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Internet Engineering Task Force (IETF)                         R. BarnesRequest for Comments: 6394                              BBN TechnologiesCategory: Informational                                     October 2011ISSN: 2070-1721Use Cases and Requirements for DNS-Based Authenticationof 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6394.Barnes                        Informational                     [Page 1]

RFC 6394                     DANE Use Cases                 October 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. Definitions .....................................................43. Use Cases .......................................................43.1. CA Constraints .............................................53.2. Service Certificate Constraints ............................63.3. Trust Anchor Assertion and Domain-Issued Certificates ......73.4. Delegated Services .........................................94. Other Requirements .............................................105. Acknowledgements ...............................................116. Security Considerations ........................................117. References .....................................................117.1. Normative References ......................................117.2. Informative References ....................................121.  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 PKIXBarnes                        Informational                     [Page 2]

RFC 6394                     DANE Use Cases                 October 2011   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.Barnes                        Informational                     [Page 3]

RFC 6394                     DANE Use Cases                 October 20112.  Definitions   This document also makes use of standard PKIX, DNSSEC, and TLS   terminology.  SeeRFC 5280 [RFC5280],RFC 4033 [RFC4033], andRFC 5246 [RFC5246], respectively, for these terms.  In addition,   terms related to TLS-protected application services and DNS names are   taken fromRFC 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.Barnes                        Informational                     [Page 4]

RFC 6394                     DANE Use Cases                 October 2011   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.Barnes                        Informational                     [Page 5]

RFC 6394                     DANE Use Cases                 October 2011   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].Barnes                        Informational                     [Page 6]

RFC 6394                     DANE Use Cases                 October 2011   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 inSection 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.Barnes                        Informational                     [Page 7]

RFC 6394                     DANE Use Cases                 October 2011   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.Barnes                        Informational                     [Page 8]

RFC 6394                     DANE Use Cases                 October 20113.4.  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.Barnes                        Informational                     [Page 9]

RFC 6394                     DANE Use Cases                 October 20114.  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.Barnes                        Informational                    [Page 10]

RFC 6394                     DANE Use Cases                 October 2011   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.  References7.1.  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.Barnes                        Informational                    [Page 11]

RFC 6394                     DANE Use Cases                 October 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 2011.7.2.  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.comBarnes                        Informational                    [Page 12]