2671
PROPOSED STANDARD
Extension Mechanisms for DNS (EDNS0) (Obsoleted)
Authors: P. Vixie
Date: August 1999
Area: int
Working Group: dnsind
Stream: IETF
Obsoleted by:
RFC 6891
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 clients to advertise their capabilities to servers. This document describes backward compatible mechanisms for allowing the protocol to grow. [STANDARDS-TRACK]
RFC 2671
Obsoleted by: 6891 PROPOSED STANDARD
Network Working Group P. Vixie
Request for Comments: 2671 ISC
Category: Standards Track August 1999
<span class="h1">Extension Mechanisms for DNS (EDNS0)</span>
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
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 clients to advertise their capabilities to servers. This
document describes backward compatible mechanisms for allowing the
protocol to grow.
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a> - Rationale and Scope</span>
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. DNS (see [<a href="./rfc1035" title=""Domain Names - Implementation and Specification"">RFC1035</a>]) specifies a Message Format and within such</span>
<span class="h3"> messages there are standard formats for encoding options, errors,</span>
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.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Existing clients will not know how to interpret the protocol</span>
<span class="h3"> extensions detailed here. In practice, these clients will be</span>
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.
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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a> - Affected Protocol Elements</span>
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. The DNS Message Header's (see [RFC1035 4.1.1]) second full 16-bit</span>
<span class="h3"> word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of</span>
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.
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. The first two bits of a wire format domain label are used to denote</span>
<span class="h3"> the type of the label. [RFC1035 4.1.4] allocates two of the four</span>
possible types and reserves the other two. Proposals for use of
the remaining types far outnumber those available. More label
types are needed.
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. DNS Messages are limited to 512 octets in size when sent over UDP.</span>
<span class="h3"> While the minimum maximum reassembly buffer size still allows a</span>
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.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a> - Extended Label Types</span>
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. The "0 1" label type will now indicate an extended label type,</span>
<span class="h3"> whose value is encoded in the lower six bits of the first octet of</span>
a label. All subsequently developed label types should be encoded
using an extended label type.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. The "1 1 1 1 1 1" extended label type will be reserved for future</span>
<span class="h3"> expansion of the extended label type code space.</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a> - OPT pseudo-RR</span>
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. One OPT pseudo-RR can be added to the additional data section of</span>
<span class="h3"> either a request or a response. An OPT is called a pseudo-RR</span>
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.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. An OPT RR has a fixed part and a variable set of options expressed</span>
<span class="h3"> as {attribute, value} pairs. The fixed part holds some DNS meta</span>
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.
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<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. The fixed part of an OPT RR is structured as follows:</span>
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
<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. The variable part of an OPT RR is encoded in its RDATA and is</span>
<span class="h3"> structured as zero or more of the following:</span>
+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.
<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>. The sender's UDP payload size (which OPT stores in the RR CLASS</span>
<span class="h3"> field) is the number of octets of the largest UDP payload that can</span>
be reassembled and delivered in the sender's network stack. Note
that path MTU, with or without fragmentation, may be smaller than
this.
<span class="h4"><a class="selflink" id="section-4.5.1" href="#section-4.5.1">4.5.1</a>. Note that a 512-octet UDP payload requires a 576-octet IP</span>
<span class="h4"> reassembly buffer. Choosing 1280 on an Ethernet connected</span>
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.
<span class="h4"><a class="selflink" id="section-4.5.2" href="#section-4.5.2">4.5.2</a>. Both requestors and responders are advised to take account of the</span>
<span class="h4"> path's discovered MTU (if already known) when considering message</span>
sizes.
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<span class="h4"><a class="selflink" id="section-4.5.3" href="#section-4.5.3">4.5.3</a>. The requestor's maximum payload size can change over time, and</span>
<span class="h4"> should therefore not be cached for use beyond the transaction in</span>
which it is advertised.
<span class="h4"><a class="selflink" id="section-4.5.4" href="#section-4.5.4">4.5.4</a>. The responder's maximum payload size can change over time, but</span>
<span class="h4"> can be reasonably expected to remain constant between two</span>
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.)
<span class="h4"><a class="selflink" id="section-4.5.5" href="#section-4.5.5">4.5.5</a>. Due to transaction overhead, it is unwise to advertise an</span>
<span class="h4"> architectural limit as a maximum UDP payload size. Just because</span>
your stack can reassemble 64KB datagrams, don't assume that you
want to spend more than about 4KB of state memory per ongoing
transaction.
<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>. The extended RCODE and flags (which OPT stores in the RR TTL field)</span>
<span class="h3"> are structured as follows:</span>
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | 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 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
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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.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a> - Transport Considerations</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. The presence of an OPT pseudo-RR in a request should be taken as an</span>
<span class="h3"> indication that the requestor fully implements the given version of</span>
EDNS, and can correctly understand any response that conforms to
that feature's specification.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Lack of use of these features in a request must be taken as an</span>
<span class="h3"> indication that the requestor does not implement any part of this</span>
specification and that the responder may make no use of any
protocol extension described here in its response.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Responders who do not understand these protocol extensions are</span>
<span class="h3"> expected to send a response with RCODE NOTIMPL, FORMERR, or</span>
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.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a> - Security Considerations</span>
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.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a> - IANA Considerations</span>
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.
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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 [<a href="./rfc1035" title=""Domain Names - Implementation and Specification"">RFC1035</a>].
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.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a> - Acknowledgements</span>
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.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a> - References</span>
[<a id="ref-RFC1035">RFC1035</a>] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, <a href="./rfc1035">RFC 1035</a>, November 1987.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a> - Author's Address</span>
Paul Vixie
Internet Software Consortium
950 Charter Street
Redwood City, CA 94063
Phone: +1 650 779 7001
EMail: [email protected]
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<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a> - Full Copyright Statement</span>
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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