DNSIND Working Group Paul Vixie

INTERNET-DRAFT ISC

<draft-dnsind-edns0-01.txt> January, 1999

Extension mechanisms for DNS (EDNS0)

Status of this Memo

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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

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.

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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.

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. The 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.

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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.

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.

OPTION-DATA Varies per OPTION-CODE.

4.5. The sender's UDP buffer 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.

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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. Requestors are advised to retry in such cases.

4.5.2. Both requestors and responders are advised to take account of the

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

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.'' Note that both requestors and responders

should set this to the highest level they implement,

that responders should send back RCODE=BADVERS and that

requestors should be prepared to probe using lower

version numbers if they receive an 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

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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 responders.

7 - IANA Considerations

IANA is hereby requested to assign an RR type code 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.

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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] P. Mockapetris, ``Domain Names - Implementation and

Specification,'' RFC 1035, USC/Information Sciences

Institute, November 1987.

10 - Author's Address

Paul Vixie

Internet Software Consortium

950 Charter Street

Redwood City, CA 94063

+1 650 779 7001

<paul@vix.com>

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