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Network Working Group D. Eastlake, 3rd
Request for Comments: 2929 Motorola
BCP: 42 E. Brunner-Williams
Category: Best Current Practice Engage
B. Manning
ISI
September 2000
Domain Name System (DNS) IANA Considerations
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
Internet Assigned Number Authority (IANA) parameter assignment
considerations are given for the allocation of Domain Name System
(DNS) classes, Resource Record (RR) types, operation codes, error
codes, etc.
Table of Contents
1. Introduction................................................. 2
2. DNS Query/Response Headers................................... 2
2.1 One Spare Bit?.............................................. 3
2.2 Opcode Assignment........................................... 3
2.3 RCODE Assignment............................................ 4
3. DNS Resource Records......................................... 5
3.1 RR TYPE IANA Considerations................................. 6
3.1.1 Special Note on the OPT RR................................ 7
3.2 RR CLASS IANA Considerations................................ 7
3.3 RR NAME Considerations...................................... 8
4. Security Considerations...................................... 9
References...................................................... 9
Authors' Addresses.............................................. 11
Full Copyright Statement........................................ 12
Eastlake, et al. Best Current Practice [Page 1]
RFC 2929 DNS IANA Considerations September 2000
1. Introduction
The Domain Name System (DNS) provides replicated distributed secure
hierarchical databases which hierarchically store "resource records"
(RRs) under domain names.
This data is structured into CLASSes and zones which can be
independently maintained. See [RFC 1034, 1035, 2136, 2181, 2535]
familiarity with which is assumed.
This document covers, either directly or by reference, general IANA
parameter assignment considerations applying across DNS query and
response headers and all RRs. There may be additional IANA
considerations that apply to only a particular RR type or
query/response opcode. See the specific RFC defining that RR type or
query/response opcode for such considerations if they have been
defined.
IANA currently maintains a web page of DNS parameters. See
<http://www.iana.org/numbers.htm>.
"IETF Standards Action", "IETF Consensus", "Specification Required",
and "Private Use" are as defined in [RFC 2434].
2. DNS Query/Response Headers
The header for DNS queries and responses contains field/bits in the
following diagram taken from [RFC 2136, 2535]:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA| Z|AD|CD| RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT/ZOCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT/PRCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT/UPCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The ID field identifies the query and is echoed in the response so
they can be matched.
Eastlake, et al. Best Current Practice [Page 2]
RFC 2929 DNS IANA Considerations September 2000
The QR bit indicates whether the header is for a query or a response.
The AA, TC, RD, RA, AD, and CD bits are each theoretically meaningful
only in queries or only in responses, depending on the bit. However,
many DNS implementations copy the query header as the initial value
of the response header without clearing bits. Thus any attempt to
use a "query" bit with a different meaning in a response or to define
a query meaning for a "response" bit is dangerous given existing
implementation. Such meanings may only be assigned by an IETF
Standards Action.
The unsigned fields query count (QDCOUNT), answer count (ANCOUNT),
authority count (NSCOUNT), and additional information count (ARCOUNT)
express the number of records in each section for all opcodes except
Update. These fields have the same structure and data type for
Update but are instead the counts for the zone (ZOCOUNT),
prerequisite (PRCOUNT), update (UPCOUNT), and additional information
(ARCOUNT) sections.
2.1 One Spare Bit?
There have been ancient DNS implementations for which the Z bit being
on in a query meant that only a response from the primary server for
a zone is acceptable. It is believed that current DNS
implementations ignore this bit.
Assigning a meaning to the Z bit requires an IETF Standards Action.
2.2 Opcode Assignment
New OpCode assignments require an IETF Standards Action.
Currently DNS OpCodes are assigned as follows:
OpCode Name Reference
0 Query [RFC 1035]
1 IQuery (Inverse Query) [RFC 1035]
2 Status [RFC 1035]
3 available for assignment
4 Notify [RFC 1996]
5 Update [RFC 2136]
6-15 available for assignment
Eastlake, et al. Best Current Practice [Page 3]
RFC 2929 DNS IANA Considerations September 2000
2.3 RCODE Assignment
It would appear from the DNS header above that only four bits of
RCODE, or response/error code are available. However, RCODEs can
appear not only at the top level of a DNS response but also inside
OPT RRs [RFC 2671], TSIG RRs [RFC 2845], and TKEY RRs [RFC 2930].
The OPT RR provides an eight bit extension resulting in a 12 bit
RCODE field and the TSIG and TKEY RRs have a 16 bit RCODE field.
Error codes appearing in the DNS header and in these three RR types
all refer to the same error code space with the single exception of
error code 16 which has a different meaning in the OPT RR from its
meaning in other contexts. See table below.
RCODE Name Description Reference
Decimal
Hexadecimal
0 NoError No Error [RFC 1035]
1 FormErr Format Error [RFC 1035]
2 ServFail Server Failure [RFC 1035]
3 NXDomain Non-Existent Domain [RFC 1035]
4 NotImp Not Implemented [RFC 1035]
5 Refused Query Refused [RFC 1035]
6 YXDomain Name Exists when it should not [RFC 2136]
7 YXRRSet RR Set Exists when it should not [RFC 2136]
8 NXRRSet RR Set that should exist does not [RFC 2136]
9 NotAuth Server Not Authoritative for zone [RFC 2136]
10 NotZone Name not contained in zone [RFC 2136]
11-15 available for assignment
16 BADVERS Bad OPT Version [RFC 2671]
16 BADSIG TSIG Signature Failure [RFC 2845]
17 BADKEY Key not recognized [RFC 2845]
18 BADTIME Signature out of time window [RFC 2845]
19 BADMODE Bad TKEY Mode [RFC 2930]
20 BADNAME Duplicate key name [RFC 2930]
21 BADALG Algorithm not supported [RFC 2930]
22-3840 available for assignment
0x0016-0x0F00
3841-4095 Private Use
0x0F01-0x0FFF
4096-65535 available for assignment
0x1000-0xFFFF
Since it is important that RCODEs be understood for interoperability,
assignment of new RCODE listed above as "available for assignment"
requires an IETF Consensus.
Eastlake, et al. Best Current Practice [Page 4]
RFC 2929 DNS IANA Considerations September 2000
3. DNS Resource Records
All RRs have the same top level format shown in the figure below
taken from [RFC 1035]:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
/ /
/ NAME /
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TYPE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| CLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TTL |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| RDLENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
/ RDATA /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
NAME is an owner name, i.e., the name of the node to which this
resource record pertains. NAMEs are specific to a CLASS as described
in section 3.2. NAMEs consist of an ordered sequence of one or more
labels each of which has a label type [RFC 1035, 2671].
TYPE is a two octet unsigned integer containing one of the RR TYPE
codes. See section 3.1.
CLASS is a two octet unsigned integer containing one of the RR CLASS
codes. See section 3.2.
TTL is a four octet (32 bit) bit unsigned integer that specifies the
number of seconds that the resource record may be cached before the
source of the information should again be consulted. Zero is
interpreted to mean that the RR can only be used for the transaction
in progress.
RDLENGTH is an unsigned 16 bit integer that specifies the length in
octets of the RDATA field.
Eastlake, et al. Best Current Practice [Page 5]
RFC 2929 DNS IANA Considerations September 2000
RDATA is a variable length string of octets that constitutes the
resource. The format of this information varies according to the
TYPE and in some cases the CLASS of the resource record.
3.1 RR TYPE IANA Considerations
There are three subcategories of RR TYPE numbers: data TYPEs, QTYPEs,
and MetaTYPEs.
Data TYPEs are the primary means of storing data. QTYPES can only be
used in queries. Meta-TYPEs designate transient data associated with
an particular DNS message and in some cases can also be used in
queries. Thus far, data TYPEs have been assigned from 1 upwards plus
the block from 100 through 103 while Q and Meta Types have been
assigned from 255 downwards (except for the OPT Meta-RR which is
assigned TYPE 41). There have been DNS implementations which made
caching decisions based on the top bit of the bottom byte of the RR
TYPE.
There are currently three Meta-TYPEs assigned: OPT [RFC 2671], TSIG
[RFC 2845], and TKEY [RFC 2930].
There are currently five QTYPEs assigned: * (all), MAILA, MAILB,
AXFR, and IXFR.
Considerations for the allocation of new RR TYPEs are as follows:
Decimal
Hexadecimal
0
0x0000 - TYPE zero is used as a special indicator for the SIG RR [RFC
2535] and in other circumstances and must never be allocated
for ordinary use.
1 - 127
0x0001 - 0x007F - remaining TYPEs in this range are assigned for data
TYPEs by IETF Consensus.
128 - 255
0x0080 - 0x00FF - remaining TYPEs in this rage are assigned for Q and
Meta TYPEs by IETF Consensus.
256 - 32767
0x0100 - 0x7FFF - assigned for data, Q, or Meta TYPE use by IETF
Consensus.
Eastlake, et al. Best Current Practice [Page 6]
RFC 2929 DNS IANA Considerations September 2000
32768 - 65279
0x8000 - 0xFEFF - Specification Required as defined in [RFC 2434].
65280 - 65535
0xFF00 - 0xFFFF - Private Use.
3.1.1 Special Note on the OPT RR
The OPT (OPTion) RR, number 41, is specified in [RFC 2671]. Its
primary purpose is to extend the effective field size of various DNS
fields including RCODE, label type, flag bits, and RDATA size. In
particular, for resolvers and servers that recognize it, it extends
the RCODE field from 4 to 12 bits.
3.2 RR CLASS IANA Considerations
DNS CLASSes have been little used but constitute another dimension of
the DNS distributed database. In particular, there is no necessary
relationship between the name space or root servers for one CLASS and
those for another CLASS. The same name can have completely different
meanings in different CLASSes although the label types are the same
and the null label is usable only as root in every CLASS. However,
as global networking and DNS have evolved, the IN, or Internet, CLASS
has dominated DNS use.
There are two subcategories of DNS CLASSes: normal data containing
classes and QCLASSes that are only meaningful in queries or updates.
The current CLASS assignments and considerations for future
assignments are as follows:
Decimal
Hexadecimal
0
0x0000 - assignment requires an IETF Standards Action.
1
0x0001 - Internet (IN).
2
0x0002 - available for assignment by IETF Consensus as a data CLASS.
3
0x0003 - Chaos (CH) [Moon 1981].
4
0x0004 - Hesiod (HS) [Dyer 1987].
Eastlake, et al. Best Current Practice [Page 7]
RFC 2929 DNS IANA Considerations September 2000
5 - 127
0x0005 - 0x007F - available for assignment by IETF Consensus as data
CLASSes only.
128 - 253
0x0080 - 0x00FD - available for assignment by IETF Consensus as
QCLASSes only.
254
0x00FE - QCLASS None [RFC 2136].
255
0x00FF - QCLASS Any [RFC 1035].
256 - 32767
0x0100 - 0x7FFF - assigned by IETF Consensus.
32768 - 65280
0x8000 - 0xFEFF - assigned based on Specification Required as defined
in [RFC 2434].
65280 - 65534
0xFF00 - 0xFFFE - Private Use.
65535
0xFFFF - can only be assigned by an IETF Standards Action.
3.3 RR NAME Considerations
DNS NAMEs are sequences of labels [RFC 1035]. The last label in each
NAME is "ROOT" which is the zero length label. By definition, the
null or ROOT label can not be used for any other NAME purpose.
At the present time, there are two categories of label types, data
labels and compression labels. Compression labels are pointers to
data labels elsewhere within an RR or DNS message and are intended to
shorten the wire encoding of NAMEs. The two existing data label
types are sometimes referred to as Text and Binary. Text labels can,
in fact, include any octet value including zero octets but most
current uses involve only [US-ASCII]. For retrieval, Text labels are
defined to treat ASCII upper and lower case letter codes as matching.
Binary labels are bit sequences [RFC 2673].
IANA considerations for label types are given in [RFC 2671].
Eastlake, et al. Best Current Practice [Page 8]
RFC 2929 DNS IANA Considerations September 2000
NAMEs are local to a CLASS. The Hesiod [Dyer 1987] and Chaos [Moon
1981] CLASSes are essentially for local use. The IN or Internet
CLASS is thus the only DNS CLASS in global use on the Internet at
this time.
A somewhat dated description of name allocation in the IN Class is
given in [RFC 1591]. Some information on reserved top level domain
names is in Best Current Practice 32 [RFC 2606].
4. Security Considerations
This document addresses IANA considerations in the allocation of
general DNS parameters, not security. See [RFC 2535] for secure DNS
considerations.
References
[Dyer 1987] Dyer, S., and F. Hsu, "Hesiod", Project Athena Technical
Plan - Name Service, April 1987,
[Moon 1981] D. Moon, "Chaosnet", A.I. Memo 628, Massachusetts
Institute of Technology Artificial Intelligence
Laboratory, June 1981.
[RFC 1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC 1035, November 1987.
[RFC 1591] Postel, J., "Domain Name System Structure and
Delegation", RFC 1591, March 1994.
[RFC 1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996.
[RFC 2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC 2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
Eastlake, et al. Best Current Practice [Page 9]
RFC 2929 DNS IANA Considerations September 2000
[RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC 2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS
Names", RFC 2606, June 1999.
[RFC 2671] Vixie, P., "Extension mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC 2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC
2672, August 1999.
[RFC 2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.
Wellington, "Secret Key Transaction Authentication for
DNS (TSIG)", RFC 2845, May 2000.
[RFC 2930] Eastlake, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[US-ASCII] ANSI, "USA Standard Code for Information Interchange",
X3.4, American National Standards Institute: New York,
1968.
Eastlake, et al. Best Current Practice [Page 10]
RFC 2929 DNS IANA Considerations September 2000
Authors' Addresses
Donald E. Eastlake 3rd
Motorola
140 Forest Avenue
Hudson, MA 01749 USA
Phone: +1-978-562-2827 (h)
+1-508-261-5434 (w)
Fax: +1-508-261-4447 (w)
EMail: Donald.Eastlake@motorola.com
Eric Brunner-Williams
Engage
100 Brickstone Square, 2nd Floor
Andover, MA 01810
Phone: +1-207-797-0525 (h)
+1-978-684-7796 (w)
Fax: +1-978-684-3118
EMail: brunner@engage.com
Bill Manning
USC/ISI
4676 Admiralty Way, #1001
Marina del Rey, CA 90292 USA
Phone: +1-310-822-1511
EMail: bmanning@isi.edu
Eastlake, et al. Best Current Practice [Page 11]
RFC 2929 DNS IANA Considerations September 2000
Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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The limited permissions granted above are perpetual and will not be
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Eastlake, et al. Best Current Practice [Page 12]