Network Working Group R. Austein
Request for Comments: 5001 ISC
Category: Standards Track August 2007
DNS Name Server Identifier (NSID) Option
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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
mechanisms 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 that 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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RFC 5001 DNS NSID August 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Resolver Behavior . . . . . . . . . . . . . . . . . . . . 3
2.2. Name Server Behavior . . . . . . . . . . . . . . . . . . . 3
2.3. The NSID Option . . . . . . . . . . . . . . . . . . . . . 4
2.4. Presentation Format . . . . . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. The NSID Payload . . . . . . . . . . . . . . . . . . . . . 4
3.2. NSID Is Not Transitive . . . . . . . . . . . . . . . . . . 7
3.3. User Interface Issues . . . . . . . . . . . . . . . . . . 7
3.4. Truncation . . . . . . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
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 that 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.
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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] option to signal the resolver's
desire for information identifying the name server and to hold the
name server's response, if any.
2.1. Resolver Behavior
A resolver signals its desire for information identifying a name
server by sending an empty NSID option (Section 2.3) in an EDNS OPT
pseudo-RR in the query message.
The resolver MUST NOT include any NSID payload data in the query
message.
The semantics of an NSID request are not transitive. That is: the
presence of an NSID option in a query is a request that the name
server which receives the query identify itself. If the name server
side of a recursive name server receives an NSID request, the client
is asking the recursive name server to identify itself; if the
resolver side of the recursive name server wishes to receive
identifying information, it is free to add NSID requests in its own
queries, but that is a separate matter.
2.2. Name Server Behavior
A name server that understands the NSID option and chooses to honor a
particular NSID request responds by including identifying information
in a NSID option (Section 2.3) in an EDNS OPT pseudo-RR in the
response message.
The name server MUST ignore any NSID payload data that might be
present in the query message.
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 add an NSID
option when sending a query, this has no effect on the presence or
absence of the NSID option in the recursive name server's response to
the original client.
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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. The NSID Option
The OPTION-CODE for the NSID option is 3.
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.
2.4. Presentation Format
User interfaces MUST read and write the contents 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.
3.1. The NSID Payload
The syntax and semantics of the content of the NSID option are
deliberately left outside the scope of this specification.
Choosing the NSID content is a prerogative of the server
administrator. The server administrator might choose to encode the
NSID content in such a way that the server operator (or clients
authorized by the server operator) can decode the NSID content to
obtain more information than other clients can. Alternatively, the
server operator might choose unencoded NSID content that is equally
meaningful to any client.
This section describes 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 specifying a simple opaque
byte string in Section 2.3.
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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 probabilistically 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 dynamically 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
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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
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 Probabilistically unique identifiers have properties similar 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 probabilistically unique
identifier across reboots, but this does not appear to be a
serious restriction, since authoritative nameservers almost always
have some form of non-volatile storage. 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
generates a new identifier every time it reboots.
o Using an arbitrary octet string gives name server operators yet
another setting 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.
o A signed blob is not particularly useful as an NSID payload unless
the signed data is dynamic and includes some kind of replay
protection, such as a timestamp or some kind of data identifying
the requestor. Signed blobs that meet these criteria could
conceivably be useful in some situations but would require
detailed security analysis beyond the scope of this document.
o A static encrypted blob would not be particularly useful, as it
would be subject to replay attacks and would, in effect, just be a
random number to any party that does not possess the decryption
key. Dynamic encrypted blobs could conceivably be useful in some
situations but, as with signed blobs, dynamic encrypted blobs
would require detailed security analysis beyond the scope of this
document.
Given all of the issues listed above, there does not appear to be a
single solution that will meet all needs. Section 2.3 therefore
defines the NSID payload to be an opaque byte string and leaves the
choice of payload up to the implementor and name server operator.
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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 probabilistically 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.
3.2. NSID Is Not Transitive
As specified in Section 2.1 and Section 2.2, the NSID option is 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,
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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 be binary data
meaningful only 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 and back again than
it is for the payload data to be pretty while in transit. In
particular, it's critical that it be straightforward for a user to
cut and paste an 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.
3.4. Truncation
In some cases, adding the NSID option to a response message may
trigger message truncation. This specification does not change the
rules for DNS message truncation in any way, but implementors will
need to pay attention to this issue.
Including the NSID option in a response is always optional, so this
specification never requires name servers to truncate response
messages.
By definition, a resolver that requests NSID responses also supports
EDNS, so a resolver that requests NSID responses can also use the
"sender's UDP payload size" field of the OPT pseudo-RR to signal a
receive buffer size large enough to make truncation unlikely.
4. IANA Considerations
IANA has allocated EDNS option code 3 for the NSID option
(Section 2.3).
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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.
Two of the possible kinds of payload data discussed in Section 3.1
involve a digital signature and encryption, respectively. While this
specification discusses some of the pitfalls that might lurk for
careless users of these kinds of payload data, full analysis of the
issues that would be involved in these kinds of payload data would
require knowledge of the content to be signed or encrypted,
algorithms to be used, and so forth, which is beyond the scope of
this document. Implementors should seek competent advice before
attempting to use these kinds of NSID payloads.
6. Acknowledgements
Thanks to: Joe Abley, Harald Alvestrand, Dean Anderson, Mark Andrews,
Roy Arends, Steve Bellovin, Alex Bligh, Randy Bush, David Conrad,
John Dickinson, Alfred Hoenes, Johan Ihren, Daniel Karrenberg, Peter
Koch, William Leibzon, Ed Lewis, Thomas Narten, Mike Patton, Geoffrey
Sisson, Andrew Sullivan, Mike StJohns, Tom Taylor, Paul Vixie, Sam
Weiler, and Suzanne Woolf, none of whom are responsible for what the
author did with their comments and suggestions. 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", RFC 2119, BCP 14, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
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[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
7.2. Informative References
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", RFC 2277, BCP 18, January 1998.
Author's Address
Rob Austein
ISC
950 Charter Street
Redwood City, CA 94063
USA
EMail: sra@isc.org
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RFC 5001 DNS NSID August 2007
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