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DNS Transport over TCP - Implementation Requirements
RFC 5966

Internet Engineering Task Force (IETF)                         R. Bellis
Request for Comments: 5966                                    Nominet UK
Updates: 1035, 1123                                          August 2010
Category: Standards Track
ISSN: 2070-1721

          DNS Transport over TCP - Implementation Requirements

Abstract

   This document updates the requirements for the support of TCP as a
   transport protocol for DNS implementations.

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/rfc5966.

Copyright Notice

   Copyright (c) 2010 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.

Bellis                       Standards Track                    [Page 1]
RFC 5966                      DNS over TCP                   August 2010

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology Used in This Document . . . . . . . . . . . . . . . 3
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Transport Protocol Selection  . . . . . . . . . . . . . . . . . 4
   5.  Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Response Reordering . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

1.  Introduction

   Most DNS [RFC1034] transactions take place over UDP [RFC0768].  TCP
   [RFC0793] is always used for zone transfers and is often used for
   messages whose sizes exceed the DNS protocol's original 512-byte
   limit.

   Section 6.1.3.2 of [RFC1123] states:

      DNS resolvers and recursive servers MUST support UDP, and SHOULD
      support TCP, for sending (non-zone-transfer) queries.

   However, some implementors have taken the text quoted above to mean
   that TCP support is an optional feature of the DNS protocol.

   The majority of DNS server operators already support TCP and the
   default configuration for most software implementations is to support
   TCP.  The primary audience for this document is those implementors
   whose failure to support TCP restricts interoperability and limits
   deployment of new DNS features.

   This document therefore updates the core DNS protocol specifications
   such that support for TCP is henceforth a REQUIRED part of a full DNS
   protocol implementation.

   Whilst this document makes no specific recommendations to operators
   of DNS servers, it should be noted that failure to support TCP (or
   the blocking of DNS over TCP at the network layer) may result in
   resolution failure and/or application-level timeouts.

Bellis                       Standards Track                    [Page 2]
RFC 5966                      DNS over TCP                   August 2010

2.  Terminology Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Discussion

   In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below),
   the normal behaviour of any DNS server needing to send a UDP response
   that would exceed the 512-byte limit is for the server to truncate

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