--- /dev/null
+
+
+
+
+DANE V. Dukhovni
+Internet-Draft Unaffiliated
+Intended status: Standards Track W. Hardaker
+Expires: February 18, 2015 Parsons
+ August 17, 2014
+
+
+ Updates to and Operational Guidance for the DANE Protocol
+ draft-ietf-dane-ops-06
+
+Abstract
+
+ This memo clarifies and updates the DANE TLSA protocol based on
+ implementation experience since the publication of the original DANE
+ specification in [RFC6698]. It also contains guidance for DANE
+ implementers and operators.
+
+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 18, 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
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+
+
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+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
+ 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
+ 2. DANE TLSA Record Overview . . . . . . . . . . . . . . . . . . 4
+ 2.1. Example TLSA record . . . . . . . . . . . . . . . . . . . 6
+ 3. DANE TLS Requirements . . . . . . . . . . . . . . . . . . . . 6
+ 4. Certificate-Usage-Specific DANE Updates and Guidelines . . . 7
+ 4.1. Certificate Usage DANE-EE(3) . . . . . . . . . . . . . . 7
+ 4.2. Certificate Usage DANE-TA(2) . . . . . . . . . . . . . . 8
+ 4.3. Certificate Usage PKIX-EE(1) . . . . . . . . . . . . . . 11
+ 4.4. Certificate Usage PKIX-TA(0) . . . . . . . . . . . . . . 12
+ 4.5. Opportunistic Security and PKIX usages . . . . . . . . . 12
+ 5. Service Provider and TLSA Publisher Synchronization . . . . . 13
+ 6. TLSA Base Domain and CNAMEs . . . . . . . . . . . . . . . . . 15
+ 7. TLSA Publisher Requirements . . . . . . . . . . . . . . . . . 16
+ 7.1. Key rollover with fixed TLSA Parameters . . . . . . . . . 17
+ 7.2. Switching to DANE-TA from DANE-EE . . . . . . . . . . . . 18
+ 7.3. Switching to New TLSA Parameters . . . . . . . . . . . . 18
+ 7.4. TLSA Publisher Requirements Summary . . . . . . . . . . . 19
+ 8. Digest Algorithm Agility . . . . . . . . . . . . . . . . . . 19
+ 9. General DANE Guidelines . . . . . . . . . . . . . . . . . . . 20
+ 9.1. DANE DNS Record Size Guidelines . . . . . . . . . . . . . 21
+ 9.2. Certificate Name Check Conventions . . . . . . . . . . . 21
+ 9.3. Design Considerations for Protocols Using DANE . . . . . 23
+ 10. Interaction with Certificate Transparency . . . . . . . . . . 23
+ 11. Note on DNSSEC Security . . . . . . . . . . . . . . . . . . . 24
+ 12. Summary of Updates to RFC6698 . . . . . . . . . . . . . . . . 25
+ 13. Security Considerations . . . . . . . . . . . . . . . . . . . 26
+ 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
+ 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
+ 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
+ 16.1. Normative References . . . . . . . . . . . . . . . . . . 27
+ 16.2. Informative References . . . . . . . . . . . . . . . . . 28
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
+
+1. Introduction
+
+ [RFC6698] specifies a new DNS resource record "TLSA" that associates
+ a public certificate or public key of a trusted leaf or issuing
+ authority with the corresponding TLS transport endpoint. These DANE
+ TLSA records, when validated by DNSSEC, can be used to augment or
+ replace the trust model of the existing public Certification
+ Authority (CA) Public Key Infrastructure (PKI).
+
+ [RFC6698] defines three TLSA record fields with respectively 4, 2 and
+ 3 currently specified values. These yield 24 distinct combinations
+ of TLSA record types. This many options have lead to implementation
+
+
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+ and operational complexity. This memo will recommend best-practice
+ choices to help simplify implementation and deployment given these
+ plethora of choices.
+
+ Implementation complexity also arises from the fact that the TLS
+ transport endpoint is often specified indirectly via Service Records
+ (SRV), Mail Exchange (MX) records, CNAME records or other mechanisms
+ that map an abstract service domain to a concrete server domain.
+ With service indirection there are multiple potential places for
+ clients to find the relevant TLSA records. Service indirection is
+ often used to implement "virtual hosting", where a single Service
+ Provider transport endpoint simultaneously supports multiple hosted
+ domain names. With services that employ TLS, such hosting
+ arrangements may require the Service Provider to deploy multiple
+ pairs of private keys and certificates with TLS clients signaling the
+ desired domain via the Server Name Indication (SNI) extension
+ ([RFC6066], section 3). This memo provides operational guidelines
+ intended to maximize interoperability between DANE TLS clients and
+ servers.
+
+ In the context of this memo, channel security is assumed to be
+ provided by TLS or DTLS. The Transport Layer Security (TLS)
+ [RFC5246] and Datagram Transport Layer Security (DTLS) [RFC6347]
+ protocols provide secured TCP and UDP communication over IP. By
+ convention, "TLS" will be used throughout this document and, unless
+ otherwise specified, the text applies equally well to the DTLS
+ protocol. Used without authentication, TLS provides protection only
+ against eavesdropping through its use of encryption. With
+ authentication, TLS also provides integrity protection and
+ authentication, which protect the transport against man-in-the-middle
+ (MITM) attacks.
+
+ Other related documents that build on [RFC6698] are
+ [I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp-with-dane]. In
+ Section 12 we summarize the updates this document makes to [RFC6698].
+
+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 are used throughout this document:
+
+ Service Provider: A company or organization that offers to host a
+ service on behalf of a Customer Domain. The original domain name
+ associated with the service often remains under the control of the
+
+
+
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+ customer. Connecting applications may be directed to the Service
+ Provider via a redirection resource record. Example redirection
+ records include MX, SRV, and CNAME. The Service Provider
+ frequently provides services for many customers and must carefully
+ manage any TLS credentials offered to connecting applications to
+ ensure name matching is handled easily by the applications.
+
+ Customer Domain: As described above, a client may be interacting
+ with a service that is hosted by a third party. We will refer to
+ the domain name used to locate the service prior to any
+ redirection, as the "Customer Domain".
+
+ TLSA Publisher: The entity responsible for publishing a TLSA record
+ within a DNS zone. This zone will be assumed DNSSEC-signed and
+ validatable to a trust anchor, unless otherwise specified. If the
+ Customer Domain is not outsourcing their DNS service, the TLSA
+ Publisher will be the customer themselves. Otherwise, the TLSA
+ Publisher is sometimes the operator of the outsourced DNS service.
+
+ public key: The term "public key" is short-hand for the
+ subjectPublicKeyInfo component of a PKIX [RFC5280] certificate.
+
+ SNI: The "Server Name Indication" (SNI) TLS protocol extension
+ allows a TLS client to request a connection to a particular
+ service name of a TLS server ([RFC6066], section 3). Without this
+ TLS extension, a TLS server has no choice but to offer a PKIX
+ certificate with a default list of server names, making it
+ difficult to host multiple Customer Domains at the same IP-
+ addressed based TLS service endpoint (i.e., "secure virtual
+ hosting").
+
+ TLSA parameters: In [RFC6698] the TLSA record is defined to consist
+ of four fields. The first three of these are numeric parameters
+ that specify the meaning of the data in fourth and final field.
+ To avoid language contortions when we need to distinguish between
+ the first three fields that together define a TLSA record "type"
+ and the fourth that provides a data value of that type, we will
+ call the first three fields "TLSA parameters", or sometimes just
+ "parameters" when obvious from context.
+
+2. DANE TLSA Record Overview
+
+
+
+
+
+
+
+
+
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+
+ DANE TLSA [RFC6698] specifies a protocol for publishing TLS server
+ certificate associations via DNSSEC [RFC4033] [RFC4034] [RFC4035].
+ The DANE TLSA specification defines multiple TLSA RR types via
+ combinations of numeric values of the first three fields of the TLSA
+ record (i.e. the "TLSA parameters"). The numeric values of these
+ parameters were later given symbolic names in [RFC7218]. These
+ parameters are:
+
+ The 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 PKIX, or public PKI issued, and DANE, or domain-issued trust
+ anchors:
+
+ o When the low bit is set (PKIX-EE(1) and DANE-EE(3)) the TLSA
+ record matches an EE certificate (also commonly referred to as a
+ leaf or server certificate.)
+
+ o When the low bit is not set (PKIX-TA(0) and DANE-TA(2)) the TLSA
+ record matches a trust anchor (a Certification Authority) that
+ issued one of the certificates in the server certificate chain.
+
+ o When the high bit is set (DANE-TA(2) and DANE-EE(3)), the server
+ certificate chain is domain-issued and may be verified without
+ reference to any pre-existing public certification authority PKI.
+ Trust is entirely placed on the content of the TLSA records
+ obtained via DNSSEC.
+
+
+
+
+
+
+
+
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+ o When the high bit is not set (PKIX-TA(0) and PKIX-EE(1)), the TLSA
+ record publishes a server policy stating that its certificate
+ chain must pass PKIX validation [RFC5280] and the DANE TLSA record
+ is used to signal an additional requirement that the PKIX
+ validated server certificate chain also contains the referenced CA
+ or EE certificate.
+
+ The selector field specifies whether the TLSA RR matches the whole
+ certificate (Cert(0)) or just its subjectPublicKeyInfo (SPKI(1)).
+ The subjectPublicKeyInfo is an ASN.1 DER encoding of the
+ certificate's algorithm id, any parameters and the public key data.
+
+ The matching type field specifies how the TLSA RR Certificate
+ Association Data field is to be compared with the certificate or
+ public key. A value of Full(0) means an exact match: the full DER
+ encoding of the certificate or public key is given in the TLSA RR. A
+ value of SHA2-256(1) means that the association data matches the
+ SHA2-256 digest of the certificate or public key, and likewise
+ SHA2-512(2) means a SHA2-512 digest is used. Of the two digest
+ algorithms, for now only SHA2-256(1) is mandatory to implement.
+ Clients SHOULD implement SHA2-512(2), but servers SHOULD NOT
+ exclusively publish SHA2-512(2) digests. The digest algorithm
+ agility protocol defined in Section 8 SHOULD be used by clients to
+ decide how to process TLSA RRsets that employ multiple digest
+ algorithms. Server operators MUST publish TLSA RRsets that are
+ compatible with digest algorithm agility.
+
+2.1. Example TLSA record
+
+ In the example TLSA record below:
+
+ _25._tcp.mail.example.com. IN TLSA PKIX-TA Cert SHA2-256 (
+ E8B54E0B4BAA815B06D3462D65FBC7C0
+ CF556ECCF9F5303EBFBB77D022F834C0 )
+
+ The TLSA Certificate Usage is DANE-TA(2), the selector is Cert(0) and
+ the matching type is SHA2-256(1). The last field is the Certificate
+ Association Data Field, which in this case contains the SHA2-256
+ digest of the server certificate.
+
+3. DANE TLS Requirements
+
+ [RFC6698] does not discuss what versions of TLS are required when
+ using DANE records. This document specifies that TLS clients that
+ support DANE/TLSA MUST support at least TLS 1.0 and SHOULD support
+ TLS 1.2. TLS clients and servers using DANE SHOULD support the
+ "Server Name Indication" (SNI) extension of TLS.
+
+
+
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+4. Certificate-Usage-Specific DANE Updates and Guidelines
+
+ The four Certificate Usage values from the TLSA record, DANE-EE(3),
+ DANE-TA(2), PKIX-EE(1) and PKIX-TA(0), are discussed below.
+
+4.1. Certificate Usage DANE-EE(3)
+
+ In this section the meaning of DANE-EE(3) is updated from [RFC6698]
+ to specify that peer identity matching and that validity interval
+ compliance is based solely on the TLSA RRset properties. We also
+ extend [RFC6698] to cover the use of DANE authentication of raw
+ public keys [I-D.ietf-tls-oob-pubkey] via TLSA records with
+ Certificate Usage DANE-EE(3) and selector SPKI(1).
+
+ 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, with DANE-EE(3), 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 signatures.
+
+ With DANE-EE(3) servers that know all the connecting clients are
+ making use of DANE, they need not employ SNI (i.e., the may ignore
+ the client's SNI message) even when the server is known under
+ multiple domain names that would otherwise require separate
+ certificates. It is instead sufficient for the TLSA RRsets for all
+ the domain names in question to match the server's primary
+ certificate. For application protocols where the server name is
+ obtained indirectly via SRV, MX or similar records, it is simplest to
+ publish a single hostname as the target server name for all the
+ hosted domains.
+
+ In organizations where it is practical to make coordinated changes in
+ DNS TLSA records before server key rotation, it is generally best to
+ publish end-entity DANE-EE(3) certificate associations in preference
+ to other choices of certificate usage. DANE-EE(3) TLSA records
+ support multiple server names without SNI, don't suddenly stop
+ working when leaf or intermediate certificates expire, and don't fail
+ when a server operator neglects to include all the required issuer
+ certificates in the server certificate chain.
+
+ TLSA records published for DANE servers SHOULD, as a best practice,
+ be "DANE-EE(3) SPKI(1) SHA2-256(1)" records. Since all DANE
+
+
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+ 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. This TLSA record type easily supports hosting
+ arrangements with a single certificate matching all hosted domains.
+ It is also the easiest to implement correctly in the client.
+
+ Another advantage of "DANE-EE(3) SPKI(1)" (with any suitable matching
+ type) TLSA records is that they are compatible with the raw public
+ key TLS extension specified in [I-D.ietf-tls-oob-pubkey]. DANE
+ clients that support this extension can use the TLSA record to
+ authenticate servers that negotiate the use of raw public keys in
+ place of X.509 certificate chains. Provided the server adheres to
+ the requirements of Section 7, the fact that raw public keys are not
+ compatible with any other TLSA record types will not get in the way
+ of successful authentication. Clients that employ DANE to
+ authenticate the peer server SHOULD NOT negotiate the use of raw
+ public keys unless the server's TLSA RRset includes compatible TLSA
+ records.
+
+ While it is, in principle, also possible to authenticate raw public
+ keys via "DANE-EE(3) Cert(0) Full(0)" records by extracting the
+ public key from the certificate in DNS, this is in conflict with the
+ indicated selector and requires extra logic on clients that not all
+ implementations are expected to provide. Servers SHOULD NOT rely on
+ "DANE-EE(3) Cert(0) Full(0)" TLSA records to publish authentication
+ data for raw public keys.
+
+4.2. Certificate Usage DANE-TA(2)
+
+ This section updates [RFC6698] by specifying a new operational
+ requirement for servers publishing TLSA records with a usage of DANE-
+ TA(2): such servers MUST include the trust-anchor certificate in
+ their TLS server certificate message.
+
+ 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:
+
+
+
+
+
+
+
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+ www1.example.com. IN A 192.0.2.1
+ www2.example.com. IN A 192.0.2.2
+ _443._tcp.www1.example.com. IN CNAME tlsa201._dane.example.com.
+ _443._tcp.www2.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 SHOULD employ SNI to select the appropriate certificate to
+ present to the client.
+
+4.2.1. Recommended record combinations
+
+ TLSA records with selector Full(0) are NOT RECOMMENDED. 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 DNS
+ interoperability issues with large TLSA records containing full
+ certificates or keys (see Section 9.1.1).
+
+ 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 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, enable a subsidiary CA to
+ issue a chain that violates the trust anchor's path length or name
+ constraints.
+
+4.2.2. Trust anchor digests and server certificate chain
+
+ With DANE-TA(2) (these TLSA records are expected to match the digest
+ of a TA certificate or public key), a complication arises when the TA
+ certificate is omitted from the server's certificate chain, perhaps
+ on the basis of Section 7.4.2 of [RFC5246]:
+
+
+
+
+
+
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+ The sender's certificate MUST come first in the list. Each
+ following certificate MUST directly certify the one preceding
+ it. Because certificate validation requires that root keys be
+ distributed independently, the self-signed certificate that
+ specifies the root certification authority MAY be omitted from
+ the chain, under the assumption that the remote end must
+ already possess it in order to validate it in any case.
+
+ With TLSA Certificate Usage DANE-TA(2), there is no expectation that
+ the client is pre-configured with the trust anchor certificate. In
+ fact, client implementations are free to ignore all locally
+ configured trust anchors when processing usage DANE-TA(2) TLSA
+ records and may rely exclusively on the certificates provided in the
+ server's certificate chain. But, with a digest in the TLSA record,
+ the TLSA record contains neither the full trust anchor certificate
+ nor the full public key. If the TLS server's certificate chain does
+ not contain the trust anchor certificate, DANE clients will be unable
+ to authenticate the server.
+
+ TLSA Publishers that publish TLSA Certificate Usage DANE-TA(2)
+ associations with a selector of SPKI(1) or using a digest-based
+ matching type (not Full(0)) MUST ensure that the corresponding server
+ is configured to also include the trust anchor certificate in its TLS
+ handshake certificate chain, even if that certificate is a self-
+ signed root CA and would have been optional in the context of the
+ existing public CA PKI.
+
+4.2.3. Trust anchor public keys
+
+ TLSA records with TLSA Certificate Usage DANE-TA(2), selector SPKI(1)
+ and a matching type of Full(0) will publish the full public key of a
+ trust anchor via DNS. In section 6.1.1 of [RFC5280] the definition
+ of a trust anchor consists of the following four parts:
+
+ 1. the trusted issuer name,
+
+ 2. the trusted public key algorithm,
+
+ 3. the trusted public key, and
+
+ 4. optionally, the trusted public key parameters associated with the
+ public key.
+
+ Items 2-4 are precisely the contents of the subjectPublicKeyInfo
+ published in the TLSA record. The issuer name is not included in the
+ subjectPublicKeyInfo.
+
+
+
+
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+ With TLSA Certificate Usage DANE-TA(2), the client may not have the
+ associated trust anchor certificate, and cannot generally verify
+ whether a particular certificate chain is "issued by" the trust
+ anchor described in the TLSA record.
+
+ When the server certificate chain includes a CA certificate whose
+ public key matches the TLSA record, the client can match that CA as
+ the intended issuer. Otherwise, the client can only check that the
+ topmost certificate in the server's chain is "signed by" the trust
+ anchor's public key in the TLSA record. Such a check may be
+ difficult to implement, and cannot be expected to be supported by all
+ clients.
+
+ Thus, servers should not rely on "DANE-TA(2) SPKI(1) Full(0)" TLSA
+ records to be sufficient to authenticate chains issued by the
+ associated public key in the absence of a corresponding certificate
+ in the server's TLS certificate message. Servers SHOULD include the
+ TA certificate in their certificate chain.
+
+ If none of the server's certificate chain elements match a public key
+ specified in a TLSA record, and at least one "DANE-TA(2) SPKI(1)
+ Full(0)" TLSA record is available, clients are encouraged to check
+ whether the topmost certificate in the chain is signed by the
+ provided public key and has not expired, and in that case consider
+ the server authenticated, provided the rest of the chain passes
+ validation including leaf certificate name checks.
+
+4.3. Certificate Usage PKIX-EE(1)
+
+ This Certificate Usage is similar to DANE-EE(3), but in addition PKIX
+ verification is required. Therefore, name checks, certificate
+ expiration, etc., apply as they would without DANE. When, for a
+ given application protocol, DANE clients support both DANE-EE(3) and
+ PKIX-EE(1) usages, it should be noted that an attacker who can
+ compromise DNSSEC can replace these with usage DANE-EE(3) or DANE-
+ TA(2) TLSA records of their choosing, and thus bypass any PKIX
+ verification requirements.
+
+ Therefore, except when applications support only the PKIX Certificate
+ Usages (0 and 1), this Certificate Usage offers only illusory
+ incremental security over usage DANE-EE(3). It provides lower
+ operational reliability than DANE-EE(3) since some clients may not be
+ configured with the required root CA, the server's chain may be
+ incomplete or name checks may fail. PKIX-EE(1) also requires more
+ complex coordination between the Customer Domain and the Service
+ Provider in hosting arrangements. This certificate usage is NOT
+ RECOMMENDED.
+
+
+
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+4.4. Certificate Usage PKIX-TA(0)
+
+ This section updates [RFC6698] by specifying new client
+ implementation requirements. Clients that trust intermediate
+ certificates MUST be prepared to construct longer PKIX chains than
+ would be required for PKIX alone.
+
+ TLSA Certificate Usage PKIX-TA(0) allows a domain to publish
+ constraints on the set of PKIX certification authorities trusted to
+ issue certificates for its TLS servers. This TLSA record matches
+ PKIX-verified trust chains which contain an issuer certificate (root
+ or intermediate) that matches its association data field (typically a
+ certificate or digest).
+
+ As with PKIX-EE(1) case, an attacker who can compromise DNSSEC can
+ replace these with usage DANE-EE(3) or DANE-TA(2) TLSA records of his
+ choosing and thus bypass the PKIX verification requirements.
+ Therefore, except when applications support only the PKIX Certificate
+ Usages (0 and 1), this Certificate Usage offers only illusory
+ incremental security over usage DANE-TA(2). It provides lower
+ operational reliability than DANE-TA(2) since some clients may not be
+ configured with the required root CA. PKIX-TA(0) also requires more
+ complex coordination between the Customer Domain and the Service
+ Provider in hosting arrangements. This certificate usage is NOT
+ RECOMMENDED.
+
+ TLSA Publishers who publish TLSA records for a particular public root
+ CA, will expect that clients will then only accept chains anchored at
+ that root. It is possible, however, that the client's trusted
+ certificate store includes some intermediate CAs, either with or
+ without the corresponding root CA. When a client constructs a trust
+ chain leading from a trusted intermediate CA to the server leaf
+ certificate, such a "truncated" chain might not contain the trusted
+ root published in the server's TLSA record.
+
+ If the omitted root is also trusted, the client may erroneously
+ reject the server chain if it fails to determine that the shorter
+ chain it constructed extends to a longer trusted chain that matches
+ the TLSA record. Thus, when matching a usage PKIX-TA(0) TLSA record,
+ a client SHOULD NOT always stop extending the chain when the first
+ locally trusted certificate is found. If no TLSA records have
+ matched any of the elements of the chain, and the trusted certificate
+ found is not self-issued, the client MUST attempt to build a longer
+ chain in the hope that a certificate closer to the root may in fact
+ match the server's TLSA record.
+
+4.5. Opportunistic Security and PKIX usages
+
+
+
+
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+
+
+ When the client's protocol design is based on Opportunistic Security
+ (OS, [I-D.dukhovni-opportunistic-security]), and authentication is
+ opportunistically employed based on the presence of server TLSA
+ records, it is especially important to avoid the PKIX-EE(1) and PKIX-
+ TA(0) certificate usages. This is because the client has no way to
+ know in advance that it and the server both trust the requisite root
+ CA. Use of authentication in OS needs to be employed only when the
+ client can be confident of success, absent an attack, or an
+ operational error on the server side.
+
+5. Service Provider and TLSA Publisher Synchronization
+
+ Complications arise when the TLSA Publisher is not the same entity as
+ the Service Provider. In this situation, the TLSA Publisher and the
+ Service Provider must cooperate to ensure that TLSA records published
+ by the TLSA Publisher don't fall out of sync with the server
+ certificate used by the Service Provider.
+
+ Whenever possible, the TLSA Publisher and the Service Provider should
+ be the same entity. Otherwise, changes in the service certificate
+ chain must be carefully coordinated between the parties involved.
+ Such coordination is difficult and service outages will result when
+ coordination fails.
+
+ Having the master TLSA record in the Service Provider's zone avoids
+ the complexity of bilateral coordination of server certificate
+ configuration and TLSA record management. Even when the TLSA RRset
+ must be published in the Customer Domain's DNS zone (perhaps the
+ client application does not "chase" CNAMEs to the TLSA base domain),
+ it is possible to employ CNAME records to delegate the content of the
+ TLSA RRset to a domain operated by the Service Provider. Certificate
+ name checks generally constrain the applicability of TLSA CNAMEs
+ across organizational boundaries to Certificate Usages DANE-EE(3) and
+ DANE-TA(2):
+
+ Certificate Usage DANE-EE(3): In this case the Service Provider can
+ publish a single TLSA RRset that matches the server certificate or
+ public key digest. The same RRset works for all Customer Domains
+ because name checks do not apply with DANE-EE(3) TLSA records (see
+ Section 4.1). A Customer Domain can create a CNAME record
+ pointing to the TLSA RRset published by the Service Provider.
+
+ Certificate Usage DANE-TA(2): When the Service Provider operates a
+ private certification authority, the Service Provider is free to
+ issue a certificate bearing any customer's domain name. Without
+ DANE, such a certificate would not pass trust verification, but
+ with DANE, the customer's TLSA RRset that is aliased to the
+ provider's TLSA RRset can delegate authority to the provider's CA
+
+
+
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+
+
+ for the corresponding service. The Service Provider can generate
+ appropriate certificates for each customer and use the SNI
+ information provided by clients to select the right certificate
+ chain to present to each client.
+
+ Below are example DNS records (assumed "secure" and shown without the
+ associated DNSSEC information, such as record signatures) that
+ illustrate both of of the above models in the case of an HTTPS
+ service whose clients all support DANE TLS. These examples work even
+ with clients that don't "chase" CNAMEs when constructing the TLSA
+ base domain (see Section 6 below).
+
+ ; The hosted web service is redirected via a CNAME alias.
+ ; The associated TLSA RRset is also redirected via a CNAME alias.
+ ;
+ ; A single certificate at the provider works for all Customer
+ ; Domains due to the use of the DANE-EE(3) Certificate Usage.
+ ;
+ www1.example.com. IN CNAME w1.example.net.
+ _443._tcp.www1.example.com. IN CNAME _443._tcp.w1.example.net.
+ _443._tcp.w1.example.net. IN TLSA DANE-EE SPKI SHA2-256 (
+ 8A9A70596E869BED72C69D97A8895DFA
+ D86F300A343FECEFF19E89C27C896BC9 )
+
+ ;
+ ; A CA at the provider can also issue certificates for each Customer
+ ; Domain, and use the DANE-TA(2) Certificate Usage type to
+ ; indicate a trust anchor.
+ ;
+ www2.example.com. IN CNAME w2.example.net.
+ _443._tcp.www2.example.com. IN CNAME _443._tcp.w2.example.net.
+ _443._tcp.w2.example.net. IN TLSA DANE-TA Cert SHA2-256 (
+ C164B2C3F36D068D42A6138E446152F5
+ 68615F28C69BD96A73E354CAC88ED00C )
+
+ With protocols that support explicit transport redirection via DNS MX
+ records, SRV records, or other similar records, the TLSA base domain
+ is based on the redirected transport end-point, rather than the
+ origin domain. With SMTP, for example, when an email service is
+ hosted by a Service Provider, the Customer Domain's MX hostnames will
+ point at the Service Provider's SMTP hosts. When the Customer
+ Domain's DNS zone is signed, the MX hostnames can be securely used as
+ the base domains for TLSA records that are published and managed by
+ the Service Provider. For example (without the required DNSSEC
+ information, such as record signatures):
+
+
+
+
+
+
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+
+
+ ; Hosted SMTP service
+ ;
+ example.com. IN MX 0 mx1.example.net.
+ example.com. IN MX 0 mx2.example.net.
+ _25._tcp.mx1.example.net. IN TLSA DANE-EE SPKI SHA2-256 (
+ 8A9A70596E869BED72C69D97A8895DFA
+ D86F300A343FECEFF19E89C27C896BC9 )
+ _25._tcp.mx2.example.net. IN TLSA DANE-EE SPKI SHA2-256 (
+ C164B2C3F36D068D42A6138E446152F5
+ 68615F28C69BD96A73E354CAC88ED00C )
+
+ If redirection to the Service Provider's domain (via MX or SRV
+ records or any similar mechanism) is not possible, and aliasing of
+ the TLSA record is not an option, then more complex coordination
+ between the Customer Domain and Service Provider will be required.
+ Either the Customer Domain periodically provides private keys and a
+ corresponding certificate chain to the Provider (after making
+ appropriate changes in its TLSA records), or the Service Provider
+ periodically generates the keys and certificates and must wait for
+ matching TLSA records to be published by its Customer Domains before
+ deploying newly generated keys and certificate chains. In Section 6
+ below, we describe an approach that employs CNAME "chasing" to avoid
+ the difficulties of coordinating key management across organization
+ boundaries.
+
+ For further information about combining DANE and SRV, please see
+ [I-D.ietf-dane-srv].
+
+6. TLSA Base Domain and CNAMEs
+
+ When the application protocol does not support service location
+ indirection via MX, SRV or similar DNS records, the service may be
+ redirected via a CNAME. A CNAME is a more blunt instrument for this
+ purpose, since unlike an MX or SRV record, it remaps the entire
+ origin domain to the target domain for all protocols.
+
+ The complexity of coordinating key management is largely eliminated
+ when DANE TLSA records are found in the Service Provider's domain, as
+ discussed in Section 5. Therefore, DANE TLS clients connecting to a
+ server whose domain name is a CNAME alias SHOULD follow the CNAME
+ hop-by-hop to its ultimate target host (noting at each step whether
+ the CNAME is DNSSEC-validated). If at each stage of CNAME expansion
+ the DNSSEC validation status is "secure", the final target name
+ SHOULD be the preferred base domain for TLSA lookups.
+
+ Implementations failing to find a TLSA record using a base name of
+ the final target of a CNAME expansion SHOULD issue a TLSA query using
+ the original destination name. That is, the preferred TLSA base
+
+
+
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+
+
+ domain should be derived from the fully expanded name, and failing
+ that should be the initial domain name.
+
+ When the TLSA base domain is the result of "secure" CNAME expansion,
+ the resulting domain name MUST be used as the HostName in SNI, and
+ MUST be the primary reference identifier for peer certificate
+ matching with certificate usages other than DANE-EE(3).
+
+ Protocol-specific TLSA specifications may provide additional guidance
+ or restrictions when following CNAME expansions.
+
+ Though CNAMEs are illegal on the right hand side of most indirection
+ records, such as MX and SRV records, they are supported by some
+ implementations. For example, if the MX or SRV host is a CNAME
+ alias, some implementations may "chase" the CNAME. If they do, they
+ SHOULD use the target hostname as the preferred TLSA base domain as
+ described above (and if the TLSA records are found there, use the
+ CNAME expanded domain also in SNI and certificate name checks).
+
+7. TLSA Publisher Requirements
+
+ This section updates [RFC6698] by specifying a requirement on the
+ TLSA Publisher to ensure that each combination of Certificate Usage,
+ selector and matching type in the server's TLSA RRset MUST include at
+ least one record that matches the server's current certificate chain.
+ TLSA records that match recently retired or yet to be deployed
+ certificate chains will be present during key rollover. Such past or
+ future records must never be the only records published for any given
+ combination of usage, selector and matching type. We describe a TLSA
+ record update algorithm that ensures this requirement is met.
+
+ While a server is to be considered authenticated when its certificate
+ chain is matched by any of the published TLSA records, not all
+ clients support all combinations of TLSA record parameters. Some
+ clients may not support some digest algorithms, others may either not
+ support, or may exclusively support, the PKIX Certificate Usages.
+ Some clients may prefer to negotiate [I-D.ietf-tls-oob-pubkey] raw
+ public keys, which are only compatible with TLSA records whose
+ Certificate Usage is DANE-EE(3) with selector SPKI(1).
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+ A consequence of the above uncertainty as to which TLSA parameters
+ are supported by any given client is that servers need to ensure that
+ each and every parameter combination that appears in the TLSA RRset
+ is, on its own, sufficient to match the server's current certificate
+ chain. In particular, when deploying new keys or new parameter
+ combinations some care is required to not generate parameter
+ combinations that only match past or future certificate chains (or
+ raw public keys). The rest of this section explains how to update
+ the TLSA RRset in a manner that ensures the above requirement is met.
+
+7.1. Key rollover with fixed TLSA Parameters
+
+ The simplest case is key rollover while retaining the same set of
+ published parameter combinations. In this case, TLSA records
+ matching the existing server certificate chain (or raw public keys)
+ are first augmented with corresponding records matching the future
+ keys, at least two TTLs or longer before the the new chain is
+ deployed. This allows the obsolete RRset to age out of client caches
+ before the new chain is used in TLS handshakes. Once sufficient time
+ has elapsed and all clients performing DNS lookups are retrieving the
+ updated TLSA records, the server administrator may deploy the new
+ certificate chain, verify that it works, and then remove any obsolete
+ records matching the no longer active chain:
+
+ ; The initial TLSA RRset
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+
+ ; The transitional TLSA RRset published at least 2*TTL seconds
+ ; before the actual key change
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+ _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
+
+ ; The final TLSA RRset after the key change
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
+
+ The next case to consider is adding or switching to a new combination
+ of TLSA parameters. In this case publish the new parameter
+ combinations for the server's existing certificate chain first, and
+ only then deploy new keys if desired:
+
+
+
+
+
+
+
+
+
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+
+
+ ; Initial TLSA RRset
+ ;
+ _443._tcp.www.example.org. IN TLSA 1 1 1 01d09d19c2139a46...
+
+ ; New TLSA RRset, same key re-published as DANE-EE(3)
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+
+7.2. Switching to DANE-TA from DANE-EE
+
+ A more complex involves switching to a trust-anchor or PKIX usage
+ from a chain that is either self-signed, or issued by a private CA
+ and thus not compatible with PKIX. Here the process is to first add
+ TLSA records matching the future chain that is issued by the desired
+ future CA (private or PKIX), but initially with the same parameters
+ as the legacy chain. Then, after deploying the new keys, switch to
+ the new TLSA parameter combination.
+
+ ; The initial TLSA RRset
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+
+ ; A transitional TLSA RRset, published at least 2*TTL before the
+ ; actual key change. The new keys are issued by a DANE-TA(2) CA,
+ ; but for now specified via a DANE-EE(3) association.
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+ _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
+
+ ; The final TLSA RRset after the key change. Now that the old
+ ; self-signed EE keys are not an impediment, specify the issuing
+ ; TA of the new keys.
+ ;
+ _443._tcp.www.example.org. IN TLSA 2 0 1 c57bce38455d9e3d...
+
+7.3. Switching to New TLSA Parameters
+
+ When employing a new digest algorithm in the TLSA RRset, for
+ compatibility with digest agility specified in Section 8 below,
+ administrators should publish the new digest algorithm with each
+ combinations of Certificate Usage and selector for each associated
+ key or chain used with any other digest algorithm. When removing an
+ algorithm, remove it entirely. Each digest algorithm employed should
+ match the same set of chains (or raw public keys).
+
+
+
+
+
+
+
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+
+
+ ; The initial TLSA RRset with EE SHA2-256 associations for two keys.
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+ _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
+
+ ; The new TLSA RRset also with SHA2-512 associations for each key
+ ;
+ _443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
+ _443._tcp.www.example.org. IN TLSA 3 1 2 d9947c35089310bc...
+ _443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
+ _443._tcp.www.example.org. IN TLSA 3 1 2 89a7486a4b6ae714...
+
+7.4. TLSA Publisher Requirements Summary
+
+ In summary, server operators updating TLSA records should make one
+ change at a time. The individual safe changes are:
+
+ o Pre-publish new certificate associations that employ the same TLSA
+ parameters (usage, selector and matching type) as existing TLSA
+ records, but match certificate chains that will be deployed in the
+ near future. Wait for stale TLSA RRsets to expire from DNS caches
+ before configuring servers to use the new certificate chain.
+
+ o Remove TLSA records matching no longer deployed certificate
+ chains.
+
+ o Extend the TLSA RRset with a new combination of parameters (usage,
+ selector and matching type) that is used to generate matching
+ associations for all certificate chains that are published with
+ some other parameter combination.
+
+ The above steps are intended to ensure that at all times and for each
+ combination of usage, selector and matching type at least one TLSA
+ record corresponds to the server's current certificate chain. No
+ combination of Certificate Usage, selector and matching type in a
+ server's TLSA RRset should ever match only some combination of future
+ or past certificate chains. As a result, no matter what combinations
+ of usage, selector and matching type may be supported by a given
+ client, they will be sufficient to authenticate the server.
+
+8. Digest Algorithm Agility
+
+
+
+
+
+
+
+
+
+
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+
+
+ While [RFC6698] specifies multiple digest algorithms, it does not
+ specify a protocol by which the TLS 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. Suppose also that
+ the server adheres to the requirements of Section 7 and ensures that
+ each combination of TLSA parameters contains at least one record that
+ matches the server's current certificate chain (or raw public keys).
+ Under the above assumptions the client can safely ignore TLSA records
+ with the weaker algorithm "WorseAlg", because it suffices to only
+ check the records with the stronger algorithm "BetterAlg".
+
+ To make digest algorithm agility possible, all published TLSA RRsets
+ for use with DANE TLS MUST conform to the requirements of Section 7.
+ With servers publishing compliant TLSA RRsets, TLS clients can, for
+ each combination of usage and selector, ignore all digest records
+ except those that employ their notion of the strongest digest
+ algorithm. (The server should only publish algorithms it deems
+ acceptable at all.) The ordering of digest algorithms by strength is
+ not specified in advance; it is entirely up to the TLS client. TLS
+ client implementations SHOULD make the digest algorithm preference
+ ordering a configurable option.
+
+ Note, TLSA records with a matching type of Full(0) that publish an
+ entire 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.
+
+ TLS clients SHOULD use digest algorithm agility when processing the
+ DANE TLSA records of an TLS 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 considered by the client to be the
+ strongest among usable records with the given usage and selector.
+
+9. General DANE Guidelines
+
+
+
+
+
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+
+
+ These guidelines provide guidance for using or designing protocols
+ for DANE.
+
+9.1. DANE DNS Record Size Guidelines
+
+ Selecting a combination of TLSA parameters to use requires careful
+ thought. One important consideration to take into account is the
+ size of the resulting TLSA record after its parameters are selected.
+
+9.1.1. UDP and TCP Considerations
+
+ Deployments SHOULD avoid TLSA record sizes that cause UDP
+ fragmentation.
+
+ Although DNS over TCP would provide the ability to more easily
+ transfer larger DNS records between clients and servers, it is not
+ universally deployed and is still prohibited by some firewalls.
+ Clients that request DNS records via UDP typically only use TCP upon
+ receipt of a truncated response in the DNS response message sent over
+ UDP. Setting the TC bit alone will be insufficient if the response
+ containing the TC bit is itself fragmented.
+
+9.1.2. Packet Size Considerations for TLSA Parameters
+
+ Server operators SHOULD NOT publish TLSA records using both a TLSA
+ Selector of Cert(0) and a TLSA Matching Type of Full(0), as even a
+ single certificate is generally too large to be reliably delivered
+ via DNS over UDP. Furthermore, two TLSA records containing full
+ certificates will need to be published simultaneously during a
+ certificate rollover, as discussed in Section 7.1.
+
+ While TLSA records using a TLSA Selector of SPKI(1) and a TLSA
+ Matching Type of Full(0) (which publish the bare public keys without
+ the overhead of a containing X.509 certificate) are generally more
+ compact, these too should be used with caution as they are still
+ larger than necessary. Rather, servers SHOULD publish digest-based
+ TLSA Matching Types in their TLSA records. The complete
+ corresponding certificate should, instead, be transmitted to the
+ client in-band during the TLS handshake, which can be easily verified
+ using the digest value.
+
+ In summary, the use of a TLSA Matching Type of Full(0) is NOT
+ RECOMMENDED and the use of a digest-based matching type, such as
+ SHA2-256(1) SHOULD be used.
+
+9.2. Certificate Name Check Conventions
+
+
+
+
+
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+
+
+ Certificates presented by a TLS server will generally contain a
+ subjectAltName (SAN) extension or a Common Name (CN) element within
+ the subject distinguished name (DN). The TLS server's DNS domain
+ name is normally published within these elements, ideally within the
+ subjectAltName extension. (The use of the CN field for this purpose
+ is deprecated.)
+
+ When a server hosts multiple domains at the same transport endpoint,
+ the server's ability to respond with the right certificate chain is
+ predicated on correct SNI information from the client. DANE clients
+ MUST send the SNI extension with a HostName value of the base domain
+ of the TLSA RRset.
+
+ Except with TLSA Certificate Usage DANE-EE(3), where name checks are
+ not applicable (see Section 4.1), DANE clients MUST verify that the
+ client has reached the correct server by checking that the server
+ name is listed in the server certificate's SAN or CN. The server
+ name used for this comparison SHOULD be the base domain of the TLSA
+ RRset. Additional acceptable names may be specified by protocol-
+ specific DANE standards. For example, with SMTP both the destination
+ domain name and the MX host name are acceptable names to be found in
+ the server certificate (see [I-D.ietf-dane-smtp-with-dane]).
+
+ It is the responsibility of the service operator, in coordination
+ with the TLSA Publisher, to ensure that at least one of the TLSA
+ records published for the service will match the server's certificate
+ chain (either the default chain or the certificate that was selected
+ based on the SNI information provided by the client).
+
+ Given the DNSSEC validated DNS records below:
+
+ example.com. IN MX 0 mail.example.com.
+ mail.example.com. IN A 192.0.2.1
+ _25._tcp.mail.example.com. IN TLSA DANE-TA Cert SHA2-256 (
+ E8B54E0B4BAA815B06D3462D65FBC7C0
+ CF556ECCF9F5303EBFBB77D022F834C0 )
+
+ The TLSA base domain is "mail.example.com" and is required to be the
+ HostName in the client's SNI extension. The server certificate chain
+ is required to be be signed by a trust anchor with the above
+ certificate SHA2-256 digest. Finally, one of the DNS names in the
+ server certificate is required to be be either "mail.example.com" or
+ "example.com" (this additional name is a concession to compatibility
+ with prior practice, see [I-D.ietf-dane-smtp-with-dane] for details).
+
+ The semantics of wildcards in server certificates are left to
+ individual application protocol specifications.
+
+
+
+
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+
+
+9.3. Design Considerations for Protocols Using DANE
+
+ When a TLS client goes to the trouble of authenticating a certificate
+ chain presented by a TLS server, it will typically not continue to
+ use that server in the event of authentication failure, or else
+ authentication serves no purpose. Some clients may, at times,
+ operate in an "audit" mode, where authentication failure is reported
+ to the user or in logs as a potential problem, but the connection
+ proceeds despite the failure. Nevertheless servers publishing TLSA
+ records MUST be configured to allow correctly configured clients to
+ successfully authenticate their TLS certificate chains.
+
+ A service with DNSSEC-validated TLSA records implicitly promises TLS
+ support. When all the TLSA records for a service are found
+ "unusable", due to unsupported parameter combinations or malformed
+ associated data, DANE clients cannot authenticate the service
+ certificate chain. When authenticated TLS is dictated by the
+ application, the client SHOULD NOT connect to the associated server.
+ If, on the other hand, the use of TLS is "opportunistic", then the
+ client SHOULD generally use the server via an unauthenticated TLS
+ connection, but if TLS encryption cannot be established, the client
+ MUST NOT use the server. Standards for DANE specific to the
+ particular application protocol may modify the above requirements, as
+ appropriate, to specify whether the connection should be established
+ anyway without relying on TLS security, with only encryption but not
+ authentication, or whether to refuse to connect entirely.
+ Application protocols need to specify when to prioritize security
+ over the ability to connect under adverse conditions.
+
+9.3.1. Design Considerations for non-PKIX Protocols
+
+ For some application protocols (such as SMTP to MX with opportunistic
+ TLS), the existing public CA PKI is not a viable alternative to DANE.
+ For these (non-PKIX) protocols, new DANE standards SHOULD NOT suggest
+ publishing TLSA records with TLSA Certificate Usage PKIX-TA(0) or
+ PKIX-EE(1), as TLS clients cannot be expected to perform [RFC5280]
+ PKIX validation or [RFC6125] identity verification.
+
+ Protocols designed for non-PKIX use SHOULD choose to treat any TLSA
+ records with TLSA Certificate Usage PKIX-TA(0) or PKIX-EE(1) as
+ unusable. After verifying that the only available TLSA Certificate
+ Usage types are PKIX-TA(0) or PKIX-EE(1), protocol specifications MAY
+ instruct clients to either refuse to initiate a connection or to
+ connect via unauthenticated TLS if no alternative authentication
+ mechanisms are available.
+
+10. Interaction with Certificate Transparency
+
+
+
+
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+
+
+ Certificate Transparency (CT) [RFC6962] defines an experimental
+ approach to mitigate the risk of rogue or compromised public CAs
+ issuing unauthorized certificates. This section clarifies the
+ interaction of CT and DANE. CT is an experimental protocol and
+ auditing system that applies only to public CAs, and only when they
+ are free to issue unauthorized certificates for a domain. If the CA
+ is not a public CA, or a DANE-EE(3) TLSA RR directly specifies the
+ end entity certificate, there is no role for CT, and clients need not
+ apply CT checks.
+
+ When a server is authenticated via a DANE TLSA RR with TLSA
+ Certificate Usage DANE-EE(3), the domain owner has directly specified
+ the certificate associated with the given service without reference
+ to any PKIX certification authority. Therefore, when a TLS client
+ authenticates the TLS server via a TLSA certificate association with
+ usage DANE-EE(3), CT checks SHOULD NOT be performed. Publication of
+ the server certificate or public key (digest) in a TLSA record in a
+ DNSSEC signed zone by the domain owner assures the TLS client that
+ the certificate is not an unauthorized certificate issued by a rogue
+ CA without the domain owner's consent.
+
+ When a server is authenticated via a DANE TLSA RR with TLSA usage
+ DANE-TA(2) and the server certificate does not chain to a known
+ public root CA, CT cannot apply (CT logs only accept chains that
+ start with a known, public root). Since TLSA Certificate Usage DANE-
+ TA(2) is generally intended to support non-PKIX trust anchors, TLS
+ clients SHOULD NOT perform CT checks with usage DANE-TA(2) using
+ unknown root CAs.
+
+ A server operator who wants clients to perform CT checks should
+ publish TLSA RRs with usage PKIX-TA(0) or PKIX-EE(1).
+
+11. Note on DNSSEC Security
+
+ Clearly the security of the DANE TLSA PKI rests on the security of
+ the underlying DNSSEC infrastructure. While this memo is not a guide
+ to DNSSEC security, a few comments may be helpful to TLSA
+ implementers.
+
+ With the existing public CA PKI, name constraints are rarely used,
+ and a public root CA can issue certificates for any domain of its
+ choice. With DNSSEC, under the Registry/Registrar/Registrant model,
+ the situation is different: only the registrar of record can update a
+ domain's DS record in the registry parent zone (in some cases,
+ however, the registry is the sole registrar). With many gTLDs, for
+ which multiple registrars compete to provide domains in a single
+ registry, it is important to make sure that rogue registrars cannot
+ easily initiate an unauthorized domain transfer, and thus take over
+
+
+
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+
+
+ DNSSEC for the domain. DNS Operators SHOULD use a registrar lock of
+ their domains to offer some protection against this possibility.
+
+ When the registrar is also the DNS operator for the domain, one needs
+ to consider whether the registrar will allow orderly migration of the
+ domain to another registrar or DNS operator in a way that will
+ maintain DNSSEC integrity. TLSA Publishers SHOULD ensure their
+ registrar publishes a suitable domain transfer policy.
+
+ DNSSEC signed RRsets cannot be securely revoked before they expire.
+ Operators should plan accordingly and not generate signatures with
+ excessively long duration periods. For domains publishing high-value
+ keys, a signature lifetime of a few days is reasonable, and the zone
+ should be resigned daily. For domains with less critical data, a
+ reasonable signature lifetime is a couple of weeks to a month, and
+ the zone should be resigned weekly. Monitoring of the signature
+ lifetime is important. If the zone is not resigned in a timely
+ manner, one risks a major outage and the entire domain will become
+ bogus.
+
+12. Summary of Updates to RFC6698
+
+ Authors note: is this section needed? Or is it sufficiently clear
+ above that we don't need to restate things here?
+
+ o In Section 3 we update [RFC6698] to specify a requirement for
+ clients to support at least TLS 1.0, and to support SNI.
+
+ o In Section 4.1 we update [RFC6698] to specify peer identity
+ matching and certificate validity interval based solely on the
+ basis of the TLSA RRset. We also specify DANE authentication of
+ raw public keys [I-D.ietf-tls-oob-pubkey] via TLSA records with
+ Certificate Usage DANE-EE(3) and selector SPKI(1).
+
+ o In Section 4.2 we update [RFC6698] to require that servers
+ publishing digest TLSA records with a usage of DANE-TA(2) MUST
+ include the trust-anchor certificate in their TLS server
+ certificate message. This extends to the case of "2 1 0" TLSA
+ records which publish a full public key.
+
+ o In Section 4.3 and Section 4.4, we explain that PKIX-EE(1) and
+ PKIX-TA(0) are generally NOT RECOMMENDED. With usage PKIX-TA(0)
+ we note that clients may need to processes extended trust chains
+ beyond the first trusted issuer, when that issuer is not self-
+ signed.
+
+
+
+
+
+
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+
+
+ o In Section 6, we recommend that DANE application protocols specify
+ that when possible securely CNAME expanded names be used to derive
+ the TLSA base domain.
+
+ o In Section 7, we specify a strategy for managing TLSA records that
+ interoperates with DANE clients regardless of what subset of the
+ possible TLSA record types (combinations of TLSA parameters) is
+ supported by the client.
+
+ o In Section 8, we propose a digest algorithm agility protocol.
+ [Note: This section does not yet represent the rough consensus of
+ the DANE working group and requires further discussion. Perhaps
+ this belongs in a separate document.]
+
+ o In Section 9.1 we recommend against the use of Full(0) TLSA
+ records, as digest records are generally much more compact.
+
+13. Security Considerations
+
+ Application protocols that cannot make use of the existing public CA
+ PKI (so called non-PKIX protocols), may choose not to implement
+ certain PKIX-dependent TLSA record types defined in [RFC6698]. If
+ such records are published despite not being supported by the
+ application protocol, they are treated as "unusable". When TLS is
+ opportunistic, the client may proceed to use the server with
+ mandatory unauthenticated TLS. This is stronger than opportunistic
+ TLS without DANE, since in that case the client may also proceed with
+ a plaintext connection. When TLS is not opportunistic, the client
+ MUST NOT connect to the server.
+
+ Therefore, when TLSA records are used with protocols where PKIX does
+ not apply, the recommended policy is for servers to not publish PKIX-
+ dependent TLSA records, and for opportunistic TLS clients to use them
+ to enforce the use of (albeit unauthenticated) TLS, but otherwise
+ treat them as unusable. Of course, when PKIX validation is supported
+ by the application protocol, clients SHOULD perform PKIX validation
+ per [RFC6698].
+
+14. IANA Considerations
+
+ This specification requires no support from IANA.
+
+15. Acknowledgements
+
+ The authors would like to thank Phil Pennock for his comments and
+ advice on this document.
+
+
+
+
+
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+
+
+ Acknowledgments from Viktor: Thanks to Tony Finch who finally prodded
+ me into participating in DANE working group discussions. Thanks to
+ Paul Hoffman who motivated me to produce this memo and provided
+ feedback on early drafts. Thanks also to Samuel Dukhovni for
+ editorial assistance.
+
+16. References
+
+16.1. Normative References
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [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.
+
+ [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.
+
+ [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
+ Security Version 1.2", RFC 6347, January 2012.
+
+ [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
+ of Named Entities (DANE) Transport Layer Security (TLS)
+ Protocol: TLSA", RFC 6698, August 2012.
+
+
+
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+
+
+ [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
+ Conversations about DNS-Based Authentication of Named
+ Entities (DANE)", RFC 7218, April 2014.
+
+16.2. Informative References
+
+ [I-D.dukhovni-opportunistic-security]
+ Dukhovni, V., "Opportunistic Security: Some Protection
+ Most of the Time", draft-dukhovni-opportunistic-
+ security-03 (work in progress), August 2014.
+
+ [I-D.ietf-dane-smtp-with-dane]
+ Dukhovni, V. and W. Hardaker, "SMTP security via
+ opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-11
+ (work in progress), August 2014.
+
+ [I-D.ietf-dane-srv]
+ Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
+ Based Authentication of Named Entities (DANE) TLSA Records
+ with SRV Records", draft-ietf-dane-srv-07 (work in
+ progress), July 2014.
+
+ [I-D.ietf-tls-oob-pubkey]
+ Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
+ T. Kivinen, "Using Raw Public Keys in Transport Layer
+ Security (TLS) and Datagram Transport Layer Security
+ (DTLS)", draft-ietf-tls-oob-pubkey-11 (work in progress),
+ January 2014.
+
+ [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
+ Transparency", RFC 6962, June 2013.
+
+Authors' Addresses
+
+ Viktor Dukhovni
+ Unaffiliated
+
+ Email: ietf-dane@dukhovni.org
+
+
+ Wes Hardaker
+ Parsons
+ P.O. Box 382
+ Davis, CA 95617
+ US
+
+ Email: ietf@hardakers.net
+
+
+
+
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--- /dev/null
+
+
+
+
+DANE V. Dukhovni
+Internet-Draft Two Sigma
+Intended status: Standards Track W. Hardaker
+Expires: February 18, 2015 Parsons
+ August 17, 2014
+
+
+ SMTP security via opportunistic DANE TLS
+ draft-ietf-dane-smtp-with-dane-12
+
+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 18, 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 . . . . . . . . . . . . . . . . . . . 27
+ 7. Note on DANE for Message User Agents . . . . . . . . . . . . 27
+ 8. Interoperability considerations . . . . . . . . . . . . . . . 28
+ 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 28
+ 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 28
+ 9. Operational Considerations . . . . . . . . . . . . . . . . . 29
+ 9.1. Client Operational Considerations . . . . . . . . . . . . 29
+ 9.2. Publisher Operational Considerations . . . . . . . . . . 30
+ 10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
+ 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 31
+ 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
+ 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
+ 13.1. Normative References . . . . . . . . . . . . . . . . . . 31
+ 13.2. Informative References . . . . . . . . . . . . . . . . . 32
+ 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 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 include 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-conformant MX record, DANE TLSA does not apply for
+ such a connection.
+
+ In the subsections that follow we explain how to locate the SMTP
+ servers and the associated TLSA records for a given next-hop
+ destination domain. We also explain which name or names are to be
+ used in identity checks of the SMTP server certificate.
+
+2.2.1. MX resolution
+
+ In this section we consider next-hop domains that are subject to MX
+ resolution and have MX records. The TLSA records and the associated
+ base domain are derived separately for each MX hostname that is used
+ to attempt message delivery. DANE TLS can authenticate message
+ delivery to the intended next-hop domain only when the MX records are
+ obtained securely via a DNSSEC validated lookup.
+
+ MX records MUST be sorted by preference; an MX hostname with a worse
+ (numerically higher) MX preference that has TLSA records MUST NOT
+ preempt an MX hostname with a better (numerically lower) preference
+ that has no TLSA records. In other words, prevention of delivery
+ loops by obeying MX preferences MUST take precedence over channel
+ security considerations. Even with two equal-preference MX records,
+ an MTA is not obligated to choose the MX hostname that offers more
+ security. Domains that want secure inbound mail delivery need to
+ ensure that all their SMTP servers and MX records are configured
+ accordingly.
+
+ In the language of [RFC5321] Section 5.1, the original next-hop
+ domain is the "initial name". If the MX lookup of the initial name
+ results in a CNAME alias, the MTA replaces the initial name with the
+ resulting name and performs a new lookup with the new name. MTAs
+ typically support recursion in CNAME expansion, so this replacement
+ is performed repeatedly (up to the MTA's recursion limit) until the
+ ultimate non-CNAME domain is found.
+
+ If the MX RRset (or any CNAME leading to it) is "insecure" (see
+ Section 2.1.1), 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 a
+ CNAME record. If the central trust anchor is compromised, all
+ servers need to be issued new keys by a new TA, and an updated DANE-
+ TA(2) TLSA RRset needs to be published containing just the new TA.
+
+ SMTP servers cannot expect broad CRL or OCSP support from SMTP
+ clients. As outlined above, with DANE, compromised server or trust
+ anchor keys can be "revoked" by removing them from the DNS without
+ the need for client-side support for OCSP or CRLs.
+
+5. Digest algorithm agility
+
+
+
+
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+
+ While [RFC6698] specifies multiple digest algorithms, it does not
+ specify a protocol by which the SMTP client and TLSA record publisher
+ can agree on the strongest shared algorithm. Such a protocol would
+ allow the client and server to avoid exposure to any deprecated
+ weaker algorithms that are published for compatibility with less
+ capable clients, but should be ignored when possible. Such a
+ protocol is specified in [I-D.ietf-dane-ops]. SMTP clients and
+ servers that implement this specification MUST comply with the
+ requirements outlined under "Digest Algorithm Agility" in
+ [I-D.ietf-dane-ops].
+
+6. Mandatory TLS Security
+
+ An MTA implementing this protocol may require a stronger security
+ assurance when sending email to selected destinations. The sending
+ organization may need to send sensitive email and/or may have
+ regulatory obligations to protect its content. This protocol is not
+ in conflict with such a requirement, and in fact can often simplify
+ authenticated delivery to such destinations.
+
+ Specifically, with domains that publish DANE TLSA records for their
+ MX hostnames, a sending MTA can be configured to use the receiving
+ domains's DANE TLSA records to authenticate the corresponding SMTP
+ server. Authentication via DANE TLSA records is easier to manage, as
+ changes in the receiver's expected certificate properties are made on
+ the receiver end and don't require manually communicated
+ configuration changes. With mandatory DANE TLS, when no usable TLSA
+ records are found, message delivery is delayed. Thus, mail is only
+ sent when an authenticated TLS channel is established to the remote
+ SMTP server.
+
+ Administrators of mail servers that employ mandatory DANE TLS, need
+ to carefully monitor their mail logs and queues. If a partner domain
+ unwittingly misconfigures their TLSA records, disables DNSSEC, or
+ misconfigures SMTP server certificate chains, mail will be delayed
+ and may bounce if the issue is not resolved in a timely manner.
+
+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].
+
+
+
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+
+ 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
+ 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
+
+
+
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+
+ more anonymous TLS cipher suites in its TLS HELLO. SMTP servers,
+ that need to interoperate with opportunistic TLS clients SHOULD be
+ prepared to interoperate with such clients by either always selecting
+ a mutually supported non-anonymous cipher suite or by correctly
+ handling client connections that negotiate anonymous cipher suites.
+
+ Note that while SMTP server operators are under no obligation to
+ enable anonymous cipher suites, no security is gained by sending
+ certificates to clients that will ignore them. Indeed support for
+ anonymous cipher suites in the server makes audit trails more
+ informative. Log entries that record connections that employed an
+ anonymous cipher suite record the fact that the clients did not care
+ to authenticate the server.
+
+9. Operational Considerations
+
+9.1. Client Operational Considerations
+
+ An operational error on the sending or receiving side that cannot be
+ corrected in a timely manner may, at times, lead to consistent
+ failure to deliver time-sensitive email. The sending MTA
+ administrator may have to choose between letting email queue until
+ the error is resolved and disabling opportunistic or mandatory DANE
+ TLS 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.
+
+
+
+
+
+
+
+
+
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+
+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 guidelines in the "TLSA Publisher
+ Requirements" section of [I-D.ietf-dane-ops].
+
+ TLSA Publishers SHOULD follow the TLSA publication size guidance
+ found in [I-D.ietf-dane-ops] under "DANE DNS Record Size Guidelines".
+
+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.
+
+ 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,
+
+
+
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+
+ implementation must not initiate a connection when this specification
+ indicates a particular SMTP server must be considered unreachable.
+
+11. IANA considerations
+
+ This specification requires no support from IANA.
+
+12. Acknowledgements
+
+ The authors would like to extend great thanks to Tony Finch, who
+ started the original version of a DANE SMTP document. His work is
+ greatly appreciated and has been incorporated into this document.
+ The authors would like to additionally thank Phil Pennock for his
+ comments and advice on this document.
+
+ Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
+ to begin work on this memo and provided feedback on early drafts.
+ Thanks to Patrick Koetter, Perry Metzger and Nico Williams for
+ valuable review comments. Thanks also to Wietse Venema who created
+ Postfix, and whose advice and feedback were essential to the
+ development of the Postfix DANE implementation.
+
+13. References
+
+13.1. Normative References
+
+ [I-D.ietf-dane-ops]
+ Dukhovni, V. and W. Hardaker, "Updates to and Operational
+ Guidance for the DANE Protocol", draft-ietf-dane-ops-06
+ (work in progress), August 2014.
+
+ [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.
+
+
+
+
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+
+
+ [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.
+
+ [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]
+
+
+
+
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+
+
+ Dukhovni, V., "Opportunistic Security: Some Protection
+ Most of the Time", draft-dukhovni-opportunistic-
+ security-03 (work in progress), August 2014.
+
+ [I-D.ietf-dane-srv]
+ Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
+ Based Authentication of Named Entities (DANE) TLSA Records
+ with SRV Records", draft-ietf-dane-srv-07 (work in
+ progress), July 2014.
+
+ [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.org
+
+
+ Wes Hardaker
+ Parsons
+ P.O. Box 382
+ Davis, CA 95617
+ US
+
+ Email: ietf@hardakers.net
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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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
+Expires: February 18, 2015 Parsons
+ August 17, 2014
SMTP security via opportunistic DANE TLS
- draft-ietf-dane-smtp-with-dane-11
+ draft-ietf-dane-smtp-with-dane-12
Abstract
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.
+ This Internet-Draft will expire on February 18, 2015.
Copyright Notice
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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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+ 6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 27
+ 7. Note on DANE for Message User Agents . . . . . . . . . . . . 27
+ 8. Interoperability considerations . . . . . . . . . . . . . . . 28
+ 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 28
+ 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 28
+ 9. Operational Considerations . . . . . . . . . . . . . . . . . 29
+ 9.1. Client Operational Considerations . . . . . . . . . . . . 29
+ 9.2. Publisher Operational Considerations . . . . . . . . . . 30
+ 10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
+ 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 31
+ 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
+ 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
+ 13.1. Normative References . . . . . . . . . . . . . . . . . . 31
+ 13.2. Informative References . . . . . . . . . . . . . . . . . 32
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
+
+
+
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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
+ follows, the term "insecure" will also include the case of
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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
+ address in the non-conformant MX record, DANE TLSA does not apply for
such a connection.
In the subsections that follow we explain how to locate the SMTP
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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.
+ compromised key expires, at which point it can return to using a
+ CNAME record. If the central trust anchor is compromised, all
+ servers need to be issued new keys by a new TA, and an updated DANE-
+ TA(2) TLSA RRset needs to be published containing just the new TA.
+
+ SMTP servers cannot expect broad CRL or OCSP support from SMTP
+ clients. As outlined above, with DANE, compromised server or trust
+ anchor keys can be "revoked" by removing them from the DNS without
+ the need for client-side support for OCSP or CRLs.
5. Digest algorithm agility
- While [RFC6698] specifies multiple digest algorithms, it does not
- specify a protocol by which the SMTP client and TLSA record publisher
- can agree on the strongest shared algorithm. Such a protocol would
- allow the client and server to avoid exposure to any deprecated
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+ 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
- 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.
+ capable clients, but should be ignored when possible. Such a
+ protocol is specified in [I-D.ietf-dane-ops]. SMTP clients and
+ servers that implement this specification MUST comply with the
+ requirements outlined under "Digest Algorithm Agility" in
+ [I-D.ietf-dane-ops].
6. Mandatory TLS Security
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
replaced by corresponding DNS Service (SRV) records
[I-D.ietf-dane-srv].
+
+
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+
+
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
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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-
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present to the client. Clients that don't send SNI information may
not see the expected certificate chain.
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
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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
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
least one TLSA record when usable TLSA records are found for that
server.
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9.2. Publisher Operational Considerations
SMTP servers that publish certificate usage DANE-TA(2) associations
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 MUST follow the guidelines in the "TLSA Publisher
+ Requirements" section of [I-D.ietf-dane-ops].
- TLSA Publishers should follow the TLSA publication size guidance
- found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".
+ TLSA Publishers SHOULD follow the TLSA publication size guidance
+ found in [I-D.ietf-dane-ops] under "DANE DNS Record Size Guidelines".
10. Security Considerations
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
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,
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implementation must not initiate a connection when this specification
indicates a particular SMTP server must be considered unreachable.
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.
+ Dukhovni, V. and W. Hardaker, "Updates to and Operational
+ Guidance for the DANE Protocol", draft-ietf-dane-ops-06
+ (work in progress), August 2014.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
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[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[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
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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+ Dukhovni, V., "Opportunistic Security: Some Protection
+ Most of the Time", draft-dukhovni-opportunistic-
+ security-03 (work in progress), August 2014.
+ [I-D.ietf-dane-srv]
+ Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
+ Based Authentication of Named Entities (DANE) TLSA Records
+ with SRV Records", draft-ietf-dane-srv-07 (work in
+ progress), July 2014.
+ [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
+ 2009.
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+ [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
+ STD 72, RFC 6409, November 2011.
+Authors' Addresses
Viktor Dukhovni
Two Sigma
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