rfc2929.txt
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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]
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