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Network Working Group                                       Y. Pettersen
Internet-Draft                                        Opera Software ASA
Updates: RFC 2965                                           July 9, 2007
(if approved)
Intended status: Standards Track
Expires: January 10, 2008

   The TLD Subdomain Structure Protocol and its use for Cookie domain

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document defines a protocol and specification format that can be
   used by a client to discover how a Top Level Domain (TLD) is
   organized in terms of what subdomains are used to place closely
   related but independent domains, e.g. commercial domains in country
   code TLDs (ccTLD) like .uk are placed in the .co.uk subTLD domain.

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   This information is then used to limit which domains an Internet
   service can set cookies for, strengthening the rules already defined
   by the cookie specifications.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.  Introduction

   The Domain Name System [RFC1034] used to name Internet hosts allows a
   wide range of hierarchical names to be used to indicate what a given
   host is, some implemented by the owners of a domain, such as creating
   subdomains for certain tasks or functions, others by the Top Level
   Domain registry owner to indicate what kind of service the domain is,
   e.g. commercial, educational, government or geographic location, e.g.
   city or state.

   While this system makes it relatively easy for TLD administrators to
   organize online services, and for the user to locate and recognize
   relevant services, this flexibility causes various security and
   privacy related problems when services located at different hosts are
   allowed to share data through functionality administrated by the
   client, e.g.  HTTP state management cookies [RFC2965] [NETSC].  Most
   information sharing mechanisms make the process of sharing easy,
   perhaps too easy, since in many cases there is no mechanism to ensure
   that the servers receiving the information really want it, and it is
   often difficult to determine the source of the information being
   shared.  To some extent [RFC2965] addresses some of these concerns
   for cookies, in that clients that supports [RFC2965] style cookies
   sends the target domain for the cookie along with the cookie so that
   the recipient can verify that the cookie has the correct domain.
   Unfortunately, [RFC2965] is not widely deployed in clients, or on
   servers.  The recipient(s) can make inappropriate information sharing
   more difficult by requiring the information to contain data
   identifying the source and assuring the integrity of the data, e.g.
   by use of cryptographic technologies.  These techniques tend,
   however, to be computationally costly.  There are two problem areas:

   o  Incorrect sharing of information between non-associated services
      e.g. example1.com and example2.com or example1.co.uk and
      example2.co.uk.  That is, the information may be distributed to
      all services within a given Top Level Domain.

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   o  Undesirable information sharing within a single service.  This is,
      in particular, a problem for services that sell hosting services
      to many different customers, such as webhotels, where the service
      itself has little or no control of the customers actions.

   While both these problems are in some ways similar, they call for
   different solutions.  This specification will only propose a solution
   for the first problem area.  The second problem area must be handled
   separately.  This specification will first define a TLS Subdomain
   Structure Protocol that can be used to discover the actual structure
   of a Top Level Domain e.g. that the TLD have several subTLDs co.tld,
   ac.tld, org.tld, then it will show how this information can be used
   to determine when information sharing through cookies is not

2.  The TLD Subdomain Structure Protocol

   The TLD Subdomain Structure Protocol is an HTTP service, managed by
   the TLD owner, and located at a well known URI location that, when
   queried, returns information about a TLD's domain structure.  The
   client can then use this information to decide what actions are
   permitted for the protocol data the client is processing.  Procedure
   for use:

   o  The client should retrieve the domain list for the Top Level
      Domain "tld" from https://www.subdomains.tld/tld/domainlist .
      [The actual location must be decided by IANA, this section
      contains the author's suggestion.  Due to security considerations
      it should be considered whether or not an https URL, or at least a
      signed file should be used]

   o  If the client is not able to retrieve a list from the "subdomains"
      server it MAY attempt to retrieve a list from a vendor specified
      location.  Alternatively, the vendor MAY mirror the list from the
      "subdomains" server(s), and only retrieve the lists from the
      vendor specified location.

   o  The Content-Type of the returned list MUST be application/

   o  The retrieved specification SHOULD be cached for at least 30 days

   o  The TLD owner SHOULD update the list at least 90 days before a new
      sub-domain becomes active.

   o  If no specification can be retrieved the user agent MAY fall back
      to alternative methods, depending on the profile.

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2.1.  Securing the domain information

   Individuals with malicious intent may wish to modify the domain list
   served by the service location to either classify a domain
   incorrectly as a subTLD or to hide a subTLD's classification.  Beside
   obviously securing the hosting locations, this also means that the
   content served will have to be secured.

   1.  Digitally sign the specification, using one of the available
       message signature methods, e.g.  S/MIME [RFC2311].  This will
       secure the content during storage both at the client and the
       server, as well as during transit.  The drawback is that the
       client must implement decoding and verification of the message
       format which it may not already support, which may be problematic
       for clients having limited resources.

   2.  Using an encrypted connection, such as HTTP over TLS [RFC2818],
       which is supported by many clients already.  Unfortunately, this
       method does not protect the content when stored by the client.

   3.  Use XML Signatures [RFC3275] to create a signature over the
       specification.  This method is currently not defined.

   This specification recommends using HTTP over TLS, and the client
   MUST use the non-anonymous cipher suites, to secure the transport of
   the specification.  The client MUST ensure that the hostname in the
   certificate matches the hostname used in the request.

2.2.  Domainlist format

   The domain list file can contain a list of subdomains that are
   considered top level domains, as well as a special list of names that
   are not top level domains.  None of the domain lists need specify the
   TLD name, since that is implicit from the request URI.  The domain
   names listed MUST be encoded in punycode, according to [RFC3490].

2.2.1.  Domainlist schema

   The domain list is an XML file that follows the following schema

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   default namespace = "http://xmlns.opera.com/tlds"

   start =
       element tld {
         attribute levels { xsd:integer }?,
         attribute name { xsd:NCName }?,
         (domain | registry)*
   registry =
     element registry {
       attribute levels { xsd:integer }?,
       attribute name { xsd:NCName },
       (domain | registry)*
   domain =
     element domain {
       attribute name { xsd:NCName }

   The domainlist file contains a single <tld> block, which may contain
   multiple registry and domain blocks, and a registry block may also
   contain multiple registry and domain blocks.

   Both domain and registry tags MUST contain a name attribute
   identifying the domain or registry.  The tld block MAY have a name
   attribute, but this name MUST be ignored by clients, which must
   instead use the name of the TLD used to request the file.

   All names MUST be punycode encoded to make it possible for clients
   not supporting IDNA to use the document.

   The tld and registry blocks MAY contain an attribute, levels,
   specifying how many levels below the current domain are registry-
   like.  The default is none, meaning that the default inside the
   current domain level is that labels are ordinary domains and not
   registry-like.  If the levels attribute is 1 (one) it means that by
   default all next-level labels within the registry/tld are registry
   like and not normal domains.

   Implementations MUST ignore attributes and syntax they do not

2.2.2.  Domainlist interpretation

   For each new registry or domain block within the tld or registry the
   effective domain name the block applies to is the name of the block
   prepended to the ".name" of the effective domain name of the
   containing block.

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   For the tld block the effective domain name is the name of the TLD
   the client is evaluating, and for the registry block named "example"
   the effective name becomes example.tld.
   <?xml version="1.0" encoding="UTF-8"?>
   <tld xmlns="http://xmlns.opera.com/tlds" name="tld" levels=1 >
       <registry name="co" levels="0">
         <registry name="state" />
       <registry name="example" levels="1"/ >
       <domain name="parliament"/>

   In the above example, the specification is for the TLD "tld".  By
   default any second level domain "x.tld" is a registry-like domain,
   although parliament.tld is not a registry-like domain

   In the example TLD, however, the co.tld registry has a sub registry
   "state.co.tld", while all other domains in the co.tld domains are
   ordinary domains.

   Also, the registry example.tld has defined all domains y.example.tld
   as registry like, with no exceptions.

3.  A TLD Subdomain Structure Protocol profile for Cookies

   HTTP State management cookies is one area where it is important, both
   for security and privacy reasons, to ensure that unauthorized
   services cannot set cookies for another service.  Inappropriate
   cookies can affect the functionality of a service, but may also be
   used to track the users across services in an undesirable fashion.
   Neither the original Netscape cookie specification [NETSC] nor
   [RFC2965] are adequate in many cases.

   The [NETSC] rules require only that the target domain must have one
   internal dot (e.g. example.com) if the TLD belong to a list of
   generic TLDs (gTLD), while for all TLDS the domain must contain two
   internal dots (e.g. example.co.uk).  The latter rule was never
   properly implemented, in particular due to the many flat ccTLD domain
   structures that are in use.

   [RFC2965] set the requirement that cookies can only be set for the
   server's parent domain.

   Unfortunately, both policies still leave open the possibility of
   setting cookies for a subTLD by setting the cookie from a host name
   example.subtld.tld to the domain subtld.tld, which is by itself
   legal, but not desirable because that means that the cookie can be

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   sent to numerous websites either revealing sensitive information, or
   interfering with those other websites without authroization.  As can
   be seen, these rules do not work satisfactorily, especially when
   applied to ccTLDs, which may have a flat domain structure similar to
   the one used by the generic .com TLD, a hierarchical subTLD structure
   like the one used by the .uk ccTLD (e.g. .co.uk), or a combination of
   both.  But there are also gTLDs, such as .name, for which cookies
   should not be allowed for the second level domains, as these are
   generally family names shared between many different users, not
   service names.  A partially effective method for distinguishing
   servicenames from subTLDs by using DNS has been defined in
   [DNSCOOKIE].  However this method is not immune to TLD regsitries
   that uses subTLDs as directories, or to services that does not define
   an IP address for the domainname.  Using the TLD Subdomain Structure
   Protcol to retrieve a list of all subTLDs in a given TLD will solve
   both those problems.

3.1.  Procedure for using the TLD Subdomain Structure Protcol for

   When receiving a cookie the client must first perform all the checks
   required by the relevant specification.  Upon completion of these
   checks the client then performs the following additional verification
   checks if the cookie is being set for the server's parent, grand-
   parent domain (or higher):

   1.  If the domain structure of the TLD is not known already, or the
       structure information has expired, the client should retrieve or
       validate the structure specification from the server hosting the
       specification, according to section 2.  If retrieval is
       unsuccessful, and no copy of the specification is known, the
       client MAY use alternative methods to decide the domain's status,
       e.g. those described in [DNSCOOKIE], or other heuristics.
       Evaluate the specification as specified in section 2.  If the
       target domain is part of the subTLD structure the cookie MUST be

   2.  If the target domain is not a subTLD, the cookie is accepted.

3.2.  Unverifiable transactions

   Use of HTTP Cookies, combined with HTTP requests to resources that
   are located in domains other than the one the user actually wants to
   visit, have caused widespread privacy concerns.  The reason is that
   multiple websites can link to the same independent website, e.g. an
   advertiser, who may then use cookies to build a profile of the
   visitor, that can be used to select advertisements that are of
   interest to the user.

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   [RFC2965] specified that if the name of the host of an included
   resource does not domain match the domain reach (defined as the
   parent domain of the host) of the URL of the document the user
   started loading, loading the resource is considered an unverifiable
   transcation, and in such third party transactions cookies should not
   be sent or accepted.  The latter point is not widely implemented,
   except when selected by especially interested users.  This means that
   server1.example.com and server2.example.com can share cookies, and
   either can be referenced automatically (e.g. by including an image)
   by the other without being considered an unverifiable transaction,
   while requests to server3.example2.com would be considered an
   unverifiable transaction.  However, like the normal domain matching
   rule for cookies, this rule opens up some holes.  If the host
   example.co.uk requests a resource from server4.example3.co.uk, the
   request to example3.co.uk server would not be considered an
   unverifiable transaction because example.co.uk's reach is co.uk,
   which domain matches server4.example3.co.uk, a conclusion which is
   obviously, to a human with some knowlegde of the .uk domain
   structure, incorrect.

   To avoid such misclassifications clients SHOULD apply the procedure
   specified in 3.1 for the reach domain used to decide if a request is
   an unverifiable, and if the reach domain is a subTLD, the reach of
   the original host must be changed to become the same as the name of
   the host itself, and requests that do not domain match the original
   host's name must be considered unverifiable transactions.  That is,
   the reach for example.co.uk becomes example.co.uk, not co.uk, and
   example3.co.uk will therefore not domain match the resulting reach.

4.  Examples

   The following examples demonstrates how the TLD Subdomain Structure
   Protcol can be used to decide cookie domain permissions.

4.1.  Example 1
   < ?xml version="1.0" encoding="UTF-8"?>
   <tld xmlns="http://xmlns.opera.com/tlds" name="tld" levels=1 >
       <domain name="example" />

   This specification means that all names at the top level are subTLDs,
   except "example.tld" for which cookies are allowed.  Cookies are also
   implicitly allowed for any y.x.tld domains.

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4.2.  Example 2
   < ?xml version="1.0" encoding="UTF-8"? >
   <tld xmlns="http://xmlns.opera.com/tlds" name="tld" >
       <registry name="example1" levels=1 />
       <registry name="example2" levels=1 />

   This specification means that example1.tld and example2.tld and any
   domains foo.example1.tld and bar.example2.tld are registry-like
   domains for which cookies are not allowed, for any other domains
   cookies are allowed.

4.3.  Example 3
   <?xml version="1.0" encoding="UTF-8"?>
   <tld xmlns="http://xmlns.opera.com/tlds" name="tld" >
       <registry name="example1" levels=1 />
       <registry name="example2" levels=1 >
          <domain name="example3" />

   This example has the same meaning as Example 2, but with the
   exception that the domain example3.example2.tld is a regular domain
   for which cookies are allowed.

5.  IANA Considerations

   This specification requires that the domain list is retrievable from
   a well-known location.  This means that a hostname or group of
   hostnames must be assigned to serve the domain list.  Suggestions for
   where to located the service are described in section 5.1 The
   specification also requires that responses are served with a specific
   media type.  Section 5.2 provides the registration of this media

5.1.  Location of the TLD Subdomain Structure specification

   The location of the domain list must be located at a location that
   can easily be deduced by the client from the name of the TLD.
   Several possibilities exist:

   1.  A reserved domain name in the TLD's name space e.g.
       https://www.subdomains.tld/domainlist or
       https://subdomains.nictld.tld/domainlist .

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   2.  A common repositiory,, e.g.
       https://subdomains.example.org/tld/domainlist, managed by the
       IANA or another Internet governance body

   The benefit of the first alternative is that the data are not located
   at a single repository which makes it more difficult to shut down the
   system completely.  On the other hand the TLD registries may find the
   overhead of maintaining such a service burdensome, and therefore
   avoid implementing it, or let the service lapse.  The second
   alternative creates a common repository, which may increase adoption.
   On the other hand, a single location makes it more susceptible to
   denial of service attacks.

5.2.  Registration of the application/subdomain-structure Media Type

   Type name : application

   Subtype name: subdomain-structure

   Required parameters: none

   Optional parameters: none

   Encoding considerations: The content of this media type is always
   transmitted in binary form.

   Security considerations: See Section 6

   Interoperability considerations: none

   Published specification: This document

   Additional information:

   Magic number(s): none

   File extension(s):

   Macintosh file type code(s):

   Person & email address to contact for further information: Yngve N.

   Email: yngve@opera.com

   Intended usage: common

   Restrictions on usage: none

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   Author/Change controller: Yngve N. Pettersen

   Email: yngve@opera.com

6.  Security Considerations

   Retrieval of the specifications are vulnerable to denial of service
   attacks or loss of network connection.  Hosting the specifications at
   a single location can increase this vulnerability, although the
   exposure can be reduced by using mirrors with the same name, but
   hosted at different network locations.  This protocol is as
   vulnerable to DNS security problems as any other [RFC2616] HTTP based
   service.  Requiring the specifications to be digitally signed or
   transmitted over a authenticated TLS connection reduces this

   Section 3 of this document describe using the domain list defined in
   section 2 as a method of increasing security.  The effectiveness of
   the domain list for this purpose, and the resulting security for the
   client depend both on the integrity of the list, and its correctness.
   The integrity of the list depends on how securely it is stored at the
   server, and how securely it is transmitted.  This specification
   recommends downloading the domain list using HTTP over TLS, which
   makes the transmission as secure as the message authentication
   mechanism used (encryption is not required), and the servers should
   be configured to use the strongest available key lengths and
   authentication mechansims.  An alternative approach would be to
   digitally sign the files.

   The correctness of the list depends on how well the TLD registry
   defined it.  A list that does not include some subTLDs may expose the
   client to potential privacy and security problems, but not any worse
   than the situation would be without this protocol and profile, while
   a subdomain incorrectly classified as a subTLD can lead to denial of
   service for the affected services.  Both of the problems can be
   prevented by careful construction and auditing of the lists, both by
   the TLD registry, and by interested thirdparties.

7.  Acknowledgements

   Anne van Kesteren assisted with defining the XML format in
   Section 2.2.1.

8.  References

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8.1.  Normative References

   [NETSC]    "Persistent Client State HTTP Cookies",

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC3275]  Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
              Language) XML-Signature Syntax and Processing", RFC 3275,
              March 2002.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

8.2.  References

              Pettersen, Y., "Enhanced validation of domains for HTTP
              State Management Cookies using DNS. Work in progress.",
              July 2006, <draft-pettersen-dns-cookie-validate-02.txt>.

   [RFC2311]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
              L. Repka, "S/MIME Version 2 Message Specification",
              RFC 2311, March 1998.

Appendix A.  Open issues

   o  Download location URI for the original domain lists

   o  Should Digital signatures be used on the files, instead of using

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Author's Address

   Yngve N. Pettersen
   Opera Software ASA
   Waldemar Thranes gate 98
   N-0175 OSLO,

   Email: yngve@opera.com

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