
+
+ If the MX RRset (or any CNAME leading to it) is "insecure" (see
+ Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via
+ pre-DANE opportunistic TLS. That said, the protocol in this memo is
+ an "opportunistic security" protocol, meaning that it strives to
+ communicate with each peer as securely as possible, while maintaining
+ broad interoperability. Therefore, the SMTP client MAY proceed to
+ use DANE TLS (as described in Section 2.2.2 below) even with MX hosts
+ obtained via an "insecure" MX RRset. For example, when a hosting
+ provider has a signed DNS zone and publishes TLSA records for its
+ SMTP servers, hosted domains that are not signed may still benefit
+ from the provider's TLSA records. Deliveries via the provider's SMTP
+ servers will not be subject to active attacks when sending SMTP
+ clients elect to make use of the provider's TLSA records.
+
+ When the MX records are not (DNSSEC) signed, an active attacker can
+ redirect SMTP clients to MX hosts of his choice. Such redirection is
+ tamper-evident when SMTP servers found via "insecure" MX records are
+ recorded as the next-hop relay in the MTA delivery logs in their
+ original (rather than CNAME expanded) form. Sending MTAs SHOULD log
+ unexpanded MX hostnames when these result from insecure MX lookups.
+ Any successful authentication via an insecurely determined MX host
+ MUST NOT be misrepresented in the mail logs as secure delivery to the
+ intended next-hop domain. When DANE TLS is mandatory (Section 6) for
+ a given destination, delivery MUST be delayed when the MX RRset is
+ not "secure".
+
+
+
+
+
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+ Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is
+ "secure", and the SMTP client MUST treat each MX hostname as a
+ separate non-MX destination for opportunistic DANE TLS as described
+ in Section 2.2.2. When, for a given MX hostname, no TLSA records are
+ found, or only "insecure" TLSA records are found, DANE TLSA is not
+ applicable with the SMTP server in question and delivery proceeds to
+ that host as with pre-DANE opportunistic TLS. To avoid downgrade
+ attacks, any errors during TLSA lookups MUST, as explained in
+ Section 2.1.1, cause the SMTP server in question to be treated as
+ unreachable.
+
+2.2.2. Non-MX destinations
+
+ This section describes the algorithm used to locate the TLSA records
+ and associated TLSA base domain for an input domain not subject to MX
+ resolution. Such domains include:
+
+ o Each MX hostname used in a message delivery attempt for an
+ original next-hop destination domain subject to MX resolution.
+ Note, MTAs are not obligated to support CNAME expansion of MX
+ hostnames.
+
+ o Any administrator configured relay hostname, not subject to MX
+ resolution. This frequently involves configuration set by the MTA
+ administrator to handle some or all mail.
+
+ o A next-hop destination domain subject to MX resolution that has no
+ MX records. In this case the domain's name is implicitly also its
+ sole SMTP server name.
+
+ Note that DNS queries with type TLSA are mishandled by load balancing
+ nameservers that serve the MX hostnames of some large email
+ providers. The DNS zones served by these nameservers are not signed
+ and contain no TLSA records, but queries for TLSA records fail,
+ rather than returning the non-existence of the requested TLSA
+ records.
+
+ To avoid problems delivering mail to domains whose SMTP servers are
+ served by the problem nameservers the SMTP client MUST perform any A
+ and/or AAAA queries for the destination before attempting to locate
+ the associated TLSA records. This lookup is needed in any case to
+ determine whether the destination domain is reachable and the DNSSEC
+ validation status of the chain of CNAME queries required to reach the
+ ultimate address records.
+
+ If no address records are found, the destination is unreachable. If
+ address records are found, but the DNSSEC validation status of the
+ first query response is "insecure" (see Section 2.1.3), the SMTP
+
+
+
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+ client SHOULD NOT proceed to search for any associated TLSA records.
+ With the problem domains, TLSA queries will lead to DNS lookup errors
+ and cause messages to be consistently delayed and ultimately returned
+ to the sender. We don't expect to find any "secure" TLSA records
+ associated with a TLSA base domain that lies in an unsigned DNS zone.
+ Therefore, skipping TLSA lookups in this case will also reduce
+ latency with no detrimental impact on security.
+
+ If the A and/or AAAA lookup of the "initial name" yields a CNAME, we
+ replace it with the resulting name as if it were the initial name and
+ perform a lookup again using the new name. This replacement is
+ performed recursively.
+
+ We consider the following cases for handling a DNS response for an A
+ or AAAA DNS lookup:
+
+ Not found: When the DNS queries for A and/or AAAA records yield
+ neither a list of addresses nor a CNAME (or CNAME expansion is not
+ supported) the destination is unreachable.
+
+ Non-CNAME: The answer is not a CNAME alias. If the address RRset
+ is "secure", TLSA lookups are performed as described in
+ Section 2.2.3 with the initial name as the candidate TLSA base
+ domain. If no "secure" TLSA records are found, DANE TLS is not
+ applicable and mail delivery proceeds with pre-DANE opportunistic
+ TLS (which, being best-effort, degrades to cleartext delivery when
+ STARTTLS is not available or the TLS handshake fails).
+
+ Insecure CNAME: The input domain is a CNAME alias, but the ultimate
+ network address RRset is "insecure" (see Section 2.1.1). If the
+ initial CNAME response is also "insecure", DANE TLS does not
+ apply. Otherwise, this case is treated just like the non-CNAME
+ case above, where a search is performed for a TLSA record with the
+ original input domain as the candidate TLSA base domain.
+
+ Secure CNAME: The input domain is a CNAME alias, and the ultimate
+ network address RRset is "secure" (see Section 2.1.1). Two
+ candidate TLSA base domains are tried: the fully CNAME-expanded
+ initial name and, failing that, then the initial name itself.
+
+
+
+
+
+
+
+
+
+
+
+
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+ In summary, if it is possible to securely obtain the full, CNAME-
+ expanded, DNSSEC-validated address records for the input domain, then
+ that name is the preferred TLSA base domain. Otherwise, the
+ unexpanded input-MX domain is the candidate TLSA base domain. When
+ no "secure" TLSA records are found at either the CNAME-expanded or
+ unexpanded domain, then DANE TLS does not apply for mail delivery via
+ the input domain in question. And, as always, errors, bogus or
+ indeterminate results for any query in the process MUST result in
+ delaying or abandoning delivery.
+
+2.2.3. TLSA record lookup
+
+ Each candidate TLSA base domain (the original or fully CNAME-expanded
+ name of a non-MX destination or a particular MX hostname of an MX
+ destination) is in turn prefixed with service labels of the form
+ "_<port>._tcp". The resulting domain name is used to issue a DNSSEC
+ query with the query type set to TLSA ([RFC6698] Section 7.1).
+
+ For SMTP, the destination TCP port is typically 25, but this may be
+ different with custom routes specified by the MTA administrator in
+ which case the SMTP client MUST use the appropriate number in the
+ "_<port>" prefix in place of "_25". If, for example, the candidate
+ base domain is "mx.example.com", and the SMTP connection is to port
+ 25, the TLSA RRset is obtained via a DNSSEC query of the form:
+
+ _25._tcp.mx.example.com. IN TLSA ?
+
+ The query response may be a CNAME, or the actual TLSA RRset. If the
+ response is a CNAME, the SMTP client (through the use of its
+ security-aware stub resolver) restarts the TLSA query at the target
+ domain, following CNAMEs as appropriate and keeping track of whether
+ the entire chain is "secure". If any "insecure" records are
+ encountered, or the TLSA records don't exist, the next candidate TLSA
+ base is tried instead.
+
+ If the ultimate response is a "secure" TLSA RRset, then the candidate
+ TLSA base domain will be the actual TLSA base domain and the TLSA
+ RRset will constitute the TLSA records for the destination. If none
+ of the candidate TLSA base domains yield "secure" TLSA records then
+ delivery MAY proceed via pre-DANE opportunistic TLS. SMTP clients
+ MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades
+ or even to skip SMTP servers that fail authentication, but MUST NOT
+ misrepresent authentication success as either a secure connection to
+ the SMTP server or as a secure delivery to the intended next-hop
+ domain.
+
+ TLSA record publishers may leverage CNAMEs to reference a single
+ authoritative TLSA RRset specifying a common Certification Authority
+
+
+
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+ or a common end entity certificate to be used with multiple TLS
+ services. Such CNAME expansion does not change the SMTP client's
+ notion of the TLSA base domain; thus, when _25._tcp.mx.example.com is
+ a CNAME, the base domain remains mx.example.com and this is still the
+ reference identifier used together with the next-hop domain in peer
+ certificate name checks.
+
+ Note, shared end entity certificate associations expose the
+ publishing domain to substitution attacks, where an MITM attacker can
+ reroute traffic to a different server that shares the same end entity
+ certificate. Such shared end entity records SHOULD be avoided unless
+ the servers in question are functionally equivalent (an active
+ attacker gains nothing by diverting client traffic from one such
+ server to another).
+
+ For example, given the DNSSEC validated records below:
+
+ example.com. IN MX 0 mx1.example.com.
+ example.com. IN MX 0 mx2.example.com.
+ _25._tcp.mx1.example.com. IN CNAME tlsa211._dane.example.com.
+ _25._tcp.mx2.example.com. IN CNAME tlsa211._dane.example.com.
+ tlsa211._dane.example.com. IN TLSA 2 1 1 e3b0c44298fc1c149a...
+
+ The SMTP servers mx1.example.com and mx2.example.com will be expected
+ to have certificates issued under a common trust anchor, but each MX
+ hostname's TLSA base domain remains unchanged despite the above CNAME
+ records. Correspondingly, each SMTP server will be associated with a
+ pair of reference identifiers consisting of its hostname plus the
+ next-hop domain "example.com".
+
+ If, during TLSA resolution (including possible CNAME indirection), at
+ least one "secure" TLSA record is found (even if not usable because
+ it is unsupported by the implementation or support is
+ administratively disabled), then the corresponding host has signaled
+ its commitment to implement TLS. The SMTP client MUST NOT deliver
+ mail via the corresponding host unless a TLS session is negotiated
+ via STARTTLS. This is required to avoid MITM STARTTLS downgrade
+ attacks.
+
+ As noted previously (in Section Section 2.2.2), when no "secure" TLSA
+ records are found at the fully CNAME-expanded name, the original
+ unexpanded name MUST be tried instead. This supports customers of
+ hosting providers where the provider's zone cannot be validated with
+ DNSSEC, but the customer has shared appropriate key material with the
+ hosting provider to enable TLS via SNI. Intermediate names that
+ arise during CNAME expansion that are neither the original, nor the
+ final name, are never candidate TLSA base domains, even if "secure".
+
+
+
+
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+3. DANE authentication
+
+ This section describes which TLSA records are applicable to SMTP
+ opportunistic DANE TLS and how to apply such records to authenticate
+ the SMTP server. With opportunistic DANE TLS, both the TLS support
+ implied by the presence of DANE TLSA records and the verification
+ parameters necessary to authenticate the TLS peer are obtained
+ together. In contrast to protocols where channel security policy is
+ set exclusively by the client, authentication via this protocol is
+ expected to be less prone to connection failure caused by
+ incompatible configuration of the client and server.
+
+3.1. TLSA certificate usages
+
+ The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
+ via combinations of 3 numeric parameters. The numeric values of
+ these parameters were later given symbolic names in
+ [I-D.ietf-dane-registry-acronyms]. The rest of the TLSA record is
+ the "certificate association data field", which specifies the full or
+ digest value of a certificate or public key. The parameters are:
+
+ The TLSA Certificate Usage field: Section 2.1.1 of [RFC6698]
+ specifies 4 values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and DANE-
+ EE(3). There is an additional private-use value: PrivCert(255).
+ All other values are reserved for use by future specifications.
+
+ The selector field: Section 2.1.2 of [RFC6698] specifies 2 values:
+ Cert(0), SPKI(1). There is an additional private-use value:
+ PrivSel(255). All other values are reserved for use by future
+ specifications.
+
+ The matching type field: Section 2.1.3 of [RFC6698] specifies 3
+ values: Full(0), SHA2-256(1), SHA2-512(2). There is an additional
+ private-use value: PrivMatch(255). All other values are reserved
+ for use by future specifications.
+
+ We may think of TLSA Certificate Usage values 0 through 3 as a
+ combination of two one-bit flags. The low bit chooses between trust
+ anchor (TA) and end entity (EE) certificates. The high bit chooses
+ between public PKI issued and domain-issued certificates.
+
+ The selector field specifies whether the TLSA RR matches the whole
+ certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1). The
+ subjectPublicKeyInfo is an ASN.1 DER encoding of the certificate's
+ algorithm id, any parameters and the public key data.
+
+ The matching type field specifies how the TLSA RR Certificate
+ Association Data field is to be compared with the certificate or
+
+
+
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+ public key. A value of Full(0) means an exact match: the full DER
+ encoding of the certificate or public key is given in the TLSA RR. A
+ value of SHA2-256(1) means that the association data matches the
+ SHA2-256 digest of the certificate or public key, and likewise
+ SHA2-512(2) means a SHA2-512 digest is used.
+
+ Since opportunistic DANE TLS will be used by non-interactive MTAs,
+ with no user to "press OK" when authentication fails, reliability of
+ peer authentication is paramount. Server operators are advised to
+ publish TLSA records that are least likely to fail authentication due
+ to interoperability or operational problems. Because DANE TLS relies
+ on coordinated changes to DNS and SMTP server settings, the best
+ choice of records to publish will depend on site-specific practices.
+
+ The certificate usage element of a TLSA record plays a critical role
+ in determining how the corresponding certificate association data
+ field is used to authenticate server's certificate chain. The next
+ two subsections explain the process for certificate usages DANE-EE(3)
+ and DANE-TA(2). The third subsection briefly explains why
+ certificate usages PKIX-TA(0) and PKIX-EE(1) are not applicable with
+ opportunistic DANE TLS.
+
+ In summary, we recommend the use of either "DANE-EE(3) SPKI(1)
+ SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records
+ depending on site needs. Other combinations of TLSA parameters are
+ either explicitly unsupported, or offer little to recommend them over
+ these two.
+
+ The mandatory to support digest algorithm in [RFC6698] is
+ SHA2-256(1). When the server's TLSA RRset includes records with a
+ matching type indicating a digest record (i.e., a value other than
+ Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be
+ provided along with any other digest published, since some SMTP
+ clients may support only SHA2-256(1). If at some point the SHA2-256
+ digest algorithm is tarnished by new cryptanalytic attacks,
+ publishers will need to include an appropriate stronger digest in
+ their TLSA records, initially along with, and ultimately in place of,
+ SHA2-256.
+
+3.1.1. Certificate usage DANE-EE(3)
+
+ Authentication via certificate usage DANE-EE(3) TLSA records involves
+ simply checking that the server's leaf certificate matches the TLSA
+ record. In particular the binding of the server public key to its
+ name is based entirely on the TLSA record association. The server
+ MUST be considered authenticated even if none of the names in the
+ certificate match the client's reference identity for the server.
+
+
+
+
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+ Similarly, the expiration date of the server certificate MUST be
+ ignored, the validity period of the TLSA record key binding is
+ determined by the validity interval of the TLSA record DNSSEC
+ signature.
+
+ With DANE-EE(3) servers need not employ SNI (may ignore the client's
+ SNI message) even when the server is known under independent names
+ that would otherwise require separate certificates. It is instead
+ sufficient for the TLSA RRsets for all the domains in question to
+ match the server's default certificate. Of course with SMTP servers
+ it is simpler still to publish the same MX hostname for all the
+ hosted domains.
+
+ For domains where it is practical to make coordinated changes in DNS
+ TLSA records during SMTP server key rotation, it is often best to
+ publish end-entity DANE-EE(3) certificate associations. DANE-EE(3)
+ certificates don't suddenly stop working when leaf or intermediate
+ certificates expire, and don't fail when the server operator neglects
+ to configure all the required issuer certificates in the server
+ certificate chain.
+
+ TLSA records published for SMTP servers SHOULD, in most cases, be
+ "DANE-EE(3) SPKI(1) SHA2-256(1)" records. Since all DANE
+ implementations are required to support SHA2-256, this record type
+ works for all clients and need not change across certificate renewals
+ with the same key.
+
+3.1.2. Certificate usage DANE-TA(2)
+
+ Some domains may prefer to avoid the operational complexity of
+ publishing unique TLSA RRs for each TLS service. If the domain
+ employs a common issuing Certification Authority to create
+ certificates for multiple TLS services, it may be simpler to publish
+ the issuing authority as a trust anchor (TA) for the certificate
+ chains of all relevant services. The TLSA query domain (TLSA base
+ domain with port and protocol prefix labels) for each service issued
+ by the same TA may then be set to a CNAME alias that points to a
+ common TLSA RRset that matches the TA. For example:
+
+ example.com. IN MX 0 mx1.example.com.
+ example.com. IN MX 0 mx2.example.com.
+ _25._tcp.mx1.example.com. IN CNAME tlsa211._dane.example.com.
+ _25._tcp.mx2.example.com. IN CNAME tlsa211._dane.example.com.
+ tlsa211._dane.example.com. IN TLSA 2 1 1 e3b0c44298fc1c14....
+
+
+
+
+
+
+
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+
+ With usage DANE-TA(2) the server certificates will need to have names
+ that match one of the client's reference identifiers (see [RFC6125]).
+ The server MAY employ SNI to select the appropriate certificate to
+ present to the client.
+
+ SMTP servers that rely on certificate usage DANE-TA(2) TLSA records
+ for TLS authentication MUST include the TA certificate as part of the
+ certificate chain presented in the TLS handshake server certificate
+ message even when it is a self-signed root certificate. At this
+ time, many SMTP servers are not configured with a comprehensive list
+ of trust anchors, nor are they expected to at any point in the
+ future. Some MTAs will ignore all locally trusted certificates when
+ processing usage DANE-TA(2) TLSA records. Thus even when the TA
+ happens to be a public Certification Authority known to the SMTP
+ client, authentication is likely to fail unless the TA certificate is
+ included in the TLS server certificate message.
+
+ TLSA records with selector Full(0) are discouraged. While these
+ potentially obviate the need to transmit the TA certificate in the
+ TLS server certificate message, client implementations may not be
+ able to augment the server certificate chain with the data obtained
+ from DNS, especially when the TLSA record supplies a bare key
+ (selector SPKI(1)). Since the server will need to transmit the TA
+ certificate in any case, server operators SHOULD publish TLSA records
+ with a selector other than Full(0) and avoid potential
+ interoperability issues with large TLSA records containing full
+ certificates or keys.
+
+ TLSA Publishers employing DANE-TA(2) records SHOULD publish records
+ with a selector of Cert(0). Such TLSA records are associated with
+ the whole trust anchor certificate, not just with the trust anchor
+ public key. In particular, the SMTP client SHOULD then apply any
+ relevant constraints from the trust anchor certificate, such as, for
+ example, path length constraints.
+
+ While a selector of SPKI(1) may also be employed, the resulting TLSA
+ record will not specify the full trust anchor certificate content,
+ and elements of the trust anchor certificate other than the public
+ key become mutable. This may, for example, allow a subsidiary CA to
+ issue a chain that violates the trust anchor's path length or name
+ constraints.
+
+3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1)
+
+ As noted in the introduction, SMTP clients cannot, without relying on
+ DNSSEC for secure MX records and DANE for STARTTLS support signaling,
+ perform server identity verification or prevent STARTTLS downgrade
+ attacks. The use of PKIX CAs offers no added security since an
+
+
+
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+
+ attacker capable of compromising DNSSEC is free to replace any PKIX-
+ TA(0) or PKIX-EE(1) TLSA records with records bearing any convenient
+ non-PKIX certificate usage.
+
+ SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX-
+ TA(0) or PKIX-EE(1). SMTP clients cannot be expected to be
+ configured with a suitably complete set of trusted public CAs.
+ Lacking a complete set of public CAs, clients would not be able to
+ verify the certificates of SMTP servers whose issuing root CAs are
+ not trusted by the client.
+
+ Opportunistic DANE TLS needs to interoperate without bilateral
+ coordination of security settings between client and server systems.
+ Therefore, parameter choices that are fragile in the absence of
+ bilateral coordination are unsupported. Nothing is lost since the
+ PKIX certificate usages cannot aid SMTP TLS security, they can only
+ impede SMTP TLS interoperability.
+
+ SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
+ or PKIX-EE(1) is undefined. SMTP clients should generally treat such
+ TLSA records as unusable.
+
+3.2. Certificate matching
+
+ When at least one usable "secure" TLSA record is found, the SMTP
+ client MUST use TLSA records to authenticate the SMTP server.
+ Messages MUST NOT be delivered via the SMTP server if authentication
+ fails, otherwise the SMTP client is vulnerable to MITM attacks.
+
+3.2.1. DANE-EE(3) name checks
+
+ The SMTP client MUST NOT perform certificate name checks with
+ certificate usage DANE-EE(3), see Section 3.1.1 above.
+
+3.2.2. DANE-TA(2) name checks
+
+ To match a server via a TLSA record with certificate usage DANE-
+ TA(2), the client MUST perform name checks to ensure that it has
+ reached the correct server. In all DANE-TA(2) cases the SMTP client
+ MUST include the TLSA base domain as one of the valid reference
+ identifiers for matching the server certificate.
+
+ TLSA records for MX hostnames: If the TLSA base domain was obtained
+ indirectly via a "secure" MX lookup (including any CNAME-expanded
+ name of an MX hostname), then the original next-hop domain used in
+ the MX lookup MUST be included as as a second reference
+ identifier. The CNAME-expanded original next-hop domain MUST be
+ included as a third reference identifier if different from the
+
+
+
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+ original next-hop domain. When the client MTA is employing DANE
+ TLS security despite "insecure" MX redirection the MX hostname is
+ the only reference identifier.
+
+ TLSA records for Non-MX hostnames: If MX records were not used
+ (e.g., if none exist) and the TLSA base domain is the CNAME-
+ expanded original next-hop domain, then the original next-hop
+ domain MUST be included as a second reference identifier.
+
+ Accepting certificates with the original next-hop domain in addition
+ to the MX hostname allows a domain with multiple MX hostnames to
+ field a single certificate bearing a single domain name (i.e., the
+ email domain) across all the SMTP servers. This also aids
+ interoperability with pre-DANE SMTP clients that are configured to
+ look for the email domain name in server certificates. For example,
+ with "secure" DNS records as below:
+
+ exchange.example.org. IN CNAME mail.example.org.
+ mail.example.org. IN CNAME example.com.
+ example.com. IN MX 10 mx10.example.com.
+ example.com. IN MX 15 mx15.example.com.
+ example.com. IN MX 20 mx20.example.com.
+ ;
+ mx10.example.com. IN A 192.0.2.10
+ _25._tcp.mx10.example.com. IN TLSA 2 0 1 ...
+ ;
+ mx15.example.com. IN CNAME mxbackup.example.com.
+ mxbackup.example.com. IN A 192.0.2.15
+ ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
+ _25._tcp.mx15.example.com. IN TLSA 2 0 1 ...
+ ;
+ mx20.example.com. IN CNAME mxbackup.example.net.
+ mxbackup.example.net. IN A 198.51.100.20
+ _25._tcp.mxbackup.example.net. IN TLSA 2 0 1 ...
+
+ Certificate name checks for delivery of mail to exchange.example.org
+ via any of the associated SMTP servers MUST accept at least the names
+ "exchange.example.org" and "example.com", which are respectively the
+ original and fully expanded next-hop domain. When the SMTP server is
+ mx10.example.com, name checks MUST accept the TLSA base domain
+ "mx10.example.com". If, despite the fact that MX hostnames are
+ required to not be aliases, the MTA supports delivery via
+ "mx15.example.com" or "mx20.example.com" then name checks MUST accept
+ the respective TLSA base domains "mx15.example.com" and
+ "mxbackup.example.net".
+
+3.2.3. Reference identifier matching
+
+
+
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+ When name checks are applicable (certificate usage DANE-TA(2)), if
+ the server certificate contains a Subject Alternative Name extension
+ ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS-
+ IDs are matched against the client's reference identifiers. The CN-
+ ID ([RFC6125]) is only considered when no DNS-IDs are present. The
+ server certificate is considered matched when one of its presented
+ identifiers ([RFC5280]) matches any of the client's reference
+ identifiers.
+
+ Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
+ The wildcard character must be entire first label of the DNS-ID or
+ CN-ID. Thus, "*.example.com" is valid, while "smtp*.example.com" and
+ "*smtp.example.com" are not. SMTP clients MUST support wildcards
+ that match the first label of the reference identifier, with the
+ remaining labels matching verbatim. For example, the DNS-ID
+ "*.example.com" matches the reference identifier "mx1.example.com".
+ SMTP clients MAY, subject to local policy allow wildcards to match
+ multiple reference identifier labels, but servers cannot expect broad
+ support for such a policy. Therefore any wildcards in server
+ certificates SHOULD match exactly one label in either the TLSA base
+ domain or the next-hop domain.
+
+4. Server key management
+
+ Two TLSA records MUST be published before employing a new EE or TA
+ public key or certificate, one matching the currently deployed key
+ and the other matching the new key scheduled to replace it. Once
+ sufficient time has elapsed for all DNS caches to expire the previous
+ TLSA RRset and related signature RRsets, servers may be configured to
+ use the new EE private key and associated public key certificate or
+ may employ certificates signed by the new trust anchor.
+
+ Once the new public key or certificate is in use, the TLSA RR that
+ matches the retired key can be removed from DNS, leaving only RRs
+ that match keys or certificates in active use.
+
+ As described in Section 3.1.2, when server certificates are validated
+ via a DANE-TA(2) trust anchor, and CNAME records are employed to
+ store the TA association data at a single location, the
+ responsibility of updating the TLSA RRset shifts to the operator of
+ the trust anchor. Before a new trust anchor is used to sign any new
+ server certificates, its certificate (digest) is added to the
+ relevant TLSA RRset. After enough time elapses for the original TLSA
+ RRset to age out of DNS caches, the new trust anchor can start
+ issuing new server certificates. Once all certificates issued under
+ the previous trust anchor have expired, its associated RRs can be
+ removed from the TLSA RRset.
+
+
+
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+ In the DANE-TA(2) key management model server operators do not
+ generally need to update DNS TLSA records after initially creating a
+ CNAME record that references the centrally operated DANE-TA(2) RRset.
+ If a particular server's key is compromised, its TLSA CNAME SHOULD be
+ replaced with a DANE-EE(3) association until the certificate for the
+ compromised key expires, at which point it can return to using CNAME
+ record. If the central trust anchor is compromised, all servers need
+ to be issued new keys by a new TA, and a shared DANE-TA(2) TLSA RRset
+ needs to be published containing just the new TA. SMTP servers
+ cannot expect broad SMTP client CRL or OCSP support.
+
+5. Digest algorithm agility
+
+ While [RFC6698] specifies multiple digest algorithms, it does not
+ specify a protocol by which the SMTP client and TLSA record publisher
+ can agree on the strongest shared algorithm. Such a protocol would
+ allow the client and server to avoid exposure to any deprecated
+ weaker algorithms that are published for compatibility with less
+ capable clients, but should be ignored when possible. We specify
+ such a protocol below.
+
+ Suppose that a DANE TLS client authenticating a TLS server considers
+ digest algorithm "BetterAlg" stronger than digest algorithm
+ "WorseAlg". Suppose further that a server's TLSA RRset contains some
+ records with "BetterAlg" as the digest algorithm. Finally, suppose
+ that for every raw public key or certificate object that is included
+ in the server's TLSA RRset in digest form, whenever that object
+ appears with algorithm "WorseAlg" with some usage and selector it
+ also appears with algorithm "BetterAlg" with the same usage and
+ selector. In that case our client can safely ignore TLSA records
+ with the weaker algorithm "WorseAlg", because it suffices to check
+ the records with the stronger algorithm "BetterAlg".
+
+ Server operators MUST ensure that for any given usage and selector,
+ each object (certificate or public key), for which a digest
+ association exists in the TLSA RRset, is published with the SAME SET
+ of digest algorithms as all other objects that published with that
+ usage and selector. In other words, for each usage and selector, the
+ records with non-zero matching types will correspond to on a cross-
+ product of a set of underlying objects and a fixed set of digest
+ algorithms that apply uniformly to all the objects.
+
+ To achieve digest algorithm agility, all published TLSA RRsets for
+ use with opportunistic DANE TLS for SMTP MUST conform to the above
+ requirements. Then, for each combination of usage and selector, SMTP
+ clients can simply ignore all digest records except those that employ
+ the strongest digest algorithm. The ordering of digest algorithms by
+ strength is not specified in advance, it is entirely up to the SMTP
+
+
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+ client. SMTP client implementations SHOULD make the digest algorithm
+ preference order configurable. Only the future will tell which
+ algorithms might be weakened by new attacks and when.
+
+ Note, TLSA records with a matching type of Full(0), that publish the
+ full value of a certificate or public key object, play no role in
+ digest algorithm agility. They neither trump the processing of
+ records that employ digests, nor are they ignored in the presence of
+ any records with a digest (i.e. non-zero) matching type.
+
+ SMTP clients SHOULD use digest algorithm agility when processing the
+ DANE TLSA records of an SMTP server. Algorithm agility is to be
+ applied after first discarding any unusable or malformed records
+ (unsupported digest algorithm, or incorrect digest length). Thus,
+ for each usage and selector, the client SHOULD process only any
+ usable records with a matching type of Full(0) and the usable records
+ whose digest algorithm is believed to be the strongest among usable
+ records with the given usage and selector.
+
+ The main impact of this requirement is on key rotation, when the TLSA
+ RRset is pre-populated with digests of new certificates or public
+ keys, before these replace or augment their predecessors. Were the
+ newly introduced RRs to include previously unused digest algorithms,
+ clients that employ this protocol could potentially ignore all the
+ digests corresponding to the current keys or certificates, causing
+ connectivity issues until the new keys or certificates are deployed.
+ Similarly, publishing new records with fewer digests could cause
+ problems for clients using cached TLSA RRsets that list both the old
+ and new objects once the new keys are deployed.
+
+ To avoid problems, server operators SHOULD apply the following
+ strategy:
+
+ o When changing the set of objects published via the TLSA RRset
+ (e.g. during key rotation), DO NOT change the set of digest
+ algorithms used; change just the list of objects.
+
+ o When changing the set of digest algorithms, change only the set of
+ algorithms, and generate a new RRset in which all the current
+ objects are re-published with the new set of digest algorithms.
+
+ After either of these two changes are made, the new TLSA RRset should
+ be left in place long enough that the older TLSA RRset can be flushed
+ from caches before making another change.
+
+6. Mandatory TLS Security
+
+
+
+
+
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+ An MTA implementing this protocol may require a stronger security
+ assurance when sending email to selected destinations. The sending
+ organization may need to send sensitive email and/or may have
+ regulatory obligations to protect its content. This protocol is not
+ in conflict with such a requirement, and in fact can often simplify
+ authenticated delivery to such destinations.
+
+ Specifically, with domains that publish DANE TLSA records for their
+ MX hostnames, a sending MTA can be configured to use the receiving
+ domains's DANE TLSA records to authenticate the corresponding SMTP
+ server. Authentication via DANE TLSA records is easier to manage, as
+ changes in the receiver's expected certificate properties are made on
+ the receiver end and don't require manually communicated
+ configuration changes. With mandatory DANE TLS, when no usable TLSA
+ records are found, message delivery is delayed. Thus, mail is only
+ sent when an authenticated TLS channel is established to the remote
+ SMTP server.
+
+ Administrators of mail servers that employ mandatory DANE TLS, need
+ to carefully monitor their mail logs and queues. If a partner domain
+ unwittingly misconfigures their TLSA records, disables DNSSEC, or
+ misconfigures SMTP server certificate chains, mail will be delayed
+ and may bounce if the issue is not resolved in a timely manner.
+
+7. Note on DANE for Message User Agents
+
+ We note that the SMTP protocol is also used between Message User
+ Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409]. In
+ [RFC6186] a protocol is specified that enables an MUA to dynamically
+ locate the MSA based on the user's email address. SMTP connection
+ security considerations for MUAs implementing [RFC6186] are largely
+ analogous to connection security requirements for MTAs, and this
+ specification could be applied largely verbatim with DNS MX records
+ replaced by corresponding DNS Service (SRV) records
+ [I-D.ietf-dane-srv].
+
+ However, until MUAs begin to adopt the dynamic configuration
+ mechanisms of [RFC6186] they are adequately served by more
+ traditional static TLS security policies. Specification of DANE TLS
+ for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
+ is left to future documents that focus specifically on SMTP security
+ between MUAs and MSAs.
+
+
+
+
+
+
+
+
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+8. Interoperability considerations
+
+8.1. SNI support
+
+ To ensure that the server sends the right certificate chain, the SMTP
+ client MUST send the TLS SNI extension containing the TLSA base
+ domain. This precludes the use of the backward compatible SSL 2.0
+ compatible SSL HELLO by the SMTP client. The minimum SSL/TLS client
+ HELLO version for SMTP clients performing DANE authentication is SSL
+ 3.0, but a client that offers SSL 3.0 MUST also offer at least TLS
+ 1.0 and MUST include the SNI extension. Servers that don't make use
+ of SNI MAY negotiate SSL 3.0 if offered by the client.
+
+ Each SMTP server MUST present a certificate chain (see [RFC5246]
+ Section 7.4.2) that matches at least one of the TLSA records. The
+ server MAY rely on SNI to determine which certificate chain to
+ present to the client. Clients that don't send SNI information may
+ not see the expected certificate chain.
+
+ If the server's TLSA records match the server's default certificate
+ chain, the server need not support SNI. In either case, the server
+ need not include the SNI extension in its TLS HELLO as simply
+ returning a matching certificate chain is sufficient. Servers MUST
+ NOT enforce the use of SNI by clients, as the client may be using
+ unauthenticated opportunistic TLS and may not expect any particular
+ certificate from the server. If the client sends no SNI extension,
+ or sends an SNI extension for an unsupported domain, the server MUST
+ simply send some fallback certificate chain of its choice. The
+ reason for not enforcing strict matching of the requested SNI
+ hostname is that DANE TLS clients are typically willing to accept
+ multiple server names, but can only send one name in the SNI
+ extension. The server's fallback certificate may match a different
+ name acceptable to the client, e.g., the original next-hop domain.
+
+8.2. Anonymous TLS cipher suites
+
+ Since many SMTP servers either do not support or do not enable any
+ anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD
+ offer to negotiate a typical set of non-anonymous cipher suites
+ required for interoperability with such servers. An SMTP client
+ employing pre-DANE opportunistic TLS MAY in addition include one or
+ more anonymous TLS cipher suites in its TLS HELLO. SMTP servers,
+ that need to interoperate with opportunistic TLS clients SHOULD be
+ prepared to interoperate with such clients by either always selecting
+ a mutually supported non-anonymous cipher suite or by correctly
+ handling client connections that negotiate anonymous cipher suites.
+
+
+
+
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+ Note that while SMTP server operators are under no obligation to
+ enable anonymous cipher suites, no security is gained by sending
+ certificates to clients that will ignore them. Indeed support for
+ anonymous cipher suites in the server makes audit trails more
+ informative. Log entries that record connections that employed an
+ anonymous cipher suite record the fact that the clients did not care
+ to authenticate the server.
+
+9. Operational Considerations
+
+9.1. Client Operational Considerations
+
+ An operational error on the sending or receiving side that cannot be
+ corrected in a timely manner may, at times, lead to consistent
+ failure to deliver time-sensitive email. The sending MTA
+ administrator may have to choose between letting email queue until
+ the error is resolved and disabling opportunistic or mandatory DANE
+ TLS for one or more destinations. The choice to disable DANE TLS
+ security should not be made lightly. Every reasonable effort should
+ be made to determine that problems with mail delivery are the result
+ of an operational error, and not an attack. A fallback strategy may
+ be to configure explicit out-of-band TLS security settings if
+ supported by the sending MTA.
+
+ SMTP clients may deploy opportunistic DANE TLS incrementally by
+ enabling it only for selected sites, or may occasionally need to
+ disable opportunistic DANE TLS for peers that fail to interoperate
+ due to misconfiguration or software defects on either end. Some
+ implementations MAY support DANE TLS in an "audit only" mode in which
+ failure to achieve the requisite security level is logged as a
+ warning and delivery proceeds at a reduced security level. Unless
+ local policy specifies "audit only" or that opportunistic DANE TLS is
+ not to be used for a particular destination, an SMTP client MUST NOT
+ deliver mail via a server whose certificate chain fails to match at
+ least one TLSA record when usable TLSA records are found for that
+ server.
+
+9.2. Publisher Operational Considerations
+
+ SMTP servers that publish certificate usage DANE-TA(2) associations
+ MUST include the TA certificate in their TLS server certificate
+ chain, even when that TA certificate is a self-signed root
+ certificate.
+
+ TLSA Publishers must follow the digest agility guidelines in
+ Section 5 and must make sure that all objects published in digest
+ form for a particular usage and selector are published with the same
+ set of digest algorithms.
+
+
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+ TLSA Publishers should follow the TLSA publication size guidance
+ found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".
+
+10. Security Considerations
+
+ This protocol leverages DANE TLSA records to implement MITM resistant
+ opportunistic channel security for SMTP. For destination domains
+ that sign their MX records and publish signed TLSA records for their
+ MX hostnames, this protocol allows sending MTAs to securely discover
+ both the availability of TLS and how to authenticate the destination.
+
+ This protocol does not aim to secure all SMTP traffic, as that is not
+ practical until DNSSEC and DANE adoption are universal. The
+ incremental deployment provided by following this specification is a
+ best possible path for securing SMTP. This protocol coexists and
+ interoperates with the existing insecure Internet email backbone.
+
+ The protocol does not preclude existing non-opportunistic SMTP TLS
+ security arrangements, which can continue to be used as before via
+ manual configuration with negotiated out-of-band key and TLS
+ configuration exchanges.
+
+ Opportunistic SMTP TLS depends critically on DNSSEC for downgrade
+ resistance and secure resolution of the destination name. If DNSSEC
+ is compromised, it is not possible to fall back on the public CA PKI
+ to prevent MITM attacks. A successful breach of DNSSEC enables the
+ attacker to publish TLSA usage 3 certificate associations, and
+ thereby bypass any security benefit the legitimate domain owner might
+ hope to gain by publishing usage 0 or 1 TLSA RRs. Given the lack of
+ public CA PKI support in existing MTA deployments, avoiding
+ certificate usages 0 and 1 simplifies implementation and deployment
+ with no adverse security consequences.
+
+ Implementations must strictly follow the portions of this
+ specification that indicate when it is appropriate to initiate a non-
+ authenticated connection or cleartext connection to a SMTP server.
+ Specifically, in order to prevent downgrade attacks on this protocol,
+ implementation must not initiate a connection when this specification
+ indicates a particular SMTP server must be considered unreachable.
+
+11. IANA considerations
+
+ This specification requires no support from IANA.
+
+12. Acknowledgements
+
+ The authors would like to extend great thanks to Tony Finch, who
+ started the original version of a DANE SMTP document. His work is
+
+
+
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+ greatly appreciated and has been incorporated into this document.
+ The authors would like to additionally thank Phil Pennock for his
+ comments and advice on this document.
+
+ Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
+ to begin work on this memo and provided feedback on early drafts.
+ Thanks to Patrick Koetter, Perry Metzger and Nico Williams for
+ valuable review comments. Thanks also to Wietse Venema who created
+ Postfix, and whose advice and feedback were essential to the
+ development of the Postfix DANE implementation.
+
+13. References
+
+13.1. Normative References
+
+ [I-D.ietf-dane-ops]
+ Dukhovni, V. and W. Hardaker, "DANE TLSA implementation
+ and operational guidance", draft-ietf-dane-ops-00 (work in
+ progress), October 2013.
+
+ [RFC1035] Mockapetris, P., "Domain names - implementation and
+ specification", STD 13, RFC 1035, November 1987.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
+ Transport Layer Security", RFC 3207, February 2002.
+
+ [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "DNS Security Introduction and Requirements", RFC
+ 4033, March 2005.
+
+ [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "Resource Records for the DNS Security Extensions",
+ RFC 4034, March 2005.
+
+ [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
+ Rose, "Protocol Modifications for the DNS Security
+ Extensions", RFC 4035, March 2005.
+
+ [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
+ (TLS) Protocol Version 1.2", RFC 5246, August 2008.
+
+ [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
+ Housley, R., and W. Polk, "Internet X.509 Public Key
+ Infrastructure Certificate and Certificate Revocation List
+ (CRL) Profile", RFC 5280, May 2008.
+
+
+
+Dukhovni & Hardaker Expires November 26, 2014 [Page 32]
+?
+Internet-Draft SMTP security via opportunistic DANE TLS May 2014
+
+
+ [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
+ October 2008.
+
+ [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
+ Extension Definitions", RFC 6066, January 2011.
+
+ [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
+ Verification of Domain-Based Application Service Identity
+ within Internet Public Key Infrastructure Using X.509
+ (PKIX) Certificates in the Context of Transport Layer
+ Security (TLS)", RFC 6125, March 2011.
+
+ [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
+ Submission/Access Services", RFC 6186, March 2011.
+
+ [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
+ DNS", RFC 6672, June 2012.
+
+ [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
+ of Named Entities (DANE) Transport Layer Security (TLS)
+ Protocol: TLSA", RFC 6698, August 2012.
+
+13.2. Informative References
+
+ [I-D.ietf-dane-registry-acronyms]
+ Gudmundsson, O., "Adding acronyms to simplify DANE
+ conversations", draft-ietf-dane-registry-acronyms-01 (work
+ in progress), October 2013.
+
+ [I-D.ietf-dane-srv]
+ Finch, T., "Using DNS-Based Authentication of Named
+ Entities (DANE) TLSA records with SRV and MX records.",
+ draft-ietf-dane-srv-02 (work in progress), February 2013.
+
+ [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
+ 2009.
+
+ [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
+ STD 72, RFC 6409, November 2011.
+
+Authors' Addresses
+
+ Viktor Dukhovni
+ Two Sigma
+
+ Email: ietf-dane@???
+
+
+
+
+
+Dukhovni & Hardaker Expires November 26, 2014 [Page 33]
+?
+Internet-Draft SMTP security via opportunistic DANE TLS May 2014
+
+
+ Wes Hardaker
+ Parsons
+ P.O. Box 382
+ Davis, CA 95617
+ US
+
+ Email: ietf@???
+
+
+
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+
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+Dukhovni & Hardaker Expires November 26, 2014 [Page 34]
