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