DANE V. Dukhovni
Internet-Draft Unaffiliated
Intended status: Best Current Practice W. Hardaker
Expires: August 18, 2014 Parsons
February 14, 2014
DANE TLSA implementation and operational guidance
draft-ietf-dane-ops-03
Abstract
This memo provides 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 using them for server
authentication. Finally, guidance is given to protocol designers who
wish to make use of TLSA records when securing protocols using a
combination of the Transport Layer Security (TLS) protocol and TLSA
records.
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 August 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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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.
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 . . . . . . . . . . . . . . . . . . . 6
3.1. TLS Requirements . . . . . . . . . . . . . . . . . . . . 6
3.2. DANE DNS Record Size Guidelines . . . . . . . . . . . . . 6
3.3. Certificate Name Check Conventions . . . . . . . . . . . 7
3.4. Service Provider and TLSA Publisher Synchronization . . . 8
3.5. TLSA Base Domain and CNAMEs . . . . . . . . . . . . . . . 10
3.6. Interaction with Certificate Transparency . . . . . . . . 11
3.7. Design Considerations for Protocols Using DANE . . . . . 11
3.8. TLSA Records and Trust Anchor Digests . . . . . . . . . . 12
3.9. Trust anchor public keys . . . . . . . . . . . . . . . . 13
4. Certificate Usage Specific DANE Guidelines . . . . . . . . . 14
4.1. Certificate Usage DANE-EE(3) Guidelines . . . . . . . . . 14
4.2. Certificate Usage DANE-TA(2) Guidelines . . . . . . . . . 15
4.3. Certificate Usage PKIX-EE(1) Guidelines . . . . . . . . . 15
4.4. Certificate Usage PKIX-TA(0) Guidelines . . . . . . . . . 15
5. Note on DNSSEC security . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Section 2 of [RFC6698] specifies a new "TLSA" DNS resource record
which associates a TLS transport endpoint with a corresponding
trusted leaf or issuing authority certificates or public keys.
DNSSEC-validated DANE TLSA records can be used to augment or replace
the trust model of the existing public Certificate Authority (CA)
Public Key Infrastructure (PKI).
[RFC6698] defines 24 combinations of TLSA record parameters.
Additional complexity arises when the TLS transport endpoint is
obtained indirectly via a Service Record (SRV), Mail Exchange (MX)
record, CNAME records or other mechanisms that map an abstract
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service domain to a concrete server domain. With service indirection
there are multiple potential places for clients to find the relevant
TLSA records. Service indirection is often used to implement
"virtual hosting", where a single Service Provider transport endpoint
simultaneously supports multiple hosted domain names. With services
that employ TLS, such hosting arrangements may require the Service
Provider to employ multiple pairs of private keys and certificates
with TLS clients signalling the desired domain via an Server Name
Indication (SNI) extension ([RFC6066], section 3). This memo
provides operational guidelines intended to maximize interoperability
between DANE TLS clients and servers.
In the context of this memo, channel security is assumed to be
provided by TLS or DTLS. The Transport Layer Security (TLS)
[RFC5246] and Datagram Transport Layer Security (DTLS) [RFC6347]
protocols provide secured TCP and UDP communication over the IP. By
convention, "TLS" will be used throughout this document and, unless
otherwise specified, the text applies equally well to the DTLS
protocol. Used without authentication, TLS provides protection only
against eavesdropping through its use of encryption. With
authentication, TLS also provides integrity protection and
authentication, which protects the transport against man-in-the-
middle (MITM) attacks.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The following terms are used throughout this document:
Service Provider: A company or organization that offers to host a
service on behalf of a Customer Domain. The original domain name
associated with the service often remains under the control of the
customer. Connecting applications may directed to the Service
Provider via a redirection resource record. Example redirection
records include MX, SRV, and CNAME. The Service Provider
frequently 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.
Customer Domain: Customers that make use of a Service Provider to
outsource their service(s) will be referred to as "Customer
Domains".
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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
Customer Domain is not outsourcing their DNS service, the TLSA
Publisher will be the customer themselves. Otherwise the TLSA
Publisher may be the operator of the outsourced DNS service.
public key: The term "public key" will be an informal short-hand for
the subjectPublicKeyInfo component of a PKIX [RFC5280]
certificate.
SNI: "Server Name Indication", or SNI, describes the TLS protocol
extension by which a TLS client requests to connect to a
particular service name of a TLS server ([RFC6066], section 3).
Without this TLS extension, a TLS server has no choice but to
offer a PKIX certificate with a default list of server names,
making it difficult to host multiple Customer Domains at the same
TLS service endpoint (i.e., "secure virtual hosting").
2. DANE TLSA record overview
DANE TLSA [RFC6698] specifies a protocol for publishing TLS server
certificate associations via DNSSEC [RFC4033] [RFC4034] [RFC4035].
The DANE TLSA specification defines multiple TLSA RR types via
combinations of 3 numeric parameters. The numeric values of these
parameters were later given symbolic names in
[I-D.ietf-dane-registry-acronyms]. These parameters are:
The TLSA Certificate Usage field: Section 2.1.1 of [RFC6698]
specifies 4 values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and DANE-
EE(3). There is an additional private-use value: PrivCert(255).
All other values are reserved for use by future specifications.
The selector field: Section 2.1.2 of [RFC6698] specifies 2 values:
Cert(0), SPKI(1). There is an additional private-use value:
PrivSel(255). All other values are reserved for use by future
specifications.
The matching type field: Section 2.1.3 of [RFC6698] specifies 3
values: Full(0), SHA2-256(1), SHA2-512(2). There is an additional
private-use value: PrivMatch(255). All other values are reserved
for use by future specifications.
We may think of TLSA Certificate Usage values 0 through 3 as a
combination of two one-bit flags. The low-bit chooses between trust
anchor (TA) and end entity (EE) certificates. The high bit chooses
between PKIX, or public PKI issued, and TA, or domain-issued trust
anchors:
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o When the low bit is set (PKIX-EE(1) and DANE-EE(3)) the TLSA
record matches an EE certificate (also commonly referred to as a
leaf or server certificate.)
o When the low bit is not set (PKIX-TA(0) and DANE-TA(2)) the TLSA
record matches a trust anchor (a Certificate Authority) that
issued one of the certificates in the server certificate chain.
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 pre-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-TA(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
value of SHA2-256(1) means that the association data matches the
SHA2-256 digest of the certificate or public key, and likewise
SHA2-512(2) means a SHA2-512 digest is used. Of the two digest
algorithms, for now only SHA2-256(1) is mandatory to implement.
Clients SHOULD implement SHA2-512(2), but servers SHOULD NOT
exclusively publish SHA2-512(2) digests. A digest algorithm agility
protocol is proposed in section 2.3.3 of
[I-D.ietf-dane-smtp-with-dane] that SHOULD be used by clients to
decide how to process TLSA RRsets that employ multiple digest
algorithms. Server operators MUST publish TLSA RRsets that are
compatible with digest algorithm agility.
2.1. Example TLSA record
In the example TLSA record below:
_25._tcp.mail.example.com. 300 IN TLSA PKIX-TA Cert SHA2-256 (
E8B54E0B4BAA815B06D3462D65FBC7C0
CF556ECCF9F5303EBFBB77D022F834C0 )
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The TLSA Certificate Usage is DANE-TA(2), the selector is Cert(0) 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
SHA2-256 digest of the server certificate.
3. General DANE Guidelines
These guidelines provide guidance for using or designing protocols
for DANE, regardless of what sort of TLSA record will be used.
3.1. TLS Requirements
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 simultaneously during a
certificate rollover.
While TLSA records using a TLSA Selector of SPKI(1) and a TLSA
Matching Type of Full(0) (which publishes the bare public key without
the overhead of a containing X.509 certificate) are generally more
compact, these too should be used with caution as they are still
larger than necessary. Rather, servers SHOULD publish digest-based
TLSA Matching Types in their TLSA records. The complete
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corresponding 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 SHA2-256(1) and SHA2-512(2) is strongly
preferred.
3.3. Certificate Name Check Conventions
Certificates presented by a TLS server will generally contain a
subjectAltName (SAN) extension or a Common Name (CN) element in the
subject distinguished name (DN). The server's DNS domain name should
be published within these elements, ideally within the subjectAltName
extension as use of the CN field for this purpose is deprecated.
Name checks SHOULD NOT consider the subject CN when SAN values of
type 'dns' are present.
When a server hosts multiple domains at the same transport endpoint,
the server's ability to respond with the right certificate chain is
predicated on correct SNI information from the client. DANE clients
MUST send the SNI extension with a HostName value of the base domain
of the TLSA RRset.
Except with TLSA Certificate Usage DANE-EE(3), where name checks are
not applicable (see Section 4.1), DANE clients MUST verify that the
client has reached the correct server by checking that the server
name is listed in the server certificate. The server name used for
this comparison SHOULD be the base domain of the TLSA RRset.
Additional acceptable names may be specified by protocol-specific
DANE standards. For example, with SMTP both the destination domain
name and the MX host name are acceptable names to be found in the
server certificate (see [I-D.ietf-dane-smtp-with-dane]).
It is the responsibility of the service operator, in coordination
with the TLSA Publisher, to ensure that at least one of the TLSA
records published for the service will match the server's certificate
chain (either the default chain or the certificate that was selected
based on the SNI information from the client). With certificate
usage values other than DANE-EE(3), the EE certificate SHOULD include
the TLSA base domain as one of its names. If other acceptable names
are specified by a protocol-specific DANE standard, one of those MAY
be used in place of the TLSA base domain.
Given the DNSSEC validated DNS records below:
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example.com. 300 IN MX 0 mail.example.com.
_25._tcp.mail.example.com. 300 IN TLSA DANE-TA Cert SHA2-256 (
E8B54E0B4BAA815B06D3462D65FBC7C0
CF556ECCF9F5303EBFBB77D022F834C0 )
The TLSA base domain is "mail.example.com" and this MUST be the
HostName in the client's SNI extension. The server certificate chain
MUST be signed by a trust anchor with the above certificate SHA2-256
digest. One of the DNS names in the server certificate MUST be
either "mail.example.com" or "example.com".
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 used by the Service Provider.
Whenever possible, the TLSA Publisher and the Service Provider should
be the same entity. Otherwise, changes in the service certificate
chain must be carefully coordinated between the parties involved.
Such coordination is difficult and service outages will result when
coordination fails.
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. Even when the TLSA RRset
must be published in the Customer Domain's DNS zone, it is possible
to employ CNAME records (see Section 3.5) to delegate the content of
the TLSA RRset to a domain operated by the Service Provider.
Certificate name checks generally constrain the applicability of TLSA
CNAMEs across organizational boundaries to Certificate Usages DANE-
EE(3) and DANE-TA(2):
Certificate Usage DANE-EE(3): In this case the Service Provider can
publish a single TLSA RRset that matches the server certificate or
public key digest. The same RRset works for all Customer Domains
because name checks do not apply with DANE-EE(3) TLSA records (see
Section 4.1). A Customer Domain can create a CNAME record
pointing to the TLSA RRset published by the Service Provider.
Certificate Usage DANE-TA(2): When the Service Provider operates a
private certificate authority, the Service Provider is free to
issue a certificate bearing any customer's domain name. Without
DANE, such a certificate would not pass trust verification, but
with DANE, the customer's TLSA RRset that is aliased to the
provider's TLSA RRset can delegate authority to the provider's CA
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for the corresponding service. The Service Provider can generate
appropriate certificates for each customer and use SNI to select
the right certificate chain to present to each client.
Below are example DNS records that illustrate both of of the above
cases in the case of an HTTPS service whose clients all support DANE
TLS.
; Hosted web service redirected via a CNAME alias.
; Associated TLSA RRset redirected via a CNAME alias.
;
; Single certificate at provider works for all Customer Domains
;
www1.example.com. 300 IN CNAME w3.example.net.
_443._tcp.www3.example.com. 300 IN CNAME _443._tcp.w3.example.net.
_443._tcp.w3.example.net. 300 IN TLSA DANE-EE SPKI SHA2-256 (
8A9A70596E869BED72C69D97A8895DFA
D86F300A343FECEFF19E89C27C896BC9 )
;
; CA at provider can issue certificates for each Customer Domain.
;
www2.example.com. 300 IN CNAME w2.example.net.
_443._tcp.www2.example.com. 300 IN CNAME _443._tcp.w2.example.net.
_443._tcp.w2.example.net. 300 IN TLSA DANE-TA Cert SHA2-256 (
C164B2C3F36D068D42A6138E446152F5
68615F28C69BD96A73E354CAC88ED00C )
With protocols that support explicit transport redirection via DNS MX
records, SRV records, or other similar records, the TLSA base domain
is based on the redirected transport end-point, rather than the
origin domain. With SMTP for example, when email service is hosted
by a Service Provider, the Customer Domain's MX hostnames will point
at the Service Provider's SMTP hosts. When the Customer Domain's DNS
zone is signed, the MX hostnames can be securely used as the base
domains for TLSA records that are published and managed by the
Service Provider. For example:
; Hosted SMTP service
;
example.com. 300 IN MX 0 mx1.example.net.
example.com. 300 IN MX 0 mx2.example.net.
_25._tcp.mx1.example.net. 300 IN TLSA DANE-EE SPKI SHA2-256 (
8A9A70596E869BED72C69D97A8895DFA
D86F300A343FECEFF19E89C27C896BC9 )
_25._tcp.mx2.example.net. 300 IN TLSA DANE-EE SPKI SHA2-256 (
C164B2C3F36D068D42A6138E446152F5
68615F28C69BD96A73E354CAC88ED00C )
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If redirection to the Service Provider's domain (via MX or SRV
records or any similar mechanism) is not possible, and aliasing of
the TLSA record is not an option, then more complex coordination
between the Customer Domain and Service Provider is required. Either
the Customer Domain periodically provides private keys and a
corresponding certificate chain to the Provider after making
appropriate changes in its TLSA records, or the Service Provider
periodically generates the keys and certificates and must wait for
matching TLSA records to be published by its Customer Domains before
deploying newly generated keys and certificate chains.
For further information about combining DANE and SRV, please see
[I-D.ietf-dane-srv].
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.
The complexity of coordinating key rollover is largely eliminated
when DANE TLSA records are found in the Service Provider's domain, as
discussed in Section 3.4. Therefore, DANE TLS 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). If at each stage of CNAME expansion
the DNSSEC validation status is "secure", the final target name
SHOULD be the preferred base domain for TLSA lookups.
Implementations failing to find a TLSA record using a base name of
the final target of a CNAME expansion SHOULD issue a TLSA query using
the original destination name. That is, the preferred TLSA base
domain should be derived from the fully expanded name, and failing
that should be the initial domain name.
Protocol-specific TLSA specifications may provide additional guidance
or restrictions when following CNAME expansions.
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 preferred TLSA base domain as well as the
HostName in SNI, provided the CNAME RR is found to be "secure" at
each step in the CNAME expansion.
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3.6. Interaction with Certificate Transparency
Certificate Transparency (CT) [RFC6962] defines an experimental
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 an experimental 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 a DANE-EE(3) TLSA RR directly specifies the
end entity certificate, 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 DANE-EE(3), the domain owner has directly specified
the certificate associated with the given service without reference
to any PKIX certificate authority. Therefore, when a TLS client
authenticates the TLS server via a TLSA certificate association with
usage DANE-EE(3), CT checks SHOULD 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 TLS client that
the certificate is not an unauthorized certificate issued by a rogue
CA without the domain owner's consent.
When a server is authenticated via a DANE TLSA RR with TLSA usage
DANE-TA(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-
TA(2) is generally intended to support non-PKIX trust anchors, TLS
clients SHOULD NOT perform CT checks with usage DANE-TA(2) using
unknown root CAs.
A server operator who wants clients to perform CT checks should
publish TLSA RRs with usage PKIX-TA(0) or PKIX-EE(1).
3.7. Design Considerations for Protocols Using DANE
When a TLS client goes to the trouble of authenticating a certificate
chain presented by a TLS server, it should not continue to use that
server in the event of authentication failure, or else authentication
serves no purpose. Servers publishing TLSA records MUST be
configured to allow correctly configured clients to successfully
authenticate their TLS certificate chains.
A service with DNSSEC-validated TLSA records implicitly promises TLS
support. When all the TLSA records for a service are found
"unusable", due to unsupported parameter combinations or malformed
associated data, DANE clients cannot authenticate the service
certificate chain. When authenticated TLS is dictated by the
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application, the client SHOULD NOT connect to the associated server.
If, on the other hand, the use of TLS is "opportunistic", then the
client SHOULD generally use the server via an unauthenticated TLS
connection, but if TLS encryption cannot be established, the client
MUST NOT use the server. Standards for DANE specific to the
particular application protocol may modify the above as appropriate
to specify whether the connection should be established anyway
without relying on TLS security, with only encryption but not
authentication, or whether to refuse to connect entirely. Protocols
must choose whether to prioritize security or robustness.
3.7.1. Design Considerations for non-PKIX Protocols
For some application protocols (such as SMTP to MX with opportunistic
TLS), the existing public CA PKI is not a viable alternative to DANE.
For these (non-PKIX) protocols, new DANE standards SHOULD NOT suggest
publishing TLSA records with TLSA Certificate Usage PKIX-TA(0) or
PKIX-EE(1), as TLS 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-TA(0) or PKIX-EE(1) as
unusable. After verifying that the only available TLSA Certificate
Usage types are PKIX-TA(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.
3.8. TLSA Records and Trust Anchor Digests
With TLSA records that match the EE certificate (i.e., DANE-EE(3) or
PKIX-EE(1)), the TLS client has no difficulty matching TLSA records
against the server certificate, as this certificate is always present
in the TLS server certificate chain.
With DANE TLSA records that match the digest of a TA certificate or
public key (i.e., DANE-TA(2) or PKIX-TA(0)), 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.
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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. With Certificate Usage
PKIX-TA(0), this is not a problem, since the client is expected to be
pre-configured with the issuing TA certificate.
With TLSA Certificate Usage DANE-TA(2), however, there is no
expectation that the client is pre-configured with the trust anchor
certificate. Rather, with TLSA Certificate Usage DANE-TA(2) clients
must be able to rely on the TLSA records alone. But, with a digest
in the TLSA record, the TLSA record contains 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, DANE
clients will be unable to authenticate the server.
TLSA Publishers that publish TLSA Certificate Usage DANE-TA(2) with a
digest (not Full(0)) matching type MUST ensure that the corresponding
server is configured to also include the 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.
3.9. Trust anchor public keys
TLSA records with TLSA Certificate Usage DANE-TA(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 DANE-TA(2), the client may not have the
associated 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
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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" 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 chains when the TLSA Certificate Usage is DANE-
TA(2).
If none of the server's certificate chain elements match a public key
specified in full in a TLSA record, clients SHOULD check whether the
topmost certificate in the chain is signed by the provided public key
and has not expired, and if that is the case, and the rest of the
chain passes validation, consider the server authenticated if name
checks are also successful.
4. Certificate Usage Specific DANE Guidelines
4.1. Certificate Usage DANE-EE(3) Guidelines
Authentication via certificate usage "3" TLSA records involves simply
checking that the server's leaf certificate matches the TLSA record.
Other than extracting the relevant certificate elements for
comparison, no other use is made of the certificate content.
Authentication via certificate usage "3" TLSA records involves no
certificate authority signature checks. It also involves no server
name checks, and thus does not impose any new requirements on the
names contained in the server certificate (servers don't require an
SNI extension to be present when the TLSA record matches the server's
default certificate).
Two TLSA records will need to be published before updating a server's
public key, one matching the currently deployed key and the other
matching the new key scheduled to replace it. Once sufficient time
has elapsed for all the previous TLSA RRsets, which contains only the
old key, to expire from DNS caches, the server may be reconfigured to
use the new private key and associated certificate chain. Once the
server is using the new key, the TLSA RR that matches the retired key
can be removed from DNS, leaving only the TLSA RR that matches the
new key.
TLSA records for servers SHOULD, when possible, be DANE-EE(3),
SPKI(1), SHA2-256(1) records. Such records specify the SHA2-256
digest of the public key of the server certificate. Since all DANE
implementations are required to support SHA2-256, this record works
for all clients and need not change across certificate renewals with
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the same key. With no name checks required, this TLSA record type
supports hosting arrangements with a single certificate matching all
client domains! It is also the easiest to implement correctly in the
client.
4.2. Certificate Usage DANE-TA(2) Guidelines
Some domains may prefer to reduce the operational complexity of
maintaining a distinct TLSA RRset for each TLS service. If the
domain employs a common issuing certificate authority to create
certificates for multiple TLS services, it may be simpler to publish
the issuing authority as a trust anchor (TA) for the certificate
chains of all relevant services. The TLSA RRs for each service
issued by the same TA may then be CNAMEs to a common TLSA RRset that
matches the TA. This certificate usage also allows Service Providers
to independently generate appropriate certificates for each Customer
Domain (see Section 3.4).
As explained in Section 3.8, servers that employ Certificate Usage
DANE-TA(2) TLSA records MUST include the TA certificate as part of
the certificate chain presented in the TLS handshake even when it is
a self-signed root certificate. TLSA Publishers should publish
either "DANE-TA(2) SPKI(1) SHA2-256(1)" or "DANE-TA(2) Cert(0)
SHA2-256(1)" TLSA parameters. As with leaf certificate rollover
discussed in Section 4.1, two such TLSA RRs need to be published to
facilitate TA certificate rollover.
4.3. Certificate Usage PKIX-EE(1) Guidelines
From a TLSA record perspective this certificate usage is similar to
DANE-EE(3), but in addition PKIX verification is required.
Therefore, name checks, certificate expiration, etc., apply as they
would without DANE. It should be noted that an attacker who can
compromise DNSSEC can replace these with usage DANE-EE(3) or DANE-
TA(2) TLSA records of his choosing, and thus bypass the PKIX
verification requirements.
Therefore, in most cases this certificate usage offers only illusory
incremental security over usage DANE-EE(3). It provides lower
operational reliability than usage 3 since when some clients may not
be configured with the required root CA, the server's chain may be
incomplete or name checks may fail. PKIX-EE(1) also requires more
complex coordination between the Customer Domain and the Service
Provider in hosting arrangements. This certificate usage is NOT
RECOMMENDED.
4.4. Certificate Usage PKIX-TA(0) Guidelines
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TLSA Certificate Usage PKIX-TA(0) allows a domain to publish
constraints on the set of certificate authorities trusted to issue
certificates for its TLS servers. Clients MUST only accept PKIX-
verified trust chains which contain a match for one of the published
TLSA records.
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 trusted
certificate store 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
certificate, such a "truncated" chain might not contain a trusted
root 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, when matching a usage PKIX-TA(0)
TLSA record, 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.
As with PKIX-EE(1) case, An attacker who can compromise DNSSEC can
replace these with usage DANE-EE(3) or DANE-TA(2) TLSA records of his
choosing and thus bypass the PKIX verification requirements.
Therefore, in most cases this certificate usage offers only illusory
incremental security over usage DANE-TA(2). It provides lower
reliability than usage 2, though, since some clients may not be
configured with the required root CA, and additionally requires more
complex coordination between the Customer Domain and the Service
Provider in hosting arrangements. This certificate usage is NOT
RECOMMENDED.
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 a public root CA 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
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(in some cases, however, the registry is the sole registrar). With
many 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 against 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 daily. For domains with less critical data, a reasonable
signature lifetime is a couple of weeks to a month, and the zone
should be resigned weekly. 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.
6. Security Considerations
Application protocols that cannot make use of the existing public CA
PKI (so called non-PKIX protocols), may choose not to implement
certain PKIX-dependent TLSA record types defined in [RFC6698]. If
such records are published despite not being supported by the
application protocol, they are treated as "unusable". When TLS is
opportunistic, the client may proceed to use the server with
mandatory unauthenticated TLS. This is stronger than opportunistic
TLS without DANE, since in that case the client may also proceed with
a plaintext connection. When TLS is not opportunistic, the client
MUST NOT connect to the server.
Therefore, when TLSA records are used with protocols where PKIX does
not apply, the recommended policy is for servers to not publish PKIX-
dependent TLSA records, and for opportunistic TLS clients to use them
to enforce the use of (albeit unauthenticated) TLS, but otherwise
treat them as unusable. Of course, when PKIX validation is supported
by the application protocol, clients SHOULD perform PKIX validation
per [RFC6698].
7. IANA considerations
This specification requires no support from IANA.
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8. 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. Thanks also to Samuel Dukhovni for
editorial assistance.
9. References
9.1. Normative References
[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.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
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[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.
9.2. Informative References
[I-D.ietf-dane-registry-acronyms]
Gudmundsson, O., "Adding acronyms to simplify DANE
conversations", draft-ietf-dane-registry-acronyms-03 (work
in progress), January 2014.
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-06
(work in progress), February 2014.
[I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA records
with SRV and MX records.", draft-ietf-dane-srv-05 (work in
progress), February 2014.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, June 2013.
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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