Network Working Group R. Austein
Internet-Draft ISC
Expires: January 19, 2006 July 18, 2005
EDNS NSID Extension
draft-austein-dnsext-nsid-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
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. While existing ad-hoc
mechanism allow an operator to send follow-up queries when it is
necessary to debug such a configuration, the only completely reliable
way to obtain the identity of the name server which actually
responded is to have the name server include this information in the
response itself. This note proposes a protocol extension to support
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this functionality.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed Mechanism . . . . . . . . . . . . . . . . . . . . . . 4
2.1 The SI Flag . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 The NSID Option . . . . . . . . . . . . . . . . . . . . . 4
3. What Should the NSID Payload Be? . . . . . . . . . . . . . . . 5
4. Should Recursive Name Servers Respond to SI? . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1 Normative References . . . . . . . . . . . . . . . . . . . 12
8.2 Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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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.
Existing ad-hoc mechanisms such as those described in [I-D.ietf-
dnsop-serverid] allow an operator to send follow-up queries when it
is necessary to debug such a configuration, but there are situations
in which this is not a totally satisfactory solution, since anycast
routing may have changed, or the server pool in question may be
behind some kind of extremely dynamic load balancing hardware. Thus,
while these ad-hoc mechanisms are certainly better than nothing (and
have the advantage of already being deployed), a better solution
seems desirable.
Given that a DNS query is an idempotent operation with no retained
state, it would appear that the only completely reliable way to
obtain the identity of the name server which actually responded to a
particular query is to have that name server include identifying
information in the response itself. This note proposes a protocol
enhancement to achieve this.
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2. Proposed Mechanism
This note proposes using an EDNS [RFC2671] flag bit to signal the
resolver's desire for information identifying the name server, and an
EDNS option to hold the name server's response (should it choose to
honor the resolver's request).
2.1 The SI Flag
A resolver signals its desire for information identifying the server
by setting the SI (Send Identification) flag in the extended flags
field of the OPT pseudo-RR.
The value of the SI flag is [TBD].
The semantics of the SI flag are not transitive. That is: the SI
flag is a request that the name server which receives the query
identify itself; in a so-called forwarding setup, the first hop name
server is the one that should identify itself. If the resolver side
of a forwarding name server wishes to receive identifying
information, it is free to set the SI flag in its own queries, but
that is a separate matter.
A name server which understands the SI flag should echo its value
back in the response message, regardless of whether the name server
chose to honor the request.
2.2 The NSID Option
A name server which understands the SI flag and chooses to honor it
responds by including identifying information in a NSID option in an
EDNS OPT pseudo-RR in the response message.
The OPTION-CODE for the NSID option is [TBD].
The OPTION-DATA for the NSID option is an opaque byte string the
semantics of which are deliberately left outside the protocol. See
Section 3 for discussion.
The NSID option is not transitive. A name server must not send an
NSID option back to a resolver which did not request it. In
particular, while a forwarder may choose to set the SI bit when
forwarding a query, this has no effect on the setting of the SI bit
or the presence or absence of the NSID option in the forwarder's
response to the original client.
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3. What Should the NSID Payload Be?
The syntax and semantics of the content of the NSID option is
deliberately left outside the scope of this specification. This
describe some of the kinds of data that server administrators might
choose to provide as the content of the NSID option, and explains the
reasoning (such as it is) behind choosing a simple opaque byte
string.
There are several possibilities for the payload of the NSID option.
o It could be the "real" name of the specific name server within the
name server pool.
o It could be the "real" IP address (IPv4 or IPv6) of the name
server within the name server pool.
o It could be some sort of pseudo-random number generated in a
predictable fashion somehow using the server's IP address or name
as a seed value.
o It could be some sort of probabilisticly unique identifier
initially derived from some sort of random number generator then
preserved across reboots of the name server.
o It could be some sort of dynamicly generated identifier so that
only the name server operator could even tell whether or not any
two queries had been answered by the same server.
o It could be a blob of signed data, with a corresponding key which
might (or might not) be available via DNS lookups.
o It could be a blob of encrypted data, the key for which presumably
would be restricted to parties with a need to know (in the opinion
of the server operator).
o It could be an arbitrary string of octets chosen at the discretion
of the name server operator.
Each of these options has advantages and disadvantages.
o Using the "real" name is simple, but assumes that the name server
has a "real" name, which it may not.
o Using the "real" address is also simple, and the name server
almost certainly does have at least one non-anycast IP address for
maintenance operations, but assumes that the operator of the name
server is willing to divulge its non-anycast address, which might
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not be the case.
o Given that one common reason for using anycast DNS techniques is
an attempt to harden a critical name server against denial of
service attacks, some name server operators are likely to want an
identifier other than the "real" name or "real" address of the
name server instance.
o Using a hash or pseudo-random number can provide a fixed length
value that the resolver can use to tell two name servers apart
without necessarily being able to tell where either one of them
"really" is, but makes debugging more difficult if one happens to
be in a friendly open environment. Furthermore, a nonce may not
add much value, since a hash based on an IPv4 address still only
involves a 32-bit search space, and DNS names used for servers
that operators might have to debug at 4am tend not to be very
random at all.
o Probabilisticly unique identifiers have similar properties to
hashed identifiers, but (given a sufficiently good random number
generator) are immune to the search space issues. However, the
strength of this approach is also its weakness: there is no
algorithmic transformation by which even the server operator can
associate name server instances with identifiers while debugging,
which might be annoying. This approach also requires the name
server instance to preserve the probabilisticly unique identifier
across reboots, but this does not appear to be a serious
restriction, since authoritative nameservers almost always have
nonvolatile storage (such as a disk drive) in any case, and in
rare cases where a name server does not have any way to store such
an identifier, nothing terrible will happen if the name server
just generates a new identifier every time it reboots.
o Using an arbitrary octet string gives name server operators yet
another thing to configure, or mis-configure, or forget to
configure. Having all the nodes in an anycast name server
constellation identify themselves as "My Name Server" would not be
particularly useful.
Given all of the issues listed above, the best approach might be:
o Define the NSID payload to be an opaque byte string, as specified
in Section 2.2.
o Operators for whom divulging the unicast address is an issue could
use the raw binary representation of a probabilisticly unique
random number. This should probably be the default implementation
behavior.
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o Operators for whom divulging the unicast address is not an issue
could just use the raw binary representation of a unicast address
for simplicity. This would only be done via an explicit
configuration choice by the operator.
o Operators who really need or want the ability to set the NSID
payload to an arbitrary value could do so, but this would only be
done via an explicit configuration choice by the operator.
This approach appears to provide enough information for useful
debugging without unintentionally leaking the maintenance addresses
of anycast name servers to nogoodniks, while also allowing name
server operators who do not find such leakage threatening to provide
more information at their own discretion.
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4. Should Recursive Name Servers Respond to SI?
Most of the discussion of name server identification to date has
focused on identifying authoritative name servers, since the best
known cases of anycast name servers are a subset of the name servers
for the root zone. However, given that anycast DNS techniques are
equally applicable to recursive name servers as well as authoritative
name servers, it may be useful for the name server side of a
recursive name server to support this mechanism as well. The
semantics proposed for the SI bit in Section 2.1 are intended to
support this model.
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5. IANA Considerations
This mechanism requires allocation of one EDNS flag bit for the SI
flag (Section 2.1).
This mechanism requires allocation of one ENDS option code for the
NSID option (Section 2.2).
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6. Security Considerations
This document describes a channel signaling mechanism, intended
primarily for debugging. Channel signaling mechanisms are outside
the scope of DNSSEC per se. Thus, applications that require
integrity protection for the data being signaled will need to use a
channel security mechanism such as TSIG [RFC2845].
Section 3 discusses a number of different kinds of information that a
name server operator might choose to provide as the value of the NSID
option. Some of these kinds of information are security sensitive in
some environments. This specification deliberately leaves the syntax
and semantics of the NSID option content up to the implementation and
the name server operator.
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7. Acknowledgements
Joe Abley, Harald Alvestrand, Roy Arends, Steve Bellovin, Randy Bush,
David Conrad, Johan Ihren, Mike Patton, Paul Vixie, Sam Weiler,
Suzanne Woolf, and the law firm of Dewey, Chetham, and Howe.
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8. References
8.1 Normative References
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
8.2 Informative References
[I-D.ietf-dnsop-serverid]
Conrad, D., "Identifying an Authoritative Name Server",
draft-ietf-dnsop-serverid-04 (work in progress),
March 2005.
Author's Address
Rob Austein
ISC
950 Charter Street
Redwood City, CA 94063
USA
Email: sra@isc.org
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