Request for Comments: 6195
Updates: 1183, 3597
Category: Best Current Practice
Domain Name System (DNS) IANA Considerations
This document specifies Internet Assigned Number Authority (IANA) parameter assignment considerations for the allocation of Domain Name System (DNS) resource record types, CLASSes, operation codes, error codes, DNS protocol message header bits, and AFSDB resource record subtypes.
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/rfc6195.
Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................2 1.1. Terminology ................................................3 2. DNS Query/Response Headers ......................................3 2.1. One Spare Bit? .............................................4 2.2. OpCode Assignment ..........................................4 2.3. RCODE Assignment ...........................................4 3. DNS Resource Records ............................................6 3.1. RRTYPE IANA Considerations .................................7 3.1.1. DNS RRTYPE Allocation Policy ........................8 3.1.2. DNS RRTYPE Expert Guidelines ........................9 3.1.3. Special Note on the OPT RR ..........................9 3.1.4. The AFSDB RR Subtype Field .........................10 3.2. RR CLASS IANA Considerations ..............................10 3.3. Label Considerations ......................................12 3.3.1. Label Types ........................................12 3.3.2. Label Contents and Use .............................12 4. Security Considerations ........................................13 5. IANA Considerations ............................................13 Appendix A. RRTYPE Allocation Template ............................14 Appendix B. Changes from RFC 5395 .................................15 Normative References ..............................................15 Informative References ............................................16
The Domain Name System (DNS) provides replicated distributed secure hierarchical databases that store "resource records" (RRs) under domain names. DNS data is structured into CLASSes and zones that can be independently maintained. Familiarity with [RFC1034], [RFC1035], [RFC2136], [RFC2181], and [RFC4033] is assumed.
This document provides, either directly or by reference, the general IANA parameter assignment considerations that apply across DNS query and response headers and all RRs. There may be additional IANA considerations that apply to only a particular RRTYPE or query/response OpCode. See the specific RFC defining that RRTYPE or query/response OpCode for such considerations if they have been defined, except for AFSDB RR considerations [RFC1183], which are included herein. This RFC obsoletes [RFC5395]; however, the only significant change is the change to the public review mailing list to firstname.lastname@example.org.
IANA currently maintains a web page of DNS parameters available from http://www.iana.org.
"Standards Action", "IETF Review", "Specification Required", and "Private Use" are as defined in [RFC5226].
2. DNS Query/Response Headers
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.
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, some 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 a Standards Action.
The unsigned integer 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 [RFC2136]. 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 a Standards Action.
2.2. OpCode Assignment
Currently, DNS OpCodes are assigned as follows:
OpCode Name Reference 0 Query [RFC1035] 1 IQuery (Inverse Query, Obsolete) [RFC3425] 2 Status [RFC1035] 3 available for assignment 4 Notify [RFC1996] 5 Update [RFC2136] 6-15 available for assignment
New OpCode assignments require a Standards Action as modified by [RFC4020].
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 [RFC2671], TSIG RRs [RFC2845], and TKEY RRs [RFC2930]. The OPT RR provides an 8-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 than in other contexts. This duplicate assignment was accidental. See table below.
RCODE Name Description Reference Decimal Hexadecimal 0 NoError No Error [RFC1035] 1 FormErr Format Error [RFC1035] 2 ServFail Server Failure [RFC1035] 3 NXDomain Non-Existent Domain [RFC1035] 4 NotImp Not Implemented [RFC1035] 5 Refused Query Refused [RFC1035] 6 YXDomain Name Exists when it should not [RFC2136] 7 YXRRSet RR Set Exists when it should not [RFC2136] 8 NXRRSet RR Set that should exist does not [RFC2136] 9 NotAuth Server Not Authoritative for zone [RFC2136] 10 NotZone Name not contained in zone [RFC2136] 11 - 15 Available for assignment 16 BADVERS Bad OPT Version [RFC2671] 16 BADSIG TSIG Signature Failure [RFC2845] 17 BADKEY Key not recognized [RFC2845] 18 BADTIME Signature out of time window [RFC2845] 19 BADMODE Bad TKEY Mode [RFC2930] 20 BADNAME Duplicate key name [RFC2930] 21 BADALG Algorithm not supported [RFC2930] 22 BADTRUC Bad Truncation [RFC4635] 23 - 3,840 0x0017 - 0x0F00 Available for assignment 3,841 - 4,095 0x0F01 - 0x0FFF Private Use 4,096 - 65,534 0x1000 - 0xFFFE Available for assignment 65,535 0xFFFF Reserved, can only be allocated by a Standards Action.
Since it is important that RCODEs be understood for interoperability, assignment of a new RCODE in the ranges listed above as "Available for assignment" requires an IETF Review.
3. DNS Resource Records
All RRs have the same top-level format, shown in the figure below taken from [RFC1035].
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 [RFC1035] [RFC2671].
TYPE is a 2-octet unsigned integer containing one of the RRTYPE codes. See Section 3.1.
CLASS is a 2-octet unsigned integer containing one of the RR CLASS codes. See Section 3.2.
TTL is a 4-octet (32-bit) unsigned integer that specifies, for data TYPEs, 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.
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. RRTYPE IANA Considerations
There are three subcategories of RRTYPE numbers: data TYPEs, QTYPEs, and Meta-TYPEs.
Data TYPEs are the means of storing data. QTYPES can only be used in queries. Meta-TYPEs designate transient data associated with a particular DNS message and, in some cases, can also be used in queries. Thus far, data TYPEs have been assigned from 1 upward, plus the block from 100 through 103, and from 32,768 upward, while Q and Meta-TYPEs have been assigned from 255 downward except for the OPT Meta-RR, which is assigned TYPE 41. There have been DNS implementations that made caching decisions based on the top bit of the bottom byte of the RRTYPE. There are currently three Meta-TYPEs assigned: OPT [RFC2671], TSIG [RFC2845], and TKEY [RFC2930]. There are currently five QTYPEs assigned: * (ALL), MAILA, MAILB, AXFR, and IXFR.
RRTYPEs have mnemonics that must be completely disjoint from the mnemonics used for CLASSes and that must match the following regular expression:
Considerations for the allocation of new RRTYPEs are as follows:
Decimal Hexadecimal 0 0x0000 - RRTYPE zero is used as a special indicator for the SIG (0) RR [RFC2931] [RFC4034] and in other circumstances, and it must never be allocated for ordinary use. 1 - 127 0x0001 - 0x007F - Remaining RRTYPEs in this range are assigned for data TYPEs by the DNS RRTYPE Allocation Policy as specified in Section 3.1.1. 128 - 255 0x0080 - 0x00FF - Remaining RRTYPEs in this range are assigned for Q and Meta-TYPEs by the DNS RRTYPE Allocation Policy as specified in Section 3.1.1. 256 - 61,439 0x0100 - 0xEFFF - Remaining RRTYPEs in this range are assigned for data RRTYPEs by the DNS RRTYPE Allocation Policy as specified in Section 3.1.1. (32,768 and 32,769 (0x8000 and 0x8001) have been assigned.)
61,440 - 65,279
0xF000 - 0xFEFF - Reserved for future use. IETF Review required to
65,280 - 65,534
0xFF00 - 0xFFFE - Private Use.
0xFFFF - Reserved, can only be assigned by a Standards Action.
3.1.1. DNS RRTYPE Allocation Policy
Parameter values specified in Section 3.1 above, as assigned based on DNS RRTYPE Allocation Policy, are allocated by Expert Review if they meet the two requirements listed below. There will be a pool of a small number of Experts appointed by the IESG. Each application will be ruled on by an Expert selected by IANA. In any case where the selected Expert is unavailable or states they have a conflict of interest, IANA may select another Expert from the pool.
Some guidelines for the Experts are given in Section 3.1.2. RRTYPEs that do not meet the requirements below may nonetheless be allocated by a Standards Action as modified by [RFC4020].
- A complete template as specified in Appendix A has been posted for three weeks to the email@example.com mailing list before the Expert Review decision.
Note that partially completed or draft templates may be posted directly by the applicant for comment and discussion, but the formal posting to start the three-week period is made by the Expert.
- The RR for which an RRTYPE code is being requested is either (a) a data TYPE that can be handled as an Unknown RR as described in [RFC3597] or (b) a Meta-TYPE whose processing is optional, i.e., it is safe to simply discard RRs with that Meta-TYPE in queries or responses.
Note that such RRs may include additional section processing, provided such processing is optional.
After the applicant posts their formal application with their template as specified in Appendix A, IANA appoints an Expert and the template is posted, with an indication that it is a formal application, to the firstname.lastname@example.org mailing list. No less than three weeks and no more than six weeks after this posting to email@example.com, the selected Expert shall post a message, explicitly accepting or rejecting the application, to IANA, firstname.lastname@example.org, and the email address provided by the applicant. If the Expert does not post such a message, the application shall be considered rejected but may be resubmitted to IANA. IANA should report non-responsive Experts to the IESG.
IANA shall maintain a public archive of approved templates.
3.1.2. DNS RRTYPE Expert Guidelines
The selected DNS RRTYPE Expert is required to monitor discussion of the proposed RRTYPE, which may occur on the email@example.com mailing list, and may consult with other technical experts as necessary. The Expert should normally reject any RRTYPE allocation request that meets one or more of the following criteria:
- Was documented in a manner that was not sufficiently clear to evaluate or implement.
- The proposed RRTYPE or RRTYPEs affect DNS processing and do not meet the criteria in point 2 of Section 3.1.1 above.
- The documentation of the proposed RRTYPE or RRTYPEs is incomplete. (Additional documentation can be provided during the public comment period or by the Expert.)
- Application use as documented makes incorrect assumptions about DNS protocol behavior, such as wild cards, CNAME, DNAME, etc.
- An excessive number of RRTYPE values is being requested when the purpose could be met with a smaller number or with Private Use values.
3.1.3. Special Note on the OPT RR
The OPT (OPTion) RR (RRTYPE 41) and its IANA considerations are specified in [RFC2671]. Its primary purpose is to extend the effective field size of various DNS fields including RCODE, label type, OpCode, flag bits, and RDATA size. In particular, for resolvers and servers that recognize it, it extends the RCODE field from 4 to 12 bits.
3.1.4. The AFSDB RR Subtype Field
The AFSDB RR [RFC1183] is a CLASS-insensitive RR that has the same RDATA field structure as the MX RR [RFC1035], but the 16-bit unsigned integer field at the beginning of the RDATA is interpreted as a subtype as follows:
Decimal Hexadecimal 0 0x0000 - Reserved; allocation requires a Standards Action. 1 0x0001 - Andrews File Service v3.0 Location Service [RFC1183]. 2 0x0002 - DCE/NCA root cell directory node [RFC1183]. 3 - 65,279 0x0003 - 0xFEFF - Allocation by IETF Review.
65,280 - 65,534
0xFF00 - 0xFFFE - Private Use.
0xFFFF - Reserved; allocation requires a Standards Action.
3.2. RR CLASS IANA Considerations
There are currently two subcategories of DNS CLASSes: normal, data- containing classes and QCLASSes that are only meaningful in queries or updates.
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 data CLASS and those for another data CLASS. The same DNS NAME can have completely different meanings in different CLASSes. The label types are the same, and the null label is usable only as root in every CLASS. As global networking and DNS have evolved, the IN, or Internet, CLASS has dominated DNS use.
As yet, there has not been a requirement for "meta-CLASSes". That would be a CLASS to designate transient data associated with a particular DNS message, which might be usable in queries. However, it is possible that there might be a future requirement for one or more "meta-CLASSes".
CLASSes have mnemonics that must be completely disjoint from the mnemonics used for RRTYPEs and that must match the following regular expression:
The current CLASS assignments and considerations for future assignments are as follows:
Decimal Hexadecimal 0 0x0000 - Reserved; assignment requires a Standards Action. 1 0x0001 - Internet (IN). 2 0x0002 - Available for assignment by IETF Review as a data CLASS. 3 0x0003 - Chaos (CH) [Moon1981]. 4 0x0004 - Hesiod (HS) [Dyer1987]. 5 - 127 0x0005 - 0x007F - Available for assignment by IETF Review for data CLASSes only. 128 - 253 0x0080 - 0x00FD - Available for assignment by IETF Review for QCLASSes and meta-CLASSes only. 254 0x00FE - QCLASS NONE [RFC2136]. 255 0x00FF - QCLASS * (ANY) [RFC1035]. 256 - 32,767 0x0100 - 0x7FFF - Assigned by IETF Review.
32,768 - 57,343
0x8000 - 0xDFFF - Assigned for data CLASSes only, based on
Specification Required as defined in [RFC5226].
57,344 - 65,279
0xE000 - 0xFEFF - Assigned for QCLASSes and meta-CLASSes only, based
on Specification Required as defined in [RFC5226].
65,280 - 65,534
0xFF00 - 0xFFFE - Private Use.
0xFFFF - Reserved; can only be assigned by a Standards Action.
3.3. Label Considerations
DNS NAMEs are sequences of labels [RFC1035].
3.3.1. Label Types
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-value octets, but many current uses involve only [US-ASCII]. For retrieval, Text labels are defined to treat ASCII upper and lower case letter codes as matching [RFC4343]. Binary labels are bit sequences [RFC2673]. The Binary label type is Experimental [RFC3363].
IANA considerations for label types are given in [RFC2671].
3.3.2. Label Contents and Use
The last label in each NAME is "ROOT", which is the zero-length label. By definition, the null or ROOT label cannot be used for any other NAME purpose.
NAMEs are local to a CLASS. The Hesiod [Dyer1987] and Chaos [Moon1981] CLASSes are for essentially local use. The IN, or Internet, CLASS is thus the only DNS CLASS in global use on the Internet at this time.
4. Security Considerations
5. IANA Considerations
This document consists entirely of DNS IANA Considerations.
IANA has established a process for accepting Appendix A templates and selecting an Expert from those appointed to review such template form applications. IANA archives and makes available all approved RRTYPE allocation templates. It is the duty of the applicant to post the formal application template to the firstname.lastname@example.org mailing list, which IANA will monitor. The email@example.com mailing list is for community discussion and comment. See Section 3.1 and Appendix A for more details.
Appendix A. RRTYPE Allocation Template
DNS RRTYPE PARAMETER ALLOCATION TEMPLATE
When ready for formal consideration, this template is to be submitted to IANA for processing by emailing the template to firstname.lastname@example.org.
A. Submission Date:
B. Submission Type:
[ ] New RRTYPE
[ ] Modification to existing RRTYPE
C. Contact Information for submitter (will be publicly posted):
International telephone number:
Other contact handles:
D. Motivation for the new RRTYPE application.
Please keep this part at a high level to inform the Expert and reviewers about uses of the RRTYPE. Most reviewers will be DNS experts that may have limited knowledge of your application space.
E. Description of the proposed RR type.
This description can be provided in-line in the template, as an attachment, or with a publicly available URL.
F. What existing RRTYPE or RRTYPEs come closest to filling that need
and why are they unsatisfactory?
G. What mnemonic is requested for the new RRTYPE (optional)?
Note: this can be left blank and the mnemonic decided after the template is accepted.
H. Does the requested RRTYPE make use of any existing IANA registry
or require the creation of a new IANA sub-registry in DNS Parameters? If so, please indicate which registry is to be used or created. If a new sub-registry is needed, specify the allocation policy for it and its initial contents. Also include what the modification procedures will be.
I. Does the proposal require/expect any changes in DNS
servers/resolvers that prevent the new type from being processed as an unknown RRTYPE (see [RFC3597])?
Appendix B. Changes From RFC 5395
Replaced "email@example.com" with "firstname.lastname@example.org".
Updated the boilerplate text.
Fixed Section 5 to say that it is the duty of the applicant, not the expert, to post the application to email@example.com.
Changed the regular expression for RRTYPE and CLASS names so as to prohibit trailing hyphen ("-") and require a minimum length of 2 characters.
Made a number of minor editorial and typos fixes.
[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. [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996. [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000. [RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY RR)", RFC 2930, September 2000. [RFC3425] Lawrence, D., "Obsoleting IQUERY", RFC 3425, November 2002. [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR) Types", RFC 3597, September 2003. [RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 4020, February 2005. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. [RFC4635] Eastlake 3rd, D., "HMAC SHA (Hashed Message Authentication Code, Secure Hash Algorithm) TSIG Algorithm Identifiers", RFC 4635, August 2006. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[US-ASCII] ANSI, "USA Standard Code for Information Interchange",
X3.4, American National Standards Institute: New York, 1968.
[Dyer1987] Dyer, S., and F. Hsu, "Hesiod", Project Athena Technical Plan - Name Service, April 1987. [Moon1981] Moon, D., "Chaosnet", A.I. Memo 628, Massachusetts Institute of Technology Artificial Intelligence Laboratory, June 1981. [RFC1183] Everhart, C., Mamakos, L., Ullmann, R., and P. Mockapetris, "New DNS RR Definitions", RFC 1183, October 1990. [RFC1591] Postel, J., "Domain Name System Structure and Delegation", RFC 1591, March 1994. [RFC2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, June 1999. [RFC2673] Crawford, M., "Binary Labels in the Domain Name System", RFC 2673, August 1999. [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures ( SIG(0)s )", RFC 2931, September 2000. [RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T. Hain, "Representing Internet Protocol version 6 (IPv6) Addresses in the Domain Name System (DNS)", RFC 3363, August 2002. [RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case Insensitivity Clarification", RFC 4343, January 2006. [RFC5395] Eastlake 3rd, D., "Domain Name System (DNS) IANA Considerations", BCP 42, RFC 5395, November 2008.
Donald E. Eastlake 3rd
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270 EMail: firstname.lastname@example.org