As per Relevance of the word assignment, we have this rfc below:
Network Working Group D. Eastlake, 3
Request for Comments: 2929
BCP: 42 E. Brunner-
Category: Best Current Practice
B.
September 2000
Domain Name System (DNS) IANA
Status of this
This document specifies an Internet Best Current Practices for
Internet Community, and requests discussion and suggestions
improvements. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
Internet Assigned Number Authority (IANA) parameter
considerations are given for the allocation of Domain Name
(DNS) classes, Resource Record (RR) types, operation codes,
codes, etc
Table of
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.
The Domain Name System (DNS) provides replicated distributed
hierarchical databases which hierarchically store "resource records
(RRs) under domain names
This data is structured into CLASSes and zones which can
independently maintained. See [RFC 1034, 1035, 2136, 2181, 2535]
familiarity with which is assumed
This document covers, either directly or by reference, general
parameter assignment considerations applying across DNS query
response headers and all RRs. There may be additional
considerations that apply to only a particular RR type
query/response opcode. See the specific RFC defining that RR type
query/response opcode for such considerations if they have
defined
IANA currently maintains a web page of DNS parameters.
.
"IETF Standards Action", "IETF Consensus", "Specification Required",
and "Private Use" are as defined in [RFC 2434].
2. DNS Query/Response
The header for DNS queries and responses contains field/bits in
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
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
only in queries or only in responses, depending on the bit. However
many DNS implementations copy the query header as the initial
of the response header without clearing bits. Thus any attempt
use a "query" bit with a different meaning in a response or to
a query meaning for a "response" bit is dangerous given
implementation. Such meanings may only be assigned by an
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
Update. These fields have the same structure and data type
Update but are instead the counts for the zone (ZOCOUNT),
prerequisite (PRCOUNT), update (UPCOUNT), and additional
(ARCOUNT) sections
2.1 One Spare Bit
There have been ancient DNS implementations for which the Z bit
on in a query meant that only a response from the primary server
a zone is acceptable. It is believed that current
implementations ignore this bit
Assigning a meaning to the Z bit requires an IETF Standards Action
2.2 Opcode
New OpCode assignments require an IETF Standards Action
Currently DNS OpCodes are assigned as follows
OpCode Name
0 Query [RFC 1035]
1 IQuery (Inverse Query) [RFC 1035]
2 Status [RFC 1035]
3 available for
4 Notify [RFC 1996]
5 Update [RFC 2136]
6-15 available for
Eastlake, et al. Best Current Practice [Page 3]
RFC 2929 DNS IANA Considerations September 2000
2.3 RCODE
It would appear from the DNS header above that only four bits
RCODE, or response/error code are available. However, RCODEs
appear not only at the top level of a DNS response but also
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
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
all refer to the same error code space with the single exception
error code 16 which has a different meaning in the OPT RR from
meaning in other contexts. See table below
RCODE Name Description
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
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
0x0016-0x0F00
3841-4095 Private
0x0F01-0x0
4096-65535 available for
0x1000-0
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
All RRs have the same top level format shown in the figure
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
resource record pertains. NAMEs are specific to a CLASS as
in section 3.2. NAMEs consist of an ordered sequence of one or
labels each of which has a label type [RFC 1035, 2671].
TYPE is a two octet unsigned integer containing one of the RR
codes. See section 3.1.
CLASS is a two octet unsigned integer containing one of the RR
codes. See section 3.2.
TTL is a four octet (32 bit) bit unsigned integer that specifies
number of seconds that the resource record may be cached before
source of the information should again be consulted. Zero
interpreted to mean that the RR can only be used for the
in progress
RDLENGTH is an unsigned 16 bit integer that specifies the length
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
resource. The format of this information varies according to
TYPE and in some cases the CLASS of the resource record
3.1 RR TYPE IANA
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
used in queries. Meta-TYPEs designate transient data associated
an particular DNS message and in some cases can also be used
queries. Thus far, data TYPEs have been assigned from 1 upwards
the block from 100 through 103 while Q and Meta Types have
assigned from 255 downwards (except for the OPT Meta-RR which
assigned TYPE 41). There have been DNS implementations which
caching decisions based on the top bit of the bottom byte of the
TYPE
There are currently three Meta-TYPEs assigned: OPT [RFC 2671],
[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
0
0x0000 - TYPE zero is used as a special indicator for the SIG RR [
2535] and in other circumstances and must never be
for ordinary use
1 - 127
0x0001 - 0x007F - remaining TYPEs in this range are assigned for
TYPEs by IETF Consensus
128 - 255
0x0080 - 0x00FF - remaining TYPEs in this rage are assigned for Q
Meta TYPEs by IETF Consensus
256 - 32767
0x0100 - 0x7FFF - assigned for data, Q, or Meta TYPE use by
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
The OPT (OPTion) RR, number 41, is specified in [RFC 2671].
primary purpose is to extend the effective field size of various
fields including RCODE, label type, flag bits, and RDATA size.
particular, for resolvers and servers that recognize it, it
the RCODE field from 4 to 12 bits
3.2 RR CLASS IANA
DNS CLASSes have been little used but constitute another dimension
the DNS distributed database. In particular, there is no
relationship between the name space or root servers for one CLASS
those for another CLASS. The same name can have completely
meanings in different CLASSes although the label types are the
and the null label is usable only as root in every CLASS. However
as global networking and DNS have evolved, the IN, or Internet,
has dominated DNS use
There are two subcategories of DNS CLASSes: normal data
classes and QCLASSes that are only meaningful in queries or updates
The current CLASS assignments and considerations for
assignments are as follows
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
CLASSes only
128 - 253
0x0080 - 0x00FD - available for assignment by IETF Consensus
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
in [RFC 2434].
65280 - 65534
0xFF00 - 0xFFFE - Private Use
65535
0xFFFF - can only be assigned by an IETF Standards Action
3.3 RR NAME
DNS NAMEs are sequences of labels [RFC 1035]. The last label in
NAME is "ROOT" which is the zero length label. By definition,
null or ROOT label can not be used for any other NAME purpose
At the present time, there are two categories of label types,
labels and compression labels. Compression labels are pointers
data labels elsewhere within an RR or DNS message and are intended
shorten the wire encoding of NAMEs. The two existing data
types are sometimes referred to as Text and Binary. Text labels can
in fact, include any octet value including zero octets but
current uses involve only [US-ASCII]. For retrieval, Text labels
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 [
1981] CLASSes are essentially for local use. The IN or
CLASS is thus the only DNS CLASS in global use on the Internet
this time
A somewhat dated description of name allocation in the IN Class
given in [RFC 1591]. Some information on reserved top level
names is in Best Current Practice 32 [RFC 2606].
4. Security
This document addresses IANA considerations in the allocation
general DNS parameters, not security. See [RFC 2535] for secure
considerations
[Dyer 1987] Dyer, S., and F. Hsu, "Hesiod", Project Athena
Plan - Name Service, April 1987,
[Moon 1981] D. Moon, "Chaosnet", A.I. Memo 628,
Institute of Technology Artificial
Laboratory, June 1981.
[RFC 1034] Mockapetris, P., "Domain Names - Concepts
Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation
Specifications", STD 13, RFC 1035, November 1987.
[RFC 1591] Postel, J., "Domain Name System Structure
Delegation", RFC 1591, March 1994.
[RFC 1996] Vixie, P., "A Mechanism for Prompt Notification of
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
Specification", RFC 2181, July 1997.
[RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
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
Names", RFC 2606, June 1999.
[RFC 2671] Vixie, P., "Extension mechanisms for DNS (EDNS0)",
2671, August 1999.
[RFC 2672] Crawford, M., "Non-Terminal DNS Name Redirection",
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
DNS (TSIG)", RFC 2845, May 2000.
[RFC 2930] Eastlake, D., "Secret Key Establishment for DNS (
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'
Donald E. Eastlake 3
140 Forest
Hudson, MA 01749
Phone: +1-978-562-2827 (h
+1-508-261-5434 (w
Fax: +1-508-261-4447 (w
EMail: Donald.Eastlake@motorola.
Eric Brunner-
100 Brickstone Square, 2nd
Andover, MA 01810
Phone: +1-207-797-0525 (h
+1-978-684-7796 (w
Fax: +1-978-684-3118
EMail: brunner@engage.
Bill
USC/
4676 Admiralty Way, #1001
Marina del Rey, CA 90292
Phone: +1-310-822-1511
EMail: bmanning@isi.
Eastlake, et al. Best Current Practice [Page 11]
RFC 2929 DNS IANA Considerations September 2000
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Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
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included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
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Eastlake, et al. Best Current Practice [Page 12]
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