DANE R. Barnes
Internet-Draft BBN Technologies
Intended status: Informational April 29, 2011
Expires: October 31, 2011
Use Cases and Requirements for DNS-based Authentication of Named
Entities (DANE)
draft-ietf-dane-use-cases-02.txt
Abstract
Many current applications use the certificate-based authentication
features in TLS to allow clients to verify that a connected server
properly represents a desired domain name. Traditionally, this
authentication has been based on PKIX trust hierarchies, rooted in
well-known CAs, but additional information can be provided via the
DNS itself. This document describes a set of use cases in which the
DNS and DNSSEC could be used to make assertions that support the TLS
authentication process.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 31, 2011.
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
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. CA Constraints . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Certificate Constraints . . . . . . . . . . . . . . . . . 5
3.3. Domain-Issued Certificates . . . . . . . . . . . . . . . . 6
3.4. Delegated Services . . . . . . . . . . . . . . . . . . . . 7
3.5. Opportunistic Security . . . . . . . . . . . . . . . . . . 8
3.6. Web Services . . . . . . . . . . . . . . . . . . . . . . . 8
4. Other Requirements . . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Transport-Layer Security or TLS is used as the basis for security
features in many modern Internet applications [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].
One feature that is common to most uses of TLS is the use of
certificates to authenticate domain names for services. The TLS
client begins the TLS connection process with the goal of connecting
to a server with a specific domain name. (The process of obtaining
this domain name is application-specific. It could be entered by a
user or found through an automated discovery process, e.g., via an
SRV or NAPTR record.) After obtaining the address of the server via
an A or AAAA record, the client conducts a TLS handshake with the
server, during which the server presents a PKIX certificate for
itself [RFC5280]. Based on this certificate, the client decides
whether the server properly represents the desired domain name, and
thus whether to proceed with the TLS connection or not.
In most current applications, this decision process is based on PKIX
validation and application-specific name matching. The client
validates that the certificate chains to a trust anchor [RFC5280],
and that the desired domain name is contained in the certificate
[RFC6125]. Within this framework, bindings between public keys and
domain names are asserted by PKIX CAs. Authentication decisions
based on these bindings rely on the authority of these CAs.
The DNS is built to provide information about domain names, and with
the advent of DNSSEC [RFC1034][RFC4033], it is possible for this
information to be provided securely, in the sense that clients can
verify that DNS information was provided by the domain owner. The
goal of technologies for DNS-based Authentication of Named Entities
(DANE) is to use the DNS and DNSSEC to provide additional information
to inform the TLS domain authentication 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.
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.
Note in particular that the term "server" in this document refers to
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the server role in TLS, rather than to a host. Multiple servers of
this type may be co-located on a single physical host, using
different ports, and each of these can use different certificates.
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
goal for DANE.
In the below use cases, we will refer to the following dramatis
personae:
Alice The operator of a TLS-protected 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
Oscar An outsourcing provider that operates TLS-protected 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 protections to TLS
server certificates, since the use of these certificates to
authenticate server domain names is very common. In applications
where TLS clients are also identified by domain names (e.g., XMPP
server-to-server connections), the same considerations and use cases
can also be applied to 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. In TLS
terms, Alice is letting Bob know that Charlie's certificate must
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appear somewhere in the server Certificate message's certificate_list
structure.
When Bob connects to alice.example.com, he uses this mechanism to
verify that 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.
Because these constraints do not increase the scope of PKIX-based
assertions about domains, there is not a strict requirement for
DNSSEC. 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. In the
worst case, tampering with these constraints increases the risk of
false authentication to the level that is now standard.
Injected or modified false records 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 case 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 owners 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. 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 the
specific certificate issued by Charlie. 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.
When Bob connects to alice.example.com, he uses this mechanism to
verify that that the certificate presented by the server is the
correct certificate. Bob also performs the normal PKIX validation
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procedure for this certificate, in particular verifying that the
certificate chains to a trust anchor.
As in Section 3.1., Alice's assertions about server certificates can
be used to constrain the behavior of an outsourcing provider Oscar as
well as the CA Charlie and other CAs. Such a certificate constraint
requires Oscar to present the specified certificate to clients and
not another.
The other security considerations for this case are the same as for
the "CA Constraints" case above.
3.3. Domain-Issued Certificates
Alice would like to be able to use 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, however, he
doesn't have a way to verify that the issuer of the certificate is
actually Alice. 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 have a mechanism for visitors to her site to
know that the certificates she issues are actually hers. When Bob
connects to alice.example.com, he uses this mechanism to verify that
the certificate presented by the server was 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.
This use case is functionally equivalent to the case where Alice
doesn't issue her own certificates, but uses a CA Trent that 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.
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 Trent advertised by
Alice is not trustworthy, which would lead him to decide not to
accept Trent as a TA, and thus to reject Alice's certificate if it is
issued under Trent.
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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. 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 owner
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 owner of the domain.
This is not a significant incremental risk, however, relative to the
current PKIX-based system. 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 owner.
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 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.
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3. Alice can put DANE records into her DNS server, but delegate the
address records to Diane's DNS server. This means that Alice can
control the usage of certificates but Diane 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 setup (i.e. a CA or an EE certificate pointer).
Which of these deployment patterns is used in a given deployment will
determine what sort of constraints can be made. 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
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 make any requirements on the certificates that Oscar uses.
3.5. Opportunistic Security
Alice would like to to publish a web site so that Bob will always
have the benefit of the best security his client is capable of,
without resulting in a negative user experience when using a legacy
browser. For example, suppose that Bob uses two browsers on
different machines, one is a legacy browser that does not support
DANE and cannot be updated, the other is a browser that has full
support for DANE. In this case, the legacy browser should continue
to work as before, while the new browser should be able to discover
DANE support. In general, the DANE mechanism must allow a clients to
determine whether DANE security is available for a site.
3.6. Web Services
A web service is an HTTP-based Internet protocol designed to support
direct machine-to-machine communication without the intervention of a
human operator or other form of supervisor. Since web services are
application protocols, the one aspect of Internet architecture that
is essential as far as a Web Service is concerned is that the DNS be
used as the naming system for service discovery. Web Services
typically evolve over time. A service provider must frequently
support legacy clients alongside new and in many cases multiple
versions of each protocol. Discovering the certificates or keys to
be used to secure the connection to the Web service represents merely
one aspect of the more general problem of Web Service property
discovery.
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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 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 a DANE information for the name
alice.example.com, it must only affect services hosted at
alice.example.com.
Predictability: Client behavior in response to DANE information must
be spelled out in the DANE specification as precisely as possible,
especially for cases where DANE information might conflict with
PKIX information.
Simple Key Management: DANE should have a mode in which the domain
owner only needs to maintain a single long-lived public/private
key pair.
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.
Wild Cards and CNAME: The mechanism for distributing DANE
information should be compatible with the use of DNS wild cards
and CNAME records for setting default properties for domains and
redirecting services.
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.
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6. IANA Considerations
This document makes no request of IANA.
7. 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.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[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.
8.2. 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,
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June 2002.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 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.
Author's Address
Richard Barnes
BBN Technologies
9861 Broken Land Parkway
Columbia, MD 21046
US
Phone: +1 410 290 6169
Email: rbarnes@bbn.com
Barnes Expires October 31, 2011 [Page 11]
