RFC 2929
Domain Name System (DNS) IANA Considerations
RFC 2929: Domain Name System (DNS) IANA Considerations 🎀
Okay bestie, this is the RFC that explains how IANA hands out DNS numbers and codes, and honestly it's iconic 💅
- RFC: 2929, from the Network Working Group 🌸
- Title: Domain Name System (DNS) IANA Considerations
- Authors: D. Eastlake, 3rd (Motorola), E. Brunner-Williams (Engage), and B. Manning (ISI) 👑
- Category: Best Current Practice, which is BCP 42 ✨
- Date: September 2000 📅
- Obsoletes and updates: the title block doesn't list anything this one obsoletes or updates 🙈
Status of this Memo 📜
So this doc is a Best Current Practice for the whole Internet Community 🌍 and it's basically asking everyone to talk about it and send ideas for making it better 💬. Distribution of this memo is unlimited, so share away 🫶
Copyright Notice 🌈
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract 🧁
This one's about IANA parameter assignment considerations: the rules for handing out 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........................................ 121. Introduction ✨
Okay so the DNS is basically a set of replicated, distributed, secure, hierarchical databases 🗂️ that store "resource records" (RRs) under domain names. Iconic architecture, no notes 💅
The data is organized into CLASSes and zones, and those can be maintained independently 🦋 Familiarity with [RFC 1034, 1035, 2136, 2181, 2535] is assumed, so go read those first, bestie 📚
This doc covers IANA parameter assignment considerations that apply across DNS query and response headers and all RRs, either directly or by reference 🎀 There may be extra IANA considerations for just one particular RR type or one query/response opcode. If those exist, they live in the specific RFC that defines that RR type or opcode, so check there too 🔍
IANA keeps a web page of DNS parameters at <http://www.iana.org/numbers.htm> 🌐
"IETF Standards Action", "IETF Consensus", "Specification Required", and "Private Use" all mean what they're defined to mean in [RFC 2434]. Love a good glossary moment 🧚
2. DNS Query/Response Headers 🍓
Every DNS query and response has a header made of fields and bits, and it looks like this 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 gets echoed back in the response, so the two can be matched up 💕
The QR bit tells you whether the header belongs to a query or a response 💫
Here's the tea 🫖 The AA, TC, RD, RA, AD, and CD bits are each only theoretically meaningful in queries or only in responses, depending on the bit. But lots of DNS implementations copy the query header as the starting value of the response header without clearing bits. So trying to use a "query" bit with a different meaning in a response, or give a query meaning to a "response" bit, is dangerous given existing implementations 😭 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) give the number of records in each section for all opcodes except Update 📊 Update uses the same structure and data types for these fields, but they're instead counts for the zone (ZOCOUNT), prerequisite (PRCOUNT), update (UPCOUNT), and additional information (ARCOUNT) sections.
2.1 One Spare Bit? 🤔
Ancient DNS implementations existed where the Z bit being on in a query meant that only a response from the primary server for a zone was 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 🔒
Right now, DNS OpCodes are assigned like this:
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 assignment2.3 RCODE Assignment 🎟️
Looking at the DNS header above, it seems like only four bits of RCODE (the response or error code) are available. But RCODEs can show up in more places than the top level of a DNS response. They also appear inside OPT RRs [RFC 2671], TSIG RRs [RFC 2845], and TKEY RRs [RFC 2930] 🙌 The OPT RR gives an eight bit extension, which makes a 12 bit RCODE field, and the TSIG and TKEY RRs have a 16 bit RCODE field.
Error codes in the DNS header and in these three RR types all refer to the same error code space. The one exception is error code 16, which means something different in the OPT RR than it does in other contexts. See the 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
0x1000-0xFFFFSince RCODEs have to be understood for interoperability, assigning a new RCODE from the ones listed above as "available for assignment" requires an IETF Consensus 🤝
3. DNS Resource Records 🦋
Every RR has 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 the owner name, meaning the name of the node this resource record is about 🏷️ NAMEs are specific to a CLASS, as described in section 3.2. A NAME is an ordered sequence of one or more labels, and each label has a label type [RFC 1035, 2671] ✨
TYPE is a two octet unsigned integer holding one of the RR TYPE codes. See section 3.1 💫
CLASS is a two octet unsigned integer holding one of the RR CLASS codes. See section 3.2 🌸
TTL is a four octet (32 bit) bit unsigned integer. It says how many seconds the resource record may be cached before the source of the information should be consulted again ⏰ A value of zero is interpreted to mean the RR can only be used for the transaction in progress 🧁
RDLENGTH is an unsigned 16 bit integer that gives the length, in octets, of the RDATA field 📏
RDATA is a variable length string of octets that makes up the resource itself 🎁 Its format depends on the TYPE and, in some cases, the CLASS of the resource record 🌈
3.1 RR TYPE IANA Considerations ✨
okay so RR TYPE numbers come in three flavours bestie 💖: data TYPEs, QTYPEs, and MetaTYPEs.
Data TYPEs are the main way we store stuff 📦. QTYPEs can only be used in queries 🔍. Meta-TYPEs are for transient data tied to one particular DNS message, and sometimes they can also show up in queries.
So far, data TYPEs have been handed out starting from 1 and going up, plus the block from 100 through 103. Q and Meta types have been assigned from 255 going down, except for the OPT Meta-RR, which got TYPE 41 🦋. Some DNS implementations even made caching decisions based on the top bit of the bottom byte of the RR TYPE 👀. literally wild.
There are currently three Meta-TYPEs assigned: OPT [RFC 2671], TSIG [RFC 2845], and TKEY [RFC 2930] 🎀. There are also five QTYPEs assigned: * (all), MAILA, MAILB, AXFR, and IXFR ✨.
Here's how new RR TYPEs get allocated 💅:
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.
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
okay the OPT (OPTion) RR, number 41 🦋, is specified in [RFC 2671]. Its main job is to stretch the effective field size of a bunch of DNS fields, including RCODE, label type, flag bits, and RDATA size 🙌. Like, for resolvers and servers that recognise it, it extends the RCODE field from 4 to 12 bits. iconic, honestly.
3.2 RR CLASS IANA Considerations
DNS CLASSes have been barely used, but they're another whole dimension of the DNS distributed database 🌈. There's no necessary relationship between the name space or root servers for one CLASS and those for another. Same name, totally different meanings in different CLASSes, even though the label types are the same and the null label only works as root in every CLASS 🙈. BUT as global networking and DNS evolved, the IN (Internet) CLASS has absolutely dominated DNS use 👑.
There are two subcategories of DNS CLASSes: normal data containing classes, and QCLASSes that only mean something in queries or updates 💜.
Here are the current CLASS assignments and what's planned for the future 📋:
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].
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
okay so 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 🚫.
Right now there are two categories of label types: data labels and compression labels. Compression labels are pointers to data labels somewhere else in an RR or DNS message, and they exist to shorten the wire encoding of NAMEs 🪄. The two existing data label types are sometimes called Text and Binary. Text labels can, in fact, include any octet value (zero octets too!), but most current uses only involve [US-ASCII] 🔤. For retrieval, Text labels 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] 📝.
NAMEs are local to a CLASS 📍. The Hesiod [Dyer 1987] and Chaos [Moon 1981] CLASSes are basically for local use. That makes the IN (Internet) CLASS the only DNS CLASS in global use on the Internet right now 🌍.
A somewhat dated description of name allocation in the IN Class is in [RFC 1591] 📜. Some info on reserved top level domain names is in Best Current Practice 32 [RFC 2606] 🎀.
4. Security Considerations ✨
🚨 bestie, listen up: this document is about IANA considerations for allocating general DNS parameters. It is not about security. For secure DNS considerations, go read [RFC 2535] 🔐.
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.
- [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.
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.eduFull Copyright Statement 📜
okay bestie, the legal bit is kept word-for-word so nothing gets twisted 🫶:
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Funding for the RFC Editor function is currently provided by the Internet Society. thank you, Internet Society 🫶💖
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