1 Network Working Group R. Austein
2 Request for Comments: 5001 ISC
3 Category: Standards Track August 2007
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6 DNS Name Server Identifier (NSID) Option
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8 Status of This Memo
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10 This document specifies an Internet standards track protocol for the
11 Internet community, and requests discussion and suggestions for
12 improvements. Please refer to the current edition of the "Internet
13 Official Protocol Standards" (STD 1) for the standardization state
14 and status of this protocol. Distribution of this memo is unlimited.
15
16 Copyright Notice
17
18 Copyright (C) The IETF Trust (2007).
19
20 Abstract
21
22 With the increased use of DNS anycast, load balancing, and other
23 mechanisms allowing more than one DNS name server to share a single
24 IP address, it is sometimes difficult to tell which of a pool of name
25 servers has answered a particular query. While existing ad-hoc
26 mechanisms allow an operator to send follow-up queries when it is
27 necessary to debug such a configuration, the only completely reliable
28 way to obtain the identity of the name server that responded is to
29 have the name server include this information in the response itself.
30 This note defines a protocol extension to support this functionality.
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52 Austein Standards Track [Page 1]
53 RFC 5001 DNS NSID August 2007
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56 Table of Contents
57
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
59 1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . . 3
60 2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
61 2.1. Resolver Behavior . . . . . . . . . . . . . . . . . . . . 3
62 2.2. Name Server Behavior . . . . . . . . . . . . . . . . . . . 3
63 2.3. The NSID Option . . . . . . . . . . . . . . . . . . . . . 4
64 2.4. Presentation Format . . . . . . . . . . . . . . . . . . . 4
65 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
66 3.1. The NSID Payload . . . . . . . . . . . . . . . . . . . . . 4
67 3.2. NSID Is Not Transitive . . . . . . . . . . . . . . . . . . 7
68 3.3. User Interface Issues . . . . . . . . . . . . . . . . . . 7
69 3.4. Truncation . . . . . . . . . . . . . . . . . . . . . . . . 8
70 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
71 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
72 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
73 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
74 7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
75 7.2. Informative References . . . . . . . . . . . . . . . . . . 10
76
77 1. Introduction
78
79 With the increased use of DNS anycast, load balancing, and other
80 mechanisms allowing more than one DNS name server to share a single
81 IP address, it is sometimes difficult to tell which of a pool of name
82 servers has answered a particular query.
83
84 Existing ad-hoc mechanisms allow an operator to send follow-up
85 queries when it is necessary to debug such a configuration, but there
86 are situations in which this is not a totally satisfactory solution,
87 since anycast routing may have changed, or the server pool in
88 question may be behind some kind of extremely dynamic load balancing
89 hardware. Thus, while these ad-hoc mechanisms are certainly better
90 than nothing (and have the advantage of already being deployed), a
91 better solution seems desirable.
92
93 Given that a DNS query is an idempotent operation with no retained
94 state, it would appear that the only completely reliable way to
95 obtain the identity of the name server that responded to a particular
96 query is to have that name server include identifying information in
97 the response itself. This note defines a protocol enhancement to
98 achieve this.
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108 RFC 5001 DNS NSID August 2007
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111 1.1. Reserved Words
112
113 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
114 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
115 document are to be interpreted as described in [RFC2119].
116
117 2. Protocol
118
119 This note uses an EDNS [RFC2671] option to signal the resolver's
120 desire for information identifying the name server and to hold the
121 name server's response, if any.
122
123 2.1. Resolver Behavior
124
125 A resolver signals its desire for information identifying a name
126 server by sending an empty NSID option (Section 2.3) in an EDNS OPT
127 pseudo-RR in the query message.
128
129 The resolver MUST NOT include any NSID payload data in the query
130 message.
131
132 The semantics of an NSID request are not transitive. That is: the
133 presence of an NSID option in a query is a request that the name
134 server which receives the query identify itself. If the name server
135 side of a recursive name server receives an NSID request, the client
136 is asking the recursive name server to identify itself; if the
137 resolver side of the recursive name server wishes to receive
138 identifying information, it is free to add NSID requests in its own
139 queries, but that is a separate matter.
140
141 2.2. Name Server Behavior
142
143 A name server that understands the NSID option and chooses to honor a
144 particular NSID request responds by including identifying information
145 in a NSID option (Section 2.3) in an EDNS OPT pseudo-RR in the
146 response message.
147
148 The name server MUST ignore any NSID payload data that might be
149 present in the query message.
150
151 The NSID option is not transitive. A name server MUST NOT send an
152 NSID option back to a resolver which did not request it. In
153 particular, while a recursive name server may choose to add an NSID
154 option when sending a query, this has no effect on the presence or
155 absence of the NSID option in the recursive name server's response to
156 the original client.
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163 RFC 5001 DNS NSID August 2007
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166 As stated in Section 2.1, this mechanism is not restricted to
167 authoritative name servers; the semantics are intended to be equally
168 applicable to recursive name servers.
169
170 2.3. The NSID Option
171
172 The OPTION-CODE for the NSID option is 3.
173
174 The OPTION-DATA for the NSID option is an opaque byte string, the
175 semantics of which are deliberately left outside the protocol. See
176 Section 3.1 for discussion.
177
178 2.4. Presentation Format
179
180 User interfaces MUST read and write the contents of the NSID option
181 as a sequence of hexadecimal digits, two digits per payload octet.
182
183 The NSID payload is binary data. Any comparison between NSID
184 payloads MUST be a comparison of the raw binary data. Copy
185 operations MUST NOT assume that the raw NSID payload is null-
186 terminated. Any resemblance between raw NSID payload data and any
187 form of text is purely a convenience, and does not change the
188 underlying nature of the payload data.
189
190 See Section 3.3 for discussion.
191
192 3. Discussion
193
194 This section discusses certain aspects of the protocol and explains
195 considerations that led to the chosen design.
196
197 3.1. The NSID Payload
198
199 The syntax and semantics of the content of the NSID option are
200 deliberately left outside the scope of this specification.
201
202 Choosing the NSID content is a prerogative of the server
203 administrator. The server administrator might choose to encode the
204 NSID content in such a way that the server operator (or clients
205 authorized by the server operator) can decode the NSID content to
206 obtain more information than other clients can. Alternatively, the
207 server operator might choose unencoded NSID content that is equally
208 meaningful to any client.
209
210 This section describes some of the kinds of data that server
211 administrators might choose to provide as the content of the NSID
212 option, and explains the reasoning behind specifying a simple opaque
213 byte string in Section 2.3.
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217 Austein Standards Track [Page 4]
218 RFC 5001 DNS NSID August 2007
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221 There are several possibilities for the payload of the NSID option:
222
223 o It could be the "real" name of the specific name server within the
224 name server pool.
225
226 o It could be the "real" IP address (IPv4 or IPv6) of the name
227 server within the name server pool.
228
229 o It could be some sort of pseudo-random number generated in a
230 predictable fashion somehow using the server's IP address or name
231 as a seed value.
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233 o It could be some sort of probabilistically unique identifier
234 initially derived from some sort of random number generator then
235 preserved across reboots of the name server.
236
237 o It could be some sort of dynamically generated identifier so that
238 only the name server operator could tell whether or not any two
239 queries had been answered by the same server.
240
241 o It could be a blob of signed data, with a corresponding key which
242 might (or might not) be available via DNS lookups.
243
244 o It could be a blob of encrypted data, the key for which could be
245 restricted to parties with a need to know (in the opinion of the
246 server operator).
247
248 o It could be an arbitrary string of octets chosen at the discretion
249 of the name server operator.
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251 Each of these options has advantages and disadvantages:
252
253 o Using the "real" name is simple, but the name server may not have
254 a "real" name.
255
256 o Using the "real" address is also simple, and the name server
257 almost certainly does have at least one non-anycast IP address for
258 maintenance operations, but the operator of the name server may
259 not be willing to divulge its non-anycast address.
260
261 o Given that one common reason for using anycast DNS techniques is
262 an attempt to harden a critical name server against denial of
263 service attacks, some name server operators are likely to want an
264 identifier other than the "real" name or "real" address of the
265 name server instance.
266
267 o Using a hash or pseudo-random number can provide a fixed length
268 value that the resolver can use to tell two name servers apart
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273 RFC 5001 DNS NSID August 2007
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276 without necessarily being able to tell where either one of them
277 "really" is, but makes debugging more difficult if one happens to
278 be in a friendly open environment. Furthermore, hashing might not
279 add much value, since a hash based on an IPv4 address still only
280 involves a 32-bit search space, and DNS names used for servers
281 that operators might have to debug at 4am tend not to be very
282 random.
283
284 o Probabilistically unique identifiers have properties similar to
285 hashed identifiers, but (given a sufficiently good random number
286 generator) are immune to the search space issues. However, the
287 strength of this approach is also its weakness: there is no
288 algorithmic transformation by which even the server operator can
289 associate name server instances with identifiers while debugging,
290 which might be annoying. This approach also requires the name
291 server instance to preserve the probabilistically unique
292 identifier across reboots, but this does not appear to be a
293 serious restriction, since authoritative nameservers almost always
294 have some form of non-volatile storage. In the rare case of a
295 name server that does not have any way to store such an
296 identifier, nothing terrible will happen if the name server
297 generates a new identifier every time it reboots.
298
299 o Using an arbitrary octet string gives name server operators yet
300 another setting to configure, or mis-configure, or forget to
301 configure. Having all the nodes in an anycast name server
302 constellation identify themselves as "My Name Server" would not be
303 particularly useful.
304
305 o A signed blob is not particularly useful as an NSID payload unless
306 the signed data is dynamic and includes some kind of replay
307 protection, such as a timestamp or some kind of data identifying
308 the requestor. Signed blobs that meet these criteria could
309 conceivably be useful in some situations but would require
310 detailed security analysis beyond the scope of this document.
311
312 o A static encrypted blob would not be particularly useful, as it
313 would be subject to replay attacks and would, in effect, just be a
314 random number to any party that does not possess the decryption
315 key. Dynamic encrypted blobs could conceivably be useful in some
316 situations but, as with signed blobs, dynamic encrypted blobs
317 would require detailed security analysis beyond the scope of this
318 document.
319
320 Given all of the issues listed above, there does not appear to be a
321 single solution that will meet all needs. Section 2.3 therefore
322 defines the NSID payload to be an opaque byte string and leaves the
323 choice of payload up to the implementor and name server operator.
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328 RFC 5001 DNS NSID August 2007
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331 The following guidelines may be useful to implementors and server
332 operators:
333
334 o Operators for whom divulging the unicast address is an issue could
335 use the raw binary representation of a probabilistically unique
336 random number. This should probably be the default implementation
337 behavior.
338
339 o Operators for whom divulging the unicast address is not an issue
340 could just use the raw binary representation of a unicast address
341 for simplicity. This should only be done via an explicit
342 configuration choice by the operator.
343
344 o Operators who really need or want the ability to set the NSID
345 payload to an arbitrary value could do so, but this should only be
346 done via an explicit configuration choice by the operator.
347
348 This approach appears to provide enough information for useful
349 debugging without unintentionally leaking the maintenance addresses
350 of anycast name servers to nogoodniks, while also allowing name
351 server operators who do not find such leakage threatening to provide
352 more information at their own discretion.
353
354 3.2. NSID Is Not Transitive
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356 As specified in Section 2.1 and Section 2.2, the NSID option is not
357 transitive. This is strictly a hop-by-hop mechanism.
358
359 Most of the discussion of name server identification to date has
360 focused on identifying authoritative name servers, since the best
361 known cases of anycast name servers are a subset of the name servers
362 for the root zone. However, given that anycast DNS techniques are
363 also applicable to recursive name servers, the mechanism may also be
364 useful with recursive name servers. The hop-by-hop semantics support
365 this.
366
367 While there might be some utility in having a transitive variant of
368 this mechanism (so that, for example, a stub resolver could ask a
369 recursive server to tell it which authoritative name server provided
370 a particular answer to the recursive name server), the semantics of
371 such a variant would be more complicated, and are left for future
372 work.
373
374 3.3. User Interface Issues
375
376 Given the range of possible payload contents described in
377 Section 3.1, it is not possible to define a single presentation
378 format for the NSID payload that is efficient, convenient,
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383 RFC 5001 DNS NSID August 2007
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386 unambiguous, and aesthetically pleasing. In particular, while it is
387 tempting to use a presentation format that uses some form of textual
388 strings, attempting to support this would significantly complicate
389 what's intended to be a very simple debugging mechanism.
390
391 In some cases the content of the NSID payload may be binary data
392 meaningful only to the name server operator, and may not be
393 meaningful to the user or application, but the user or application
394 must be able to capture the entire content anyway in order for it to
395 be useful. Thus, the presentation format must support arbitrary
396 binary data.
397
398 In cases where the name server operator derives the NSID payload from
399 textual data, a textual form such as US-ASCII or UTF-8 strings might
400 at first glance seem easier for a user to deal with. There are,
401 however, a number of complex issues involving internationalized text
402 which, if fully addressed here, would require a set of rules
403 significantly longer than the rest of this specification. See
404 [RFC2277] for an overview of some of these issues.
405
406 It is much more important for the NSID payload data to be passed
407 unambiguously from server administrator to user and back again than
408 it is for the payload data to be pretty while in transit. In
409 particular, it's critical that it be straightforward for a user to
410 cut and paste an exact copy of the NSID payload output by a debugging
411 tool into other formats such as email messages or web forms without
412 distortion. Hexadecimal strings, while ugly, are also robust.
413
414 3.4. Truncation
415
416 In some cases, adding the NSID option to a response message may
417 trigger message truncation. This specification does not change the
418 rules for DNS message truncation in any way, but implementors will
419 need to pay attention to this issue.
420
421 Including the NSID option in a response is always optional, so this
422 specification never requires name servers to truncate response
423 messages.
424
425 By definition, a resolver that requests NSID responses also supports
426 EDNS, so a resolver that requests NSID responses can also use the
427 "sender's UDP payload size" field of the OPT pseudo-RR to signal a
428 receive buffer size large enough to make truncation unlikely.
429
430 4. IANA Considerations
431
432 IANA has allocated EDNS option code 3 for the NSID option
433 (Section 2.3).
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441 5. Security Considerations
442
443 This document describes a channel signaling mechanism intended
444 primarily for debugging. Channel signaling mechanisms are outside
445 the scope of DNSSEC, per se. Applications that require integrity
446 protection for the data being signaled will need to use a channel
447 security mechanism such as TSIG [RFC2845].
448
449 Section 3.1 discusses a number of different kinds of information that
450 a name server operator might choose to provide as the value of the
451 NSID option. Some of these kinds of information are security
452 sensitive in some environments. This specification deliberately
453 leaves the syntax and semantics of the NSID option content up to the
454 implementation and the name server operator.
455
456 Two of the possible kinds of payload data discussed in Section 3.1
457 involve a digital signature and encryption, respectively. While this
458 specification discusses some of the pitfalls that might lurk for
459 careless users of these kinds of payload data, full analysis of the
460 issues that would be involved in these kinds of payload data would
461 require knowledge of the content to be signed or encrypted,
462 algorithms to be used, and so forth, which is beyond the scope of
463 this document. Implementors should seek competent advice before
464 attempting to use these kinds of NSID payloads.
465
466 6. Acknowledgements
467
468 Thanks to: Joe Abley, Harald Alvestrand, Dean Anderson, Mark Andrews,
469 Roy Arends, Steve Bellovin, Alex Bligh, Randy Bush, David Conrad,
470 John Dickinson, Alfred Hoenes, Johan Ihren, Daniel Karrenberg, Peter
471 Koch, William Leibzon, Ed Lewis, Thomas Narten, Mike Patton, Geoffrey
472 Sisson, Andrew Sullivan, Mike StJohns, Tom Taylor, Paul Vixie, Sam
473 Weiler, and Suzanne Woolf, none of whom are responsible for what the
474 author did with their comments and suggestions. Apologies to anyone
475 inadvertently omitted from the above list.
476
477 7. References
478
479 7.1. Normative References
480
481 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
482 Requirement Levels", RFC 2119, BCP 14, March 1997.
483
484 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
485 RFC 2671, August 1999.
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496 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
497 Wellington, "Secret Key Transaction Authentication for DNS
498 (TSIG)", RFC 2845, May 2000.
499
500 7.2. Informative References
501
502 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
503 Languages", RFC 2277, BCP 18, January 1998.
504
505 Author's Address
506
507 Rob Austein
508 ISC
509 950 Charter Street
510 Redwood City, CA 94063
511 USA
512
513 EMail: sra@isc.org
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551 Full Copyright Statement
552
553 Copyright (C) The IETF Trust (2007).
554
555 This document is subject to the rights, licenses and restrictions
556 contained in BCP 78, and except as set forth therein, the authors
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590
591 Acknowledgement
592
593 Funding for the RFC Editor function is currently provided by the
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