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Network Working Group                                            P. VixieRequest for Comments: 2671                                            ISCCategory: Standards Track                                     August 1999Extension 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 Scope1.1. 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 19992 - Affected Protocol Elements2.1. 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 Types3.1. 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-RR4.1. 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     greater.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 19994.3. 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} pairs4.4. 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 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.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       sizes.Vixie                       Standards Track                     [Page 3]

RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 19994.5.3. 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       transaction.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 theVixie                       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 Considerations5.1. 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     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.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     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: vixie@isc.orgVixie                       Standards Track                     [Page 6]

RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 199911 - 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   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   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   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.Vixie                       Standards Track                     [Page 7]