DANE V. Dukhovni
Internet-Draft Unaffiliated
Intended status: Best Current Practice W. Hardaker
Expires: April 24, 2014 Parsons
October 21, 2013
DANE TLSA implementation and operational guidance
draft-ietf-dane-ops-01
Abstract
This memo provides operational guidance to server operators to help
ensure that clients will be able to authenticate a server's
certificate chain via published TLSA records. Guidance is also
provided to clients for selecting reliable TLSA record parameters and
how to use them for server authentication. Finally, guidance is
given to protocol designers who wish to make use of TLSA records when
securing protocols using a TLS and TLSA combination.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2014.
Copyright Notice
Copyright (c) 2013 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
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. DANE TLSA record overview . . . . . . . . . . . . . . . . . . 4
2.1. Example TLSA record . . . . . . . . . . . . . . . . . . . 5
3. General DANE Guidelines . . . . . . . . . . . . . . . . . . . 5
3.1. TLS Requirements . . . . . . . . . . . . . . . . . . . . 5
3.2. DANE DNS Record Size Guidelines . . . . . . . . . . . . . 6
3.3. Certificate Name Check Conventions . . . . . . . . . . . 6
3.4. Service Provider and TLSA Publisher Synchronization . . . 7
3.5. TLSA Base Domain and CNAMEs . . . . . . . . . . . . . . . 8
3.6. TLSA Base Name Priorities . . . . . . . . . . . . . . . . 10
3.7. Interaction with Certificate Transparency . . . . . . . . 10
3.8. Design Considerations for Protocols Using DANE . . . . . 11
3.9. TLSA Records and Trust Anchor Digests . . . . . . . . . . 12
3.10. Trust anchor public keys . . . . . . . . . . . . . . . . 14
4. Type Specific DANE Guidelines . . . . . . . . . . . . . . . . 15
4.1. Type 3 Guidelines . . . . . . . . . . . . . . . . . . . . 15
4.2. Type 2 Guidelines . . . . . . . . . . . . . . . . . . . . 15
4.3. Type 1 Guidelines . . . . . . . . . . . . . . . . . . . . 15
4.4. Type 0 Guidelines . . . . . . . . . . . . . . . . . . . . 15
5. Note on DNSSEC security . . . . . . . . . . . . . . . . . . . 16
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
The Domain Name System Security Extensions (DNSSEC) add data origin
authentication and data integrity to the Domain Name System. DNSSEC
is defined in [RFC4033], [RFC4034] and [RFC4035].
In the context of this memo, channel security is assumed to be
provided by TLS or DTLS. The Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS) protocols provide secured
TCP and UDP communication over the Internet Protocol. By convention,
"TLS" will be used throughout this document and, unless otherwise
specified, the text applies equally as well to the DTLS protocol.
Used without authentication, TLS provides protection only against
eavesdropping. With authentication, TLS also provides protection
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against man-in-the-middle (MITM) attacks. Since the publication of
the TLS 1.0 specification in [RFC2246], two updates to the protocol
have been published: TLS 1.1 [RFC4346] and TLS 1.2 [RFC5246]. The
DTLS protocol was later documented in [RFC6347].
As described in the introduction of [RFC6698], TLS authentication via
the existing public Certificate Authority (CA) Public Key
Infrastructure (PKI) suffers from an over-abundance of trusted
certificate authorities capable of issuing certificates for any
domain of their choice. DNS-Based Authentication of Named Entities
(DANE) leverages the DNSSEC infrastructure to publish trusted keys
and certificates of end-entities or certificate-authorities for use
with TLS via a new TLSA record type. DNSSEC validated DANE TLSA
records have created a new PKI designed to augment or replace the
trust model of the existing public CA PKI.
1.1. 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 [RFC2119].
The following terms are used throughout this document:
Service Provider: A company or organization that offers to host a
service on behalf of a Client Domain. The original domain name
associated with the service is typically still within the control
of the client and the service provider is frequently referred to
by a redirection resource record. Example redirection records
include MX, SRV, and CNAME. Many times, the Service Provider
provides services for many customers and must carefully manage any
TLS credentials offered to connecting applications to ensure name
matching is handled easily by the applications.
Client Domain: Clients that make use of a Service Provider to
outsource their services will be referred to as "Client Domains".
TLSA Publisher: The entity responsible for publishing a TLSA record
within a DNS zone. This zone will be considered DNSSEC signed and
validatable to a trust anchor, unless otherwise specified. If the
Client Domain is not outsourcing their DNS service, the TLSA
Publisher will be the client themselves. Otherwise the TLSA
Publisher may be the outsourced DNS service instead.
public key: The term "public key" will be an informal short-hand for
the subjectPublicKeyInfo component of a PKIX certificate.
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SNI: The "Server Name Indication", or SNI, describes the process by
which a TLS client requests to connect to a particular service
name of a TLS server ([RFC3546]). Without this TLS extension, a
TLS server has no choice but to offer a PKIX certificate with a
default list of server names. Service Providers that are expected
to host services for many clients need to present the correct
certificate for the correct client, and the SNI extension provides
a hint to the server which certificate should be transmitted to
the client.
2. DANE TLSA record overview
[RFC6698] specifies a protocol for publishing TLS server certificate
associations via DNSSEC. The DANE TLSA specification defines
multiple TLSA RR types via combinations of 3 numeric parameters.
These integer values of these parameters were later given symbolic
names in [I-D.ietf-dane-registry-acronyms]. These parameters are:
o The TLSA Certificate Usage field. Section 2.1.1 of [RFC6698]
specifies 4 values ranging from 0 to 3: PKIX-CA(0), PKIX-EE(1),
DANE-TA(2), and DANE-EE(3). There is an additional private-use
value: PrivCert(255). All other values are reserved for use by
future specifications.
o The selector field. Section 2.1.2 of [RFC6698] specifies 2 values
ranging from 0 to 1: Cert(0), SPKI(1). There is an additional
private-use value: PrivSel(255). All other values are reserved
for use by future specifications.
o The matching type field. Section 2.1.3 of [RFC6698] specifies 3
values ranging from 0 to 2: Full(0), SHA2-256(1), SHA2-512(2)
There is an additional private-use value: PrivMatch(255). All
other values are reserved for use by future specifications.
We may consider the TLSA Certificate Usage values 0 through 3 to be a
combination of two one-bit flags. The low-bit chooses between
referencing trust-anchor (TA) and end-entity (EE) certificates. The
high bit chooses between public PKI issued and domain-issued
certificates:
o When the low bit is set (PKIX-EE(1) and DANE-EE(3)) the TLSA
record matches an EE (server) certificate.
o When the low bit is not set (PKIX-CA(0) and DANE-TA(2)) the TLSA
record matches a trust-anchor (a certificate authority) that
issued a certificate somewhere in the certificate chain that
authenticates the final end-entity certificate.
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o When the high bit is set (DANE-TA(2) and DANE-EE(3)), the server
certificate chain is domain-issued and may be verified without
reference to any existing public certificate authority PKI. Trust
is entirely placed on the content of the TLSA records obtained via
DNSSEC.
o When the high bit is not set (PKIX-CA(0) and PKIX-EE(1)), the TLSA
record publishes a server policy stating that its certificate
chain must pass PKIX validation [RFC5280] and the DANE TLSA record
is used to constrain the server certificate chain to contain the
referenced CA or EE certificate.
The selector field specifies whether the TLSA RR matches the whole
certificate (Cert(0)) or just its subjectPublicKeyInfo (SPKI(1)).
The subjectPublicKeyInfo is an ASN.1 DER encoding of the
certificate's algorithm id, any parameters and the public key data).
The matching type field specifies how the TLSA RR Certificate
Association Data field is to be compared with the certificate or
public key. A value of Full(0) means an exact match: the full DER
encoding of the certificate or public key is given in the TLSA RR. A
SHA2-256(1) value means a SHA-256 digest of the certificate or public
key, and likewise SHA-512(2) means a SHA-512 digest is used. Of
these, only SHA-256(1) is mandatory to implement. Clients SHOULD
implement SHA-512(2), but servers SHOULD NOT exclusively publish
SHA-512(2) digests. Unless a "second preimage" attack is found
against SHA-256(1), servers should only publish SHA-256(1) digests.
2.1. Example TLSA record
In the example TLSA record below:
_25._tcp.mail.example.com. IN TLSA 3 0 1 (
E8B54E0B4BAA815B06D3462D65FBC7C0
CF556ECCF9F5303EBFBB77D022F834C0 )
The TLSA Certificate Usage is DANE-EE(3), the selector is Cert(0)
(Cert) and the matching type is SHA2-256(1). The rest of the record
is the certificate association data field, which is in this case the
SHA-256 digest of the server certificate.
3. General DANE Guidelines
These guidelines provide guidance for using or designing protocols
for DANE, regardless of what of TLSA record will be used.
3.1. TLS Requirements
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TLS clients that support DANE/TLSA MUST support at least TLS 1.0 and
SHOULD support TLS 1.2. TLS clients and servers using DANE SHOULD
support the "Server Name Indication" extension of TLS.
3.2. DANE DNS Record Size Guidelines
Selecting a combination of TLSA parameters to use requires careful
thought. One important consideration to take into account is the
size of the resulting TLSA record after its parameters are selected.
3.2.1. UDP and TCP Considerations
Deployments SHOULD avoid TLSA record sizes that cause UDP
fragmentation.
Although DNS over TCP would provide the ability to transfer larger
DNS records between clients and servers, it is not universally
deployed and is still blocked by some firewalls. Clients that
request DNS records via UDP typically only use TCP upon receipt of a
truncated response in TCP.
3.2.2. Packet Size Considerations for TLSA Parameters
Server operators SHOULD NOT publish TLSA records using both a TLSA
Selector of Cert(0) and a TLSA Matching Type of Full(0), as even a
single certificate is generally too large to be reliably delivered
via DNS over UDP. Furthermore, two TLSA records containing full
certificates may need to be published in during certificate rollover.
While TLSA records using a TLSA Selector of SPKI(1) and a TLSA
Matching Type of Full(0) publish full public keys without the full
X.509 wrapping, are generally more compact, these too should be used
with caution as they are still larger than necessary. Instead,
servers SHOULD make use of the digest-based TLSA Matching Types
within TLSA records instead. The complete certificate should,
instead, be transmitted to the client in-band during the TLS
handshake.
In summary, the use of a TLSA Matching Type of Full(0) is NOT
RECOMMENDED and the use of SHA-256(1) and SHA-512(2), instead, are
encouraged.
3.3. Certificate Name Check Conventions
Certificates presented by a TLS server will contain either a Common
Name (CN) or subjectAltName (or both). The server's "hostname"
should be published within these fields, ideally within the
subjectAltName as usage of the Common Name field is depreciated.
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This section discusses what must steps must be taken to match an
expected name against the name found within a certificate, if
checking is required.
The TLSA Publisher for TLSA records for a given service MUST ensure
that at least one of these TLSA records will match the server's
certificate chain. If SNI is not employed for a TLS connection, the
TLSA record must match the server's default certificate. If the SNI
extension is sent by the client with a host_name (see [RFC3546]
Section 3.1) equal to the base domain of the TLSA RRset, at least one
TLSA record must match the certificate presented by the server for
that host_name.
When, for example, the TLSA RRset is published at
"_25._tcp.mail.example.com", the TLSA base domain is
"mail.example.com". At least one of the TLSA records in the
_25._tcp.mail.example.com RRset MUST match the server certificate
chain, provided the client TLS hanshake included the SNI extension
with a host_name of "mail.example.com".
Note: Except with TLSA Certificate Usage DANE-EE(3), where name
checks are not applicable (see Section 4.1), DANE aware clients
SHOULD use the base domain of the TLSA RRset to verify that the
client has reached the correct server by checking that the TLSA base
domain is matched by one of the subjectAltName ([RFC5280]) values in
the server certificate. The commonName from the certificate subject
DN SHOULD only be used when no subjectAltNames of type 'dns' are
present. Additional acceptable names may be specified by protocol
specific DANE specifications. For example, with SMTP both the
destination domain name and the MX host name are acceptable names to
be found in the server certificate.
Since the server's ability to respond with the right certificate
chain requires the TLS client to provide the correct SNI information,
clients SHOULD send the SNI extension with a host_name value of the
base domain of the TLSA RRset. Clients failing to transmit SNI
information may be unable to properly authenticate the presented
certificate due to certificate naming mismatches.
3.4. Service Provider and TLSA Publisher Synchronization
Complications arise when the TLSA Publisher is not the same entity as
the Service Provider. In this situation, the TLSA Publisher and the
Service Provider must cooperate to ensure that TLSA records published
by the TLSA Publisher don't fall out of sync with the server
certificate configuration used by the Service Provider.
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Ideally, the TLSA Publisher and the Service Provider should be the
same entity. If a TLSA record must be published in the Client
Domain's base domain, CNAME records can easily point at the real TLSA
record in the Service Provider's zone assuming TLSA Certificate Usage
DANE-EE(3) TLSA records are published by the Service Provider (see
Section 3.5). Having the master TLSA record in the Service
Provider's zone avoids the complexity of bilateral coordination of
server certificate configuration and TLSA record management.
For example, with SMTP, the Client Domain's MX record's exchange name
can point directly at the Service Provider's SMTP hosts. When the
Client Domain's DNS zone is signed, the MX record's exchange name can
be securely used as the base name for TLSA records that are published
and managed by the Service Provider.
If directly pointing at the Service Provider's domain is not
possible, then care must be taken during a Service Provider's
certificate rollover. Before a Service Provider publishes a new
certificate, it should make that certificate available to all of its
Client Domains and the Client Domains should publish a new TLSA
record in advance of the certificate usage date. Only once the new
certificate is in place and in-use globally may the older TLSA record
be removed.
3.5. TLSA Base Domain and CNAMEs
When the protocol does not support service location indirection via
MX, SRV or similar DNS records, the service may be redirected via a
CNAME. A CNAME is a more blunt instrument for this purpose, since
unlike an MX or SRV record, it remaps the origin domain to the target
domain for all protocols, not just a singular one. Also Unlike MX or
SRV records, CNAME records may chain (though DNS resolving
implementations will generally impose an implementation dependent
maximum nesting depth).
When CNAMEs are employed, the best place to seek DANE TLSA records is
in the Service Provider's domain, as discussed in Section 3.4.
Therefore, DANE PKI clients connecting to a server whose domain name
is a CNAME alias SHOULD follow the CNAME hop-by-hop to its ultimate
target host (noting at each step whether the CNAME is DNSSEC
validated) and use the final target host as the base domain for TLSA
lookups.
Implementations failing to find a TLSA record using a base name of
the final target of a CNAME expansion MAY choose to issue a TLSA
query using the original destination name. I.e, the preferred tlsa
base name would derived for the most-expanded name, and failing that
would be the initial query name.
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Protocol-specific TLSA specifications may provide additional guidance
or restrictions when following CNAME expansions.
3.5.1. Redirecting TLSA lookups in the Client Domain
If CNAMEs are not followed, Client Domains will need to publish TLSA
records that match the Service Provider's certificate chain or always
use an entity that was both the Service Provider and the TLSA
publisher. Having the TLSA base domain be different than the Service
Provider's domain imposes a difficult key management burden on the
Client Domain and the Service Provider.
Fortunately, it is possible to publish CNAMEs in the Client Domain
pointing to the Service Provider's TLSA RRset if the TLSA certificate
usage field is set to DANE-EE(3). Otherwise, a client that used the
alias name (from the hosted domain rather than the Service Provider's
domain) as the base domain to obtain the TLSA RRset would look for
the hosted domain in the server certificate when performing name
checks, and would generally fail to authenticate the server except in
the rare cases when the server's certificate does include the Client
Domain. SNI SHOULD be used to help perform the right certificate
selection by the server, although this imposes a management burden on
the TLS server that could be avoided by ensuring the TLSA base domain
is within the Service Provider's control in the first place.
Example CNAME record for a TLSA domain:
; TLSA RRs aliased to Service Provider, but the base domain is
; the hosted domain. Likely to fail name check unless Service
; Provider usage is "3".
;
_25._tcp.mail.example.com. IN CNAME _25._tcp.mail.example.net.
_25._tcp.mail.example.net. IN TLSA 3 1 1 ...
Note: when the TLSA RRset query domain (base domain plus port and
protocol prefixes) resolves to a DNSSEC validated CNAME that points
to a DNSSEC signed zone with the actual TLSA records, as the above
example indicates, it has no effect on the value of the base domain,
which remains the original domain to which the client prefixed the
port and protocol. In the example above, the base domain is
"mail.example.com" and not "mail.example.net".
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Though CNAMEs are illegal on the right hand side of most indirection
records, such as MX and SRV records, they are supported by some
implementations. For example, if the MX or SRV host is a CNAME
alias, some implementations may "chase" the CNAME. They SHOULD use
the target hostname as the base domain for TLSA records as well as
the host_name in SNI, provided the CNAME RR is found to be "secure"
at each step in the CNAME expansion.
3.6. TLSA Base Name Priorities
There are multiple steps within a chaining DNS lookup process that
TLSA base names can be pulled from. This section will discuss what
the preferred selection points are. TBD.
1. Final Domain Name
2. Redirect Name
3. Initial Name
3.7. Interaction with Certificate Transparency
[RFC6962] Certificate Transparency or CT for short, defines an
approach to mitigate the risk of rogue or compromised public CAs
issuing unauthorized certificates. This section clarifies the
interaction of CT and DANE. CT is a protocol and auditing system
that applies only to public CAs, and only when they are free to issue
unauthorized certificates for a domain. If the CA is not a public
CA, or DANE TLSA RRs constrain the end-entity certificate to a fixed
public key, there is no role for CT, and clients need not apply CT
checks.
When a server is authenticated via a DANE TLSA RR with TLSA
Certificate Usage PKIX-EE(1) or DANE-EE(3), the domain owner has
unambiguously specified the certificate associated with the given
service. Even if a rogue CA were able to issue an unauthorized end-
entity certificate that binds a public key to a name in that domain,
barring "second preimage" attacks on the hashing algorithms in use,
any such certificate would not match the TLSA record and would be
rejected. Therefore, when a TLS client authenticates the TLS server
via a TLSA certificate association with usage PKIX-EE(1) or DANE-
EE(3), CT checks need not be performed. Publication of the server
certificate or public key (digest) in a TLSA record in a DNSSEC
signed zone by the domain owner assures the client that the
certificate is not an unauthorized certificate issued by a rogue CA
without the domain owner's consent.
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When a server is authenticated via a DANE TLSA RR with TLSA usage
DANE-CA(2) and the server certificate does not chain to a known
public root CA, CT cannot apply (CT logs only accept chains that
start with a known, public root). Since TLSA Certificate Usage DANE-
CA(2) is generally intended to support non-PKIX trust anchors,
clients need not perform CT checks with usage DANE-CA(2) using
unknown root CAs.
A server operator that wants to perform CT checks should use TLSA RRs
with usage PKIX-CA(0) and use a known, trusted public PKIX root
issuer.
3.8. Design Considerations for Protocols Using DANE
When a TLS client goes to the trouble of authenticating a certificate
presented by a TLS server, it should not continue to use the server
in case of authentication failure or else authentication serves no
purpose. Servers publishing TLSA records MUST be configured to allow
correctly configured clients to successfully authenticate the
server's TLS certificate.
If all the TLSA records for a service are found unusable (possibly
due to unsupported parameter combinations), it is application
protocol specific as to whether the connection should be established
anyway without relying on TLS security, with only TLS encryption but
not authentication, or whether to refuse to connect entirely.
Protocols must choose whether to prioritize security or robustness.
Refusing to connect in the case of unusable parameters is clearly the
better option if transport security is critical, but some protocols
may value operational robustness when transport security is merely a
"nice to have" rather than a requirement.
3.8.1. Design Considerations for non-PKIX Protocols
For some application protocols, the existing public CA PKI may not be
viable (such as with SMTP over TLS). For these (non-PKIX) protocols,
protocol documents SHOULD NOT suggest publishing TLSA records with
TLSA Certificate Usage PKIX-CA(0) or PKIX-EE(1), as clients cannot be
expected to perform [RFC5280] PKIX validation or [RFC6125] identity
verification.
Protocols designed for non-PKIX use SHOULD choose to treat any TLSA
records with TLSA Certificate Usage PKIX-CA(0) or PKIX-EE(1) as
unusable. After verifying that the only available TLSA Certificate
Usage types are PKIX-CA(0) or PKIX-EE(1), protocol specifications MAY
instruct clients to either refuse to initiate a connection or to
connect via unauthenticated TLS if no alternative authentication
mechanisms are available.
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If non-PKIX protocols do allow for publication of TLSA records with
TLSA Certificate Usage PKIX-CA(0) or PKIX-EE(1), clients SHOULD make
use of the TLSA verification to the fullest extent possible.
3.8.1.1. TLSA Certificate Usage PKIX-EE(1)
With non-PKIX protocols, clients using TLSA Certificate Usage PKIX-
EE(1) records MAY ignore the PKIX validation requirement, and
authenticate the server per the content of the TLSA record alone.
Since servers will hopefully rely on SNI to select the correct
certificate for presentation, the client SHOULD use the SNI extension
to signal the base domain of the TLSA RRset.
3.8.1.2. TLSA Certificate Usage PKIX-CA(0)
With TLSA Certificate Usage PKIX-CA(0) in non-PKIX protocols, the
usability of the TLSA records depends on its matching type.
If the matching type is Full(0), the client has all the information
it needs to match the server trust-chain to the TLSA record. The
client MAY ignore the PKIX validation requirement and authenticate
the server via its DANE TLSA records alone (sending SNI with the base
domain as usual). The client SHOULD use the base domain of the TLSA
record(s) in certificate name checks.
If the matching type is not Full(0), the TLSA record contains only a
digest of the trust certificate authority certificate or public key.
The full certificate may not be included in the server's certificate
chain and the client may not be able to match the server trust chain
against the TLSA record when a non-PKIX protocol is being used, as
the client won't have the needed CA trust list. See Section 3.9.1
for a more complete discussion of this case. The client cannot
reliably authenticate the server in this case and SHOULD treat the
TLSA record as unusable.
If the client is configured with a set of trusted CAs that are
believed to be sufficiently complete to authenticate all the servers
it expects to communicate with, then it MAY elect to honor
certificate usage PKIX-CA(0) TLSA records that publish digests of the
trusted CA certificate or public key.
3.9. TLSA Records and Trust Anchor Digests
With TLSA records that match the EE certificate, the TLS client has
no difficulty matching the TLS record against the server certificate,
as this certificate is always present in the TLS server certificate
chain. The TLS client can, if necessary, extract the public key from
the server certificate, and can compute the appropriate digest.
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With DANE TLSA records that match the digest of a TA certificate or
public key, a complication arises when the TA certificate is omitted
from the server's certificate chain. This can happen when the trust-
anchor is a root certificate authority, as stated in section 7.4.2 of
[RFC5246]:
The sender's certificate MUST come first in the list. Each
following certificate MUST directly certify the one preceding
it. Because certificate validation requires that root keys be
distributed independently, the self-signed certificate that
specifies the root certificate authority MAY be omitted from the
chain, under the assumption that the remote end must already
possess it in order to validate it in any case.
This means that TLSA records that match a TA certificate or public
key digest are not entirely sufficient to validate the peer
certificate chain. If no matching certificate is found in the
server's certificate chain, the chain may be signed by an omitted
root CA whose digest matches the TLSA record. We will consider each
trust-anchor TLSA Certificate Usage in turn.
3.9.1. Trust Anchor Digests With TLSA Certificate Usage 0
In this case, from the server's perspective, the omission of the root
CA seems reasonable, since in addition to authentication via DANE
TLSA records, the client is expected to perform [RFC5280] PKIX
validation of the server's trust chain and thus to already have a
copy of the omitted root certificate.
From the client's perspective the situation is more nuanced. Despite
the server's indicated preference for PKIX validation, the client may
not possess (or may not fully trust) a complete set of public root
CAs. This is especially likely in protocols where the existing
public CA PKI is not applicable, as described in Section 3.8.1. If
it is likely that a client lacks a sufficiently complete list of
trusted CAs, and that a non-negligible number of DNS servers publish
TLSA Certificate Usage PKIX-CA(0) TLSA records with digests of
omitted root CAs, then such a client SHOULD treat such TLSA records
as "unusable". Simply ignoring PKIX validation is not an option,
since the client will also be unable to match the TLSA record without
position of the root certificate. The client MAY choose fall back to
unauthenticated TLS, if PKIX is also not an option (see
[I-D.ietf-dane-srv]) or refuse to initiate a connection.
3.9.2. Trust Anchor Digests With TLSA Certificate Usage 2
With TLSA Certificate Usage DANE-CA(2), there is no expectation that
the client is pre-configured with the trust anchor certificate. With
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TLSA Certificate Usage DANE-CA(2) clients are expecting to rely on
the TLSA records alone. But, with a matching type other than PKIX-
CA(0) the TLSA records contain neither the full trust anchor
certificate nor the full public key. If the TLS server's certificate
chain does not contain the trust-anchor certificate, clients will be
unable to authenticate the server.
TLSA Publishers that publish TLSA Certificate Usage DANE-CA(2) with a
non-zero matching type MUST ensure that the corresponding server is
configured to include the associated trust anchor certificate in its
TLS handshake certificate chain, even if that certificate is a self-
signed root CA and would have been optional in the context of the
existing public CA PKI.
Since servers are expected to always provide usage DANE-CA(2) trust
anchor certificates (either via DNS or else via the TLS hanshake),
clients SHOULD fully support this TLSA Certificate Usage. Clients
MAY choose to treat it as unusable if experience proves that servers
don't consistently live up to their obligations.
3.10. Trust anchor public keys
TLSA records with TLSA Certificate Usage PKIX-CA(0) or DANE-CA(2),
selector SPKI(1) and a matching type of Full(0) publish the full
public key of a trust anchor via DNS. In section 6.1.1 of [RFC5280]
the definition of a trust anchor consists of the following four
parts:
1. the trusted issuer name,
2. the trusted public key algorithm,
3. the trusted public key, and
4. optionally, the trusted public key parameters associated with the
public key.
Items 2-4 are precisely the contents of the subjectPublicKeyInfo
published in the TLSA record, but the issuer name is not included in
the public key.
With TLSA Certificate Usage PKIX-CA(0), when the client is able to
perform PKIX validation, the client can construct a complete PKIX
trust chain as it will have access to the trust anchor name. So in
that case, the client can verify that the server certificate chain is
issued by a trust anchor that matches the TLSA record.
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With TLSA Certificate Usage DANE-CA(2), the client may not have the
missing trust anchor certificate, and cannot generally verify whether
a particular certificate chain is "issued by" the trust anchor
described in the TLSA record. If the server certificate chain
includes a CA certificate whose public key matches the TLSA record,
the client can match that CA as the intended issuer. Otherwise, the
client can only check that the topmost certificate in the server's
chain is "signed by" by the trust anchor public key in the TLSA
record.
Since trust chain validation via bare public keys rather than trusted
CA certificates may be difficult to implement using existing TLS
libraries, servers SHOULD include the trust anchor certificate in
their certificate chain when the TLSA Certificate Usage is DANE-
CA(2).
If none of the server's certificate chain elements match a public key
specified in full (selector = Cert(0), match type = Full(0)) in a
TLSA record, clients SHOULD attempt to check whether the topmost
certificate in the chain is signed by the provided public key, and if
so consider the server trust chain valid, with authentication
complete if name checks are also successful.
4. Type Specific DANE Guidelines
4.1. Type 3 Guidelines
4.2. Type 2 Guidelines
4.3. Type 1 Guidelines
4.4. Type 0 Guidelines
TLSA Certificate Usage PKIX-CA(0) allows a domain to publish
constraints on the set of certificate authorities trusted to issue
certificates for its TLS servers. It is expected that clients will
only accept trust chains which contain a match for one of the
published TLSA records. This is simple for TLSA Certificate Usage
PKIX-EE(1) where the PKIX trust chain always contains the leaf server
certificate. The situation for TLSA Certificate Usage PKIX-CA(0) is
more subtle.
TLSA Publishers may publish TLSA records for a particular public root
CA, expecting that clients will then only accept chains anchored at
that root. It is possible, however, that the client's set of trusted
certificates includes some intermediate CAs, either with or without
the corresponding root CA. When a client constructs a trust chain
leading from a trusted intermediate CA to the server leaf
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certificate, such a chain may omit any trusted roots published in the
server's TLSA records.
If the omitted root is also trusted, the client may erroneously
reject the server chain if it fails to determine that the shorter
chain it constructed extends to a longer trusted chain that matches
the TLSA records. This means that a client SHOULD not always stop
extending the chain when the first locally trusted certificate is
found. If no TLSA records have matched any of the elements of the
chain, it MUST attempt to build a longer chain if the trusted
certificate found is not self-issued, in the hope that a certificate
closer to the root may in fact match the server's TLSA records.
5. Note on DNSSEC security
Clearly the security of the DANE TLSA PKI rests on the security of
the underlying DNSSEC infrastructure. While this memo is not a guide
to DNSSEC security, a few comments may be helpful to TLSA
implementors.
With the existing public CA PKI, name constraints are rarely used and
public root CAs can issue certificates for any domain of its choice.
With DNSSEC, the situation is different. Only the registrar of
record can update a domain's DS record in the registry parent zone
(in some cases, however, the registry is the sole registrar). With
gTLDs, for which multiple registrars compete to provide domains in a
single registry, it is important to make sure that rogue registrars
cannot easily initiate an unauthorized domain transfer, and thus take
over DNSSEC for the domain. DNS Operators SHOULD use a registrar
lock of their domains to offer some protection of this possibility.
When the registrar is also the DNS operator for the domain, one needs
to consider whether the registrar will allow orderly migration of the
domain to another registrar or DNS operator in a way that will
maintain DNSSEC integrity. TLSA Publishers SHOULD ensure their
registrar publishes a suitable domain transfer policy.
DNSSEC signed RRsets cannot be securely revoked before they expire.
Operators should plan accordingly and not generate signatures with
excessively long duration. For domains publishing high-value keys, a
signature lifetime of a few days is reasonable, and the zone should
be resigned every day. For more domains with less critical data, a
reasonable signature lifetime is a couple of weeks to a month, and
the zone should be resigned every week. Monitoring of the signature
lifetime is important. If the zone is not resigned in a timely
manner, one risks a major outage with the entire domain becoming
invalid.
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6. Acknowledgements
The authors would like to thank Phil Pennock for his comments and
advice on this document.
Acknowledgments from Viktor: Thanks to Tony Finch who finally prodded
me into participating in DANE working group discussions. Thanks to
Paul Hoffman who motivated me to produce this memo and provided
feedback on early drafts.
7. Security Considerations
Application protocols that cannot make use of the existing public CA
PKI (so called non-PKIX protocols), may choose to not implement
certain PKIX-dependent TLSA record types defined in [RFC6698], or may
choose to make a best-effort use of such records. In neither case is
security compromised, since by assumption PKIX verification is simply
not an option for these protocols. When the TLS server is
authenticated based on the TLSA records alone, the client is as well
authenticated as possible, treating the TLSA records as unusable
would lead to weaker security.
Therefore, when TLSA records are used with protocols where PKIX does
not apply, the recommended trade-off is for servers to not publish
PKIX-dependent TLSA records, and for clients to use them as best they
can, but otherwise treat them unusable. Of course when PKIX
validation is an option clients SHOULD perform PKIX validation per
[RFC6698].
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
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[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.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[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.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, June 2013.
8.2. Informative References
[I-D.ietf-dane-registry-acronyms]
Gudmundsson, O., "Adding acronyms to simplify DANE
conversations", draft-ietf-dane-registry-acronyms-01 (work
in progress), October 2013.
[I-D.ietf-dane-srv]
Finch, T., "Using DNS-Based Authentication of Named
Entities (DANE) TLSA records with SRV and MX records.",
draft-ietf-dane-srv-02 (work in progress), February 2013.
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Authors' Addresses
Viktor Dukhovni
Unaffiliated
Email: ietf-dane@dukhovni.org
Wes Hardaker
Parsons
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
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