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Network Working Group                                           S. WoolfRequest for Comments: 4892             Internet Systems Consortium, Inc.Category: Informational                                        D. Conrad                                                                   ICANN                                                               June 2007Requirements for a Mechanism Identifying a Name Server InstanceStatus 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 anWoolf & 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 CLASSWoolf & 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.  References7.1.  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",RFC4033, March 2005.Woolf & Conrad               Informational                      [Page 6]

RFC 4892                        Serverid                       June 2007Authors' 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 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Woolf & Conrad               Informational                      [Page 8]