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rfc:rfc4892

Network Working Group S. Woolf Request for Comments: 4892 Internet Systems Consortium, Inc. Category: Informational D. Conrad

                                                                 ICANN
                                                             June 2007
  Requirements for a Mechanism Identifying a Name Server Instance

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 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.  A standardized mechanism to
 determine the identity of a name server responding to a particular
 query would be useful, particularly as a diagnostic aid for
 administrators.  Existing ad hoc mechanisms for addressing this need
 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
 some widely deployed implementations of the DNS protocol, including
 advantages and disadvantages, and discusses some attributes of an
 improved 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 in similar and other situations.  Furthermore, the ability
 to identify which server is responding to a query has become more
 useful as DNS has become more critical to more Internet users, and as
 network and server deployment topologies have become more complex.

Woolf & Conrad Informational [Page 1] RFC 4892 Serverid June 2007

 The conventional means for determining which of several possible
 servers is answering a query has traditionally been based on the use
 of the server's IP address as a unique identifier.  However, the
 modern Internet has seen the deployment of various load balancing,
 fault-tolerance, or attack-resistance schemes such as shared use of
 unicast IP addresses as documented in [RFC3258].  An unfortunate side
 effect of these schemes has been to make the use of IP addresses as
 identifiers associated with DNS (or any other) service somewhat
 problematic.  Specifically, multiple dedicated DNS queries may not go
 to the same server even though sent to the same IP address.  Non-DNS
 methods such as ICMP ping, TCP connections, or non-DNS UDP packets
 (such as those generated by tools like "traceroute"), etc., may well
 be even less certain to reach the same server as the one 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 "server that answered the query I sent to IP address A.B.C.D".
 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

 For some time, the commonly deployed Berkeley Internet Name Domain
 (BIND) implementation of the DNS protocol suite from the Internet
 Systems Consortium [BIND] has supported a way of identifying a
 particular server via the use of a standards-compliant, if somewhat
 unusual, DNS query.  Specifically, a query to a recent 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.  (The value defaults to the result 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 the BIND-based mechanism, which dropped the
 implementation-specific label, replaces "BIND." with "SERVER.".  Thus
 the query label to learn the unique name of a server may appear as
 "ID.SERVER.".
 (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 CHAOS TXT RR for this name will return an

Woolf & Conrad Informational [Page 2] RFC 4892 Serverid June 2007

 administratively defined string which defaults to the software
 version of the server responding.  This is, however, not generally
 implemented by other vendors.)

2.1. Advantages

 There are several valuable attributes to this mechanism, which
 account for its usefulness.
 1.  The "HOSTNAME.BIND." or "ID.SERVER." 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 system 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 is likely to
     reach the same server over a path subject to the same routing,
     resource, and security policy as the query, without 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 serious drawbacks to the CHAOS/TXT
 query mechanism that argue against standardizing it as it currently
 operates.
 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
     effort.

Woolf & Conrad Informational [Page 3] RFC 4892 Serverid June 2007

 3.  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 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 aforementioned refinement changes
     the query label but preserves the ad hoc CHAOS/TXT mechanism.
 4.  There is no convention or shared understanding of what
     information an answer to such a query for a server identity could
     or should contain, including a possible encoding or
     authentication mechanism.
 5.  Hypothetically, since DNSSEC has been defined to cover all DNS
     classes, the TXT RRs returned in response to the "ID.SERVER."
     query could be signed, which has the advantages described in
     [RFC4033].  However, since DNSSEC deployment for the CHAOS class
     is neither existent nor foreseeable, and since the "ID.SERVER."
     TXT RR is expected to be unique per server, this would be
     impossible in practice.
 The first of the listed disadvantages may be technically the most
 serious.  It argues for an attempt to design a good answer to the
 problem, "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
 extensions:
 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

Woolf & Conrad Informational [Page 4] RFC 4892 Serverid June 2007

     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 and considered sensitive by the operator, such as a
     hostname or unicast IP address maintained for administrative
     purposes.
 5.  It should be possible to authenticate the received data by some
     mechanism analogous to those provided by DNSSEC.  In this
     context, the need could be met by including encryption options in
     the specification of a new mechanism.
 6.  The identification mechanism should not be implementation-
     specific.

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.  A proposed extension, specified and adopted by normal IETF
 process, is described in [NSID], including relevant IANA action.

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 server identification 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 the 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
 identification.

Woolf & Conrad Informational [Page 5] RFC 4892 Serverid June 2007

 Both authentication and confidentiality of server identification data
 are potentially of interest to administrators -- that is, operators
 may wish to make such data available and reliable to themselves and
 their chosen associates only.  This constraint would imply both an
 ability to authenticate it to themselves and to keep it private from
 arbitrary other parties, which leads to characteristics 4 and 5 of an
 improved solution.

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 versions 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.

7. References

7.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
            Specification", STD 13, RFC 1035, November 1987.
 [RFC3258]  Hardie, T., "Distributing Authoritative Name Servers via
            Shared Unicast Addresses", RFC 3258, April 2002.

7.2. Informative References

 [BIND]     ISC, "BIND 9 Configuration Reference".
 [NSID]     Austein, R., "DNS Name Server Identifier Option (NSID)",
            Work in Progress, June 2006.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements", RFC
            4033, March 2005.

Woolf & Conrad Informational [Page 6] RFC 4892 Serverid June 2007

Authors' Addresses

 Suzanne Woolf
 Internet Systems Consortium, Inc.
 950 Charter Street
 Redwood City, CA  94063
 US
 Phone: +1 650 423-1333
 EMail: woolf@isc.org
 URI:   http://www.isc.org/
 David Conrad
 ICANN
 4676 Admiralty Way
 Marina del Rey, CA  90292
 US
 Phone: +1 310 823 9358
 EMail: david.conrad@icann.org
 URI:   http://www.iana.org/

Woolf & Conrad Informational [Page 7] RFC 4892 Serverid June 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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

Woolf & Conrad Informational [Page 8]

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