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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.

Vixie Standards Track [Page 1] 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.

Vixie Standards Track [Page 2] 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)
0: |                          OPTION-CODE                          |
2: |                         OPTION-LENGTH                         |
4: |                                                               |
   /                          OPTION-DATA                          /
   /                                                               /
 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

Vixie Standards Track [Page 3] 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)
 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

Vixie Standards Track [Page 4] 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.

Vixie Standards Track [Page 5] 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

Vixie Standards Track [Page 6] 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.

Vixie Standards Track [Page 7]

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