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Austein 3 Internet-Draft ISC 4 Expires: July 15, 2006 January 11, 2006 6 DNS Name Server Identifier Option (NSID) 7 draft-ietf-dnsext-nsid-01 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on July 15, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2006). 38 Abstract 40 With the increased use of DNS anycast, load balancing, and other 41 mechanisms allowing more than one DNS name server to share a single 42 IP address, it is sometimes difficult to tell which of a pool of name 43 servers has answered a particular query. While existing ad-hoc 44 mechanism allow an operator to send follow-up queries when it is 45 necessary to debug such a configuration, the only completely reliable 46 way to obtain the identity of the name server which responded is to 47 have the name server include this information in the response itself. 48 This note defines a protocol extension to support this functionality. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . . 3 54 2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 2.1. Resolver Behavior . . . . . . . . . . . . . . . . . . . . 4 56 2.2. Name Server Behavior . . . . . . . . . . . . . . . . . . . 4 57 2.3. The NSID Option . . . . . . . . . . . . . . . . . . . . . 4 58 2.4. Presentation Format . . . . . . . . . . . . . . . . . . . 5 59 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 6 60 3.1. The NSID Payload . . . . . . . . . . . . . . . . . . . . . 6 61 3.2. NSID Is Not Transitive . . . . . . . . . . . . . . . . . . 8 62 3.3. User Interface Issues . . . . . . . . . . . . . . . . . . 8 63 3.4. Truncation . . . . . . . . . . . . . . . . . . . . . . . . 9 64 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 66 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 67 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 68 7.1. Normative References . . . . . . . . . . . . . . . . . . . 13 69 7.2. Informative References . . . . . . . . . . . . . . . . . . 13 70 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 71 Intellectual Property and Copyright Statements . . . . . . . . . . 15 73 1. Introduction 75 With the increased use of DNS anycast, load balancing, and other 76 mechanisms allowing more than one DNS name server to share a single 77 IP address, it is sometimes difficult to tell which of a pool of name 78 servers has answered a particular query. 80 Existing ad-hoc mechanisms allow an operator to send follow-up 81 queries when it is necessary to debug such a configuration, but there 82 are situations in which this is not a totally satisfactory solution, 83 since anycast routing may have changed, or the server pool in 84 question may be behind some kind of extremely dynamic load balancing 85 hardware. Thus, while these ad-hoc mechanisms are certainly better 86 than nothing (and have the advantage of already being deployed), a 87 better solution seems desirable. 89 Given that a DNS query is an idempotent operation with no retained 90 state, it would appear that the only completely reliable way to 91 obtain the identity of the name server which responded to a 92 particular query is to have that name server include identifying 93 information in the response itself. This note defines a protocol 94 enhancement to achieve this. 96 1.1. Reserved Words 98 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 99 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 100 document are to be interpreted as described in [RFC2119]. 102 2. Protocol 104 This note uses an EDNS [RFC2671] option to signal the resolver's 105 desire for information identifying the name server and to hold the 106 name server's response, if any. 108 2.1. Resolver Behavior 110 A resolver signals its desire for information identifying a name 111 server by sending an empty NSID option (Section 2.3) in an EDNS OPT 112 pseudo-RR in the query message. 114 The resolver MUST NOT include any NSID payload data in the query 115 message. 117 The semantics of an NSID request are not transitive. That is: the 118 presence of an NSID option in a query is a request that the name 119 server which receives the query identify itself. If the name server 120 side of a recursive name server receives an NSID request, the client 121 is asking the recursive name server to identify itself; if the 122 resolver side of the recursive name server wishes to receive 123 identifying information, it is free to add NSID requests in its own 124 queries, but that is a separate matter. 126 2.2. Name Server Behavior 128 A name server which understands the NSID option and chooses to honor 129 a particular NSID request responds by including identifying 130 information in a NSID option (Section 2.3) in an EDNS OPT pseudo-RR 131 in the response message. 133 The name server MUST ignore any NSID payload data that might be 134 present in the query message. 136 The NSID option is not transitive. A name server MUST NOT send an 137 NSID option back to a resolver which did not request it. In 138 particular, while a recursive name server may choose to add an NSID 139 option when sending a query, this has no effect on the presence or 140 absence of the NSID option in the recursive name server's response to 141 the original client. 143 As stated in Section 2.1, this mechanism is not restricted to 144 authoritative name servers; the semantics are intended to be equally 145 applicable to recursive name servers. 147 2.3. The NSID Option 149 The OPTION-CODE for the NSID option is [TBD]. 151 The OPTION-DATA for the NSID option is an opaque byte string the 152 semantics of which are deliberately left outside the protocol. See 153 Section 3.1 for discussion. 155 2.4. Presentation Format 157 User interfaces MUST read and write the content of the NSID option as 158 a sequence of hexadecimal digits, two digits per payload octet. 160 The NSID payload is binary data. Any comparison between NSID 161 payloads MUST be a comparison of the raw binary data. Copy 162 operations MUST NOT assume that the raw NSID payload is null- 163 terminated. Any resemblance between raw NSID payload data and any 164 form of text is purely a convenience, and does not change the 165 underlying nature of the payload data. 167 See Section 3.3 for discussion. 169 3. Discussion 171 This section discusses certain aspects of the protocol and explains 172 considerations that led to the chosen design. 174 3.1. The NSID Payload 176 The syntax and semantics of the content of the NSID option is 177 deliberately left outside the scope of this specification. This 178 section describe some of the kinds of data that server administrators 179 might choose to provide as the content of the NSID option, and 180 explains the reasoning behind choosing a simple opaque byte string. 182 There are several possibilities for the payload of the NSID option: 184 o It could be the "real" name of the specific name server within the 185 name server pool. 187 o It could be the "real" IP address (IPv4 or IPv6) of the name 188 server within the name server pool. 190 o It could be some sort of pseudo-random number generated in a 191 predictable fashion somehow using the server's IP address or name 192 as a seed value. 194 o It could be some sort of probabilisticly unique identifier 195 initially derived from some sort of random number generator then 196 preserved across reboots of the name server. 198 o It could be some sort of dynamicly generated identifier so that 199 only the name server operator could tell whether or not any two 200 queries had been answered by the same server. 202 o It could be a blob of signed data, with a corresponding key which 203 might (or might not) be available via DNS lookups. 205 o It could be a blob of encrypted data, the key for which could be 206 restricted to parties with a need to know (in the opinion of the 207 server operator). 209 o It could be an arbitrary string of octets chosen at the discretion 210 of the name server operator. 212 Each of these options has advantages and disadvantages: 214 o Using the "real" name is simple, but the name server may not have 215 a "real" name. 217 o Using the "real" address is also simple, and the name server 218 almost certainly does have at least one non-anycast IP address for 219 maintenance operations, but the operator of the name server may 220 not be willing to divulge its non-anycast address. 222 o Given that one common reason for using anycast DNS techniques is 223 an attempt to harden a critical name server against denial of 224 service attacks, some name server operators are likely to want an 225 identifier other than the "real" name or "real" address of the 226 name server instance. 228 o Using a hash or pseudo-random number can provide a fixed length 229 value that the resolver can use to tell two name servers apart 230 without necessarily being able to tell where either one of them 231 "really" is, but makes debugging more difficult if one happens to 232 be in a friendly open environment. Furthermore, hashing might not 233 add much value, since a hash based on an IPv4 address still only 234 involves a 32-bit search space, and DNS names used for servers 235 that operators might have to debug at 4am tend not to be very 236 random. 238 o Probabilisticly unique identifiers have similar properties to 239 hashed identifiers, but (given a sufficiently good random number 240 generator) are immune to the search space issues. However, the 241 strength of this approach is also its weakness: there is no 242 algorithmic transformation by which even the server operator can 243 associate name server instances with identifiers while debugging, 244 which might be annoying. This approach also requires the name 245 server instance to preserve the probabilisticly unique identifier 246 across reboots, but this does not appear to be a serious 247 restriction, since authoritative nameservers almost always have 248 some form of nonvolatile storage in any case, and in the rare case 249 of a name server that does not have any way to store such an 250 identifier, nothing terrible will happen if the name server just 251 generates a new identifier every time it reboots. 253 o Using an arbitrary octet string gives name server operators yet 254 another thing to configure, or mis-configure, or forget to 255 configure. Having all the nodes in an anycast name server 256 constellation identify themselves as "My Name Server" would not be 257 particularly useful. 259 Given all of the issues listed above, there does not appear to be a 260 single solution that will meet all needs. Section 2.3 therefore 261 defines the NSID payload to be an opaque byte string and leaves the 262 choice up to the implementor and name server operator. The following 263 guidelines may be useful to implementors and server operators: 265 o Operators for whom divulging the unicast address is an issue could 266 use the raw binary representation of a probabilisticly unique 267 random number. This should probably be the default implementation 268 behavior. 270 o Operators for whom divulging the unicast address is not an issue 271 could just use the raw binary representation of a unicast address 272 for simplicity. This should only be done via an explicit 273 configuration choice by the operator. 275 o Operators who really need or want the ability to set the NSID 276 payload to an arbitrary value could do so, but this should only be 277 done via an explicit configuration choice by the operator. 279 This approach appears to provide enough information for useful 280 debugging without unintentionally leaking the maintenance addresses 281 of anycast name servers to nogoodniks, while also allowing name 282 server operators who do not find such leakage threatening to provide 283 more information at their own discretion. 285 3.2. NSID Is Not Transitive 287 As specified in Section 2.1 and Section 2.2, the NSID option is not 288 transitive. This is strictly a hop-by-hop mechanism. 290 Most of the discussion of name server identification to date has 291 focused on identifying authoritative name servers, since the best 292 known cases of anycast name servers are a subset of the name servers 293 for the root zone. However, given that anycast DNS techniques are 294 also applicable to recursive name servers, the mechanism may also be 295 useful with recursive name servers. The hop-by-hop semantics support 296 this. 298 While there might be some utility in having a transitive variant of 299 this mechanism (so that, for example, a stub resolver could ask a 300 recursive server to tell it which authoritative name server provided 301 a particular answer to the recursive name server), the semantics of 302 such a variant would be more complicated, and are left for future 303 work. 305 3.3. User Interface Issues 307 Given the range of possible payload contents described in 308 Section 3.1, it is not possible to define a single presentation 309 format for the NSID payload that is efficient, convenient, 310 unambiguous, and aesthetically pleasing. In particular, while it is 311 tempting to use a presentation format that uses some form of textual 312 strings, attempting to support this would significantly complicate 313 what's intended to be a very simple debugging mechanism. 315 In some cases the content of the NSID payload may be binary data 316 meaningful only to the name server operator, and may not be 317 meaningful to the user or application, but the user or application 318 must be able to capture the entire content anyway in order for it to 319 be useful. Thus, the presentation format must support arbitrary 320 binary data. 322 In cases where the name server operator derives the NSID payload from 323 textual data, a textual form such as US-ASCII or UTF-8 strings might 324 at first glance seem easier for a user to deal with. There are, 325 however, a number of complex issues involving internationalized text 326 which, if fully addressed here, would require a set of rules 327 significantly longer than the rest of this specification. See 328 [RFC2277] for an overview of some of these issues. 330 It is much more important for the NSID payload data to be passed 331 unambiguously from server administrator to user and back again than 332 it is for the payload data data to be pretty while in transit. In 333 particular, it's critical that it be straightforward for a user to 334 cut and paste an exact copy of the NSID payload output by a debugging 335 tool into other formats such as email messages or web forms without 336 distortion. Hexadecimal strings, while ugly, are also robust. 338 3.4. Truncation 340 In some cases, adding the NSID option to a response message may 341 trigger message truncation. This specification does not change the 342 rules for DNS message truncation in any way, but implementors will 343 need to pay attention to this issue. 345 Including the NSID option in a response is always optional, so this 346 specification never requires name servers to truncate response 347 messages. 349 By definition, a resolver that requests NSID responses also supports 350 EDNS, so a resolver that requests NSID responses can also use the 351 "sender's UDP payload size" field of the OPT pseudo-RR to signal a 352 receive buffer size large enough to make truncation unlikely. 354 4. IANA Considerations 356 This mechanism requires allocation of one ENDS option code for the 357 NSID option (Section 2.3). 359 5. Security Considerations 361 This document describes a channel signaling mechanism, intended 362 primarily for debugging. Channel signaling mechanisms are outside 363 the scope of DNSSEC per se. Applications that require integrity 364 protection for the data being signaled will need to use a channel 365 security mechanism such as TSIG [RFC2845]. 367 Section 3.1 discusses a number of different kinds of information that 368 a name server operator might choose to provide as the value of the 369 NSID option. Some of these kinds of information are security 370 sensitive in some environments. This specification deliberately 371 leaves the syntax and semantics of the NSID option content up to the 372 implementation and the name server operator. 374 6. Acknowledgements 376 Joe Abley, Harald Alvestrand, Mark Andrews, Roy Arends, Steve 377 Bellovin, Randy Bush, David Conrad, Johan Ihren, Daniel Karrenberg, 378 Peter Koch, Mike Patton, Mike StJohns, Paul Vixie, Sam Weiler, and 379 Suzanne Woolf. Apologies to anyone inadvertently omitted from the 380 above list. 382 7. References 384 7.1. Normative References 386 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 387 Requirement Levels", RFC 2119, BCP 14, March 1997. 389 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 390 RFC 2671, August 1999. 392 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 393 Wellington, "Secret Key Transaction Authentication for DNS 394 (TSIG)", RFC 2845, May 2000. 396 7.2. Informative References 398 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and 399 Languages", RFC 2277, BCP 18, January 1998. 401 Author's Address 403 Rob Austein 404 ISC 405 950 Charter Street 406 Redwood City, CA 94063 407 USA 409 Email: sra@isc.org 411 Intellectual Property Statement 413 The IETF takes no position regarding the validity or scope of any 414 Intellectual Property Rights or other rights that might be claimed to 415 pertain to the implementation or use of the technology described in 416 this document or the extent to which any license under such rights 417 might or might not be available; nor does it represent that it has 418 made any independent effort to identify any such rights. 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