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Versions: (draft-deccio-domain-name-relationships) 00

Internet Engineering Task Force                                C. Deccio
Internet-Draft                                             Verisign Labs
Intended status: Informational                                 J. Levine
Expires: January 7, 2016                            Taughannock Networks
                                                            July 6, 2015

                 Concepts for Domain Name Relationships


   Various Internet protocols and applications require some mechanism
   for identifying relationships between Domain Name System (DNS) names.
   In this document we provide examples of protocols and applications
   for which knowledge of these relationships is useful, if not
   required.  Further we discuss the various types of domain name
   relationships, review current needs and solutions, and identify
   considerations for solution sets.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 7, 2016.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Domain Name Concepts . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Domain Names . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Domain Name Scope  . . . . . . . . . . . . . . . . . . . .  4
       2.2.1.  Public/private Boundaries  . . . . . . . . . . . . . .  4
     2.3.  Domain Name Relationships  . . . . . . . . . . . . . . . .  4
   3.  Policy-based Domain Name Relationships . . . . . . . . . . . .  5
     3.1.  Cross-Scope Policy Relationships . . . . . . . . . . . . .  5
     3.2.  Intra-Scope Policy Relationships . . . . . . . . . . . . .  5
       3.2.1.  Public-public Policy Relationships . . . . . . . . . .  6
       3.2.2.  Private-private Policy Relationships . . . . . . . . .  6
   4.  Known Applications Requiring Identification of
       Policy-based Domain Relationships  . . . . . . . . . . . . . .  6
     4.1.  HTTP Cookies . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Email sender verification  . . . . . . . . . . . . . . . .  7
     4.3.  SSL certificate requests . . . . . . . . . . . . . . . . .  7
   5.  Public Suffix List . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Known Application Usage  . . . . . . . . . . . . . . . . .  9
   6.  Solution Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   The use of various Internet protocols and applications has introduced
   the desire and need for designated relationships between Domain Name
   System (DNS) names, beyond the lineal relationship inherent in the
   names themselves.  While protocols, such as that used by HTTP
   Cookies, have traditionally used ancestral relationships to determine
   allowable scope of information sharing and authorization, there is an
   increasing need to identify relationships between arbitrary domains.

   We begin by establishing terminology and concepts, after which we
   discuss known applications for which the identification of domain
   name relationships are desirable or required.  We then discuss the
   Public Suffix List, the primary solution for domain relationships
   currently available.  Finally, we recommend considerations for
   solutions in this problem space.

2.  Domain Name Concepts

   For consistency in language we define terms and concepts surrounding
   domain names.

2.1.  Domain Names

   A DNS domain name is represented as sequence of dot-separated labels,
   such as www.example.com (i.e., comprised of labels "www", "example",
   and "com").  This sequence corresponds to the list of the labels
   formed by traversing the tree representing the domain name space,
   from the node representing the name itself to the root (top) of the
   tree ([RFC1034]).  In this tree context, we thus refer to domain
   name's parent as the domain name formed by removing the leftmost
   label (i.e., the domain name corresponding to the node directly above
   it in the tree).  The parent of www.example.com is example.com.

   As there are no requirements or inferences surrounding delegation
   (i.e., zone cut) at any point in the DNS tree, there are no
   assumptions in this document about administrative boundaries drawn by
   delegations, unless explicitly stated otherwise.  That is to say that
   this document considers DNS names independently from their
   administration, as defined by the DNS.

   As noted in [RFC1034], the term "domain name" is used in contexts
   outside the DNS.  The scope of this document is limited to domain
   names as defined by the DNS.

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2.2.  Domain Name Scope

   The use of domain names in various applications over time has
   produced a notion of scope, which we use to refer to the general
   ability of arbitrary entities to register children of a domain name
   (i.e., create child nodes in the domain name tree).  In some contexts
   these are called "public suffixes" or "registry controlled domains"
   ([RFC6265]).  For example, children of the top-level domain (TLD)
   com, are generally registrable by arbitrary entities, which puts the
   com domain name in the public scope.  However, com's children are
   typically not used in the same fashion (though certainly there are
   exceptions), which puts them largely in the private scope.

   The children of public domain names may either be in public or
   private scope; likewise the children of private domain names may
   either be in public or private scope.

   While zone cuts often exist along public/private scope boundaries
   (e.g., between com and example.com), they are not required at these
   boundaries, nor are scope boundaries required at zone cuts.  In this
   document public/private scope is considered independent of
   administrative boundaries defined by the DNS (i.e., zone cuts).

   The most well-known delineator of public/private scope is the Public
   Suffix List (PSL) [PSL], which is described later in this document.

2.2.1.  Public/private Boundaries

   If we consider the root domain name itself to be public, then between
   the root domain name and any private domain name (below), there must
   exist at least one boundary going from some public parent to private
   child.  The first such boundary encountered upon downward traversal
   from the root is the first-level public boundary.  Subsequent public-
   to-private boundaries are referred to as lower-level public
   boundaries.  For example, because the com domain name is considered
   public, if we assume that example.com is private, then the first-
   level public boundary is between com and example.com.  If the
   public.example.com domain name is considered public (i.e., children
   domain names can be registered by arbitrary third parties) and
   foo.public.example.com is a private domain name, then a lower-level
   public boundary exists between public.example.com and

2.3.  Domain Name Relationships

   In this document two types of domain name relationships are
   identified: ancestry and policy.  An ancestral relationship exists
   between two domains if one domain name is an ancestor of the other.

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   A policy relationship exists between two domain names if their
   relationship is such that application policy should treat them as
   equivalent.  For example, the two names might be administered by the
   operating organization, or there might business or other
   relationships between the two operating entities.

   In the simplest case, two domain names might be policy-related for
   all applications or purposes.  However, it is possible that two
   domains are related only for explicitly defined purposes.

   An ancestral relationship between two names can be identified merely
   by comparing the names themselves to determine whether one is a
   substring of the other.  However, there is no inherent way to
   determine policy relationships neither by examination of the names
   themselves, nor by examining the administrative boundaries (i.e.,
   zone cuts) defined in the DNS.  This is the problem being considered
   in this document.

3.  Policy-based Domain Name Relationships

   Because policy-based domain name relationships are not inherently
   apparent based on the names themselves or DNS protocol, mechanisms
   outside the DNS namespace and base protocol are necessary to
   advertise and detect those relationships.

   In this section we enumerate the different types of ancestral and
   scope relationships upon which policy-based relationships can be

3.1.  Cross-Scope Policy Relationships

   If scope of one domain name is public and another is private, then it
   can be inferred, by the definition of their respective scopes, that
   there exists no policy-based relationship between the two.  That is,
   a public domain name cannot be related, for policy purposes, to a
   private domain name.

   Note that this doesn't prohibit policy relationships between two
   domain names of the same scope but having (an even number) of scope
   boundaries in between.

3.2.  Intra-Scope Policy Relationships

   We now consider the existence of a policy relationship between two
   domains names of the same scope.

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3.2.1.  Public-public Policy Relationships

   The connotation of a public domain name in the context of policy is
   that it should not be used for purposes normally associated with
   private domain names.  For example, it would be unreasonable to
   expect legitimate mail to come from an email address having the exact
   suffix of org.au (a domain name currently identified by [PSL] as
   being public).  This is especially true of domain names above the
   first-level public boundary.

   Because of this connotation, one consideration for policy amongst two
   domain names, both public, is that no effective relationship exists
   because they are ineligible by definition.  Other than that, there is
   insufficient information from only domain names and scope alone to
   confirm or deny a policy relationship.

3.2.2.  Private-private Policy Relationships

   There are two classes of potential private-private policy
   relationships: ancestral and cross-domain (non-ancestral).  In
   neither case can the presence or absence of a policy relationship be
   confirmed using only the names and scope information.

4.  Known Applications Requiring Identification of Policy-based Domain

   In this section we discuss the current state of known applications
   requiring identification of policy-based domain name relationships.

4.1.  HTTP Cookies

   Domain names are used extensively in conjunction with the Hypertext
   Transfer Protocol (HTTP) ([RFC7230], [RFC7231]).  The domain names
   used in Uniform Resource Identifiers (URIs) [RFC3986] are used by
   HTTP clients not only for resolution to an HTTP server Internet
   Protocol (IP) address, but also for enforcing policy.

   HTTP clients maintain local state in the form of key/value pairs
   known as cookies ([RFC6265]).  While most often cookies are initially
   set by HTTP servers, HTTP clients send all cookies in HTTP requests
   for which the domain name in the URI is within the cookies' scope.
   The scope of a cookie is defined using a domain name in the "domain"
   attribute of the cookie.  When a cookie's "domain" attribute is
   specified as a domain name (as opposed to an IP address), the domain
   name in the URL is considered within scope if it is a descendant of
   the "domain" attribute.

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   RFC 2965 [RFC2965] (now obsolete) required that the value of the
   "domain" field carry at least one embedded dot.  This was to prohibit
   TLDs--which were almost exclusively public--from being associated, by
   policy, with other domains.  Cookies having public scope would enable
   the association of HTTP requests across different, independently
   operated domains, which policy association raises concerns of user
   privacy and security.

   In the current specification ([RFC6265]), the semantic requirements
   were modified to match "public suffixes" because it was recognized
   that TLDs are not the only domain names with public scope--and that
   not all TLDs are public suffixes.  The notion that all TLDs are
   inherently public has been challenged by the many and diverse domain
   names that have been delegated since 2013 as part of the new generic
   top-level domain (gTLD) program ([NewgTLDs]).

4.2.  Email sender verification

   An emerging sender verification called Domain-based Message
   Authentication, Reporting and Conformance (DMARC)
   [I-D.kucherawy-dmarc-base] attempts to validate the domain name of
   the author's address on the message's "From:" header using the
   DomainKeys Identified Email (DKIM) [RFC5585] and Sender Policy
   Framework (SPF) [RFC7208] authentication schemes.  A DKIM signature
   and SPF check each validate a specific domain name.  For DKIM it is
   the domain name corresponding the DKIM signature.  For SPF the domain
   name of the message's bounce address is validated.  DMARC allows
   approximate matching between the author's domain and the validated
   domain name, where one can be an ancestor or descendant of the other.

   DMARC validators are supposed to ensure that the two domain names are
   under the same management, the specifics of which are deliberately
   left out of the spec.

4.3.  SSL certificate requests

   Secure Socket Layer (SSL) certificate authorities typically validate
   certificate signing requests by sending a confirmation message to one
   of the WHOIS contacts for the (private scope) domain name (CA/B
   Ballot 74 [CA/B-Ballot-74]).  In cases where there are multiple
   levels of delegation (i.e., crossing public/private scopes), the
   WHOIS contact needs to be the one for the registrant of the domain,
   not a higher level registration.

   When an SSL certificate is for a wildcard domain name, the entire
   range of names covered by the wildcard needs to be under the same
   control.  Authorities do not (knowingly) issue certificates for
   public domain names such as *.org.au.

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5.  Public Suffix List

   The most well-known resource currently available for identifying
   public domain names is the Public Suffix List (PSL) [PSL].  The PSL
   is explicitly referenced as an example of an up-to-date public suffix
   list in [RFC6265].  The PSL was developed by Mozilla Firefox
   developers to further address HTTP security and privacy concerns
   surrounding cookie scope when the "no embedded dot" rule of [RFC2965]
   was the upper limit.

   The PSL contains a list of known public suffixes, and includes
   placeholder public domains designated by "wildcard" notation in the
   file.  A wildcard implies that all children of the wildcard's parent
   are in fact public domain names themselves--except where otherwise
   noted as a wildcard exception.  For example, we use the contrived
   entries in Table 1 to demonstrate this use of the PSL.

           | Entry        | Meaning                            |
           | example      | example is public                  |
           | *.example    | All children of example are public |
           | !foo.example | foo.example is private             |

                      Table 1: Contrived PSL Entries

   These entries result in the scopes shown in Table 2:

                     | Name                | Scope   |
                     | example             | Public  |
                     | foo.example         | Private |
                     | baz.foo.example     | Private |
                     | bar.example         | Public  |
                     | baz.bar.example     | Private |
                     | www.baz.bar.example | Private |

         Domain name scope based on the PSL entries from Table 1.

                      Table 2: Contrived PSL Entries

   The PSL effectively identifies scope, insomuch as the list is
   accurate.  Of the 6,823 entries in the PSL at the time of this
   writing, all but 50 are used to designate first-level public
   boundaries; the remainder designate lower-level boundaries.  The

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   primary function of the PSL, therefore, is to delineate first-level
   public boundaries.

   Matters of policy that can be settled simply by identifying the scope
   of the names in question are thus addressed by the PSL.  However, the
   question of determining whether a policy-based relationship between
   intra-scope names (with the possible exception of those of public
   scope) are unaddressed.

5.1.  Known Application Usage

   The PSL is used by several browsers, including Mozilla Firefox, to
   identify domain names as public or private.  This is used for
   validating the domain attribute of cookies.  Additionally, it
   provides visual and organizational convenience for readily
   identifying the highest intra-scope private ancestor for a given
   private domain name (i.e., the child of the domain name's nearest
   public ancestor).  This is useful for organizing names and URIs by
   domain name, as in bookmarks, and for highlighting key parts of URIs
   or certificates in the address bar or other parts of the browser

   Existing DMARC implementations are known to use the PSL to assert
   policy-based relationships between SPF- or DKIM-authenticated
   validated domain names and domain name corresponding to the address
   in the "From:" header.  Such a relationship is identified if two
   domain names are both of private scope and share an ancestral

   DMARC implementations also use the PSL to identify the highest intra-
   scope ancestor of a (private) domain name for the purpose of looking
   up the DMARC DNS record.  The the appropriate ancestor name is
   identified it is appended to the label "_dmarc" to find the
   appropriate information in the DNS.

   SSL certificate authorities use the PSL to ensure that wildcards are
   not issued for domain names having public scope.

6.  Solution Considerations

   The problem discussed in this document is the association of domain
   names for policy purposes.  The PSL has been the de-facto
   supplementary resource utilized for identifying such relationships.
   The shortcomings of only having domain names and their scope (e.g.,
   via the PSL) have been treated in Section Section 5.

   An alternate paradigm for addressing the problem involves a system

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   wherein policy-based relationships are explicitly defined on a per-
   domain name (pair) basis.  For scalability and dynamic response this
   is most effectively achieved through defining these relationships in
   the DNS itself, e.g., through special records included in the DNS at
   (or near) the domain names themselves, such as the mechanism proposed
   in [I-D.sullivan-domain-origin-assert].  One benefit to this paradigm
   is that it allows the definition of policy-based relationships
   between arbitrary names at any locations in the DNS domain name tree,
   and the notion of scope becomes moot.  Another benefit is that it
   puts the definition of those relationships in the hands of the
   administrators and operators of the domain names themselves, rather
   than a third party.

   There are several challenges with the domain name-centric paradigm as
   well.  One challenge is that it requires correct, consistent, and
   coordinated efforts by affected domain name operators.  The number of
   involved parties, moving parts, and dependencies introduces more
   chance for error.  Additionally, having the information available
   online (e.g., in the DNS) means that consumption by local
   applications is dependent on real-time Internet connectivity, which
   is not always possible nor desirable.

   Another solution set is that which includes both a scope definition
   resource (e.g., the PSL) and a mechanism for explicit definition of
   policy-based relationships on a per-domain name basis.  In this case
   the scope definitions are consulted first to determine whether a
   policy-based relationship is possible, after which (if necessary)
   special domain name-specific lookups are issued to further determine
   whether such a relationship exists.  This addresses what might be the
   most common issues using a central, relatively simple, and
   established mechanism, leaving the flexibility for additional
   extensibility with domain name-specific relationship definitions.

   We recommend that the cost and the value of the different solution
   paradigms be considered when developing solutions for the problem of
   defining policy-based relationships between domain names.  As part of
   this, the model of domain name relationships outlined in Section
   Section 2.3 should be analyzed to consider which types of
   relationships are most in demand, and which solutions are sufficient
   for the circumstances in highest demand.  Such will enable an
   appropriate and usable balance of efficiency, robustness,
   flexibility, and autonomy.

7.  IANA Considerations

   This document includes no requests for IANA.

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8.  Security Considerations

   This document does not specify a protocol or usage and, therefore,
   there are no new security considerations for it.  There are security
   considerations for major cases in which domain boundaries are used,
   such as HTTP Cookies and DMARC, both discussed here.  See the
   Security Considerations of RFC 6265 [RFC6265] and

9.  Informative References

              Certificate Authority(CA)/Browser Forum, "Ballot 74",
              2015, <https://cabforum.org/2012/05/31/

              Kucherawy, M. and E. Zwicky, "Domain-based Message
              Authentication, Reporting and Conformance (DMARC)",
              draft-kucherawy-dmarc-base-13 (work in progress),
              February 2015.

              Sullivan, A., "Asserting DNS Administrative Boundaries
              Within DNS Zones", draft-sullivan-domain-origin-assert-02
              (work in progress), October 2012.

              ICANN, "New Generic Top-Level Domains", 2015,

   [PSL]      Mozilla Foundation, "Public Suffix List", 2015,

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

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

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

   [RFC5585]  Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
              Identified Mail (DKIM) Service Overview", RFC 5585,

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              July 2009.

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

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              April 2014.

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

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

Authors' Addresses

   Casey Deccio
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703-948-3200
   Email: cdeccio@verisign.com

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 831 480 2300
   Email: standards@taugh.com
   URI:   http://jl.ly

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