DNS Name Server Identifier (NSID) Option
RFC 5001

Network Working Group                                         R. Austein
Request for Comments: 5001                                           ISC
Category: Standards Track                                    August 2007

                DNS Name Server Identifier (NSID) Option

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

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
   mechanisms 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 that responded is to
   have the name server include this information in the response itself.
   This note defines a protocol extension to support this functionality.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Reserved Words . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Resolver Behavior  . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Name Server Behavior . . . . . . . . . . . . . . . . . . .  3
     2.3.  The NSID Option  . . . . . . . . . . . . . . . . . . . . .  4
     2.4.  Presentation Format  . . . . . . . . . . . . . . . . . . .  4
   3.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  The NSID Payload . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  NSID Is Not Transitive . . . . . . . . . . . . . . . . . .  7
     3.3.  User Interface Issues  . . . . . . . . . . . . . . . . . .  7
     3.4.  Truncation . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10

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 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 that responded to a particular
   query is to have that name server include identifying information in
   the response itself.  This note defines a protocol enhancement to
   achieve this.

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1.1.  Reserved Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Protocol

   This note uses an EDNS [RFC2671] option to signal the resolver's
   desire for information identifying the name server and to hold the
   name server's response, if any.

2.1.  Resolver Behavior

   A resolver signals its desire for information identifying a name
   server by sending an empty NSID option (Section 2.3) in an EDNS OPT
   pseudo-RR in the query message.

   The resolver MUST NOT include any NSID payload data in the query
   message.

   The semantics of an NSID request are not transitive.  That is: the
   presence of an NSID option in a query is a request that the name
   server which receives the query identify itself.  If the name server
   side of a recursive name server receives an NSID request, the client
   is asking the recursive name server to identify itself; if the
   resolver side of the recursive name server wishes to receive
   identifying information, it is free to add NSID requests in its own
   queries, but that is a separate matter.

2.2.  Name Server Behavior

   A name server that understands the NSID option and chooses to honor a
   particular NSID request responds by including identifying information
   in a NSID option (Section 2.3) in an EDNS OPT pseudo-RR in the
   response message.

   The name server MUST ignore any NSID payload data that might be
   present in the query message.

   The NSID option is not transitive.  A name server MUST NOT send an
   NSID option back to a resolver which did not request it.  In
   particular, while a recursive name server may choose to add an NSID
   option when sending a query, this has no effect on the presence or
   absence of the NSID option in the recursive name server's response to
   the original client.

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   As stated in Section 2.1, this mechanism is not restricted to
   authoritative name servers; the semantics are intended to be equally
   applicable to recursive name servers.

2.3.  The NSID Option

   The OPTION-CODE for the NSID option is 3.

   The OPTION-DATA for the NSID option is an opaque byte string, the
   semantics of which are deliberately left outside the protocol.  See
   Section 3.1 for discussion.

2.4.  Presentation Format

   User interfaces MUST read and write the contents of the NSID option
   as a sequence of hexadecimal digits, two digits per payload octet.

   The NSID payload is binary data.  Any comparison between NSID
   payloads MUST be a comparison of the raw binary data.  Copy
   operations MUST NOT assume that the raw NSID payload is null-
   terminated.  Any resemblance between raw NSID payload data and any
   form of text is purely a convenience, and does not change the
   underlying nature of the payload data.

   See Section 3.3 for discussion.

3.  Discussion

   This section discusses certain aspects of the protocol and explains
   considerations that led to the chosen design.

3.1.  The NSID Payload

   The syntax and semantics of the content of the NSID option are
   deliberately left outside the scope of this specification.

   Choosing the NSID content is a prerogative of the server
   administrator.  The server administrator might choose to encode the
   NSID content in such a way that the server operator (or clients
   authorized by the server operator) can decode the NSID content to
   obtain more information than other clients can.  Alternatively, the
   server operator might choose unencoded NSID content that is equally
   meaningful to any client.

   This section describes some of the kinds of data that server
   administrators might choose to provide as the content of the NSID
   option, and explains the reasoning behind specifying a simple opaque
   byte string in Section 2.3.

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   There are several possibilities 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 pseudo-random number generated in a
      predictable fashion somehow using the server's IP address or name
      as a seed value.

   o  It could be some sort of probabilistically unique identifier
      initially derived from some sort of random number generator then
      preserved across reboots of the name server.

   o  It could be some sort of dynamically generated identifier so that
      only the name server operator could tell whether or not any two
      queries had been answered by the same server.

   o  It could be a blob of signed data, with a corresponding key which
      might (or might not) be available via DNS lookups.

   o  It could be a blob of encrypted data, the key for which could be
      restricted to parties with a need to know (in the opinion of the
      server operator).

   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 the name server may not have
      a "real" name.

   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 the operator of the name server may
      not be willing to divulge its non-anycast address.

   o  Given that one common reason for using anycast DNS techniques is
      an attempt to harden a critical name server against denial of
      service attacks, some name server operators are likely to want an
      identifier other than the "real" name or "real" address of the
      name server instance.

   o  Using a hash or pseudo-random number can provide a fixed length
      value that the resolver can use to tell two name servers apart

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      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, hashing might 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.

   o  Probabilistically unique identifiers have properties similar 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 probabilistically unique
      identifier across reboots, but this does not appear to be a
      serious restriction, since authoritative nameservers almost always
      have some form of non-volatile storage.  In the rare case of a
      name server that does not have any way to store such an
      identifier, nothing terrible will happen if the name server
      generates a new identifier every time it reboots.

   o  Using an arbitrary octet string gives name server operators yet
      another setting to configure, or mis-configure, or forget to
      configure.  Having all the nodes in an anycast name server
      constellation identify themselves as "My Name Server" would not be
      particularly useful.

   o  A signed blob is not particularly useful as an NSID payload unless
      the signed data is dynamic and includes some kind of replay
      protection, such as a timestamp or some kind of data identifying
      the requestor.  Signed blobs that meet these criteria could
      conceivably be useful in some situations but would require
      detailed security analysis beyond the scope of this document.

   o  A static encrypted blob would not be particularly useful, as it
      would be subject to replay attacks and would, in effect, just be a
      random number to any party that does not possess the decryption
      key.  Dynamic encrypted blobs could conceivably be useful in some
      situations but, as with signed blobs, dynamic encrypted blobs
      would require detailed security analysis beyond the scope of this
      document.

   Given all of the issues listed above, there does not appear to be a
   single solution that will meet all needs.  Section 2.3 therefore
   defines the NSID payload to be an opaque byte string and leaves the
   choice of payload up to the implementor and name server operator.

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   The following guidelines may be useful to implementors and server
   operators:

   o  Operators for whom divulging the unicast address is an issue could
      use the raw binary representation of a probabilistically 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 should 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 should 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.

3.2.  NSID Is Not Transitive

   As specified in Section 2.1 and Section 2.2, the NSID option is not
   transitive.  This is strictly a hop-by-hop mechanism.

   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
   for the root zone.  However, given that anycast DNS techniques are
   also applicable to recursive name servers, the mechanism may also be
   useful with recursive name servers.  The hop-by-hop semantics support
   this.

   While there might be some utility in having a transitive variant of
   this mechanism (so that, for example, a stub resolver could ask a
   recursive server to tell it which authoritative name server provided
   a particular answer to the recursive name server), the semantics of
   such a variant would be more complicated, and are left for future
   work.

3.3.  User Interface Issues

   Given the range of possible payload contents described in
   Section 3.1, it is not possible to define a single presentation
   format for the NSID payload that is efficient, convenient,

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   unambiguous, and aesthetically pleasing.  In particular, while it is
   tempting to use a presentation format that uses some form of textual
   strings, attempting to support this would significantly complicate
   what's intended to be a very simple debugging mechanism.

   In some cases the content of the NSID payload may be binary data
   meaningful only to the name server operator, and may not be
   meaningful to the user or application, but the user or application
   must be able to capture the entire content anyway in order for it to
   be useful.  Thus, the presentation format must support arbitrary
   binary data.

   In cases where the name server operator derives the NSID payload from
   textual data, a textual form such as US-ASCII or UTF-8 strings might
   at first glance seem easier for a user to deal with.  There are,
   however, a number of complex issues involving internationalized text
   which, if fully addressed here, would require a set of rules
   significantly longer than the rest of this specification.  See
   [RFC2277] for an overview of some of these issues.

   It is much more important for the NSID payload data to be passed
   unambiguously from server administrator to user and back again than
   it is for the payload data to be pretty while in transit.  In
   particular, it's critical that it be straightforward for a user to
   cut and paste an exact copy of the NSID payload output by a debugging
   tool into other formats such as email messages or web forms without
   distortion.  Hexadecimal strings, while ugly, are also robust.

3.4.  Truncation

   In some cases, adding the NSID option to a response message may
   trigger message truncation.  This specification does not change the
   rules for DNS message truncation in any way, but implementors will
   need to pay attention to this issue.

   Including the NSID option in a response is always optional, so this
   specification never requires name servers to truncate response
   messages.

   By definition, a resolver that requests NSID responses also supports
   EDNS, so a resolver that requests NSID responses can also use the
   "sender's UDP payload size" field of the OPT pseudo-RR to signal a
   receive buffer size large enough to make truncation unlikely.

4.  IANA Considerations

   IANA has allocated EDNS option code 3 for the NSID option
   (Section 2.3).

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5.  Security Considerations

   This document describes a channel signaling mechanism intended
   primarily for debugging.  Channel signaling mechanisms are outside
   the scope of DNSSEC, per se.  Applications that require integrity
   protection for the data being signaled will need to use a channel
   security mechanism such as TSIG [RFC2845].

   Section 3.1 discusses a number of different kinds of information that
   a name server operator might choose to provide as the value of the
   NSID option.  Some of these kinds of information are security
   sensitive in some environments.  This specification deliberately
   leaves the syntax and semantics of the NSID option content up to the
   implementation and the name server operator.

   Two of the possible kinds of payload data discussed in Section 3.1
   involve a digital signature and encryption, respectively.  While this
   specification discusses some of the pitfalls that might lurk for
   careless users of these kinds of payload data, full analysis of the
   issues that would be involved in these kinds of payload data would
   require knowledge of the content to be signed or encrypted,
   algorithms to be used, and so forth, which is beyond the scope of
   this document.  Implementors should seek competent advice before
   attempting to use these kinds of NSID payloads.

6.  Acknowledgements

   Thanks to: Joe Abley, Harald Alvestrand, Dean Anderson, Mark Andrews,
   Roy Arends, Steve Bellovin, Alex Bligh, Randy Bush, David Conrad,
   John Dickinson, Alfred Hoenes, Johan Ihren, Daniel Karrenberg, Peter
   Koch, William Leibzon, Ed Lewis, Thomas Narten, Mike Patton, Geoffrey
   Sisson, Andrew Sullivan, Mike StJohns, Tom Taylor, Paul Vixie, Sam
   Weiler, and Suzanne Woolf, none of whom are responsible for what the
   author did with their comments and suggestions.  Apologies to anyone
   inadvertently omitted from the above list.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP 14, March 1997.

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

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   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, May 2000.

7.2.  Informative References

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", RFC 2277, BCP 18, January 1998.

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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