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DNSEXT Working Group M. Graff
Internet-Draft P. Vixie
Obsoletes: 2671 (if approved) Internet Systems Consortium
Intended status: Standards Track July 28, 2009
Expires: January 29, 2010
Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-02
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 29, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
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
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allow requestors to advertise their capabilities to responders. This
document describes backward compatible mechanisms for allowing the
protocol to grow.
This document updates the EDNS0 specification based on 10 years of
operational experience.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. Affected Protocol Elements . . . . . . . . . . . . . . . . . . 3
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4
6. OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. OPT Record Behavior . . . . . . . . . . . . . . . . . . . 4
6.2. OPT Record Format . . . . . . . . . . . . . . . . . . . . 5
6.3. Requestor's Payload Size . . . . . . . . . . . . . . . . . 6
6.4. Responder's Payload Size . . . . . . . . . . . . . . . . . 6
6.5. Payload Size Selection . . . . . . . . . . . . . . . . . . 7
6.6. Middleware Boxes . . . . . . . . . . . . . . . . . . . . . 7
6.7. Extended RCODE . . . . . . . . . . . . . . . . . . . . . . 7
6.8. OPT Options Type Allocation Procedure . . . . . . . . . . 8
7. Transport Considerations . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
DNS [RFC1035] specifies a Message Format and within such messages
there are standard formats for encoding options, errors, and name
compression. The maximum allowable size of a DNS Message is fixed.
Many of DNS's protocol limits are too small for uses which are or
which are desired to become common. There is no way for
implementations to advertise their capabilities.
Unextended agents will not know how to interpret the protocol
extensions detailed here. In practice, these clients will be
upgraded when they have need of a new feature, and only new features
will make use of the extensions. Extended agents must be prepared
for behaviour of unextended clients in the face of new protocol
elements, and fall back gracefully to unextended DNS. [RFC2671]
originally proposed extensions to the basic DNS protocol to overcome
these deficiencies. This memo refines that specification and
obsoletes [RFC2671].
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. EDNS Support Requirement
EDNS support is manditory in a modern world. DNSSEC requires EDNS
support, and many other featres are made possible only by EDNS
support to request or advertise them.
4. Affected Protocol Elements
4.1. Message Header
The DNS Message Header's (see , section 4.1.1 [RFC1035]) second full
16-bit word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a
number of 1-bit flags. The original reserved Z bits have been
allocated to various purposes, and most of the RCODE values are now
in use. More flags and more possible RCODEs are needed. The OPT
pseudo-RR specified below contains subfields that carry a bit field
extension of the RCODE field and additional flag bits, respectively.
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4.2. Label Types
The first two bits of a wire format domain label are used to denote
the type of the label. ,section 4.1.4 [RFC1035] allocates two of the
four possible types and reserves the other two. More label types
were proposed in [RFC2671] section 3.
4.3. UDP Message Size
DNS Messages are limited to 512 octets in size when sent over UDP.
While the minimum maximum reassembly buffer size still allows a limit
of 512 octets of UDP payload, most of the hosts now connected to the
Internet are able to reassemble larger datagrams. Some mechanism
must be created to allow requestors to advertise larger buffer sizes
to responders. To this end, the OPT pseudo-RR specified below
contains a maximum payload size field.
5. Extended Label Types
The first octet in the on-the-wire representation of a DNS label
specifies the label type; the basic DNS specification [RFC1035]
dedicates the two most significant bits of that octet for this
purpose.
This document reserves DNS label type 0b01 for use as an indication
for Extended Label Types. A specific extended label type is selected
by the 6 least significant bits of the first octet. Thus, Extended
Label Types are indicated by the values 64-127 (0b01xxxxxx) in the
first octet of the label.
This document does not describe any specific Extended Label Type.
In practice, Extended Label Types are difficult to use due to support
in clients and intermediate gateways. Therefore, the registry of
Extended Label Types is requested to be closed. They cause
interoperability problems and at present no defined label types are
in use.
6. OPT pseudo-RR
6.1. OPT Record Behavior
One OPT pseudo-RR (RR type 41) MAY be added to the additional data
section of a request. If present in requests, compliant responders
which implement EDNS MUST include an OPT record in non-truncated
responses, and SHOULD attempt to include them in all responses. An
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OPT is called a pseudo-RR because it pertains to a particular
transport level message and not to any actual DNS data. OPT RRs MUST
NOT be cached, forwarded, or stored in or loaded from master files.
The quantity of OPT pseudo-RRs per message MUST be either zero or
one, but not greater.
6.2. OPT Record Format
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
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
encode them as {attribute, value} pairs.
The fixed part of an OPT RR is structured as follows:
+------------+--------------+------------------------------+
| Field Name | Field Type | Description |
+------------+--------------+------------------------------+
| NAME | domain name | empty (root domain) |
| TYPE | u_int16_t | OPT |
| CLASS | u_int16_t | requestor's UDP payload size |
| TTL | u_int32_t | extended RCODE and flags |
| RDLEN | u_int16_t | describes RDATA |
| RDATA | octet stream | {attribute,value} pairs |
+------------+--------------+------------------------------+
OPT RR Format
The variable part of an OPT RR is encoded in its RDATA and is
structured as zero or more of the following:
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | |
/ OPTION-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
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OPTION-CODE
Assigned by Expert Review.
OPTION-LENGTH
Size (in octets) of OPTION-DATA.
OPTION-DATA
Varies per OPTION-CODE.
Order of appearance of option tuples is never relevant. Any option
whose meaning is affected by other options is so affected no matter
which one comes first in the OPT RDATA.
Any OPTION-CODE values not understood by a responder or requestor
MUST be ignored. Specifications of such options might wish to
include some kind of signalled acknowledgement. For example, an
option specification might say that if a responder sees option XYZ,
it SHOULD include option XYZ in its response.
6.3. Requestor's Payload Size
The requestor's UDP payload size (which OPT stores in the RR CLASS
field) is the number of octets of the largest UDP payload that can be
reassembled and delivered in the requestor's network stack. Note
that path MTU, with or without fragmentation, may be smaller than
this. Values lower than 512 MUST be treated as equal to 512.
Note that a 512-octet UDP payload requires a 576-octet IP reassembly
buffer. Choosing 1280 for IPv4 over Ethernet would be reasonable.
The consequence of choosing too large a value may be an ICMP message
from an intermediate gateway, or even a silent drop of the response
message.
The requestor's maximum payload size can change over time, and MUST
therefore not be cached for use beyond the transaction in which it is
advertised.
6.4. Responder's Payload Size
The responder's maximum payload size can change over time, but can be
reasonably expected to remain constant between two sequential
transactions; for example, a meaningless QUERY to discover a
responder's maximum UDP payload size, followed immediately by an
UPDATE which takes advantage of this size. (This is considered
preferrable to the outright use of TCP for oversized requests, if
there is any reason to suspect that the responder implements EDNS,
and if a request will not fit in the default 512 payload size limit.)
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6.5. Payload Size Selection
Due to transaction overhead, it is unwise to advertise an
architectural limit as a maximum UDP payload size. Just because your
stack can reassemble 64KB datagrams, don't assume that you want to
spend more than about 4KB of state memory per ongoing transaction.
A requestor MAY choose to implement a fallback to smaller advertised
sizes to work around firewall or other network limitations. A
requestor SHOULD choose to use a fallback mechanism which begins with
a large size, such as 4096. If that fails, a fallback around the
1220 byte range SHOULD be tried, as it has a reasonable chance 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 retry.
6.6. Middleware Boxes
Middleware boxes MUST NOT limit DNS messages over UDP to 512 bytes.
Middleware boxes which simply forward requests to a recursive
resolver MUST NOT modify the OPT record contents in either direction.
6.7. Extended RCODE
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. Note that
EXTENDED-RCODE value "0" indicates that an unextended RCODE is
in use (values "0" through "15").
VERSION
Indicates the implementation level of whoever sets it. 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.
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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.
DO
DNSSEC OK bit as defined by [RFC3225].
Z
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6.8. OPT Options Type Allocation Procedure
Allocations assigned by expert review. TBD
7. Transport Considerations
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
EDNS, and can correctly understand any response that conforms to that
feature's specification.
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
specification and that the responder MUST NOT use any protocol
extension described here in its response.
Responders who do not implement these protocol extensions MUST
respond with FORMERR messages without any OPT record.
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
values, a FORMERR MAY be retured. If this occurs the response MUST
include an OPT record. This MAY be used to distinguish between
servers whcih do not implement EDNS and format errors within EDNS.
If EDNS is used in a request, and the response arrives with TC set
and with no EDNS OPT RR, a requestor SHOULD assume that truncation
prevented the OPT RR from being appended by the responder, and
further, that EDNS is not used in the response. Correspondingly, an
EDNS responder who cannot fit all necessary elements (including an
OPT RR) into a response, SHOULD respond with a normal (unextended)
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DNS response, possibly setting TC if the response will not fit in the
unextended response message's 512-octet size.
8. Security Considerations
Requestor-side specification of the maximum buffer size may open a
new DNS denial of service attack if responders can be made to send
messages which are too large for intermediate gateways to forward,
thus leading to potential ICMP storms between gateways and
responders.
Announcing very large UDP buffer sizes may result in dropping by
firewalls. This could cause retransmissions with no hope of success.
Some devices reject fragmented UDP packets.
Announcing too small UDP buffer sizes may result in fallback to TCP.
This is especially important with DNSSEC, where answers are much
larger.
9. IANA Considerations
The IANA has assigned RR type code 41 for OPT.
[RFC2671] specified a number of IANA sub-registries within "DOMAIN
NAME SYSTEM PARAMETERS:" "EDNS Extended Label Type", "EDNS Option
Codes", "EDNS Version Numbers", and "Domain System Response Code."
IANA is advised to re-parent these subregistries to this document.
RFC 2671 created an extended label type registry. We request that
this registry be closed.
This document assigns extended label type 0bxx111111 as "Reserved for
future extended label types." We request that IANA record this
assignment.
This document assigns option code 65535 to "Reserved for future
expansion."
This document expands the RCODE space from 4 bits to 12 bits. This
will allow IANA to assign more than the 16 distinct RCODE values
allowed in RFC 1035 [RFC1035].
This document assigns EDNS Extended RCODE "16" to "BADVERS".
IESG approval should be required to create new entries in the EDNS
Extended Label Type or EDNS Version Number registries, while any
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published RFC (including Informational, Experimental, or BCP) should
be grounds for allocation of an EDNS Option Code.
10. Acknowledgements
Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley,
Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and Thomas
Narten were each instrumental in creating and refining this
specification.
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.
11.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Michael Graff
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1304
Email: mgraff@isc.org
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Paul Vixie
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1301
Email: vixie@isc.org
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DNSEXT Working Group J. Damas
Internet-Draft M. Graff
Obsoletes: 2671, 2673 P. Vixie
(if approved) Internet Systems Consortium
Intended status: Standards Track March 7, 2011
Expires: September 8, 2011
Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-05
Abstract
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
allow requestors to advertise their capabilities to responders. This
document describes backward compatible mechanisms for allowing the
protocol to grow.
This document updates the EDNS0 specification [RFC2671] based on 10
years of deployment experience.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 8, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Damas, et al. Expires September 8, 2011 [Page 1]
Internet-Draft EDNS0 Extensions March 2011
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 4
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4
6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 5
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 5
6.2. OPT Record Wire Format . . . . . . . . . . . . . . . . . . 5
6.3. Cache behaviour . . . . . . . . . . . . . . . . . . . . . 7
6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.5. Requestor's Payload Size . . . . . . . . . . . . . . . . . 7
6.6. Responder's Payload Size . . . . . . . . . . . . . . . . . 7
6.7. Payload Size Selection . . . . . . . . . . . . . . . . . . 8
6.8. Middleware Boxes . . . . . . . . . . . . . . . . . . . . . 8
6.9. OPT Record TTL Field Use . . . . . . . . . . . . . . . . . 8
6.10. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.11. OPT Options Code Allocation Procedure . . . . . . . . . . 9
7. Transport Considerations . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Document Editing History . . . . . . . . . . . . . . 11
Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . . 11
Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
DNS [RFC1035] specifies a Message Format and within such messages
there are standard formats for encoding options, errors, and name
compression. The maximum allowable size of a DNS Message is limited
to 512 bytes. Many of DNS's protocol limits are too small for uses
which are commom or desired to become common. RFC 1035 does not
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
extensions defined in [RFC2671] and restated here. 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. [RFC2671] proposed extensions to the
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
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
Obsoletes Extended Labels.
2. Terminology
"Requestor" is the side which sends a request. "Responder" is an
authoritative, recursive resolver, or other DNS component which
responds to questions.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. EDNS Support Requirement
EDNS support is practically mandatory in a modern world. DNSSEC
requires EDNS support, and many other features are made possible only
by EDNS support to request or advertise them. Many organizations are
requiring DNSSEC. Without common interoperability, DNSSEC cannot be
as easily deployed.
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DNS publishers are wanting to put more data in answers. DNSSEC
DNSKEY records, negative answers, and many other DNSSEC queries cause
larger answers to be returned. In order to support this, DNS
servers, middleware, and stub resolvers MUST support larger packet
sizes advertised via EDNS0.
4. DNS Message changes
4.1. Message Header
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 ,
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
values are now in use. The OPT pseudo-RR specified below contains
extensions to the RCODE bit field as well as additional flag bits.
4.2. Label Types
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
types and reserves the other two. More label types were defined in
[RFC2671]. This document obsoletes the use of the 2-bit combination
defined by [RFC2671] to identify extended label types.
4.3. UDP Message Size
Traditional DNS Messages are limited to 512 octets in size when sent
over UDP ([RFC1035]). Today, many organizations wish to return many
records in a single reply, and special tricks are needed to make the
responses fit in this 512-byte limit. Additionally, inclusion of
DNSSEC records frequently requires a much larger response than a 512
byte message can hold.
EDNS0 is intended to address these larger packet sizes and continue
to use UDP. It specifies a way to advertise additional features such
as larger response size capability, which is intended to help avoid
truncated UDP responses which then cause retry over TCP.
5. Extended Label Types
The first octet in the on-the-wire representation of a DNS label
specifies the label type; the basic DNS specification [RFC1035]
dedicates the two most significant bits of that octet for this
purpose.
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[RFC2671] defined DNS label type 0b01 for use as an indication for
Extended Label Types. A specific Extended Label Type is selected by
the 6 least significant bits of the first octet. Thus, Extended
Label Types are indicated by the values 64-127 (0b01xxxxxx) in the
first octet of the label.
Extended Label Types are difficult to use due to support in clients
and intermediate gateways as described in [RFC3364] which moves them
to experimental status and [RFC3363], which describes the pros and
cons.
Therefore, this document moves them from experimental to historical,
making them obsoleted. Additionally, the registry of Extended Label
Types is requested to be closed.
6. The OPT pseudo-RR
6.1. OPT Record Definition
An OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the
additional data section of a request.
The OPT RR has RR type 41.
If present in requests, compliant responders MUST include an OPT
record in their respective responses.
An OPT record does not carry any DNS data. It is used only to
contain control information pertaining to the question and answer
sequence of a specific transaction. OPT RRs MUST NOT be cached,
forwarded, or stored in or loaded from master files.
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
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 Wire Format
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
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
encode them as {attribute, value} pairs.
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The fixed part of an OPT RR is structured as follows:
+------------+--------------+------------------------------+
| Field Name | Field Type | Description |
+------------+--------------+------------------------------+
| NAME | domain name | Must be 0 (root domain) |
| TYPE | u_int16_t | OPT (42) |
| CLASS | u_int16_t | requestor's UDP payload size |
| TTL | u_int32_t | extended RCODE and flags |
| RDLEN | u_int16_t | length of all RDATA |
| RDATA | octet stream | {attribute,value} pairs |
+------------+--------------+------------------------------+
OPT RR Format
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
encoded as:
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | |
/ OPTION-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE
Assigned by Expert Review.
OPTION-LENGTH
Size (in octets) of OPTION-DATA.
OPTION-DATA
Varies per OPTION-CODE. MUST be treated as binary.
The order of appearance of option tuples is not defined. If one
option modifies the behavior of another or multiple options are
related to one another in some way, they have the same effect
regardless of ordering in the RDATA wire encoding.
Any OPTION-CODE values not understood by a responder or requestor
MUST be ignored. Specifications of such options might wish to
include some kind of signaled acknowledgement. For example, an
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option specification might say that if a responder sees option XYZ,
it MUST include option XYZ in its response.
6.3. Cache behaviour
The OPT record MUST NOT be cached.
6.4. Fallback
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
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
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
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
reassembled and delivered in the requestor's network stack. Note
that path MTU, with or without fragmentation, could be smaller than
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
receive. For example, if a requestor sits behind a firewall which
will block fragmented IP packets, a requestor SHOULD not choose a
value which will cause fragmentation. Doing so will prevent large
responses from being received, and can cause fallback to occur.
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
Ethernet would be reasonable. 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
not be cached for use beyond the transaction in which it is
advertised.
6.6. Responder's Payload Size
The responder's maximum payload size can change over time, but can be
reasonably expected to remain constant between two closely spaced
sequential transactions; for example, a meaningless QUERY to discover
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a responder's maximum UDP payload size, followed immediately by an
UPDATE which takes advantage of this size. This is considered
preferable to the outright use of TCP for oversized requests, if
there is any reason to suspect that the responder implements EDNS,
and if a request will not fit in the default 512 payload size limit.
6.7. Payload Size Selection
Due to transaction overhead, it is unwise to advertise an
architectural limit as a maximum UDP payload size. Just because your
stack can reassemble 64KB datagrams, don't assume that you want to
spend more than about 4KB of state memory per ongoing transaction.
A requestor MAY choose to implement a fallback to smaller advertised
sizes to work around firewall or other network limitations. A
requestor SHOULD choose to use a fallback mechanism which begins with
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
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
retry.
6.8. Middleware Boxes
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)
are structured as follows:
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+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 whoever sets it. 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 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
DO
DNSSEC OK bit as defined by [RFC3225].
Z
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6.11. OPT Options Code Allocation Procedure
Allocations assigned by expert review. Assignment of Option Codes
should be liberal, but duplicate functionality is to be avoided.
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7. Transport Considerations
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
EDNS, and can correctly understand any response that conforms to that
feature's specification.
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
specification and that the responder MUST NOT include an OPT record
in its response.
Responders who do not implement these protocol extensions MUST
respond with FORMERR messages without any OPT record.
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
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
distinguish between servers which do not implement EDNS and format
errors within EDNS.
The minimal response must be the DNS header, question section, and an
OPT record. This must also occur when an truncated response (using
the DNS header's TC bit) is returned.
8. Security Considerations
Requestor-side specification of the maximum buffer size may open a
DNS denial of service attack if responders can be made to send
messages which are too large for intermediate gateways to forward,
thus leading to potential ICMP storms between gateways and
responders.
Announcing very large UDP buffer sizes may result in dropping by
middleboxes (see Section 6.8). This could cause retransmissions with
no hope of success. Some devices have been found to reject
fragmented UDP packets.
Announcing too small UDP buffer sizes may result in fallback to TCP
with a corresponding load impact on DNS servers. This is especially
important with DNSSEC, where answers are much larger.
9. IANA Considerations
The IANA has assigned RR type code 41 for OPT.
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[RFC2671] specified a number of IANA sub-registries within "DOMAIN
NAME SYSTEM PARAMETERS:"
o EDNS Extended Label Type
o EDNS Option Codes
o EDNS Version Numbers
o Domain System Response Code
IANA is advised to re-parent these sub-registries to this document.
[RFC2671] created the "EDNS Extended Label Type Registry". We
request that this registry be closed.
This document assigns option code 65535 in the "EDNS Option Codes"
registry to "Reserved for future expansion."
[RFC2671] expands the RCODE space from 4 bits to 12 bits. This
allows more than the 16 distinct RCODE values allowed in [RFC1035].
IETF Standards Action is required to add a new RCODE. Adding new
RCODEs should be avoided due to the difficulty in upgrading the
installed base.
This document assigns EDNS Extended RCODE 16 to "BADVERS".
IETF Standards Action is required for assignments of new EDNS0 flags.
Flags SHOULD be used only when necessary for DNS resolution to
function. For many uses, a EDNS Option Code may be preferred.
IETF Standards Action is required to create new entries in the EDNS
Version Number registry. Expert Review is required for allocation of
an EDNS Option Code.
Appendix A. Document Editing History
Following is a list of high-level changes made to the original
RFC2671.
Appendix A.1. Changes since RFC2671
o Support for the OPT record is now mandatory.
o Extended label types obsoleted and the registry is closed.
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o The bitstring label type, which was already moved from draft to
experimental, is requested to be moved to historical.
o Changes in how EDNS buffer sizes are selected, with
recommendations on how to select them.
o Front material (IPR notice and such) was updated to current
requirements.
Appendix A.2. Changes since -02
o Specified the method for allocation of constants.
o Cleaned up a lot of wording, along with quite a bit of document
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.
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Authors' Addresses
Joao Damas
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1312
Email: joao@isc.org
Michael Graff
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1304
Email: mgraff@isc.org
Paul Vixie
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1301
Email: vixie@isc.org
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