Internet DRAFT - draft-ietf-dnsext-rfc2671bis-edns0
draft-ietf-dnsext-rfc2671bis-edns0
DNSEXT Working Group J. Damas
Internet-Draft Bond Internet Systems
Obsoletes: 2671, 2673 M. Graff
(if approved)
Intended status: Standards Track P. Vixie
Expires: July 4, 2013 Internet Systems Consortium
December 31, 2012
Extension Mechanisms for DNS (EDNS(0))
draft-ietf-dnsext-rfc2671bis-edns0-10
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 EDNS(0) specification (and obsoletes RFC
2671) based on feedback from deployment experience in several
implementations. It also obsoletes RFC 2673 ("Binary Labels in the
Domain Name System") and adds considerations on the use of extended
labels in the DNS.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on July 4, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 5
4. DNS Message changes . . . . . . . . . . . . . . . . . . . . . 5
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 5
5. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 6
6. The OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . 6
6.1. OPT Record Definition . . . . . . . . . . . . . . . . . . 6
6.1.1. Basic elements . . . . . . . . . . . . . . . . . . . . 6
6.1.2. Wire Format . . . . . . . . . . . . . . . . . . . . . 7
6.1.3. OPT Record TTL Field Use . . . . . . . . . . . . . . . 8
6.1.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Behaviour . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.1. Cache behaviour . . . . . . . . . . . . . . . . . . . 9
6.2.2. Fallback . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.3. Requestor's Payload Size . . . . . . . . . . . . . . . 10
6.2.4. Responder's Payload Size . . . . . . . . . . . . . . . 10
6.2.5. Payload Size Selection . . . . . . . . . . . . . . . . 11
6.2.6. Support in MiddleBoxes . . . . . . . . . . . . . . . . 11
7. Transport Considerations . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9.1. OPT Option Code Allocation Procedure . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Document Editing History . . . . . . . . . . . . . . 15
A.1. Changes since RFC2671 . . . . . . . . . . . . . . . . . . 15
A.2. Changes since -02 . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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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 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 other 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].
Unextended agents will not know how to interpret the protocol
extensions defined in [RFC2671] and restated here. Extended agents
need to be prepared for handling the interactions with unextended
clients in the face of new protocol elements, and fall back
gracefully to unextended DNS.
EDNS is a hop-by-hop extension to DNS. This means the use of EDNS is
negotiated between each pair of hosts in a DNS resolution process.
For instance the stub resolver communicating with the recursive
resolver or the recursive resolver communicating with an
authoritative server.
[RFC2671] specified extended label types. The only such label
proposed was in [RFC2673] for a label type called "Bitstring Labels."
For various reasons introducing a new label type was found to be
extremely difficult, and [RFC2673] was moved to Experimental. This
document deprecates Extended Labels, and therefore Binary Labels,
obsoleting [RFC2673].
2. Terminology
"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.
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].
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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). However, implementation
experiencing indicates that adding new RCODEs should be avoided due
to the difficulty in upgrading the installed base. Flags SHOULD be
used only when necessary for DNS resolution to function.
For many uses, a EDNS Option Code may be preferred.
With time, some applications of DNS have made EDNS a requirement for
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 widespread 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
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
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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.
EDNS(0) 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 was selected by
the 6 least significant bits of the first octet. Thus, Extended
Label Types were indicated by the values 64-127 (0b01xxxxxx) in the
first octet of the label.
Extended Label Types are extremely difficult to deploy due to lack of
support in clients and intermediate gateways as described in
[RFC3363] which moved [RFC2673] to experimental status, and
[RFC3364], which describes the pros and cons. As such proposals that
contemplate extended labels SHOULD weigh this deployment cost against
the possibility of implementing functionality in other ways.
Finally, implementations MUST NOT generate or pass Binary Labels in
their communications as they are now deprecated.
6. The OPT pseudo-RR
6.1. OPT Record Definition
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.
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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.
When an OPT RR MUST is included within any DNS message only ONE OPT
RR can be present. 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 |
+------------+--------------+------------------------------+
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 a bit field. Each option
is encoded as:
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+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | |
/ OPTION-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE
Assigned by the Expert Review process as defined by the dnsext
working group and the IESG.
OPTION-LENGTH
Size (in octets) of OPTION-DATA.
OPTION-DATA
Varies per OPTION-CODE. MUST be treated as a bit field.
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 and supports
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 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
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EXTENDED-RCODE
Forms upper 8 bits of extended 12-bit RCODE (together with the
4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
indicates that an unextended RCODE is in use (values 0 through
15).
VERSION
Indicates the implementation level of the setter. 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 EDNS(0),
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 signalled
through EDNS. If an implementation detects that some servers for the
zone support EDNS(0) while others would force the use of TCP to fetch
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all data, preference MAY be given to those which support EDNS(0).
Implementers SHOULD analyse this choice and the impact on both
endpoints.
6.2.3. Requestor's Payload Size
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. 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 by implementers to be used where
fragmentation is not a concern. Implementations SHOULD use their
largest configured or implemented values as a starting point in an
EDNS transaction in the absence of previous knowledge about the
destination server.
Choosing a very large value will guarantee fragmentation at the IP
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
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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 a EDNS maximum payload size of 4096 octets as a starting
point.
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 MUST be treated as equal to 512 bytes.
6.2.6. Support in 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.
Conformant 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 be able to
process the OPT record and act based on its contents. 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
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in [RFC5625]
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.
Extended agents MUST be prepared for handling the interactions with
unextended clients in the face of new protocol elements, and fall
back gracefully to unextended DNS when needed.
Responders which choose not to implement the protocol extensions
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.
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 6.2.6). This could cause retransmissions
with no hope of success. Some devices have been found to reject
fragmented UDP packets.
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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.
[RFC2671] specified a number of IANA sub-registries within "DOMAIN
NAME SYSTEM PARAMETERS:"
o DNS EDNS(0) Options
o EDNS Version Number
o EDNS Header Flags
Additionally, two entries were generated in existing registries:
EDNS Extended Label Type in the DNS Label Types Registry
Bad OPT Version in the DNS RCODES registry
IANA is advised to update 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.
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 implied a request to open a new registry for Extended Label
Types but due to the possibility of ambiguity new text registrations
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
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Label Types in the "DNS Label Types" Registry.
IETF Standards Action is required for assignments of new EDNS(0)
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. Within the EDNS Option Code space Expert
Review is required for allocation of an EDNS Option Code. IANA is
requested to keep a registry for the EDNS Option Code space.
9.1. OPT Option Code Allocation Procedure
OPT Option Codes are assigned by expert review.
Assignment of Option Codes should be liberal, but duplicate
functionality is to be avoided.
10. References
10.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
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.
10.2. 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.
[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.
[RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
Support for Internet Protocol version 6 (IPv6)", RFC 3364,
August 2002.
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[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.
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.
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
Joao Damas
Bond Internet Systems
Av Albufera 14
S.S. Reyes, Madrid 28701
ES
Phone: +1 650.423.1312
Email: joao@bondis.org
Michael Graff
Email: explorer@flame.org
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Paul Vixie
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
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