Network Working Group                                       Y. Pettersen
Internet-Draft                                        Opera Software ASA
Updates: 6265 (if approved)                                March 6, 2012
Intended status: Experimental
Expires: September 7, 2012

 The Public Suffix Structure file format and its use for Cookie domain


   This document defines the term "Public Suffix domain" as meaning a
   domain under which multiple parties that are unaffiliated with the
   owner of the Public Suffix domain may register subdomains.  Examples
   of Public Suffix domains include "org", "", "" and
   "".  It also defines a file format that can be used to
   distribute information about such Public Suffix domains to relying
   parties.  As an example, this information is then used to limit which
   domains an Internet service can set HTTP cookies for, strengthening
   the rules already defined by the cookie specification.  This
   specification updates RFC 6265 [RFC6265] by defining the term "Public
   Suffix domain".

Requirements Language

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

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

   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 September 7, 2012.

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Copyright Notice

   Copyright (c) 2012 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
   ( 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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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   The Domain Name System (DNS) [RFC1034] used to name Internet hosts
   allows a wide range of hierarchical names to be used to indicate what
   a kind of business a given host is engaged in.  Some are implemented
   by the owners of a domain, such as by creating subdomains for certain
   tasks or functions, while others, called Public Suffixes (or
   registry-like domains), are created by the Top Level Domain (TLD)
   registry owner or individual domain owners to indicate what kind of
   service the hosts under the domain provides, e.g., commercial,
   educational, governmental or geographical location, such as city or

   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 [RFC6265] and cross-
   document information sharing in ECMAScript DOM.  Most of these
   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.  It

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   is also often difficult to determine the original source of the
   information being shared.  To some extent, [RFC2965] tried to address
   some of these concerns for cookies, in that clients that send
   [RFC2965]-style cookies also send 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] was never
   widely deployed in clients or on servers.  The recipient server(s)
   can make inappropriate information sharing more detectable by
   requiring the information to contain data identifying the source, as
   well as by assuring the integrity of the data, e.g., by using
   cryptographic technologies.  However, these techniques tend to be
   computationally costly.

   There are two problem areas:

   o  Incorrect sharing of information between non-associated services
      e.g., and or and -- That is, the information may be distributed to
      all services within a given Top Level Domain or Public Suffix
      domain.  Sharing within a TLD is usually prevented by a simple
      rule that does not permit it, but that is more difficult for
      general Public Suffix domains, since they do not have a well
      defined pattern.

   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 web hotels, where
      the service itself has little or no control of the customers'

   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 what Public
   Suffixes are; then it will propose a file format that can be used to
   distribute information about the Public Suffixes within a Top Level
   Domain, e.g., that the TLD have several Public Suffix domains, such
   as co.tld, ac.tld, org.tld.  Finally it will show how this
   information can be used to determine when information sharing through
   cookies is not desirable.

2.  Public Suffix domains

   A Public Suffix domain is used very much like a Top Level Domain.
   The owner or operator of the domain allow unaffiliated third parties
   to register domain names in the Public Suffix domain and to control
   all activity inside the registered domain.

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   While most such domains are open to registration by the general
   public, the owner of the Public Suffix domain may have defined
   restrictions on which third parties may register a domain, such as
   only private persons, schools, or government agencies, to mention a
   few.  Such limitations do not change the fact that each domain
   registrant is nominally independent of all other domain registrants
   in the Public Suffix domain, as well as of the owner of the Public
   Suffix domain.

   Just like a domain under a TLD may be a Public Suffix domain, a
   domain registered under a Public Suffix domain may also be a Public
   Suffix domain.  Examples of this are the and
   Public Suffix domains in the dot-US ccTLD.

   There are various categories of Public Suffix domains.  The most
   common category is the second-level domain, used by many ccTLDs to
   group content, such as co.tld for commercial, ac.tld for academic
   institution, and gov.tld for government.  Another common category is
   Public Suffixes dedicated to geographical locations, such as as
   states, provinces, and cities, such as the domain
   organization used by the US ccTLD, possibly with more Public Suffix
   domains within these domains.  A third category is ISP shared
   hosting, and "vanity" domain names, e.g.,  In addition,
   a number of social websites provide their users with direct access
   names under their domains (e.g.,,
   rather than using URLs).

   Information about Public Suffix domains can be used in several
   security features in clients:

   o  Blocking websites' ability to set cookies to the Public Suffix

   o  Limiting the ability of active content, such as EcmaScript, from
      affecting content in independent domains that happen to share the
      same Public Suffix domain as the source domain

   o  Highlighting the actual domain in the displayed URL in the
      client's UI, to reduce the potential of a malicious site
      misleading the user with a URL that identifies the host as, which might
      lead users to think they are visiting
      rather than a site in the domain example.pubsuffix-domain.tld

   As there is currently no reliable method in DNS or other protocols
   that allows clients to automatically recognize a Public Suffix
   domain, the owner of such a domain must self-declare the domain's
   status as a Public Suffix domain and register it with each of the

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   repositories that track such information.  Such self-registration may
   lead to inconsistencies between the various repositories, which could
   cause security problems to develop.  A more reliable method might be
   that the TLD registrar collect such information from their registered
   domains, and make it available to relying parties.  Section 3
   presents a XML-based format for how this information can be published
   and shared by the TLD registrar.

3.  The Public Suffix Structure file format

   The Public Suffix Structure file format specifies how to encode
   information about Public Suffix domains inside a TLD.  It is based on
   XML and is able to specify Public Suffixes and exceptions to any
   level of the domain hierarchy that is desirable.

3.1.  Domain list format

   The domain list file can contain a list of subdomains that are
   considered Public Suffix 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 implied either by the file that
   is parsed or by the content of the <tld> tag.  The domain names
   listed MUST be encoded in punycode, as specified by [RFC5891].

3.1.1.  Domain list schema

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

       default namespace = ""

       start =
           element tld {
             attribute levels { xsd:nonNegativeInteger | "all"},
             attribute name { xsd:NCName },
             (domain | registry)*
       registry =
           element registry {
             attribute levels { xsd:nonNegativeInteger },
             attribute name { xsd:NCName },
             attribute all { string "true" | string "false" },
             (domain | registry)*
       domain =
           element domain {
             attribute name { xsd:NCName }

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   The domain list file usually contains a single <tld>-block (but may
   contain multiple entries), which may contain multiple registry and
   domain blocks, and a registry block, which define a Public Suffix
   domain, may also contain multiple registry and domain blocks.  When
   used alone, the <tld>-block MAY contain a name field identifying the
   TLD name; if there are multiple <tld>-blocks, each block MUST specify
   the name field.

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

   All names SHOULD be punycode encoded [RFC5891] to make it possible
   for clients unaware of either Unicode or IDNA to use the document.

   The <tld>- and <registry>-blocks MAY contain an attribute, "levels",
   specifying how many levels below the current domain are Public
   Suffixes.  The default is "none", meaning that the default inside the
   current domain level is that labels are ordinary domains and not
   Public Suffix domains.  If the value of the "levels" attribute is 1
   (one) by default all next-level labels within the registry/TLD are
   Public Suffix domains, not normal domains.  If the value of the
   "levels" attribute is the case-insensitive token "all", then all
   subdomains domains below the current domain are Public Suffix
   domains, by default.

   A <registry>-block with the attribute "all" set to "true" inside the
   declaration for the registry domain example.tld indicates that all
   domains x.example.tld are also Public Suffix domains, by default,
   unless a domain is specified differently by a different declaration.
   The registry-all block may contain additional <registry>- or
   <domain>-blocks, which then apply to domains foo.x.example.tld, for
   all domains x, except those that have separate entries.  This allows
   specification of wildcard structures, where the structure for lower
   domains are similar for all domains.

   Implementations MUST ignore attributes and syntax they do not

3.1.2.  Domainlist interpretation

   For each new <registry>- or <domain>-block within the <tld>- or
   <registry>-block, the effective domain name to which the block
   applies is the name of the block prepended to the ".parentdomain" 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="" name="tld" levels="1" >
       <registry name="co" levels="0">
         <registry name="state" />
       <registry name="province">
         <registry all level="1">

           <domain name="school" />
       <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 Public Suffix domain;
   although, parliament.tld is not a Public Suffix domain, but a normal

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

   Additionally, all domains x.province.tld are Public Suffixes, and all
   school.x.province.tld are normal domains for all domains x in

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

3.2.  Public Suffix Structure as a web service

   The Public Suffix structure file can be provided as an HTTP service,
   managed by either the application vendor, the TLD owners, or some
   other trusted organization, and it can be located at a 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.
   The procedure for use as a service is as follows:

   o  The client retrieves the domain list for the Top Level Domain
      "tld" from the vendor specified URI .  Multiple

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      alternative URIs for a fallback procedure may be specified.

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

   o  The retrieved specification SHOULD be cached by the client 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, undefined methods, depending on the profile.

3.3.  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 Public Suffix domain or to hide a Public Suffix
   domain's classification.  Besides obviously securing the hosting
   locations, this also means that the content served will have to be

   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 that it may not already support, which may be problematic
       for clients having limited resources.

   2.  Use 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.

4.  A Public Suffix Structure file format profile for HTTP Cookies

   The HTTP State management cookies area is one where it is important,
   both for security and privacy reasons, to ensure that unauthorized

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   services cannot set cookies for another service.  Inappropriate
   cookies can affect the functionality of a service, but they may also
   be used to track the users across services in an undesirable fashion.
   Neither the original Netscape cookie specification[NETSC], [RFC2965]
   or [RFC6265] are adequate in many cases.

   The original Netscape specification's rules required only that the
   target domain must have one internal dot (e.g., if the
   TLD belongs to a list of generic TLDs (gTLD), while for all other
   TLDs the domain must contain two internal dots (e.g.,
   The latter rule was never properly implemented, in particular due to
   the many flat ccTLD domain structures that are in use.  (The
   successor [RFC6265] has since expanded this policy to exclude domains
   listed in client-specific lists of "public suffixes").[RFC2965] set
   the requirement that cookies can only be set for the server's parent

   Unfortunately, both the [NETSC] and [RFC2965] policies still left
   open the possibility of setting cookies for a Public Suffix domain by
   setting the cookie from a host name example.pubsuf.tld to the domain
   pubsuf.tld, which is by itself legal, but not desirable, because that
   means that the cookie can be sent to numerous websites either
   revealing sensitive information, or interfering with those other
   websites without authorization.  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 Public Suffix domain structure like the one
   used by the .uk ccTLD (e.g.,, or a combination of both.
   However, 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
   service names from Public Suffix domains by using DNS was developed
   by Opera Software ASA.  However, this method was not immune to TLD
   registries that use Public Suffix domains as directories or to
   services that do not define an IP address for the domain name.  Using
   the Public Suffix Structure file format to retrieve a list of all
   Public Suffix domains in a given TLD will solve both those problems.

4.1.  Procedure for using the Public Suffix Structure file format 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):

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   1.  If the Public Suffix domain structure of the TLD is not known
       already, or the structure information has expired, according to
       the client's policies, the client should retrieve or revalidate
       the structure specification from the server hosting the
       specification, according to Section 3.  If retrieval is
       unsuccessful, and no copy of the specification is known, the
       client MAY use alternative information or heuristics to decide
       the domain's status.  Upon successful retrieval the specification
       is evaluated as specified in Section 3.  If the target domain is
       designated as a Public Suffix domain, then the cookie MUST be
       discarded or, alternatively, processed as if it had not specified
       a domain attribute.

   2.  If the target domain is not a Public Suffix domain, the cookie is
       accepted (unless other policies configured for the client prevent

4.2.  Third party cookies

   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, which can be used to select advertisements that might be of
   interest to the user.

   Some clients have therefore implemented restrictions on what cookie
   related activities are accepted in relation to a third-party domain.
   Frequently, such restrictions are based on determining whether the
   two hosts share the same immediate parent domain as a domain suffix,
   or if the first domain of the two is a parent domain (suffix) of the

   This determination method might incorrectly classify a third-party
   server as a first party if the immediate parent domain of the first
   party server is a Public Suffix domain, and possibly break the user's
   privacy expectations.

   To avoid such misclassifications, when the two servers have the first
   party server's immediate parent domain as a shared suffix, clients
   SHOULD apply the procedure specified in Section 4.1 for this domain,
   and, if this domain is determined to be a Public Suffix domain, the
   second host must be considered a third party.  That is, if the first
   party server causes a request for a resource at the parent domain is determined to be a Public
   Suffix domain, and is therefore a third-party server,
   even if they share the same immediate parent domain.

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5.  Examples

   The following examples demonstrate how the Public Suffix Structure
   file format can be used to decide cookie domain permissions.

5.1.  Example 1
   <?xml version="1.0" encoding="UTF-8"?>
   <tld xmlns="" name="tld" levels="1" >
       <domain name="example" />

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

5.2.  Example 2
   <?xml version="1.0" encoding="UTF-8"? >
   <tld xmlns="" 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 Public Suffix
   domains for which cookies are not allowed; for any other domains
   cookies are allowed.

5.3.  Example 3
   <?xml version="1.0" encoding="UTF-8"?>
   <tld xmlns="" 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.

6.  IANA Considerations

   This specification also requires that responses are served with a
   specific media type.  Below is the registration information for this

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   media type.

6.1.  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 7.

   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.


   Intended usage: common

   Restrictions on usage: none

   Author/Change controller: Yngve N. Pettersen


7.  Security Considerations

   Retrieval of the Public Suffix Structure specifications is vulnerable
   to denial of service attacks or loss of network connection.  Hosting
   the specifications at a single location can increase this

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   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 vulnerability.

   Section 4 of this document describes using the domain list defined in
   Section 3 as a method of increasing security and privacy.  The
   effectiveness of the domain list for this purpose, and the resulting
   security and privacy improvements for the user, depend both on the
   integrity of the list, and its correctness.  The integrity of the
   list depends on how securely it is stored in the repository and how
   securely it is transmitted.  This specification recommends
   downloading the domain list using HTTP over TLS [RFC2818], 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 mechanisms.  An alternative or complimentary approach
   would be to digitally sign the files.

   The correctness of the list depends on how well the TLD registry
   defined it, or how well the list maintainer have been able to collect
   correct information.  A list that does not include some Public Suffix
   domains may expose the client to potential privacy and security
   problems, but the situation would not be any worse than it would be
   without this protocol and profile, while a subdomain incorrectly
   classified as a Public Suffix domain 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 regis
   try and by interested third parties.

8.  Acknowledgments

   Anne van Kesteren assisted with defining the XML format in
   Section 3.1.1.

   The Public Suffix List project [PUBSUFFIX] was initiated by members
   of the Mozilla Community, and members of the project, in particular
   Gervase Markham, have provided input to this document.

9.  References

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9.1.  Normative References

   [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.

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

   [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.

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

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

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

9.2.  Non-normative references

   [NETSC]    "Persistent Client State HTTP Cookies", <http://

              "The Homepage of the Public Suffix List, a list of
              registry-like domains gathered by volunteers.",

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

Appendix A.  Collection of information for the TLD structure

   This document does not define how the information encoded in the TLD
   Structure Specification is gathered.

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   There are several methods available for collecting the information
   encoded in the TLD Structure Specification, the two main ones being:

      Data provided by the TLD registry owner through a machine readable
      repository at well known locations

      Data gathered by one or more application vendors based on publicly
      available information, such as the Mozilla Project's Public Suffix

Appendix B.  Alternative solutions

   A possible alternative to the format specified in Section 3, encoding
   the information directly in the DNS records for the Public Suffix
   domain, using a DNS extension.

   Accessing this type of information requires that the client or its
   environment is able to directly access the DNS network.  In many
   environments, e.g., firewalled systems, this may not be possible.
   Also, not all runtime environments can provide this information,
   which may lead to a DNS client embedded directly in the client.

   For some applications, it may be necessary, due to system
   limitations, to access this information through an online web service
   in order to provide the necessary information for each hostname or
   domain visited.  A web service may, however, introduce unnecessary
   privacy problems, as well as delays each time a new domain is tested.

Appendix C.  Open issues

   o  Download location URI for the original domain lists

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

Author's Address

   Yngve N. Pettersen
   Opera Software ASA
   P.O. Box 2648 St Hanshaugen
   NO-0131 OSLO,


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