Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: July 7, 2012 Kirei AB
January 4, 2012
Using Secure DNS to Associate Certificates with Domain Names For TLS
draft-ietf-dane-protocol-14
Abstract
TLS and DTLS use PKIX certificates for authenticating the server.
Users want their applications to verify that the certificate provided
by the TLS server is in fact associated with the domain name they
expect. TLSA provides bindings of keys to domains that are asserted
not by external entities, but by the entities that operate the DNS.
This document describes how to use secure DNS to associate the TLS
server's certificate with the intended domain name.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 7, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Certificate Associations . . . . . . . . . . . . . . . . . 4
1.2. Securing Certificate Associations . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. The TLSA Resource Record . . . . . . . . . . . . . . . . . . . 5
2.1. TLSA RDATA Wire Format . . . . . . . . . . . . . . . . . . 6
2.1.1. The Certificate Usage Field . . . . . . . . . . . . . 6
2.1.2. The Selector Field . . . . . . . . . . . . . . . . . . 6
2.1.3. The Matching Type Field . . . . . . . . . . . . . . . 7
2.1.4. The Certificate for Association Field . . . . . . . . 7
2.2. TLSA RR Presentation Format . . . . . . . . . . . . . . . 7
2.3. TLSA RR Examples . . . . . . . . . . . . . . . . . . . . . 8
3. Domain Names for TLS Certificate Associations . . . . . . . . 8
4. Semantics and Features of TLSA Certificate Usages . . . . . . 9
4.1. Pass PKIX Validation and Chain Through CA . . . . . . . . 9
4.2. Pass PKIX Validation and Match End Entity Certificate . . 9
4.3. Pass PKIX Validation and Use Trust Anchor . . . . . . . . 9
4.4. Use of TLS Certificate Associations in TLS . . . . . . . . 9
5. TLSA and DANE Use Cases and Requirements . . . . . . . . . . . 10
6. Mandatory-to-Implement Algorithms . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . 12
7.2. TLSA Usages . . . . . . . . . . . . . . . . . . . . . . . 12
7.3. TLSA Selectors . . . . . . . . . . . . . . . . . . . . . . 13
7.4. TLSA Matching Types . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Operational Considerations for Deploying TLSA
Records . . . . . . . . . . . . . . . . . . . . . . . 16
A.1. Creating TLSA Records . . . . . . . . . . . . . . . . . . 16
A.1.1. Ambiguities and Corner Cases When TLS Clients
Build Trust Chains . . . . . . . . . . . . . . . . . . 16
A.1.2. Choosing a Selector Type . . . . . . . . . . . . . . . 17
A.2. Provisioning TLSA Records in DNS . . . . . . . . . . . . . 18
A.2.1. Provisioning TLSA Records with Aliases . . . . . . . . 18
A.2.2. Provisioning with NS Records . . . . . . . . . . . . . 21
A.3. Securing the Last Hop . . . . . . . . . . . . . . . . . . 21
A.4. Handling Certificate Rollover . . . . . . . . . . . . . . 21
Appendix B. Pseudocode for Using TLSA . . . . . . . . . . . . . . 21
B.1. Helper Functions . . . . . . . . . . . . . . . . . . . . . 21
B.2. Main TLSA Pseudo Code . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
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
certificate, must trust a trust anchor upon which the server's
certificate is rooted, and must successfully validate the
certificate.
Some people want a different way to authenticate the association of
the server's certificate with the intended domain name without
trusting an external certificate authority (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.
There are many use cases for such functionality. [DANEUSECASES]
lists the ones that the protocol in this document is meant to apply
to. [DANEUSECASES] also lists many requirements, most of which the
protocol in this document is believed to meet. Section 5 covers the
applicability of this document to the use cases in detail.
This document applies to both TLS [RFC5246] and DTLS [RFC4347bis].
In order to make the document more readable, it mostly only talks
about "TLS", but in all cases, it means "TLS or DTLS". This document
only relates to securely associating certificates for TLS and DTLS
with host names; other security protocols and other forms of
identification of TLS servers (such as IP addresses) are handled in
other documents. For example, keys for IPsec are covered in
[RFC4025] and keys for SSH are covered in [RFC4255].
1.1. Certificate Associations
A certificate association is formed from a piece of information
identifying a certificate with the domain name where the data is
found. The data used to identify the certificate consists of either
a PKIX certificate or a hash of a PKIX certificate. When using the
certificate itself in the certificate association, the entire
certificate in the normal format is used. This document only applies
to PKIX [RFC5280] certificates, not certificates of other formats.
A DNS query can return multiple certificate associations, such as in
the case of different server software on a single host using
different certificates, or in the case that a server is changing from
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one certificate to another.
1.2. Securing Certificate Associations
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 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.
DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
[RFC4034], and [RFC4035]), uses cryptographic keys and digital
signatures to provide authentication of DNS data. Information
retrieved from the DNS and that is validated using DNSSEC is thereby
proved to be the authoritative data. The DNSSEC signature MUST 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.
This document only relates to securely getting the DNS information
for the certificate association using DNSSEC; other secure DNS
mechanisms are out of scope.
1.3. 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, and TLS
terminology. See [RFC5280], [RFC4033], and [RFC5246] respectively,
for these terms. In addition, terms related to TLS-protected
application services and DNS names are taken from [RFC6125].
2. The TLSA Resource Record
The TLSA DNS resource record (RR) is used to associate a certificate
with the domain name where the record is found. The semantics of how
the TLSA RR is interpreted are given later in this document.
The type value for the TLSA RR type is TBD.
The TLSA RR is class independent.
The TLSA RR has no special TTL requirements.
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2.1. TLSA RDATA Wire Format
The RDATA for a TLSA RR consists of a one octet usage type field, a
one octet selector field, a one octet matching type field and the
certificate for association 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Usage | Selector | Matching Type | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ Certificate for Association /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1.1. The Certificate Usage Field
A one-octet value, called "certificate usage" or just "usage",
specifying the provided association that will be used to match the
target certificate from TLS. This will be an IANA registry in order
to make it easier to add additional certificate usages in the future.
The usages defined in this document are:
0 -- The target certificate MUST pass PKIX validation and MUST
chain through a CA certificate matching the TLSA record
1 -- The target certificate MUST pass PKIX validation and MUST
match the TLSA record
2 -- The target certificate MUST pass PKIX validation, with any
certificate matching the TLSA record considered to be a trust
anchor for this validation
The three certificate usages defined in this document explicitly only
apply to PKIX-formatted certificates. If TLS allows other formats
later, or if extensions to this protocol are made that accept other
formats for certificates, those certificates will need their own
certificate types.
The use of this field is described in greater detail in Section 4.
2.1.2. The Selector Field
A one-octet value, called "selector", specifying what will be
matched. This value is defined in a new IANA registry. The
selectors defined in this document are:
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0 -- Full certificate
1 -- SubjectPublicKeyInfo
2.1.3. The Matching Type Field
A one-octet value, called "matching type", specifying how the
certificate association is presented. This value is defined in a new
IANA registry. The types defined in this document are:
0 -- Exact match on selected content
1 -- SHA-256 hash of selected content
2 -- SHA-512 hash of selected content
Using the same hash algorithm as is used in the signature in the
certificate will make it more likely that the TLS client will
understand this TLSA data.
2.1.4. The Certificate for Association Field
The "association data" to be matched. The field contains the bytes
to be matched or the hash of the bytes to be matched. The source of
the data to be matched is controlled by the Matching Type field. 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 is as follows:
o The certificate usage field MUST be represented as an unsigned
decimal integer.
o The selector field MUST be represented as an unsigned decimal
integer.
o The matching type field MUST be represented as an unsigned decimal
integer.
o The certificate for association field MUST be represented as a
string of hexadecimal characters. Whitespace is allowed within
the string of hexadecimal characters.
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2.3. TLSA RR Examples
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 trust anchor:
_443._tcp.www.example.com. IN TLSA
2 0 0 30820307308201efa003020102020... )
3. Domain Names for TLS Certificate Associations
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.
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 domain name is appended to the result of step 2 to complete
the prepared domain name.
For example, to request a TLSA resource record for an HTTP server
running TLS on port 443 at "www.example.com", you would use
"_443._tcp.www.example.com" in the request. To request a TLSA
resource record for an SMTP server running the STARTTLS protocol on
port 25 at "mail.example.com", you would use
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"_25._tcp.mail.example.com".
4. Semantics and Features of TLSA Certificate Usages
The three certificate usages have very different semantics, but also
have features common to all three types.
4.1. Pass PKIX Validation and Chain Through CA
Certificate usage 0 is used to specify a CA certificate, or the
public key of such a certificate, that the must be found in any of
the PKIX validation chains for the end entity certificate given by
the server in TLS. This usage is sometimes referred to as "CA
constraint" because it limits which CA can be used to issue
certificates for a given host name.
4.2. Pass PKIX Validation and Match End Entity Certificate
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 usage is
sometimes referred to as "service certificate constraints" because it
limits which end entity certificate may be used by a given host name.
4.3. Pass PKIX Validation and Use Trust Anchor
Certificate usage 2 is used to specify a certificate, or the public
key of such a certificate, that must be used as a trust anchor when
validating the end entity certificate given by the server in TLS.
This 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.
4.4. Use of TLS Certificate Associations in TLS
Section 2.1 of this document defines the mandatory matching rules for
the data from the TLS 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. The
matching or chaining MUST be done within the life of the TTL on the
TLSA record.
Some specifications for applications that run under TLS, such as
[RFC2818] for HTTP, require the server's certificate to have a domain
name that matches the host name expected by the client. Some
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specifications such at [RFC6125] detail how to match the identify
given in a PKIX certificate with those expected by the user.
An application that complies with this document first requests TLSA
records in order to make certificate associations.
Determining whether a TLSA RRset can be used depends 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, to 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 marked as unusable.
If an application receives zero usable certificate associations, it
processes TLS in the normal fashion without any input from the TLSA
records; otherwise, that application attempts to match each
certificate association with the TLS server's end entity certificate.
Clients that validate the DNSSEC signatures themselves MUST either
use standard DNSSEC validation procedures or a secure transport (such
as TSIG [RFC2845], SIG(0) [RFC2931], or IPsec [RFC6071]) between
themselves and the entity performing the DNSSEC signature validation.
Note that it is not sufficient to use secure transport to a DNS
resolver that does not do DNSSEC signature validation.
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 marked as unusable. If the
comparison data for a certificate is malformed, the certificate
association MUST be marked as unusable.
5. TLSA and DANE Use Cases and Requirements
The different types of certificates for association defined in TLSA
are matched with various sections of [DANEUSECASES]. The three use
cases from section 3 of [DANEUSECASES] are covered in this protocol
as follows:
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3.1 CA Constraints -- Implemented using certificate usage 0.
3.2 Certificate Constraints -- Implemented using certificate usage
1.
3.3 Domain-Issued Certificates -- Implemented using certificate
usage 2.
The requirements from section 4 of [DANEUSECASES] are covered in this
protocol as follows (note that some of these might be excessively
glib):
Multiple Ports -- Covered in the TLSA request syntax.
No Downgrade -- Covered by DNSSEC itself.
Encapsulation -- Covered in the TLSA response semantics.
Predictability -- Covered by this spec.
Opportunistic Security -- Covered in the TLSA request syntax.
Combination -- Covered in the TLSA response semantics. No support
is provided to combine two TLSA certificate associations in a
single operation. Support exists for having multiple TLSA
certificate associations that are treated independently.
Roll-over -- Covered by the TTLs on the TLSA records.
Simple Key Management -- Implemented using the SubjectPublicKeyInfo
selector.
Minimal Dependencies -- Covered in the TLSA response semantics.
Minimal Options -- Covered in the TLSA response semantics.
Wild Cards -- Covered in a limited manner in the TLSA request
syntax; see Appendix A.
Redirection -- Covered in the TLSA request syntax; see Appendix A.
6. Mandatory-to-Implement Algorithms
A system creating TLSA records that conforms to this specification
MUST be able to create TLSA records containing certificate usages 0,
1 and 2. A system creating TLSA records that conforms to this
specification MUST be able to create TLSA records with selectors 0
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(full certificate) and 1 (SubjectPublicKeyInfo). A system creating
TLSA records that conforms to this specification MUST be able to
create TLSA records using matching type 0 (no hash used) and matching
type 1 (SHA-256), and SHOULD be able to create TLSA records using
matching type 2 (SHA-512).
TLS clients conforming to this specification MUST be able to
correctly interpret TLSA records with certificate usages 0, 1 and 2.
TLS clients conforming to this specification MUST be able to compare
a certificate for association with a certificate from TLS using
selectors type 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).
At the time this is written, it is expected that there will be a new
family of hash algorithms called SHA-3 within the next few years. It
is expected that some of the SHA-3 algorithms will be mandatory
and/or recommended for TLSA records after the algorithms are fully
defined. At that time, this specification will be updated.
7. IANA Considerations
7.1. TLSA RRtype
This document uses a new DNS RR type, TLSA, whose value is TBD. A
separate request for the RR type will be submitted to the expert
reviewer, and future versions of this document will have that value
instead of TBD.
7.2. TLSA Usages
This document creates a new registry, "Certificate Usages for TLSA
Resource Records". The registry policy is "RFC Required". The
initial entries in the registry are:
Value Short description Reference
----------------------------------------------------------
0 Pass PKIX and chain through CA [This]
1 Pass PKIX and match EE [This]
2 Pass PKIX and trusted via certificate [This]
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, "Selectors for TLSA Resource
Records". The registry policy is "Specification Required". The
initial entries in the registry are:
Value Short description Reference
----------------------------------------------------------
0 Full Certificate [This]
1 SubjectPublickeyInfo [This]
2-254 Unassigned
255 Private use
7.4. TLSA Matching Types
This document creates a new registry, "Matching Types for TLSA
Resource Records". The registry policy is "Specification Required".
The initial entries in the registry are:
Value Short description Reference
---------------------------------------------
0 No hash used [This]
1 SHA-256 NIST FIPS 180-3
2 SHA-512 NIST FIPS 180-3
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 protocols described in this document relies on
the security of DNSSEC as used by the client requesting A/AAAA and
TLSA records.
A DNS administrator who goes rogue and changes both the A/AAAA and
TLSA records for a domain name can cause the user to go to an
unauthorized server that will appear authorized, unless the client
performs certificate validation and rejects the certificate. That
administrator could probably get a certificate issued anyway, so this
is not an additional threat.
If the authentication mechanism for adding or changing TLSA data in a
zone is weaker than the authentication mechanism for changing the
A/AAAA records, a man-in-the-middle who can redirect traffic to their
site may be able to impersonate the attacked host in TLS if they can
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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 host.
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,
the TLSA protocol 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. Thus,
such proxies might choose to aggressively block TLSA requests and/or
responses.
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 domain name
administrator.
If a server's certificate is revoked, or if an intermediate CA in a
chain between the end entity and a trust anchor has its certificate
revoked, a TLSA record with a certificate type 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 key or
certificate used in TLSA records with a certificate type of 2 are in
fact able to be used as reliable trust anchors.
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 here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley, Ben
Laurie, Ilari Liusvaara, Ondrej Mikle, Scott Schmit, Ondrej Sury,
Richard Barnes, Jim Schaad, and Stephen Farrell.
This document has also been greatly helped by many active
participants of the DANE Working Group.
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10. References
10.1. Normative References
[DANEUSECASES]
Barnes, R., "Use Cases and Requirements for DNS-based
Authentication of Named Entities (DANE)",
draft-ietf-dane-use-cases (work in progress), 2011.
[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.
[RFC4347bis]
Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
progress), July 2010.
[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.
10.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
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[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2931] Eastlake, 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.
[RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
February 2011.
Appendix A. Operational Considerations for Deploying TLSA Records
A.1. Creating TLSA Records
When creating TLSA records with certificate usage type 0 or 2, care
needs to be taken when choosing between selector type 0 (full
certificate) and 1 (SubjectPublicKeyInfo) because of the algorithms
that various TLS clients employ to build their trust-chain. The
following outlines some important cases and discusses implications of
the choice of selector type.
Note that certificate usage 2 is not affected by this discussion if
the association is made to an end entity certificate.
A.1.1. Ambiguities and Corner Cases When TLS Clients Build Trust Chains
TLS clients are known to implement methods that may cause any
certificate (except the end entity certificate in the original
certificate chain sent by server) to be exchanged or removed from the
trust chain when client builds trust chain.
Certificates the client can use to replace certificates from original
chain include:
o Client's trust anchors
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o Certificates cached locally
o Certificates retrieved from a URI listed in an Authority
Information Access X.509v3 extension
CAs frequently reissue certificates with a different validity period,
a hash in the signature algorithm or PKIX extensions; only the public
key, issuer and subject remain intact. Thes reissued certificates
are certificates that the TLS client can use in place of original
certificate.
Clients are known to exchange or remove certificates that could cause
TLSA association that rely on the full certificate to fail. For
example:
o The client considers the hash in signature algorithm of a
certificate no longer sufficiently secure
o A cross-certificate issued to CA2 by CA1 is represented by a self-
signed root certificate of CA2 as a trust anchor in client's trust
store
o A CA certificate above the cross-certificate in original chain may
be removed from the chain because the client found a trust anchor
below that CA certificate
A.1.2. Choosing a Selector Type
A.1.2.1. Selector Type 0
The "Full certificate" selector provides most precise specification
of a trust anchor. Such non-ambiguity foils a class of hypothetical
future attacks on a CA where the CA issues new certificate with an
identical SubjectPublicKeyInfo, but a different issuer, subject, or
extensions that would allow redirection of a trust chain. This "Full
certificate" selector would also foil bad practices or negligence of
a CA if the CA uses the same key for unrelated CA certificates.
For a DNS administrator, the best course to avoid false-positive
failures at client's side when using this selector is:
o Do not associate to CA certificates that have a signature
algorithm with hash that is considered weak if that CA has issued
a replacement certificate.
o Determine how common client applications process the TLSA
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
A SubjectPublicKeyInfo selector gives greater flexibility in avoiding
many false-positive failures caused by trust-chain-building
algorithms used in many clients.
One specific use-case should be noted: creating a TLSA association to
certificate I1 that directly signed end entity certificate S1 of the
server. Because the key used to sign S1 is fixed, the association to
I1 must succeed: if the client swaps I1 for I2 (a different
certificate), I2's SubjectPublicKeyInfo must match the
SubjectPublicKeyInfo of I1. Such association would not suffer from
false-positive failure on the client if the client uses a reissued CA
certificate I2 in place of I1.
The attack surface is a bit broader compared to "full certificate"
selector:
o The administrator must know or trust the CA not to engage in bad
practices, such as not sharing key of I1 for unrelated CA
certificates leading to trust-chain redirect
o The administrator should understand whether some PKIX extension
may adversely affect security of the association. If possible,
administrators should review all CA certificates that share the
SubjectPublicKeyInfo.
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.
In practice, sharing keys in differently-purposed CA certificates is
rare, but certainly happens sometimes. An attack on an association
by SubjectPublicKeyInfo would require either gross negligence on the
part of the CA or an attacker gaining control of CA.
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.
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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.
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 10.0.0.0
If the owner of the orginal 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 10.0.0.0
_443._tcp.sub6.example.com. IN TLSA 1 1 1 536a570ac49d9ba4...
Note that it is acceptable for both the original domain and the
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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 10.0.0.0
_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
"308202c5308201ab"; the TLSA record whose value starts
"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 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 10.0.0.0
_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 you can only wildcard at a layer below the host
name. For example, if you want to have the same TLSA record for
every TCP port for www.example.com, you might have
*._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.
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A.2.2. Provisioning with NS Records
[[ This was proposed, and questioned, and not yet followed through
on. ]]
A.3. Securing the Last Hop
[[ Need to add text here about the various ways that a client who is
pulling in TLSA records can be sure that they are protected by
DNSSEC. ]]
A.4. Handling Certificate Rollover
[[ Need to add text here about how to handle a change in certificate.
It would cover using two TLSA records at the same time, the TTL on
the RRset, and coordinating that with the use of the certificates in
the TLS server. ]]
Appendix B. Pseudocode for Using TLSA
This appendix describes the interactions given earlier in this
specification in pseudocode format. This appendix is non-normative.
If the steps below disagree with the text earlier in the document,
the steps earlier in the document should be considered correct and
this text incorrect.
B.1. Helper Functions
// implement the function for exiting
function Finish (F) = {
if (F == 0) {
abort the TLS handshake or prevent TLS from starting
exit
}
if (F == 1) {
fall back to non-TLSA certificate validation
exit
}
if (F == 2) {
accept the TLS connection
exit
}
// unreachable
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}
// implement the selector function
function Select (S, X) = {
// Full certificate
if (S == 0) {
return X
}
// SubjectPublicKeyInfo
if (S == 1) {
return X.SubjectPublicKeyInfo
}
return undef
}
// 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)
}
return undef
}
B.2. Main TLSA Pseudo Code
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, class=IN)
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// check for states that would change processing
if (ValState == BOGUS) {
Finish(0)
}
if ((ValState == INDETERMINATE) or (ValState == INSECURE)) {
Finish(1)
}
// if here, ValState must be SECURE
for each R in TLSArecords {
// unusable records include unknown certUsage, unknown
// selectorType, unknown matchingType, and erroneous RDATA
if (R is unusable) remove it from TLSArecords
}
if (length(TLSArecords) == 0) {
Finish(1)
}
// A TLS client might have multiple trust anchors that it might use
// when validating the TLS server's end entity certificate. Also,
// there can be multiple PKIX validation chains for the
// certificates given by the server in TLS. Thus, there are
// possibly many chains that might need to be tested during
// PKIX validation.
for each R in TLSArecords {
// pass PKIX validation and chain through CA cert from TLSA
if (R.certUsage == 0) {
for each PKIX validation chain H {
if (C passes PKIX validation in H) {
for each D in H {
if (D is a CA certificate) and
Match(matchingType, Select(selectorType, D), R) {
Finish(2)
}
}
}
}
// pass PKIX validation and match EE cert from TLSA
if (R.certUsage == 1) {
for each PKIX validation chain H {
if (C passes PKIX validation in H) {
if (Match(matchingType, Select(selectorType, C), R)) {
Finish(2)
}
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}
}
}
// pass PKIX validation using TLSA record as trust anchor
if (R.certUsage == 2) {
for each PKIX validation chain H that has R as the trust anchor {
if (C passes PKIX validation in H) {
Finish(2)
}
}
}
}
// if here, the none of the TLSA records was sufficient for TLS
Finish(0)
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
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