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IETF DNSOPS working group                                    T. Hardie
Internet draft                                            Equinix, Inc
Category: Work-in-progress                             June 2000


Distributing Root or Authoritative Name Servers via Shared Unicast Addresses

Status of this memo

  This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 of RFC 2026.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups.  Note that
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  Internet-Drafts are draft documents valid for a maximum of six
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  The list of current Internet-Drafts can be accessed at

  To view the list Internet-Draft Shadow Directories, see

Copyright Notice

  Copyright (C) The Internet Society 1999.  All Rights Reserved.


  This memo describes a set of practices intended to enable an
  authoritative name server operator to provide access to a single
  named server in multiple locations.  It was originally written to
  apply particularly to root server operations and later expanded to
  include the more general case of authoritative name servers.  In
  both cases, the primary motivation for the development and
  deployment of these practices is to increase the distribution of DNS
  servers to previously under-served areas of the network topology and
  to reduce the latency for DNS query responses in those areas.  This
  document presumes a one-to-one mapping between named authoritative
  servers and administrative entities (operators).  This document
  contains no guidelines or recommendations for caching name servers.

1. Architecture

1.1 Server Requirements

  Root servers must meet the host requirements listed in [1], and
  operators of other authoritative name servers may also wish to refer
  to it for guidance on appropriate practice.  In addition to meeting
  those requirements, each of the hosts participating in a
  shared-unicast system should be configured with two network
  interfaces.  These interfaces may be either two physical interfaces
  or one physical interface mapped to two logical interfaces.
  One of the network interfaces should use the shared
  unicast address associated with the authoritative name server.  The
  other interface, referred to as the administrative interface below,
  should use a distinct address specific to that host.  The host
  should respond to DNS queries only on the shared-unicast interface.
  Responses on that interface should only relate to zones for which
  the host is authoritative; the host should not be configured as a
  caching name server.  The host should use the administrative
  interface and address for all mesh coordination.

1.2 Zone file delivery

  In order to minimize the risk of man-in-the-middle attacks, zone
  files should be delivered to the administrative interface of the
  servers participating in the mesh.  Secure file transfer methods and
  strong authentication should be used for all transfers.  If the hosts
  in the mesh make their zones available for zone transer, the administrative
  interfaces should be used for those transfers as well, in order to avoid
  the problems with potential routing changes for TCP traffic
  noted in section 1.5 below.

1.3 Synchronization

  The root name servers traditionally form a loosely synchronized
  system and some delay in propagation of a specific zone file is an
  expected part of the current operational environment.  Authoritative
  name servers may be loosely or tightly synchronized, depending on
  the practices set by the operating organization.  As noted below in
  section 3.1.2, lack of synchronization among servers using the same
  shared unicast address could create problems for some users of this
  service.  In order to minimize that risk, switch-overs from one data
  set to another data set should be coordinated as much as possible.
  The use of synchronized clocks on the participating hosts and set
  times for switch-overs provides a basic level of coordination.  A
  more complete coordination process would involve:

       a) receipt of zones at a distribution host
       b) confirmation of the integrity of zones received
       c) distribution of the zones to all of the servers in the
       d) confirmation of the integrity of the zones at each server
       e) coordination of the switchover times for the servers in the
       f) institution of a failure process to ensure that servers that
          did not receive correct data or could not switchover to the
          new data ceased to respond to incoming queries until the
          problem could be resolved.

  Depending on the size of the mesh, the distribution host may also be
  a participant; for authoritative servers, it may also be the host on
  which zones are generated.

1.4 Server Placement

  Though the geographic diversity of server placement helps reduce the
  effects of service disruptions due to local problems, it is
  diversity of placement in the network topology which is the driving
  force behind these distribution practices.  Server placement should
  emphasize that diversity.  Ideally, servers should be placed
  topologically near the points at which the operator exchanges routes
  and traffic with other networks.

1.5 Routing

  The organization administering the mesh of servers sharing a unicast
  address must have an autonomous system number and speak BGP to its
  peers.  To those peers, the organization announces a route to the
  network containing the shared-unicast address of the name server.
  The organization's border routers must then deliver the traffic
  destined for the name server to the nearest instantiation.  Routing
  to the administrative interfaces for the servers can use the normal
  routing methods for the administering organization.

  One potential problem with using shared unicast addresses is that
  routers forwarding traffic to them may have more than one available
  route, and those routes may, in fact, reach different instances of
  the shared unicast address.  Because UDP is self-contained, UDP
  traffic from a single source reaching different instances presents
  no problem.  TCP traffic, in contrast, may fail or present
  unworkable performance characteristics in a limited set of
  circumstances.  For split-destination failures to occur, the router
  forwarding the traffic must both have equal cost routes to the two
  differentinstances and use a load sharing algorithm which does
  per-packet rather than per-destination load sharing.

  Four things mitigate the severity of this problem.  The first is
  that UDP is a fairly high proportion of the query traffic to name
  servers.  The second is that the aim of this proposal is to
  diversify topological placement; for most users, this means that the
  coordination of placement will ensure that new instances of a name
  server will be at a significantly different cost metric from
  existing instances.  Some set of users may end up in the middle, but
  that should be relatively rare.  The third is that per packet load
  sharing is only one of the possible load sharing mechanisms, and
  other mechanisms are increasing in popularity.

  Lastly, in the case where the traffic is TCP, per packet load
  sharing is used, and equal cost routes to different instances of a
  name server are available, any implementation which measures the
  performance of servers to select a preferred server will quickly
  prefer a server for which this problem does not occur.  The root
  server system distributes the root servers among multiple
  organizations, which automatically mitigates the problem by ensuring
  that no single AS is announcing all of the salient servers.  For
  authoritative servers, care must be taken that all of the servers
  for a specific zone are not participants in the same shared-unicast
  mesh.  To guard even against the case where multiple meshes have
  a set of users affected by per packet load sharing along equal cost
  routes, organizations implementing these practices should always
  provide at least one authoritative server which is not a participant
  in any shared unicast mesh.  Those deploying shared-unicast meshes
  should note that any specific host may become unreachable to a client
  should a server fail, a path fail, or the route to that host be withdrawn;
  these error conditions are not specific to shared-unicast

  Appendix A. contains an ASCII diagram of a simple implementation of
  this system.  In it, the odd numbered routers deliver traffic to the
  shared-unicast interface network and filter traffic from the
  administrative network; the even numbered routers deliver traffic to
  the administrative network and filter traffic from the shared-unicast
  network.  These are depicted as separate routers for the ease this
  gives in explanation, but they could easily be separate interfaces
  on the same router.  Similarly, a local NTP source is depicted for
  synchronization, but the level of synchronization needed would not
  require that source to be either local or a stratum one NTP server.

2. Administration

2.1 Points of Contact

   A single point of contact for reporting problems is crucial to the
   correct administration of this system.  If an external user of the
   system needs to report a problem related to the service, there must
   be no ambiguity about whom to contact.  If internal monitoring does
   not indicate a problem, the contact may, of course, need to work
   with the external user to identify which server generated the

3. Security Considerations

   As a core piece of internet infrastructure, the root servers are a
   common target of attack; authoritative name servers may also be
   targets of attack.  The practices outlined here increase the risk
   of certain kinds of attack and reduce the risk of others.

3.1 Increased Risks

3.1.1 Increase in physical servers

   The architecture outlined in this document increases the number of
   physical servers, which could increase the possibility that a
   server mis-configuration will occur which allows for a security
   breach.  In general, the entity administering a mesh should ensure
   that patches and security mechanisms applied to a single member of
   the mesh are appropriate for and applied to all of the members of a

3.1.2 Data synchronization problems

   The level of systemic synchronization described above should be
   augmented by synchronization of the data present at each of the
   servers.  While the DNS itself is a loosely coupled system,
   debugging problems with data in specific zones would be far more
   difficult if two different servers sharing a single unicast address
   might return different responses to the same query.  For example,
   if the data associated with example.com has changed and the
   administrators of the domain are testing for the changes at the
   root name servers, they should not need to check each instance of a
   named root server.  The use of ntp to provide a synchronized time
   for switch-over eliminates some aspects of this problem, but
   mechanisms to handle failure during the switchover are required.
   In particular, a server which cannot make the switchover must not
   roll-back to a previous version; it must cease to respond to
   queries so that other servers are queried.

3.1.3 Distribution risks

   If the mechanism used to distribute zone files among the servers is
   not well secured, a man-in-the-middle attack could result in the
   injection of false information.  Digital signatures will alleviate
   this risk, but encrypted transport and tight access lists are a
   necessary adjunct to them.  Since zone files will be distributed to
   the administrative interfaces of meshed servers, the access control
   list for distribution of the zone files should include the
   administrative interface of the server or servers, rather than
   their shared unicast addresses.

3.2 Decreased Risks

   The increase in number of physical servers reduces, however, the
   likelihood that a denial-of-service attack will take out a
   significant portion of the DNS infrastructure.  The increase in
   servers also reduces the effect of machine crashes, fiber cuts, and
   localized disasters by reducing the number of users dependent on
   a specific machine.

4. IANA Considerations

   Any root server operator choosing to employ the practices described
   in this document should do so in coordination with the Root Server
   System Advisory Committee.  Since the aim of this set of practices
   for root server operations is to increase the availability of root
   servers in under-served areas of the network topology, coordination
   of the deployment of new servers would also be of benefit.

5. Full copyright statement

  Copyright (C) The Internet Society 1999.  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain
  it or assist in its implementation may be prepared, copied,
  published and distributed, in whole or in part, without restriction
  of any kind, provided that the above copyright notice and this
  paragraph are included on all such copies and derivative works.
  However, this document itself may not be modified in any way, such
  as by removing the copyright notice or references to the Internet
  Society or other Internet organizations, except as needed for the
  purpose of developing Internet standards in which case the
  procedures for copyrights defined in the Internet Standards process
  must be followed, or as required to translate it into languages
  other than English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on

5. Acknowledgements

   Masataka Ohta, Bill Manning, Randy Bush, Chris Yarnell, Ray Plzak,
   Mark Andrews, Robert Elz, Geoff Houston, Bill Norton, Akira Kato,
   Suzanne Woolf, and Gunnar Lindberg all provided input and
   commentary on this work.

6. References

[1] "Root Name Server Operational Requirements". Randy Bush, Daniel
Karrenberg, Mark Kosters, Raymond Plzak,

7. Editor's address

   Ted Hardie
   Equinix, Inc.
   901 Marshall St.
   Redwood City, CA 94063
   Tel: 1.650.817.2226
   Fax: 1.650.298.0420

Appendix A.

Peer 1-|                |
Peer 2-|                |
Peer 3-|     Switch     |
Transit|                |  _________                       _________
etc    |                |--|Router1|---|----|--------------|Router2|---WAN-|
       |                |  ---------   |    |              ---------       |
       |                |              |    |                              |
       |                |              |    |                              |
       ------------------            [NTP] [DNS]                           |
       __________________                                                  |
Peer 1-|                |                                                  |
Peer 2-|                |                                                  |
Peer 3-|     Switch     |                                                  |
Transit|                |  _________                       _________       |
etc    |                |--|Router3|---|----|--------------|Router4|---WAN-|
       |                |  ---------   |    |              ---------       |
       |                |              |    |                              |
       |                |              |    |                              |
       ------------------            [NTP] [DNS]                           |
       __________________                                                  |
Peer 1-|                |                                                  |
Peer 2-|                |                                                  |
Peer 3-|     Switch     |                                                  |
Transit|                |  _________                       _________       |
etc    |                |--|Router5|---|----|--------------|Router6|---WAN-|
       |                |  ---------   |    |              ---------       |
       |                |              |    |                              |
       |                |              |    |                              |
       ------------------            [NTP] [DNS]                           |
       __________________                                                  |
Peer 1-|                |                                                  |
Peer 2-|                |                                                  |
Peer 3-|     Switch     |                                                  |
Transit|                |  _________                       _________       |
etc    |                |--|Router7|---|----|--------------|Router8|---WAN-|
       |                |  ---------   |    |              ---------
       |                |              |    |
       |                |              |    |
       ------------------            [NTP] [DNS]

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