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IETF RFC 5855
Nameservers for IPv4 and IPv6 Reverse Zones
Last modified on Monday, May 24th, 2010
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Internet Engineering Task Force (IETF) J. Abley
Request for Comments: 5855 T. Manderson
BCP: 155 ICANN
Category: Best Current Practice May 2010
ISSN: 2070-1721
Nameservers for IPv4 and IPv6 Reverse Zones
 Abstract
This document specifies a stable naming scheme for the nameservers
that serve the zones IN-ADDR.ARPA and IP6.ARPA in the DNS. These
zones contain data that facilitate reverse mapping (address to name).
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/RFC 5855.
Copyright Notice
Copyright (c) 2010 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
(http://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.
Abley & Manderson Best Current Practice PAGE 1 
RFC 5855 Nameservers for Reverse Zones May 2010
Table of Contents
1. Introduction ....................................................2
2. Nameservers for IN-ADDR.ARPA ....................................3
3. Nameservers for IP6.ARPA ........................................3
4. IAB Statement ...................................................4
5. IANA Considerations .............................................4
6. Security Considerations .........................................4
7. References ......................................................4
7.1. Normative References .......................................4
7.2. Informative References .....................................5
Appendix A. Existing NS RRSets ....................................6
Appendix B. Performance Characteristics ...........................7
B.1. Label Compression ..........................................7
B.2. Query Patterns .............................................9
B.2.1. QNAME under IN-ADDR.ARPA ..............................10
B.2.2. QNAME under IP6.ARPA ..................................10
1. Introduction
The Domain Name System (DNS) is described in [RFC 1034] and [RFC 1035].
The DNS currently supports keyed data retrieval using three
namespaces -- domain names, IPv4 addresses, and IPv6 addresses.
Mapping of IPv4 addresses to names is accomplished using data
published in the IN-ADDR.ARPA zone. For IPv6, the IP6.ARPA zone is
used (see [RFC 3596]). The process of mapping an address to a name is
generally known as a "reverse lookup", and the IN-ADDR.ARPA and
IP6.ARPA zones are said to support the "reverse DNS".
The secure and stable hosting of the IN-ADDR.ARPA and IP6.ARPA zones
is critical to the operation of the Internet, since many applications
rely upon timely responses to reverse lookups to be able to operate
normally.
At the time of this writing, the IN-ADDR.ARPA zone is served by a
subset of the DNS root servers, and IP6.ARPA by servers operated by
APNIC, ARIN, ICANN, LACNIC, and the RIPE NCC (see Appendix A).
This document specifies a dedicated and stable set of nameserver
names for each of the IN-ADDR.ARPA and IP6.ARPA zones.
The naming scheme specified in this document allows IN-ADDR.ARPA and
IP6.ARPA to be delegated to two different sets of nameservers, to
facilitate operational separation of the infrastructure used to serve
each zone. This separation might help ensure that an operational
failure of IN-ADDR.ARPA servers does not impact IPv6 reverse lookups
as collateral damage, for example.
Abley & Manderson Best Current Practice PAGE 2
RFC 5855 Nameservers for Reverse Zones May 2010
The choice of operators for individual nameservers is beyond the
scope of this document and is an IANA function that falls under the
scope of Section 4 of the Memorandum of Understanding (MoU) between
the IETF and ICANN [RFC 2860].
2. Nameservers for IN-ADDR.ARPA
This document specifies the following naming scheme for servers that
host the IN-ADDR.ARPA zone:
A.IN-ADDR-SERVERS.ARPA
B.IN-ADDR-SERVERS.ARPA
C.IN-ADDR-SERVERS.ARPA
D.IN-ADDR-SERVERS.ARPA
E.IN-ADDR-SERVERS.ARPA
F.IN-ADDR-SERVERS.ARPA
...
The IN-ADDR-SERVERS.ARPA zone has been delegated to the same set of
servers as IN-ADDR.ARPA. IPv4 and IPv6 glue records for each of
those servers has been added to the ARPA zone.
The IN-ADDR-SERVERS.ARPA and IN-ADDR.ARPA zones are delegated to the
same servers, since they are both dedicated for a single purpose and
hence can reasonably share fate.
All servers in the set are named under the same domain to facilitate
label compression. Since glue for all servers exist in the ARPA
zone, the use of a single domain does not present a practical single
point of failure.
3. Nameservers for IP6.ARPA
This document specifies the following nameserver set for the IP6.ARPA
zone:
A.IP6-SERVERS.ARPA
B.IP6-SERVERS.ARPA
C.IP6-SERVERS.ARPA
D.IP6-SERVERS.ARPA
E.IP6-SERVERS.ARPA
F.IP6-SERVERS.ARPA
...
The IP6-SERVERS.ARPA zone has been delegated to the same set of
servers as IP6.ARPA. IPv4 and IPv6 glue records for each of those
servers has been added to the ARPA zone.
Abley & Manderson Best Current Practice PAGE 3
RFC 5855 Nameservers for Reverse Zones May 2010
4. IAB Statement
In its capacity as the body that provides technical guidance to ICANN
for the administration of the ARPA top-level domain as described in
[RFC 3172], the IAB has reviewed this proposal and supports it as an
operational change that is in line with the respective roles of ICANN
and the IAB.
5. IANA Considerations
With due consideration to the approval of the IAB (see Section 4),
the IANA has delegated:
1. IN-ADDR-SERVERS.ARPA to the nameservers listed in Section 2;
2. IP6-SERVERS.ARPA to the nameservers listed in Section 3.
Additionally, IANA has installed IPv4 and IPv6 glue records for the
nameservers concerned in the ARPA zone.
The choice of operators for all nameservers concerned is beyond the
scope of this document and is an IANA function that falls under the
scope of Section 4 of the MoU between the IETF and ICANN [RFC 2860].
6. Security Considerations
This document introduces no additional security risks for the
Internet.
7. References
7.1. Normative References
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC 3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, September 2001.
Abley & Manderson Best Current Practice PAGE 4
RFC 5855 Nameservers for Reverse Zones May 2010
7.2. Informative References
[RFC 2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority", RFC 2860,
June 2000.
[RFC 3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
Abley & Manderson Best Current Practice PAGE 5
RFC 5855 Nameservers for Reverse Zones May 2010
Appendix A. Existing NS RRSets
The NS RRSet for the IN-ADDR.ARPA zone at the time of this writing is
as follows:
IN-ADDR.ARPA. 86400 IN NS A.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS B.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS C.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS D.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS E.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS F.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS G.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS H.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS I.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS K.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS L.ROOT-SERVERS.NET.
IN-ADDR.ARPA. 86400 IN NS M.ROOT-SERVERS.NET.
The NS RRSet for the IP6.ARPA zone at the time of this writing is as
follows:
IP6.ARPA. 84600 IN NS NS-SEC.RIPE.NET.
IP6.ARPA. 86400 IN NS SEC1.APNIC.NET.
IP6.ARPA. 86400 IN NS NS2.LACNIC.NET.
IP6.ARPA. 86400 IN NS NS.ICANN.ORG.
IP6.ARPA. 86400 IN NS TINNIE.ARIN.NET.
For completeness, the NS RRSet for the ARPA zone at the time of this
writing is as follows:
ARPA. 86400 IN NS A.ROOT-SERVERS.NET.
ARPA. 86400 IN NS B.ROOT-SERVERS.NET.
ARPA. 86400 IN NS C.ROOT-SERVERS.NET.
ARPA. 86400 IN NS D.ROOT-SERVERS.NET.
ARPA. 86400 IN NS E.ROOT-SERVERS.NET.
ARPA. 86400 IN NS F.ROOT-SERVERS.NET.
ARPA. 86400 IN NS G.ROOT-SERVERS.NET.
ARPA. 86400 IN NS H.ROOT-SERVERS.NET.
ARPA. 86400 IN NS I.ROOT-SERVERS.NET.
ARPA. 86400 IN NS K.ROOT-SERVERS.NET.
ARPA. 86400 IN NS L.ROOT-SERVERS.NET.
ARPA. 86400 IN NS M.ROOT-SERVERS.NET.
Abley & Manderson Best Current Practice PAGE 6
RFC 5855 Nameservers for Reverse Zones May 2010
Appendix B. Performance Characteristics
B.1. Label Compression
The choice of names for the respective NS RRSets of the IN-ADDR.ARPA
and IP6.ARPA zones have a relatively minor impact on the delegation
response sizes from their parent zones, given other anticipated
contributors such as DNSSEC. However, it is still considered good
practice to use a naming scheme that is reasonably compressible:
doing so for frequently queried zones such as these is likely to have
at least measurable impact on aggregate DNS traffic in the Internet
as a whole, and has potential transport benefits to clients whose
queries will not result in secure replies.
The naming schemes described in Sections 2 and 3 are highly
compressible. That is, once a single nameserver name has been
encoded in a DNS message, subsequent nameservers can be specified
with substantially smaller encoding.
In the DNS, a complete encoding of an a-label involves a one-byte
length field, plus a one-byte-per-character encoding of the a-label
itself. A domain name's encoding consists of one or more a-labels,
so-encoded, plus a single terminating zero byte. Where a terminating
series of a-labels has already been encoded as described above,
subsequent terminating references to the same series can be made
using a two-byte pointer to that full encoding.
The non-compressed representation of the nameserver A.IN-ADDR-
SERVERS.ARPA fills (1 + 1) + (15 + 1) + (4 + 1) + 1 = 24 bytes.
The non-compressed representation of A.IP6-SERVERS.ARPA fills
(1 + 1) + (10 + 1) + (4 + 1) + 1 = 19 bytes.
Subsequent nameservers under either domain are encoded with the
initial label, plus two bytes for a pointer to the repeated domain
elsewhere in the message, i.e., (1 + 1) + 2 = 4 bytes.
Abley & Manderson Best Current Practice PAGE 7
RFC 5855 Nameservers for Reverse Zones May 2010
The encoded size of the a-labels in a twelve-record NS RRSet named
according to Section 2 for IN-ADDR.ARPA is as follows:
+------------------------+---------------------------------------+
| Nameserver | Encoded Size |
+------------------------+---------------------------------------+
| A.IN-ADDR-SERVERS.ARPA | (1 + 1) + (15 + 1) + (4 + 1) + 1 = 24 |
| | |
| B.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| C.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| D.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| E.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| F.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| G.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| H.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| I.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| J.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| K.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| L.IN-ADDR-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| Total | 68 bytes |
+------------------------+---------------------------------------+
Abley & Manderson Best Current Practice PAGE 8
RFC 5855 Nameservers for Reverse Zones May 2010
The encoded size of the a-labels in a six-record NS RRSet named
according to Section 3 for IP6.ARPA is, hence, as follows:
+--------------------+---------------------------------------+
| Nameserver | Encoded Size |
+--------------------+---------------------------------------+
| A.IP6-SERVERS.ARPA | (1 + 1) + (10 + 1) + (4 + 1) + 1 = 19 |
| | |
| B.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| C.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| D.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| E.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| F.IP6-SERVERS.ARPA | (1 + 1) + 2 = 4 |
| | |
| Total | 39 bytes |
+--------------------+---------------------------------------+
By way of comparison, the encoded size of the labels in the NS RRSet
for IP6.ARPA (shown in Appendix A) is as follows:
+-----------------+--------------------------------------+
| Nameserver | Encoded Size |
+-----------------+--------------------------------------+
| NS-SEC.RIPE.NET | (6 + 1) + (4 + 1) + (3 + 1) + 1 = 17 |
| | |
| SEC1.APNIC.NET | (4 + 1) + (5 + 1) + 2 + 1 = 14 |
| | |
| NS2.LANIC.NET | (3 + 1) + (6 + 1) + 2 + 1 = 14 |
| | |
| NS.ICANN.ORG | (2 + 1) + (5 + 1) + (3 + 1) + 1 = 14 |
| | |
| TINNIE.ARIN.NET | (6 + 1) + (4 + 1) + 2 + 1 = 15 |
| | |
| Total | 74 bytes |
+-----------------+--------------------------------------+
B.2. Query Patterns
A brief description of likely query patterns for an empty cache with
the existing and new NS RRSets follows.
Abley & Manderson Best Current Practice PAGE 9
RFC 5855 Nameservers for Reverse Zones May 2010
B.2.1. QNAME under IN-ADDR.ARPA
Consider the IN-ADDR.ARPA NS RRSet (described in Appendix A) and a
QNAME that is delegated beneath the IN-ADDR.ARPA zone:
1. Query sent to root server that is also authoritative for
IN-ADDR.ARPA; response is a referral from the IN-ADDR.ARPA zone.
In the case where the initial query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
the IN-ADDR.ARPA zone); response is a referral to an ARPA server
with additional-section glue.
2. Query sent to an ARPA server (all of which are also authoritative
in this case for IN-ADDR.ARPA); response is a referral from the
IN-ADDR.ARPA zone.
Consider the same query with the IN-ADDR.ARPA NS RRSet (described in
Section 2):
1. Query sent to a root server that is also authoritative for ARPA;
response is a referral to an IN-ADDR.ARPA server, with additional-
section glue.
2. Query sent to an IN-ADDR.ARPA server; response is a referral from
the IN-ADDR.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
ARPA); response is a referral to an ARPA server, with additional-
section glue.
2. Query sent to an ARPA server; response is a referral to an
IN-ADDR.ARPA server, with additional-section glue.
3. Query sent to an IN-ADDR.ARPA server; response is a referral from
the IN-ADDR.ARPA zone.
B.2.2. QNAME under IP6.ARPA
Consider the IP6.ARPA NS RRSet (described in Appendix A) and a QNAME
that is delegated beneath the IP6.ARPA zone:
1. Query sent to root server that is also authoritative for ARPA;
response is a referral from the ARPA zone to an IP6.ARPA server
with no additional-section glue.
Abley & Manderson Best Current Practice PAGE 10
RFC 5855 Nameservers for Reverse Zones May 2010
2. A recursive lookup for one of the nameservers specified in the
referral must now be performed in order to obtain an address for
an IP6.ARPA server. In all cases, three queries are required.
Successive recursive lookups may be performed in the event that a
server is unresponsive.
3. Query sent to IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET; response is a referral to an
ARPA server with additional-section glue.
2. Query sent to an ARPA server; response is a referral from the ARPA
zone to an IP6.ARPA server with no additional-section glue.
3. A recursive lookup for one of the nameservers specified in the
referral must now be performed in order to obtain an address for
an IP6.ARPA server. In all cases, three queries are required.
Successive recursive lookups may be performed in the event that a
server is unresponsive.
4. Query sent to IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
Consider the same query with the IP6.ARPA NS RRSet (described in
Section 3):
1. Query sent to a root server that is also authoritative for ARPA;
response is a referral to an IP6.ARPA server, with additional-
section glue.
2. Query sent to an IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
In the case where the first query is sent to the J root server:
1. Query sent to J.ROOT-SERVERS.NET (which is not authoritative for
ARPA); response is a referral to an ARPA server, with additional-
section glue.
2. Query sent to an ARPA server; response is a referral to an
IP6.ARPA server with additional-section glue.
3. Query sent to an IP6.ARPA server; response is a referral from the
IP6.ARPA zone.
Abley & Manderson Best Current Practice PAGE 11
RFC 5855 Nameservers for Reverse Zones May 2010
Authors' Addresses
Joe Abley
ICANN
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
USA
Phone: +1 310 463 9062
EMail: joe.abley@icann.org
Terry Manderson
ICANN
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
USA
Phone: +61 4 1127 5673
EMail: terry.manderson@icann.org
Abley & Manderson Best Current Practice PAGE 12
Nameservers for IPv4 and IPv6 Reverse Zones
RFC TOTAL SIZE: 23027 bytes
PUBLICATION DATE: Monday, May 24th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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