INTERNET-DRAFT David Conrad
draft-ietf-dnsop-serverid-00.txt Nominum, Inc.
May, 2002
Identifying an Authoritative Name Server
Status of this Memo
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Abstract
A standardized mechanism to determine the identity of a name server
responding to a particular query would be useful, particularly as a
diagnostic aid. This document describes an identification convention
used in one widely deployed implementation of the DNS protocol and
proposes a slight modification to that convention aimed at addressing
some implementation concerns.
1. Introduction
Determining the identity of the name server responding to a query has
become more complex due primarily to the proliferation of various
load balancing techniques. This document describes a convention used
by one particular DNS server implementation to provide identifying
information and proposes a slight modification to that convention to
address concerns regarding implementation neutrality.
Note that this document makes no value judgements as to whether or
not the convention in current use is good or bad; it merely documents
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the covention's existence and proposes a slight redefinition of the
convention to address non-technical implementation concerns.
2. Rationale
Identifying which name server is responding to queries is often
useful, particularly in attempting to diagnose name server
difficulties. 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 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 (e.g., as generated by tools such
as "traceroute"), etc., can end up going to a different server than
that which receives the DNS queries.
This proposal makes the assumption that 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.
3. Historical Conventions
Recent versions of the commonly deployed Berkeley Internet Name
Domain implementation of the DNS protocol suite from the Internet
Software Consortium [BIND] support a way of identifying a particular
server via the use of a standard, 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.
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 re-
definable string which defaults to the version of the server
responding.
4. An Implementation Neutral Convention
The previously described use of the CHAOS class "BIND." domain has
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rightly been viewed by many implementors as not being standardized
nor being implementation neutral. As such, a standard mechanism to
identify a particular machine among a shared unicast set of machines
serving the same DNS data does not currently exist.
Since a name server conforming to [RFC1034] and [RFC1035] should
support the CHAOS class and the use of TXT resource record queries in
the CHAOS class to derive information about a name server has been
used in several independent name server implementations, the quickest
way of supporting the identification of a particular name server out
of a set of name servers all sharing the same unicast prefix would
likely be to standardize on the BIND convention, albeit with a slight
modification to address implementation neutrality concerns.
The convention proposed here simply redefines the top level CHAOS
domain to be "SERVER." instead of "BIND.". Since using the actual
hostname may be considered an information leakage security risk, the
use of the actual hostname of the server is discouraged and instead a
unique per-server identifier should be used. As the BIND convention
of "HOSTNAME" implies the use of a hostname, the domain name
"ID.SERVER" is proposed. That is, a TXT RR query for "ID.SERVER." in
the CHAOS class will return an administratively defined string that
can be used to differentiate among multiple servers.
To make this convention useful, DNS operators wishing to identify
their servers MUST put a unique string for the RDATA of the TXT
record associated with the "ID.SERVER." domain in class CHAOS.
Implementors MUST provide a way to disable returning identifying
information. Implementors SHOULD provide a way to limit who can
query for the identifying information.
The use of other names in the CHAOS class "SERVER." domain are beyond
the scope of this document.
IANA Considerations
The "SERVER." domain in the CHAOS class should be reserved by IANA
and a registry should be created that reserves the "ID" name. In the
future, requests may be submitted for other sub-domains of "SERVER.",
e.g., "VERSION.SERVER." and the IANA should take appropriate action.
Security Considerations
Providing identifying information as to which server is responding
can be seen as information leakage and thus a security risk. It may
be appropriate to restrict who can query for the "ID.SERVER."
domain. Filtering on source address would be one way in which
restrictions can be applied.
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The identifer returned via an "ID.SERVER." query SHOULD NOT contain
the hostname or other information that could be considered sensitive.
Acknowledgements
The technique for host identification documented here derive from
practices implemented by Paul Vixie of the Internet Software
Consortium in the Berkeley Internet Name Domain package. Useful
comments on earlier drafts were provided by Bob Halley, Brian
Wellington, Andreas Gustafsson, Ted Hardie, Chris Yarnell, and
members of the ICANN Root Server System Advisory Council.
References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3258] Hardie, T., "Distributing Authoritative Name Servers via
Shared Unicast Addresses", RFC 3258, April, 2002.
Author's Address
David Conrad
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
USA
Phone: +1 650 381 6003
Fax: +1 650 381 6055
Email: david.conrad@nominum.com
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