Network Working Group S. Woolf
Internet-Draft Internet Systems Consortium, Inc.
Expires: January 16, 2005 D. Conrad
Nominum, Inc.
July 18, 2004
Identifying an Authoritative Name `Server
draft-ietf-dnsop-serverid-02
Status of this Memo
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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. Existing ad
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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 disadvantasges, and to
characterize an improved mechanism.
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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 widely deployed implementation of the DNS
protocol and discusses requirements for an improved solution to the
problem.
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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.
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.
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3. Existing 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.
3.1 Advantages
There are several valuable attributes to this mechanism, which
account for its usefulness.
1. This 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. It is simple to configure. An administrator can easily turn on
this feature and control the results of the relevant query.
3. 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.
3.2 Disadvantages
At the same time, there are some forbidding drawbacks to the
VERSION.BIND 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
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purpose is a good use of the namespace or of implementation
effort.
3. It is implementation specific. BIND is one DNS implementation.
At the time of this writing, it is probably the most prevalent,
for authoritative servers anyway. This does not justify
standardizing on its ad hoc solution to a problem shared across
many operators and implementors.
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.
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4. 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.
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.
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. The identification mechanism SHOULD NOT be
implementation-specific.
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5. 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
IANA.
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6. Security Considerations
Providing identifying information as to which server is responding
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
identification.
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7. 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 draft 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.
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