Username / Password : Request For Comments

RFC Number : 2671

Title : Extension Mechanisms for DNS (EDNS0).

Network Working Group P. Vixie
Request for Comments: 2671 ISC
Category: Standards Track August 1999

Extension Mechanisms for DNS (EDNS0)

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.


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.

1 - Rationale and Scope

1.1. DNS (see [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.

1.2. 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.

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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

2 - Affected Protocol Elements

2.1. The DNS Message Header's (see [RFC1035 4.1.1]) 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.

2.2. The first two bits of a wire format domain label are used to denote
the type of the label. [RFC1035 4.1.4] 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.

2.3. 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.

3 - Extended Label Types

3.1. 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.

3.2. The '1 1 1 1 1 1' extended label type will be reserved for future
expansion of the extended label type code space.

4 - OPT pseudo-RR

4.1. 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

4.2. 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.

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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

4.3. 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 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

4.4. 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)
4: | |
/ /

OPTION-CODE (Assigned by IANA.)

OPTION-LENGTH Size (in octets) of OPTION-DATA.


4.5. 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

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.

4.5.2. Both requestors and responders are advised to take account of the
path's discovered MTU (if already known) when considering message

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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

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.

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.)

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

4.6. The extended RCODE and flags (which OPT stores in the RR TTL field)
are structured as follows:

+0 (MSB) +1 (LSB)
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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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

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.

5 - Transport Considerations

5.1. 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.

5.2. 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.

5.3. 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.

6 - 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

7 - 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.

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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

This document assigns extended label type 0bxx111111 as 'Reserved
for future extended label types.' We request that IANA record this

This document assigns option code 65535 to 'Reserved for future

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 [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.

8 - 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

9 - References

[RFC1035] Mockapetris, P., 'Domain Names - Implementation and
Specification', STD 13, RFC 1035, November 1987.

10 - Author's Address

Paul Vixie
Internet Software Consortium
950 Charter Street
Redwood City, CA 94063

Phone: +1 650 779 7001

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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999

11 - Full Copyright Statement

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
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than

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


Funding for the RFC Editor function is currently provided by the
Internet Society.

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