Publishing Organization Boundaries in the DNS

Network Working Group                                          J. Levine
Internet-Draft                                      Taughannock Networks
Intended status: Informational                             10 April 2020
Expires: 12 October 2020

             Publishing Organization Boundaries in the DNS


   The organization that manages a subtree in the DNS is often different
   from the one that manages the tree above it.  We describe an
   architecture to publish in the DNS the boundaries between
   organizations that can be adapted to various policy models and can be
   queried with a small number of DNS lookups.

Status of This Memo

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   This Internet-Draft will expire on 12 October 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Design Issues . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  TXT record format . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Lookup Process  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  DNS Records . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Application scenarios . . . . . . . . . . . . . . . . . . . .   7
     6.1.  DMARC . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Cookies . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.3.  SSL Certificates  . . . . . . . . . . . . . . . . . . . .   8
   7.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  ABNF syntax of bound records  . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Variations  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Often, the organization that manages a subtree in the DNS is
   different from the one that manages the tree above it.  Many
   applications use information about such boundaries to implement
   security policies.  For example, web browsers use them to limit the
   names where HTTP cookies can be set, and Secure Socket Layer (SSL)
   certificate services use them to determine the party responsible for
   the domain in a signing request.  Mail security applications such as
   Domain-based Messaging Authentication, Reporting and Conformance
   (DMARC) use them to locate an organization's policy records in the
   DNS.  This specification is intended to provide boundaries usable for
   DMARC, and possibly for other applications.

   [[Please direct discussion of this draft to the dbound working group

2.  Design Issues

   Organization boundaries can be assigned on what one could call an
   opt-in or opt-out basis.  "Opt-in" means that two names are only
   managed by the same organization if both actively assert that they
   are related.  "Opt-out" means that if there is any boundary
   information at all for a DNS subtree, each name is assumed to be
   under the same management as its parent unless there is a boundary
   assertion to the contrary.  This design describes an opt-out model.

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   Within the opt-out model, this design can adapt to a variety of

   *  Policies can be published by the domains themselves, or by a third
      party.  In the former case, each domain might assert its own
      boundary policies.  In the latter case, the third party makes the
      assertions, which may or may not agree with what the domains
      themselves would want.

   *  Multiple levels of delegation may be implemented, which is
      different from irregular boundaries.  For example, "us", "",
      and "" are irregular boundaries, because they're all
      handled by the US registry operator Neustar.  CentralNIC's
      "" would be a second level of delegation below Verisign's

   *  Different sets of boundary rules can be published for different
      applications.  For example, the boundaries for SSL certificates
      might be different from the boundaries for e-mail policies, or for
      HTTP cookie setting policies.

   In the lookup process below, the boundary point data is stored in the
   DNS tree in a TXT record.  The boundary is considered to be directly
   below the name that the process returns, similarly to the names in
   the [PSL].  If the process returned "abc.example", then
   "" and "" are separated by the
   boundary, but "" and "" are not.

   Each domain can publish its own policies within its own domain name
   space, or a separate authority can publish a global set of policies
   in a separate name space.

3.  TXT record format

     * IN TXT "bound=1" "." "." ""

   The bound TXT record contains at least four text strings: a tag, two
   keyword strings and a domain name.  Strings beyond the fourth are

   The "bound=1" tag is to prevent confusion when a domain publishes a
   wildcard such as * that could match a _bound name.
   Records that do not start with the correct tag or that have fewer
   than four strings are ignored.

   Each keyword string is a series of comma separated keywords.  If the
   string would otherwise be empty, it is a single dot.

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   The first keyword string expresses policy options.  It can include
   NOLOWER which means that no lower level boundaries can exist below
   this one, and NOBOUND which means that this name is not a boundary
   for this application.

   The second keyword string identifies the application(s) to which this
   boundary applies.  The strings DMARC, COOKIE, and CERT mean that the
   applications re DMARC, HTTP cookies, and SSL certificate signing
   respectively; a dot means it is a default for any applications not
   otherwise specified.

   The domain name is an absolute domain name, without the final dot.
   The first label in the name may be "*" to indicate that the boundary
   is at any name one label deeper than the rest of the name.  That is,
   the asterisk matches a single label, not the usual DNS sense of
   matching any string of labels.

4.  Lookup Process

   In general, the lookup process takes as input a domain name and
   application.  It returns the name of the boundary node in the DNS.
   This may be the domain itself or a parent.  If there is no policy for
   the domain the lookup fails; there are no defaults, and the DNS root
   is not within any organization boundary.  (Applications may apply
   defaults of their own, but that is beyond the scope of this

   Names of boundary information records use the tag "_bound" which is
   intended to be unique.

   For the first lookup, the client extracts the top level component
   (i.e., the rightmost label, as "label" is defined in Section 3 of
   [RFC1034]) of the domain name from the subcomponents, if any, and
   inserts the prefix in front of that component, after other components
   if any.  For example, if the domain to be checked is "" or
   "", the client issues a DNS query for
   "" or "".  If the domain is a
   dotless one such as "example", the client looks up "_bound.example".

   If the boundary information is published by a third party, the client
   appends the base name of the third party's domain to the name to be
   looked up.  Then the client does a DNS lookup of TXT records at that
   name, which will return zero or more TXT records.  A failure such as
   NXDOMAIN is considered to return zero records.  A lookup can return
   multiple records if different applications have different boundaries
   or policy options.  The lookup process discards any records that do
   not start with "bound=1" or contain less than four strings.

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   If a relevant policy record is returned, and the record does not
   contain the NOBOUND keyword, the domain name in the record is the
   policy boundary.  A policy record is relevant if it lists the desired
   application, or it is a default policy and there is no record with
   the application's keyword.  For example, a check for a boundary above
   "" would be issued at "", and the
   expected TXT record could be "bound=1 . . com".

   If there are no boundaries at all in a TLD, the policy record
   contains "bound=1" "." "." "." indicating the root.  For example, if
   all subdomains of the "example" top-level domain (TLD) are under the
   same management as the TLD itself, checks for "_bound.example" or
   "www._bound.example" would return "bound=1" "." "." ".".

   If the relevant record has the NOLOWER keyword set, the process
   stops.  Otherwise, the client inserts the prefix tag into the name
   just below (i.e., to the left of) the name at the largest matching
   boundary indicated in the lookup result, and repeats the lookup.  For

   *  When evaluating "", the first query would be to
      "".  If the reply to this is "bound=1 .
      . com", then the second query would be to

   *  When evaluating "", the first query would be to
      "".  If the reply to this is "bound=1" "."
      "." "us", the next lookup would be to "".
      If it returned "bound=1 "." "." "", the third lookup would be
      to "".

   This process repeats until a DNS lookup returns a relevant record
   with the NOLOWER keyword, or a lookup returns no relevant records, at
   which point the boundary is the domain name in the last retrieved
   relevant record.

   If an otherwise relevant record has the NOBOUND keyword, the process
   continues if the NOLOWER keyword is not present, but there is no
   boundary at the name with the NOBOUND keyword.  These NOBOUND keyword
   enables a name in the hierarchy to be a boundary for some
   applications but not for others.

   If the domain name in a boundary record starts with a *, e.g.,
   "*.example", the client replaces the * with the corresponding element
   of the domain being matched.  If the domain were "www.test.example",
   the boundary record domain would be treated as though it were

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5.  DNS Records

   The publishing entity uses wildcards and prefixed names that parallel
   the regular names under a TLD to cover the domain's name space.

   If there is a boundary at a given name, an entry in the TLD record
   covers the names below it.  For example, if there is a boundary at
   ".TEST", a suitable record would be:

     *._bound.test IN TXT "bound= "." "." "test"

   If the boundary is above the TEST domain, i.e., TEST is under the
   same management as FOO.TEST, the record would indicate no boundaries,
   and an additional non-wildcard record is needed to cover TEST itself:

     *._bound.test IN TXT "bound=1 "." "." "."
     _bound.test   IN TXT "bound=1 "." "." "."

   In domains with irregular policy boundaries, multiple records in the
   record describe the boundary points.  For example, in the US (United
   States) TLD, there are legacy domains under XX.US where XX is a two-
   letter state abbreviation, and there are some further points such as
   "k12" for schools within a state, with a boundary such as such as
   "".  A suitable set of of records can cover this structure.
   The closest encloser rule in RFC 4592 [RFC4592] makes the wildcards
   match the appropriate names.

   *            IN TXT "bound=1 "." "." "us"
   *         IN TXT "bound=1 "." "." ""
   *     IN TXT "bound=1 "." "." ""

   In the usual case that only the boundary closest to the looked up
   domain name is of interest, the publishing entity can publish
   "shadow" wildcard records for lower level boundaries that are used
   rather than the higher level records for names below those

   *         IN TXT "bound=1 "." "." ""
   *     IN TXT "bound=1 "." "." ""

   [TBD: What if anything to put at the shadow point?  Currently if you
   look up "" you'll get nothing because it's an ENT.
   Is that OK?  If not should it point to or ]

   In some cases, a domain may assert that every name below a given name
   is a boundary, or that every name other than a specific set of
   exceptions is a boundary.  For example (adapted from the Mozilla PSL)

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   every name below is a boundary other than  This
   could be expressed as:

   *       IN TXT "bound=1 . . *"    IN TXT "bound=1 NOBOUND ."
   *  IN TXT "bound=1 NOBOUND ."

   For any set of policy boundaries in a tree of DNS names, a suitable
   set of policy records can describe the boundaries, so a client can
   find the boundary for any name in the tree with a single policy
   lookup per level of delegation.

   Since the delegation structure is unlikely to change frequently, long
   time-to-live (TTL) values in the TXT records are appropriate.

   If different applications have different boundaries or policy
   options, the policy records for each application are put at the
   appropriate names for the boundaries.  Due to the way DNS wildcards
   work, each name with any policy records MUST have records for all
   policies, with the NOBOUND bit for policies for which the name is not
   in fact a boundary.

6.  Application scenarios

   Here are some ways that DMARC and potentially other applications can
   use BOUND data.

6.1.  DMARC

   If a DMARC lookup for the domain in a message's From: header fails,
   the client would do a boundary check for the domain name using the
   "DMARC" application.  The organizational domain is the immediate
   subdomain of the boundary domain.  (Note that the boundary will
   always be the one looked up or an ancestor.)

6.2.  Cookies

   If an http request attempts to set a cookie for a domain other than
   the request's own domain, the client would do boundary check for a
   "cookie" application for both the request's domain and the cookie
   domain.  If they are not separated by a boundary, the request is

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6.3.  SSL Certificates

   The client would do a boundary check for the domain name in a normal
   certificate, or the name after the "*." in a wildcard certificate for
   a "cert" application.  If the boundary is above the name, the name is

7.  Discussion

   The total number of DNS lookups is no more than the number of levels
   of boundary delegation, plus one if the last boundary doesn't have
   the NOLOWER keyword.  That is unlikely to be more than 2 or 3 in
   realistic scenarios, and depends on the number of boundaries, not the
   number of components in the names that are looked up.  With shadow
   records, it will typically be one lookup that matches a shadow record
   and a second to check below it that gets NXDOMAIN if the shadow
   record doesn't contain NOLOWER.

   Some domains have very irregular boundaries.  This may require a
   relatively large number of records to describe all the boundaries,
   but it doesn't seem like a number that would challenge modern DNS
   servers, or need unduly complex scripts to create them.  A mechanical
   translation of the boundary information the Mozilla PSL as of April
   2020 creates about 16,000 records.

   The wildcard lookup means that each time an application looks up the
   boundaries for a hostname, the lookup results create DNS cache
   entries that will not be reused other than for subsequent lookups for
   the identical hostname.  This might cause cache churn, but it seems
   at worst no more than we already tolerate from DNSBL lookups.  If the
   boundary zone is signed, DNS caches should be able to syntheize some
   answers from cached woldcards.

8.  ABNF syntax of bound records

   The syntax of bound records is something like this:

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   ; each token in BOUND is a separate string in the TXT record


   BTAG = %s"bound=1"

   BFLAGS = ( BFLAG *("," BFLAG) ) / "."


   BKWDS = ( BKWD *("," BKWD) ) / "."

   BKWD  = s%"DMARC" / s%"COOKIE" / s%"CERT" / MISCTOK

   ; miscellaneous tokens for future expansion


9.  Security Considerations

   The purpose of publishing organization boundaries is to provide
   advice to third parties that wish to know whether two names are
   managed by the same organization, allowing those names to be treated
   "as the same" in some sense.  Clients that rely on published
   boundaries are outsourcing some part of their own security policy to
   the publisher, so their own security depends on the publisher's
   boundaries being accurate.

   Although in some sense domains are always in control of their
   subdomains, there are many situations in which parent domains are not
   expected to influence subdomains.  For example, second level domains
   in global TLDs (gTLDs) operated by registries with contracts with the
   Internet Corporation for Assigned Names and Numers (ICANN) Since
   there is no technical bar to a parent publishing records that shadow
   part or all of the boundary record namespace for delegated
   subdomains, correct operation depends on the parent and subdomains
   agreeing about who publishes what.

   The DNS is subject to a variety of attacks.  DBOUND records are
   subject to the same ones as any other bit of the DNS, and the same
   countermeasures, such as using DNSSEC, apply.

10.  Variations

   Some boundary schemes distinguish between public and private
   subtrees.  If that were useful, a PUBLIC flag keyword could indicate
   that the subtrees below a boundary were public rather than the
   default of private.

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   Since nothing but the boundary records should be published at names
   with _bound components, one could get the same effect with a new
   DBOUND RRTYPE, which would avoid the problem of confusion with other
   TXT wildcards.

   If third parties want to publish boundary information, they can do it
   in their own subtree of the DNS.  For example, if policy.example were
   publishing boundary information about boundaries, the records for the
   test domain described above would be:

     *._bound.test.policy.exaple IN TXT "bound=1" "." "." "."
     _bound.test.policy.example  IN TXT "bound=1" "." "." "."

11.  IANA considerations

   IANA is requested to add this entry to the "Underscored and Globally
   Scoped DNS Node Names" Registry.

                | RR Type | _NODE NAME | Reference       |
                |   TXT   | _bound     | (this document) |

                                 Table 1

   This document requests that IANA create a registry of dbound Flag
   keywords.  Its registration policy is IETF Review.  Its initial
   contents are as follows.  [[NOTE: new flags are likely to change the
   lookup algorithm]]

         | Keyword | Reference       | Description              |
         | NOLOWER | (this document) | No lower level policies  |
         | NOBOUND | (this document) | No boundary at this name |

               Table 2: BOUND Flag Keywords Initial Values

   This document requests that IANA create a registry of BOUND
   Application keywords.  Its registration policy is First Come First
   Served.  Its initial contents are as follows.  [[Note: New
   applications don't affect the lookup process, and shouldn't affect
   existing applications.]]

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         |  Value  | Reference       | Description               |
         | . (Any) | (this document) | Any application without a |
         |         |                 | specific boundary record  |
         |  DMARC  | (this document) | DMARC organizational      |
         |         |                 | domains                   |
         |  COOKIE | (this document) | HTTP cookies              |
         |   CERT  | (this document) | Owner of certificate      |
         |         |                 | requests                  |

                 Table 3: BOUND Applications Initial Values

12.  References

12.1.  Normative References

   [RFC1034]  Mockapetris, P.V., "Domain names - concepts and
              facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034,
              November 1987, <>.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

12.2.  Informative References

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

Appendix A.  Change Log

   *NOTE TO RFC EDITOR: This section may be removed upon publication of
   this document as an RFC.*

   03 to -04  Make TXT fields separate strings, add shadow records,
      update ABND

   02 to -03  Add wildcard labels like in the PSL.

   01 to -02  Make TXT record the proposal, new RR as alternative.

   -00 to -01  Editorial changes to limit standard use to DMARC.

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   non-WG to -00  Add NOBOUND record to make wildcard matches do the
      right thing

      Rename to match WG name

Author's Address

   John Levine
   Taughannock Networks
   PO Box 727


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