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Versions: 00

Independent Submission                                         C. Mehner
Internet-Draft                                                      USAA
Intended status: Standards Track                       November 20, 2014
Expires: May 24, 2015


                     HTTP DANE Validation Assertion
                      draft-cem-dane-assertion-00

Abstract

   This document defines a new HTTP header that allows web host
   operators to instruct user agents to remember the hosts' request for
   DANE (DNS-Based Authentication of Named Entities) validation over a
   period of time.  During that time, UAs will require that the host
   presents a certificate chain that will authenticate the Transport
   Layer Security connection using DANE.  By having hosts explicitly
   state that they have adopted DANE, UAs will only expend resources
   attempting DANE validation on hosts that request it.  Comments are
   solicited and should be addressed to the author

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 24, 2015.

Copyright Notice

   Copyright (c) 2014 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



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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Server and Client Behavior  . . . . . . . . . . . . . . . . .   3
     2.1.  Response Header Field Syntax  . . . . . . . . . . . . . .   3
       2.1.1.  The max-age Directive . . . . . . . . . . . . . . . .   4
       2.1.2.  The includeSubDomains Directive . . . . . . . . . . .   5
       2.1.3.  The required Directive  . . . . . . . . . . . . . . .   5
       2.1.4.  Examples  . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   6
       2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   6
       2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   6
     2.3.  User Agent Processing Model . . . . . . . . . . . . . . .   6
       2.3.1.  DANE-Validation Response Header Field Processing  . .   7
       2.3.2.  Noting a DANE Host - Storage Model  . . . . . . . . .   7
       2.3.3.  HTTP-Equiv <Meta> Element Attribute . . . . . . . . .   9
     2.4.  Noting DANE Hosts . . . . . . . . . . . . . . . . . . . .   9
     2.5.  Validating DANE Connections . . . . . . . . . . . . . . .   9
     2.6.  Interactions With Preloaded DANE Host Lists . . . . . . .  10
     2.7.  TLSA Certificate Usages in DANE . . . . . . . . . . . . .  10
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     3.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Using includeSubDomains Safely  . . . . . . . . . . . . .  11
     3.3.  Interactions With Cookie Scoping  . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   5.  Usability Considerations  . . . . . . . . . . . . . . . . . .  13
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document defines a new HTTP header that enables user agents
   (UAs) a web host to know which hosts upon which they should perform a
   DANE [RFC6698] validation.  This is called an "HTTP DANE Validation
   Assertion" (HDVA).  The goal of this proposal is to raise the
   adoption of DANE in web hosts by addressing the cost of attempting
   DANE Validation on every host via a mechanism that allows web hosts
   to declare that they use DANE.  Using this header also can give hosts
   the functionality of HTTP-based public key pinning
   [I-D.ietf-websec-key-pinning] while gaining the greater flexibility
   of DANE.



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   UAs performing DANE validation on every HTTPS connection will not
   benefit from this header, however conformant UAs will use DANE on
   connections subsequent to the initial time the host is noted.  Those
   hosts will not be able to detect and thwart a MITM attacking the UA's
   first connection to the host.  However, the requirement that the MITM
   provide an X.509 certificate chain that can pass the UA's validation
   requirements without error (UAs are assumed to use either [RFC5280]
   or [RFC6698] to verify cryptographic identity) or control over a DNS-
   SEC zone mitigates this risk somewhat.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Server and Client Behavior

2.1.  Response Header Field Syntax

   The DANE-Validation HTTP response header field (DVA header field)
   indicates to a UA that it should perform DANE Validation ([RFC6698])
   in regards to the host emitting the response message containing this
   header field.

   Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
   header fields, using the grammar defined in [RFC5234] and the rules
   defined in Section 3.2 of [RFC7230].

   DANE-Validation-Directives = directive *( OWS ";" OWS directive )

   directive             = directive-name [ "=" directive-value ]
   directive-name        = token
   directive-value       = token
                         / quoted-string


                       Figure 1: HDVA Header Syntax

   Optional white space (OWS) is used as defined in Section 3.2.3 of
   [RFC7230].  Token and quoted-string are used as defined in
   Section 3.2.6 of [RFC7230].

   The directives defined in this specification are described below.
   The overall requirements for directives are:

   1.  The order of appearance of directives is not significant.




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   2.  A given directive MUST NOT appear more than once in a given
       header field.  Directives are either optional or required, as
       stipulated in their definitions.

   3.  Directive names are case-insensitive.

   4.  UAs MUST ignore any header fields containing directives, or other
       header field value data, that do not conform to the syntax
       defined in this specification.  In particular, UAs must not
       attempt to fix malformed header fields.

   5.  If a header field contains any directive(s) the UA does not
       recognize, the UA MUST ignore those directives.

   6.  If the DVA header field otherwise satisfies the above
       requirements (1 through 5), the UA MUST process the directives it
       recognizes.

   Additional directives extending the semantic functionality of the
   header fields can be defined in other specifications.  The first such
   specification will need to define a registry for such directives.
   Such future directives will be ignored by UAs implementing only this
   specification, as well as by generally non-conforming UAs.

   When a connection passes DANE Validation after noting the DVA header,
   the host becomes a Known DANE Host.

2.1.1.  The max-age Directive

   The REQUIRED "max-age" directive specifies the number of seconds,
   after the reception of the DVA header field, during which the UA
   SHOULD regard the host (from whom the message was received) as a
   Known DANE Host.

   The syntax of the max-age directive's REQUIRED value (after quoted-
   string unescaping, if necessary) is defined as:

   max-age-value = delta-seconds
   delta-seconds = 1*DIGIT

                      Figure 2: max-age Value Syntax

   delta-seconds is used as defined in [RFC7234], Section 1.2.1.

   See Section 2.3.2 for limitations on the range of values for max-age.






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2.1.2.  The includeSubDomains Directive

   The OPTIONAL includeSubDomains directive is a valueless directive
   that, if present (i.e., it is "asserted"), signals to the UA that the
   DANE Policy applies to this DANE Host as well as any subdomains of
   the host's domain name.

2.1.3.  The required Directive

   The OPTIONAL required directive is a valueless directive that, if
   present (i.e., it is "asserted"), signals to the UA that it MUST
   perform a DANE validation and if no DANE information is received (via
   network lookup or cache) the UA MUST NOT start a TLS connection or it
   MUST abort the TLS handshake.  Because Section 4.1 of [RFC6698]
   allows fallback from DANE to PKIX, this directive is in place for
   Host operators to force both PKIX and DANE validation to take place
   to provide the additional protection on the connection.

2.1.4.  Examples

   The HDVA header field below stipulates that the HDVA Policy is to
   remain in effect for one year (there are approximately 31536000
   seconds in a year), and the policy applies only to the domain of the
   HDVA Host issuing it:

   DANE-Validation: max-age=31536000

   The HDVA header field below stipulates that the HDVA Policy is to
   remain in effect for approximately six months and that the policy
   applies to the domain of the issuing HDVA Host and all of its
   subdomains:

   DANE-Validation: max-age=15768000 ; includeSubDomains

   The max-age directive value can optionally be quoted:

   DANE-Validation: max-age="31536000"

   The HDVA header field below indicates that the UA must delete the
   entire HDVA Policy associated with the HDVA Host that sent the header
   field:

   DANE-Validation: max-age=0

   The HDVA header field below has exactly the same effect as the one
   immediately above because the includeSubDomains directive's presence
   in the HDVA header field is ignored when max-age is zero:




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   DANE-Validation: max-age=0; includeSubDomains

   The HDVA header field below states that if the UA MUST receive and
   validate DANE information, and if it does not it MUST close the
   connection:

   DANE-Validation: max-age=15768000; required

2.2.  Server Processing Model

   This section describes the processing model that DANE Hosts
   implement.  The model has 2 parts: (1) the processing rules for HTTP
   request messages received over a secure transport (e.g.
   authenticated, non-anonymous TLS); and (2) the processing rules for
   HTTP request messages received over non-secure transports, such as
   TCP.

2.2.1.  HTTP-over-Secure-Transport Request Type

   When replying to an HTTP request that was conveyed over a secure
   transport, a DANE Host SHOULD include in its response exactly one DVA
   header field and MUST satisfy the grammar specified in Section 2.1.

   Establishing a given host as a Known DANE Host, in the context of a
   given UA, is accomplished as follows:

   1.  Over the HTTP protocol running over secure transport, by
       correctly returning (per this specification) at least one valid
       DVA header field to the UA.

   2.  Through other mechanisms, such as a client-side pre-loaded Known
       DANE Host List.

2.2.2.  HTTP Request Type

   DANE Hosts MAY include the DVA header field in HTTP redirects
   conveyed over non-secure transport.  Hosts may choose to do this if
   they wish to operate using the TLSA usages of DANE-EE or DANE-TA (as
   defined in [RFC7218].

2.3.  User Agent Processing Model

   The UA processing model relies on parsing domain names.  Note that
   internationalized domain names SHALL be canonicalized according to
   the scheme in Section 10 of [RFC6797].






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2.3.1.  DANE-Validation Response Header Field Processing

   If the UA receives, over a secure transport, an HTTP response that
   includes a DVA header field conforming to the grammar specified in
   Section 2.1, and there are no underlying secure transport errors or
   warnings (see Section 2.4), the UA MUST either:

   o  Note the host as a Known DANE Host if it is not already so noted
      (see Section 2.3.2)

   or

   o  Update the UA's cached information for the Known DANE Host if any
      of the max-age, includeSubDomains, or required header field value
      directives convey information different from that already
      maintained by the UA.

   The max-age value is essentially a "time to live" value relative to
   the time of the most recent observation of the DVA header field.  If
   the max-age header field value token has a value of 0, the UA MUST
   remove its cached DANE Policy information (including the
   includeSubDomains directive, if asserted) if the DANE Host is Known,
   or, MUST NOT note this DANE Host if it is not yet Known.

   If a UA receives more than one DVA header field in an HTTP response
   message over secure transport, then the UA MUST process only the
   first DVA header field.

   If the UA receives the HTTP response over non-secure transport it
   MUST NOT note the host as a DVA host.  To allow the validation of
   DANE-EE and DANE-TA TLSA usages, a UA MAY accept DVA headers as a
   'hint' to perform DANE Validation on the connection.

   If the DVA header is not a Valid DANE Header (see Section 2.4), the
   UA MUST ignore any present DVA header field(s).  The UA MUST ignore
   any DVA header fields not conforming to the grammar specified in
   Section 2.1.

2.3.2.  Noting a DANE Host - Storage Model

   The Effective Date of a Known DANE Host is the time that the UA
   observed a Valid DANE Header for the host.  The Effective Expiration
   Date of a Known DANE Host is the Effective Date plus the max-age.  A
   Known DANE Host is "expired" if the Effective Expiration Date refers
   to a date in the past.  The UA MUST ignore any expired Known DANE
   Hosts in its cache.





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   For example, if a UA is beginning to perform DANE Validation for a
   Known DANE Host and finds that the cached information for the host
   indicates an Effective Expiration Date in the past, the UA MUST NOT
   continue with DANE Validation for the host, and must consider the
   host to no longer be a Known DANE Host.

   Known DANE Hosts are identified only by domain names, and never IP
   addresses.  If the substring matching the host production from the
   Request-URI (of the message to which the host responded)
   syntactically matches the IP-literal or IPv4address productions from
   Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
   Known DANE Host.

   Otherwise, if the substring does not congruently match an existing
   Known DANE Host's domain name, per the matching procedure specified
   in Section 8.2 of [RFC6797], then the UA MUST add this host to the
   Known DANE Host cache.  The UA caches the following information:

   o  the DANE Host's domain name

   o  the Effective Expiration Date, or enough information to calculate
      it (the Effective Date and the value of the max-age directive)

   o  whether or not the includeSubDomains directive is asserted

   o  whether or not the required directive is asserted

   If any other metadata from optional or future DVA header directives
   are present in the Valid DANE Header, and the UA understands them,
   the UA MAY note them as well.

   UAs MAY set an upper limit on the value of max-age, so that UAs that
   have noted erroneous DANE Validation Assertions (whether by accident
   or due to attack) have some chance of aging out over time.  If the
   server sets a max-age greater than the UA's upper limit, the UA MAY
   behave as if the server set the max-age to the UA's upper limit.  For
   example, if the UA caps max-age at 5184000 seconds (60 days), and a
   DANE Host sets a max-age directive of 90 days in its Valid DANE
   Header, the UA MAY behave as if the max-age were effectively 60 days.
   (One way to achieve this behavior is for the UA to simply store a
   value of 60 days instead of the 90 day value provided by the DANE
   Host.)  For UA implementation guidance on how to select a maximum
   max-age, see Section 3.1.

   The UA MUST NOT modify any metadata of any superdomain matched Known
   DANE Host.





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2.3.3.  HTTP-Equiv <Meta> Element Attribute

   UAs MUST NOT heed http-equiv="DANE-Validation" attribute settings on
   <meta> elements [W3C.REC-html401-19991224] in received content.

2.4.  Noting DANE Hosts

   Upon receipt of the DVA response header field, the UA notes the host
   as a Known DANE Host, storing the Host and associated directives in
   non-volatile storage (for example, along with the HSTS or HPKP
   metadata).  The associated directives are collectively known as DANE
   Metadata.

   The UA MUST note the Host as a DANE Host if and only if it received
   the DVA response header field over an error-free TLS connection.  If
   the host is a DANE Host, this includes the validation added in
   Section 2.5.

   If the DVA response header field does not meet the above criteria,
   the UA MUST NOT note the host as a DANE Host.  A DVA response header
   field that meets all these criteria is known as a Valid DANE Header.

   Whenever a UA receives a Valid DANE Header, it MUST set its DANE
   Metadata to the exact Effective Expiration Date (computed from max-
   age), and note any associated directives if present.

   For forward compatibility, the UA MUST ignore any unrecognized DVA
   header directives, while still processing those directives it does
   recognize.  Section 2.1 specifies the directives max-age,
   includeSubDomains, and required but future specifications and
   implementations might use additional directives.

2.5.  Validating DANE Connections

   When a UA connects to a Known DANE Host using a TLS connection, the
   UA SHOULD perform a DANE Validation on the Host, as soon as possible
   (e.g. immediately after receiving the Server Certificate message).  A
   DANE Validation follows the procedure for comparing a certificate
   association from a TLSA record and a certificate from the TLS
   handshake as defined in [RFC6698].)

   If no TLSA records were received for evaluation and the host's DANE
   Metadata includes an asserted required directive, the UA MUST
   terminate the connection.

   If the connection has no errors in the DANE Validation, the UA will
   determine whether to apply any additional correctness checks such as




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   Pin Validation [I-D.ietf-websec-key-pinning], or applying an HTTP
   Strict Transport Security Policy [RFC6797].

2.6.  Interactions With Preloaded DANE Host Lists

   UAs MAY choose to implement additional sources of DANE Host
   information, such as through built-in lists of host information.
   Such UAs should allow users to override such additional sources,
   including disabling them from consideration.

   The effective policy for a Known DANE Host that has both built-in
   hosts and hosts from previously observed DVA header response fields
   is implementation-defined.

2.7.  TLSA Certificate Usages in DANE

   HDVA is able to interoperate with UAs that support DANE and those
   that do not.  For Hosts that use PKIX-TA or PKIX-EE certificate
   validation will occur both with and without DANE.  Hosts that expect
   to use DANE-TA or DANE-EE should not expect to interoperate with UAs
   that do not support DANE.  Conversely, hosts that choose PKIX-TA or
   PKIX-EE should not expect full interoperation with UAs that do not
   include a full list of trust authorities.

   UAs that choose to accept and validate DVA headers over non-secure
   transport as a 'hint' to perform a DANE Validation MUST do so in
   according with Section 2.3.1 and MUST allow DANE-TA and DANE-EE
   usages for the initial connection and given a successful DANE
   Validation note the TLS connection as error-free.

3.  Security Considerations

   HTTP DANE-Validation Assertions allow hosts to strongly assert their
   intention for additional validation of their cryptographic identity.
   This document concerns the expression, conveyance, and enforcement of
   the DANE Validation policy.

   See [RFC6698] for security considerations relating to a DANE
   Validation.

   This document adds the concept of a required directive that requires
   the UA to receive TLSA records when communicating with a DANE Host
   over secure transport.  When the host asserts the required directive
   there is additional risk of an active network attacker blocking DANE
   information from reaching the UA.  This scenario would effectively
   create a denial of service for the victim of the attack.  This is
   also a concern in some networks, which are configured in such a
   manner as to effectively block DANE information.  If a host chooses



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   to assert the required directive, they should consider clients that
   may not be able to get DANE information and consider the associated
   risks when asserting that directive.  Without the required directive,
   an active network attacker could potentially block DANE information
   from reaching the victim and force validation of the connection to
   precede without any DANE information this would circumvent the
   additional out-of-band checks and rely on the UA's normal
   cryptographic identity validation, which could allow an attacker to
   man-in-the-middle the connection with a certificate that would
   otherwise fail DANE Validation.

   To help mitigate this risk UAs SHOULD cache TLSA records as mentioned
   in Section 8.2 of [RFC6698].  In addition, if a UA does a pre-fetch
   for IP address, they should also prefetch TLSA records for DANE
   Hosts.  Furthermore, UAs can receive TLSA records through another
   medium such as TLS extensions, HTTP, or other methods, which may not
   be blocked or be faster than DNS itself.

   To discover attacks on a host that does not assert the required
   directive, over a network where an attacker is blocking the TLSA
   records from reaching a UA, the host may also employ the Report-Only
   directive from [I-D.ietf-websec-key-pinning].

3.1.  Maximum max-age

   As mentioned in Section 2.3.2, UAs MAY cap the max-age value at some
   upper limit.  There is a security trade-off in that low maximum
   values provide a narrow window of protection for users who visit the
   Known DANE Host only infrequently, while high maximum values may
   potentially result in a UA's inability to successfully perform DANE
   Validation on hosts that assert the required directive should they
   choose to remove the TLSA record from the domain.  Also, if a host
   has removed the TLSA records, a long max-age would create longer
   initial connection times while the UA attempts to retrieve a non-
   existent TLSA record.

   There is likely no ideal upper limit to the max-age directive that
   would satisfy all use cases.  However, a value on the order of 60
   days (5,184,000 seconds) may be considered a balance between the two
   competing security concerns.

3.2.  Using includeSubDomains Safely

   It may happen that DANE Hosts whose hostnames share a parent domain
   use different Valid DVA Headers.  If a host whose hostname is a
   parent domain for another host sets the includeSubDomains directive,
   the two hosts' DANE policies may conflict with each other.  For
   example, consider two Known DANE Hosts, example.com and



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   subdomain.example.com.  Assume example.com sets a Valid DVA Header
   such as this:

   DANE-Validation: max-age=12000; required; includeSubDomains

                  Figure 3: example.com Valid DVA Header

   Assume subdomain.example.com sets a Valid DVA Header such as this:

   DANE-Validation: max-age=12000;

             Figure 4: subdomain.example.com Valid DVA Header

   Assume a UA that has not previously noted either of these hosts as
   DANE Hosts.  If the UA first contacts subdomain.example.com, it will
   note the host as a DANE Host, and attempt DANE Validation as normal
   on subsequent connections.  If the UA does not receive a TLSA record
   for subdomain.example.com it will fall back to the UA's normal
   certificate path validation.  If the UA then contacts example.com, it
   will note the DANE Host and require DANE Validation on future
   connections.

   However, if the UA happened to visit example.com before
   subdomain.example.com, the UA would, due to example.com's use of the
   includeSubDomains and required directives, require a valid TLSA
   record to perform DANE Validation for subdomain.example.com.  If such
   a record was not present or available, the UA will cancel the
   connection.  Thus, depending on the order in which the UA observes
   the Valid DVA Headers for hosts example.com and
   subdomain.example.com, DANE Validation might or might not fail for
   subdomain.example.com, if it cannot receive any TLSA records.  This
   can occur even if the certificate chain the UA receives for
   subdomain.example.com is perfectly valid.

   Thus, DANE Host operators must use the includeSubDomains directive
   with care.  For example, they may choose to use the required
   directive only on Hosts that do not assert the includeSubDomains
   directive, those that do not have any child domains, or to only use
   the required directive on HOSTs whose child domains are all assured
   to receive TLSA records via TLS extensions or some other pre-arranged
   means.

3.3.  Interactions With Cookie Scoping

   HTTP cookies [RFC6265] set by a Known DANE Host can be stolen by a
   network attacker who can forge web and DNS responses so as to cause a
   client to send the cookies to a phony subdomain of the host.  To
   prevent this, hosts SHOULD set the "secure" attribute and precisely



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   scope the "domain" attribute on all security-sensitive cookies, such
   as session cookies.  These settings tell the browser that the cookie
   should only be sent back to the specific host(s) (not any arbitrary
   subdomain of a given domain), and should only be sent over HTTPS (not
   HTTP).

4.  IANA Considerations

   IANA is requested to register the response headers described in this
   document in the "Message Headers" registry ([permanent-headers]),
   with the following parameters:

   o  Header Field Names should be "DANE-Validation"

   o  Protocol should be "http"

   o  Status should be "standard"

   o  Reference should be this document

5.  Usability Considerations

   To keep backwards compatibility with non-conforming UAs a host may
   choose to provide PKIX-TA or PKIX-EE TLSA records combined with the
   required directive in the DVA Header in order to provide a protection
   against imposter certificates.  When the UA encounters a situation
   like that where it would prevent the connection from continuing,
   users will experience a denial of service.  It is advisable for UAs
   to explain the reason why, i.e. that it was impossible to verify the
   confirmed cryptographic identity of the host.

   It is advisable that UAs have a way for users to clear the current
   list of DANE Hosts, and to allow users to query the current state of
   DANE Hosts.

6.  Acknowledgements

   Thanks to the websec working group for the Public-Key-Pinning draft,
   from which this document draws heavily.

7.  Normative References

   [I-D.ietf-websec-key-pinning]
              Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", draft-ietf-websec-key-pinning-21
              (work in progress), October 2014.





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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 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.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, April 2014.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
              2014.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

   [permanent-headers]
              Klyne, G., "Permanent Message Header Field Names", July
              2014, <http://www.iana.org/assignments/message-headers/
              message-headers.xml#perm-headers/>.




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Author's Address

   Carl Mehner
   USAA
   9800 Fredericksburg
   San Antonio, TX  78288
   US

   Email: carl.mehner@usaa.com










































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