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IETF DNSOPS working group                                    T. Hardie
Internet draft                                            Equinix, Inc
Category: Work-in-progress                                 April, 2001


  Distributing 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
  other groups may also distribute working documents as Internet-

  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

  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.  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.  The shared unicast system described here is specific
  to IPv4; IPv6 uses anycast differently from IPv4 and those
  differences prevent this system from being used in IPv6
  environments.  It should also be noted that the system described
  here is related to that describe in [ANYCAST], but it does not
  require dedicated address space, routing changes, or the other
  elements of a full anycast infrastructure which that document

1. Architecture

1.1 Server Requirements

  Operators of authoritative name servers may wish to refer to
  [SECONDARY] and [ROOT] for general guidance on appropriate practice
  for authoritative name servers.  In addition to proper configuration
  as a standard authoritative name server, 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 IPv4
  shared unicast address associated with the authoritative name
  server.  The other interface, referred to as the administrative
  interface below, should use a distinct IPv4 address specific to that
  host.  The host should respond to DNS queries only on the
  shared-unicast interface.  In order to provide the most consistent
  set of responses from the mesh of anycast hosts, it is good practice
  to limit responses on that interface to zones for which the host is

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

  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.

  This document presumes that the usual DNS failover methods are the
  only ones used to ensure reachability of the data for clients.  It
  does not advise that the routes be withdrawn in the case of failure;
  it advises instead the the DNS process shutdown so that servers on
  other addresses are queried.  This recommendation reflects a choice
  between performance and operational complexity.  While it would be
  possible to have some process withdraw the route for a specific
  server instance when it is not available, there is considerable
  operational complexity involved in ensuring that this occurs
  reliably.  Given the existing DNS failover methods, the marginal
  improvement in performance will not be sufficient to justify
  the additional complexity for most uses.

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
  different instances 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.  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, however, not specific to
  shared-unicast distributions, but would occur for standard 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, authoritative name
   servers are common targets of attack.  The practices outlined here
   increase the risk of certain kinds of attack and reduce the risk of

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
   mesh.  "Genetic diversity" (code from different code bases) can be
   a useful security measure in avoiding attacks based on
   vulnerabilities in a specific code base; in order to ensure
   consistency of responses from a single named server, however, that
   diversity should be applied to different shared-unicast meshes or
   between a mesh and a related unicast authoritative server.

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 www.example.com has changed and the
   administrators of the domain are testing for the changes at the
   example.com authoritative 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 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. 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, Scott Tucker, Bernard Aboba, Casey Ajalat and Gunnar
   Lindberg all provided input and commentary on this work.

6. References

[SECONDARY] "Selection and Operation of Secondary Name Servers".
R. Elz, R. Bush, S Bradner, M. Patton, BCP0016.

[ROOT] "Root Name Server Operational Requirements". R. Bush,
D. Karrenberg, M. Kosters, R. Plzak, BCP0040.

[ANYCAST] "Host Anycasting Service".  C. Patridge, T. Mendez, W. Milliken,

7. Editor's address

   Ted Hardie
   Equinix, Inc.
   2450 Bayshore Parkway
   Mountain View, CA 94043-1107
   Tel: 1.650.316.6226
   Fax: 1.650.315.6903

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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