DNSEXT Working Group P. Vixie
Internet-Draft M. Graff
Obsoletes: 2671 (if approved) Internet Systems Consortium
Intended status: Standards Track July 13, 2009
Expires: January 14, 2010
Extension Mechanisms for DNS (EDNS0)
draft-graff-dnsext-edns0bis-00
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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 clients to advertise their capabilities to servers. This
document describes backward compatible mechanisms for allowing the
protocol to grow.
This document is a starting point to update the EDNS0 RFC after 10
years of operational experience.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Affected Protocol Elements . . . . . . . . . . . . . . . . . . 3
3.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . . 3
3.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 3
4. Extended Label Types . . . . . . . . . . . . . . . . . . . . . 4
4.1. Extended Label Type . . . . . . . . . . . . . . . . . . . . 4
4.2. Reserved Label Type . . . . . . . . . . . . . . . . . . . . 4
5. OPT pseudo-RR . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. OPT Record Behavior . . . . . . . . . . . . . . . . . . . . 4
5.2. OPT Record Format . . . . . . . . . . . . . . . . . . . . . 4
5.2.1. Fixed Content . . . . . . . . . . . . . . . . . . . . . 4
5.2.2. Variable Content . . . . . . . . . . . . . . . . . . . 5
5.3. Sender's Payload Size . . . . . . . . . . . . . . . . . . . 5
5.3.1. Reassembly Considerations . . . . . . . . . . . . . . . 6
5.3.2. Path MTU . . . . . . . . . . . . . . . . . . . . . . . 6
5.3.3. No Caching . . . . . . . . . . . . . . . . . . . . . . 6
5.3.4. Oversize Requests . . . . . . . . . . . . . . . . . . . 6
5.3.5. Be Reasonable . . . . . . . . . . . . . . . . . . . . . 6
5.4. Extended RCODE . . . . . . . . . . . . . . . . . . . . . . 6
6. Transport Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Meaning of OPT Presense . . . . . . . . . . . . . . . . . . 7
6.2. Meaning of OPT Absence . . . . . . . . . . . . . . . . . . 7
6.3. Refusing Message with OPT Records . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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.
Existing clients 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. We must however take account of
client behaviour in the face of extra fields, and design a fallback
scheme for interoperability with these clients.
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. Affected Protocol Elements
3.1. Message Header
The DNS Message Header's (see RFC 1035, 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.
3.2. Label Types
The first two bits of a wire format domain label are used to denote
the type of the label. RFC 1035, 4.1.4 [RFC1035] allocates two of
the four possible types and reserves the other two. Proposals for
use of the remaining types far outnumber those available. More label
types are needed.
3.3. 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
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must be created to allow requestors to advertise larger buffer sizes
to responders.
4. Extended Label Types
4.1. Extended Label Type
The "0 1" label type will now indicate an extended label type, whose
value is encoded in the lower six bits of the first octet of a label.
All subsequently developed label types should be encoded using an
extended label type.
4.2. Reserved Label Type
The "1 1 1 1 1 1" extended label type will be reserved for future
expansion of the extended label type code space.
5. OPT pseudo-RR
5.1. OPT Record Behavior
One OPT pseudo-RR can be added to the additional data section of
either a request or a response. An OPT is called a pseudo-RR because
it pertains to a particular transport level message and not to any
actual DNS data. OPT RRs shall never be cached, forwarded, or stored
in or loaded from master files. The quantity of OPT pseudo-RRs per
message shall be either zero or one, but not greater.
5.2. 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 new protocol elements which we expect
to be so popular that it would be a waste of wire space to encode
them as {attribute, value} pairs.
5.2.1. Fixed Content
The fixed part of an OPT RR is structured as follows:
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+------------+--------------+---------------------------+
| Field Name | Field Type | Description |
+------------+--------------+---------------------------+
| NAME | domain name | empty (root domain) |
| TYPE | u_int16_t | OPT |
| CLASS | u_int16_t | sender'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
5.2.2. Variable Content
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 /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE
Assigned by IANA.
OPTION-LENGTH
Size (in octets) of OPTION-DATA.
OPTION-DATA
Varies per OPTION-CODE.
5.3. Sender's Payload Size
The sender'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 sender's network stack. Note that
path MTU, with or without fragmentation, may be smaller than this.
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5.3.1. Reassembly Considerations
4.5.1 Note that a 512-octet UDP payload requires a 576-octet IP
reassembly buffer. Choosing 1280 on an Ethernet connected requestor
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.
5.3.2. Path MTU
Both requestors and responders are advised to take account of the
path's discovered MTU (if already known) when considering message
sizes.
5.3.3. No Caching
4.5.3. The requestor's maximum payload size can change over time,
and should therefore not be cached for use beyond the transaction in
which it is advertised.
5.3.4. Oversize Requests
4.5.4. 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.)
5.3.5. Be Reasonable
4.5.5. 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.
5.4. Extended RCODE
4.6. 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: | Z |
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+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
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 should
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
will be limited in format to the VERSION level of the request,
but the VERSION of each response will 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, including
RCODE=BADVERS.
Z
Set to zero by senders and ignored by receivers, unless
modified in a subsequent specification.
6. Transport Considerations
6.1. Meaning of OPT Presense
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.
6.2. Meaning of OPT Absence
Lack of use of these features in a request must be taken as an
indication that the requestor does not implement any part of this
specification and that the responder may make no use of any protocol
extension described here in its response.
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6.3. Refusing Message with OPT Records
Responders who do not understand these protocol extensions are
expected to send a response with RCODE NOTIMPL, FORMERR, or SERVFAIL.
Therefore use of extensions should be ``probed'' such that a
responder who isn't known to support them be allowed a retry with no
extensions if it responds with such an RCODE. If a responder's
capability level is cached by a requestor, a new probe should be sent
periodically to test for changes to responder capability.
7. 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.
8. IANA Considerations
The IANA has assigned RR type code 41 for OPT.
It is the recommendation of this document and its working group that
IANA create a registry for EDNS Extended Label Types, for EDNS Option
Codes, and for EDNS Version Numbers.
This document assigns label type 0b01xxxxxx as "EDNS Extended Label
Type." We request that IANA record this assignment.
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
published RFC (including Informational, Experimental, or BCP) should
be grounds for allocation of an EDNS Option Code.
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9. 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.
10. References
10.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
10.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Paul Vixie
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1301
Email: vixie@isc.org
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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