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Keywords: TLS, PKIX
Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 6394 BBN Technologies
Category: Informational October 2011
ISSN: 2070-1721
Use Cases and Requirements for DNS-Based Authentication
of Named Entities (DANE)
Abstract
Many current applications use the certificate-based authentication
features in Transport Layer Security (TLS) to allow clients to verify
that a connected server properly represents a desired domain name.
Typically, this authentication has been based on PKIX certificate
chains rooted in well-known certificate authorities (CAs), but
additional information can be provided via the DNS itself. This
document describes a set of use cases in which the DNS and DNS
Security Extensions (DNSSEC) could be used to make assertions that
support the TLS authentication process. The main focus of this
document is TLS server authentication, but it also covers TLS client
authentication for applications where TLS clients are identified by
domain names.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6394.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Definitions .....................................................4
3. Use Cases .......................................................4
3.1. CA Constraints .............................................5
3.2. Service Certificate Constraints ............................6
3.3. Trust Anchor Assertion and Domain-Issued Certificates ......7
3.4. Delegated Services .........................................9
4. Other Requirements .............................................10
5. Acknowledgements ...............................................11
6. Security Considerations ........................................11
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................12
1. Introduction
Transport Layer Security (TLS) is used as the basis for security
features in many modern Internet application service protocols to
provide secure client-server connections [RFC5246]. It underlies
secure HTTP and secure email [RFC2818] [RFC2595] [RFC3207], and
provides hop-by-hop security in real-time multimedia and instant-
messaging protocols [RFC3261] [RFC6120].
Application service clients typically establish TLS connections to
application servers identified by DNS domain names. The process of
obtaining this "source" domain is application specific [RFC6125].
The name could be entered by a user or found through an automated
discovery process such as an SRV or NAPTR record. After obtaining
the address of the server via an A or AAAA DNS record, the client
conducts a TLS handshake with the server, during which the server
presents a PKIX certificate [RFC5280]. The TLS layer performs PKIX
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validation of the certificate, including verification that the
certificate chains to one of the client's trust anchors. If this
validation is successful, then the application layer determines
whether the DNS name for the application service presented in the
certificate matches the source domain name [RFC6125]. Typically, if
the name matches, then the client proceeds with the TLS connection.
The certificate authorities (CAs) that issue PKIX certificates are
asserting bindings between domain names and the public keys they
certify. Application service clients are verifying these bindings
and making authorization decisions -- whether to proceed with
connections -- based on them.
Clients thus rely on CAs to correctly assert bindings between public
keys and domain names, in the sense that the holder of the
corresponding private key should be the domain holder. Today, an
attacker can successfully authenticate as a given application service
domain if he can obtain a "mis-issued" certificate from one of the
widely used CAs -- a certificate containing the victim application
service's domain name and a public key whose corresponding private
key is held by the attacker. If the attacker can additionally insert
himself as a "man in the middle" between a client and server (e.g.,
through DNS cache poisoning of an A or AAAA record), then the
attacker can convince the client that a server of the attacker's
choice legitimately represents the victim's application service.
With the advent of DNSSEC [RFC4033], it is now possible for DNS name
resolution to provide its information securely, in the sense that
clients can verify that DNS information was provided by the domain
operator and not tampered with in transit. The goal of technologies
for DNS-based Authentication of Named Entities (DANE) is to use the
DNS and DNSSEC to provide additional information about the
cryptographic credentials associated with a domain, so that clients
can use this information to increase the level of assurance they
receive from the TLS handshake process. This document describes a
set of use cases that capture specific goals for using the DNS in
this way, and a set of requirements that the ultimate DANE mechanism
should satisfy.
Finally, it should be noted that although this document will
frequently use HTTPS as an example application service, DANE is
intended to apply equally to all applications that make use of TLS to
connect to application services identified by domain names.
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2. Definitions
This document also makes use of standard PKIX, DNSSEC, and TLS
terminology. See RFC 5280 [RFC5280], RFC 4033 [RFC4033], and
RFC 5246 [RFC5246], respectively, for these terms. In addition,
terms related to TLS-protected application services and DNS names are
taken from RFC 6125 [RFC6125].
Note in particular that the term "server" in this document refers to
the server role in TLS, rather than to a host. Multiple servers of
this type may be co-located on a single physical host, often using
different ports, and each of these can use different certificates.
This document refers several times to the notion of a "domain
holder". This term is understood to mean the entity that is
authorized to control the contents of a particular zone. For
example, the registrants of 2nd- or 3rd-level domains are the holders
of those domains. The holder of a particular domain is not
necessarily the entity that operates the zone.
It should be noted that the presence of a valid DNSSEC signature in a
DNS reply does not necessarily imply that the records protected by
that signature were authorized by the domain holder. The distinction
between the holder of a domain and the operator of the corresponding
zone has several security implications, which are discussed in the
individual use cases below.
3. Use Cases
In this section, we describe the major use cases that the DANE
mechanism should support. This list is not intended to represent all
possible ways that the DNS can be used to support TLS authentication.
Rather, it represents the specific cases that comprise the initial
goals for DANE.
In the use cases below, we will refer to the following dramatis
personae:
Alice: The operator of a TLS-protected application service on the
host alice.example.com, and administrator of the corresponding
DNS zone.
Bob: A client connecting to alice.example.com.
Charlie: A well-known CA that issues certificates with domain names
as identifiers.
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Oscar: An outsourcing provider that operates TLS-protected
application services on behalf of customers.
Trent: A CA that issues certificates with domain names as
identifiers, but is not generally well-known.
These use cases are framed in terms of adding verification steps to
TLS server identity checking on the part of application service
clients. In application services where the clients are also
identified by domain names (e.g., Extensible Messaging and Presence
Protocol (XMPP) server-to-server connections), the same
considerations and use cases are applicable to the application
server's checking of identities in TLS client certificates.
3.1. CA Constraints
Alice runs a website on alice.example.com and has obtained a
certificate from the well-known CA Charlie. She is concerned that
other well-known CAs might issue certificates for alice.example.com
without her authorization, which clients would accept. Alice would
like to provide a mechanism for visitors to her site to know that
they should expect alice.example.com to use a certificate issued
under the CA that she uses (Charlie) and not another CA. That is,
Alice is recommending that the client verify that there is a valid
certificate chain from the server certificate to Charlie before
accepting the server certificate. (For example, in the TLS
handshake, the server might include Charlie's certificate in the
server Certificate message's certificate_list structure [RFC5246]).
When Bob connects to alice.example.com, he uses this mechanism to
verify that the certificate presented by the server was issued under
the proper CA, Charlie. Bob also performs the normal PKIX validation
procedure for this certificate, in particular verifying that the
certificate chains to a trust anchor (possibly Charlie's CA, if Bob
accepts Charlie's CA as a trust anchor).
Alice may wish to provide similar information to an external CA
operator, Charlie. Prior to issuing a certificate for
alice.example.com to someone claiming to be Alice, Charlie needs to
verify that Alice is actually requesting a certificate. Alice could
indicate her preferred CA using DANE to CAs as well as relying
parties. Charlie could then check to see whether Alice said that her
certificates should be issued by Charlie or another CA. Note that
this check does not guarantee that the precise entity requesting a
certification from Charlie actually represents Alice -- only that
Alice has authorized Charlie to issue certificates for her domain to
properly authorized individuals.
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In principle, DANE information expressing CA constraints can be
presented with or without DNSSEC protection. Presenting DANE
information without DNSSEC protection does not introduce any new
vulnerabilities, but neither does it add much assurance. Deletion of
records removes the protection provided by this constraint, but the
client is still protected by CA practices (as now). Injected or
modified false records are not useful unless the attacker can also
obtain a certificate for the target domain. Thus, in the worst case,
tampering with these constraints increases the risk of false
authentication to the level that is now standard.
Using DANE information for CA constraints without DNSSEC provides a
very small incremental security feature. Many common attacks against
TLS connections already require the attacker to inject false A or
AAAA records in order to steer the victim client to the attacker's
server. An attacker that can already inject false DNS records can
also provide fake DANE information (without DNSSEC) by simply
spoofing the additional records required to carry the DANE
information.
Injected or modified false DANE information of this type can be used
for denial of service, even if the attacker does not have a
certificate for the target domain. If an attacker can modify DNS
responses that a target host receives, however, there are already
much simpler ways of denying service, such as providing a false A or
AAAA record. In this case, DNSSEC is not helpful, since an attacker
could still cause a denial of service by blocking all DNS responses
for the target domain.
Continuing to require PKIX validation also limits the degree to which
DNS operators (as distinct from the holders of domains) can interfere
with TLS authentication through this mechanism. As above, even if a
DNS operator falsifies DANE records, it cannot masquerade as the
target server unless it can also obtain a certificate for the target
domain.
3.2. Service Certificate Constraints
Alice runs a website on alice.example.com and has obtained a
certificate from the well-known CA Charlie. She is concerned about
additional, unauthorized certificates being issued by Charlie as well
as by other CAs. She would like to provide a way for visitors to her
site to know that they should expect alice.example.com to present a
specific certificate. In TLS terms, Alice is letting Bob know that
this specific certificate must be the first certificate in the server
Certificate message's certificate_list structure [RFC5246].
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When Bob connects to alice.example.com, he uses this mechanism to
verify that the certificate presented by the server is the correct
certificate. Bob also performs the normal PKIX validation procedure
for this certificate, in particular verifying that the certificate
chains to a trust anchor.
The security implications for this case are the same as for the "CA
Constraints" case above.
3.3. Trust Anchor Assertion and Domain-Issued Certificates
Alice would like to be able to generate and use certificates for her
website on alice.example.com without involving an external CA at all.
Alice can generate her own certificates today, making self-signed
certificates and possibly certificates subordinate to those
certificates. When Bob receives such a certificate in a TLS
handshake, however, he doesn't automatically have a way to verify
that the issuer of the certificate is actually Alice, because he
doesn't necessarily possess Alice's corresponding trust anchor. This
concerns him because an attacker could present a different
certificate and perform a man-in-the-middle attack. Bob would like
to protect against this.
Alice would thus like to publish information so that visitors to her
site can know that the certificates presented by her application
services are legitimately hers. When Bob connects to
alice.example.com, he uses this information to verify that the
certificate presented by the server has been issued by Alice. Since
Bob can bind certificates to Alice in this way, he can use Alice's CA
as a trust anchor for purposes of validating certificates for
alice.example.com. Alice can additionally recommend that clients
accept only her certificates using the CA constraints described
above.
As in Section 3.1 above, Alice may wish to represent this information
to potential third-party CAs (Charlie) as well as to relying parties
(Bob). Since publishing a certificate in a DANE record of this form
authorizes the holder of the corresponding private key to represent
alice.example.com, a CA that has received a request to issue a
certificate from alice.example.com could use the DANE information to
verify the requestor's authorization to receive a certificate for
that domain. For example, a CA might choose to issue a certificate
for a given domain name and public key only when the holder of the
domain name has provisioned DANE information with a certificate
containing the public key.
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Note that this use case is functionally equivalent to the case where
Alice doesn't issue her own certificates, but uses Trent's CA, which
is not well-known. In this case, Alice would be advising Bob that he
should treat Trent as a trust anchor for purposes of validating
Alice's certificates, rather than a CA operated by Alice herself.
Bob would thus need a way to securely obtain Trent's trust anchor
information, namely through DANE information.
Alice's advertising of trust anchor material in this way does not
guarantee that Bob will accept the advertised trust anchor. For
example, Bob might have out-of-band information (such as a
pre-existing local policy) that indicates that the CA advertised by
Alice (Trent's CA) is not trustworthy, which would lead him to decide
not to accept Trent as a trust anchor, and thus to reject Alice's
certificate if it is issued under Trent's CA.
Providing trust anchor material in this way clearly requires DNSSEC,
since corrupted or injected records could be used by an attacker to
cause clients to trust an attacker's certificate (assuming that the
attacker's certificate is not rejected by some other local policy).
Deleted records will only result in connection failure and denial of
service, although this could result in clients re-connecting without
TLS (a downgrade attack), depending on the application. Therefore,
in order for this use case to be safe, applications must forbid
clients from falling back to unsecured channels when records appear
to have been deleted (e.g., when a missing record has no NSEC or
NSEC3 record).
By the same token, this use case puts the most power in the hands of
DNS operators. Since the operator of the appropriate DNS zone has
de facto control over the content and signing of the zone, he can
create false DANE records that bind a malicious party's certificate
to a domain. This risk is especially important to keep in mind in
cases where the operator of a DNS zone is a different entity than the
holder of the domain, as in DNS hosting/outsourcing arrangements,
since in these cases the DNS operator might be able to make changes
to a domain that are not authorized by the holder of the domain.
It should be noted that DNS operators already have the ability to
obtain certificates for domains under their control, under certain CA
policies. In the current system, CAs need to verify that an entity
requesting a certificate for a domain is actually the legitimate
holder of that domain. Typically, this is done using information
published about that domain, such as WHOIS email addresses or special
records inserted into a domain. By manipulating these values, it is
possible for DNS operators to obtain certificates from some well-
known certificate authorities today without authorization from the
true domain holder.
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3.4. Delegated Services
In addition to guarding against CA mis-issue, CA constraints and
certificate constraints can also be used to constrain the set of
certificates that can be used by an outsourcing provider. Suppose
that Oscar operates alice.example.com on behalf of Alice. In
particular, Oscar then has de facto control over what certificates to
present in TLS handshakes for alice.example.com. In such cases,
there are a few ways that DNS-based information about TLS
certificates could be configured; for example:
1. Alice has the A/AAAA records in her DNS and can sign them along
with the DANE record, but Oscar and Alice now need to have tight
coordination if the addresses and/or the certificates change.
2. Alice refers to Oscar's DNS by delegating a sub-domain name to
Oscar, and has no control over the A/AAAA, DANE, or any other
pieces under Oscar's control.
3. Alice can put DANE records into her DNS server but delegate the
address records to Oscar's DNS server. This means that Alice can
control the usage of certificates, but Oscar is free to move the
servers around as needed. The only coordination needed is when
the certificates change, and then it would depend on how the DANE
record is set up (i.e., a CA or an end-entity certificate
pointer).
Which of these deployment patterns is used in a given deployment will
determine what sort of constraints can be expressed by which actors.
In cases where Alice controls DANE records (1 and 3), she can use CA
and certificate constraints to control what certificates Oscar
presents for Alice's application services. For instance, Alice might
require Oscar to use certificates under a given set of CAs. This
control, however, requires that Alice update DANE records when Oscar
needs to change certificates. Cases where Oscar controls DANE
records allow Oscar to maintain more autonomy from Alice, but by the
same token, Alice cannot enforce any requirements on the certificates
that Oscar presents in TLS handshakes.
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4. Other Requirements
In addition to supporting the above use cases, the DANE mechanism
must satisfy several lower-level operational and protocol
requirements and goals.
Multiple Ports: DANE should be able to support multiple application
services with different credentials on the same named host,
distinguished by port number.
No Downgrade: An attacker who can tamper with DNS responses must not
be able to make a DANE-compliant client treat a site that has
deployed DANE and DNSSEC like a site that has deployed neither.
Encapsulation: If there is DANE information for the name
alice.example.com, it must only affect application services hosted
at alice.example.com.
Predictability: Client behavior in response to DANE information must
be defined in the DANE specification as precisely as possible,
especially for cases where DANE information might conflict with
PKIX information.
Opportunistic Security: The DANE mechanism must allow a client to
determine whether DANE information is available for a site, so
that a client can provide the highest level of security possible
for a given application service. Clients that do not support DANE
should continue to work as specified, regardless of whether DANE
information is present or not.
Combination: The DANE mechanism must allow multiple DANE statements
of the above forms to be combined. For example, a domain holder
should be able to specify that clients should accept a particular
certificate (Section 3.2) as well as any certificate issued by its
own CA (Section 3.3). The precise types of combination allowed
will be defined by the DANE protocol.
Roll-over: The DANE mechanism must allow a site to transition from
using one DANE mechanism to another. For example, a domain holder
should be able to migrate from using DANE to assert a domain-
issued certificate (Section 3.3) to using DANE to require an
external CA (Section 3.1), or vice versa. The DANE mechanism must
also allow roll-over between records of the same type, e.g., when
changing CAs.
Simple Key Management: DANE should have a mode in which the domain
holder only needs to maintain a single long-lived public/private
key pair.
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Minimal Dependencies: It should be possible for a site to deploy
DANE without also deploying anything else, except DNSSEC.
Minimal Options: Ideally, DANE should have only one operating mode.
Practically, DANE should have as few operating modes as possible.
Wildcards: The mechanism for distributing DANE information should
allow the use of DNS wildcard labels (*) for setting DANE
information for all names within a wildcard expansion.
Redirection: The mechanism for distributing DANE information should
work when the application service name is the result of following
a DNS redirection chain (e.g., via CNAME or DNAME).
5. Acknowledgements
Thanks to Eric Rescorla for the initial formulation of the use cases,
Zack Weinberg and Phillip Hallam-Baker for contributing other
requirements, and the whole DANE working group for helpful comments
on the mailing list.
6. Security Considerations
The primary focus of this document is the enhancement of TLS
authentication procedures using the DNS. The general effect of such
mechanisms is to increase the role of DNS operators in authentication
processes, either in place of or in addition to traditional third-
party actors such as commercial certificate authorities. The
specific security implications of the respective use cases are
discussed in their respective sections above.
7. References
7.1. Normative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, 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.
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[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.
7.2. Informative References
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
Author's Address
Richard Barnes
BBN Technologies
9861 Broken Land Parkway
Columbia, MD 21046
US
Phone: +1 410 290 6169
EMail: rbarnes@bbn.com
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