draft-ietf-dnsext-rfc2671bis-edns0-08.txt   draft-ietf-dnsext-rfc2671bis-edns0-09.txt 
DNSEXT Working Group J. Damas DNSEXT Working Group J. Damas
Internet-Draft M. Graff Internet-Draft M. Graff
Obsoletes: 2671, 2673 P. Vixie Obsoletes: 2671, 2673 P. Vixie
(if approved) Internet Systems Consortium (if approved) Internet Systems Consortium
Intended status: Standards Track February 7, 2012 Intended status: Standards Track August 13, 2012
Expires: August 10, 2012 Expires: February 14, 2013
Extension Mechanisms for DNS (EDNS0) Extension Mechanisms for DNS (EDNS(0))
draft-ietf-dnsext-rfc2671bis-edns0-08 draft-ietf-dnsext-rfc2671bis-edns0-09
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 (RFC 2671) based on This document updates the EDNS(0) specification (and obsoletes RFC
feedback from deployment experience in several implementations. It 2671) based on feedback from deployment experience in several
also closes the IANA registry for extended labels created as part of implementations. It also closes the IANA registry for extended
RFC 2671 and obsoletes RFC 2673 ("Binary Labels in the Domain Name labels created as part of RFC 2671 and obsoletes RFC 2673 ("Binary
System") which depends on the existence of extended labels. Labels in the Domain Name System") which depends on the existence of
extended labels.
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 August 10, 2012. This Internet-Draft will expire on February 14, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 9 skipping to change at page 3, line 9
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 4 3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 5
4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 5 4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 5
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 5 4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 5
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 6 5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 6
6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 6 6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 6
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 6 6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 6
6.1.1. Basic elements . . . . . . . . . . . . . . . . . . . . 6 6.1.1. Basic elements . . . . . . . . . . . . . . . . . . . . 6
6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 7 6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 7
6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 8 6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 8
6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 9 6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.1. Cache behaviour . . . . . . . . . . . . . . . . . . . 9 6.2.1. Cache behaviour . . . . . . . . . . . . . . . . . . . 9
6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 9 6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 9 6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 10
6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 10 6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 10
6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 10 6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 11
6.2.6. Support in MiddleBoxes . . . . . . . . . . . . . . . . 11 6.2.6. Support in MiddleBoxes . . . . . . . . . . . . . . . . 11
7. OPT Option Code Allocation Procedure . . . . . . . . . . . . . 11 7. Transport Considerations . . . . . . . . . . . . . . . . . . . 12
8. Transport Considerations . . . . . . . . . . . . . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 9.1. OPT Option Code Allocation Procedure . . . . . . . . . . . 14
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
11.2. Informative References . . . . . . . . . . . . . . . . . . 14 10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Document Editing History . . . . . . . . . . . . . . 14 Appendix A. Document Editing History . . . . . . . . . . . . . . 15
A.1. Changes since RFC2671 . . . . . . . . . . . . . . . . . . 14 A.1. Changes since RFC2671 . . . . . . . . . . . . . . . . . . 15
A.2. Changes since -02 . . . . . . . . . . . . . . . . . . . . 15 A.2. Changes since -02 . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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 over UDP compression. The maximum allowable size of a DNS Message over UDP
not using the extensions described in this document is limited to 512 not using the extensions described in this document is limited to 512
bytes. Many of DNS's protocol limits, such as the maximum message bytes. Many of DNS's protocol limits, such as the maximum message
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any way for implementations to advertise their capabilities to any of any way for implementations to advertise their capabilities to any of
the other actors they interact with. the other actors they interact with.
[RFC2671] added an extension mechanism to DNS. This mechanism is [RFC2671] added an extension mechanism to DNS. This mechanism is
widely supported and a number of new DNS uses and protocol extensions widely supported and a number of new DNS uses and protocol extensions
depend 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 need to 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. gracefully to unextended DNS.
[RFC2671] specified extended label types. The only one proposed was EDNS is a hop-by-hop extension to DNS. This means the use of EDNS is
in [RFC2673] for a label type called "Bitstring Labels." For various negotiated between each pair of hosts in a DNS resolution process.
reasons introducing a new label type was found to be extremely For instance the stub resolver communicating with the recursive
difficult, and [RFC2673] was moved to Experimental. This document resolver or the recursive resolver communicating with an
deprecates Extended Labels, and therefore Binary Labels, obsoleting authoritative server.
[RFC2673].
[RFC2671] specified extended label types. The only such label
proposed was in [RFC2673] for a label type called "Bitstring Labels."
For various reasons introducing a new label type was found to be
extremely difficult, and [RFC2673] was moved to Experimental. This
document deprecates Extended Labels, and therefore Binary Labels,
obsoleting [RFC2673].
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. Other terminology is used as per its use in responds to questions. Other terminology is used as per its use in
the references (e.g. middleboxes as in [RFC5625]) the references.
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 provides a mechanism to improve the scalability of DNS as its EDNS provides a mechanism to improve the scalability of DNS as its
uses get more diverse on the Internet. It does this by enabling the uses get more diverse on the Internet. It does this by enabling the
use of UDP transport for DNS messages with sizes beyond the limits use of UDP transport for DNS messages with sizes beyond the limits
specified in RFC 1035 as well as providing extra data space for specified in RFC 1035 as well as providing extra data space for
additional flags and return codes (RCODEs). additional flags and return codes (RCODEs). However, implementation
experiencing indicates that adding new RCODEs should be avoided due
to the difficulty in upgrading the installed base. Flags SHOULD be
used only when necessary for DNS resolution to function.
For many uses, a EDNS Option Code may be preferred.
With time, some applications of DNS have made EDNS a requirement for With time, some applications of DNS have made EDNS a requirement for
their deployment. For instance, DNSSEC uses the additional flag their deployment. For instance, DNSSEC uses the additional flag
space introduced in EDNS to signal the request to include DNSSEC data space introduced in EDNS to signal the request to include DNSSEC data
in a DNS response. in a DNS response.
Given the increase in DNS response sizes when including larger data Given the increase in DNS response sizes when including larger data
items such as AAAA Records, DNSSEC information (e.g. RRSIG or items such as AAAA Records, DNSSEC information (e.g. RRSIG or
DNSKEY) or large TXT Records, the additional UDP payload capabilities DNSKEY) or large TXT Records, the additional UDP payload capabilities
provided by EDNS can help improve the scalability of the DNS by provided by EDNS can help improve the scalability of the DNS by
avoiding generalized use of TCP for DNS transport. avoiding widespread 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
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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]. Fitting the increasing amounts of data that can over UDP [RFC1035]. Fitting the increasing amounts of data that can
be transported in DNS in this 512-byte limit is becoming more be transported in DNS in this 512-byte limit is becoming more
difficult. For instance, inclusion of DNSSEC records frequently difficult. For instance, inclusion of DNSSEC records frequently
requires a much larger response than a 512 byte message can hold. requires a much larger response than a 512 byte message can hold.
EDNS0 is intended to provide support for transporting these larger EDNS(0) is intended to provide support for transporting these larger
packet sizes while continuing to use UDP. It specifies a way to packet sizes while continuing to use UDP. It specifies a way to
advertise additional features such as larger response size advertise additional features such as larger response size
capability, which is intended to help avoid truncated UDP responses capability, which is intended to help avoid truncated UDP responses
which then cause retry over TCP. 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 was 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 were 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 lack of support in
and intermediate gateways as described in [RFC3364] which moves them clients and intermediate gateways as described in [RFC3363] which
to experimental status and [RFC3363], which describes the pros and moved [RFC2673] to experimental status, and [RFC3364], which
cons. describes the pros and cons.
Therefore, this document moves them from experimental to historical, Therefore, this document obsoletes [RFC2673] and deprecates the use
making them deprecated. of Extended Label Types.
Implementations MUST NOT generate or pass Extended Labels in their Implementations MUST NOT generate or pass Extended Labels in their
communications. Additionally, no further registrations of Extended communications. Additionally, IANA has been requested to close
Label Types are permitted. registration of further Extended Label Types in the "DNS Label Types"
Registry so that no further registrations will be 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 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.
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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.
No more than one OPT RR MUST be included within any DNS message. If When an OPT RR MUST is included within any DNS message only ONE OPT
a query message with more than one OPT RR is received, a FORMERR RR can be present. If a query message with more than one OPT RR is
(RCODE=1) MUST be returned. The placement flexibility for the OPT RR received, a FORMERR (RCODE=1) MUST be returned. The placement
does not override the need for the TSIG or SIG(0) RRs to be the last flexibility for the OPT RR does not override the need for the TSIG or
in the additional section whenever they are present. SIG(0) RRs to be the last in the additional section whenever they are
present.
6.1.2. 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 (41) | | 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 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 a bit field. Each option
encoded as: is 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 the Expert Review process as defined by the dnsext
working group and the IESG.
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 a bit field.
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 and supports
it MUST include option XYZ in its response. option XYZ, it MUST include option XYZ in its response.
6.1.3. OPT Record TTL Field Use 6.1.3. OPT Record TTL Field Use
The extended RCODE and flags (which OPT stores in the RR TTL field) The extended RCODE and flags (which OPT stores in the RR TTL field)
are structured as follows: are structured as follows:
+0 (MSB) +1 (LSB) +0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION | 0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
skipping to change at page 9, line 26 skipping to change at page 9, line 46
modified in a subsequent specification. modified in a subsequent specification.
6.2. Behaviour 6.2. Behaviour
6.2.1. Cache behaviour 6.2.1. Cache behaviour
The OPT record MUST NOT be cached. The OPT record MUST NOT be cached.
6.2.2. 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 EDNS(0),
MAY issue queries without an OPT record. It MAY cache this knowledge it MAY issue queries without an OPT record. It MAY cache this
for a brief time in order to avoid fallback delays in the future. knowledge for a brief time in order to avoid fallback delays in the
However, if DNSSEC or any future option using EDNS is required, no future. However, if DNSSEC or any future option using EDNS is
fallback should be performed as they are only signaled through EDNS0. required, no fallback should be performed as they are only signalled
If an implementation detects that some servers for the zone support through EDNS. If an implementation detects that some servers for the
EDNS(0) while others would force the use of TCP to fetch all data, zone support EDNS(0) while others would force the use of TCP to fetch
preference SHOULD be given to those support EDNS(0). all data, preference MAY be given to those which support EDNS(0).
Implementers SHOULD analyse this choice and the impact on both
endpoints.
6.2.3. 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. this.
Values lower than 512 MUST be treated as equal to 512. Values lower than 512 MUST be treated as equal to 512.
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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. This responses from being received, and can cause fallback to occur. This
knowledge may be auto-detected by the implementation or provided by a knowledge may be auto-detected by the implementation or provided by a
human administrator. 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. Ethernet would be reasonable.
Bigger values SHOULD be considered where fragmentation is not a Bigger values SHOULD be considered by implementers to be used where
concern. fragmentation is not a concern. Implementations SHOULD use their
largest configured or implemented values as a starting point in an
EDNS transaction in the absence of previous knowledge about the
destination server.
Choosing a very large value will guarantee fragmentation at the IP Choosing a very large value will guarantee fragmentation at the IP
layer, and may prevent answers from being received due to a single layer, and may prevent answers from being received due to a single
fragment loss or misconfigured firewalls. 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.
6.2.4. Responder's Payload Size 6.2.4. Responder's Payload Size
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requests, if there is any reason to suspect that the responder requests, if there is any reason to suspect that the responder
implements EDNS, and if a request will not fit in the default 512 implements EDNS, and if a request will not fit in the default 512
payload size limit. payload size limit.
6.2.5. Payload Size Selection 6.2.5. Payload Size Selection
Due to transaction overhead, it is not recommended to advertise an Due to transaction overhead, it is not recommended to advertise an
architectural limit as a maximum UDP payload size. Even on system architectural limit as a maximum UDP payload size. Even on system
stacks capable of reassembling 64KB datagrams, memory usage at low stacks capable of reassembling 64KB datagrams, memory usage at low
levels in the system will be a concern. A good compromise may be the 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 use of a EDNS maximum payload size of 4096 octets as a starting
maximum payload size of 4096 octets. point.
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.
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proxies, etc) referred to here as MiddleBoxes. proxies, etc) referred to here as MiddleBoxes.
Conformant MiddleBoxes MUST NOT limit DNS messages over UDP to 512 Conformant MiddleBoxes MUST NOT limit DNS messages over UDP to 512
bytes. bytes.
MiddleBoxes which simply forward requests to a recursive resolver MiddleBoxes which simply forward requests to a recursive resolver
MUST NOT modify and MUST NOT delete the OPT record contents in either MUST NOT modify and MUST NOT delete the OPT record contents in either
direction. direction.
MiddleBoxes which have additional functionality, such as answering MiddleBoxes which have additional functionality, such as answering
queries or acting as intelligent forwarders, SHOULD understand the queries or acting as intelligent forwarders, SHOULD be able to
OPT record. These boxes MUST consider the incoming request and any process the OPT record and act based on its contents. These boxes
outgoing requests as separate transactions if the characteristics of MUST consider the incoming request and any outgoing requests as
the messages are different. separate transactions if the characteristics of the messages are
different.
A more in depth discussion of this type of equipment and other A more in depth discussion of this type of equipment and other
considerations regarding their interaction with DNS traffic is found considerations regarding their interaction with DNS traffic is found
in [RFC5625] in [RFC5625]
7. OPT Option Code Allocation Procedure 7. Transport Considerations
Allocations are assigned by expert review.
Assignment of Option Codes should be liberal, but duplicate
functionality is to be avoided.
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.
Extended agents MUST be prepared for handling the interactions with
unextended clients in the face of new protocol elements, and fall
back gracefully to unextended DNS when needed.
Responders which choose not to implement the protocol extensions Responders which choose not to implement the protocol extensions
defined in this document MUST respond with a return code (RCODE) of defined in this document MUST respond with a return code (RCODE) of
FORMERR to messages containing an OPT RR in the additional section FORMERR to messages containing an OPT RR in the additional section
and MUST NOT include an OPT record in the response. 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 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.
9. Security Considerations 8. 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 6.2.6). This could cause retransmissions middleboxes (see Section 6.2.6). This could cause retransmissions
with 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.
10. IANA Considerations 9. 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 DNS EDNS0 Options o DNS EDNS(0) Options
o EDNS Version Number o EDNS Version Number
o EDNS Header Flags o EDNS Header Flags
Additionally, several entries were generated in existing registries: Additionally, two entries were generated in existing registries:
EDNS Extended Label Type in the DNS Label Types Registry EDNS Extended Label Type in the DNS Label Types Registry
Bad OPT Version in the DNS RCODES registry Bad OPT Version in the DNS RCODES registry
IANA is advised to udpates references to [RFC2671] in these entries IANA is advised to update references to [RFC2671] in these entries
and sub-registries to this document. 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.
RCODEs should be avoided due to the difficulty in upgrading the
installed base.
This document assigns EDNS Extended RCODE 16 to "BADVERS" in the DNS This document assigns EDNS Extended RCODE 16 to "BADVERS" in the DNS
RCODES registry. RCODES registry.
[RFC2671] called for the recording of assignment of extended label [RFC2671] called for the recording of assignment of extended label
types 0bxx111111 as "Reserved for future extended label types". This types 0bxx111111 as "Reserved for future extended label types". This
request was implicitly a request to open a new registry for Extended request implied a request to open a new registry for Extended Label
Label Types but due to possible ambiguous text registrations were Types but due to the possibility of ambiguity new text registrations
instead made within the general "DNS Label Types" registry which also were instead made within the general "DNS Label Types" registry which
registers entries originally defined by [RFC1035]. also registers entries originally defined by [RFC1035].
This document requests IANA to close registration of further Extended This document requests IANA to close registration of further Extended
Label Types in the "DNS Label Types" Registry. 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 EDNS(0)
Flags SHOULD be used only when necessary for DNS resolution to flags. Flags SHOULD be used only when necessary for DNS resolution
function. For many uses, a EDNS Option Code may be preferred. to 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. Within the EDNS Option Code space Expert
an EDNS Option Code. Review is required for allocation of an EDNS Option Code. IANA is
requested to keep a registry for the EDNS Option Code space.
11. References 9.1. OPT Option Code Allocation Procedure
11.1. Normative References OPT Option Codes are assigned by expert review.
Assignment of Option Codes should be liberal, but duplicate
functionality is to be avoided.
10. References
10.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. RFC 2671, August 1999.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, December 2001. RFC 3225, December 2001.
11.2. Informative References 10.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System", [RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999. RFC 2673, August 1999.
[RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T. [RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
Hain, "Representing Internet Protocol version 6 (IPv6) Hain, "Representing Internet Protocol version 6 (IPv6)
Addresses in the Domain Name System (DNS)", RFC 3363, Addresses in the Domain Name System (DNS)", RFC 3363,
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