Selection and Operation of Secondary DNS Servers
RFC 2182

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Network Working Group                                             R. Elz
Request for Comments: 2182                       University of Melbourne
BCP: 16                                                          R. Bush
Category: Best Current Practice                              RGnet, Inc.
                                                              S. Bradner
                                                      Harvard University
                                                               M. Patton
                                                              Consultant
                                                               July 1997

            Selection and Operation of Secondary DNS Servers

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   The Domain Name System requires that multiple servers exist for every
   delegated domain (zone).  This document discusses the selection of
   secondary servers for DNS zones.  Both the physical and topological
   location of each server are material considerations when selecting
   secondary servers.  The number of servers appropriate for a zone is
   also discussed, and some general secondary server maintenance issues
   considered.

Elz, et al.              Best Current Practice                  [Page 1]
RFC 2182    Selection and Operation of Secondary DNS Servers   July 1997

Contents

       Abstract  ...................................................   1
    1  Introduction  ...............................................   2
    2  Definitions  ................................................   2
    3  Secondary Servers  ..........................................   3
    4  Unreachable servers  ........................................   5
    5  How many secondaries?  ......................................   7
    6  Finding Suitable Secondary Servers  .........................   8
    7  Serial Number Maintenance  ..................................   9
       Security Considerations  ....................................  11
       References  .................................................  11
       Acknowledgements  ...........................................  11
       Authors' Addresses  .........................................  11

1. Introduction

   A number of problems in DNS operations today are attributable to poor
   choices of secondary servers for DNS zones.  The geographic placement
   as well as the diversity of network connectivity exhibited by the set
   of DNS servers for a zone can increase the reliability of that zone
   as well as improve overall network performance and access
   characteristics.  Other considerations in server choice can
   unexpectedly lower reliability or impose extra demands on the
   network.

   This document discusses many of the issues that should be considered
   when selecting secondary servers for a zone.  It offers guidance in
   how to best choose servers to serve a given zone.

2. Definitions

   For the purposes of this document, and only this document, the
   following definitions apply:

   DNS                    The Domain Name System [RFC1034, RFC1035].

   Zone                   A part of the DNS tree, that is treated as a
                          unit.

   Forward Zone           A zone containing data mapping names to host
                          addresses, mail exchange targets, etc.

Elz, et al.              Best Current Practice                  [Page 2]
RFC 2182    Selection and Operation of Secondary DNS Servers   July 1997

   Reverse Zone           A zone containing data used to map addresses
                          to names.

   Server                 An implementation of the DNS protocols able to
                          provide answers to queries.  Answers may be
                          from information known by the server, or
                          information obtained from another server.

   Authoritative Server   A server that knows the content of a DNS zone
                          from local knowledge, and thus can answer
                          queries about that zone without needing to
                          query other servers.

   Listed Server          An Authoritative Server for which there is an
                          "NS" resource record (RR) in the zone.

   Primary Server         An authoritative server for which the zone
                          information is locally configured.  Sometimes
                          known as a Master server.

   Secondary Server       An authoritative server that obtains
                          information about a zone from a Primary Server
                          via a zone transfer mechanism.  Sometimes
                          known as a Slave Server.

   Stealth Server         An authoritative server, usually secondary,