Clarifications to the DNS Specification
RFC 2181

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Network Working Group                                             R. Elz
Request for Comments: 2181                       University of Melbourne
Updates: 1034, 1035, 1123                                        R. Bush
Category: Standards Track                                    RGnet, Inc.
                                                               July 1997

                Clarifications to the DNS Specification

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.

1. Abstract

   This document considers some areas that have been identified as
   problems with the specification of the Domain Name System, and
   proposes remedies for the defects identified.  Eight separate issues
   are considered:

     + IP packet header address usage from multi-homed servers,
     + TTLs in sets of records with the same name, class, and type,
     + correct handling of zone cuts,
     + three minor issues concerning SOA records and their use,
     + the precise definition of the Time to Live (TTL)
     + Use of the TC (truncated) header bit
     + the issue of what is an authoritative, or canonical, name,
     + and the issue of what makes a valid DNS label.

   The first six of these are areas where the correct behaviour has been
   somewhat unclear, we seek to rectify that.  The other two are already
   adequately specified, however the specifications seem to be sometimes
   ignored.  We seek to reinforce the existing specifications.

Elz & Bush                  Standards Track                     [Page 1]
RFC 2181        Clarifications to the DNS Specification        July 1997

Contents

    1  Abstract  ...................................................   1
    2  Introduction  ...............................................   2
    3  Terminology  ................................................   3
    4  Server Reply Source Address Selection  ......................   3
    5  Resource Record Sets  .......................................   4
    6  Zone Cuts  ..................................................   8
    7  SOA RRs  ....................................................  10
    8  Time to Live (TTL)  .........................................  10
    9  The TC (truncated) header bit  ..............................  11
   10  Naming issues  ..............................................  11
   11  Name syntax  ................................................  13
   12  Security Considerations  ....................................  14
   13  References  .................................................  14
   14  Acknowledgements  ...........................................  15
   15  Authors' Addresses  .........................................  15

2. Introduction

   Several problem areas in the Domain Name System specification
   [RFC1034, RFC1035] have been noted through the years [RFC1123].  This
   document addresses several additional problem areas.  The issues here
   are independent.  Those issues are the question of which source
   address a multi-homed DNS server should use when replying to a query,
   the issue of differing TTLs for DNS records with the same label,
   class and type, and the issue of canonical names, what they are, how
   CNAME records relate, what names are legal in what parts of the DNS,
   and what is the valid syntax of a DNS name.

   Clarifications to the DNS specification to avoid these problems are
   made in this memo.  A minor ambiguity in RFC1034 concerned with SOA
   records is also corrected, as is one in the definition of the TTL
   (Time To Live) and some possible confusion in use of the TC bit.

Elz & Bush                  Standards Track                     [Page 2]
RFC 2181        Clarifications to the DNS Specification        July 1997

3. Terminology

   This memo does not use the oft used expressions MUST, SHOULD, MAY, or
   their negative forms.  In some sections it may seem that a
   specification is worded mildly, and hence some may infer that the
   specification is optional.  That is not correct.  Anywhere that this
   memo suggests that some action should be carried out, or must be
   carried out, or that some behaviour is acceptable, or not, that is to
   be considered as a fundamental aspect of this specification,
   regardless of the specific words used.  If some behaviour or action
   is truly optional, that will be clearly specified by the text.

4. Server Reply Source Address Selection

   Most, if not all, DNS clients, expect the address from which a reply
   is received to be the same address as that to which the query
   eliciting the reply was sent.  This is true for servers acting as