draft-ietf-dnsext-rfc2671bis-edns0-04.txt   draft-ietf-dnsext-rfc2671bis-edns0-05.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 November 8, 2010 Intended status: Standards Track March 7, 2011
Expires: May 12, 2011 Expires: September 8, 2011
Extension Mechanisms for DNS (EDNS0) Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-04 draft-ietf-dnsext-rfc2671bis-edns0-05
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 10
years of deployment experience. years of deployment experience.
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 May 12, 2011. This Internet-Draft will expire on September 8, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 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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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3 3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. Affected Protocol Elements . . . . . . . . . . . . . . . . . . 4 4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 4
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4
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. OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . . . 5 6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 5
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 5 6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 5
6.2. OPT Record Format . . . . . . . . . . . . . . . . . . . . 5 6.2. OPT Record Wire Format . . . . . . . . . . . . . . . . . . 5
6.3. Caching behavior . . . . . . . . . . . . . . . . . . . . . 7 6.3. Cache behaviour . . . . . . . . . . . . . . . . . . . . . 7
6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.5. Requestor's Payload Size . . . . . . . . . . . . . . . . . 7 6.5. Requestor's Payload Size . . . . . . . . . . . . . . . . . 7
6.6. Responder's Payload Size . . . . . . . . . . . . . . . . . 7 6.6. Responder's Payload Size . . . . . . . . . . . . . . . . . 7
6.7. Payload Size Selection . . . . . . . . . . . . . . . . . . 8 6.7. Payload Size Selection . . . . . . . . . . . . . . . . . . 8
6.8. Middleware Boxes . . . . . . . . . . . . . . . . . . . . . 8 6.8. Middleware Boxes . . . . . . . . . . . . . . . . . . . . . 8
6.9. OPT Record TTL Field Use . . . . . . . . . . . . . . . . . 8 6.9. OPT Record TTL Field Use . . . . . . . . . . . . . . . . . 8
6.10. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.10. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.11. OPT Options Code Allocation Procedure . . . . . . . . . . 9 6.11. OPT Options Code Allocation Procedure . . . . . . . . . . 9
7. Transport Considerations . . . . . . . . . . . . . . . . . . . 9 7. Transport Considerations . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Document Editing History . . . . . . . . . . . . . . 11 Appendix A. Document Editing History . . . . . . . . . . . . . . 11
Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . . 11 Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . . 11
Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . . 11 Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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 fixed. compression. The maximum allowable size of a DNS Message is limited
Many of DNS's protocol limits are too small for uses which are or to 512 bytes. Many of DNS's protocol limits are too small for uses
which are desired to become common. There is no way for which are commom or desired to become common. RFC 1035 does not
implementations to advertise their capabilities. define any way for implementations to advertise their capabilities.
[RFC2671] added extension mechanism to DNS, this mechanism is widely
supported and number of new DNS uses and protocol extensions depend
on the presence of these extensions. This memo refines that
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 detailed here. In practice, these clients will be extensions defined in [RFC2671] and restated here. Extended agents
upgraded when they have need of a new feature, and only new features must be prepared for handling the interactions with unextended
will make use of the extensions. Extended agents must be prepared clients in the face of new protocol elements, and fall back
for behavior of unextended clients in the face of new protocol gracefully to unextended DNS. [RFC2671] proposed extensions to the
elements, and fall back gracefully to unextended DNS. [RFC2671] basic DNS protocol to overcome these deficiencies. This memo refines
proposed extensions to the basic DNS protocol to overcome these that specification and obsoletes [RFC2671].
deficiencies. This memo refines that specification and obsoletes
[RFC2671].
[RFC2671] specified extended label types. The only one ever proposed [RFC2671] specified extended label types. The only one proposed was
was in RFC2673 for a label type called "Bitstring Labels." For in RFC2673 for a label type called "Bitstring Labels." For various
various reasons introducing a new label type was found to be reasons introducing a new label type was found to be extremely
extremely difficult, and RFC2673 was moved to Experimental. This difficult, and RFC2673 was moved to Experimental. This document
document Obsoletes Extended Labels. Obsoletes 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.
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 support is practically mandatory in a modern world. DNSSEC
requires EDNS support, and many other Features are made possible only requires EDNS support, and many other features are made possible only
by EDNS support to request or advertise them. Many organizations are by EDNS support to request or advertise them. Many organizations are
beginning to require DNSSEC. Without common interoperability, DNSSEC requiring DNSSEC. Without common interoperability, DNSSEC cannot be
cannot be as easily deployed. as easily deployed.
DNS publishers are wanting to put more data in answers. DNSSEC DNS publishers are wanting to put more data in answers. DNSSEC
DNSKEY records, negative answers, and many other DNSSEC queries cause DNSKEY records, negative answers, and many other DNSSEC queries cause
larger answers to be returned. In order to support this, DNS larger answers to be returned. In order to support this, DNS
servers, middleware, and stub resolvers MUST support larger packet servers, middleware, and stub resolvers MUST support larger packet
sizes advertised via EDNS0. sizes advertised via EDNS0.
4. Affected Protocol Elements 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.
skipping to change at page 4, line 31 skipping to change at page 4, line 35
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]). Today, many organizations wish to return many
records in a single reply, and special tricks are needed to make the records in a single reply, and special tricks are needed to make the
responses fit in this 512-byte limit. Additionally, DNSSEC responses fit in this 512-byte limit. Additionally, inclusion of
signatures can easily generate a much larger response than a 512 byte DNSSEC records frequently requires a much larger response than a 512
message can hold. byte message can hold.
EDNS0 is intended to address these larger packet sizes and continue EDNS0 is intended to address these larger packet sizes and continue
to use UDP. It specifies a way to advertise additional features such to use UDP. It specifies a way to advertise additional features such
as larger response size capability, which is intended to help avoid as larger response size capability, which is intended to help avoid
truncated UDP responses which then cause retry over TCP. 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.
This document does not describe any specific Extended Label Type. Extended Label Types are difficult to use due to support in clients
and intermediate gateways as described in [RFC3364] which moves them
In practice, Extended Label Types are difficult to use due to support to experimental status and [RFC3363], which describes the pros and
in clients and intermediate gateways. Therefore, the registry of cons.
Extended Label Types is requested to be closed. They cause
interoperability problems and at present no defined label types are
in use.
Bitstring labels were originally created to solve problems with IPv6 Therefore, this document moves them from experimental to historical,
reverse zones. Due to the problems of introducing a new label type making them obsoleted. Additionally, the registry of Extended Label
they were moved to experimental. This document moves them from Types is requested to be closed.
experimental to historical, making them obsoleted.
6. OPT pseudo-RR 6. The OPT pseudo-RR
6.1. OPT Record Definition 6.1. OPT Record Definition
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 been assigned 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 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 Only one OPT RR MAY be included within any DNS message. If a message
with more than one OPT RR is received, a FORMERR MUST be returned. with more than one OPT RR is received, a FORMERR (RCODE=1) MUST be
returned. The placement flexibility for the OPT RR does not override
the need for the TSIG or SIG(0) RRs to be the last in the additional
section whenever they are present.
6.2. OPT Record Format 6.2. OPT Record 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 | empty (root domain) | | NAME | domain name | Must be 0 (root domain) |
| TYPE | u_int16_t | OPT | | TYPE | u_int16_t | OPT (42) |
| 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 | describes 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 is encoded in its RDATA and is The variable part of an OPT RR may contain zero or more options in
structured as zero or more of the following: the RDATA. Each option must be treated as binary. Each option 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 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. Varies per OPTION-CODE. MUST be treated as binary.
The order of appearance of option tuples is not guaranteed. 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. Caching behavior 6.3. Cache behaviour
The OPT record must not be cached. The OPT record MUST NOT be cached.
6.4. Fallback 6.4. 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
EDNS(0) while others would force the use of TCP to fetch all data,
preference SHOULD be given to those support EDNS(0).
6.5. Requestor's Payload Size 6.5. Requestor's Payload Size
The requestor's UDP payload size (which OPT stores in the RR CLASS The requestor's UDP payload size (encoded in the RR CLASS field) is
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, may 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.
Requestors 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.
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. Choosing a very large value will
guarantee fragmentation at the IP layer, and may prevent answers from guarantee fragmentation at the IP layer, and may prevent answers from
being received due to a single fragment loss or misconfigured being received due to a single fragment loss or misconfigured
skipping to change at page 8, line 16 skipping to change at page 8, line 21
Due to transaction overhead, it is unwise to advertise an Due to transaction overhead, it is unwise to advertise an
architectural limit as a maximum UDP payload size. Just because your architectural limit as a maximum UDP payload size. Just because your
stack can reassemble 64KB datagrams, don't assume that you want to stack can reassemble 64KB datagrams, don't assume that you want to
spend more than about 4KB of state memory per ongoing transaction. spend more than about 4KB of state memory per ongoing transaction.
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 byte range SHOULD be tried, as it has a reasonable chance to fit 1280-1410 byte range SHOULD be tried, as it has a reasonable chance
within a single Ethernet frame. Failing that, a requestor MAY choose to fit within a single Ethernet frame. Failing that, a requestor MAY
a 512 byte packet, which with large answers may cause a TCP retry. choose a 512 byte packet, which with large answers may cause a TCP
retry.
6.8. Middleware Boxes 6.8. Middleware Boxes
Middleware boxes (e.g. firewalls, SOHO routers, load balancers, etc) Middleware boxes (e.g. firewalls, SOHO routers, load balancers, etc)
MUST NOT limit DNS messages over UDP to 512 bytes. MUST NOT limit DNS messages over UDP to 512 bytes.
Middleware boxes which simply forward requests to a recursive Middleware boxes which simply forward requests to a recursive
resolver MUST NOT modify and MUST NOT delete the OPT record contents resolver MUST NOT modify and MUST NOT delete the OPT record contents
in either direction. in either direction.
Middleware boxes which have additional functionality, such as Middleware boxes which have additional functionality, such as
answering certain queries or acting like an intelligent forwarder, answering certain queries or acting like an intelligent forwarder,
MUST understand the OPT record. These boxes MUST consider the MUST understand the OPT record. These boxes MUST consider the
incoming request and any outgoing requests as separate transactions incoming request and any outgoing requests as separate transactions
if the characteristics of the messages are different. 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 6.9. 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 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO| Z | 2: | DO| Z |
skipping to change at page 12, line 18 skipping to change at page 12, line 34
[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.
[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 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.
[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
 End of changes. 40 change blocks. 
69 lines changed or deleted 92 lines changed or added

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