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RFC 2119 keyword, line 346: '...ror the response MUST be generated as ...' RFC 2119 keyword, line 354: '... TSIG MUST be included on the first ...' RFC 2119 keyword, line 356: '...nvelopes, but it MUST be placed on at ...' RFC 2119 keyword, line 366: '...ails, the client MUST close the connec...' RFC 2119 keyword, line 367: '... enough (as specified above) it SHOULD...' (10 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 167 has weird spacing: '...hm Name doma...' == Line 172 has weird spacing: '...re Size u_in...' == Line 173 has weird spacing: '... stream defi...' == Line 179 has weird spacing: '... stream unde...' == Line 191 has weird spacing: '... RdLen as ap...' == (4 more instances...) -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (June 1999) is 9075 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Missing reference section? 'RFC1034' on line 515 looks like a reference -- Missing reference section? 'RFC1035' on line 518 looks like a reference -- Missing reference section? 'RFC2535' on line 539 looks like a reference -- Missing reference section? 'RFC2136' on line 532 looks like a reference -- Missing reference section? 'RFC2137' on line 536 looks like a reference -- Missing reference section? 'RFC1321' on line 521 looks like a reference -- Missing reference section? 'RFC2104' on line 528 looks like a reference -- Missing reference section? 'RFC1750' on line 524 looks like a reference Summary: 9 errors (**), 0 flaws (~~), 8 warnings (==), 10 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 DNSIND Working Group Paul Vixie (Ed.) (ISC) 2 INTERNET-DRAFT Olafur Gudmundsson (TISLabs) 3 Donald Eastlake 3rd (IBM) 4 Brian Wellington (TISLabs) 5 June 1999 7 Amends: RFC 1035 9 Secret Key Transaction Signatures for DNS (TSIG) 11 Status of this Memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 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 Abstract 34 This protocol allows for transaction level authentication using 35 shared secrets and one way hashing. It can be used to authenticate 36 dynamic updates as coming from an approved client, or to authenticate 37 responses as coming from an approved recursive name server. 39 No provision has been made here for distributing the shared secrets; 40 it is expected that a network administrator will statically configure 41 name servers and clients using some out of band mechanism such as 42 sneaker-net until a secure automated mechanism for key distribution 43 is available. 45 1 - Introduction 47 1.1. The Domain Name System (DNS) [RFC1034, RFC1035] is a replicated 48 hierarchical distributed database system that provides information 49 fundamental to Internet operations, such as name <=> address translation 50 and mail handling information. DNS has recently been extended [RFC2535] 51 to provide for data origin authentication, and public key distribution, 52 all based on public key cryptography and public key based digital 53 signatures. To be practical, this form of security generally requires 54 extensive local caching of keys and tracing of authentication through 55 multiple keys and signatures to a pre-trusted locally configured key. 57 1.2. One difficulty with the [RFC2535] scheme is that common DNS 58 implementations include simple ``stub'' resolvers which do not have 59 caches. Such resolvers typically rely on a caching DNS server on 60 another host. It is impractical for these stub resolvers to perform 61 general [RFC2535] authentication and they would naturally depend on 62 their caching DNS server to perform such services for them. To do so 63 securely requires secure communication of queries and responses. 64 [RFC2535] provides public key transaction signatures to support this but 65 such signatures are very expensive computationally to generate. In 66 general, these require the same complex public key logic that is 67 impractical for stubs. This document specifies an efficient secret key 68 based transaction signature that avoids these difficulties. 70 1.3. A second area where use of straight [RFC2535] public key based 71 mechanisms may be impractical is authenticating dynamic update [RFC2136] 72 requests. [RFC2535] provides for request signatures but with [RFC2535] 73 they, like transaction signatures, require computationally expensive 74 public key cryptography and complex authentication logic. Secure Domain 75 Name System Dynamic Update ([RFC2137]) describes how different keys are 76 used in dynamically updated zones. This document's secret key based 77 signatures can be used to authenticate DNS update requests as well as 78 transaction responses, providing a lightweight alternative to the 79 protocol described by [RFC2137]. 81 1.4. A further use of this mechanishm is to protect zone transfers. In 82 this case the data covered would be the whole zone transfer including 83 any glue records sent. The protocol described by [RFC2535] does not 84 protect glue records and unsigned records unless SIG(0) (transaction 85 signature) is used. 87 1.5. The signature mechanism proposed in this document uses shared 88 secret keys to establish trust relationship between two entities. Such 89 keys must be protected in a fashion similar to private keys, lest a 90 third party masquerade as one of the intended parties (forge 91 signatures). There is an urgent need to provide simple and efficient 92 authentication between clients and local servers and this proposal 93 addresses that need. This proposal is unsuitable for general server to 94 server authentication for servers which speak with many other servers, 95 since key management would become unwieldy with the number of shared 96 keys going up quadratically. But it is suitable for many resolvers on 97 hosts that only talk to few recursive servers. 99 1.6. A server acting as an indirect caching resolver -- a ``forwarder'' 100 in common usage -- might use transaction signatures when communicating 101 with its small number of preconfigured ``upstream'' servers. Other uses 102 of DNS secret key signatures and possible systems for automatic secret 103 key distribution may be proposed in separate future documents. 105 1.7. New Assigned Numbers 107 RRTYPE = TSIG (250) 108 ERROR = 0..15 (a DNS RCODE) 109 ERROR = 16 (BADSIG) 110 ERROR = 17 (BADKEY) 111 ERROR = 18 (BADTIME) 113 2 - TSIG RR Format 115 2.1 TSIG RR Type 117 To provide secret key signatures, we use a new RR type whose mnemonic is 118 TSIG and whose type code is 250. TSIG is a meta-RR and can not be 119 stored. TSIG RRs can be used for authentication between DNS entities 120 that have established a shared secret key. TSIG RRs are dynamically 121 computed to cover a particular DNS transaction and are not DNS RRs in 122 the usual sense. 124 2.2 TSIG Calculation 126 As the TSIG RRs are related to one DNS request/response, there is no 127 value in storing or retransmitting them, thus the TSIG RR should be 128 discarded once it has been used to authenticate DNS message. The only 129 Message Digest algorithm specified in this document is ``HMAC-MD5'' (see 130 [RFC1321], [RFC2104]). Other algorithms can be specified at later date. 131 Names and definitions of new algorithms should be registered with IANA. 132 All multi-octet integers in TSIG Record are sent in network byte order 133 (see [RFC1035 2.3.2]). 135 2.3. Record Format 137 NAME A domain-like name of the key used. The name should reflect 138 the names of the hosts and uniquely identify the key among a 139 set of keys these two hosts may share at any given time. If 140 hosts A and B in same zone share a key then the key name could 141 be A-B-.. If two hosts in different zones share the 142 key the key name could be .A..B. It should 143 be possible for more than one key to be in simultaneous use 144 among a set of interacting hosts. The name only needs to be 145 meaningful to the communicating hosts but a meaningful 146 mnemonic name as above is strongly recommended. 148 The name may be used as a local index to the key involved and 149 it is recommended that it be globally unique. Where a key is 150 just shared between two hosts, its name actually only need 151 only be meaningful to them but it is recommended that the key 152 name be mnemonic and incorporate the resolver and server host 153 names in that order. 155 TYPE TSIG (250: Transaction SIGnature) 157 CLASS ANY 159 TTL 0 161 RdLen (variable) 162 RDATA 164 Field Name Data Type Notes 166 ------------------------------------------------------------------ 167 Algorithm Name domain-name Name of the algorithm 168 expressed as a domain name. 169 Time Signed u_int48_t seconds since 1-Jan-70 UTC. 170 Fudge u_int16_t seconds of error permitted 171 in Time Signed. 172 Signature Size u_int16_t number of octets in Signature. 173 Signature octet stream defined by Algorithm Name. 174 Original ID u_int16_t original message ID 175 Error u_int16_t expanded RCODE covering 176 signature processing. 177 Other Len u_int16_t length, in octets, of Other 178 Data. 179 Other Data octet stream undefined by this protocol. 181 2.4. Example 183 NAME GW-DENVAX-0042.HOME.VIX.COM. 185 TYPE TSIG 187 CLASS ANY 189 TTL 0 191 RdLen as appropriate 193 RDATA 195 Field Name Contents 196 ------------------------------------------- 197 Algorithm Name HMAC-MD5.SIG-ALG.REG.INT. 198 Time Signed 853804800 199 Fudge 300 200 Signature Size as appropriate 201 Signature as appropriate 202 Original ID as appropriate 203 Error 0 (NOERROR) 204 Other Len 0 205 Other Data empty 207 3 - Protocol Operation 209 3.1. Effects of adding TSIG to outgoing message 211 Once the outgoing message has been constructed, the keyed message digest 212 operation can be performed. The resulting message digest will then be 213 stored in a TSIG which is appended to the additional data section. 214 Appending a transaction signature to an DNS message is not allowed to 215 result in a truncated response; a TCP connection must be used to prevent 216 the truncation. To force a TCP connection, the server is permitted to 217 return an answer with no data only TSIG attached and TC bit set and 218 RCODE 0 (NOERROR). The client should at this point retry the request 219 using TCP (per [RFC1035 4.2.2]). 221 3.2. TSIG processing on incoming messages 223 Upon receipt of a message with a TSIG RR, the TSIG RR is copied to a 224 safe location, removed from the DNS Message, and decremented out of the 225 DNS Message Headers ARCOUNT. At this point the keyed message digest 226 operation is performed. If the algorithm name or key name is unknown to 227 the recipient, or if the message digests do not match, the whole DNS 228 Message must be discarded. A response with RCODE 9 (NOTAUTH) should be 229 sent back to the originator with TSIG ERROR 17 (BADKEY), if no key is 230 available to sign this message it must be sent unsigned (Signature Size 231 == 0 and empty signature). A message to the system operations log 232 should to be generated, to warn the operations staff of a possible 233 security incident in progress. Care should be taken to ensure that 234 logging of this type of event does not open the system to a denial of 235 service attack. 237 3.3. Time values used in TSIG calculations 239 The data digested includes the two timer values in the TSIG header in 240 order to prevent replay attacks. If this were not done an attacker 241 could replay old messages but update the ``Time Signed'' and ``Fudge'' 242 fields to make the message look new. This data is named ``TSIG 243 Timers'', and for the purpose of digest calculation they are invoked in 244 their ``on the wire'' format, in the following order: first Time Signed, 245 then Fudge. For example: 247 Field Name Value Wire Format Meaning 249 --------------------------------------------------------------------------- 250 Time Signed 853804800 00 00 32 e4 07 00 Tue Jan 21 00:00:00 1997 251 Fudge 300 01 2C 5 minutes 252 3.4. TSIG Variables and Coverage 254 When generating or verifying a transaction signature, the following data 255 are digested, in network byte order or wire format, as appropriate: 257 3.4.1. DNS Message 259 A whole and complete DNS message in wire format, before the TSIG RR has 260 been added to the additional data section and before the DNS Message 261 Header's ARCOUNT field has been incremented to contain the TSIG RR. If 262 the message ID differs from the original message ID, the original 263 message ID is substituted for the message ID. 265 3.4.2. TSIG Variables 267 Source Field Name Notes 268 ------------------------------------------------------------------------ 269 TSIG RR NAME Key name, in canonical wire format 270 TSIG RR CLASS (Always ANY in the current specification) 271 TSIG RR TTL (Always 0 in the current specification) 272 TSIG RDATA Algorithm Name in canonical wire format 273 TSIG RDATA Time Signed in network byte order 274 TSIG RDATA Fudge in network byte order 275 TSIG RDATA Error in network byte order 276 TSIG RDATA Other Len in network byte order 277 TSIG RDATA Other Data exactly as transmitted 279 The RR RDLEN and RDATA Signature Length are not included in the hash 280 since they are not guaranteed to be knowable before the signature is 281 generated. 283 The Original ID field is not included in this section, as it has already 284 been substituted for the message ID in the DNS header and hashed. 286 ``Canonical wire format'' means uncompressed labels shifted to lower 287 case. The use of label types other than 00 is not defined for this 288 specification. 290 3.4.3. Request Signature 292 Response signatures will include the request signature in their digest. 293 The request's signature is digested in wire format, including the 294 following fields: 296 Field Type Description 297 --------------------------------------------------------- 298 Signature Length u_int16_t in network byte order 299 Signature Data octet stream exactly as transmitted 301 3.5. Padding 303 Digested components are fed into the hashing function as a continuous 304 octet stream with no interfield padding. 306 4 - Protocol Details 308 4.1. TSIG generation on requests 310 Client performs the message digest operation and appends TSIG to 311 additional data section and transmits request to server. The client 312 must store the message digest from the request while awaiting an answer. 313 Digest components for requests are: 315 DNS Message (request) 316 TSIG Variables (response) 318 Note that some older name servers will not accept requests with a 319 nonempty additional data section, but clients should only attempt signed 320 transactions against servers who are known to support TSIG and share 321 some secret key with the client -- so, this is not a problem in 322 practice. 324 4.2. TSIG on Answers 326 When a server has generated a response to a signed request, it signs the 327 response using the same algorithm and key. Digest components are: 329 Request Signature 330 DNS Message (response) 331 TSIG Variables (response) 333 4.3. TSIG on TSIG Error returns 335 When a server detects an error in TSIG checks relating to the key or 336 signature, the server should send back an unsigned error message. If an 337 error is detected that does not relate to the key or signature, the 338 server should send back a signed error message. Digest components are: 340 Request signature (if the request signature validated) 341 DNS Message (response) 342 TSIG Variables (response) 344 The reason that the request is not included in this digest in some cases 345 is to make it possible for the client to verify the error. If the error 346 is not a TSIG error the response MUST be generated as specified in 347 [4.2]. 349 4.4. TSIG on TCP connection 351 A DNS TCP session can include multiple DNS envelopes. This is, for 352 example commonly used by AXFR. TSIG on such a connection can be used to 353 protect the connection from hijacking and provide data integrity. The 354 TSIG MUST be included on the first and last DNS envelopes. It can be 355 optionally placed on any intermediary envelopes. It is expensive to 356 include it on every envelopes, but it MUST be placed on at least every 357 100'th envelopes. The first envelope is processed as a standard answer, 358 and subsequent messages have the following digest components: 360 Prior Digest (running) 361 DNS Message (current message) 362 TSIG Timers (current message) 364 This allows client to rapidly detect when a transfer has been altered 365 and it can close the connection at that point and retry. Once client 366 TSIG check fails, the client MUST close the connection. If the client 367 does not get TSIG frequently enough (as specified above) it SHOULD 368 assume the connection has been hijacked and it SHOULD close the 369 connection. Client should treat this the same way as any other 370 interrupted transfer. 372 4.5. Server TSIG checks 374 Upon receipt of a message, server will check if there is a TSIG RR. If 375 one exists, the server is required to return a TSIG RR in the response. 376 The server MUST perform the following checks in the following order, 377 check KEY, check TIME values, check Signature. 379 4.5.1. KEY check and error handling 381 If a non-forwarding server does not recognize the key used by the client 382 the server MUST generate an error response with RCODE 9 (NOTAUTH) and 383 TSIG ERROR 17 (BADKEY). This response should be unsigned as specified 384 in [4.3]. The server should log the error. 386 4.5.2. TIME check and error handling 388 If the server time is outside the time interval specified by the request 389 (which is: Time Signed, plus/minus Fudge), the server MUST generate an 390 error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18 (BADTIME). This 391 response MUST be signed by the same key. It MUST include the client's 392 current time in the time signed field, the server's current time in the 393 other data field, and 6 in the other data length field. This is done so 394 that the client can verify a message with a BADTIME error without the 395 verification detecting another BADTIME error. The data signed is 396 specified in [4.3]. 398 4.5.3. Signature check and error handling 400 If TSIG fails to verify, the server MUST generate an error response as 401 specified in [4.3] with RCODE of 9 (NOTAUTH) and TSIG ERROR 16 (BADSIG). 402 This response should be unsigned as specified in [4.3]. The server 403 should log the error. 405 4.6. Client processing of answer 407 When a client receives a response from a server it expects a TSIG from, 408 it first checks if the TSIG RR is present in the response. Otherwise 409 the response is treated as having a format error and discarded. The 410 client then extracts the TSIG, adjusts the ARCOUNT, and calculates the 411 keyed digest in the same way as the server. If the TSIG does not 412 validate, that response must be discarded, unless the RCODE is 9 413 (NOTAUTH), in which case the client should attempt to verify the 414 response as it was TSIG error as specified in [4.3]. An message 415 containing an unsigned TSIG record or a TSIG record which fails 416 verification should not be considered an acceptable response; the client 417 should log an error and continue to wait for a signed response until the 418 request times out. 420 4.6.1. Key error handling 422 If an RCODE on a response is 9 (NOTAUTH), and the response TSIG 423 validates, and the TSIG key is different from the key used on the 424 request, then this is a key error. Client should retry the request 425 using the key specified by server. This should never occur, as a server 426 should never sign a response with a different key than signed the 427 request. 429 4.6.2. Time error handling 431 If the response RCODE is 9 (NOTAUTH), and TSIG ERROR is 18 (BADTIME) or 432 the TSIG times in request and answer do not overlap, then this is a TIME 433 error. This is an indication that client and server are not clock 434 synchronized. In this case the client should log the event. DNS 435 resolvers MUST NOT adjust any clocks in the client based on BADTIME 436 errors, but the server's time in other data field should be logged. 438 4.6.3. Signature error handling 440 If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG), this 441 is a signature error, and client MAY retry the request with a new 442 request ID but it would be better to try a different shared key if one 443 is available. Client SHOULD keep track of how many times each key has 444 Signature errors. Clients should log this event. 446 4.7. Special considerations for forwarding servers 448 A server acting as a Forwarding Server of a DNS message should check for 449 the existence of the TSIG record. If the name on the TSIG is not of a 450 secret that the server shares with the originator the server will 451 forward the message unchanged including the TSIG. If the name of the 452 TSIG is of a key this server shares with the originator it processes the 453 TSIG. If the TSIG passes all checks, the forwarding server has the 454 obligation of including a TSIG of his own, to the destination or the 455 next forwarder. If no transaction security is available to the 456 destination and response has the AD flag (see [RFC2535]), the forwarder 457 MUST unset the AD flag before adding the TSIG to the answer. 459 5 - Shared Secrets 461 5.1. Secret keys are very sensitive information and all available steps 462 should be taken to protect them on every host on which they are stored. 463 Generally such hosts need to be physically protected. If they are 464 multi-user machines, great care should be taken that unprivileged users 465 have no access to keying material. Resolvers usually run unprivileged, 466 which means all users of a host will usually be able to see whatever 467 configuration data is used by the resolver. 469 5.2. A name server usually runs privileged, which means its 470 configuration data need not be visible to all users of the host. For 471 this reason, a host that implements transaction signatures should 472 probably be configured with a ``stub resolver'' and a local caching and 473 forwarding name server. This presents a special problem for [RFC2136] 474 which otherwise depends on clients to communicate only with a zone's 475 authoritative name servers. 477 5.3. Use of strong random shared secrets is essential to the security of 478 TSIG. See [RFC1750] for a discussion of this issue. The secret should 479 be at least as long as the keyed message digest , i.e., 16 bytes for 480 HMAC-MD5 or 20 bytes for HMAC-SHA1. 482 6 - Security Considerations 484 6.1. The approach specified here is computationally much less expensive 485 than the signatures specified in [RFC2535]. As long as the shared 486 secret key is not compromised, strong authentication is provided for the 487 last hop from a local name server to the user resolver. 489 6.2. Secret keys should be changed periodically. If the client host has 490 been compromised, the server should suspend the use of all secrets known 491 to that client. If possible, secrets should be stored in encrypted 492 form. Secrets should never be transmitted in the clear over any 493 network. This document does not address the issue on how to distribute 494 secrets. Secrets should never be shared by more than two entities. 496 6.3. This mechanism does not authenticate source data, only its 497 transmission between two parties who share some secret. The original 498 source data can come from a compromised zone master or can be corrupted 499 during transit from an authentic zone master to some ``caching 500 forwarder.'' However, if the server is faithfully performing the full 501 [RFC2535] security checks, then only security checked data will be 502 available to the client. 504 7 - IANA Considerations 506 A new algorithm name should be a valid domain name of the type 507 algorithm-name.SIG-ALG.REG.INT. This requires an IETF consensus. 509 Adding new error codes requires an IETF consensus. 511 IANA must maintain control over the SIG-ALG.REG.INT domain. 513 7 - References 515 [RFC1034] P. Mockapetris, ``Domain Names - Concepts and Facilities,'' 516 RFC 1034, ISI, November 1987. 518 [RFC1035] P. Mockapetris, ``Domain Names - Implementation and 519 Specification,'' RFC 1034, ISI, November 1987. 521 [RFC1321] R. Rivest, ``The MD5 Message-Digest Algorithm,'' RFC 1321, 522 MIT LCS & RSA Data Security, Inc., April 1992. 524 [RFC1750] D. Eastlake, S. Crocker, J. Schiller, ``Randomness 525 Recommendations for Security,'' RFC 1750, DEC, CyberCash & 526 MIT, December 1995. 528 [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, ``HMAC-MD5: Keyed-MD5 529 for Message Authentication,'' RFC 2104 , IBM, UCSD & IBM, 530 February 1997. 532 [RFC2136] P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ``Dynamic 533 Updates in the Domain Name System,'' RFC 2136, ISC & Bellcore 534 & Cisco & DEC, April 1997. 536 [RFC2137] D. Eastlake 3rd ``Secure Domain Name System Dynamic Update,'' 537 CyberCash, April 1997. 539 [RFC2535] D. Eastlake, ``Domain Name System Security Extensions,'' RFC 540 2535, IBM, March 1999. 542 9 - Authors' Addresses 544 Paul Vixie Olafur Gudmundsson 545 Internet Software Consortium TIS Labs at Network Associates 546 950 Charter Street 3060 Washington Road, Route 97 547 Redwood City, CA 94063 Glenwood, MD 21738 548 +1 650 779 7001 +1 443 259 2389 549 551 Donald E. Eastlake 3rd Brian Wellington 552 IBM TIS Labs at Network Associates 553 65 Shindegan Hill Road, RR #1 3060 Washington Road, Route 97 554 Carmel, NY 10512 USA Glenwood, MD 21738 555 +1 914 783 7913 +1 443 259 2369 556