Extension Mechanisms for DNS (EDNS(0))
RFC 6891

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Internet Engineering Task Force (IETF)                          J. Damas
Request for Comments: 6891                         Bond Internet Systems
STD: 75                                                         M. Graff
Obsoletes: 2671, 2673
Category: Standards Track                                       P. Vixie
ISSN: 2070-1721                              Internet Systems Consortium
                                                              April 2013

                 Extension Mechanisms for DNS (EDNS(0))

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 requestors to advertise their capabilities to responders.  This
   document describes backward-compatible mechanisms for allowing the
   protocol to grow.

   This document updates the Extension Mechanisms for DNS (EDNS(0))
   specification (and obsoletes RFC 2671) based on feedback from
   deployment experience in several implementations.  It also obsoletes
   RFC 2673 ("Binary Labels in the Domain Name System") and adds
   considerations on the use of extended labels in the DNS.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6891.

Damas, et al.                Standards Track                    [Page 1]
RFC 6891                   EDNS(0) Extensions                 April 2013

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Damas, et al.                Standards Track                    [Page 2]
RFC 6891                   EDNS(0) Extensions                 April 2013

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  EDNS Support Requirement . . . . . . . . . . . . . . . . . . .  5
   4.  DNS Message Changes  . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Message Header . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Label Types  . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  UDP Message Size . . . . . . . . . . . . . . . . . . . . .  6
   5.  Extended Label Types . . . . . . . . . . . . . . . . . . . . .  6
   6.  The OPT Pseudo-RR  . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  OPT Record Definition  . . . . . . . . . . . . . . . . . .  6
       6.1.1.  Basic Elements . . . . . . . . . . . . . . . . . . . .  6
       6.1.2.  Wire Format  . . . . . . . . . . . . . . . . . . . . .  7
       6.1.3.  OPT Record TTL Field Use . . . . . . . . . . . . . . .  9
       6.1.4.  Flags  . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . . 10
       6.2.1.  Cache Behaviour  . . . . . . . . . . . . . . . . . . . 10
       6.2.2.  Fallback . . . . . . . . . . . . . . . . . . . . . . . 10
       6.2.3.  Requestor's Payload Size . . . . . . . . . . . . . . . 10
       6.2.4.  Responder's Payload Size . . . . . . . . . . . . . . . 11
       6.2.5.  Payload Size Selection . . . . . . . . . . . . . . . . 11
       6.2.6.  Support in Middleboxes . . . . . . . . . . . . . . . . 11