Extension Mechanisms for DNS (EDNS0)
RFC 2671

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
       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 the

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                   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
     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.

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     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.org

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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
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   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
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Acknowledgement

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

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