One document matched: draft-ietf-dnsind-2ndry-00.txt

Network Working Group                                         Robert Elz
Internet Draft                                   University of Melbourne
Expiration Date: August 1996
                                                              Randy Bush
                                                             RGnet, Inc.

                                                           February 1996


                   Selection of Secondary DNS Servers

                     draft-ietf-dnsind-2ndry-00.txt


1. Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

2. Abstract

   This draft discusses the selection of secondary servers for DNS
   zones.

3. Introduction

   Poor choice of secondary servers for DNS zones seems to currently be
   an endemic problem.  This draft discusses some of the issues, and
   attempts to give some guidance in the matter of the selection of the
   required secondary DNS server(s).








kre/randy                                                       [Page 1]

Internet Draft       draft-ietf-dnsind-2ndry-00.txt        February 1996


4. 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.

   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.

5. Secondary Servers

   A prime purpose of secondary servers is to allow information from the
   Domain Name System to be available widely and reliably to clients
   throughout the Internet (that is, throughout the world), even when
   the primary server is unavailable or unreachable.  They can also
   spread the name resolution load, that purpose is not treated further
   here.

   When selecting secondary servers, attention should be given to the
   various likely failure modes, and servers should be placed so that it
   is likely that at least one server will be available to all
   significant parts of the Internet, for any likely failure.

   Consequently, placing all servers at the local site, while easy to
   arrange, and easy to manage, is not a good policy, as they are



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   susceptible, together, to being disconnected from the Internet due to
   a single link failure, or a site (or sometimes, building or even
   room) wide power failure.

   Secondary servers should be placed at both topologically and
   geographically dispersed locations on the Internet, to minimise the
   likelihood of a single failure disabling all of them.

6. Unreachable servers

   Similarly, listing as a server, or as the address of a server, a name
   or value that cannot be reached from much of the network is
   undesirable.  Not only does such a server add no reliability at all,
   it actually causes the network as a whole (the Internet) problems, as
   the lack of reachability cannot be ascertained other than by
   attempted use and the subsequent lack or response (or occasionally
   ICMP error response).  Further, even that is generally
   indistinguishable from a simple packet loss, so the sequence must be
   repeated, several times, to give any real evidence of an unreachable
   server.  Further, the whole thing needs to be repeated from time to
   time to distinguish a permanently unreachable server from a
   temporarily unreachable one.  Worst of all, all this may potentially
   be done by every other DNS server on the Internet.

   To avoid this problem, NS records for a zone returned in any response
   should list only servers that the client requesting the information,
   and others to which it may forward the reply, are likely to be able
   to reach.  Additionally, addresses of the servers returned must all
   be reachable.  As the addresses of each server form a Resource Record
   Set [KRE1996], all must be returned (or none), thus it is not
   acceptable to elide addresses of servers that are unreachable, or to
   return them with a low TTL (while returning others with a higher
   TTL).

   In particular, when some servers are behind a firewall, which
   disallows DNS queries or responses, their names, or addresses, should
   not be returned to clients outside the firewall.  Similarly, servers
   outside the firewall should not be made known to clients inside it,
   if the clients would be unable to query those servers.  Implementing
   this usually requires dual DNS setups, one for internal use, the
   other for external use.  Such a setup often solves other problems
   with environments like this.

   When a server is at a firewall boundary, reachable from both sides,
   but using different addresses, that server should be given two names,
   each name associated with appropriate A records, such that each
   appears to be reachable only on the appropriate side of the firewall.
   This should then be treated just like two servers, one on each side



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Internet Draft       draft-ietf-dnsind-2ndry-00.txt        February 1996


   of the firewall.  Special care will need to be taken to allow such a
   server to return the correct responses to clients on each side.

   A similar problem occurs with DNS servers located in parts of the net
   that are often disconnected from the Internet as a whole, for
   example, those which connect via an intermittent connection that is
   often down.  A similar solution may need to be adopted, though here
   much of the zone information can often be replicated, with only NS
   records being adjusted.

   Servers in this environment often need special provision to give them
   access to the root servers.  Often this is accomplished via "fake
   root" configurations.  In such a case the servers should be kept well
   isolated from the rest of the DNS, lest their unusual configuration
   pollute others.

7. How many secondaries?

   The DNS specification requires at least two servers for every zone.
   That is, usually, the primary and one secondary.  While two,
   carefully placed, are usually sufficient, occasions where two are
   insufficient are frequent enough that we advise the use of more than
   two listed servers.  Various problems can cause a server to be
   unavailable for extended periods - during such a period, a zone with
   only two listed servers is actually running with just one.  Since any
   server may occasionally be unavailable, for all kinds of reasons,
   this zone is likely, at times, to have no functional servers at all.

   On the other hand, having large numbers of servers adds little
   benefit, while adding costs.  At the simplest, more servers cause
   packets to be larger, so requiring more bandwidth.  This may seem,
   and realistically is, trivial, however there is a limit to the size
   of a DNS packet, and causing that limit to be reached has more
   serious performance implications.  It is wise to stay well clear of
   it.  More servers also increase both the likelihood that one server
   will be misconfigured, or malfunction, without being detected.

   Consequently, it is recommended for most forward zones, that three,
   four, or five listed servers, depending on circumstances and
   placement of the servers, be established.  Reverse zones, that is,
   sub-domains of .IN-ADDR.ARPA, tend to be less crucial, and less
   servers, less distributed, will often suffice.  Servers which are
   authoritative for the zone, but not listed in NS records (also known
   as "stealth" servers) are not included above - it can often be useful
   for all servers at a site to be authoritative, but unlisted, for all
   local zones.





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8. Security Considerations

   This document does not consider security.

   The mention of firewalls in section 6 is purely because they are a
   fact of life (and an impediment to orderly communications).  It is
   not intended to imply that a firewall is in any way useful for
   security purposes.

   It is not believed that anything in this document adds to any
   security issues that may exist with the DNS, nor does it do anything
   to lessen them.

9. References

[RFC1034]   Domain Names - Concepts and Facilities,
            P. Mockapetris, ISI, November 1987.

[RFC1035]   Domain Names - Implementation and Specification,
            P. Mockapetris, ISI, November 1987

[KRE1996]   Clarifications to the DNS specification,
            R. Elz, R. Bush,
            Work In Progress (internet-draft), February 1996.

10. Acknowledgements

   To be supplied.























kre/randy                                                       [Page 5]