Locally Served DNS Zones
Why DNS servers should answer reverse lookups for private address space themselves instead of passing the queries on.
Andrews, M., "Locally Served DNS Zones", BCP 163, RFC 6303, DOI 10.17487/RFC6303, July 2011, <https://www.rfc-editor.org/info/rfc6303>.
@misc{rfc6303,
series = {Request for Comments},
number = 6303,
howpublished = {RFC 6303},
publisher = {RFC Editor},
doi = {10.17487/RFC6303},
url = {https://www.rfc-editor.org/info/rfc6303},
author = {M. Andrews},
title = {{Locally Served DNS Zones}},
pagetotal = 10,
year = 2011,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Andrews
Request for Comments: 6303 ISC
BCP: 163 July 2011
Category: Best Current Practice
ISSN: 2070-1721
Locally Served DNS Zones
Abstract
Experience with the Domain Name System (DNS) has shown that there are
a number of DNS zones that all iterative resolvers and recursive
nameservers should automatically serve, unless configured otherwise.
RFC 4193 specifies that this should occur for D.F.IP6.ARPA. This
document extends the practice to cover the IN-ADDR.ARPA zones for RFC
1918 address space and other well-known zones with similar
characteristics.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6303.
Copyright Notice
Copyright (c) 2011 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
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Table of Contents
1. Introduction ....................................................2
1.1. Reserved Words .............................................3
2. Effects on Sites Using RFC 1918 Addresses .......................3
3. Changes to Iterative Resolver Behaviour .........................4
4. Lists Of Zones Covered ..........................................5
4.1. RFC 1918 Zones .............................................5
4.2. RFC 5735 and RFC 5737 Zones ................................5
4.3. Local IPv6 Unicast Addresses ...............................6
4.4. IPv6 Locally Assigned Local Addresses ......................6
4.5. IPv6 Link-Local Addresses ..................................7
4.6. IPv6 Example Prefix ........................................7
5. Zones That Are Out of Scope .....................................7
6. IANA Considerations .............................................8
7. Security Considerations .........................................8
8. Acknowledgements ................................................9
9. References ......................................................9
9.1. Normative References .......................................9
9.2. Informative References ....................................10
1. Introduction
Experience with the Domain Name System (DNS, [RFC1034] and [RFC1035])
has shown that there are a number of DNS zones that all iterative
resolvers and recursive nameservers SHOULD automatically serve,
unless intentionally configured otherwise. These zones include, but
are not limited to, the IN-ADDR.ARPA zones for the address space
allocated by [RFC1918] and the IP6.ARPA zones for locally assigned
unique local IPv6 addresses defined in [RFC4193].
This recommendation is made because data has shown that significant
leakage of queries for these namespaces is occurring, despite
instructions to restrict them, and because it has therefore become
necessary to deploy sacrificial nameservers to protect the immediate
parent nameservers for these zones from excessive, unintentional
query load [AS112] [RFC6304] [RFC6305]. There is every expectation
that the query load will continue to increase unless steps are taken
as outlined here.
Additionally, queries from clients behind badly configured firewalls
that allow outgoing queries for these namespaces, but drop the
responses, put a significant load on the root servers (forward zones
but not reverse zones are configured). They also cause operational
load for the root server operators, as they have to reply to
enquiries about why the root servers are "attacking" these clients.
Changing the default configuration will address all these issues for
the zones listed in Section 4.
[RFC4193] recommends that queries for D.F.IP6.ARPA be handled
locally. This document extends the recommendation to cover the
IN-ADDR.ARPA zones for [RFC1918] and other well-known IN-ADDR.ARPA
and IP6.ARPA zones for which queries should not appear on the public
Internet.
It is hoped that by doing this the number of sacrificial servers
[AS112] will not have to be increased, and may in time be reduced.
This recommendation should also help DNS responsiveness for sites
that are using [RFC1918] addresses but do not follow the last
paragraph in Section 3 of [RFC1918].
1.1. Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Effects on Sites Using RFC 1918 Addresses
For most sites using [RFC1918] addresses, the changes here will have
little or no detrimental effect. If the site does not already have
the reverse tree populated, the only effect will be that the name
error responses will be generated locally rather than remotely.
For sites that do have the reverse tree populated, most will either
have a local copy of the zones or will be forwarding the queries to
servers that have local copies of the zone. Therefore, this
recommendation will not be relevant.
The most significant impact will be felt at sites that make use of
delegations for [RFC1918] addresses and have populated these zones.
These sites will need to override the default configuration expressed
in this document to allow resolution to continue. Typically, such
sites will be fully disconnected from the Internet and have their own
root servers for their own non-Internet DNS tree.
3. Changes to Iterative Resolver Behaviour
Unless configured otherwise, an iterative resolver will now return
authoritatively (AA=1) name errors (RCODE=3) for queries within the
zones in Section 4, with the obvious exception of queries for the
zone name itself where SOA, NS, and "no data" responses will be
returned as appropriate to the query type. One common way to do this
all at once is to serve empty (SOA and NS only) zones.
An implementation of this recommendation MUST provide a mechanism to
disable this new behaviour, and SHOULD allow this decision on a zone-
by-zone basis.
If using empty zones one SHOULD NOT use the same NS and SOA records
as used on the public Internet servers, as that will make it harder
to detect the origin of the responses and thus any leakage to the
public Internet servers. It is RECOMMENDED that the NS record
defaults to the name of the zone and the SOA MNAME defaults to the
name of the only NS RR's (Resource Record's) target. The SOA RNAME
SHOULD default to "nobody.invalid." [RFC2606]. Implementations
SHOULD provide a mechanism to set these values. No address records
need to be provided for the nameserver.
Below is an example of a generic empty zone in master file format.
It will produce a negative cache Time to Live (TTL) of 3 hours.
@ 10800 IN SOA @ nobody.invalid. 1 3600 1200 604800 10800
@ 10800 IN NS @
The SOA RR is needed to support negative caching [RFC2308] of name
error responses and to point clients to the primary master for DNS
dynamic updates.
SOA values of particular importance are the MNAME, the SOA RR's TTL,
and the negTTL value. Both TTL values SHOULD match. The rest of the
SOA timer values MAY be chosen arbitrarily since they are not
intended to control any zone transfer activity.
The NS RR is needed as some UPDATE [RFC2136] clients use NS queries
to discover the zone to be updated. Having no address records for
the nameserver is expected to abort UPDATE processing in the client.
4. Lists Of Zones Covered
The following subsections are the initial contents of the IANA
registry as described in the IANA Considerations section. Following
the caveat in that section, the list contains only reverse zones
corresponding to permanently assigned address space. The zone name
is the entity to be registered.
4.1. RFC 1918 Zones
The following zones correspond to the IPv4 address space reserved in
[RFC1918].
+----------------------+
| Zone |
+----------------------+
| 10.IN-ADDR.ARPA |
| 16.172.IN-ADDR.ARPA |
| 17.172.IN-ADDR.ARPA |
| 18.172.IN-ADDR.ARPA |
| 19.172.IN-ADDR.ARPA |
| 20.172.IN-ADDR.ARPA |
| 21.172.IN-ADDR.ARPA |
| 22.172.IN-ADDR.ARPA |
| 23.172.IN-ADDR.ARPA |
| 24.172.IN-ADDR.ARPA |
| 25.172.IN-ADDR.ARPA |
| 26.172.IN-ADDR.ARPA |
| 27.172.IN-ADDR.ARPA |
| 28.172.IN-ADDR.ARPA |
| 29.172.IN-ADDR.ARPA |
| 30.172.IN-ADDR.ARPA |
| 31.172.IN-ADDR.ARPA |
| 168.192.IN-ADDR.ARPA |
+----------------------+
4.2. RFC 5735 and RFC 5737 Zones
The following zones correspond to those address ranges from [RFC5735]
and [RFC5737] that are not expected to appear as source or
destination addresses on the public Internet; as such, there are no
globally unique names associated with the addresses in these ranges.
The recommendation to serve an empty zone 127.IN-ADDR.ARPA is not an
attempt to discourage any practice to provide a PTR RR for
1.0.0.127.IN-ADDR.ARPA locally. In fact, a meaningful reverse
mapping should exist, but the exact setup is out of the scope of this
document. Similar logic applies to the reverse mapping for ::1
(Section 4.3). The recommendations made here simply assume that no
other coverage for these domains exists.
+------------------------------+-----------------------+
| Zone | Description |
+------------------------------+-----------------------+
| 0.IN-ADDR.ARPA | IPv4 "THIS" NETWORK |
| 127.IN-ADDR.ARPA | IPv4 Loopback NETWORK |
| 254.169.IN-ADDR.ARPA | IPv4 LINK LOCAL |
| 2.0.192.IN-ADDR.ARPA | IPv4 TEST-NET-1 |
| 100.51.198.IN-ADDR.ARPA | IPv4 TEST-NET-2 |
| 113.0.203.IN-ADDR.ARPA | IPv4 TEST-NET-3 |
| 255.255.255.255.IN-ADDR.ARPA | IPv4 BROADCAST |
+------------------------------+-----------------------+
4.3. Local IPv6 Unicast Addresses
The reverse mappings ([RFC3596], Section 2.5 ("IP6.ARPA Domain")) for
the IPv6 Unspecified (::) and Loopback (::1) addresses ([RFC4291],
Sections 2.4, 2.5.2, and 2.5.3) are covered by these two zones:
+-------------------------------------------+
| Zone |
+-------------------------------------------+
| 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.\ |
| 0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA |
| 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.\ |
| 0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA |
+-------------------------------------------+
Note: Line breaks and escapes ('\') have been inserted above for
readability and to adhere to line width constraints. They are not
parts of the zone names.
4.4. IPv6 Locally Assigned Local Addresses
Section 4.4 of [RFC4193] already required special treatment of:
+--------------+
| Zone |
+--------------+
| D.F.IP6.ARPA |
+--------------+
4.5. IPv6 Link-Local Addresses
IPv6 Link-Local Addresses as described in [RFC4291], Section 2.5.6
are covered by four distinct reverse DNS zones:
+----------------+
| Zone |
+----------------+
| 8.E.F.IP6.ARPA |
| 9.E.F.IP6.ARPA |
| A.E.F.IP6.ARPA |
| B.E.F.IP6.ARPA |
+----------------+
4.6. IPv6 Example Prefix
IPv6 example prefix [RFC3849].
+--------------------------+
| Zone |
+--------------------------+
| 8.B.D.0.1.0.0.2.IP6.ARPA |
+--------------------------+
Note: 8.B.D.0.1.0.0.2.IP6.ARPA is not being used as an example here.
5. Zones That Are Out of Scope
IPv6 site-local addresses (deprecated, see [RFC4291] Sections 2.4 and
2.5.7), and IPv6 non-locally assigned local addresses ([RFC4193]) are
not covered here.
It is expected that IPv6 site-local addresses will be self correcting
as IPv6 implementations remove support for site-local addresses.
However, sacrificial servers for the zones C.E.F.IP6.ARPA through
F.E.F.IP6.ARPA may still need to be deployed in the short term if the
traffic becomes excessive.
For IPv6 non-locally assigned local addresses (L = 0) [RFC4193],
there has been no decision made about whether the Regional Internet
Registries (RIRs) will provide delegations in this space or not. If
they don't, then C.F.IP6.ARPA will need to be added to the list in
Section 4.4. If they do, then registries will need to take steps to
ensure that nameservers are provided for these addresses.
IP6.INT was once used to provide reverse mapping for IPv6. IP6.INT
was deprecated in [RFC4159] and the delegation removed from the INT
zone in June 2006. While it is possible that legacy software
continues to send queries for names under the IP6.INT domain, this
document does not specify that IP6.INT be considered a local zone.
This document has also deliberately ignored names immediately under
the root domain. While there is a subset of queries to the root
nameservers that could be addressed using the techniques described
here (e.g., .local, .workgroup, and IPv4 addresses), there is also a
vast amount of traffic that requires a different strategy (e.g.,
lookups for unqualified hostnames, IPv6 addresses).
6. IANA Considerations
IANA has established a registry of zones that require this default
behaviour. The initial contents of this registry are defined in
Section 4. Implementors are encouraged to periodically check this
registry and adjust their implementations to reflect changes therein.
This registry can be amended through "IETF Review" as per [RFC5226].
As part of this review process, it should be noted that once a zone
is added it is effectively added permanently; once an address range
starts being configured as a local zone in systems on the Internet,
it will be impossible to reverse those changes.
IANA should coordinate with the RIRs to ensure that, as DNS Security
(DNSSEC) is deployed in the reverse tree, delegations for these zones
are made in the manner described in Section 7.
7. Security Considerations
During the initial deployment phase, particularly where [RFC1918]
addresses are in use, there may be some clients that unexpectedly
receive a name error rather than a PTR record. This may cause some
service disruption until their recursive nameserver(s) have been
re-configured.
As DNSSEC is deployed within the IN-ADDR.ARPA and IP6.ARPA
namespaces, the zones listed above will need to be delegated as
insecure delegations, or be within insecure zones. This will allow
DNSSEC validation to succeed for queries in these spaces despite not
being answered from the delegated servers.
It is recommended that sites actively using these namespaces secure
them using DNSSEC [RFC4035] by publishing and using DNSSEC trust
anchors. This will protect the clients from accidental import of
unsigned responses from the Internet.
8. Acknowledgements
This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC.
9. References
9.1. Normative References
[RFC1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION", STD 13, RFC 1035, November 1987.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4159] Huston, G., "Deprecation of "ip6.int"", BCP 109, RFC 4159,
August 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
9.2. Informative References
[AS112] "AS112 Project", <http://www.as112.net/>.
[RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004.
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
[RFC6304] Abley, J. and W. Maton, "AS112 Nameserver Operations",
RFC 6304, July 2011.
[RFC6305] Abley, J. and W. Maton, "I'm Being Attacked by
PRISONER.IANA.ORG!", RFC 6305, July 2011.
Author's Address
Mark P. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
EMail: marka@isc.org
ISO Plain Language
On this page
§1Why Some DNS Queries Should Stay Local
Experience with the Domain Name System has shown that there are a number of DNS zones that all iterative resolvers and recursive nameservers SHOULD serve automatically, unless they are configured otherwise. RFC 4193 already asks for this to happen for the zone D.F.IP6.ARPA, and this document extends the practice to the IN-ADDR.ARPA zones for RFC 1918 address space and to other well-known zones with similar characteristics.
The reason is measured leakage. Despite instructions to restrict them, queries for these namespaces are still escaping onto the public Internet, and the load has grown large enough that sacrificial nameservers have had to be deployed to protect the parent nameservers for these zones from excessive, unintentional query load. Every expectation is that the load will continue to increase unless steps are taken as described here.
Clients behind badly configured firewalls make the problem worse. A firewall that allows outgoing queries for these namespaces but drops the responses puts a significant load on the root servers, since forward zones are configured but reverse zones are not. It also creates work for the root server operators, who have to reply to enquiries about why the root servers appear to be attacking those clients. Changing the default configuration addresses all of this for the zones on the list.
Lookups for addresses that only exist inside one network should never travel across the public Internet. This document tells DNS servers to answer those lookups themselves, so the traffic stays where it belongs.
§2Effect on Sites Using Private Addresses
For most sites that use RFC 1918 addresses, the change has little or no detrimental effect. If the site has not already populated its reverse tree, the only effect is that name error responses are generated locally rather than remotely. For sites that do have the reverse tree populated, most either keep a local copy of the zones or forward the queries to servers that have a local copy, so the recommendation does not apply to them.
The most significant impact falls on sites that use delegations for RFC 1918 addresses and have populated those zones. These sites need to override the default configuration described in the document so that resolution continues to work. Typically such sites are fully disconnected from the Internet and run their own root servers for their own non-Internet DNS tree.
Most networks will not notice the change. A network that runs its own private DNS tree, with its own copies of the private reverse zones, has to switch the new behaviour off for those zones.
§3How a Resolver Should Answer
Unless configured otherwise, an iterative resolver returns authoritatively, with the AA flag set, a name error with RCODE 3 for queries inside the zones listed in Section 4. The exception is a query for the zone name itself, where SOA, NS and "no data" responses are returned as appropriate to the query type. One common way to do all of this at once is to serve empty zones, that is, zones with only an SOA and an NS record.
An implementation of this recommendation MUST provide a mechanism to disable the new behaviour, and SHOULD allow that decision on a zone-by-zone basis. When using empty zones, an implementation SHOULD NOT use the same NS and SOA records as the public Internet servers use, because that would make it harder to detect where the responses came from and therefore harder to detect any leakage to the public Internet servers. It is RECOMMENDED that the NS record default to the name of the zone and that the SOA MNAME default to the name of the only NS record's target. The SOA RNAME SHOULD default to nobody.invalid.. Implementations SHOULD provide a mechanism to set these values, and no address records need to be provided for the nameserver.
The SOA record is needed to support negative caching of the name error responses and to point clients to the primary master for dynamic updates. The SOA values of particular importance are the MNAME, the TTL of the SOA record and the negative TTL, and both TTL values SHOULD match. The remaining SOA timer values MAY be chosen arbitrarily, because they are not intended to control any zone transfer activity. The NS record is needed because some UPDATE clients use NS queries to discover the zone to be updated, and having no address records for the nameserver is expected to abort UPDATE processing in the client.
The server should say "no such name" itself, and it should say it authoritatively so that other servers cache the answer. It also needs a switch to turn this off for zones a network really does serve.
@ 10800 IN SOA @ nobody.invalid. 1 3600 1200 604800 10800
@ 10800 IN NS @§4The Zones on the List
The initial list of zones is the content of the IANA registry described in Section 6, and it contains only reverse zones for permanently assigned address space. The zone name is the thing registered.
The RFC 1918 zones cover the IPv4 address space reserved for private internets: 10.IN-ADDR.ARPA, the sixteen zones 16.172.IN-ADDR.ARPA through 31.172.IN-ADDR.ARPA, and 168.192.IN-ADDR.ARPA. The zones from RFC 5735 and RFC 5737 cover ranges that are not expected to appear as source or destination addresses on the public Internet, so there are no globally unique names for them: 0.IN-ADDR.ARPA for the IPv4 "THIS" NETWORK, 127.IN-ADDR.ARPA for the IPv4 loopback network, 254.169.IN-ADDR.ARPA for IPv4 link local, 2.0.192.IN-ADDR.ARPA for TEST-NET-1, 100.51.198.IN-ADDR.ARPA for TEST-NET-2, 113.0.203.IN-ADDR.ARPA for TEST-NET-3 and 255.255.255.255.IN-ADDR.ARPA for IPv4 broadcast.
The IPv6 zones cover the reverse mappings for the IPv6 unspecified and loopback addresses, the zone D.F.IP6.ARPA required by Section 4.4 of RFC 4193, the four IPv6 link-local zones 8.E.F.IP6.ARPA, 9.E.F.IP6.ARPA, A.E.F.IP6.ARPA and B.E.F.IP6.ARPA, and the IPv6 example prefix zone 8.B.D.0.1.0.0.2.IP6.ARPA.
The document adds a caution about the loopback zone: serving an empty 127.IN-ADDR.ARPA is not an attempt to discourage a network from providing a PTR record for 1.0.0.127.IN-ADDR.ARPA locally. A meaningful reverse mapping should exist, but the exact setup is out of scope, and the same logic applies to the reverse mapping for ::1. The recommendations simply assume that no other coverage for these domains exists.
The list covers the reverse zones for private IPv4 space, several special-use IPv4 ranges, and the IPv6 zones for loopback, link-local and locally assigned addresses.
- Locally served zones Reverse zones for address space with no global names
- RFC 1918 private space 10.IN-ADDR.ARPA, 16.172 to 31.172, 168.192.IN-ADDR.ARPA
- RFC 5735 and RFC 5737 special use 0, 127, 254.169, 2.0.192, 100.51.198, 113.0.203, 255.255.255.255
- IPv6 unspecified and loopback Two IP6.ARPA zones
- IPv6 locally assigned D.F.IP6.ARPA
- IPv6 link-local 8.E.F, 9.E.F, A.E.F and B.E.F
- IPv6 example prefix 8.B.D.0.1.0.0.2.IP6.ARPA
§5Zones That Are Out of Scope
IPv6 site-local addresses, which are deprecated, and IPv6 non-locally assigned local addresses from RFC 4193 are not covered. IPv6 site-local addresses are expected to correct themselves as implementations drop support for them, although sacrificial servers for the zones C.E.F.IP6.ARPA through F.E.F.IP6.ARPA may still be needed in the short term if the traffic becomes excessive.
For IPv6 non-locally assigned local addresses, where L is 0, no decision has been made about whether the Regional Internet Registries will provide delegations in that space. If they do not, C.F.IP6.ARPA will need to be added to the list. If they do, the registries will need to make sure that nameservers are provided for those addresses.
IP6.INT was once used to provide reverse mapping for IPv6. It was deprecated in RFC 4159 and the delegation was removed from the INT zone in June 2006. Legacy software may still send queries for names under the IP6.INT domain, but this document does not treat IP6.INT as a local zone. The document has also deliberately left alone the names immediately under the root domain. Some of that traffic, such as lookups for .local or .workgroup and for IPv4 addresses, could be handled with these techniques, but there is a far larger amount of traffic that needs a different strategy, such as lookups for unqualified hostnames and for IPv6 addresses.
Some reverse zones are deliberately left out, including the old IPv6 site-local range and the IP6.INT zone that was withdrawn years ago.
§6The Registry of Zones
IANA has established a registry of the zones that require this default behaviour, and the initial contents of that registry are the zones in Section 4. Implementors are encouraged to check the registry periodically and adjust their implementations to follow any change.
The registry can be amended through IETF Review as described in RFC 5226. As part of that review, it should be noted that once a zone is added it is effectively added permanently: once an address range starts being configured as a local zone in systems across the Internet, reversing those changes becomes impossible.
IANA should coordinate with the Regional Internet Registries to ensure that, as DNSSEC is deployed in the reverse tree, delegations for these zones are made in the manner described in Section 7.
The list of zones lives in an IANA registry, and software is expected to keep up with it. Adding a zone is a one-way step in practice, because once networks start answering for it locally there is no going back.
§7Security Considerations
During the initial deployment phase, particularly where RFC 1918 addresses are in use, some clients may unexpectedly receive a name error rather than a PTR record. That may cause service disruption until their recursive nameservers have been reconfigured.
As DNSSEC is deployed within the IN-ADDR.ARPA and IP6.ARPA namespaces, the zones on the list will need to be delegated as insecure delegations, or be within insecure zones. That allows DNSSEC validation to succeed for queries in these spaces even though the answers do not come from the delegated servers.
It is recommended that sites actively using these namespaces secure them with DNSSEC (RFC 4035) by publishing and using DNSSEC trust anchors. That protects clients from accidentally importing unsigned responses from the Internet.
Turning this on can briefly break lookups that used to work, and the reverse tree needs DNSSEC to be handled carefully so that unsigned answers cannot slip in from elsewhere.