DNSEXT Working Group J.L.S.D. Damas
Internet-Draft M.G. Graff
Obsoletes: 2671, 2673 (if approved) P.V. Vixie
Intended status: Standards Track Internet Systems Consortium
Expires: June 01, 2012 December 2011
Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-06
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
feedback from deployment experience in several implementations.
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 June 01, 2012.
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 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
Damas, Graff & Vixie Expires June 01, 2012 [Page 1]
Internet-Draft EDNS0 Extensions December 2011
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 3
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4
6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 4
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 4
6.1.1. Basic elements . . . . . . . . . . . . . . . . . . . . 5
6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 5
6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 6
6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2.1. Cache behaviour . . . . . . . . . . . . . . . . . . . 7
6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 7
6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 7
6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 8
6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 8
6.2.6. MiddleBoxes . . . . . . . . . . . . . . . . . . . . . 9
7. OPT Option Code Allocation Procedure . . . . . . . . . . . . . 9
8. Transport Considerations . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Document Editing History . . . . . . . . . . . . . . . 12
Appendix A.1. Changes since RFC2671 . . . . . . . . . . . . . . 12
Appendix A.2. Changes since -02 . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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 over UDP
not using the extensions described in this document is limited to 512
bytes. Many of DNS's protocol limits, such as the maximum message
size over UDP, are too small to efficiently support the additional
information that can be conveyed in the DNS (e.g. several IPv6
addresses or DNSSEC signatures). Finally, RFC 1035 does not define
any way for implementations to advertise their capabilities to any of
the others actors they interact with.
[RFC2671] added an extension mechanism to DNS. This mechanism is
widely supported and a number of new DNS uses and protocol extensions
depend on the presence of these extensions. This memo refines that
specification and obsoletes [RFC2671].
Damas, Graff & Vixie Expires June 01, 2012 [Page 2]
Internet-Draft EDNS0 Extensions December 2011
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] 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
deprecates 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. Other terminology is used as per its use in
the references (e.g. middleboxes as in [RFC5625])
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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 provides a mechanism to improve the scalability of DNS as its
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
specified in RFC 1035 as well as providing extra data space for
additional flags and return codes (RCODEs).
With time, some applications of DNS have made EDNS a requirement for
their deployment. For instance, DNSSEC uses the additional flag
space introduced in EDNS to signal the request to include DNSSEC data
in a DNS response.
Given the increase in DNS response sizes when including larger data
items such as AAAA Records, DNSSEC information (e.g. RRSIG or
DNSKEY) or large TXT Records, the additional UDP payload capabilities
provided by EDNS can help improve the scalability of the DNS by
avoiding generalized use of TCP for DNS transport.
4. DNS Message changes
4.1. 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
Damas, Graff & Vixie Expires June 01, 2012 [Page 3]
Internet-Draft EDNS0 Extensions December 2011
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]. Fitting the increasing amounts of data that can
be transported in DNS in this 512-byte limit is becoming more
difficult. For instance, inclusion of DNSSEC records frequently
requires a much larger response than a 512 byte message can hold.
EDNS0 is intended to provide support for transporting these larger
packet sizes while continuing 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.
[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 deprecated.
Implementations MUST NOT generate or pass Extended Labels in their
communications. Additionally, no further registrations of Extended
Label Types are permitted.
6. The OPT pseudo-RR
6.1. OPT Record Definition
Damas, Graff & Vixie Expires June 01, 2012 [Page 4]
Internet-Draft EDNS0 Extensions December 2011
6.1.1. Basic elements
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.
No more than one OPT RR MUST be included within any DNS message. If
a query 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.
6.1.2. 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.
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 (41) |
| 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 |
+------------+--------------+------------------------------+
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:
Damas, Graff & Vixie Expires June 01, 2012 [Page 5]
Internet-Draft EDNS0 Extensions December 2011
+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
option specification might say that if a responder sees option XYZ,
it MUST include option XYZ in its response.
6.1.3. OPT Record TTL Field Use
The extended RCODE and flags (which OPT stores in the RR TTL field)
are structured as follows:
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO| Z |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE
Forms upper 8 bits of extended 12-bit RCODE (together with the
4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
indicates that an unextended RCODE is in use (values 0 through
Damas, Graff & Vixie Expires June 01, 2012 [Page 6]
Internet-Draft EDNS0 Extensions December 2011
15).
VERSION
Indicates the implementation level of the settter. Full
conformance with this specification is indicated by version
``0.'' Requestors are encouraged to set this to the lowest
implemented level capable of expressing a transaction, to
minimize the responder and network load of discovering the
greatest common implementation level between requestor and
responder. A requestor's version numbering strategy MAY
ideally be a run time configuration option.
If a responder does not implement the VERSION level of the
request, then it MUST respond with RCODE=BADVERS. All
responses MUST be limited in format to the VERSION level of the
request, but the VERSION of each response SHOULD be the highest
implementation level of the responder. In this way a requestor
will learn the implementation level of a responder as a side
effect of every response, including error responses and
including RCODE=BADVERS.
6.1.4. Flags
DO
DNSSEC OK bit as defined by [RFC3225].
Z
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6.2. Behaviour
6.2.1. Cache behaviour
The OPT record MUST NOT be cached.
6.2.2. 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).
6.2.3. 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
Damas, Graff & Vixie Expires June 01, 2012 [Page 7]
Internet-Draft EDNS0 Extensions December 2011
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. This
knowledge may be auto-detected by the implementation or provided by a
human administrator.
Note that a 512-octet UDP payload requires a 576-octet IP reassembly
buffer. Choosing between 1280 and 1410 bytes for IP (v4 or v6) over
Ethernet would be reasonable.
Bigger values SHOULD be considered here fragmentation is not a
concern.
Choosing a very large value will guarantee fragmentation at the IP
layer, and may prevent answers from being received due to a single
fragment loss or misconfigured firewalls.
The requestor's maximum payload size can change over time. It MUST
not be cached for use beyond the transaction in which it is
advertised.
6.2.4. Responder's Payload Size
The responder's maximum payload size can change over time, but can be
reasonably expected to remain constant between two closely spaced
sequential transactions; for example, an arbitrary QUERY used as a
probe to discover 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.
6.2.5. Payload Size Selection
Due to transaction overhead, it is not recommended to advertise an
architectural limit as a maximum UDP payload size. Even on system
stacks capable of reassembling 64KB datagrams, memory usage at low
levels in the system will be a concern. A good compromise may be the
use of about 4KB of state memory per ongoing transaction, or a EDNS
maximum payload size of 4096 octets.
Damas, Graff & Vixie Expires June 01, 2012 [Page 8]
Internet-Draft EDNS0 Extensions December 2011
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.
Values of less than 512 bytes SHOULD be treated as equal to 512
bytes.
6.2.6. MiddleBoxes
In a network that carries DNS traffic there could be active equipment
other than that participating directly in the DNS resolution process
(stub and caching resolvers, authoritative servers) that affect the
transmission of DNS messages (e.g. firewalls, load balancers,
proxies, etc) referred to here as MiddleBoxes.
MiddleBoxes MUST NOT limit DNS messages over UDP to 512 bytes.
MiddleBoxes which simply forward requests to a recursive resolver
MUST NOT modify and MUST NOT delete the OPT record contents in either
direction.
MiddleBoxes which have additional functionality, such as answering
queries or acting as intelligent forwarders, SHOULD understand the
OPT record. These boxes MUST consider the incoming request and any
outgoing requests as separate transactions if the characteristics of
the messages are different.
A more in depth discussion of this type of equipment and other
considerations regarding their interaction with DNS traffic is found
in [RFC5625]
7. OPT Option Code Allocation Procedure
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
indication that the requestor fully implements the given version of
EDNS, and can correctly understand any response that conforms to that
feature's specification.
Damas, Graff & Vixie Expires June 01, 2012 [Page 9]
Internet-Draft EDNS0 Extensions December 2011
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 which choose not to implement the protocol extensions
defined in this document MUST respond with a return code (RCODE) of
FORMERR to messages containing an OPT RR in the additional section
and MUST NOT include an OPT record in the response.
If there is a problem with processing the OPT record itself, such as
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
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.
9. 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 6.2.6). 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.
10. 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:"
o DNS EDNS0 Options
o EDNS Version Number
o EDNS Header Flags
Additionally, several entries were generated in existing registries:
Damas, Graff & Vixie Expires June 01, 2012 [Page 10]
Internet-Draft EDNS0 Extensions December 2011
EDNS Extended Label Type in the DNS Label Types Registry
Bad OPT Version in the DNS RCODES registry
IANA is advised to udpates references to [RFC2671] in these entries
and sub-registries to this document.
[RFC2671] created the "EDNS Extended Label Type Registry". We
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" in the DNS
RCODES registry.
[RFC2671] called for the recording of assignment of extended label
types 0bxx111111 as "Reserved for future extended label types". This
request was implicitly a request to open a new registry for Extended
Label Types but due to possible ambiguous text registrarions were
instead made within the general "DNS Label Types" registry which also
registers entries originally defined by [RFC1035].
This document requests IANA to close registration of further Extended
Label Types in the "DNS Label Types" Registry.
IETF Standards Action is required for assignments of new EDNS0 flags.
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.
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.
Damas, Graff & Vixie Expires June 01, 2012 [Page 11]
Internet-Draft EDNS0 Extensions December 2011
[RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O. and T.
Hain, "Representing Internet Protocol version 6 (IPv6)
Addresses in the Domain Name System (DNS)", RFC 3363,
August 2002.
11.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
Support for Internet Protocol version 6 (IPv6)", RFC 3364,
August 2002.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP
152, RFC 5625, August 2009.
Appendix A. Document Editing History
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.
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.
Authors' Addresses
Damas, Graff & Vixie Expires June 01, 2012 [Page 12]
Internet-Draft EDNS0 Extensions December 2011
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
Damas, Graff & Vixie Expires June 01, 2012 [Page 13]