RFC2671

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.

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.

1 - Rationale and Scope

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.

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 - Affected Protocol Elements

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.

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.

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

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.

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

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.

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.

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

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.

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.

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.

Both requestors and responders are advised to take account of the

   path's discovered MTU (if already known) when considering message
   sizes.

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.

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.)

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.

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

               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

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 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.

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
 responders.

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.

 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 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
 specification.

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 EMail: [email protected]

11 - Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

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Acknowledgement

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