Зашифрованная связь в Интернете часто использует транспортный уровень
Безопасность (TLS), которая зависит от третьих сторон для сертификации ключей
используемый. Этот документ улучшает эту ситуацию,
администраторам доменных имен, чтобы указать ключи, используемые в этом
доменных TLS-серверов. Для этого требуются соответствующие улучшения в TLS
клиентское программное обеспечение, но без изменения программного обеспечения сервера TLS.

Статус этой заметки

Это документ стандарта стандартов Интернета.

Этот документ является продуктом Целевой группы Internet Engineering
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Руководящая группа интернет-инженеров (IESG). Дополнительная информация о
Стандарты Интернета доступны в Разделе 2 RFC 5741.

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и как обеспечить обратную связь по нему, можно получить на
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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.

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]

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]

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]

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]

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.

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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]

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]

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]

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]

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.

Hoffman & Schlyter Standards Track [Page 10]

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.

Hoffman & Schlyter Standards Track [Page 11]

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]

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.

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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.

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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.

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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.

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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.

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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].

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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.

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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.

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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.

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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]

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.

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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.

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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),

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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.

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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.

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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.

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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]

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]

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]

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]

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]

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]

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]

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]

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]

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