Network Working Group                                           S. Woolf
Internet-Draft                         Internet Systems Consortium, Inc.
Expires: September 14, 2005 April 30, 2006                                        D. Conrad
                                                           Nominum, Inc.
                                                          March 13,
                                                        October 27, 2005

    Requirements for a Mechanism Identifying an Authoritative a Name Server
                      draft-ietf-dnsop-serverid-04 Instance

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   This document is an Internet-Draft and is subject to all provisions
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   Copyright (C) The Internet Society (2005).


   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.  A standardized mechanism to
   determine the identity of a name server responding to a particular
   query would be useful, particularly as a diagnostic aid. aid for
   administrators.  Existing ad hoc mechanisms for addressing this
   concern are not adequate.  This
   document attempts to describe the common ad hoc solution to this
   problem, including its advantages and disadvantages, and to
   characterize an improved mechanism.

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.  A standardized mechanism to
   determine the identity of a name server responding to a particular
   query would be useful, particularly as a diagnostic aid.

   Unfortunately, existing ad-hoc mechanisms for providing such
   identification have some shortcomings, not the least of which is the lack of
   prior analysis of exactly how such a mechanism should be designed and
   deployed.  This document describes the existing convention used in one
   some widely deployed implementation implementations of the DNS
   protocol protocol, including
   advantages and disadvantages, and discusses requirements for some attributes of an
   improved solution to the

2. mechanism.

1.  Introduction and Rationale

   Identifying which name server is responding to queries is often
   useful, particularly in attempting to diagnose name server
   difficulties.  This is most obviously useful for authoritative
   nameservers in the attempt to diagnose the source or prevalence of
   inaccurate data, but can also conceivably be useful for caching
   resolvers.  However, relying on the IP address of the name server has
   become more problematic due the deployment of various load balancing
   solutions, including the use of shared unicast addresses as
   documented in [RFC3258].

   An unfortunate side effect of these load balancing solutions, and
   some changes in management practices as the public Internet has
   evolved, is that traditional methods of determining which server is
   responding can be unreliable.  Specifically, non-DNS methods such as
   ICMP ping, TCP connections, or non-DNS UDP packets (such as those
   generated by tools such as like "traceroute"), etc., can end up going to a
   different server than that which receives the DNS queries.

   There is a well-known and frequently-used technique for determining
   an identity for a nameserver more specific than the
   possibly-non-unique possibly-non-
   unique "server that answered my query".  The widespread use of the
   existing convention suggests a need for a documented, interoperable
   means of querying the identity of a nameserver that may be part of an
   anycast or load-balancing cluster.  At the same time, however, it
   also has some drawbacks that argue against standardizing it as it's
   been practiced so far.


2.  Existing Conventions

   Recent versions of the commonly deployed Berkeley Internet Name
   Domain implementation of the DNS protocol suite from the Internet
   Systems Consortium [BIND] support a way of identifying a particular
   server via the use of a standard, standards-compliant, if somewhat unusual, DNS
   query.  Specifically, a query to a late model BIND server for a TXT
   resource record in class 3 (CHAOS) for the domain name
   "HOSTNAME.BIND." will return a string that can be configured by the
   name server administrator to provide a unique identifier for the
   responding server (defaulting to the value of a gethostname() call).
   This mechanism, which is an extension of the BIND convention of using
   CHAOS class TXT RR queries to sub-domains of the "BIND." domain for
   version information, has been copied by several name server vendors.


   A refinement to this, which dropped the implementation-specific
   string, replaces ".BIND" with ".ID".  Thus the query string to learn
   the unique name of a server may be queried as "SERVER.ID".

   (For reference, the other well-known name used by recent versions of
   BIND within the CHAOS class "BIND." domain is "VERSION.BIND."  A
   query for a TXT RR for this name will return an administratively
   defined string which defaults to the version of the server
   responding.  This is, however, not generally implemented by other


2.1.  Advantages

   There are several valuable attributes to this mechanism, which
   account for its usefulness.

   1.  The "hostname.bind" or "" query response mechanism is
       within the DNS protocol itself.  An identification mechanism that
       relies on the DNS protocol is more likely to be successful
       (although not guaranteed) in going to the same machine as a
       "normal" DNS query.

   2.  Since the identity information is requested and returned within
       the DNS protocol, it doesn't require allowing any other query
       mechanism to the server, such as holes in firewalls for
       otherwise-unallowed ICMP Echo requests.  Thus it does not require
       any special exceptions to site security policy.

   3.  It is simple to configure.  An administrator can easily turn on
       this feature and control the results of the relevant query.

   4.  It allows the administrator complete control of what information
       is given out in the response, minimizing passive leakage of
       implementation or configuration details.  Such details are often
       considered sensitive by infrastructure operators.


2.2.  Disadvantages

   At the same time, there are some forbidding serious drawbacks to the
   query mechanism that argue against standardizing it as it currently

   1.  It requires an additional query to correlate between the answer
       to a DNS query under normal conditions and the supposed identity
       of the server receiving the query.  There are a number of
       situations in which this simply isn't reliable.

   2.  It reserves an entire class in the DNS (CHAOS) for what amounts
       to one zone.  While CHAOS class is defined in [RFC1034] and
       [RFC1035], it's not clear that supporting it solely for this
       purpose is a good use of the namespace or of implementation

   3.  It  The initial and still common form, using .BIND, is implementation
       specific.  BIND is one DNS implementation.  At the time of this
       writing, it is probably the most prevalent for authoritative
       servers.  This does not justify standardizing on its ad hoc
       solution to a problem shared across many operators and
       implementors.  Meanwhile, the proposed refinement changes the
       string but preserves the ad hoc CHAOS/TXT mechanism.

   There is no convention or shared understanding of what information an
   answer to such a query for a server identity could or should include,
   including a possible encoding or authentication mechanism.

   The first of the listed disadvantages is technically the most
   serious.  It argues for an attempt to design a good answer to the
   problem that "I need to know what nameserver is answering my
   queries", not simply a convenient one.


3.  Characteristics of an Implementation Neutral Convention

   The discussion above of advantages and disadvantages to the
   HOSTNAME.BIND mechanism suggest some requirements for a better
   solution to the server identification problem.  These are summarized
   here as guidelines for any effort to provide appropriate protocol

   1.  The mechanism adopted MUST be in-band for the DNS protocol.  That
       is, it needs to allow the query for the server's identifying
       information to be part of a normal, operational query.  It SHOULD
       also permit a separate, dedicated query for the server's
       identifying information.  But it should preserve the ability of
       the CHAOS/TXT query-based mechanism to work through firewalls and
       in other situations where only DNS can be relied upon to reach
       the server of interest.

   2.  The new mechanism SHOULD not require dedicated namespaces or
       other reserved values outside of the existing protocol mechanisms
       for these, i.e. the OPT pseudo-RR.  In particular, it should not
       propagate the existing drawback of requiring support for a CLASS
       and top level domain in the authoritative server (or the querying
       tool) to be useful.

   3.  Support for the identification functionality SHOULD be easy to
       implement and easy to enable.  It MUST be easy to disable and
       SHOULD lend itself to access controls on who can query for it.

   4.  It should be possible to return a unique identifier for a server
       without requiring the exposure of information that may be
       non-public non-
       public and considered sensitive by the operator, such as a
       hostname or unicast IP address maintained for administrative

   5.  The identification mechanism SHOULD NOT be

5. implementation-

4.  IANA Considerations

   This document proposes no specific IANA action.  Protocol extensions,
   if any, to meet the requirements described are out of scope for this
   document.  Should such extensions be specified and adopted by normal
   IETF process, the specification will include appropriate guidance to


5.  Security Considerations

   Providing identifying information as to which server is responding to
   a particular query from a particular location in the Internet can be
   seen as information leakage and thus a security risk.  This motivates
   the suggestion above that a new mechanism for server identification
   allow the administrator to disable the functionality altogether or
   partially restrict availability of the data.  It also suggests that
   the serverid data should not be readily correlated with a hostname or
   unicast IP address that may be considered private to the nameserver
   operator's management infrastructure.

   Propagation of protocol or service meta-data can sometimes expose the
   application to denial of service or other attack.  As DNS is a
   critically important infrastructure service for the production
   Internet, extra care needs to be taken against this risk for
   designers, implementors, and operators of a new mechanism for server


6.  Acknowledgements

   The technique for host identification documented here was initially
   implemented by Paul Vixie of the Internet Software Consortium in the
   Berkeley Internet Name Daemon package.  Comments and questions on
   earlier drafts were provided by Bob Halley, Brian Wellington, Andreas
   Gustafsson, Ted Hardie, Chris Yarnell, Randy Bush, and members of the
   ICANN Root Server System Advisory Committee.  The newest version
   takes a significantly different direction from previous versions,
   owing to discussion among contributors to the DNSOP working group and
   others, particularly Olafur Gudmundsson, Ed Lewis, Bill Manning, Sam
   Weiler, and Rob Austein.

Authors' Addresses

   Suzanne Woolf
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423-1333

   David Conrad
   Nominum, Inc.
   2385 Bay Road
   Redwood City, CA  94063

   Phone: +1 1 650 381 6003

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