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diff --git a/DANE-draft-notes b/doc/doc-txt/DANE-draft-notes index 21b3992cc..21b3992cc 100644 --- a/DANE-draft-notes +++ b/doc/doc-txt/DANE-draft-notes diff --git a/doc/doc-txt/draft-ietf-dane-smtp-with-dane-10.txt b/doc/doc-txt/draft-ietf-dane-smtp-with-dane-10.txt deleted file mode 100644 index 99d17e88e..000000000 --- a/doc/doc-txt/draft-ietf-dane-smtp-with-dane-10.txt +++ /dev/null @@ -1,1904 +0,0 @@ - - - - -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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 1] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 2] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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. - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 3] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 4] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 5] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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. - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 6] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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 - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 7] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 8] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 9] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 10] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 11] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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: - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 12] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 13] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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". - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 14] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 15] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - - - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 16] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 17] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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". - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 18] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 19] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 20] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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.... - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 21] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 22] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 23] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 24] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 25] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 26] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 27] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 28] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - -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. - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 29] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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. - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 30] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - 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 - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 31] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - greatly appreciated and has been incorporated into this document. - The authors would like to additionally thank Phil Pennock for his - comments and advice on this document. - - Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me - to begin work on this memo and provided feedback on early drafts. - Thanks to Patrick Koetter, Perry Metzger and Nico Williams for - valuable review comments. Thanks also to Wietse Venema who created - Postfix, and whose advice and feedback were essential to the - development of the Postfix DANE implementation. - -13. References - -13.1. Normative References - - [I-D.ietf-dane-ops] - Dukhovni, V. and W. Hardaker, "DANE TLSA implementation - and operational guidance", draft-ietf-dane-ops-00 (work in - progress), October 2013. - - [RFC1035] Mockapetris, P., "Domain names - implementation and - specification", STD 13, RFC 1035, November 1987. - - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, March 1997. - - [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over - Transport Layer Security", RFC 3207, February 2002. - - [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. - Rose, "DNS Security Introduction and Requirements", RFC - 4033, March 2005. - - [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. - Rose, "Resource Records for the DNS Security Extensions", - RFC 4034, March 2005. - - [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. - Rose, "Protocol Modifications for the DNS Security - Extensions", RFC 4035, March 2005. - - [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security - (TLS) Protocol Version 1.2", RFC 5246, August 2008. - - [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., - Housley, R., and W. Polk, "Internet X.509 Public Key - Infrastructure Certificate and Certificate Revocation List - (CRL) Profile", RFC 5280, May 2008. - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 32] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, - October 2008. - - [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: - Extension Definitions", RFC 6066, January 2011. - - [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and - Verification of Domain-Based Application Service Identity - within Internet Public Key Infrastructure Using X.509 - (PKIX) Certificates in the Context of Transport Layer - Security (TLS)", RFC 6125, March 2011. - - [RFC6186] Daboo, C., "Use of SRV Records for Locating Email - Submission/Access Services", RFC 6186, March 2011. - - [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the - DNS", RFC 6672, June 2012. - - [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication - of Named Entities (DANE) Transport Layer Security (TLS) - Protocol: TLSA", RFC 6698, August 2012. - -13.2. Informative References - - [I-D.ietf-dane-registry-acronyms] - Gudmundsson, O., "Adding acronyms to simplify DANE - conversations", draft-ietf-dane-registry-acronyms-01 (work - in progress), October 2013. - - [I-D.ietf-dane-srv] - Finch, T., "Using DNS-Based Authentication of Named - Entities (DANE) TLSA records with SRV and MX records.", - draft-ietf-dane-srv-02 (work in progress), February 2013. - - [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July - 2009. - - [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", - STD 72, RFC 6409, November 2011. - -Authors' Addresses - - Viktor Dukhovni - Two Sigma - - Email: ietf-dane@dukhovni.org - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 33] - -Internet-Draft SMTP security via opportunistic DANE TLS May 2014 - - - Wes Hardaker - Parsons - P.O. Box 382 - Davis, CA 95617 - US - - Email: ietf@hardakers.net - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Dukhovni & Hardaker Expires November 26, 2014 [Page 34] diff --git a/doc/doc-txt/draft-ietf-dane-smtp-with-dane-11.txt b/doc/doc-txt/draft-ietf-dane-smtp-with-dane-11.txt deleted file mode 100644 index 26bed33a5..000000000 --- a/doc/doc-txt/draft-ietf-dane-smtp-with-dane-11.txt +++ /dev/null @@ -1,1960 +0,0 @@ - - - - -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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 1] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 2] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - -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. - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 3] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 4] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 5] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - -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. - - - - - - - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 6] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 7] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 8] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 9] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - "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. - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 10] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 11] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - -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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 12] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 13] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 14] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 15] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 16] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 ? - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 17] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 18] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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: - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 19] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 20] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 21] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 22] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 23] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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: - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 24] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 25] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 26] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 27] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 28] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 29] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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 - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 30] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 31] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - 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. - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 32] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - -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. - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 33] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - [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 - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 34] - -Internet-Draft SMTP security via opportunistic DANE TLS August 2014 - - - Viktor Dukhovni - Two Sigma - - Email: ietf-dane@dukhovni.org - - - Wes Hardaker - Parsons - P.O. Box 382 - Davis, CA 95617 - US - - Email: ietf@hardakers.net - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Dukhovni & Hardaker Expires February 3, 2015 [Page 35] |