DANE R. Barnes
Internet-Draft BBN Technologies
Intended status: Informational June 12, 2011
Expires: December 14, 2011
Use Cases and Requirements for DNS-based Authentication of Named
Entities (DANE)
draft-ietf-dane-use-cases-03.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
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This Internet-Draft will expire on December 14, 2011.
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. CA Constraints . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Certificate Constraints . . . . . . . . . . . . . . . . . 6
3.3. Domain-Issued Certificates . . . . . . . . . . . . . . . . 6
3.4. Delegated Services . . . . . . . . . . . . . . . . . . . . 8
4. Other Requirements . . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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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. 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 validation of the
certificate, including verification that the certificate chains to a
trust anchor. 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.
Thus 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.
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
holder 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 named 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, 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 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
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., XMPP server-to-server connections),
the same considerations and use cases can applied to the application
server's checking of identities in TLS client certificates.
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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 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 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 RPs. 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 guaranteed 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.
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.
Nonetheless, using DANE in this way without also using DNSSEC
represents 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
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DNS records can also fake DANE information (without DNSSEC) by simply
spoofing the additional records required to carry the DANE
information.
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 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. 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 [RFC5246].
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
procedure for this certificate, in particular verifying that the
certificate chains to a trust anchor.
The 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 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
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that the issuer of the certificate is actually Alice, since 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 have a mechanism for visitors to her site to
know that the certificates presented by her application services are
legitimately hers. When Bob connects to alice.example.com, he uses
this mechanism 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 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.
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 TA, 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).
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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.
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 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 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
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.
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 spelled out 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 clients 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 if DANE information were not present.
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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 its own CA (Section Section 3.3) and
require that no other be used (Section Section 3.1).
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 Section 3.3) to using DANE to require
an external CA (Section 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.
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: The mechanism for distributing DANE information should
allow the use of DNS wild card labels (*) for setting DANE
information for all names within a wild card expansion.
Redirection: The mechanism for distributing DANE information should
work when the application service name is the result of following
a DNS redirection time (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. IANA Considerations
This document makes no request of IANA.
7. Security Considerations
The primary focus of this document is the enhancement of TLS
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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
[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,
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.
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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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