Network Working Group                                          T. Hardie
Request for Comments: 3258                                 Nominum, Inc.
Category: Informational                                       April 2002

  Distributing Authoritative Name Servers via Shared Unicast Addresses

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  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 Domain Name System (DNS) servers to previously
   under-served areas of the network topology and to reduce the latency
   for DNS  query responses in those areas.

1.  Introduction

   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; applicability to IPv6 is an area for further study.  It
   should also be noted that the system described here is related to
   that described in [ANYCAST], but it does not require dedicated
   address space, routing changes, or the other elements of a full
   anycast infrastructure which that document describes.

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2.  Architecture

2.1 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 authoritative.

2.2 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 transfer, 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 2.5 below.

2.3 Synchronization

   Authoritative name servers may be loosely or tightly synchronized,
   depending on the practices set by the operating organization.  As
   noted below in section 4.1.2, 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 mesh
      d) confirmation of the integrity of the zones at each server

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      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 that 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.

2.4 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.

2.5 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

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   the shared unicast address.  Applications like the DNS, whose
   communication typically consists of independent request-response
   messages each fitting in a single UDP packet present no problem.
   Other applications, in which multiple packets must reach the same
   endpoint (e.g., TCP) may fail or present unworkable performance
   characteristics in some circumstances.  Split-destination failures
   may occur when a router does per-packet (or round-robin) load
   sharing, a topology change occurs that changes the relative metrics
   of two paths to the same anycast destination, etc.

   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 DNS 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 the DNS
   failover mechanisms to reliably avoid this problem, however, those
   using shared unicast distribution mechanisms must take care 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 hosts.

   Since ICMP response packets might go to a different member of the
   mesh than that sending a packet, packets sent with a shared unicast
   source address should also avoid using path MTU discovery.

   Appendix A. contains an ASCII diagram of an example 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

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   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.

3. Administration

3.1 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 error.

4. 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 attacks and reduce the risk of

4.1 Increased Risks

4.1.1 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.

4.1.2 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

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   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 has changed and the administrators of
   the domain are testing for the changes at the
   authoritative name servers, they should not need to check each
   instance of a named authoritative 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.

4.1.3 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.

4.2 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.

5. Acknowledgments

   Masataka Ohta, Bill Manning, Randy Bush, Chris Yarnell, Ray Plzak,
   Mark Andrews, Robert Elz, Geoff Huston, Bill Norton, Akira Kato,
   Suzanne Woolf, Bernard Aboba, Casey Ajalat, and Gunnar Lindberg all
   provided input and commentary on this work.  The editor wishes to
   remember in particular the contribution of the late Scott Tucker,
   whose extensive systems experience and plain common sense both
   contributed greatly to the editor's own deployment experience and are
   missed by all who knew him.

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6. References

   [SECONDARY] Elz, R., Bush, R., Bradner, S. and M. Patton, "Selection
               and Operation of Secondary DNS Servers", BCP 16, RFC
               2182, July 1997.

   [ROOT]      Bush, R., Karrenberg, D., Kosters, M. and R. Plzak, "Root
               Name Server Operational Requirements", BCP 40, RFC 2870,
               June 2000.

   [ANYCAST]   Patridge, C., Mendez, T. and W. Milliken, "Host
               Anycasting Service", RFC 1546, November 1993.

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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]                       |

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       __________________                                              |
Peer 1-|                |                                              |
Peer 2-|                |                                              |
Peer 3-|     Switch     |                                              |
Transit|                |  _________                   _________       |
etc    |                |--|Router7|---|----|----------|Router8|---WAN-|
       |                |  ---------   |    |          ---------
       |                |              |    |
       |                |              |    |
       ------------------            [NTP] [DNS]

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7. Editor's Address

   Ted Hardie
   Nominum, Inc.
   2385 Bay Road.
   Redwood City, CA 94063

   Phone: 1.650.381.6226

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8.  Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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
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   followed, or as required to translate it into languages other than

   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 an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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