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
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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 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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