    Internet Engineering Task Force (IETF)                        P. Pfister
    Request for Comments: 8375                                 Cisco Systems
    Updates: 7788                                                   T. Lemon
    Category: Standards Track                            Nibbhaya Consulting
    ISSN: 2070-1721                                                 May 2018

                        Special-Use Domain 'home.arpa.'

    Abstract

       This document specifies the behavior that is expected from the Domain
       Name System with regard to DNS queries for names ending with
       '.home.arpa.' and designates this domain as a special-use domain
       name. 'home.arpa.' is designated for non-unique use in residential
       home networks.  The Home Networking Control Protocol (HNCP) is
       updated to use the 'home.arpa.' domain instead of '.home'.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8375.

    Copyright Notice

       Copyright (c) 2018 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
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
       3.  General Guidance  . . . . . . . . . . . . . . . . . . . . . .   4
       4.  Domain Name Reservation Considerations  . . . . . . . . . . .   4
       5.  Updates to Home Networking Control Protocol . . . . . . . . .   7
       6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
         6.1.  Local Significance  . . . . . . . . . . . . . . . . . . .   7
         6.2.  Insecure Delegation . . . . . . . . . . . . . . . . . . .   8
         6.3.  Bypassing Manually Configured Resolvers . . . . . . . . .   9
       7.  Delegation of 'home.arpa.'  . . . . . . . . . . . . . . . . .   9
       8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
       9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
         9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
         9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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    1.  Introduction

       Users and devices within a home network (hereafter referred to as
       "homenet") require devices and services to be identified by names
       that are unique within the boundaries of the homenet [RFC7368].  The
       naming mechanism needs to function without configuration from the
       user.  While it may be possible for a name to be delegated by an ISP,
       homenets must also function in the absence of such a delegation.
       This document reserves the name 'home.arpa.' to serve as the default
       name for this purpose, with a scope limited to each individual
       homenet.

       This document corrects an error in [RFC7788] by replacing '.home'
       with 'home.arpa.' as the default domain name for homenets. '.home'
       was selected as the most user-friendly option; however, there are
       existing uses of '.home' that may be in conflict with this use.
       Evidence indicates that '.home' queries frequently leak out and reach
       the root name servers [ICANN1] [ICANN2].

       In addition, for compatibility with DNSSEC (see Section 6), it's
       necessary that an insecure delegation (see Section 4.3 of [RFC4035])
       be present for the name.  There is an existing process for allocating
       names under '.arpa.'  [RFC3172].  No such process is available for
       requesting a similar delegation in the root at the request of the
       IETF, which does not administer that zone.  As a result, all
       unregistered uses of '.home' (that is, all current uses at the time
       of this document's publication), particularly as specified in
       [RFC7788], are deprecated.

       This document registers the domain 'home.arpa.' as a special-use
       domain name [RFC6761] and specifies the behavior that is expected
       from the Domain Name System with regard to DNS queries for names
       whose rightmost non-terminal labels are 'home.arpa.'.  Queries for
       names ending with '.home.arpa.' are of local significance within the
       scope of a homenet, meaning that identical queries will result in
       different results from one homenet to another.  In other words, a
       name ending in '.home.arpa.' is not globally unique.

       Although this document makes specific reference to [RFC7788], it is
       not intended that the use of 'home.arpa.' be restricted solely to
       networks where HNCP is deployed.  Rather, 'home.arpa.' is intended to
       be the correct domain for uses like the one described for '.home' in
       [RFC7788]: local name service in residential homenets.

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    2.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    3.  General Guidance

       The domain name 'home.arpa.' is to be used for naming within
       residential homenets.  Names ending with '.home.arpa.' reference a
       zone that is served locally, the contents of which are unique only to
       a particular homenet and are not globally unique.  Such names refer
       to nodes and/or services that are located within a homenet (e.g., a
       printer or a toaster).

       DNS queries for names ending with '.home.arpa.' are resolved using
       local resolvers on the homenet.  Such queries MUST NOT be recursively
       forwarded to servers outside the logical boundaries of the homenet.

       Some service discovery user interfaces that are expected to be used
       on homenets conceal information such as domain names from end users.
       However, in some cases, it is still expected that users will need to
       see, remember, and even type names ending with '.home.arpa.'.  The
       Homenet Working Group hopes that this name will in some way indicate
       to as many readers as possible that such domain names are referring
       to devices in the home, but we recognize that it is an imperfect
       solution.

    4.  Domain Name Reservation Considerations

       This section specifies considerations for systems involved in domain
       name resolution when resolving queries for names ending with
       '.home.arpa.'.  Each item in this section addresses some aspect of
       the DNS or the process of resolving domain names that would be
       affected by this special-use allocation.  Detailed explanations of
       these items can be found in Section 5 of [RFC6761].  Although the
       term 'homenet' in [RFC7788] refers to home networks that implement a
       particular set of features, in this document the term is used to mean
       any home network, regardless of the set of features it implements.

       1.  Users can use names ending with '.home.arpa.' just as they would
           use any other domain name.  The 'home.arpa.' name is chosen to be
           readily recognized by users as signifying that the name is
           addressing a service on the homenet to which the user's device is
           connected.

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       2.  Application software SHOULD NOT treat names ending in
           '.home.arpa.' differently than other names.  In particular, there
           is no basis for trusting names that are subdomains of
           'home.arpa.' (see Section 6).

       3.  Name resolution APIs and libraries MUST NOT recognize names that
           end in '.home.arpa.' as special and MUST NOT treat them as having
           special significance, except that it may be necessary that such
           APIs not bypass the locally configured recursive resolvers.

           One or more IP addresses for recursive DNS servers will usually
           be supplied to the client through router advertisements or DHCP.
           For an administrative domain that uses subdomains of
           'home.arpa.', such as a homenet, the recursive resolvers provided
           by that domain will be able to answer queries for subdomains of
           'home.arpa.'; other resolvers will not, or they will provide
           answers that are not correct within that administrative domain.

           A host that is configured to use a resolver other than one that
           has been provided by the local network may be unable to resolve,
           or may receive incorrect results for, subdomains of 'home.arpa.'.
           In order to avoid this, it is permissible that hosts use the
           resolvers that are locally provided for resolving 'home.arpa.',
           even when they are configured to use other resolvers.

       4.  There are three considerations for recursive resolvers that
           follow this specification:

           A.  Recursive resolvers at sites using 'home.arpa.'  MUST
               transparently support DNSSEC queries: queries for DNSSEC
               records and queries with the DNSSEC OK (DO) bit set (see
               Section 3.2.1 of [RFC4035]).  While validation is not
               required, it is strongly encouraged: a caching recursive
               resolver that does not validate answers that can be validated
               may cache invalid data.  This, in turn, will prevent
               validating stub resolvers from successfully validating
               answers.

           B.  Unless configured otherwise, recursive resolvers and DNS
               proxies MUST behave as described in Section 3 of the Locally
               Served Zones document [RFC6303].  That is, queries for
               'home.arpa.' and subdomains of 'home.arpa.'  MUST NOT be
               forwarded, with one important exception: a query for a DS
               record with the DO bit set MUST return the correct answer for
               that question, including correct information in the authority
               section that proves that the record is nonexistent.

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               So, for example, a query for the NS record for 'home.arpa.'
               MUST NOT result in that query being forwarded to an upstream
               cache nor to the authoritative DNS server for '.arpa.'.
               However, as necessary to provide accurate authority
               information, a query for the DS record MUST result in
               forwarding whatever queries are necessary; typically, this
               will just be a query for the DS record, since the necessary
               authority information will be included in the authority
               section of the response if the DO bit is set.

           C.  In addition to the behavior specified above, recursive
               resolvers that can be used in a homenet MUST be configurable
               to forward queries for 'home.arpa.' and subdomains of
               'home.arpa.' to an authoritative server for 'home.arpa.'.
               This server will provide authoritative data for 'home.arpa.'
               within a particular homenet.  The special handling for DS
               records for the 'home.arpa.' delegation is still required.

               It is permissible to combine the recursive resolver function
               for general DNS lookups with an authoritative resolver for
               'home.arpa.'; in this case, rather than forwarding queries
               for subdomains of 'home.arpa.' to an authoritative server,
               the resolver answers them authoritatively.  The behavior with
               respect to forwarding queries specifically for 'home.arpa.'
               remains the same.

       5.  No special processing of 'home.arpa.' is required for
           authoritative DNS server implementations.  It is possible that an
           authoritative DNS server might attempt to check the authoritative
           servers for 'home.arpa.' for a delegation beneath that name
           before answering authoritatively for such a delegated name.  In
           such a case, because the name always has only local significance,
           there will be no such delegation in the 'home.arpa.' zone, and so
           the server would refuse to answer authoritatively for such a
           zone.  A server that implements this sort of check MUST be
           configurable so that either it does not do this check for the
           'home.arpa.' domain or it ignores the results of the check.

       6.  DNS server operators MAY configure an authoritative server for
           'home.arpa.' for use in homenets and other home networks.  The
           operator for the DNS servers authoritative for 'home.arpa.' in
           the global DNS will configure any such servers as described in
           Section 7.

       7.  'home.arpa.' is a subdomain of the 'arpa' top-level domain, which
           is operated by IANA under the authority of the Internet
           Architecture Board according to the rules established in
           [RFC3172].  There are no other registrars for '.arpa'.

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    5.  Updates to Home Networking Control Protocol

       The final paragraph in Section 8 of [RFC7788], the Home Networking
       Control Protocol, is updated as follows:

       OLD:
          Names and unqualified zones are used in an HNCP network to provide
          naming and service discovery with local significance.  A network-
          wide zone is appended to all single labels or unqualified zones in
          order to qualify them. ".home" is the default; however, an
          administrator MAY configure the announcement of a Domain-Name TLV
          (Section 10.6) for the network to use a different one.  In case
          multiple are announced, the domain of the node with the greatest
          node identifier takes precedence.

       NEW:
          Names and unqualified zones are used in an HNCP network to provide
          naming and service discovery with local significance.  A network-
          wide zone is appended to all single labels or unqualified zones in
          order to qualify them. 'home.arpa.' is the default; however, an
          administrator MAY configure the announcement of a Domain-Name TLV
          (Section 10.6) for the network to use a different one.  In case
          multiple TLVs are announced, the domain of the node with the
          greatest node identifier takes precedence.

          The 'home.arpa.' special-use name does not require a special
          resolution protocol.  Names for which the rightmost two labels are
          'home.arpa.' are resolved using the DNS protocol [RFC1035].

    6.  Security Considerations

    6.1.  Local Significance

       A DNS record that is returned as a response to a query for a Fully
       Qualified Domain Name (FQDN) that is a subdomain of 'home.arpa.' is
       expected to have local significance.  It is expected to be returned
       by a server involved in name resolution for the homenet the device is
       connected in.  However, such a response MUST NOT be considered more
       trustworthy than a similar response for any other DNS query.

       Because 'home.arpa.' is not globally scoped and cannot be secured
       using DNSSEC based on the root domain's trust anchor, there is no way
       to tell, using a standard DNS query, in which homenet scope an answer
       belongs.  Consequently, users may experience surprising results with
       such names when roaming to different homenets.

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       To prevent this from happening, it could be useful for the resolver
       on the host to securely differentiate between different homenets and
       between identical names on different homenets.  However, a mechanism
       for doing this has not yet been standardized and doing so is out of
       scope for this document.  It is expected that this will be explored
       in future work.

       The advice in [RFC6303], Section 7, to install local trust anchors
       for locally served zones can only work if there is some way of
       configuring the trust anchor in the host.  Homenet currently
       specifies no mechanism for configuring such trust anchors.  As a
       result, while this advice sounds good, it is not practicable.

       Also, although it might be useful to install a trust anchor for a
       particular instance of 'home.arpa.', it's reasonable to expect that a
       host with such a trust anchor might, from time to time, connect to
       more than one network with its own instance of 'home.arpa.'.  Such a
       host would be unable to access services on any instance of
       'home.arpa.' other than the one for which a trust anchor was
       configured.

       It is, in principle, possible to attach an identifier to an instance
       of 'home.arpa.' that could be used to identify which trust anchor to
       rely on for validating names in that particular instance.  However,
       the security implications of this are complicated, and such a
       mechanism, as well as a discussion of those implications, is out of
       scope for this document.

    6.2.  Insecure Delegation

       It is not possible to install a trust anchor (a DS RR) for this zone
       in the '.arpa' zone.  The reason for this is that in order to do so,
       it would be necessary to have the key-signing key for the zone (see
       Section 5 of [RFC4034]).  Since the zone is not globally unique, no
       one key would work.

       An alternative would be to provide an authenticated denial of
       existence (see Section 3.2 of [RFC4033]).  This would be done simply
       by not having a delegation from the 'arpa.' zone.  However, this
       requires the validating resolver to treat 'home.arpa.' specially.  If
       a validating resolver that doesn't treat 'home.arpa.' specially
       attempts to validate a name in 'home.arpa.', an authenticated denial
       of existence of 'home' as a subdomain of 'arpa.' would cause the
       validation to fail.  Therefore, the only delegation that will allow
       names under 'home.arpa.' to be resolved by all validating resolvers
       is an insecure delegation, as in Section 7 of [RFC6303].

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       Consequently, unless a trust anchor for the particular instance of
       the 'home.arpa.' zone being validated is manually configured on the
       validating resolver, DNSSEC signing and validation of names within
       the 'home.arpa.' zone is not possible.

    6.3.  Bypassing Manually Configured Resolvers

       In item 3 of Section 4, an exception is made to the behavior of stub
       resolvers that allows them to query local resolvers for subdomains of
       'home.arpa.' even when they have been manually configured to use
       other resolvers.  This behavior obviously has security and privacy
       implications and may not be desirable depending on the context.  It
       may be better to simply ignore this exception and, when one or more
       recursive resolvers are configured manually, simply fail to provide
       correct answers for subdomains of 'home.arpa.'.  At this time, we do
       not have operational experience that would guide us in making this
       decision; implementors are encouraged to consider the context in
       which their software will be deployed when deciding how to resolve
       this question.

    7.  Delegation of 'home.arpa.'

       In order to be fully functional, there must be a delegation of
       'home.arpa.' in the '.arpa.' zone [RFC3172].  This delegation MUST
       NOT include a DS record and MUST point to one or more black hole
       servers, for example, 'blackhole-1.iana.org.' and 'blackhole-
       2.iana.org.'.  The reason that this delegation must not be signed is
       that not signing the delegation breaks the DNSSEC chain of trust,
       which prevents a validating stub resolver from rejecting names
       published under 'home.arpa.' on a homenet name server.

    8.  IANA Considerations

       IANA has recorded the domain name 'home.arpa.' in the "Special-Use
       Domain Names" registry [SUDN].  IANA, with the approval of the IAB,
       has implemented the delegation requested in Section 7.

       IANA has created a new subregistry within the "Locally-Served DNS
       Zones" registry [LSDZ], titled "Transport-Independent Locally-Served
       DNS Zone Registry", with the same format as the other subregistries.
       IANA has added an entry in this new registry for 'home.arpa.' with
       the description "Homenet Special-Use Domain", listing this document
       as the reference.  The registration procedure for this subregistry
       should be the same as for the others, currently "IETF Review" (see
       Section 4.8 of [RFC8126]).

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    9.  References

    9.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
                  Requirements for the Address and Routing Parameter Area
                  Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
                  September 2001, <https://www.rfc-editor.org/info/rfc3172>.

       [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "Protocol Modifications for the DNS Security
                  Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
                  <https://www.rfc-editor.org/info/rfc4035>.

       [RFC6303]  Andrews, M., "Locally Served DNS Zones", BCP 163,
                  RFC 6303, DOI 10.17487/RFC6303, July 2011,
                  <https://www.rfc-editor.org/info/rfc6303>.

       [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
                  RFC 6761, DOI 10.17487/RFC6761, February 2013,
                  <https://www.rfc-editor.org/info/rfc6761>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    9.2.  Informative References

       [ICANN1]   "New gTLD Collision Risk Mitigation", August 2013,
                  <https://www.icann.org/en/system/files/files/
                  new-gtld-collision-mitigation-05aug13-en.pdf>.

       [ICANN2]   "New gTLD Collision Occurence Management", October 2013,
                  <https://www.icann.org/en/system/files/files/
                  resolutions-new-gtld-annex-1-07oct13-en.pdf>.

       [LSDZ]     "Locally-Served DNS Zones", July 2011,
                  <https://www.iana.org/assignments/
                  locally-served-dns-zones/>.

       [RFC1035]  Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
                  November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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       [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "DNS Security Introduction and Requirements",
                  RFC 4033, DOI 10.17487/RFC4033, March 2005,
                  <https://www.rfc-editor.org/info/rfc4033>.

       [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "Resource Records for the DNS Security Extensions",
                  RFC 4034, DOI 10.17487/RFC4034, March 2005,
                  <https://www.rfc-editor.org/info/rfc4034>.

       [RFC7368]  Chown, T., Ed., Arkko, J., Brandt, A., Troan, O., and J.
                  Weil, "IPv6 Home Networking Architecture Principles",
                  RFC 7368, DOI 10.17487/RFC7368, October 2014,
                  <https://www.rfc-editor.org/info/rfc7368>.

       [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
                  Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
                  2016, <https://www.rfc-editor.org/info/rfc7788>.

       [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

       [SUDN]     "Special-Use Domain Names", July 2012,
                  <https://www.iana.org/assignments/
                  special-use-domain-names/>.

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    Acknowledgments

       The authors would like to thank Stuart Cheshire, as well as the
       homenet chairs, Mark Townsley and Ray Bellis, for their prior work on
       '.home'.  We would also like to thank Paul Hoffman for providing
       review and comments on the IANA Considerations section, Andrew
       Sullivan for his review and proposed text, and Suzanne Woolf and Ray
       Bellis for their very detailed review comments and process insights.
       Thanks to Mark Andrews for providing an exhaustive reference list on
       the topic of insecure delegations.  Thanks to Dale Worley for
       catching a rather egregious mistake and for the Gen-Art review, and
       thanks to Daniel Migault for a thorough SecDir review.  Thanks to
       Warren Kumari for catching some additional issues and to Adam Roach
       for some helpful clarifications.

    Authors' Addresses

       Pierre Pfister
       Cisco Systems
       Paris
       France

       Email: pierre.pfister@darou.fr

       Ted Lemon
       Nibbhaya Consulting
       P.O. Box 958
       Brattleboro, Vermont  05301-0958
       United States of America

       Email: mellon@fugue.com

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