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INFORMATIONAL

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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