Network Working Group                                         R. Austein
Internet-Draft                                                       ISC
Expires: January 15, 2005                                  July 17, 2004


                          EDNS NSID Extension
                      draft-austein-dnsext-nsid-01

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   This Internet-Draft will expire on January 15, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   With the increased use of DNS anycast, load balancing, and other
   mechanisms allowing more than one DNS name server to share a single
   IP address, it is sometimes difficult to tell which of a pool of name
   servers has answered a particular query.  While existing ad-hoc
   mechanism allow an operator to send follow-up queries when it is
   necessary to debug such a configuration, the only completely reliable
   way to obtain the identity of the name server which actually
   responded is to have the name server include this information in the
   response itself.  This note proposes a protocol enhancement to
   support this functionality.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Proposed Mechanism . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   The SI Flag  . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2   The NSID Option  . . . . . . . . . . . . . . . . . . . . .  4
   3.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1   What Should the NSID Payload Be? . . . . . . . . . . . . .  5
     3.2   Should Recursive Name Servers Respond to SI? . . . . . . .  6
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.1   Normative References . . . . . . . . . . . . . . . . . . . .  9
   5.2   Informative References . . . . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10




































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1.  Introduction

   With the increased use of DNS anycast, load balancing, and other
   mechanisms allowing more than one DNS name server to share a single
   IP address, it is sometimes difficult to tell which of a pool of name
   servers has answered a particular query.

   Existing ad-hoc mechanisms such as those described in
   [I-D.ietf-dnsop-serverid] allow an operator to send follow-up queries
   when it is necessary to debug such a configuration, but there are
   situations in which this is not a totally satisfactory solution,
   since anycast routing may have changed, or the server pool in
   question may be behind some kind of extremely dynamic load balancing
   hardware.  Thus, while these ad-hoc mechanisms are certainly better
   than nothing (and have the advantage of already being deployed), a
   better solution seems desirable.

   Given that a DNS query is an idempotent operation with no retained
   state, it would appear that the only completely reliable way to
   obtain the identity of the name server which actually responded to a
   particular query is to have that name server include identifying
   information in the response itself. This note proposes a protocol
   enhancement to achieve this.




























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2.  Proposed Mechanism

   This note proposes using an EDNS [RFC2671] flag bit to signal the
   resolver's desire for information identifying the name server, and an
   EDNS option to hold the name server's response (should it chose to
   honor the resolver's request).

2.1  The SI Flag

   A resolver signals its desire for information identifying the server
   by setting the SI (Send Identification) flag in the extended flags
   field of the OPT pseudo-RR.

   The value of the SI flag is [TBD].

   The semantics of the SI flag are not transitive.  That is: the SI
   flag is a request that the name server which receives the query
   identify itself; in a so-called forwarding setup, the first hop name
   server is the one that should identify itself.  If the resolver side
   of a forwarding name server wishes to receive identifying
   information, it is free to set the SI flag in its own queries, but
   that is a separate matter.

   A name server which understands the SI flag should echo its value
   back in the response message, regardless of whether the name server
   chose to honor the request.

2.2  The NSID Option

   A name server which understands the SI flag and chooses to honor it
   responds by including identifying information in a NSID option in an
   EDNS OPT pseudo-RR in the response message.

   The OPTION-CODE for the NSID option is [TBD].

   The precise format of the identifying information is still an open
   issue at this point, and is discussed further in Section 3.1.














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3.  Open Issues

   There are a couple of open issues in this proposal which would need
   to be settled before it could be used.  The author has opinions on
   both of these and has stated those opinions below, but would
   appreciate feedback from the community.

3.1  What Should the NSID Payload Be?

   There are several options for the payload of the NSID option.
   o  It could be the "real" name of the specific name server within the
      name server pool.
   o  It could be the "real" IP address (IPv4 or IPv6) of the name
      server within the name server pool.
   o  It could be some sort of hash of the DNS name or IP address,
      perhaps including some kind of nonce.
   o  It could be some sort of probabilisticly unique identifier
      initially derived from some sort of random number generator then
      preserved across reboots of the name server.
   o  It could be an arbitrary string of octets chosen at the discretion
      of the name server operator.

   Each of these options has advantages and disadvantages.
   o  Using the "real" name is simple, but assumes that the name server
      has a "real" name, which it may not.
   o  Using the "real" address is also simple, and the name server
      almost certainly does have at least one non-anycast IP address for
      maintenance operations, but assumes that the operator of the name
      server is willing to divulge its non-anycast address, which might
      not be the case.
   o  Using a hash (with or without a nonce) provides a fixed length
      value that the resolver can use to tell two name servers apart
      without necessarily being able to tell where either one of them
      "really" is, but makes debugging more difficult if one happens to
      be in a friendly open environment.  Furthermore, a nonce may not
      add much value, since a hash based on an IPv4 address still only
      involves a 32-bit search space, and DNS names used for servers
      that operators might have to debug at 4am tend not to be very
      random at all.
   o  Probabilisticly unique identifiers have similar properties to
      hashed identifiers, but (given a sufficiently good random number
      generator) are immune to the search space issues. However, the
      strength of this approach is also its weakness: there is no
      algorithmic transformation by which even the server operator can
      associate name server instances with identifiers while debugging,
      which might be annoying.  This approach also requires the name
      server instance to preserve the probabilisticly unique identifier
      across reboots, but this does not appear to be a serious



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      restriction, since authoritative nameservers almost always have
      nonvolatile storage (such as a disk drive) in any case, and in
      rare cases where an authoritative name server does not have any
      way to store such an identifier, nothing terrible will happen if
      the name server just generates a new identifier every time it
      reboots.
   o  Using an arbitrary octet string means that at least half of the
      name servers that support this option will probably end up
      identifying themselves as "My Name Server", which is not
      particularly useful.

   Given that one common reason for using anycast DNS techniques is an
   attempt to harden a critical name server against denial of service
   attacks, the author believes that at least some name server operators
   are likely to want an identifier other than the "real" name or "real"
   address of the name server instance.  Given all of the issues listed
   above, the best approach might be a combination of several of the
   above approaches.  Tentative proposal:
   o  Define the NSID payload to be an opaque byte string.
   o  Operators for whom divulging the unicast address is an issue could
      use the raw binary representation of a probabilisticly unique
      random number.  This should probably be the default implementation
      behavior.
   o  Operators for whom divulging the unicast address is not an issue
      could just use the raw binary representation of a unicast address
      for simplicity.  This would only be done via an explicit
      configuration choice by the operator.
   o  Operators who really need or want the ability to set the NSID
      payload to an arbitrary value could do so, but this would only be
      done via an explicit configuration choice by the operator.

   This approach appears to provide enough information for useful
   debugging without unintentionally leaking the maintenance addresses
   of anycast name servers to nogoodniks, while also allowing name
   server operators who do not find such leakage threatening to provide
   more information at their own discretion.

   This proposal begs the question of whether the NSID payload would
   also need to include a type octet indicating which of these three
   options the name server operator had chosen: the author suspects that
   such an octet would not be necessary, but this is another subject on
   which the author would welcome feedback.

3.2  Should Recursive Name Servers Respond to SI?

   Most of the discussion of name server identification to date has
   focused on identifying authoritative name servers, since the best
   known cases of anycast name servers are a subset of the name servers



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   for the root zone.  However, given that anycast DNS techniques are
   equally applicable to recursive name servers as well as authoritative
   name servers, it may be useful for the name server side of a
   recursive name server to support this mechanism as well.  The
   semantics proposed for the SI bit in Section 2.1 are intended to
   support this model.













































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4.  Acknowledgements

   Steve Bellovin, Randy Bush, David Conrad, Mike Patton, Paul Vixie,
   Suzanne Woolf, and the law firm of Dewey, Chetham, and Howe.















































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5.  References

5.1  Normative References

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
              2671, August 1999.

5.2  Informative References

   [I-D.ietf-dnsop-serverid]
              Conrad, D., "Identifying an Authoritative Name Server",
              draft-ietf-dnsop-serverid-01 (work in progress), November
              2002.


Author's Address

   Rob Austein
   ISC
   950 Charter Street
   Redwood City, CA  94063
   USA

   EMail: sra@isc.org



























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   Internet Society.




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