Internet Engineering Task Force (IETF)                         L. Howard
Request for Comments: 8501                                       Retevia
Category: Informational                                    November 2018
ISSN: 2070-1721


           Reverse DNS in IPv6 for Internet Service Providers

Abstract

   In IPv4, Internet Service Providers (ISPs) commonly provide
   IN-ADDR.ARPA information for their customers by prepopulating the
   zone with one PTR record for every available address.  This practice
   does not scale in IPv6.  This document analyzes different approaches
   and considerations for ISPs in managing the IP6.ARPA zone.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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/rfc8501.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Reverse DNS in IPv4 . . . . . . . . . . . . . . . . . . .   3
     1.2.  Reverse DNS Considerations in IPv6  . . . . . . . . . . .   4
   2.  Alternatives in IPv6  . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Negative Response . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Wildcard Match  . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  Dynamic DNS from Individual Hosts . . . . . . . . . .   6
       2.3.2.  Dynamic DNS through Residential Gateways  . . . . . .   7
       2.3.3.  Automatic DNS Delegations . . . . . . . . . . . . . .   7
       2.3.4.  Generate Dynamic Records  . . . . . . . . . . . . . .   8
       2.3.5.  Populate from DHCP Server . . . . . . . . . . . . . .   8
       2.3.6.  Populate from RADIUS Server . . . . . . . . . . . . .   8
     2.4.  Delegate DNS  . . . . . . . . . . . . . . . . . . . . . .   9
     2.5.  Dynamically Generate PTR When Queried ("On the Fly")  . .   9
   3.  Manual User Updates . . . . . . . . . . . . . . . . . . . . .  10
   4.  Considerations and Recommendations  . . . . . . . . . . . . .  10
   5.  Security and Privacy Considerations . . . . . . . . . . . . .  11
     5.1.  Using Reverse DNS for Security  . . . . . . . . . . . . .  11
     5.2.  DNS Security with Dynamic DNS . . . . . . . . . . . . . .  11
     5.3.  Considerations for Other Uses of the DNS  . . . . . . . .  12
     5.4.  Privacy Considerations  . . . . . . . . . . . . . . . . .  12
     5.5.  User Creativity . . . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   [RFC1912] recommended that "every Internet-reachable host should have
   a name" and says "Failure to have matching PTR and A records can
   cause loss of Internet services similar to not being registered in
   the DNS at all".  While the need for a PTR record and for it to match
   is debatable as a best practice, some network services (see
   Section 4) still do rely on PTR lookups, and some check the source
   address of incoming connections and verify that the PTR and A records
   match before providing service.

   Individual Internet users on the residential or consumer scale,
   including small and home businesses, are constantly connecting to or
   moving around the Internet.  For large ISPs who serve residential
   users, maintenance of individual PTR records is impractical.

   Administrators at ISPs should consider whether customer PTR records
   are needed, and if so, evaluate methods for responding to reverse DNS
   queries in IPv6.

1.1.  Reverse DNS in IPv4

   ISPs that provide access to many residential users typically assign
   one or a few IPv4 addresses to each of those users and populate an
   IN-ADDR.ARPA zone with one PTR record for every IPv4 address.  Some
   ISPs also configure forward zones with matching A records so that
   lookups match.  For instance, if an ISP Example.com aggregated
   192.0.2.0/24 at a network hub in one region, the reverse zone might
   look like:

   1.2.0.192.IN-ADDR.ARPA.  IN PTR 1.string.region.example.com.

   2.2.0.192.IN-ADDR.ARPA.  IN PTR 2.string.region.example.com.

   3.2.0.192.IN-ADDR.ARPA.  IN PTR 3.string.region.example.com.

   .

   .

   .

   254.2.0.192.IN-ADDR.ARPA.  IN PTR 254.string.region.example.com.

   The conscientious Example.com might then also have a zone:

   1.string.region.example.com.  IN A 192.0.2.1

   2.string.region.example.com.  IN A 192.0.2.2

   3.string.region.example.com.  IN A 192.0.2.3

   .

   .

   .

   254.string.region.example.com.  IN A 192.0.2.254

   Many ISPs generate PTR records for all IP addresses used for
   customers, and many create the matching A record.

1.2.  Reverse DNS Considerations in IPv6

   A sample entry for 2001:0db8:0f00:0000:0012:34ff:fe56:789a might be:

   a.9.8.7.6.5.e.f.f.f.4.3.2.1.0.0.0.0.0.0.0.0.f.0.8.b.d.0.1.0.0.2
   .IP6.ARPA.  IN PTR 1.string.region.example.com.

   ISPs will often delegate an IPv6 prefix to their customers.  Since
   2^^80 possible addresses could be configured in a single /48 zone
   alone, it is impractical to write a zone with every possible address
   entered, even with automation.  If 1000 entries could be written per
   second, the zone would still not be complete after 38 trillion years.

   Furthermore, it is often impossible to associate hostnames and
   addresses, since the 64 bits in the Interface Identifier portion of
   the address are frequently assigned using Stateless Address
   Autoconfiguration (SLAAC) [RFC4862] when the host comes online, and
   they may be short-lived.

   [RFC1912] is an Informational RFC that says "PTR records must point
   back to a valid A record" and further that the administrator should
   "Make sure your PTR and A records match."  Herein are considerations
   for how to follow this advice for AAAA and PTR records.

2.  Alternatives in IPv6

   Several options exist for providing reverse DNS in IPv6.  All of
   these options also exist for IPv4, but the scaling problem is much
   less severe in IPv4.  Each option should be evaluated for its scaling
   ability, compliance with existing standards and best practices, and
   availability in common systems.

2.1.  Negative Response

   Some ISP DNS administrators may choose to provide only an NXDOMAIN
   response to PTR queries for subscriber addresses.  In some ways, this
   is the most accurate response, since no name information is known
   about the host.  However, providing a negative response to PTR
   queries does not satisfy the expectation in [RFC1912] for entries to
   match.  Users of services that are dependent on a successful lookup
   will have a poor experience.  For instance, some web services and
   Secure Shell (SSH) connections wait for a DNS response, even
   NXDOMAIN, before responding.  For the best user experience, then, it
   is important to return a response, rather than time out with no
   answer.  On the other hand, external mail servers are likely to
   reject connections, which might be an advantage in fighting spam.

   When evaluating this option, DNS administrators should consider the
   uses for reverse DNS records and the number of services affecting the
   number of users.

2.2.  Wildcard Match

   The use of wildcards in the DNS is described in [RFC4592], and their
   use in IPv6 reverse DNS is described in [RFC4472].

   While recording all possible addresses is not scalable, it may be
   possible to record a wildcard entry for each prefix assigned to a
   customer.  Consider also that "inclusion of wildcard NS RRSets in a
   zone is discouraged, but not barred".  [RFC4592]

   This solution generally scales well.  However, since the response
   will match any address in the wildcard range (/48, /56, /64, etc.), a
   forward DNS lookup on the response given will not be able to return
   the same hostname.  This method therefore fails the expectation in
   [RFC1912] that forward and reverse will match.  DNSSEC [RFC4035]
   scalability is limited to signing the wildcard zone, which may be
   satisfactory.

2.3.  Dynamic DNS

   One way to ensure that forward and reverse records match is for hosts
   to update DNS servers dynamically once interface configuration is
   complete (whether by SLAAC, DHCPv6, or other means), as described in
   [RFC4472].  Hosts would need to provide both AAAA and PTR updates and
   would need to know which servers would accept the information.

   This option should scale as well or as poorly as IPv4 dynamic DNS
   (DDNS) does.  Dynamic DNS may not scale effectively in large ISP
   networks that have no single master name server, but a single master
   server is not best practice.  The ISP's DNS system may provide a
   point for Denial-of-Service attacks, including many attempted DDNS
   updates.  Accepting updates only from authenticated sources may
   mitigate this risk, but only if authentication itself does not
   require excessive overhead.  No authentication of dynamic DNS updates
   is inherently provided.  Implementers should therefore consider use
   of TSIG [RFC2845], or at least ingress filtering, so that updates are
   only accepted from customer address space from internal network
   interfaces.  They should also rate limit the number of updates from a
   customer per second and consider impacts on scalability.  UDP is
   allowed per [RFC2136], so connection reliability is not assured,
   though the host should expect an ERROR or NOERROR message from the
   server; TCP provides transmission control, but the updating host
   would need to be configured to use TCP.

   Administrators may want to consider user creativity if they provide
   hostnames, as described in Section 5.5, "User Creativity".

   There is no assurance of uniqueness if multiple hosts try to update
   with the same name ("mycomputer.familyname.org").  There is no
   standard way to indicate to a host what server it should send DDNS
   updates to; the master listed in the SOA is often assumed to be a
   DDNS server, but this may not scale.

2.3.1.  Dynamic DNS from Individual Hosts

   In the simplest case, a residential user will have a single host
   connected to the ISP.  Since the typical residential user cannot
   configure IPv6 addresses or resolving name servers on their hosts,
   the ISP should provide address information conventionally -- i.e.,
   using their normal combination of Router Advertisements (RAs), DHCP,
   etc.  The ISP should also provide a DNS Recursive Name Server and
   Domain Search List as described in [RFC3646] or [RFC8106].  In
   determining its Fully Qualified Domain Name (FQDN), a host will
   typically use a domain from the Domain Search List.  This is an
   overloading of the parameter; multiple domains could be listed, since
   hosts may need to search for unqualified names in multiple domains
   without necessarily being a member of those domains.  Administrators
   should consider whether the Domain Search List actually provides an
   appropriate DNS suffix(es) when considering use of this option.  For
   the purposes of dynamic DNS, the host would concatenate its local
   hostname (e.g., "hostname") plus the domain(s) in the Domain Search
   List (e.g., "customer.example.com"), as in
   "hostname.customer.example.com".

   Once it learns its address and has a resolving name server, the host
   must perform an SOA lookup on the IP6.ARPA record to be added in
   order to find the owner and eventually the server that is
   authoritative for the zone (which might accept dynamic updates).
   Several recursive lookups may be required to find the longest prefix
   that has been delegated.  The DNS administrator must designate the
   Primary Master Server for the longest match required.  Once found,
   the host sends dynamic AAAA and PTR updates using the concatenation
   defined above ("hostname.customer.example.com").

   In order to use this alternative, hosts must be configured to use
   dynamic DNS.  This is not default behavior for many hosts, which is
   an inhibitor for a large ISP.  This option may be scalable, although
   registration following an outage may cause significant load, and
   hosts using privacy extensions [RFC4941] may update records daily.
   It is up to the host to provide matching forward and reverse records
   and update them when the address changes.

2.3.2.  Dynamic DNS through Residential Gateways

   Residential customers may have a gateway, which may provide DHCPv6
   service to hosts from a delegated prefix.  ISPs should provide a DNS
   Recursive Name Server and Domain Search List to the gateway, as
   described above and in [RFC3646] and [RFC8106].  There are two
   options for how the gateway uses this information.  The first option
   is for the gateway to respond to DHCPv6 requests with the same DNS
   Recursive Name Server and Domain Search List provided by the ISP.
   The alternate option is for the gateway to relay dynamic DNS updates
   from hosts to the servers and domain provided by the ISP.  Host
   behavior is unchanged; the host sends the same dynamic updates,
   either to the ISP's server (as provided by the gateway) or to the
   gateway for it to forward.  The gateway would need to be capable of
   and configured to use dynamic DNS.

2.3.3.  Automatic DNS Delegations

   An ISP may delegate authority for a subdomain, such as
   "customer12345.town.AW.customer.example.com" or
   "customer12345.example.com", to the customer's gateway.  Each domain
   thus delegated must be unique within the DNS.  The ISP may also then
   delegate the IP6.ARPA zone for the prefix delegated to the customer,
   as in (for 2001:db8:f00::/48) "0.0.f.0.8.b.d.0.1.0.0.2.IP6.ARPA".
   Then, the customer could provide updates to their own gateway, with
   forward and reverse.  However, individual hosts connected directly to
   the ISP rarely have the capability to run DNS for themselves;
   therefore, an ISP can only delegate to customers with gateways
   capable of being authoritative name servers.  If a device requests a
   DHCPv6 Prefix Delegation, that may be considered a reasonably
   reliable indicator that it is a gateway, rather than an individual
   host.  It is not necessarily an indicator that the gateway is capable
   of providing DNS services and therefore cannot be relied upon as a
   way to test whether this option is feasible.  In fact, this kind of
   delegation will not work for devices complying with [RFC6092], which
   includes the requirement, "By DEFAULT, inbound DNS queries received
   on exterior interfaces MUST NOT be processed by any integrated DNS
   resolving server".

   If the customer's gateway is the name server, it provides its own
   information to hosts on the network, as described in [RFC2136], which
   is often done for enterprise networks.

   An ISP could provide authoritative responses as a secondary server to
   the customer's master server.  For instance, the home gateway name
   server could be the master server, with the ISP providing the only
   published NS authoritative servers.

   To implement this alternative, users' residential gateways must be
   capable of acting as authoritative name servers capable of dynamic
   DNS updates.  There is no mechanism for an ISP to dynamically
   communicate to a user's equipment that a zone has been delegated, so
   user action would be required.  Most users have neither the equipment
   nor the expertise to run DNS servers, so this option is unavailable
   to the residential ISP.

2.3.4.  Generate Dynamic Records

   An ISP's name server that receives a dynamic forward or reverse DNS
   update may create a matching entry.  Since a host capable of updating
   one is generally capable of updating the other, this should not be
   required, but redundant record creation will ensure that a record
   exists.  ISPs implementing this method should check whether a record
   already exists before accepting or creating updates.

   This method is also dependent on hosts being capable of providing
   dynamic DNS updates, which is not default behavior for many hosts.

2.3.5.  Populate from DHCP Server

   An ISP's DHCPv6 server may populate the forward and reverse zones
   when the DHCP request is received if the request contains enough
   information [RFC4704].

   However, this method will only work for a single host address
   (IA_NA); the ISP's DHCP server would not have enough information to
   update all records for a prefix delegation.  If the zone authority is
   delegated to a home gateway that used this method, the gateway could
   update records for residential hosts.  To implement this alternative,
   users' residential gateways would have to support the FQDN DHCP
   option and would have to either have the zones configured or send
   DDNS messages to the ISP's name server.

2.3.6.  Populate from RADIUS Server

   A user may receive an address or prefix from a RADIUS server
   [RFC2865], the details of which may be recorded via RADIUS Accounting
   data [RFC2866].  The ISP may populate the forward and reverse zones
   from the accounting data if it contains enough information.  This
   solution allows the ISP to populate data concerning allocated
   prefixes as per Section 2.2 (wildcards) and customer premise
   equipment (CPE) endpoints.  However, as with Section 2.3.5, it does
   not allow the ISP to populate information concerning individual
   hosts.

2.4.  Delegate DNS

   For customers who are able to run their own DNS servers, such as
   commercial customers, often the best option is to delegate the
   reverse DNS zone to them, as described in [RFC2317] (for IPv4).
   However, since most residential users have neither the equipment nor
   the expertise to run DNS servers, this method is unavailable to
   residential ISPs.

   This is a general case of the specific case described in
   Section 2.3.3.  All of the same considerations still apply.

2.5.  Dynamically Generate PTR When Queried ("On the Fly")

   Common practice in IPv4 is to provide PTR records for all addresses,
   regardless of whether a host is actually using the address.  In IPv6,
   ISPs may generate PTR records for all IPv6 addresses as the records
   are requested.  Several DNS servers are capable of this.

   An ISP using this option should generate a PTR record on demand and
   cache or prepopulate the forward (AAAA) entry for the duration of the
   Time to Live of the PTR.  Similarly, the ISP would prepopulate the
   PTR following a AAAA query.  To reduce exposure to a Denial-of-
   Service attack, state or storage should be limited.  Alternatively,
   if an algorithm is used to generate a unique name, it can be employed
   on the fly in both directions.  This option has the advantage of
   assuring matching forward and reverse entries while being simpler
   than dynamic DNS.  Administrators should consider whether the lack of
   user-specified hostnames is a drawback.  It may be possible to allow
   user-specified hostnames for some records and generate others on the
   fly; looking up a record before generating on the fly may slow
   responses or may not scale well.

   DNSSEC [RFC4035] signing records on the fly may increase load;
   unsigned records can indicate that these records are less trusted,
   which might be acceptable.

   Another consideration is that the algorithm used for generating the
   record must be the same on all servers for a zone.  In other words,
   any server for the zone must produce the same response for a given
   query.  Administrators managing a variety of rules within a zone
   might find it difficult to keep those rules synchronized on all
   servers.

3.  Manual User Updates

   It is possible to create a user interface, such as a web page, that
   would allow end users to enter a hostname to associate with an
   address.  Such an interface would need to be authenticated; only the
   authorized user could add/change/delete entries.  If the ISP changes
   prefixes assigned to customers, the interface would need to specify
   only the host bits.  The backend would therefore need to be
   integrated with prefix assignments so that when a new prefix was
   assigned to a customer, the DNS service would look up user-generated
   hostnames, delete the old record, and create the new one.

   Considerations about some records being static and others dynamic or
   dynamically generated (Section 2.5) and the creativity of users
   (Section 5.5) still apply.

4.  Considerations and Recommendations

   There are six common uses for PTR lookups:

   Rejecting mail: A PTR with a certain string may indicate "This host
   is not a mail server", which may be useful for rejecting probable
   spam.  The absence of a PTR leads to the desired behavior.

   Serving ads: "This host is probably in town.province."  An ISP that
   does not provide PTR records might affect somebody else's geolocation
   (also see privacy consideration about location).

   Accepting SSH connections: The presence of a PTR may be inferred to
   mean "This host has an administrator with enough clue to set up
   forward and reverse DNS".  This is a poor inference.

   Log files: Many systems will record the PTR of remote hosts in their
   log files to make it easier when reading logs later to see what
   network the remote host uses.

   Traceroute: The ability to identify an interface and name of any
   intermediate node or router is important for troubleshooting.

   Service discovery: [RFC6763] specifies "DNS-Based Service Discovery",
   and Section 11 specifically describes how PTRs are used.

   As a general guideline, when address assignment and name are under
   the same authority, or when a host has a static address and name,
   AAAA and PTR records should exist and match.  For residential users,
   if these use cases are important to the ISP, the administrator will
   then need to consider how to provide PTR records.

   The best accuracy would be achieved if ISPs delegated authority along
   with address delegation, but residential users rarely have domain
   names or authoritative name servers.

   Dynamic DNS updates can provide accurate data, but there is no
   standard way to indicate to residential devices where to send
   updates, whether the hosts support DDNS, or whether it scales.

   An ISP has no knowledge of its residential users' hostnames and
   therefore can either provide a wildcard response or a dynamically
   generated response.  A valid negative response (such as NXDOMAIN) is
   a valid response if the four cases above are not essential;
   delegation where no name server exists should be avoided.

5.  Security and Privacy Considerations

5.1.  Using Reverse DNS for Security

   Some people think the existence of reverse DNS records, or matching
   forward and reverse DNS records, provides useful information about
   the hosts with those records.  For example, one might infer that the
   administrator of a network with properly configured DNS records was
   better informed, and by further inference more responsible, than the
   administrator of a less thoroughly configured network.  For instance,
   most email providers will not accept incoming connections on port 25
   unless forward and reverse DNS entries match.  If they match, but
   information higher in the stack (for instance, mail source) is
   inconsistent, the packet is questionable.  However, these records may
   be easily forged unless DNSSEC or other measures are taken.  The
   string of inferences is questionable and may become unneeded if other
   means for evaluating trustworthiness (such as positive reputations)
   become predominant in IPv6.

   Providing location information in PTR records is useful for
   troubleshooting, law enforcement, and geolocation services, but for
   the same reasons, it can be considered sensitive information.

5.2.  DNS Security with Dynamic DNS

   The security considerations for using dynamic DNS are described in
   [RFC3007].  DNS Security Extensions are documented in [RFC4033].

   Interactions with DNSSEC are described throughout this document.

5.3.  Considerations for Other Uses of the DNS

   Several methods exist for providing encryption keys in the DNS.  Any
   of the options presented here may interfere with these key
   techniques.

5.4.  Privacy Considerations

   Given the considerations in [RFC8117], hostnames that provide
   information about a user compromise that user's privacy.  Some users
   may want to identify their hosts using user-specified hostnames, but
   the default behavior should not be to identify a user, their
   location, their connectivity, or other information in a PTR record.

5.5.  User Creativity

   Though not precisely a security consideration, administrators may
   want to consider what domain will contain the records and who will
   provide the names.  If subscribers provide hostnames, they may
   provide inappropriate strings.  Consider "ihate.example.com" or
   "badword.customer.example.com" or
   "celebrityname.committed.illegal.acts.example.com".

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [RFC1912]  Barr, D., "Common DNS Operational and Configuration
              Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
              <https://www.rfc-editor.org/info/rfc1912>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and
              B. Wellington, "Secret Key Transaction Authentication for
              DNS (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
              <https://www.rfc-editor.org/info/rfc2845>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <https://www.rfc-editor.org/info/rfc2865>.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866,
              DOI 10.17487/RFC2866, June 2000,
              <https://www.rfc-editor.org/info/rfc2866>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,
              <https://www.rfc-editor.org/info/rfc3646>.

   [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>.

   [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>.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
              <https://www.rfc-editor.org/info/rfc4592>.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,
              <https://www.rfc-editor.org/info/rfc4704>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

7.2.  Informative References

   [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless
              IN-ADDR.ARPA delegation", BCP 20, RFC 2317,
              DOI 10.17487/RFC2317, March 1998,
              <https://www.rfc-editor.org/info/rfc2317>.

   [RFC4472]  Durand, A., Ihren, J., and P. Savola, "Operational
              Considerations and Issues with IPv6 DNS", RFC 4472,
              DOI 10.17487/RFC4472, April 2006,
              <https://www.rfc-editor.org/info/rfc4472>.

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.

   [RFC8117]  Huitema, C., Thaler, D., and R. Winter, "Current Hostname
              Practice Considered Harmful", RFC 8117,
              DOI 10.17487/RFC8117, March 2017,
              <https://www.rfc-editor.org/info/rfc8117>.

Acknowledgements

   The author would like to thank Alain Durand, JINMEI Tatuya, David
   Freedman, Andrew Sullivan, Chris Griffiths, Darryl Tanner, Ed Lewis,
   John Brzozowski, Chris Donley, Wes George, Jason Weil, John Spence,
   Ted Lemon, Stephan Lagerholm, Steinar Haug, Mark Andrews, Chris
   Roosenraad, Fernando Gont, John Levine, and many others who discussed
   and provided suggestions for this document.

Author's Address

   Lee Howard
   Retevia
   Fairfax, VA  22032
   United States of America

   Email: lee.howard@retevia.net