Network Working Group R. Austein
Internet-Draft ISC
Expires: August 9, 2004 February 9, January 15, 2005 July 17, 2004

EDNS NSID Extension
draft-austein-dnsext-nsid-00
draft-austein-dnsext-nsid-01

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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 enhancement to
support this functionality.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed Mechanism . . . . . . . . . . . . . . . . . . . . . . 4
2.1 The SI Flag . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 The NSID Option . . . . . . . . . . . . . . . . . . . . . . . 4
3. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 What Should the NSID Payload Be? . . . . . . . . . . . . . . . 5
3.2 Should Recursive Name Servers Respond to SI? . . . . . . . . . 5 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
Normative 8
5. References . . . . . . . . . . . . . . . . . . . . . 8
Informative . . . . . 9
5.1 Normative References . . . . . . . . . . . . . . . . . . . . 9
5.2 Informative References . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10

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1. Introduction

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 chose 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 precise format of the identifying information is still an open
issue at this point, and is discussed further in Section 3.1.

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3. Open Issues

There are a couple of open issues in this proposal which would need
to be settled before it could be used. The author has opinions on
both of these and has stated those opinions below, but would
appreciate feedback from the community.

3.1 What Should the NSID Payload Be?

There are several options 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 hash of the DNS name or IP address,
perhaps including some kind of nonce.
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 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 or "real" address 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 (it probably server
almost certainly does have at least one non-anycast IP address, address for
maintenance operations). Using the
"real" IP address operations, but assumes that the operator of an anycast the name
server is willing to divulge a its non-anycast address for the name server, address, which might
not be the case.
o Using a hash (with or without a nonce) provides 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

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restriction, since authoritative nameservers almost always have
nonvolatile storage (such as a disk drive) in any case, and in
rare cases where an authoritative 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 means that at least half of the
name servers that support this option will probably end up
identifying themselves as "My Name Server", which is not
particularly useful.

Given that one of the reasons common reason for using anycast DNS techniques is
often an
attempt to harden a critical name server against denial of service
attacks, the author believes that at least some name server operators
are likely to want an identifier other than the hash with nonce option "real" name or "real"
address of the name server instance. Given all of the issues listed
above, the best approach might be a combination of several of the
above approaches. Tentative proposal:
o Define the NSID payload to be an opaque byte string.
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 right default implementation
behavior.
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 here, since it will 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
address addresses
of anycast name servers to nogoodniks. nogoodniks, while also allowing name
server operators who do not find such leakage threatening to provide
more information at their own discretion.

This proposal begs the question of whether the NSID payload would
also need to include a type octet indicating which of these three
options the name server operator had chosen: the author suspects that
such an octet would not be necessary, but this is another subject on
which the author would welcome feedback.

3.2 Should Recursive Name Servers Respond to SI?

Most of the discussion of name server identification to date has

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focused on identifying authoritative name servers, since the best
known cases of anycast name servers are a subset of the name servers

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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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4. Acknowledgements

Steve Bellovin, Randy Bush, David Conrad, Mike Patton, Paul Vixie, Randy Bush,
Suzanne Woolf, and the law firm of Dewey, Chetham, and Howe.

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5. References

5.1 Normative References

[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.

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5.2 Informative References

[I-D.ietf-dnsop-serverid]
Conrad, D., "Identifying an Authoritative Name Server",
draft-ietf-dnsop-serverid-01 (work in progress), November
2002.

Author's Address

Rob Austein
ISC
950 Charter Street
Redwood City, CA 94063
USA

EMail: sra@isc.org

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