draft-ietf-dnsext-rfc2671bis-edns0-05.txt   draft-ietf-dnsext-rfc2671bis-edns0-06.txt 
DNSEXT Working Group J. Damas DNSEXT Working Group J.L.S.D. Damas
Internet-Draft M. Graff Internet-Draft M.G. Graff
Obsoletes: 2671, 2673 P. Vixie Obsoletes: 2671, 2673 (if approved) P.V. Vixie
(if approved) Internet Systems Consortium Intended status: Standards Track Internet Systems Consortium
Intended status: Standards Track March 7, 2011 Expires: June 01, 2012 December 2011
Expires: September 8, 2011
Extension Mechanisms for DNS (EDNS0) Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-05 draft-ietf-dnsext-rfc2671bis-edns0-06
Abstract Abstract
The Domain Name System's wire protocol includes a number of fixed The Domain Name System's wire protocol includes a number of fixed
fields whose range has been or soon will be exhausted and does not fields whose range has been or soon will be exhausted and does not
allow requestors to advertise their capabilities to responders. This allow requestors to advertise their capabilities to responders. This
document describes backward compatible mechanisms for allowing the document describes backward compatible mechanisms for allowing the
protocol to grow. protocol to grow.
This document updates the EDNS0 specification [RFC2671] based on 10 This document updates the EDNS0 specification [RFC2671] based on
years of deployment experience. feedback from deployment experience in several implementations.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 8, 2011. This Internet-Draft will expire on June 01, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3 3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 4 4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 3
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4 4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4 5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4
6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 5 6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 4
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 5 6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 4
6.2. OPT Record Wire Format . . . . . . . . . . . . . . . . . . 5 6.1.1. Basic elements . . . . . . . . . . . . . . . . . . . . 5
6.3. Cache behaviour . . . . . . . . . . . . . . . . . . . . . 7 6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 5
6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 6
6.5. Requestor's Payload Size . . . . . . . . . . . . . . . . . 7 6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 7
6.6. Responder's Payload Size . . . . . . . . . . . . . . . . . 7 6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 7
6.7. Payload Size Selection . . . . . . . . . . . . . . . . . . 8 6.2.1. Cache behaviour . . . . . . . . . . . . . . . . . . . 7
6.8. Middleware Boxes . . . . . . . . . . . . . . . . . . . . . 8 6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 7
6.9. OPT Record TTL Field Use . . . . . . . . . . . . . . . . . 8 6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 7
6.10. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 8
6.11. OPT Options Code Allocation Procedure . . . . . . . . . . 9 6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 8
7. Transport Considerations . . . . . . . . . . . . . . . . . . . 10 6.2.6. MiddleBoxes . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. OPT Option Code Allocation Procedure . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Transport Considerations . . . . . . . . . . . . . . . . . . . 9
Appendix A. Document Editing History . . . . . . . . . . . . . . 11 9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . . 11 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . . 12 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 11.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12 11.2. Informative References . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12 Appendix A. Document Editing History . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . 12
Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
DNS [RFC1035] specifies a Message Format and within such messages DNS [RFC1035] specifies a Message Format and within such messages
there are standard formats for encoding options, errors, and name there are standard formats for encoding options, errors, and name
compression. The maximum allowable size of a DNS Message is limited compression. The maximum allowable size of a DNS Message over UDP
to 512 bytes. Many of DNS's protocol limits are too small for uses not using the extensions described in this document is limited to 512
which are commom or desired to become common. RFC 1035 does not bytes. Many of DNS's protocol limits, such as the maximum message
define any way for implementations to advertise their capabilities. size over UDP, are too small to efficiently support the additional
information that can be conveyed in the DNS (e.g. several IPv6
addresses or DNSSEC signatures). Finally, RFC 1035 does not define
any way for implementations to advertise their capabilities to any of
the others actors they interact with.
[RFC2671] added extension mechanism to DNS, this mechanism is widely [RFC2671] added an extension mechanism to DNS. This mechanism is
supported and number of new DNS uses and protocol extensions depend widely supported and a number of new DNS uses and protocol extensions
on the presence of these extensions. This memo refines that depend on the presence of these extensions. This memo refines that
specification and obsoletes [RFC2671]. specification and obsoletes [RFC2671].
Unextended agents will not know how to interpret the protocol Unextended agents will not know how to interpret the protocol
extensions defined in [RFC2671] and restated here. Extended agents extensions defined in [RFC2671] and restated here. Extended agents
must be prepared for handling the interactions with unextended MUST be prepared for handling the interactions with unextended
clients in the face of new protocol elements, and fall back clients in the face of new protocol elements, and fall back
gracefully to unextended DNS. [RFC2671] proposed extensions to the gracefully to unextended DNS.
basic DNS protocol to overcome these deficiencies. This memo refines
that specification and obsoletes [RFC2671].
[RFC2671] specified extended label types. The only one proposed was [RFC2671] specified extended label types. The only one proposed was
in RFC2673 for a label type called "Bitstring Labels." For various in [RFC2673] for a label type called "Bitstring Labels." For various
reasons introducing a new label type was found to be extremely reasons introducing a new label type was found to be extremely
difficult, and RFC2673 was moved to Experimental. This document difficult, and [RFC2673] was moved to Experimental. This document
Obsoletes Extended Labels. deprecates Extended Labels.
2. Terminology 2. Terminology
"Requestor" is the side which sends a request. "Responder" is an "Requestor" is the side which sends a request. "Responder" is an
authoritative, recursive resolver, or other DNS component which authoritative, recursive resolver, or other DNS component which
responds to questions. responds to questions. Other terminology is used as per its use in
the references (e.g. middleboxes as in [RFC5625])
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. EDNS Support Requirement 3. EDNS Support Requirement
EDNS support is practically mandatory in a modern world. DNSSEC EDNS provides a mechanism to improve the scalability of DNS as its
requires EDNS support, and many other features are made possible only uses get more diverse on the Internet. It does this by enabling the
by EDNS support to request or advertise them. Many organizations are use of UDP transport for DNS messages with sizes beyond the limits
requiring DNSSEC. Without common interoperability, DNSSEC cannot be specified in RFC 1035 as well as providing extra data space for
as easily deployed. additional flags and return codes (RCODEs).
DNS publishers are wanting to put more data in answers. DNSSEC With time, some applications of DNS have made EDNS a requirement for
DNSKEY records, negative answers, and many other DNSSEC queries cause their deployment. For instance, DNSSEC uses the additional flag
larger answers to be returned. In order to support this, DNS space introduced in EDNS to signal the request to include DNSSEC data
servers, middleware, and stub resolvers MUST support larger packet in a DNS response.
sizes advertised via EDNS0.
Given the increase in DNS response sizes when including larger data
items such as AAAA Records, DNSSEC information (e.g. RRSIG or
DNSKEY) or large TXT Records, the additional UDP payload capabilities
provided by EDNS can help improve the scalability of the DNS by
avoiding generalized use of TCP for DNS transport.
4. DNS Message changes 4. DNS Message changes
4.1. Message Header 4.1. Message Header
The DNS Message Header's second full 16-bit word is divided into a The DNS Message Header's second full 16-bit word is divided into a
4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see , 4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see ,
section 4.1.1 [RFC1035]). Some of these were marked for future use, section 4.1.1 [RFC1035]). Some of these were marked for future use,
and most these have since been allocated. Also, most of the RCODE and most these have since been allocated. Also, most of the RCODE
values are now in use. The OPT pseudo-RR specified below contains values are now in use. The OPT pseudo-RR specified below contains
extensions to the RCODE bit field as well as additional flag bits. extensions to the RCODE bit field as well as additional flag bits.
4.2. Label Types 4.2. Label Types
The first two bits of a wire format domain label are used to denote The first two bits of a wire format domain label are used to denote
the type of the label. [RFC1035] allocates two of the four possible the type of the label. [RFC1035] allocates two of the four possible
types and reserves the other two. More label types were defined in types and reserves the other two. More label types were defined in
[RFC2671]. This document obsoletes the use of the 2-bit combination [RFC2671]. This document obsoletes the use of the 2-bit combination
defined by [RFC2671] to identify extended label types. defined by [RFC2671] to identify extended label types.
4.3. UDP Message Size 4.3. UDP Message Size
Traditional DNS Messages are limited to 512 octets in size when sent Traditional DNS Messages are limited to 512 octets in size when sent
over UDP ([RFC1035]). Today, many organizations wish to return many over UDP [RFC1035]. Fitting the increasing amounts of data that can
records in a single reply, and special tricks are needed to make the be transported in DNS in this 512-byte limit is becoming more
responses fit in this 512-byte limit. Additionally, inclusion of difficult. For instance, inclusion of DNSSEC records frequently
DNSSEC records frequently requires a much larger response than a 512 requires a much larger response than a 512 byte message can hold.
byte message can hold.
EDNS0 is intended to address these larger packet sizes and continue EDNS0 is intended to provide support for transporting these larger
to use UDP. It specifies a way to advertise additional features such packet sizes while continuing to use UDP. It specifies a way to
as larger response size capability, which is intended to help avoid advertise additional features such as larger response size
truncated UDP responses which then cause retry over TCP. capability, which is intended to help avoid truncated UDP responses
which then cause retry over TCP.
5. Extended Label Types 5. Extended Label Types
The first octet in the on-the-wire representation of a DNS label The first octet in the on-the-wire representation of a DNS label
specifies the label type; the basic DNS specification [RFC1035] specifies the label type; the basic DNS specification [RFC1035]
dedicates the two most significant bits of that octet for this dedicates the two most significant bits of that octet for this
purpose. purpose.
[RFC2671] defined DNS label type 0b01 for use as an indication for [RFC2671] defined DNS label type 0b01 for use as an indication for
Extended Label Types. A specific Extended Label Type is selected by Extended Label Types. A specific Extended Label Type is selected by
the 6 least significant bits of the first octet. Thus, Extended the 6 least significant bits of the first octet. Thus, Extended
Label Types are indicated by the values 64-127 (0b01xxxxxx) in the Label Types are indicated by the values 64-127 (0b01xxxxxx) in the
first octet of the label. first octet of the label.
Extended Label Types are difficult to use due to support in clients Extended Label Types are difficult to use due to support in clients
and intermediate gateways as described in [RFC3364] which moves them and intermediate gateways as described in [RFC3364] which moves them
to experimental status and [RFC3363], which describes the pros and to experimental status and [RFC3363], which describes the pros and
cons. cons.
Therefore, this document moves them from experimental to historical, Therefore, this document moves them from experimental to historical,
making them obsoleted. Additionally, the registry of Extended Label making them deprecated.
Types is requested to be closed.
Implementations MUST NOT generate or pass Extended Labels in their
communications. Additionally, no further registrations of Extended
Label Types are permitted.
6. The OPT pseudo-RR 6. The OPT pseudo-RR
6.1. OPT Record Definition 6.1. OPT Record Definition
6.1.1. Basic elements
An OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the An OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the
additional data section of a request. additional data section of a request.
The OPT RR has RR type 41. The OPT RR has RR type 41.
If present in requests, compliant responders MUST include an OPT If present in requests, compliant responders MUST include an OPT
record in their respective responses. record in their respective responses.
An OPT record does not carry any DNS data. It is used only to An OPT record does not carry any DNS data. It is used only to
contain control information pertaining to the question and answer contain control information pertaining to the question and answer
sequence of a specific transaction. OPT RRs MUST NOT be cached, sequence of a specific transaction. OPT RRs MUST NOT be cached,
forwarded, or stored in or loaded from master files. forwarded, or stored in or loaded from master files.
The OPT RR MAY be placed anywhere within the additional data section. The OPT RR MAY be placed anywhere within the additional data section.
Only one OPT RR MAY be included within any DNS message. If a message No more than one OPT RR MUST be included within any DNS message. If
with more than one OPT RR is received, a FORMERR (RCODE=1) MUST be a query message with more than one OPT RR is received, a FORMERR
returned. The placement flexibility for the OPT RR does not override (RCODE=1) MUST be returned. The placement flexibility for the OPT RR
the need for the TSIG or SIG(0) RRs to be the last in the additional does not override the need for the TSIG or SIG(0) RRs to be the last
section whenever they are present. in the additional section whenever they are present.
6.2. OPT Record Wire Format 6.1.2. Wire Format
An OPT RR has a fixed part and a variable set of options expressed as An OPT RR has a fixed part and a variable set of options expressed as
{attribute, value} pairs. The fixed part holds some DNS meta data {attribute, value} pairs. The fixed part holds some DNS meta data
and also a small collection of basic extension elements which we and also a small collection of basic extension elements which we
expect to be so popular that it would be a waste of wire space to expect to be so popular that it would be a waste of wire space to
encode them as {attribute, value} pairs. encode them as {attribute, value} pairs.
The fixed part of an OPT RR is structured as follows: The fixed part of an OPT RR is structured as follows:
+------------+--------------+------------------------------+ +------------+--------------+------------------------------+
| Field Name | Field Type | Description | | Field Name | Field Type | Description |
+------------+--------------+------------------------------+ +------------+--------------+------------------------------+
| NAME | domain name | Must be 0 (root domain) | | NAME | domain name | Must be 0 (root domain) |
| TYPE | u_int16_t | OPT (42) | | TYPE | u_int16_t | OPT (41) |
| CLASS | u_int16_t | requestor's UDP payload size | | CLASS | u_int16_t | requestor's UDP payload size |
| TTL | u_int32_t | extended RCODE and flags | | TTL | u_int32_t | extended RCODE and flags |
| RDLEN | u_int16_t | length of all RDATA | | RDLEN | u_int16_t | length of all RDATA |
| RDATA | octet stream | {attribute,value} pairs | | RDATA | octet stream | {attribute,value} pairs |
+------------+--------------+------------------------------+ +------------+--------------+------------------------------+
OPT RR Format
The variable part of an OPT RR may contain zero or more options in The variable part of an OPT RR may contain zero or more options in
the RDATA. Each option must be treated as binary. Each option is the RDATA. Each option MUST be treated as binary. Each option is
encoded as: encoded as:
+0 (MSB) +1 (LSB) +0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE | 0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH | 2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | | 4: | |
/ OPTION-DATA / / OPTION-DATA /
/ / / /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE OPTION-CODE
Assigned by Expert Review. Assigned by Expert Review.
OPTION-LENGTH OPTION-LENGTH
Size (in octets) of OPTION-DATA. Size (in octets) of OPTION-DATA.
OPTION-DATA OPTION-DATA
Varies per OPTION-CODE. MUST be treated as binary. Varies per OPTION-CODE. MUST be treated as binary.
The order of appearance of option tuples is not defined. If one The order of appearance of option tuples is not defined. If one
option modifies the behavior of another or multiple options are option modifies the behavior of another or multiple options are
related to one another in some way, they have the same effect related to one another in some way, they have the same effect
regardless of ordering in the RDATA wire encoding. regardless of ordering in the RDATA wire encoding.
Any OPTION-CODE values not understood by a responder or requestor Any OPTION-CODE values not understood by a responder or requestor
MUST be ignored. Specifications of such options might wish to MUST be ignored. Specifications of such options might wish to
include some kind of signaled acknowledgement. For example, an include some kind of signaled acknowledgement. For example, an
option specification might say that if a responder sees option XYZ, option specification might say that if a responder sees option XYZ,
it MUST include option XYZ in its response. it MUST include option XYZ in its response.
6.3. Cache behaviour 6.1.3. OPT Record TTL Field Use
The extended RCODE and flags (which OPT stores in the RR TTL field)
are structured as follows:
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO| Z |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE
Forms upper 8 bits of extended 12-bit RCODE (together with the
4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
indicates that an unextended RCODE is in use (values 0 through
15).
VERSION
Indicates the implementation level of the settter. Full
conformance with this specification is indicated by version
``0.'' Requestors are encouraged to set this to the lowest
implemented level capable of expressing a transaction, to
minimize the responder and network load of discovering the
greatest common implementation level between requestor and
responder. A requestor's version numbering strategy MAY
ideally be a run time configuration option.
If a responder does not implement the VERSION level of the
request, then it MUST respond with RCODE=BADVERS. All
responses MUST be limited in format to the VERSION level of the
request, but the VERSION of each response SHOULD be the highest
implementation level of the responder. In this way a requestor
will learn the implementation level of a responder as a side
effect of every response, including error responses and
including RCODE=BADVERS.
6.1.4. Flags
DO
DNSSEC OK bit as defined by [RFC3225].
Z
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6.2. Behaviour
6.2.1. Cache behaviour
The OPT record MUST NOT be cached. The OPT record MUST NOT be cached.
6.4. Fallback 6.2.2. Fallback
If a requestor detects that the remote end does not support EDNS0, it If a requestor detects that the remote end does not support EDNS0, it
MAY issue queries without an OPT record. It MAY cache this knowledge MAY issue queries without an OPT record. It MAY cache this knowledge
for a brief time in order to avoid fallback delays in the future. for a brief time in order to avoid fallback delays in the future.
However, if DNSSEC or any future option using EDNS is required, no However, if DNSSEC or any future option using EDNS is required, no
fallback should be performed as they are only signaled through EDNS0. fallback should be performed as they are only signaled through EDNS0.
If an implementation detects that some servers for the zone support If an implementation detects that some servers for the zone support
EDNS(0) while others would force the use of TCP to fetch all data, EDNS(0) while others would force the use of TCP to fetch all data,
preference SHOULD be given to those support EDNS(0). preference SHOULD be given to those support EDNS(0).
6.5. Requestor's Payload Size 6.2.3. Requestor's Payload Size
The requestor's UDP payload size (encoded in the RR CLASS field) is The requestor's UDP payload size (encoded in the RR CLASS field) is
the number of octets of the largest UDP payload that can be the number of octets of the largest UDP payload that can be
reassembled and delivered in the requestor's network stack. Note reassembled and delivered in the requestor's network stack. Note
that path MTU, with or without fragmentation, could be smaller than that path MTU, with or without fragmentation, could be smaller than
this. Values lower than 512 MUST be treated as equal to 512. this.
Values lower than 512 MUST be treated as equal to 512.
The requestor SHOULD place a value in this field that it can actually The requestor SHOULD place a value in this field that it can actually
receive. For example, if a requestor sits behind a firewall which receive. For example, if a requestor sits behind a firewall which
will block fragmented IP packets, a requestor SHOULD not choose a will block fragmented IP packets, a requestor SHOULD not choose a
value which will cause fragmentation. Doing so will prevent large value which will cause fragmentation. Doing so will prevent large
responses from being received, and can cause fallback to occur. responses from being received, and can cause fallback to occur. This
knowledge may be auto-detected by the implementation or provided by a
human administrator.
Note that a 512-octet UDP payload requires a 576-octet IP reassembly Note that a 512-octet UDP payload requires a 576-octet IP reassembly
buffer. Choosing between 1280 and 1410 bytes for IP (v4 or v6) over buffer. Choosing between 1280 and 1410 bytes for IP (v4 or v6) over
Ethernet would be reasonable. Choosing a very large value will Ethernet would be reasonable.
guarantee fragmentation at the IP layer, and may prevent answers from
being received due to a single fragment loss or misconfigured Bigger values SHOULD be considered here fragmentation is not a
firewalls. concern.
Choosing a very large value will guarantee fragmentation at the IP
layer, and may prevent answers from being received due to a single
fragment loss or misconfigured firewalls.
The requestor's maximum payload size can change over time. It MUST The requestor's maximum payload size can change over time. It MUST
not be cached for use beyond the transaction in which it is not be cached for use beyond the transaction in which it is
advertised. advertised.
6.6. Responder's Payload Size 6.2.4. Responder's Payload Size
The responder's maximum payload size can change over time, but can be The responder's maximum payload size can change over time, but can be
reasonably expected to remain constant between two closely spaced reasonably expected to remain constant between two closely spaced
sequential transactions; for example, a meaningless QUERY to discover sequential transactions; for example, an arbitrary QUERY used as a
a responder's maximum UDP payload size, followed immediately by an probe to discover a responder's maximum UDP payload size, followed
UPDATE which takes advantage of this size. This is considered immediately by an UPDATE which takes advantage of this size. This is
preferable to the outright use of TCP for oversized requests, if considered preferable to the outright use of TCP for oversized
there is any reason to suspect that the responder implements EDNS, requests, if there is any reason to suspect that the responder
and if a request will not fit in the default 512 payload size limit. implements EDNS, and if a request will not fit in the default 512
payload size limit.
6.7. Payload Size Selection 6.2.5. Payload Size Selection
Due to transaction overhead, it is unwise to advertise an Due to transaction overhead, it is not recommended to advertise an
architectural limit as a maximum UDP payload size. Just because your architectural limit as a maximum UDP payload size. Even on system
stack can reassemble 64KB datagrams, don't assume that you want to stacks capable of reassembling 64KB datagrams, memory usage at low
spend more than about 4KB of state memory per ongoing transaction. levels in the system will be a concern. A good compromise may be the
use of about 4KB of state memory per ongoing transaction, or a EDNS
maximum payload size of 4096 octets.
A requestor MAY choose to implement a fallback to smaller advertised A requestor MAY choose to implement a fallback to smaller advertised
sizes to work around firewall or other network limitations. A sizes to work around firewall or other network limitations. A
requestor SHOULD choose to use a fallback mechanism which begins with requestor SHOULD choose to use a fallback mechanism which begins with
a large size, such as 4096. If that fails, a fallback around the a large size, such as 4096. If that fails, a fallback around the
1280-1410 byte range SHOULD be tried, as it has a reasonable chance 1280-1410 byte range SHOULD be tried, as it has a reasonable chance
to fit within a single Ethernet frame. Failing that, a requestor MAY to fit within a single Ethernet frame. Failing that, a requestor MAY
choose a 512 byte packet, which with large answers may cause a TCP choose a 512 byte packet, which with large answers may cause a TCP
retry. retry.
6.8. Middleware Boxes Values of less than 512 bytes SHOULD be treated as equal to 512
bytes.
Middleware boxes (e.g. firewalls, SOHO routers, load balancers, etc)
MUST NOT limit DNS messages over UDP to 512 bytes.
Middleware boxes which simply forward requests to a recursive
resolver MUST NOT modify and MUST NOT delete the OPT record contents
in either direction.
Middleware boxes which have additional functionality, such as
answering certain queries or acting like an intelligent forwarder,
MUST understand the OPT record. These boxes MUST consider the
incoming request and any outgoing requests as separate transactions
if the characteristics of the messages are different.
A complete discussion of middleware boxes acting as DNS proxies and
the impact of EDNS in those implementations is described in
[RFC5625].
6.9. OPT Record TTL Field Use
The extended RCODE and flags (which OPT stores in the RR TTL field) 6.2.6. MiddleBoxes
are structured as follows:
+0 (MSB) +1 (LSB) In a network that carries DNS traffic there could be active equipment
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ other than that participating directly in the DNS resolution process
0: | EXTENDED-RCODE | VERSION | (stub and caching resolvers, authoritative servers) that affect the
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ transmission of DNS messages (e.g. firewalls, load balancers,
2: | DO| Z | proxies, etc) referred to here as MiddleBoxes.
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE MiddleBoxes MUST NOT limit DNS messages over UDP to 512 bytes.
Forms upper 8 bits of extended 12-bit RCODE (together with the
4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
indicates that an unextended RCODE is in use (values 0 through
15).
VERSION MiddleBoxes which simply forward requests to a recursive resolver
Indicates the implementation level of whoever sets it. Full MUST NOT modify and MUST NOT delete the OPT record contents in either
conformance with this specification is indicated by version direction.
``0.'' Requestors are encouraged to set this to the lowest
implemented level capable of expressing a transaction, to
minimize the responder and network load of discovering the
greatest common implementation level between requestor and
responder. A requestor's version numbering strategy MAY
ideally be a run time configuration option.
If a responder does not implement the VERSION level of the
request, then it answers with RCODE=BADVERS. All responses
MUST be limited in format to the VERSION level of the request,
but the VERSION of each response SHOULD be the highest
implementation level of the responder. In this way a requestor
will learn the implementation level of a responder as a side
effect of every response, including error responses and
including RCODE=BADVERS.
6.10. Flags MiddleBoxes which have additional functionality, such as answering
queries or acting as intelligent forwarders, SHOULD understand the
OPT record. These boxes MUST consider the incoming request and any
outgoing requests as separate transactions if the characteristics of
the messages are different.
DO A more in depth discussion of this type of equipment and other
DNSSEC OK bit as defined by [RFC3225]. considerations regarding their interaction with DNS traffic is found
in [RFC5625]
Z 7. OPT Option Code Allocation Procedure
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6.11. OPT Options Code Allocation Procedure Allocations are assigned by expert review.
Allocations assigned by expert review. Assignment of Option Codes Assignment of Option Codes should be liberal, but duplicate
should be liberal, but duplicate functionality is to be avoided. functionality is to be avoided.
7. Transport Considerations 8. Transport Considerations
The presence of an OPT pseudo-RR in a request should be taken as an The presence of an OPT pseudo-RR in a request should be taken as an
indication that the requestor fully implements the given version of indication that the requestor fully implements the given version of
EDNS, and can correctly understand any response that conforms to that EDNS, and can correctly understand any response that conforms to that
feature's specification. feature's specification.
Lack of presence of an OPT record in a request MUST be taken as an Lack of presence of an OPT record in a request MUST be taken as an
indication that the requestor does not implement any part of this indication that the requestor does not implement any part of this
specification and that the responder MUST NOT include an OPT record specification and that the responder MUST NOT include an OPT record
in its response. in its response.
Responders who do not implement these protocol extensions MUST Responders which choose not to implement the protocol extensions
respond with FORMERR messages without any OPT record. defined in this document MUST respond with a return code (RCODE) of
FORMERR to messages containing an OPT RR in the additional section
and MUST NOT include an OPT record in the response.
If there is a problem with processing the OPT record itself, such as If there is a problem with processing the OPT record itself, such as
an option value that is badly formatted or includes out of range an option value that is badly formatted or includes out of range
values, a FORMERR MUST be returned. If this occurs the response MUST values, a FORMERR MUST be returned. If this occurs the response MUST
include an OPT record. This is intended to allow the requestor to to include an OPT record. This is intended to allow the requestor to
distinguish between servers which do not implement EDNS and format distinguish between servers which do not implement EDNS and format
errors within EDNS. errors within EDNS.
The minimal response must be the DNS header, question section, and an The minimal response MUST be the DNS header, question section, and an
OPT record. This must also occur when an truncated response (using OPT record. This MUST also occur when an truncated response (using
the DNS header's TC bit) is returned. the DNS header's TC bit) is returned.
8. Security Considerations 9. Security Considerations
Requestor-side specification of the maximum buffer size may open a Requestor-side specification of the maximum buffer size may open a
DNS denial of service attack if responders can be made to send DNS denial of service attack if responders can be made to send
messages which are too large for intermediate gateways to forward, messages which are too large for intermediate gateways to forward,
thus leading to potential ICMP storms between gateways and thus leading to potential ICMP storms between gateways and
responders. responders.
Announcing very large UDP buffer sizes may result in dropping by Announcing very large UDP buffer sizes may result in dropping by
middleboxes (see Section 6.8). This could cause retransmissions with middleboxes (see Section 6.2.6). This could cause retransmissions
no hope of success. Some devices have been found to reject with no hope of success. Some devices have been found to reject
fragmented UDP packets. fragmented UDP packets.
Announcing too small UDP buffer sizes may result in fallback to TCP Announcing too small UDP buffer sizes may result in fallback to TCP
with a corresponding load impact on DNS servers. This is especially with a corresponding load impact on DNS servers. This is especially
important with DNSSEC, where answers are much larger. important with DNSSEC, where answers are much larger.
9. IANA Considerations 10. IANA Considerations
The IANA has assigned RR type code 41 for OPT. The IANA has assigned RR type code 41 for OPT.
[RFC2671] specified a number of IANA sub-registries within "DOMAIN [RFC2671] specified a number of IANA sub-registries within "DOMAIN
NAME SYSTEM PARAMETERS:" NAME SYSTEM PARAMETERS:"
o EDNS Extended Label Type o DNS EDNS0 Options
o EDNS Option Codes o EDNS Version Number
o EDNS Version Numbers o EDNS Header Flags
o Domain System Response Code Additionally, several entries were generated in existing registries:
IANA is advised to re-parent these sub-registries to this document. EDNS Extended Label Type in the DNS Label Types Registry
Bad OPT Version in the DNS RCODES registry
IANA is advised to udpates references to [RFC2671] in these entries
and sub-registries to this document.
[RFC2671] created the "EDNS Extended Label Type Registry". We [RFC2671] created the "EDNS Extended Label Type Registry". We
request that this registry be closed. request that this registry be closed.
This document assigns option code 65535 in the "EDNS Option Codes" This document assigns option code 65535 in the "EDNS Option Codes"
registry to "Reserved for future expansion." registry to "Reserved for future expansion."
[RFC2671] expands the RCODE space from 4 bits to 12 bits. This [RFC2671] expands the RCODE space from 4 bits to 12 bits. This
allows more than the 16 distinct RCODE values allowed in [RFC1035]. allows more than the 16 distinct RCODE values allowed in [RFC1035].
IETF Standards Action is required to add a new RCODE. Adding new IETF Standards Action is required to add a new RCODE. Adding new
RCODEs should be avoided due to the difficulty in upgrading the RCODEs should be avoided due to the difficulty in upgrading the
installed base. installed base.
This document assigns EDNS Extended RCODE 16 to "BADVERS". This document assigns EDNS Extended RCODE 16 to "BADVERS" in the DNS
RCODES registry.
[RFC2671] called for the recording of assignment of extended label
types 0bxx111111 as "Reserved for future extended label types". This
request was implicitly a request to open a new registry for Extended
Label Types but due to possible ambiguous text registrarions were
instead made within the general "DNS Label Types" registry which also
registers entries originally defined by [RFC1035].
This document requests IANA to close registration of further Extended
Label Types in the "DNS Label Types" Registry.
IETF Standards Action is required for assignments of new EDNS0 flags. IETF Standards Action is required for assignments of new EDNS0 flags.
Flags SHOULD be used only when necessary for DNS resolution to Flags SHOULD be used only when necessary for DNS resolution to
function. For many uses, a EDNS Option Code may be preferred. function. For many uses, a EDNS Option Code may be preferred.
IETF Standards Action is required to create new entries in the EDNS IETF Standards Action is required to create new entries in the EDNS
Version Number registry. Expert Review is required for allocation of Version Number registry. Expert Review is required for allocation of
an EDNS Option Code. an EDNS Option Code.
11. References
11.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
3225, December 2001.
[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.
11.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
Support for Internet Protocol version 6 (IPv6)", RFC 3364,
August 2002.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP
152, RFC 5625, August 2009.
Appendix A. Document Editing History Appendix A. Document Editing History
Following is a list of high-level changes made to the original Following is a list of high-level changes made to the original
RFC2671. RFC2671.
Appendix A.1. Changes since RFC2671 Appendix A.1. Changes since RFC2671
o Support for the OPT record is now mandatory. o Support for the OPT record is now mandatory.
o Extended label types obsoleted and the registry is closed. o Extended label types obsoleted and the registry is closed.
skipping to change at page 12, line 21 skipping to change at page 12, line 52
o Front material (IPR notice and such) was updated to current o Front material (IPR notice and such) was updated to current
requirements. requirements.
Appendix A.2. Changes since -02 Appendix A.2. Changes since -02
o Specified the method for allocation of constants. o Specified the method for allocation of constants.
o Cleaned up a lot of wording, along with quite a bit of document o Cleaned up a lot of wording, along with quite a bit of document
structure changes. structure changes.
10. References
10.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, December 2001.
[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.
10.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
Support for Internet Protocol version 6 (IPv6)", RFC 3364,
August 2002.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
Authors' Addresses Authors' Addresses
Joao Damas Joao Damas
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, California 94063 Redwood City, California 94063
US US
Phone: +1 650.423.1312 Phone: +1 650.423.1312
Email: joao@isc.org Email: joao@isc.org
Michael Graff Michael Graff
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, California 94063 Redwood City, California 94063
US US
Phone: +1 650.423.1304 Phone: +1 650.423.1304
Email: mgraff@isc.org Email: mgraff@isc.org
Paul Vixie Paul Vixie
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, California 94063 Redwood City, California 94063
US US
Phone: +1 650.423.1301 Phone: +1 650.423.1301
Email: vixie@isc.org Email: vixie@isc.org
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