Network Working Group R. Austein
Internet-Draft ISC
Expires: March 13, 2006 September 9, 2005
DNS Name Server Identifier Option (NSID)
draft-ietf-dnsext-nsid-00
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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 responded is to
have the name server include this information in the response itself.
This note defines a protocol extension to support this functionality.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Reserved Words . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 The SI Flag . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 The NSID Option . . . . . . . . . . . . . . . . . . . . . 4
2.3 Presentation Format . . . . . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 The NSID Payload . . . . . . . . . . . . . . . . . . . . . 5
3.2 SI and NSID Are Not Transitive . . . . . . . . . . . . . . 7
3.3 User Interface Issues . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative References . . . . . . . . . . . . . . . . . . . 8
7.2 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 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 responded to a
particular query is to have that name server include identifying
information in the response itself. This note defines a protocol
enhancement to achieve this.
1.1 Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Protocol
This note uses 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, if any.
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. If the name server side of a recursive name server
receives the SI bit, the client is asking the recursive name server
to identify itself; if the resolver side of the recursive name server
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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 3.1 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 recursive name server may choose to set the SI
bit when sending a query, this has no effect on the setting of the SI
bit or the presence or absence of the NSID option in the recursive
name server's response to the original client.
As stated in Section 2.1, this mechanism is not restricted to
authoritative name servers; the semantics are intended to be equally
applicable to recursive name servers.
2.3 Presentation Format
User interfaces MUST read and write the content of the NSID option as
a sequence of hexadecimal digits, two digits per payload octet.
The NSID payload is binary data. Any comparison between NSID
payloads MUST be a comparison of the raw binary data. Copy
operations MUST NOT assume that the raw NSID payload is null-
terminated. Any resemblance between raw NSID payload data and any
form of text is purely a convenience, and does not change the
underlying nature of the payload data.
See Section 3.3 for discussion.
3. Discussion
This section discusses certain aspects of the protocol and explains
considerations that led to the chosen design.
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3.1 The NSID Payload
The syntax and semantics of the content of the NSID option is
deliberately left outside the scope of this specification. This
section 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 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 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 could 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 the name server may not have
a "real" name.
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 the operator of the name server may
not be willing to divulge its non-anycast address.
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, hashing might not
add much value, since a hash based on an IPv4 address still only
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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.
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
some form of nonvolatile storage in any case, and in the rare case
of a name server that 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, there does not appear to be a
single solution that will meet all needs. Section 2.2 therefore
defines the NSID payload to be an opaque byte string and leaves the
choice up to the implementor and name server operator. The following
guidelines may be useful to implementors and server operators:
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.
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 should 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 should 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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3.2 SI and NSID Are Not Transitive
As specified in Section 2.1 and Section 2.2, the SI flag and NSID
option are not transitive. This is strictly a hop-by-hop mechanism.
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
also applicable to recursive name servers, the mechanism may also be
useful with recursive name servers. The hop-by-hop semantics support
this.
While there might be some utility in having a transitive variant of
this mechanism (so that, for example, a stub resolver could ask a
recursive server to tell it which authoritative name server provided
a particular answer to the recursive name server), the semantics of
such a variant would be more complicated, and are left for future
work.
3.3 User Interface Issues
Given the range of possible payload contents described in
Section 3.1, it is not possible to define a single presentation
format for the NSID payload that is efficient, convenient,
unambiguous, and aesthetically pleasing. In particular, while it is
tempting to use a presentation format that uses some form of textual
strings, attempting to support this would significantly complicate
what's intended to be a very simple debugging mechanism.
In some cases the content of the NSID payload may binary data only be
meaningful to the name server operator, and may not be meaningful to
the user or application, but the user or application must be able to
capture the entire content anyway in order for it to be useful.
Thus, the presentation format must support arbitrary binary data.
In cases where the name server operator derives the NSID payload from
textual data, a textual form such as US-ASCII or UTF-8 strings might
at first glance seem easier for a user to deal with. There are,
however, a number of complex issues involving internationalized text
which, if fully addressed here, would require a set of rules
significantly longer than the rest of this specification. See
[RFC2277] for an overview of some of these issues.
It is much more important for the NSID payload data to be passed
unambiguously from server administrator to user than it is for the
payload data data to be pretty while in transit. In particular, it's
critical that it be straightforward for a user to cut and paste an
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exact copy of the NSID payload output by a debugging tool into other
formats such as email messages or web forms without distortion.
Hexadecimal strings, while ugly, are also robust.
4. 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).
5. Security Considerations
This document describes a channel signaling mechanism, intended
primarily for debugging. Channel signaling mechanisms are outside
the scope of DNSSEC per se. Applications that require integrity
protection for the data being signaled will need to use a channel
security mechanism such as TSIG [RFC2845].
Section 3.1 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.
6. Acknowledgements
Joe Abley, Harald Alvestrand, Mark Andrews, Roy Arends, Steve
Bellovin, Randy Bush, David Conrad, Johan Ihren, Daniel Karrenberg,
Mike Patton, Paul Vixie, Sam Weiler, and Suzanne Woolf. Apologies to
anyone inadvertently omitted from the above list.
7. References
7.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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7.2 Informative References
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
Author's Address
Rob Austein
ISC
950 Charter Street
Redwood City, CA 94063
USA
Email: sra@isc.org
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