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+Internet Engineering Task Force (IETF) P. Hoffman
+Request for Comments: 6698 VPN Consortium
+Category: Standards Track J. Schlyter
+ISSN: 2070-1721 Kirei AB
+ August 2012
+
+
+ The DNS-Based Authentication of Named Entities (DANE)
+ Transport Layer Security (TLS) Protocol: TLSA
+
+Abstract
+
+ Encrypted communication on the Internet often uses Transport Layer
+ Security (TLS), which depends on third parties to certify the keys
+ used. This document improves on that situation by enabling the
+ administrators of domain names to specify the keys used in that
+ domain's TLS servers. This requires matching improvements in TLS
+ client software, but no change in TLS server software.
+
+Status of This Memo
+
+ This is an Internet Standards Track document.
+
+ This document is a product of the Internet Engineering Task Force
+ (IETF). It represents the consensus of the IETF community. It has
+ received public review and has been approved for publication by the
+ Internet Engineering Steering Group (IESG). Further information on
+ Internet Standards is available in Section 2 of RFC 5741.
+
+ Information about the current status of this document, any errata,
+ and how to provide feedback on it may be obtained at
+ http://www.rfc-editor.org/info/rfc6698.
+
+Copyright Notice
+
+ Copyright (c) 2012 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 1]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+Table of Contents
+
+ 1. Introduction ....................................................3
+ 1.1. Background and Motivation ..................................3
+ 1.2. Securing the Association of a Domain Name with a
+ Server's Certificate .......................................4
+ 1.3. Method for Securing Certificate Associations ...............5
+ 1.4. Terminology ................................................6
+ 2. The TLSA Resource Record ........................................7
+ 2.1. TLSA RDATA Wire Format .....................................7
+ 2.1.1. The Certificate Usage Field .........................7
+ 2.1.2. The Selector Field ..................................8
+ 2.1.3. The Matching Type Field .............................9
+ 2.1.4. The Certificate Association Data Field ..............9
+ 2.2. TLSA RR Presentation Format ................................9
+ 2.3. TLSA RR Examples ..........................................10
+ 3. Domain Names for TLSA Certificate Associations .................10
+ 4. Use of TLSA Records in TLS .....................................11
+ 4.1. Usable Certificate Associations ...........................11
+ 5. TLSA and DANE Use Cases and Requirements .......................13
+ 6. Mandatory-to-Implement Features ................................15
+ 7. IANA Considerations ............................................15
+ 7.1. TLSA RRtype ...............................................15
+ 7.2. TLSA Certificate Usages ...................................15
+ 7.3. TLSA Selectors ............................................16
+ 7.4. TLSA Matching Types .......................................16
+ 8. Security Considerations ........................................16
+ 8.1. Comparing DANE to Public CAs ..............................18
+ 8.1.1. Risk of Key Compromise .............................19
+ 8.1.2. Impact of Key Compromise ...........................20
+ 8.1.3. Detection of Key Compromise ........................20
+ 8.1.4. Spoofing Hostnames .................................20
+ 8.2. DNS Caching ...............................................21
+ 8.3. External DNSSEC Validators ................................21
+ 9. Acknowledgements ...............................................22
+ 10. References ....................................................22
+ 10.1. Normative References .....................................22
+ 10.2. Informative References ...................................23
+ Appendix A. Operational Considerations for Deploying TLSA
+ Records ...............................................25
+ A.1. Creating TLSA Records ......................................25
+ A.1.1. Ambiguities and Corner Cases When TLS Clients
+ Build Trust Chains .....................................26
+ A.1.2. Choosing a Selector Type ...............................26
+ A.2. Provisioning TLSA Records in DNS ...........................28
+ A.2.1. Provisioning TLSA Records with Aliases .................28
+ A.3. Securing the Last Hop ......................................30
+ A.4. Handling Certificate Rollover ..............................31
+
+
+
+Hoffman & Schlyter Standards Track [Page 2]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Appendix B. Pseudocode for Using TLSA .............................32
+ B.1. Helper Functions ...........................................32
+ B.2. Main TLSA Pseudocode .......................................33
+ Appendix C. Examples ..............................................35
+
+1. Introduction
+
+1.1. Background and Motivation
+
+ Applications that communicate over the Internet often need to prevent
+ eavesdropping, tampering, or forgery of their communications. The
+ Transport Layer Security (TLS) protocol provides this kind of
+ communications security over the Internet, using channel encryption.
+
+ The security properties of encryption systems depend strongly on the
+ keys that they use. If secret keys are revealed, or if public keys
+ can be replaced by fake keys (that is, a key not corresponding to the
+ entity identified in the certificate), these systems provide little
+ or no security.
+
+ TLS uses certificates to bind keys and names. A certificate combines
+ a published key with other information such as the name of the
+ service that uses the key, and this combination is digitally signed
+ by another key. Having a key in a certificate is only helpful if one
+ trusts the other key that signed the certificate. If that other key
+ was itself revealed or substituted, then its signature is worthless
+ in proving anything about the first key.
+
+ On the Internet, this problem has been solved for years by entities
+ called "Certification Authorities" (CAs). CAs protect their secret
+ key vigorously, while supplying their public key to the software
+ vendors who build TLS clients. They then sign certificates, and
+ supply those to TLS servers. TLS client software uses a set of these
+ CA keys as "trust anchors" to validate the signatures on certificates
+ that the client receives from TLS servers. Client software typically
+ allows any CA to usefully sign any other certificate.
+
+ The public CA model upon which TLS has depended is fundamentally
+ vulnerable because it allows any of these CAs to issue a certificate
+ for any domain name. A single trusted CA that betrays its trust,
+ either voluntarily or by providing less-than-vigorous protection for
+ its secrets and capabilities, can undermine the security offered by
+ any certificates employed with TLS. This problem arises because a
+ compromised CA can issue a replacement certificate that contains a
+ fake key. Recent experiences with compromises of CAs or their
+ trusted partners have led to very serious security problems, such as
+ the governments of multiple countries attempting to wiretap and/or
+ subvert major TLS-protected web sites trusted by millions of users.
+
+
+
+Hoffman & Schlyter Standards Track [Page 3]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ The DNS Security Extensions (DNSSEC) provide a similar model that
+ involves trusted keys signing the information for untrusted keys.
+ However, DNSSEC provides three significant improvements. Keys are
+ tied to names in the Domain Name System (DNS), rather than to
+ arbitrary identifying strings; this is more convenient for Internet
+ protocols. Signed keys for any domain are accessible online through
+ a straightforward query using the standard DNSSEC protocol, so there
+ is no problem distributing the signed keys. Most significantly, the
+ keys associated with a domain name can only be signed by a key
+ associated with the parent of that domain name; for example, the keys
+ for "example.com" can only be signed by the keys for "com", and the
+ keys for "com" can only be signed by the DNS root. This prevents an
+ untrustworthy signer from compromising anyone's keys except those in
+ their own subdomains. Like TLS, DNSSEC relies on public keys that
+ come built into the DNSSEC client software, but these keys come only
+ from a single root domain rather than from a multiplicity of CAs.
+
+ DNS-Based Authentication of Named Entities (DANE) offers the option
+ to use the DNSSEC infrastructure to store and sign keys and
+ certificates that are used by TLS. DANE is envisioned as a
+ preferable basis for binding public keys to DNS names, because the
+ entities that vouch for the binding of public key data to DNS names
+ are the same entities responsible for managing the DNS names in
+ question. While the resulting system still has residual security
+ vulnerabilities, it restricts the scope of assertions that can be
+ made by any entity, consistent with the naming scope imposed by the
+ DNS hierarchy. As a result, DANE embodies the security "principle of
+ least privilege" that is lacking in the current public CA model.
+
+1.2. Securing the Association of a Domain Name with a Server's
+ Certificate
+
+ A TLS client begins a connection by exchanging messages with a TLS
+ server. For many application protocols, it looks up the server's
+ name using the DNS to get an Internet Protocol (IP) address
+ associated with the name. It then begins a connection to a
+ particular port at that address, and sends an initial message there.
+ However, the client does not yet know whether an adversary is
+ intercepting and/or altering its communication before it reaches the
+ TLS server. It does not even know whether the real TLS server
+ associated with that domain name has ever received its initial
+ messages.
+
+ The first response from the server in TLS may contain a certificate.
+ In order for the TLS client to authenticate that it is talking to the
+ expected TLS server, the client must validate that this certificate
+ is associated with the domain name used by the client to get to the
+ server. Currently, the client must extract the domain name from the
+
+
+
+Hoffman & Schlyter Standards Track [Page 4]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ certificate and must successfully validate the certificate, including
+ chaining to a trust anchor.
+
+ There is a different way to authenticate the association of the
+ server's certificate with the intended domain name without trusting
+ an external CA. Given that the DNS administrator for a domain name
+ is authorized to give identifying information about the zone, it
+ makes sense to allow that administrator to also make an authoritative
+ binding between the domain name and a certificate that might be used
+ by a host at that domain name. The easiest way to do this is to use
+ the DNS, securing the binding with DNSSEC.
+
+ There are many use cases for such functionality. [RFC6394] lists the
+ ones to which the DNS RRtype in this document apply. [RFC6394] also
+ lists many requirements, most of which this document is believed to
+ meet. Section 5 covers the applicability of this document to the use
+ cases in detail. The protocol in this document can generally be
+ referred to as the "DANE TLSA" protocol. ("TLSA" does not stand for
+ anything; it is just the name of the RRtype.)
+
+ This document applies to both TLS [RFC5246] and Datagram TLS (DTLS)
+ [RFC6347]. In order to make the document more readable, it mostly
+ only talks about "TLS", but in all cases, it means "TLS or DTLS".
+ Although the references in this paragraph are to TLS and DTLS
+ version 1.2, the DANE TLSA protocol can also be used with earlier
+ versions of TLS and DTLS.
+
+ This document only relates to securely associating certificates for
+ TLS and DTLS with host names; retrieving certificates from DNS for
+ other protocols is handled in other documents. For example, keys for
+ IPsec are covered in [RFC4025], and keys for Secure SHell (SSH) are
+ covered in [RFC4255].
+
+1.3. Method for Securing Certificate Associations
+
+ A certificate association is formed from a piece of information
+ identifying a certificate and the domain name where the server
+ application runs. The combination of a trust anchor and a domain
+ name can also be a certificate association.
+
+ A DNS query can return multiple certificate associations, such as in
+ the case of a server that is changing from one certificate to another
+ (described in more detail in Appendix A.4).
+
+ This document only applies to PKIX [RFC5280] certificates, not
+ certificates of other formats.
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 5]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ This document defines a secure method to associate the certificate
+ that is obtained from the TLS server with a domain name using DNS;
+ the DNS information needs to be protected by DNSSEC. Because the
+ certificate association was retrieved based on a DNS query, the
+ domain name in the query is by definition associated with the
+ certificate. Note that this document does not cover how to associate
+ certificates with domain names for application protocols that depend
+ on SRV, NAPTR, and similar DNS resource records. It is expected that
+ future documents will cover methods for making those associations,
+ and those documents may or may not need to update this one.
+
+ DNSSEC, which is defined in [RFC4033], [RFC4034], and [RFC4035], uses
+ cryptographic keys and digital signatures to provide authentication
+ of DNS data. Information that is retrieved from the DNS and that is
+ validated using DNSSEC is thereby proved to be the authoritative
+ data. The DNSSEC signature needs to be validated on all responses
+ that use DNSSEC in order to assure the proof of origin of the data.
+
+ This document does not specify how DNSSEC validation occurs because
+ there are many different proposals for how a client might get
+ validated DNSSEC results, such as from a DNSSEC-aware resolver that
+ is coded in the application, from a trusted DNSSEC resolver on the
+ machine on which the application is running, or from a trusted DNSSEC
+ resolver with which the application is communicating over an
+ authenticated and integrity-protected channel or network. This is
+ described in more detail in Section 7 of [RFC4033].
+
+ This document only relates to getting the DNS information for the
+ certificate association securely using DNSSEC; other secure DNS
+ mechanisms are out of scope.
+
+1.4. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in RFC 2119 [RFC2119].
+
+ This document also makes use of standard PKIX, DNSSEC, TLS, and DNS
+ terminology. See [RFC5280], [RFC4033], [RFC5246], and STD 13
+ [RFC1034] [RFC1035], respectively, for these terms. In addition,
+ terms related to TLS-protected application services and DNS names are
+ taken from [RFC6125].
+
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 6]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+2. The TLSA Resource Record
+
+ The TLSA DNS resource record (RR) is used to associate a TLS server
+ certificate or public key with the domain name where the record is
+ found, thus forming a "TLSA certificate association". The semantics
+ of how the TLSA RR is interpreted are given later in this document.
+
+ The type value for the TLSA RR type is defined in Section 7.1.
+
+ The TLSA RR is class independent.
+
+ The TLSA RR has no special Time to Live (TTL) requirements.
+
+2.1. TLSA RDATA Wire Format
+
+ The RDATA for a TLSA RR consists of a one-octet certificate usage
+ field, a one-octet selector field, a one-octet matching type field,
+ and the certificate association data field.
+
+ 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Cert. Usage | Selector | Matching Type | /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
+ / /
+ / Certificate Association Data /
+ / /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+2.1.1. The Certificate Usage Field
+
+ A one-octet value, called "certificate usage", specifies the provided
+ association that will be used to match the certificate presented in
+ the TLS handshake. This value is defined in a new IANA registry (see
+ Section 7.2) in order to make it easier to add additional certificate
+ usages in the future. The certificate usages defined in this
+ document are:
+
+ 0 -- Certificate usage 0 is used to specify a CA certificate, or
+ the public key of such a certificate, that MUST be found in any of
+ the PKIX certification paths for the end entity certificate given
+ by the server in TLS. This certificate usage is sometimes
+ referred to as "CA constraint" because it limits which CA can be
+ used to issue certificates for a given service on a host. The
+ presented certificate MUST pass PKIX certification path
+ validation, and a CA certificate that matches the TLSA record MUST
+ be included as part of a valid certification path. Because this
+ certificate usage allows both trust anchors and CA certificates,
+
+
+
+Hoffman & Schlyter Standards Track [Page 7]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ the certificate might or might not have the basicConstraints
+ extension present.
+
+ 1 -- Certificate usage 1 is used to specify an end entity
+ certificate, or the public key of such a certificate, that MUST be
+ matched with the end entity certificate given by the server in
+ TLS. This certificate usage is sometimes referred to as "service
+ certificate constraint" because it limits which end entity
+ certificate can be used by a given service on a host. The target
+ certificate MUST pass PKIX certification path validation and MUST
+ match the TLSA record.
+
+ 2 -- Certificate usage 2 is used to specify a certificate, or the
+ public key of such a certificate, that MUST be used as the trust
+ anchor when validating the end entity certificate given by the
+ server in TLS. This certificate usage is sometimes referred to as
+ "trust anchor assertion" and allows a domain name administrator to
+ specify a new trust anchor -- for example, if the domain issues
+ its own certificates under its own CA that is not expected to be
+ in the end users' collection of trust anchors. The target
+ certificate MUST pass PKIX certification path validation, with any
+ certificate matching the TLSA record considered to be a trust
+ anchor for this certification path validation.
+
+ 3 -- Certificate usage 3 is used to specify a certificate, or the
+ public key of such a certificate, that MUST match the end entity
+ certificate given by the server in TLS. This certificate usage is
+ sometimes referred to as "domain-issued certificate" because it
+ allows for a domain name administrator to issue certificates for a
+ domain without involving a third-party CA. The target certificate
+ MUST match the TLSA record. The difference between certificate
+ usage 1 and certificate usage 3 is that certificate usage 1
+ requires that the certificate pass PKIX validation, but PKIX
+ validation is not tested for certificate usage 3.
+
+ The certificate usages defined in this document explicitly only apply
+ to PKIX-formatted certificates in DER encoding [X.690]. If TLS
+ allows other formats later, or if extensions to this RRtype are made
+ that accept other formats for certificates, those certificates will
+ need their own certificate usage values.
+
+2.1.2. The Selector Field
+
+ A one-octet value, called "selector", specifies which part of the TLS
+ certificate presented by the server will be matched against the
+ association data. This value is defined in a new IANA registry (see
+ Section 7.3). The selectors defined in this document are:
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 8]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ 0 -- Full certificate: the Certificate binary structure as defined
+ in [RFC5280]
+
+ 1 -- SubjectPublicKeyInfo: DER-encoded binary structure as defined
+ in [RFC5280]
+
+ (Note that the use of "selector" in this document is completely
+ unrelated to the use of "selector" in DomainKeys Identified Mail
+ (DKIM) [RFC6376].)
+
+2.1.3. The Matching Type Field
+
+ A one-octet value, called "matching type", specifies how the
+ certificate association is presented. This value is defined in a new
+ IANA registry (see Section 7.4). The types defined in this document
+ are:
+
+ 0 -- Exact match on selected content
+
+ 1 -- SHA-256 hash of selected content [RFC6234]
+
+ 2 -- SHA-512 hash of selected content [RFC6234]
+
+ If the TLSA record's matching type is a hash, having the record use
+ the same hash algorithm that was used in the signature in the
+ certificate (if possible) will assist clients that support a small
+ number of hash algorithms.
+
+2.1.4. The Certificate Association Data Field
+
+ This field specifies the "certificate association data" to be
+ matched. These bytes are either raw data (that is, the full
+ certificate or its SubjectPublicKeyInfo, depending on the selector)
+ for matching type 0, or the hash of the raw data for matching types 1
+ and 2. The data refers to the certificate in the association, not to
+ the TLS ASN.1 Certificate object.
+
+2.2. TLSA RR Presentation Format
+
+ The presentation format of the RDATA portion (as defined in
+ [RFC1035]) is as follows:
+
+ o The certificate usage field MUST be represented as an 8-bit
+ unsigned integer.
+
+ o The selector field MUST be represented as an 8-bit unsigned
+ integer.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 9]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ o The matching type field MUST be represented as an 8-bit unsigned
+ integer.
+
+ o The certificate association data field MUST be represented as a
+ string of hexadecimal characters. Whitespace is allowed within
+ the string of hexadecimal characters, as described in [RFC1035].
+
+2.3. TLSA RR Examples
+
+ In the following examples, the domain name is formed using the rules
+ in Section 3.
+
+ An example of a hashed (SHA-256) association of a PKIX CA
+ certificate:
+
+ _443._tcp.www.example.com. IN TLSA (
+ 0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
+ 7983a1d16e8a410e4561cb106618e971 )
+
+ An example of a hashed (SHA-512) subject public key association of a
+ PKIX end entity certificate:
+
+ _443._tcp.www.example.com. IN TLSA (
+ 1 1 2 92003ba34942dc74152e2f2c408d29ec
+ a5a520e7f2e06bb944f4dca346baf63c
+ 1b177615d466f6c4b71c216a50292bd5
+ 8c9ebdd2f74e38fe51ffd48c43326cbc )
+
+ An example of a full certificate association of a PKIX end entity
+ certificate:
+
+ _443._tcp.www.example.com. IN TLSA (
+ 3 0 0 30820307308201efa003020102020... )
+
+3. Domain Names for TLSA Certificate Associations
+
+ Unless there is a protocol-specific specification that is different
+ than this one, TLSA resource records are stored at a prefixed DNS
+ domain name. The prefix is prepared in the following manner:
+
+ 1. The decimal representation of the port number on which a TLS-
+ based service is assumed to exist is prepended with an underscore
+ character ("_") to become the left-most label in the prepared
+ domain name. This number has no leading zeros.
+
+
+
+
+
+
+
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ 2. The protocol name of the transport on which a TLS-based service
+ is assumed to exist is prepended with an underscore character
+ ("_") to become the second left-most label in the prepared domain
+ name. The transport names defined for this protocol are "tcp",
+ "udp", and "sctp".
+
+ 3. The base domain name is appended to the result of step 2 to
+ complete the prepared domain name. The base domain name is the
+ fully qualified DNS domain name [RFC1035] of the TLS server, with
+ the additional restriction that every label MUST meet the rules
+ of [RFC0952]. The latter restriction means that, if the query is
+ for an internationalized domain name, it MUST use the A-label
+ form as defined in [RFC5890].
+
+ For example, to request a TLSA resource record for an HTTP server
+ running TLS on port 443 at "www.example.com",
+ "_443._tcp.www.example.com" is used in the request. To request a
+ TLSA resource record for an SMTP server running the STARTTLS protocol
+ on port 25 at "mail.example.com", "_25._tcp.mail.example.com" is
+ used.
+
+4. Use of TLSA Records in TLS
+
+ Section 2.1 of this document defines the mandatory matching rules for
+ the data from the TLSA certificate associations and the certificates
+ received from the TLS server.
+
+ The TLS session that is to be set up MUST be for the specific port
+ number and transport name that was given in the TLSA query.
+
+ Some specifications for applications that run over TLS, such as
+ [RFC2818] for HTTP, require that the server's certificate have a
+ domain name that matches the host name expected by the client. Some
+ specifications, such as [RFC6125], detail how to match the identity
+ given in a PKIX certificate with those expected by the user.
+
+ If a TLSA record has certificate usage 2, the corresponding TLS
+ server SHOULD send the certificate that is referenced just like it
+ currently sends intermediate certificates.
+
+4.1. Usable Certificate Associations
+
+ An implementation of this protocol makes a DNS query for TLSA
+ records, validates these records using DNSSEC, and uses the resulting
+ TLSA records and validation status to modify its responses to the TLS
+ server.
+
+
+
+
+
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Determining whether a TLSA RRSet can be used MUST be based on the
+ DNSSEC validation state (as defined in [RFC4033]).
+
+ o A TLSA RRSet whose DNSSEC validation state is secure MUST be used
+ as a certificate association for TLS unless a local policy would
+ prohibit the use of the specific certificate association in the
+ secure TLSA RRSet.
+
+ o If the DNSSEC validation state on the response to the request for
+ the TLSA RRSet is bogus, this MUST cause TLS not to be started or,
+ if the TLS negotiation is already in progress, MUST cause the
+ connection to be aborted.
+
+ o A TLSA RRSet whose DNSSEC validation state is indeterminate or
+ insecure cannot be used for TLS and MUST be considered unusable.
+
+ Clients that validate the DNSSEC signatures themselves MUST use
+ standard DNSSEC validation procedures. Clients that rely on another
+ entity to perform the DNSSEC signature validation MUST use a secure
+ mechanism between themselves and the validator. Examples of secure
+ transports to other hosts include TSIG [RFC2845], SIG(0) [RFC2931],
+ and IPsec [RFC6071]. Note that it is not sufficient to use secure
+ transport to a DNS resolver that does not do DNSSEC signature
+ validation. See Section 8.3 for more security considerations related
+ to external validators.
+
+ If a certificate association contains a certificate usage, selector,
+ or matching type that is not understood by the TLS client, that
+ certificate association MUST be considered unusable. If the
+ comparison data for a certificate is malformed, the certificate
+ association MUST be considered unusable.
+
+ If a certificate association contains a matching type or certificate
+ association data that uses a cryptographic algorithm that is
+ considered too weak for the TLS client's policy, the certificate
+ association MUST be considered unusable.
+
+ If an application receives zero usable certificate associations from
+ a DNS request or from its cache, it processes TLS in the normal
+ fashion without any input from the TLSA records. If an application
+ receives one or more usable certificate associations, it attempts to
+ match each certificate association with the TLS server's end entity
+ certificate until a successful match is found. During the TLS
+ handshake, if none of the certificate associations matches the
+ certificate given by the TLS server, the TLS client MUST abort the
+ handshake.
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 12]
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+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ An attacker who is able to divert a user to a server under his
+ control is also likely to be able to block DNS requests from the user
+ or DNS responses being sent to the user. Thus, in order to achieve
+ any security benefit from certificate usage 0 or 1, an application
+ that sends a request for TLSA records needs to get either a valid
+ signed response containing TLSA records or verification that the
+ domain is insecure or indeterminate. If a request for a TLSA record
+ does not meet one of those two criteria but the application continues
+ with the TLS handshake anyway, the application has gotten no benefit
+ from TLSA and SHOULD NOT make any internal or external indication
+ that TLSA was applied. If an application has a configuration setting
+ that has turned on TLSA use, or has any indication that TLSA is in
+ use (regardless of whether or not this is configurable), that
+ application either MUST NOT start a TLS connection or it MUST abort a
+ TLS handshake if both of the two criteria above are not met.
+
+ The application can perform the TLSA lookup before initiating the TLS
+ handshake, or do it during the TLS handshake: the choice is up to the
+ client.
+
+5. TLSA and DANE Use Cases and Requirements
+
+ The different types of certificate associations defined in TLSA are
+ matched with various sections of [RFC6394]. The use cases from
+ Section 3 of [RFC6394] are covered in this document as follows:
+
+ 3.1 CA Constraints -- Implemented using certificate usage 0.
+
+ 3.2 Certificate Constraints -- Implemented using certificate usage 1.
+
+ 3.3 Trust Anchor Assertion and Domain-Issued Certificates --
+ Implemented using certificate usages 2 and 3, respectively.
+
+ The requirements from Section 4 of [RFC6394] are covered in this
+ document as follows:
+
+ Multiple Ports -- The TLSA records for different application services
+ running on a single host can be distinguished through the service
+ name and port number prefixed to the host name (see Section 3).
+
+ No Downgrade -- Section 4 specifies the conditions under which a
+ client can process and act upon TLSA records. Specifically, if
+ the DNSSEC status for the TLSA resource record set is determined
+ to be bogus, the TLS connection (if started) will fail.
+
+ Encapsulation -- Encapsulation is covered in the TLSA response
+ semantics.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 13]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Predictability -- The appendices of this specification provide
+ operational considerations and implementation guidance in order to
+ enable application developers to form a consistent interpretation
+ of the recommended client behavior.
+
+ Opportunistic Security -- If a client conformant to this
+ specification can reliably determine the presence of a TLSA
+ record, it will attempt to use this information. Conversely, if a
+ client can reliably determine the absence of any TLSA record, it
+ will fall back to processing TLS in the normal fashion. This is
+ discussed in Section 4.
+
+ Combination -- Multiple TLSA records can be published for a given
+ host name, thus enabling the client to construct multiple TLSA
+ certificate associations that reflect different assertions. No
+ support is provided to combine two TLSA certificate associations
+ in a single operation.
+
+ Roll-over -- TLSA records are processed in the normal manner within
+ the scope of the DNS protocol, including the TTL expiration of the
+ records. This ensures that clients will not latch onto assertions
+ made by expired TLSA records, and will be able to transition from
+ using one public key or certificate usage to another.
+
+ Simple Key Management -- The SubjectPublicKeyInfo selector in the
+ TLSA record provides a mode that enables a domain holder to only
+ have to maintain a single long-lived public/private key pair
+ without the need to manage certificates. Appendix A outlines the
+ usefulness and the potential downsides to using this mode.
+
+ Minimal Dependencies -- This specification relies on DNSSEC to
+ protect the origin authenticity and integrity of the TLSA resource
+ record set. Additionally, if DNSSEC validation is not performed
+ on the system that wishes to use TLSA certificate bindings, this
+ specification requires that the "last mile" be over a secure
+ transport. There are no other deployment dependencies for this
+ approach.
+
+ Minimal Options -- The operating modes map precisely to the DANE use
+ cases and requirements. DNSSEC use is mandatory in that this
+ specification encourages applications to use only those TLSA
+ records that are shown to be validated.
+
+ Wildcards -- Wildcards are covered in a limited manner in the TLSA
+ request syntax; see Appendix A.
+
+ Redirection -- Redirection is covered in the TLSA request syntax; see
+ Appendix A.
+
+
+
+Hoffman & Schlyter Standards Track [Page 14]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+6. Mandatory-to-Implement Features
+
+ TLS clients conforming to this specification MUST be able to
+ correctly interpret TLSA records with certificate usages 0, 1, 2,
+ and 3. TLS clients conforming to this specification MUST be able to
+ compare a certificate association with a certificate from the TLS
+ handshake using selector types 0 and 1, and matching type 0 (no hash
+ used) and matching type 1 (SHA-256), and SHOULD be able to make such
+ comparisons with matching type 2 (SHA-512).
+
+7. IANA Considerations
+
+ IANA has made the assignments in this section.
+
+ In the following sections, "RFC Required" was chosen for TLSA
+ certificate usages and "Specification Required" for selectors and
+ matching types because of the amount of detail that is likely to be
+ needed for implementers to correctly implement new certificate usages
+ as compared to new selectors and matching types.
+
+7.1. TLSA RRtype
+
+ This document uses a new DNS RR type, TLSA, whose value (52) was
+ allocated by IANA from the Resource Record (RR) TYPEs subregistry of
+ the Domain Name System (DNS) Parameters registry.
+
+7.2. TLSA Certificate Usages
+
+ This document creates a new registry, "TLSA Certificate Usages". The
+ registry policy is "RFC Required". The initial entries in the
+ registry are:
+
+ Value Short description Reference
+ ----------------------------------------------------------
+ 0 CA constraint RFC 6698
+ 1 Service certificate constraint RFC 6698
+ 2 Trust anchor assertion RFC 6698
+ 3 Domain-issued certificate RFC 6698
+ 4-254 Unassigned
+ 255 Private use
+
+ Applications to the registry can request specific values that have
+ yet to be assigned.
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 15]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+7.3. TLSA Selectors
+
+ This document creates a new registry, "TLSA Selectors". The registry
+ policy is "Specification Required". The initial entries in the
+ registry are:
+
+ Value Short description Reference
+ ----------------------------------------------------------
+ 0 Full certificate RFC 6698
+ 1 SubjectPublicKeyInfo RFC 6698
+ 2-254 Unassigned
+ 255 Private use
+
+ Applications to the registry can request specific values that have
+ yet to be assigned.
+
+7.4. TLSA Matching Types
+
+ This document creates a new registry, "TLSA Matching Types". The
+ registry policy is "Specification Required". The initial entries in
+ the registry are:
+
+ Value Short description Reference
+ ----------------------------------------------------------
+ 0 No hash used RFC 6698
+ 1 SHA-256 RFC 6234
+ 2 SHA-512 RFC 6234
+ 3-254 Unassigned
+ 255 Private use
+
+ Applications to the registry can request specific values that have
+ yet to be assigned.
+
+8. Security Considerations
+
+ The security of the DNS RRtype described in this document relies on
+ the security of DNSSEC to verify that the TLSA record has not been
+ altered.
+
+ A rogue DNS administrator who changes the A, AAAA, and/or TLSA
+ records for a domain name can cause the client to go to an
+ unauthorized server that will appear authorized, unless the client
+ performs PKIX certification path validation and rejects the
+ certificate. That administrator could probably get a certificate
+ issued by some CA anyway, so this is not an additional threat.
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 16]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ If the authentication mechanism for adding or changing TLSA data in a
+ zone is weaker than the authentication mechanism for changing the A
+ and/or AAAA records, a man-in-the-middle who can redirect traffic to
+ his site may be able to impersonate the attacked host in TLS if he
+ can use the weaker authentication mechanism. A better design for
+ authenticating DNS would be to have the same level of authentication
+ used for all DNS additions and changes for a particular domain name.
+
+ Secure Socket Layer (SSL) proxies can sometimes act as a man-in-the-
+ middle for TLS clients. In these scenarios, the clients add a new
+ trust anchor whose private key is kept on the SSL proxy; the proxy
+ intercepts TLS requests, creates a new TLS session with the intended
+ host, and sets up a TLS session with the client using a certificate
+ that chains to the trust anchor installed in the client by the proxy.
+ In such environments, using TLSA records will prevent the SSL proxy
+ from functioning as expected because the TLS client will get a
+ certificate association from the DNS that will not match the
+ certificate that the SSL proxy uses with the client. The client,
+ seeing the proxy's new certificate for the supposed destination, will
+ not set up a TLS session.
+
+ Client treatment of any information included in the trust anchor is a
+ matter of local policy. This specification does not mandate that
+ such information be inspected or validated by the server's domain
+ name administrator.
+
+ If a server's certificate is revoked, or if an intermediate CA in a
+ chain between the server and a trust anchor has its certificate
+ revoked, a TLSA record with a certificate usage of 2 that matches the
+ revoked certificate would in essence override the revocation because
+ the client would treat that revoked certificate as a trust anchor and
+ thus not check its revocation status. Because of this, domain
+ administrators need to be responsible for being sure that the keys or
+ certificates used in TLSA records with a certificate usage of 2 are
+ in fact able to be used as reliable trust anchors.
+
+ Certificates that are delivered in TLSA with certificate usage 2
+ fundamentally change the way the TLS server's end entity certificate
+ is evaluated. For example, the server's certificate might chain to
+ an existing CA through an intermediate CA that has certain policy
+ restrictions, and the certificate would not pass those restrictions
+ and thus normally be rejected. That intermediate CA could issue
+ itself a new certificate without the policy restrictions and tell its
+ customers to use that certificate with certificate usage 2. This in
+ essence allows an intermediate CA to become a trust anchor for
+ certificates that the end user might have expected to chain to an
+ existing trust anchor.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 17]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ If an administrator wishes to stop using a TLSA record, the
+ administrator can simply remove it from the DNS. Normal clients will
+ stop using the TLSA record after the TTL has expired. Replay attacks
+ against the TLSA record are not possible after the expiration date on
+ the RRsig of the TLSA record that was removed.
+
+ Generators of TLSA records should be aware that the client's full
+ trust of a certificate association retrieved from a TLSA record may
+ be a matter of local policy. While such trust is limited to the
+ specific domain name, protocol, and port for which the TLSA query was
+ made, local policy may decline to accept the certificate (for reasons
+ such as weak cryptography), as is also the case with PKIX trust
+ anchors.
+
+8.1. Comparing DANE to Public CAs
+
+ As stated above, the security of the DNS RRtype described in this
+ document relies on the security of DNSSEC to verify that the TLSA
+ record has not been altered. This section describes where the
+ security of public CAs and the security of TLSA are similar and
+ different. This section applies equally to other security-related
+ DNS RRtypes such as keys for IPsec and SSH.
+
+ DNSSEC forms certificates (the binding of an identity to a key) by
+ combining a DNSKEY, DS, or DLV resource record with an associated
+ RRSIG record. These records then form a signing chain extending from
+ the client's trust anchors to the RR of interest.
+
+ Although the DNSSEC protocol does not enforce it, DNSKEYs are often
+ marked with a SEP flag indicating whether the key is a Zone Signing
+ Key (ZSK) or a Key Signing Key (KSK). ZSKs protect records in the
+ zone (including DS and DLV records), and KSKs protect ZSK DNSKEY
+ records. This allows KSKs to be stored offline.
+
+ The TLSA RRtype allows keys from the DNSSEC PKI hierarchy to
+ authenticate keys wrapped in PKIX certificates for a particular host
+ name, protocol, and port.
+
+ With the exception of the DLV RRtype, all of these certificates
+ constrain the keys they identify to names that are within the zone
+ signing the certificate. In order for a domain's DLV resource
+ records to be honored, the domain must be configured as a DLV domain,
+ and the domain's DNSKEYs must be configured as trust anchors or be
+ authentic [RFC5074].
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 18]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+8.1.1. Risk of Key Compromise
+
+ The risk that a given certificate that has a valid signing chain is
+ fake is related to the number of keys that can contribute to the
+ validation of the certificate, the quality of protection each private
+ key receives, the value of each key to an attacker, and the value of
+ falsifying the certificate.
+
+ DNSSEC allows any set of domains to be configured as trust anchors
+ and/or DLVs, but most clients are likely to use the root zone as
+ their only trust anchor. Also, because a given DNSKEY can only sign
+ resource records for that zone, the number of private keys capable of
+ compromising a given TLSA resource record is limited to the number of
+ zones between the TLSA resource record and the nearest trust anchor,
+ plus any configured DLV domains. Typically, this will be six keys,
+ half of which will be KSKs.
+
+ PKIX only describes how to validate a certificate based on a client-
+ chosen set of trust anchors, but says nothing about how many trust
+ anchors to use or how they should be constrained. As currently
+ deployed, most PKIX clients use a large number of trust anchors
+ provided with the client or operating system software. These trust
+ anchors are selected carefully, but with a desire for broad
+ interoperability. The trust anchors and CA certificates for public
+ CAs rarely have name constraints applied.
+
+ A combination of technical protections, process controls, and
+ personnel experience contribute to the quality of security that keys
+ receive.
+
+ o The security surrounding DNSSEC DNSKEYs varies significantly. The
+ KSK/ZSK split allows the KSK to be stored offline and protected
+ more carefully than the ZSK, but not all domains do so. The
+ security applied to a zone's DNSKEYs should be proportional to the
+ value of the domain, but that is difficult to estimate. For
+ example, the root DNSKEY has protections and controls comparable
+ to or exceeding those of public CAs. On the other end of the
+ spectrum, small domains might provide no more protection to their
+ keys than they do to their other data.
+
+ o The security surrounding public CAs also varies. However, due to
+ financial incentives and standards imposed by clients for
+ acceptance into their trust anchor stores, CAs generally employ
+ security experts and protect their keys carefully, though highly
+ public compromises have occurred.
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 19]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+8.1.2. Impact of Key Compromise
+
+ The impact of a key compromise differs significantly between the two
+ models.
+
+ o DNSKEYs are inherently limited in what they can sign, so a
+ compromise of the DNSKEY for "example.com" provides no avenue of
+ attack against "example.org". Even the impact of a compromise of
+ .com's DNSKEY, while considerable, would be limited to .com
+ domains. Only the compromise of the root DNSKEY would have the
+ equivalent impact of an unconstrained public CA.
+
+ o Public CAs are not typically constrained in what names they can
+ sign, and therefore a compromise of even one CA allows the
+ attacker to generate a certificate for any name in the DNS. A
+ domain holder can get a certificate from any willing CA, or even
+ multiple CAs simultaneously, making it impossible for a client to
+ determine whether the certificate it is validating is legitimate
+ or fraudulent.
+
+ Because a TLSA certificate association is constrained to its
+ associated name, protocol, and port, the PKIX certificate is
+ similarly constrained, even if its public CAs signing the certificate
+ (if any) are not.
+
+8.1.3. Detection of Key Compromise
+
+ If a key is compromised, rapid and reliable detection is important in
+ order to limit the impact of the compromise. In this regard, neither
+ model prevents an attacker from near-invisibly attacking their
+ victim, provided that the necessary keys are compromised.
+
+ If a public CA is compromised, only the victim will see the
+ fraudulent certificate, as there is typically no publicly accessible
+ directory of all the certificates issued by a CA that can be
+ inspected. DNS resource records are typically published publicly.
+ However, the attacker could also allow the uncompromised records to
+ be published to the Internet as usual but provide a compromised DNS
+ view to the victim to achieve the same effect.
+
+8.1.4. Spoofing Hostnames
+
+ Some CAs implement technical controls to ensure that certificates are
+ not issued to domains with names similar to domains that are popular
+ and prone to attack. Of course, an attacker can attempt to
+ circumvent this restriction by finding a CA willing to issue the
+ certificate anyway. However, by using DNSSEC and TLSA, the attacker
+ can circumvent this check completely.
+
+
+
+Hoffman & Schlyter Standards Track [Page 20]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+8.2. DNS Caching
+
+ Implementations of this protocol rely heavily on the DNS, and are
+ thus prone to security attacks based on the deliberate
+ mis-association of TLSA records and DNS names. Implementations need
+ to be cautious in assuming the continuing validity of an association
+ between a TLSA record and a DNS name.
+
+ In particular, implementations SHOULD rely on their DNS resolver for
+ confirmation of an association between a TLSA record and a DNS name,
+ rather than caching the result of previous domain name lookups. Many
+ platforms already can cache domain name lookups locally when
+ appropriate, and they SHOULD be configured to do so. It is proper
+ for these lookups to be cached, however, only when the TTL (Time To
+ Live) information reported by the DNS makes it likely that the cached
+ information will remain useful.
+
+ If implementations cache the results of domain name lookups in order
+ to achieve a performance improvement, they MUST observe the TTL
+ information reported by DNS. Implementations that fail to follow
+ this rule could be spoofed or have access denied when a previously
+ accessed server's TLSA record changes, such as during a certificate
+ rollover.
+
+8.3. External DNSSEC Validators
+
+ Due to a lack of DNSSEC support in the most commonly deployed stub
+ resolvers today, some ISPs have begun checking DNSSEC in the
+ recursive resolvers they provide to their customers, setting the
+ Authentic Data (AD) flag as appropriate. DNSSEC-aware clients could
+ use that data, ignoring the fact that the DNSSEC data has been
+ validated externally. Because there is typically no authentication
+ of the recursive resolver or integrity protection of the data and AD
+ flag between the client and the recursive resolver, this can be
+ trivially spoofed by an attacker.
+
+ However, even with secure communications between a host and the
+ external validating resolver, there is a risk that the external
+ validator could become compromised. Nothing prevents a compromised
+ external DNSSEC validator from claiming that all the records it
+ provides are secure, even if the data is falsified, unless the client
+ checks the DNSSEC data itself (rendering the external validator
+ unnecessary).
+
+ For this reason, DNSSEC validation is best performed on-host, even
+ when a secure path to an external validator is available.
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 21]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+9. Acknowledgements
+
+ Many of the ideas in this document have been discussed over many
+ years. More recently, the ideas have been discussed by the authors
+ and others in a more focused fashion. In particular, some of the
+ ideas and words here originated with Paul Vixie, Dan Kaminsky, Jeff
+ Hodges, Phillip Hallam-Baker, Simon Josefsson, Warren Kumari, Adam
+ Langley, Ben Laurie, Ilari Liusvaara, Ondrej Mikle, Scott Schmit,
+ Ondrej Sury, Richard Barnes, Jim Schaad, Stephen Farrell, Suresh
+ Krishnaswamy, Peter Palfrader, Pieter Lexis, Wouter Wijngaards, John
+ Gilmore, and Murray Kucherawy.
+
+ This document has also been greatly helped by many active
+ participants of the DANE Working Group.
+
+10. References
+
+10.1. Normative References
+
+ [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
+ STD 13, RFC 1034, November 1987.
+
+ [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.
+
+ [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.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 22]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
+ Verification of Domain-Based Application Service Identity
+ within Internet Public Key Infrastructure Using X.509
+ (PKIX) Certificates in the Context of Transport Layer
+ Security (TLS)", RFC 6125, March 2011.
+
+ [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
+ Security Version 1.2", RFC 6347, January 2012.
+
+10.2. Informative References
+
+ [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
+ host table specification", RFC 952, October 1985.
+
+ [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
+ specifying the location of services (DNS SRV)", RFC 2782,
+ February 2000.
+
+ [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
+
+ [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
+ Wellington, "Secret Key Transaction Authentication for DNS
+ (TSIG)", RFC 2845, May 2000.
+
+ [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
+ ( SIG(0)s)", RFC 2931, September 2000.
+
+ [RFC4025] Richardson, M., "A Method for Storing IPsec Keying
+ Material in DNS", RFC 4025, March 2005.
+
+ [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
+ Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
+ January 2006.
+
+ [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
+ RFC 4641, September 2006.
+
+ [RFC5074] Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
+ November 2007.
+
+ [RFC5890] Klensin, J., "Internationalized Domain Names for
+ Applications (IDNA): Definitions and Document Framework",
+ RFC 5890, August 2010.
+
+ [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
+ Extensions: Extension Definitions", RFC 6066,
+ January 2011.
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 23]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ [RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
+ Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
+ February 2011.
+
+ [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
+ (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
+
+ [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
+ "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
+ September 2011.
+
+ [RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based
+ Authentication of Named Entities (DANE)", RFC 6394,
+ October 2011.
+
+ [X.690] "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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 24]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+Appendix A. Operational Considerations for Deploying TLSA Records
+
+A.1. Creating TLSA Records
+
+ When creating TLSA records, care must be taken to avoid
+ misconfigurations. Section 4 of this document states that a TLSA
+ RRSet whose validation state is secure MUST be used. This means that
+ the existence of such a RRSet effectively disables other forms of
+ name and path validation. A misconfigured TLSA RRSet will
+ effectively disable access to the TLS server for all conforming
+ clients, and this document does not provide any means of making a
+ gradual transition to using TLSA.
+
+ When creating TLSA records with certificate usage 0 (CA certificate)
+ or usage 2 (trust anchor), one needs to understand the implications
+ when choosing between selector type 0 (Full certificate) and 1
+ (SubjectPublicKeyInfo). A careful choice is required because
+ different methods for building trust chains are used by different TLS
+ clients. The following outlines the cases that one ought to be aware
+ of and discusses the implications of the choice of selector type.
+
+ Certificate usage 2 is not affected by the different types of chain
+ building when the end entity certificate is the same as the trust
+ anchor certificate.
+
+A.1.1. Ambiguities and Corner Cases When TLS Clients Build Trust Chains
+
+ TLS clients can implement their own chain-building code rather than
+ rely on the chain presented by the TLS server. This means that,
+ except for the end entity certificate, any certificate presented in
+ the suggested chain might or might not be present in the final chain
+ built by the client.
+
+ Certificates that the client can use to replace certificates from the
+ original chain include:
+
+ o Client's trust anchors
+
+ o Certificates cached locally
+
+ o Certificates retrieved from a URI listed in an Authority
+ Information Access X.509v3 extension
+
+ CAs frequently reissue certificates with different validity periods,
+ signature algorithms (such as a different hash algorithm in the
+ signature algorithm), CA key pairs (such as for a cross-certificate),
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 25]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ or PKIX extensions where the public key and subject remain the same.
+ These reissued certificates are the certificates that the TLS client
+ can use in place of an original certificate.
+
+ Clients are known to exchange or remove certificates that could cause
+ TLSA certificate associations that rely on the full certificate to
+ fail. For example:
+
+ o The client considers the signature algorithm of a certificate to
+ no longer be sufficiently secure.
+
+ o The client might not have an associated root certificate in its
+ trust store and instead uses a cross-certificate with an identical
+ subject and public key.
+
+A.1.2. Choosing a Selector Type
+
+ In this section, "false-negative failure" means that a client will
+ not accept the TLSA certificate association for a certificate
+ designated by the DNS administrator. Also, "false-positive
+ acceptance" means that the client accepts a TLSA association for a
+ certificate that is not designated by the DNS administrator.
+
+A.1.2.1. Selector Type 0 (Full Certificate)
+
+ The "Full certificate" selector provides the most precise
+ specification of a TLSA certificate association, capturing all
+ fields of the PKIX certificate. For a DNS administrator, the best
+ course to avoid false-negative failures in the client when using this
+ selector is:
+
+ 1. If a CA issued a replacement certificate, don't associate to CA
+ certificates that have a signature algorithm with a hash that is
+ considered weak by local policy.
+
+ 2. Determine how common client applications process the TLSA
+ certificate association using a fresh client installation -- that
+ is, with the local certificate cache empty.
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 26]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+A.1.2.2. Selector Type 1 (SubjectPublicKeyInfo)
+
+ A SubjectPublicKeyInfo selector gives greater flexibility in avoiding
+ some false-negative failures caused by trust-chain-building
+ algorithms used in clients.
+
+ One specific use case ought to be noted: creating a TLSA certificate
+ association to CA certificate I1 that directly signed end entity
+ certificate S1 of the server. The case can be illustrated by the
+ following graph:
+
+ +----+ +----+
+ | I1 | | I2 |
+ +----+ +----+
+ | |
+ v v
+ +----+ +----+
+ | S1 | | S1 |
+ +----+ +----+
+ Certificate chain sent by A different validation path
+ server in TLS handshake built by the TLS client
+
+ I2 is a reissued version of CA certificate I1 (that is, it has a
+ different hash in its signature algorithm).
+
+ In the above scenario, both certificates I1 and I2 that sign S1 need
+ to have identical SubjectPublicKeyInfo fields because the key used to
+ sign S1 is fixed. An association to SubjectPublicKeyInfo (selector
+ type 1) will always succeed in such a case, but an association with a
+ full certificate (selector type 0) might not work due to a false-
+ negative failure.
+
+ The attack surface is a bit broader compared to the "Full
+ certificate" selector: the DNS administrator might unintentionally
+ specify an association that would lead to false-positive acceptance.
+
+ o The administrator must know or trust that the CA does not engage
+ in bad practices, such as not sharing the key of I1 for unrelated
+ CA certificates (which would lead to trust-chain redirection). If
+ possible, the administrator ought to review all CA certificates
+ that have the same SubjectPublicKeyInfo field.
+
+ o The administrator ought to understand whether some PKIX extension
+ may adversely affect security of the association. If possible,
+ administrators ought to review all CA certificates that share the
+ SubjectPublicKeyInfo.
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 27]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ o The administrator ought to understand that any CA could, in the
+ future, issue a certificate that contains the same
+ SubjectPublicKeyInfo. Therefore, new chains can crop up in the
+ future without any warning.
+
+ Using the SubjectPublicKeyInfo selector for association with a
+ certificate in a chain above I1 needs to be decided on a case-by-case
+ basis: there are too many possibilities based on the issuing CA's
+ practices. Unless the full implications of such an association are
+ understood by the administrator, using selector type 0 is a better
+ option from a security perspective.
+
+A.2. Provisioning TLSA Records in DNS
+
+A.2.1. Provisioning TLSA Records with Aliases
+
+ The TLSA resource record is not special in the DNS; it acts exactly
+ like any other RRtype where the queried name has one or more labels
+ prefixed to the base name, such as the SRV RRtype [RFC2782]. This
+ affects the way that the TLSA resource record is used when aliasing
+ in the DNS.
+
+ Note that the IETF sometimes adds new types of aliasing in the DNS.
+ If that happens in the future, those aliases might affect TLSA
+ records, hopefully in a good way.
+
+A.2.1.1. Provisioning TLSA Records with CNAME Records
+
+ Using CNAME to alias in DNS only aliases from the exact name given,
+ not any zones below the given name. For example, assume that a zone
+ file has only the following:
+
+ sub1.example.com. IN CNAME sub2.example.com.
+
+ In this case, a request for the A record at "bottom.sub1.example.com"
+ would not return any records because the CNAME given only aliases the
+ name given. Assume, instead, the zone file has the following:
+
+ sub3.example.com. IN CNAME sub4.example.com.
+ bottom.sub3.example.com. IN CNAME bottom.sub4.example.com.
+
+ In this case, a request for the A record at bottom.sub3.example.com
+ would in fact return whatever value for the A record exists at
+ bottom.sub4.example.com.
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 28]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Application implementations and full-service resolvers request DNS
+ records using libraries that automatically follow CNAME (and DNAME)
+ aliasing. This allows hosts to put TLSA records in their own zones
+ or to use CNAME to do redirection.
+
+ If the owner of the original domain wants a TLSA record for the same,
+ they simply enter it under the defined prefix:
+
+ ; No TLSA record in target domain
+ ;
+ sub5.example.com. IN CNAME sub6.example.com.
+ _443._tcp.sub5.example.com. IN TLSA 1 1 1 308202c5308201ab...
+ sub6.example.com. IN A 192.0.2.1
+ sub6.example.com. IN AAAA 2001:db8::1
+
+ If the owner of the original domain wants to have the target domain
+ host the TLSA record, the original domain uses a CNAME record:
+
+ ; TLSA record for original domain has CNAME to target domain
+ ;
+ sub5.example.com. IN CNAME sub6.example.com.
+ _443._tcp.sub5.example.com. IN CNAME _443._tcp.sub6.example.com.
+ sub6.example.com. IN A 192.0.2.1
+ sub6.example.com. IN AAAA 2001:db8::1
+ _443._tcp.sub6.example.com. IN TLSA 1 1 1 536a570ac49d9ba4...
+
+ Note that it is acceptable for both the original domain and the
+ target domain to have TLSA records, but the two records are
+ unrelated. Consider the following:
+
+ ; TLSA record in both the original and target domain
+ ;
+ sub5.example.com. IN CNAME sub6.example.com.
+ _443._tcp.sub5.example.com. IN TLSA 1 1 1 308202c5308201ab...
+ sub6.example.com. IN A 192.0.2.1
+ sub6.example.com. IN AAAA 2001:db8::1
+ _443._tcp.sub6.example.com. IN TLSA 1 1 1 ac49d9ba4570ac49...
+
+ In this example, someone looking for the TLSA record for
+ sub5.example.com would always get the record whose value starts with
+ "308202c5308201ab"; the TLSA record whose value starts with
+ "ac49d9ba4570ac49" would only be sought by someone who is looking for
+ the TLSA record for sub6.example.com, and never for sub5.example.com.
+ Note that deploying different certificates for multiple services
+ located at a shared TLS listener often requires the use of TLS SNI
+ (Server Name Indication) [RFC6066].
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 29]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Note that these methods use the normal method for DNS aliasing using
+ CNAME: the DNS client requests the record type that they actually
+ want.
+
+A.2.1.2. Provisioning TLSA Records with DNAME Records
+
+ Using DNAME records allows a zone owner to alias an entire subtree of
+ names below the name that has the DNAME. This allows the wholesale
+ aliasing of prefixed records such as those used by TLSA, SRV, and so
+ on without aliasing the name itself. However, because DNAME can only
+ be used for subtrees of a base name, it is rarely used to alias
+ individual hosts that might also be running TLS.
+
+ ; TLSA record in target domain, visible in original domain via DNAME
+ ;
+ sub5.example.com. IN CNAME sub6.example.com.
+ _tcp.sub5.example.com. IN DNAME _tcp.sub6.example.com.
+ sub6.example.com. IN A 192.0.2.1
+ sub6.example.com. IN AAAA 2001:db8::1
+ _443._tcp.sub6.example.com. IN TLSA 1 1 1 536a570ac49d9ba4...
+
+A.2.1.3. Provisioning TLSA Records with Wildcards
+
+ Wildcards are generally not terribly useful for RRtypes that require
+ prefixing because one can only wildcard at a layer below the host
+ name. For example, if one wants to have the same TLSA record for
+ every TCP port for www.example.com, the result might be:
+
+ *._tcp.www.example.com. IN TLSA 1 1 1 5c1502a6549c423b...
+
+ This is possibly useful in some scenarios where the same service is
+ offered on many ports or the same certificate and/or key is used for
+ all services on a host. Note that the domain being searched for is
+ not necessarily related to the domain name found in the certificate,
+ so a certificate with a wildcard in it is not searched for using a
+ wildcard in the search request.
+
+A.3. Securing the Last Hop
+
+ As described in Section 4, an application processing TLSA records
+ must know the DNSSEC validity of those records. There are many ways
+ for the application to determine this securely, and this
+ specification does not mandate any single method.
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 30]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ Some common methods for an application to know the DNSSEC validity of
+ TLSA records include:
+
+ o The application can have its own DNS resolver and DNSSEC
+ validation stack.
+
+ o The application can communicate through a trusted channel (such as
+ requests to the operating system under which the application is
+ running) to a local DNS resolver that does DNSSEC validation.
+
+ o The application can communicate through a secured channel (such as
+ requests running over TLS, IPsec, TSIG, or SIG(0)) to a non-local
+ DNS resolver that does DNSSEC validation.
+
+ o The application can communicate through a secured channel (such as
+ requests running over TLS, IPsec, TSIG, or SIG(0)) to a non-local
+ DNS resolver that does not do DNSSEC validation, but gets
+ responses through a secured channel from a different DNS resolver
+ that does DNSSEC validation.
+
+A.4. Handling Certificate Rollover
+
+ Certificate rollover is handled in much the same way as for rolling
+ DNSSEC zone signing keys using the pre-publish key rollover method
+ [RFC4641]. Suppose example.com has a single TLSA record for a TLS
+ service on TCP port 990:
+
+ _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
+
+ To start the rollover process, obtain or generate the new certificate
+ or SubjectPublicKeyInfo to be used after the rollover and generate
+ the new TLSA record. Add that record alongside the old one:
+
+ _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
+ _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
+
+ After the new records have propagated to the authoritative
+ nameservers and the TTL of the old record has expired, switch to the
+ new certificate on the TLS server. Once this has occurred, the old
+ TLSA record can be removed:
+
+ _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
+
+ This completes the certificate rollover.
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 31]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+Appendix B. Pseudocode for Using TLSA
+
+ This appendix describes, in pseudocode format, the interactions given
+ earlier in this specification. If the steps below disagree with the
+ text earlier in the document, the steps earlier in the document ought
+ to be considered correct and this text incorrect.
+
+ Note that this pseudocode is more strict than the normative text.
+ For instance, it forces an order on the evaluation of criteria, which
+ is not mandatory from the normative text.
+
+B.1. Helper Functions
+
+ // implement the function for exiting
+ function Finish (F) = {
+ if (F == ABORT_TLS) {
+ abort the TLS handshake or prevent TLS from starting
+ exit
+ }
+
+ if (F == NO_TLSA) {
+ fall back to non-TLSA certificate validation
+ exit
+ }
+
+ if (F == ACCEPT) {
+ accept the TLS connection
+ exit
+ }
+
+ // unreachable
+ }
+
+ // implement the selector function
+ function Select (S, X) = {
+ // Full certificate
+ if (S == 0) {
+ return X in DER encoding
+ }
+
+ // SubjectPublicKeyInfo
+ if (S == 1) {
+ return X.SubjectPublicKeyInfo in DER encoding
+ }
+
+ // unreachable
+ }
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 32]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ // implement the matching function
+ function Match (M, X, Y) {
+ // Exact match on selected content
+ if (M == 0) {
+ return (X == Y)
+ }
+
+ // SHA-256 hash of selected content
+ if (M == 1) {
+ return (SHA-256(X) == Y)
+ }
+
+ // SHA-512 hash of selected content
+ if (M == 2) {
+ return (SHA-512(X) == Y)
+ }
+
+ // unreachable
+ }
+
+B.2. Main TLSA Pseudocode
+
+ TLS connect using [transport] to [name] on [port] and receiving end
+ entity cert C for the TLS server:
+
+ (TLSArecords, ValState) = DNSSECValidatedLookup(
+ domainname=_[port]._[transport].[name], RRtype=TLSA)
+
+ // check for states that would change processing
+ if (ValState == BOGUS) {
+ Finish(ABORT_TLS)
+ }
+ if ((ValState == INDETERMINATE) or (ValState == INSECURE)) {
+ Finish(NO_TLSA)
+ }
+ // if here, ValState must be SECURE
+
+ for each R in TLSArecords {
+ // unusable records include unknown certUsage, unknown
+ // selectorType, unknown matchingType, erroneous RDATA, and
+ // prohibited by local policy
+ if (R is unusable) {
+ remove R from TLSArecords
+ }
+ }
+ if (length(TLSArecords) == 0) {
+ Finish(NO_TLSA)
+ }
+
+
+
+Hoffman & Schlyter Standards Track [Page 33]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ // A TLS client might have multiple trust anchors that it might use
+ // when validating the TLS server's end entity (EE) certificate.
+ // Also, there can be multiple PKIX certification paths for the
+ // certificates given by the server in TLS. Thus, there are
+ // possibly many chains that might need to be tested during
+ // PKIX path validation.
+
+ for each R in TLSArecords {
+
+ // pass PKIX certificate validation and chain through a CA cert
+ // that comes from TLSA
+ if (R.certUsage == 0) {
+ for each PKIX certification path H {
+ if (C passes PKIX certification path validation in H) {
+ for each D in H {
+ if ((D is a CA certificate) and
+ Match(R.matchingType, Select(R.selectorType, D),
+ R.cert)) {
+ Finish(ACCEPT)
+ }
+ }
+ }
+ }
+ }
+
+ // pass PKIX certificate validation and match EE cert from TLSA
+ if (R.certUsage == 1) {
+ for each PKIX certification path H {
+ if ((C passes PKIX certificate validation in H) and
+ Match(R.matchingType, Select(R.selectorType, C),
+ R.cert)) {
+ Finish(ACCEPT)
+ }
+ }
+ }
+
+ // pass PKIX certification validation using TLSA record as the
+ // trust anchor
+ if (R.certUsage == 2) {
+ // the following assert() is merely a formalization of the
+ // "trust anchor" condition for a certificate D matching R
+ assert(Match(R.matchingType, Select(R.selectorType, D), R.cert))
+
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 34]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ for each PKIX certification path H that has certificate D
+ matching R as the trust anchor {
+ if (C passes PKIX validation in H) {
+ Finish(ACCEPT);
+ }
+ }
+ }
+
+ // match the TLSA record and the TLS certificate
+ if (R.certUsage == 3) {
+ if Match(R.matchingType, Select(R.selectorType, C), R.cert)
+ Finish(ACCEPT)
+ }
+ }
+
+ }
+
+ // if here, then none of the TLSA records ended in "Finish(ACCEPT)"
+ // so abort TLS
+ Finish(ABORT_TLS)
+
+Appendix C. Examples
+
+ The following are examples of self-signed certificates that have been
+ generated with various selectors and matching types. They were
+ generated with one piece of software, and validated by an individual
+ using other tools.
+
+ S = Selector
+ M = Matching Type
+
+ S M Association Data
+ 0 0 30820454308202BC020900AB58D24E77AD2AF6300D06092A86
+ 4886F70D0101050500306C310B3009060355040613024E4C31163014
+ 0603550408130D4E6F6F72642D486F6C6C616E643112301006035504
+ 071309416D7374657264616D310C300A060355040A13034F53333123
+ 30210603550403131A64616E652E6B6965762E70726163746963756D
+ 2E6F73332E6E6C301E170D3132303131363136353730335A170D3232
+ 303131333136353730335A306C310B3009060355040613024E4C3116
+ 30140603550408130D4E6F6F72642D486F6C6C616E64311230100603
+ 5504071309416D7374657264616D310C300A060355040A13034F5333
+ 312330210603550403131A64616E652E6B6965762E70726163746963
+ 756D2E6F73332E6E6C308201A2300D06092A864886F70D0101010500
+ 0382018F003082018A0282018100E62C84A5AFE59F0A2A6B250DEE68
+ 7AC8C5C604F57D26CEB2119140FFAC38C4B9CBBE8923082E7F81626B
+ 6AD5DEA0C8771C74E3CAA7F613054AEFA3673E48FFE47B3F7AF987DE
+ 281A68230B24B9DA1A98DCBE51195B60E42FD7517C328D983E26A827
+ C877AB914EE4C1BFDEAD48BD25BE5F2C473BA9C1CBBDDDA0C374D0D5
+
+
+
+Hoffman & Schlyter Standards Track [Page 35]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ 8C389CC3D6D8C20662E19CF768F32441B7F7D14AEA8966CE7C32A172
+ 2AB38623D008029A9E4702883F8B977A1A1E5292BF8AD72239D40393
+ 37B86A3AC60FA001290452177BF1798609A05A130F033457A5212629
+ FBDDB8E70E2A9E6556873C4F7CA46AE4A8B178F05FB319005E1C1C7D
+ 4BD77DFA34035563C126AA2C3328B900E7990AC9787F01DA82F74C3D
+ 4B6674CCECE1FD4C6EF9E6644F4635EDEDA39D8B0E2F7C8E06DAE775
+ 6213BD3D60831175BE290442B4AFC5AE6F46B769855A067C1097E617
+ 962529E166F22AEE10DDB981B8CD6FF17D3D70723169038DBFBC1A44
+ 9C8D0D31BC683C5F3CE26148E42EC9BBD4D9F261569B25B53C1D7FC2
+ DDFF6B4CAC050203010001300D06092A864886F70D01010505000382
+ 0181002B2ABE063E9C86AC4A1F7835372091079C8276A9C2C5D1EC57
+ 64DE523FDDABDEAB3FD34E6FE6CBA054580A6785A663595D90132B93
+ D473929E81FA0887D2FFF78A81C7D014B97778AB6AC9E5E690F6F5A9
+ E92BB5FBAB71B857AE69B6E18BDCCB0BA6FCD9D4B084A34F3635148C
+ 495D48FE635903B888EC1DEB2610548EDD48D63F86513A4562469831
+ 48C0D5DB82D73A4C350A42BB661D763430FC6C8E5F9D13EA1B76AA52
+ A4C358E5EA04000F794618303AB6CEEA4E9A8E9C74D73C1B0B7BAF16
+ DEDE7696B5E2F206F777100F5727E1684D4132F5E692F47AF6756EA8
+ B421000BE031B5D8F0220E436B51FB154FE9595333C13A2403F9DE08
+ E5DDC5A22FD6182E339593E26374450220BC14F3E40FF33F084526B0
+ 9C34250702E8A352B332CCCB0F9DE2CF2B338823B92AFC61C0B6B8AB
+ DB5AF718ED8DDA97C298E46B82A01B14814868CFA4F2C36268BFFF4A
+ 591F42658BF75918902D3E426DFE1D5FF0FC6A212071F6DA8BD833FE
+ 2E560D87775E8EE9333C05B6FB8EB56589D910DB5EA903
+
+ 0 1 EFDDF0D915C7BDC5782C0881E1B2A95AD099FBDD06D7B1F779
+ 82D9364338D955
+
+ 0 2 81EE7F6C0ECC6B09B7785A9418F54432DE630DD54DC6EE9E3C
+ 49DE547708D236D4C413C3E97E44F969E635958AA410495844127C04
+ 883503E5B024CF7A8F6A94
+
+ 1 0 308201A2300D06092A864886F70D01010105000382018F0030
+ 82018A0282018100E62C84A5AFE59F0A2A6B250DEE687AC8C5C604F5
+ 7D26CEB2119140FFAC38C4B9CBBE8923082E7F81626B6AD5DEA0C877
+ 1C74E3CAA7F613054AEFA3673E48FFE47B3F7AF987DE281A68230B24
+ B9DA1A98DCBE51195B60E42FD7517C328D983E26A827C877AB914EE4
+ C1BFDEAD48BD25BE5F2C473BA9C1CBBDDDA0C374D0D58C389CC3D6D8
+ C20662E19CF768F32441B7F7D14AEA8966CE7C32A1722AB38623D008
+ 029A9E4702883F8B977A1A1E5292BF8AD72239D4039337B86A3AC60F
+ A001290452177BF1798609A05A130F033457A5212629FBDDB8E70E2A
+ 9E6556873C4F7CA46AE4A8B178F05FB319005E1C1C7D4BD77DFA3403
+ 5563C126AA2C3328B900E7990AC9787F01DA82F74C3D4B6674CCECE1
+ FD4C6EF9E6644F4635EDEDA39D8B0E2F7C8E06DAE7756213BD3D6083
+ 1175BE290442B4AFC5AE6F46B769855A067C1097E617962529E166F2
+ 2AEE10DDB981B8CD6FF17D3D70723169038DBFBC1A449C8D0D31BC68
+ 3C5F3CE26148E42EC9BBD4D9F261569B25B53C1D7FC2DDFF6B4CAC05
+ 0203010001
+
+
+
+Hoffman & Schlyter Standards Track [Page 36]
+\f
+RFC 6698 DNS-Based Authentication for TLS August 2012
+
+
+ 1 1 8755CDAA8FE24EF16CC0F2C918063185E433FAAF1415664911
+ D9E30A924138C4
+
+ 1 2 D43165B4CDF8F8660AECCCC5344D9D9AE45FFD7E6AAB7AB9EE
+ C169B58E11F227ED90C17330CC17B5CCEF0390066008C720CEC6AAE5
+ 33A934B3A2D7E232C94AB4
+
+Authors' Addresses
+
+ Paul Hoffman
+ VPN Consortium
+
+ EMail: paul.hoffman@vpnc.org
+
+
+ Jakob Schlyter
+ Kirei AB
+
+ EMail: jakob@kirei.se
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hoffman & Schlyter Standards Track [Page 37]
+\f
git://git.exim.org / exim.git / commitdiff