7Internet Engineering Task Force (IETF) J. Damas
8Request for Comments: 6891 Bond Internet Systems
11Category: Standards Track P. Vixie
12ISSN: 2070-1721 Internet Systems Consortium
16 Extension Mechanisms for DNS (EDNS(0))
20 The Domain Name System's wire protocol includes a number of fixed
21 fields whose range has been or soon will be exhausted and does not
22 allow requestors to advertise their capabilities to responders. This
23 document describes backward-compatible mechanisms for allowing the
26 This document updates the Extension Mechanisms for DNS (EDNS(0))
27 specification (and obsoletes RFC 2671) based on feedback from
28 deployment experience in several implementations. It also obsoletes
29 RFC 2673 ("Binary Labels in the Domain Name System") and adds
30 considerations on the use of extended labels in the DNS.
34 This is an Internet Standards Track document.
36 This document is a product of the Internet Engineering Task Force
37 (IETF). It represents the consensus of the IETF community. It has
38 received public review and has been approved for publication by the
39 Internet Engineering Steering Group (IESG). Further information on
40 Internet Standards is available in Section 2 of RFC 5741.
42 Information about the current status of this document, any errata,
43 and how to provide feedback on it may be obtained at
44 http://www.rfc-editor.org/info/rfc6891.
58Damas, et al. Standards Track [Page 1]
60RFC 6891 EDNS(0) Extensions April 2013
65 Copyright (c) 2013 IETF Trust and the persons identified as the
66 document authors. All rights reserved.
68 This document is subject to BCP 78 and the IETF Trust's Legal
69 Provisions Relating to IETF Documents
70 (http://trustee.ietf.org/license-info) in effect on the date of
71 publication of this document. Please review these documents
72 carefully, as they describe your rights and restrictions with respect
73 to this document. Code Components extracted from this document must
74 include Simplified BSD License text as described in Section 4.e of
75 the Trust Legal Provisions and are provided without warranty as
76 described in the Simplified BSD License.
78 This document may contain material from IETF Documents or IETF
79 Contributions published or made publicly available before November
80 10, 2008. The person(s) controlling the copyright in some of this
81 material may not have granted the IETF Trust the right to allow
82 modifications of such material outside the IETF Standards Process.
83 Without obtaining an adequate license from the person(s) controlling
84 the copyright in such materials, this document may not be modified
85 outside the IETF Standards Process, and derivative works of it may
86 not be created outside the IETF Standards Process, except to format
87 it for publication as an RFC or to translate it into languages other
114Damas, et al. Standards Track [Page 2]
116RFC 6891 EDNS(0) Extensions April 2013
121 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
122 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
123 3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 5
124 4. DNS Message Changes . . . . . . . . . . . . . . . . . . . . . 5
125 4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 5
126 4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 5
127 4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 6
128 5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 6
129 6. The OPT Pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 6
130 6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 6
131 6.1.1. Basic Elements . . . . . . . . . . . . . . . . . . . . 6
132 6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 7
133 6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 9
134 6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 9
135 6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 10
136 6.2.1. Cache Behaviour . . . . . . . . . . . . . . . . . . . 10
137 6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 10
138 6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 10
139 6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 11
140 6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 11
141 6.2.6. Support in Middleboxes . . . . . . . . . . . . . . . . 11
142 7. Transport Considerations . . . . . . . . . . . . . . . . . . . 12
143 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
144 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
145 9.1. OPT Option Code Allocation Procedure . . . . . . . . . . . 15
146 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
147 10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
148 10.2. Informative References . . . . . . . . . . . . . . . . . . 15
149 Appendix A. Changes since RFCs 2671 and 2673 . . . . . . . . . . 16
170Damas, et al. Standards Track [Page 3]
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177 DNS [RFC1035] specifies a message format, and within such messages
178 there are standard formats for encoding options, errors, and name
179 compression. The maximum allowable size of a DNS message over UDP
180 not using the extensions described in this document is 512 bytes.
181 Many of DNS's protocol limits, such as the maximum message size over
182 UDP, are too small to efficiently support the additional information
183 that can be conveyed in the DNS (e.g., several IPv6 addresses or DNS
184 Security (DNSSEC) signatures). Finally, RFC 1035 does not define any
185 way for implementations to advertise their capabilities to any of the
186 other actors they interact with.
188 [RFC2671] added extension mechanisms to DNS. These mechanisms are
189 widely supported, and a number of new DNS uses and protocol
190 extensions depend on the presence of these extensions. This memo
191 refines and obsoletes [RFC2671].
193 Unextended agents will not know how to interpret the protocol
194 extensions defined in [RFC2671] and restated here. Extended agents
195 need to be prepared for handling the interactions with unextended
196 clients in the face of new protocol elements and fall back gracefully
199 EDNS is a hop-by-hop extension to DNS. This means the use of EDNS is
200 negotiated between each pair of hosts in a DNS resolution process,
201 for instance, the stub resolver communicating with the recursive
202 resolver or the recursive resolver communicating with an
203 authoritative server.
205 [RFC2671] specified extended label types. The only such label
206 proposed was in [RFC2673] for a label type called "Bit-String Label"
207 or "Binary Labels", with this latest term being the one in common
208 use. For various reasons, introducing a new label type was found to
209 be extremely difficult, and [RFC2673] was moved to Experimental.
210 This document obsoletes [RFC2673], deprecating Binary Labels.
211 Extended labels remain defined, but their use is discouraged due to
212 practical difficulties with deployment; their use in the future
213 SHOULD only be considered after careful evaluation of the deployment
218 "Requestor" refers to the side that sends a request. "Responder"
219 refers to an authoritative, recursive resolver or other DNS component
220 that responds to questions. Other terminology is used here as
221 defined in the RFCs cited by this document.
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228RFC 6891 EDNS(0) Extensions April 2013
231 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
232 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
233 document are to be interpreted as described in RFC 2119 [RFC2119].
2353. EDNS Support Requirement
237 EDNS provides a mechanism to improve the scalability of DNS as its
238 uses get more diverse on the Internet. It does this by enabling the
239 use of UDP transport for DNS messages with sizes beyond the limits
240 specified in RFC 1035 as well as providing extra data space for
241 additional flags and return codes (RCODEs). However, implementation
242 experience indicates that adding new RCODEs should be avoided due to
243 the difficulty in upgrading the installed base. Flags SHOULD be used
244 only when necessary for DNS resolution to function.
246 For many uses, an EDNS Option Code may be preferred.
248 Over time, some applications of DNS have made EDNS a requirement for
249 their deployment. For instance, DNSSEC uses the additional flag
250 space introduced in EDNS to signal the request to include DNSSEC data
253 Given the increase in DNS response sizes when including larger data
254 items such as AAAA records, DNSSEC information (e.g., RRSIG or
255 DNSKEY), or large TXT records, the additional UDP payload
256 capabilities provided by EDNS can help improve the scalability of the
257 DNS by avoiding widespread use of TCP for DNS transport.
2594. DNS Message Changes
263 The DNS message header's second full 16-bit word is divided into a
264 4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see Section
265 4.1.1 of [RFC1035]). Some of these flag values were marked for
266 future use, and most of these have since been allocated. Also, most
267 of the RCODE values are now in use. The OPT pseudo-RR specified
268 below contains extensions to the RCODE bit field as well as
269 additional flag bits.
273 The first 2 bits of a wire format domain label are used to denote the
274 type of the label. [RFC1035] allocates 2 of the 4 possible types and
275 reserves the other 2. More label types were defined in [RFC2671].
276 The use of the 2-bit combination defined by [RFC2671] to identify
277 extended label types remains valid. However, it has been found that
278 deployment of new label types is noticeably difficult and so is only
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284RFC 6891 EDNS(0) Extensions April 2013
287 recommended after careful evaluation of alternatives and the need for
292 Traditional DNS messages are limited to 512 octets in size when sent
293 over UDP [RFC1035]. Fitting the increasing amounts of data that can
294 be transported in DNS in this 512-byte limit is becoming more
295 difficult. For instance, inclusion of DNSSEC records frequently
296 requires a much larger response than a 512-byte message can hold.
298 EDNS(0) specifies a way to advertise additional features such as
299 larger response size capability, which is intended to help avoid
300 truncated UDP responses, which in turn cause retry over TCP. It
301 therefore provides support for transporting these larger packet sizes
302 without needing to resort to TCP for transport.
3045. Extended Label Types
306 The first octet in the on-the-wire representation of a DNS label
307 specifies the label type; the basic DNS specification [RFC1035]
308 dedicates the 2 most significant bits of that octet for this purpose.
310 [RFC2671] defined DNS label type 0b01 for use as an indication for
311 extended label types. A specific extended label type was selected by
312 the 6 least significant bits of the first octet. Thus, extended
313 label types were indicated by the values 64-127 (0b01xxxxxx) in the
314 first octet of the label.
316 Extended label types are extremely difficult to deploy due to lack of
317 support in clients and intermediate gateways, as described in
318 [RFC3363], which moved [RFC2673] to Experimental status; and
319 [RFC3364], which describes the pros and cons. As such, proposals
320 that contemplate extended labels SHOULD weigh this deployment cost
321 against the possibility of implementing functionality in other ways.
323 Finally, implementations MUST NOT generate or pass Binary Labels in
324 their communications, as they are now deprecated.
3286.1. OPT Record Definition
332 An OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the
333 additional data section of a request.
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343 The OPT RR has RR type 41.
345 If an OPT record is present in a received request, compliant
346 responders MUST include an OPT record in their respective responses.
348 An OPT record does not carry any DNS data. It is used only to
349 contain control information pertaining to the question-and-answer
350 sequence of a specific transaction. OPT RRs MUST NOT be cached,
351 forwarded, or stored in or loaded from master files.
353 The OPT RR MAY be placed anywhere within the additional data section.
354 When an OPT RR is included within any DNS message, it MUST be the
355 only OPT RR in that message. If a query message with more than one
356 OPT RR is received, a FORMERR (RCODE=1) MUST be returned. The
357 placement flexibility for the OPT RR does not override the need for
358 the TSIG or SIG(0) RRs to be the last in the additional section
359 whenever they are present.
363 An OPT RR has a fixed part and a variable set of options expressed as
364 {attribute, value} pairs. The fixed part holds some DNS metadata,
365 and also a small collection of basic extension elements that we
366 expect to be so popular that it would be a waste of wire space to
367 encode them as {attribute, value} pairs.
369 The fixed part of an OPT RR is structured as follows:
371 +------------+--------------+------------------------------+
372 | Field Name | Field Type | Description |
373 +------------+--------------+------------------------------+
374 | NAME | domain name | MUST be 0 (root domain) |
375 | TYPE | u_int16_t | OPT (41) |
376 | CLASS | u_int16_t | requestor's UDP payload size |
377 | TTL | u_int32_t | extended RCODE and flags |
378 | RDLEN | u_int16_t | length of all RDATA |
379 | RDATA | octet stream | {attribute,value} pairs |
380 +------------+--------------+------------------------------+
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399 The variable part of an OPT RR may contain zero or more options in
400 the RDATA. Each option MUST be treated as a bit field. Each option
404 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
406 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
408 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
412 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
415 Assigned by the Expert Review process as defined by the DNSEXT
416 working group and the IESG.
419 Size (in octets) of OPTION-DATA.
422 Varies per OPTION-CODE. MUST be treated as a bit field.
424 The order of appearance of option tuples is not defined. If one
425 option modifies the behaviour of another or multiple options are
426 related to one another in some way, they have the same effect
427 regardless of ordering in the RDATA wire encoding.
429 Any OPTION-CODE values not understood by a responder or requestor
430 MUST be ignored. Specifications of such options might wish to
431 include some kind of signaled acknowledgement. For example, an
432 option specification might say that if a responder sees and supports
433 option XYZ, it MUST include option XYZ in its response.
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452RFC 6891 EDNS(0) Extensions April 2013
4556.1.3. OPT Record TTL Field Use
457 The extended RCODE and flags, which OPT stores in the RR Time to Live
458 (TTL) field, are structured as follows:
461 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
462 0: | EXTENDED-RCODE | VERSION |
463 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
465 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
468 Forms the upper 8 bits of extended 12-bit RCODE (together with the
469 4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
470 indicates that an unextended RCODE is in use (values 0 through
474 Indicates the implementation level of the setter. Full
475 conformance with this specification is indicated by version '0'.
476 Requestors are encouraged to set this to the lowest implemented
477 level capable of expressing a transaction, to minimise the
478 responder and network load of discovering the greatest common
479 implementation level between requestor and responder. A
480 requestor's version numbering strategy MAY ideally be a run-time
481 configuration option.
482 If a responder does not implement the VERSION level of the
483 request, then it MUST respond with RCODE=BADVERS. All responses
484 MUST be limited in format to the VERSION level of the request, but
485 the VERSION of each response SHOULD be the highest implementation
486 level of the responder. In this way, a requestor will learn the
487 implementation level of a responder as a side effect of every
488 response, including error responses and including RCODE=BADVERS.
493 DNSSEC OK bit as defined by [RFC3225].
496 Set to zero by senders and ignored by receivers, unless modified
497 in a subsequent specification.
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5136.2.1. Cache Behaviour
515 The OPT record MUST NOT be cached.
519 If a requestor detects that the remote end does not support EDNS(0),
520 it MAY issue queries without an OPT record. It MAY cache this
521 knowledge for a brief time in order to avoid fallback delays in the
522 future. However, if DNSSEC or any future option using EDNS is
523 required, no fallback should be performed, as these options are only
524 signaled through EDNS. If an implementation detects that some
525 servers for the zone support EDNS(0) while others would force the use
526 of TCP to fetch all data, preference MAY be given to servers that
527 support EDNS(0). Implementers SHOULD analyse this choice and the
528 impact on both endpoints.
5306.2.3. Requestor's Payload Size
532 The requestor's UDP payload size (encoded in the RR CLASS field) is
533 the number of octets of the largest UDP payload that can be
534 reassembled and delivered in the requestor's network stack. Note
535 that path MTU, with or without fragmentation, could be smaller than
538 Values lower than 512 MUST be treated as equal to 512.
540 The requestor SHOULD place a value in this field that it can actually
541 receive. For example, if a requestor sits behind a firewall that
542 will block fragmented IP packets, a requestor SHOULD NOT choose a
543 value that will cause fragmentation. Doing so will prevent large
544 responses from being received and can cause fallback to occur. This
545 knowledge may be auto-detected by the implementation or provided by a
548 Note that a 512-octet UDP payload requires a 576-octet IP reassembly
549 buffer. Choosing between 1280 and 1410 bytes for IP (v4 or v6) over
550 Ethernet would be reasonable.
552 Where fragmentation is not a concern, use of bigger values SHOULD be
553 considered by implementers. Implementations SHOULD use their largest
554 configured or implemented values as a starting point in an EDNS
555 transaction in the absence of previous knowledge about the
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564RFC 6891 EDNS(0) Extensions April 2013
567 Choosing a very large value will guarantee fragmentation at the IP
568 layer, and may prevent answers from being received due to loss of a
569 single fragment or to misconfigured firewalls.
571 The requestor's maximum payload size can change over time. It MUST
572 NOT be cached for use beyond the transaction in which it is
5756.2.4. Responder's Payload Size
577 The responder's maximum payload size can change over time but can
578 reasonably be expected to remain constant between two closely spaced
579 sequential transactions, for example, an arbitrary QUERY used as a
580 probe to discover a responder's maximum UDP payload size, followed
581 immediately by an UPDATE that takes advantage of this size. This is
582 considered preferable to the outright use of TCP for oversized
583 requests, if there is any reason to suspect that the responder
584 implements EDNS, and if a request will not fit in the default
585 512-byte payload size limit.
5876.2.5. Payload Size Selection
589 Due to transaction overhead, it is not recommended to advertise an
590 architectural limit as a maximum UDP payload size. Even on system
591 stacks capable of reassembling 64 KB datagrams, memory usage at low
592 levels in the system will be a concern. A good compromise may be the
593 use of an EDNS maximum payload size of 4096 octets as a starting
596 A requestor MAY choose to implement a fallback to smaller advertised
597 sizes to work around firewall or other network limitations. A
598 requestor SHOULD choose to use a fallback mechanism that begins with
599 a large size, such as 4096. If that fails, a fallback around the
600 range of 1280-1410 bytes SHOULD be tried, as it has a reasonable
601 chance to fit within a single Ethernet frame. Failing that, a
602 requestor MAY choose a 512-byte packet, which with large answers may
605 Values of less than 512 bytes MUST be treated as equal to 512 bytes.
6076.2.6. Support in Middleboxes
609 In a network that carries DNS traffic, there could be active
610 equipment other than that participating directly in the DNS
611 resolution process (stub and caching resolvers, authoritative
612 servers) that affects the transmission of DNS messages (e.g.,
613 firewalls, load balancers, proxies, etc.), referred to here as
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620RFC 6891 EDNS(0) Extensions April 2013
623 Conformant middleboxes MUST NOT limit DNS messages over UDP to 512
626 Middleboxes that simply forward requests to a recursive resolver MUST
627 NOT modify and MUST NOT delete the OPT record contents in either
630 Middleboxes that have additional functionality, such as answering
631 queries or acting as intelligent forwarders, SHOULD be able to
632 process the OPT record and act based on its contents. These
633 middleboxes MUST consider the incoming request and any outgoing
634 requests as separate transactions if the characteristics of the
635 messages are different.
637 A more in-depth discussion of this type of equipment and other
638 considerations regarding their interaction with DNS traffic is found
6417. Transport Considerations
643 The presence of an OPT pseudo-RR in a request should be taken as an
644 indication that the requestor fully implements the given version of
645 EDNS and can correctly understand any response that conforms to that
646 feature's specification.
648 Lack of presence of an OPT record in a request MUST be taken as an
649 indication that the requestor does not implement any part of this
650 specification and that the responder MUST NOT include an OPT record
653 Extended agents MUST be prepared for handling interactions with
654 unextended clients in the face of new protocol elements and fall back
655 gracefully to unextended DNS when needed.
657 Responders that choose not to implement the protocol extensions
658 defined in this document MUST respond with a return code (RCODE) of
659 FORMERR to messages containing an OPT record in the additional
660 section and MUST NOT include an OPT record in the response.
662 If there is a problem with processing the OPT record itself, such as
663 an option value that is badly formatted or that includes out-of-range
664 values, a FORMERR MUST be returned. If this occurs, the response
665 MUST include an OPT record. This is intended to allow the requestor
666 to distinguish between servers that do not implement EDNS and format
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679 The minimal response MUST be the DNS header, question section, and an
680 OPT record. This MUST also occur when a truncated response (using
681 the DNS header's TC bit) is returned.
6838. Security Considerations
685 Requestor-side specification of the maximum buffer size may open a
686 DNS denial-of-service attack if responders can be made to send
687 messages that are too large for intermediate gateways to forward,
688 thus leading to potential ICMP storms between gateways and
691 Announcing very large UDP buffer sizes may result in dropping of DNS
692 messages by middleboxes (see Section 6.2.6). This could cause
693 retransmissions with no hope of success. Some devices have been
694 found to reject fragmented UDP packets.
696 Announcing UDP buffer sizes that are too small may result in fallback
697 to TCP with a corresponding load impact on DNS servers. This is
698 especially important with DNSSEC, where answers are much larger.
7009. IANA Considerations
702 The IANA has assigned RR type code 41 for OPT.
704 [RFC2671] specified a number of IANA subregistries within "DOMAIN
705 NAME SYSTEM PARAMETERS":
707 o DNS EDNS(0) Options
709 o EDNS Version Number
713 Additionally, two entries were generated in existing registries:
715 o EDNS Extended Label Type in the DNS Label Types registry
717 o Bad OPT Version in the DNS RCODES registry
719 IANA has updated references to [RFC2671] in these entries and
720 subregistries to this document.
722 [RFC2671] created the DNS Label Types registry. This registry is to
725 The registration procedure for the DNS Label Types registry is
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732RFC 6891 EDNS(0) Extensions April 2013
735 This document assigns option code 65535 in the DNS EDNS0 Options
736 registry to "Reserved for future expansion".
738 The current status of the IANA registry for EDNS Option Codes at the
739 time of publication of this document is
741 o 0-4 assigned, per references in the registry
743 o 5-65000 Available for assignment, unassigned
745 o 65001-65534 Local/Experimental use
747 o 65535 Reserved for future expansion
749 [RFC2671] expands the RCODE space from 4 bits to 12 bits. This
750 allows more than the 16 distinct RCODE values allowed in [RFC1035].
751 IETF Review is required to add a new RCODE.
753 This document assigns EDNS Extended RCODE 16 to "BADVERS" in the DNS
756 [RFC2671] called for the recording of assignment of extended label
757 types 0bxx111111 as "Reserved for future extended label types"; the
758 IANA registry currently contains "Reserved for future expansion".
759 This request implied, at that time, a request to open a new registry
760 for extended label types, but due to the possibility of ambiguity,
761 new text registrations were instead made within the general DNS Label
762 Types registry, which also registers entries originally defined by
763 [RFC1035]. There is therefore no Extended Label Types registry, with
764 all label types registered in the DNS Label Types registry.
766 This document deprecates Binary Labels. Therefore, the status for
767 the DNS Label Types registration "Binary Labels" is now "Historic".
769 IETF Standards Action is required for assignments of new EDNS(0)
770 flags. Flags SHOULD be used only when necessary for DNS resolution
771 to function. For many uses, an EDNS Option Code may be preferred.
773 IETF Standards Action is required to create new entries in the EDNS
774 Version Number registry. Within the EDNS Option Code space, Expert
775 Review is required for allocation of an EDNS Option Code. Per this
776 document, IANA maintains a registry for the EDNS Option Code space.
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788RFC 6891 EDNS(0) Extensions April 2013
7919.1. OPT Option Code Allocation Procedure
793 OPT Option Codes are assigned by Expert Review.
795 Assignment of Option Codes should be liberal, but duplicate
796 functionality is to be avoided.
80010.1. Normative References
802 [RFC1035] Mockapetris, P., "Domain names - implementation and
803 specification", STD 13, RFC 1035, November 1987.
805 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
806 Requirement Levels", BCP 14, RFC 2119, March 1997.
808 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
809 RFC 2671, August 1999.
811 [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
812 RFC 3225, December 2001.
81410.2. Informative References
816 [RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
817 RFC 2673, August 1999.
819 [RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
820 Hain, "Representing Internet Protocol version 6 (IPv6)
821 Addresses in the Domain Name System (DNS)", RFC 3363,
824 [RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
825 Support for Internet Protocol version 6 (IPv6)", RFC 3364,
828 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
829 BCP 152, RFC 5625, August 2009.
842Damas, et al. Standards Track [Page 15]
844RFC 6891 EDNS(0) Extensions April 2013
847Appendix A. Changes since RFCs 2671 and 2673
849 Following is a list of high-level changes to RFCs 2671 and 2673.
851 o Support for the OPT record is now mandatory.
853 o Extended label types remain available, but their use is
854 discouraged as a general solution due to observed difficulties in
855 their deployment on the Internet, as illustrated by the work with
856 the "Binary Labels" type.
858 o RFC 2673, which defined the "Binary Labels" type and is currently
859 Experimental, is requested to be moved to Historic.
861 o Made changes in how EDNS buffer sizes are selected, and provided
862 recommendations on how to select them.
867 Bond Internet Systems
869 S.S. Reyes, Madrid 28701
872 Phone: +1 650.423.1312
873 EMail: joao@bondis.org
878 EMail: explorer@flame.org
882 Internet Systems Consortium
884 Redwood City, California 94063
887 Phone: +1 650.423.1301
898Damas, et al. Standards Track [Page 16]