Network Working Group R. Austein
Internet-Draft ISC
Expires: August 9, 2004 February 9, 2004
EDNS NSID Extension
draft-austein-dnsext-nsid-00
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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. 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
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . 8
Informative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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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 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.
It could be the "real" name of the specific name server within the
name server pool.
It could be the "real" IP address (IPv4 or IPv6) of the name
server within the name server pool.
It could be some sort of hash of the DNS name or IP address,
perhaps including some kind of nonce.
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. Using the
"real" name or "real" address is simple, but assumes that the name
server has a "real" name (it probably does have at least one
non-anycast IP address, for maintenance operations). Using the
"real" IP address assumes that the operator of an anycast name server
is willing to divulge a non-anycast address for the name server,
which might not be the case. 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. 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 for using anycast DNS techniques is
often an attempt to harden a critical name server against denial of
service attacks, the author believes that the hash with nonce option
is probably the right choice here, since it will provide enough
information for useful debugging without leaking the maintenance
address of anycast name servers to nogoodniks.
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
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
David Conrad, Paul Vixie, Randy Bush, Suzanne Woolf, and the law firm
of Dewey, Chetham, and Howe.
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Normative References
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
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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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