Network Working Group S. Woolf
Internet-Draft Internet Systems Consortium, Inc.
Expires: September 6, 2006 D. Conrad
Nominum, Inc.
March 5, 2006
Requirements for a Mechanism Identifying a Name Server Instance
draft-ietf-dnsop-serverid-06
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Copyright (C) The Internet Society (2006).
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
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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.
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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.
The traditional 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 somewhat problematic. Specifically, a dedicated DNS
query may not go to the same server as answered a previous query,
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 XXX".
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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2. Existing Conventions
For some time, the commonly deployed Berkeley Internet Name Domain
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 string, replaces ".BIND" with ".SERVER".
Thus the query string to learn the unique name of a server may be
queried 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
administratively defined string which defaults to the 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.
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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.
5. Hypothetically, since it's an ordinary DNS record and the
relevant DNSSEC RRs are class independent, the id.server response
RR could be signed, which has the advantages described in
[RFC4033].
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.
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 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.
4. 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 may be 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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2.3. 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
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.
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3. 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.
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4. 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
identification.
Both authentication and confidentiality of serverid data are
potentially of interest to administrators-- that is, operators may
wish to make serverid data available and reliable to themselves and
their chosen associates only. This would imply both an ability to
authenticate it to themselves and keep it private from arbitrary
other parties. This led to Characteristics 4 and 5 of an improved
solution.
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5. 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.
6. References
[1] Mockapetris, P., "Domain Names - Concepts and Facilities",
RFC 1034, STD 0013, November 1987.
[2] Mockapetris, P., "Domain Names - Implementation and
Specification", RFC 1035, STD 0013, November 1987.
[3] Hardie, T., "Distributing Authoritative Name Servers via Shared
Unicast Addresses", RFC 3258, April 2002.
[4] ISC, "BIND 9 Configuration Reference".
[5] Austein, S., "DNS Name Server Identifier Option (NSID)",
Internet Drafts http://www.ietf.org/internet-drafts/
draft-ietf-dnsext-nsid-01.txt, January 2006.
[6] Arends, R., Austein, S., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
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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
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
US
Phone: +1 1 650 381 6003
Email: david.conrad@nominum.com
URI: http://www.nominum.com/
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