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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNSIND Working Group Paul Vixie (Ed.) (ISC) 3 INTERNET-DRAFT Olafur Gudmundsson (TISLabs) 4 Donald Eastlake 3rd (IBM) 5 Brian Wellington (TISLabs) 6 November 1998 8 Amends: RFC 1035 10 Secret Key Transaction Signatures for DNS (TSIG) 12 Status of this Memo 14 This document is an Internet-Draft. Internet-Drafts are working 15 documents of the Internet Engineering Task Force (IETF), its areas, 16 and its working groups. Note that other groups may also distribute 17 working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six months 20 and may be updated, replaced, or obsoleted by other documents at any 21 time. It is inappropriate to use Internet-Drafts as reference 22 material or to cite them other than as ``work in progress.'' 24 To view the entire list of current Internet-Drafts, please check the 25 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 26 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern 27 Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 28 Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 30 Abstract 32 This protocol allows for transaction level authentication using 33 shared secrets and one way hashing. It can be used to authenticate 34 dynamic updates as coming from an approved client, or to authenticate 35 responses as coming from an approved recursive name server. 37 No provision has been made here for distributing the shared secrets; 38 it is expected that a network administrator will statically configure 39 name servers and clients using some out of band mechanism such as 40 sneaker-net until a secure automated mechanism for key distribution 41 is available. 43 1 - Introduction 45 1.1. The Domain Name System (DNS) [RFC1034, RFC1035] is a replicated 46 hierarchical distributed database system that provides information 47 fundamental to Internet operations, such as name <=> address translation 48 and mail handling information. DNS has recently been extended [RFC2065] 49 to provide for data origin authentication, and public key distribution, 50 all based on public key cryptography and public key based digital 51 signatures. To be practical, this form of security generally requires 52 extensive local caching of keys and tracing of authentication through 53 multiple keys and signatures to a pre-trusted locally configured key. 55 1.2. One difficulty with the [RFC2065] scheme is that common DNS 56 implementations include simple ``stub'' resolvers which do not have 57 caches. Such resolvers typically rely on a caching DNS server on 58 another host. It is impractical for these stub resolvers to perform 59 general [RFC2065] authentication and they would naturally depend on 60 their caching DNS server to perform such services for them. To do so 61 securely requires secure communication of queries and responses. 62 [RFC2065] provides public key transaction signatures to support this but 63 such signatures are very expensive computationally to generate. In 64 general, these require the same complex public key logic that is 65 impractical for stubs. This document specifies an efficient secret key 66 based transaction signature that avoids these difficulties. 68 1.3. A second area where use of straight [RFC2065] public key based 69 mechanisms may be impractical is authenticating dynamic update [RFC2136] 70 requests. [RFC2065] provides for request signatures but with [RFC2065] 71 they, like transaction signatures, require computationally expensive 72 public key cryptography and complex authentication logic. Secure Domain 73 Name System Dynamic Update ([RFC2137]) describes how different keys are 74 used in dynamically updated zones. This document's secret key based 75 signatures can be used to authenticate DNS update requests as well as 76 transaction responses, providing a lightweight alternative to the 77 protocol described by [RFC2137]. 79 1.4. A further use of this mechanishm is to protect zone transfers. In 80 this case the data covered would be the whole zone transfer including 81 any glue records sent. The protocol described by [RFC2065] does not 82 protect glue records and unsigned records unless SIG(0) (transaction 83 signature) is used. 85 1.5. The signature mechanism proposed in this document uses shared 86 secret keys to establish trust relationship between two entities. Such 87 keys must be protected in a fashion similar to private keys, lest a 88 third party masquerade as one of the intended parties (forge 89 signatures). There is an urgent need to provide simple and efficient 90 authentication between clients and local servers and this proposal 91 addresses that need. This proposal is unsuitable for general server to 92 server authentication for servers which speak with many other servers, 93 since key management would become unwieldy with the number of shared 94 keys going up quadratically. But it is suitable for many resolvers on 95 hosts that only talk to few recursive servers. 97 1.6. A server acting as an indirect caching resolver -- a ``forwarder'' 98 in common usage -- might use transaction signatures when communicating 99 with its small number of preconfigured ``upstream'' servers. Other uses 100 of DNS secret key signatures and possible systems for automatic secret 101 key distribution may be proposed in separate future documents. 103 1.7. New Assigned Numbers 105 RRTYPE = TSIG (250) 106 ERROR = 0..15 (a DNS RCODE) 107 ERROR = 16 (BADSIG) 108 ERROR = 17 (BADKEY) 109 ERROR = 18 (BADTIME) 111 2 - TSIG RR Format 113 2.1 TSIG RR Type 115 To provide secret key signatures, we use a new RR type whose mnemonic is 116 TSIG and whose type code is 250. TSIG is a meta-RR and can not be 117 stored. TSIG RRs can be used for authentication between DNS entities 118 that have established a shared secret key. TSIG RRs are dynamically 119 computed to cover a particular DNS transaction and are not DNS RRs in 120 the usual sense. 122 2.2 TSIG Calculation 124 As the TSIG RRs are related to one DNS request/response, there is no 125 value in storing or retransmitting them, thus the TSIG RR should be 126 discarded once it has been used to authenticate DNS message. The only 127 Message Digest algorithm specified in this document is ``HMAC-MD5'' (see 128 [RFC1321], [RFC2104]). Other algorithms can be specified at later date. 129 Names and definitions of new algorithms should be registered with IANA. 130 All multi-octet integers in TSIG Record are sent in network byte order 131 (see [RFC1035 2.3.2]). 133 2.3. Record Format 135 NAME A domain-like name of the key used. The name should reflect 136 the names of the hosts and uniquely identify the key among a 137 set of keys these two hosts may share at any given time. If 138 hosts A and B in same zone share a key then the key name could 139 be A-B-.. If two hosts in different zones share the 140 key the key name could be .A..B. It should 141 be possible for more than one key to be in simultaneous use 142 among a set of interacting hosts. The name only needs to be 143 meaningful to the communicating hosts but a meaningful 144 mnemonic name as above is strongly recommended. 146 The name may be used as a local index to the key involved and 147 it is recommended that it be globally unique. Where a key is 148 just shared between two hosts, its name actually only need 149 only be meaningful to them but it is recommended that the key 150 name be mnemonic and incorporate the resolver and server host 151 names in that order. 153 TYPE TSIG (250: Transaction SIGnature) 155 CLASS ANY 157 TTL 0 159 RdLen (variable) 160 RDATA 162 Field Name Data Type Notes 163 ------------------------------------------------------------------ 164 Algorithm Name domain-name Name of the algorithm 165 expressed as a domain name. 166 Time Signed u_int48_t seconds since 1-Jan-70 UTC. 167 Fudge u_int16_t seconds of error permitted 168 in Time Signed. 169 Signature Size u_int16_t number of octets in Signature. 170 Signature octet stream defined by Algorithm Name. 171 Original ID u_int16_t original message ID 172 Error u_int16_t expanded RCODE covering 173 signature processing. 174 Other Len u_int16_t length, in octets, of Other Data. 175 Other Data octet stream undefined by this protocol. 177 2.4. Example 179 NAME GW-DENVAX-0042.HOME.VIX.COM. 181 TYPE TSIG 183 CLASS ANY 185 TTL 0 187 RdLen as appropriate 189 RDATA 191 Field Name Contents 192 ------------------------------------------- 193 Algorithm Name HMAC-MD5.SIG-ALG.REG.INT. 194 Time Signed 853804800 195 Fudge 300 196 Signature Size as appropriate 197 Signature as appropriate 198 Original ID as appropriate 199 Error 0 (NOERROR) 200 Other Len 0 201 Other Data empty 203 3 - Protocol Operation 205 3.1. Effects of adding TSIG to outgoing message 207 Once the outgoing message has been constructed, the keyed message digest 208 operation can be performed. The resulting message digest will then be 209 stored in a TSIG which is appended to the additional data section. 210 Appending a transaction signature to an DNS message is not allowed to 211 result in a truncated response; a TCP connection must be used to prevent 212 the truncation. To force a TCP connection, the server is permitted to 213 return an answer with no data only TSIG attached and TC bit set and 214 RCODE 0 (NOERROR). The client should at this point retry the request 215 using TCP (per [RFC1035 4.2.2]). 217 3.2. TSIG processing on incoming messages 219 Upon receipt of a message with a TSIG RR, the TSIG RR is copied to a 220 safe location, removed from the DNS Message, and decremented out of the 221 DNS Message Headers ARCOUNT. At this point the keyed message digest 222 operation is performed. If the algorithm name or key name is unknown to 223 the recipient, or if the message digests do not match, the whole DNS 224 Message must be discarded. A response with RCODE 9 (NOTAUTH) should be 225 sent back to the originator with TSIG ERROR 17 (BADKEY), if no key is 226 available to sign this message it must be sent unsigned (Signature Size 227 == 0 and empty signature). A message to the system operations log 228 should to be generated, to warn the operations staff of a possible 229 security incident in progress. Care should be taken to ensure that 230 logging of this type of event does not open the system to a denial of 231 service attack. 233 3.3. Time values used in TSIG calculations 235 The data digested includes the two timer values in the TSIG header in 236 order to prevent replay attacks. If this were not done an attacker 237 could replay old messages but update the ``Time Signed'' and ``Fudge'' 238 fields to make the message look new. This data is named ``TSIG 239 Timers'', and for the purpose of digest calculation they are invoked in 240 their ``on the wire'' format, in the following order: first Time Signed, 241 then Fudge. For example: 243 Field Name Value Wire Format Meaning 244 --------------------------------------------------------------------------- 245 Time Signed 853804800 00 00 32 e4 07 00 Tue Jan 21 00:00:00 1997 246 Fudge 300 01 2C 5 minutes 247 3.4. TSIG Variables and Coverage 249 When generating or verifying a transaction signature, the following data 250 are digested, in network byte order or wire format, as appropriate: 252 3.4.1. DNS Message 254 A whole and complete DNS message in wire format, before the TSIG RR has 255 been added to the additional data section and before the DNS Message 256 Header's ARCOUNT field has been incremented to contain the TSIG RR. If 257 the message is of a type that can be forwarded (i.e. update) and the 258 message ID differs from the original message ID, the original message ID 259 is substituted for the message ID. 261 3.4.2. TSIG Variables 263 Source Field Name Notes 264 ------------------------------------------------------------------------ 265 TSIG RR NAME Key name, in canonical wire format 266 TSIG RR CLASS (Always ANY in the current specification) 267 TSIG RR TTL (Always 0 in the current specification) 268 TSIG RDATA Algorithm Name in canonical wire format 269 TSIG RDATA Time Signed in network byte order 270 TSIG RDATA Fudge in network byte order 271 TSIG RDATA Error in network byte order 272 TSIG RDATA Other Len in network byte order 273 TSIG RDATA Other Data exactly as transmitted 275 The RR RDLEN and RDATA Signature Length are not included in the hash 276 since they are not guaranteed to be knowable before the signature is 277 generated. 279 ``Canonical wire format'' means uncompressed labels shifted to lower 280 case. The use of label types other than 00 is not defined for this 281 specification. 283 3.4.3. Request Signature 285 Response signatures will include the request signature in their digest. 286 The request's signature is digested in wire format, including the 287 following fields: 289 Field Type Description 290 --------------------------------------------------------- 291 Signature Length u_int16_t in network byte order 292 Signature Data octet stream exactly as transmitted 294 3.5. Padding 296 Digested components are fed into the hashing function as a continuous 297 octet stream with no interfield padding. 299 4 - Protocol Details 301 4.1. TSIG generation on requests 303 Client performs the message digest operation and appends TSIG to 304 additional data section and transmits request to server. The client 305 must store the message digest from the request while awaiting an answer. 306 Digest components for requests are: 308 DNS Message (request) 309 TSIG Variables (response) 311 Note that some older name servers will not accept requests with a 312 nonempty additional data section, but clients should only attempt signed 313 transactions against servers who are known to support TSIG and share 314 some secret key with the client -- so, this is not a problem in 315 practice. 317 4.2. TSIG on Answers 319 When a server has generated a response to a signed request, it signs the 320 response using the same algorithm and key. Digest components are: 322 Request Signature 323 DNS Message (response) 324 TSIG Variables (response) 326 4.3. TSIG on TSIG Error returns 328 When a server detects an error in TSIG checks relating to the key or 329 signature, the server should send back an unsigned error message. If an 330 error is detected that does not relate to the key or signature, the 331 server should send back a signed error message. Digest components are: 333 Request signature (if the request signature validated) 334 DNS Message (response) 335 TSIG Variables (response) 337 The reason that the request is not included in this digest in some cases 338 is to make it possible for the client to verify the error. If the error 339 is not a TSIG error the response MUST be generated as specified in 340 [4.2]. 342 4.4. TSIG on TCP connection 344 A DNS TCP session can include multiple DNS envelopes. This is, for 345 example commonly used by AXFR. TSIG on such a connection can be used to 346 protect the connection from hijacking and provide data integrity. The 347 TSIG MUST be included on the first and last DNS envelopes. It can be 348 optionally placed on any intermediary envelopes. It is expensive to 349 include it on every envelopes, but it MUST be placed on at least every 350 100'th envelopes. The first envelope is processed as a standard answer, 351 and subsequent messages have the following digest components: 353 Prior Digest (running) 354 DNS Message (current message) 355 TSIG Timers (current message) 357 This allows client to rapidly detect when a transfer has been altered 358 and it can close the connection at that point and retry. Once client 359 TSIG check fails, the client MUST close the connection. If the client 360 does not get TSIG frequently enough (as specified above) it SHOULD 361 assume the connection has been hijacked and it SHOULD close the 362 connection. Client should treat this the same way as any other 363 interrupted transfer. 365 4.5. Server TSIG checks 367 Upon receipt of a message, server will check if there is a TSIG RR. If 368 one exists, the server is required to return a TSIG RR in the response. 369 The server MUST perform the following checks in the following order, 370 check KEY, check TIME values, check Signature. 372 4.5.1. KEY check and error handling 374 If a non-forwarding server does not recognize the key used by the client 375 the server MUST generate an error response with RCODE 9 (NOTAUTH) and 376 TSIG ERROR 17 (BADKEY). This response should be unsigned as specified 377 in [4.3]. The server should log the error. 379 4.5.2. TIME check and Error handling 381 If the server time is outside the time interval specified by the request 382 (which is: Time Signed, plus/minus Fudge), the server MUST generate an 383 error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18 (BADTIME). This 384 response MUST be signed by the same key. It MUST include the client's 385 current time in the time signed field, the server's current time in the 386 other data field, and 6 in the other data length field. This is done so 387 that the client can verify a message with a BADTIME error without the 388 verification detecting another BADTIME error. The data signed is 389 specified in [4.3]. 391 4.5.3. Signature check and error handling 393 If TSIG fails to verify, the server MUST generate an error response as 394 specified in [4.3] with RCODE of 9 (NOTAUTH) and TSIG ERROR 16 (BADSIG). 395 The server MUST sign this error response with the same key the client 396 used. 398 4.6. Client processing of answer 400 When a client receives a response from a server it expects a TSIG from, 401 it first checks if the TSIG RR is present in the response. Otherwise 402 the response is treated as having a format error and discarded. The 403 client then extracts the TSIG, adjusts the ARCOUNT, and calculates the 404 keyed digest in the same way as the server. If the TSIG does not 405 validate, that response must be discarded, unless the RCODE is 9 406 (NOTAUTH), in which case the client should attempt to verify the 407 response as it was TSIG error as specified in [4.3]. An message 408 containing an unsigned TSIG record or a TSIG record which fails 409 verification should not be considered an acceptable response; the client 410 should log an error and continue to wait for a signed response until the 411 request times out. 413 4.6.1. Key error handling 415 If an RCODE on a response is 9 (NOTAUTH), and the response TSIG 416 validates, and the TSIG key is different from the key used on the 417 request, then this is a key error. Client should retry the request 418 using the key specified by server. This should never occur, as a server 419 should never sign a response with a different key than signed the 420 request. 422 4.6.2. Time error handling 424 If the response RCODE is 9 (NOTAUTH), and TSIG ERROR is 18 (BADTIME) or 425 the TSIG times in request and answer do not overlap, then this is a TIME 426 error. This is an indication that client and server are not clock 427 synchronized. In this case the client should log the event. DNS 428 resolvers MUST NOT adjust any clocks in the client based on BADTIME 429 errors, but the server's time in other data field should be logged. 431 4.6.3. Signature error handling 433 If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG), this 434 is a signature error, and client MAY retry the request with a new 435 request ID but it would be better to try a different shared key if one 436 is available. Client SHOULD keep track of how many times each key has 437 Signature errors. Clients should log this event. 439 4.7. Special considerations for forwarding servers 441 A server acting as a Forwarding Server of a DNS message should check for 442 the existence of the TSIG record. If the name on the TSIG is not of a 443 secret that the server shares with the originator the server will 444 forward the message unchanged including the TSIG. If the name of the 445 TSIG is of a key this server shares with the originator it processes the 446 TSIG. If the TSIG passes all checks, the forwarding server has the 447 obligation of including a TSIG of his own, to the destination or the 448 next forwarder. If no transaction security is available to the 449 destination and response has the AD flag (see [RFC2065]), the forwarder 450 MUST unset the AD flag before adding the TSIG to the answer. 452 5 - Shared Secrets 454 5.1. Secret keys are very sensitive information and all available steps 455 should be taken to protect them on every host on which they are stored. 456 Generally such hosts need to be physically protected. If they are 457 multi-user machines, great care should be taken that unprivileged users 458 have no access to keying material. Resolvers usually run unprivileged, 459 which means all users of a host will usually be able to see whatever 460 configuration data is used by the resolver. 462 5.2. A name server usually runs privileged, which means its 463 configuration data need not be visible to all users of the host. For 464 this reason, a host that implements transaction signatures should 465 probably be configured with a ``stub resolver'' and a local caching and 466 forwarding name server. This presents a special problem for [RFC2136] 467 which otherwise depends on clients to communicate only with a zone's 468 authoritative name servers. 470 5.3. Use of strong random shared secrets is essential to the security of 471 TSIG. See [RFC1750] for a discussion of this issue. The secret should 472 be at least as long as the keyed message digest , i.e., 16 bytes for 473 HMAC-MD5 or 20 bytes for HMAC-SHA1. 475 6 - Security Considerations 477 6.1. The approach specified here is computationally much less expensive 478 than the signatures specified in [RFC2065]. As long as the shared 479 secret key is not compromised, strong authentication is provided for the 480 last hop from a local name server to the user resolver. 482 6.2. Secret keys should be changed periodically. If the client host has 483 been compromised, the server should suspend the use of all secrets known 484 to that client. If possible, secrets should be stored in encrypted 485 form. Secrets should never be transmitted in the clear over any 486 network. This document does not address the issue on how to distribute 487 secrets. Secrets should never be shared by more than two entities. 489 6.3. This mechanism does not authenticate source data, only its 490 transmission between two parties who share some secret. The original 491 source data can come from a compromised zone master or can be corrupted 492 during transit from an authentic zone master to some ``caching 493 forwarder.'' However, if the server is faithfully performing the full 494 [RFC2065] security checks, then only security checked data will be 495 available to the client. 497 7 - References 499 [RFC1034] P. Mockapetris, ``Domain Names - Concepts and Facilities,'' 500 RFC 1034, ISI, November 1987. 502 [RFC1035] P. Mockapetris, ``Domain Names - Implementation and 503 Specification,'' RFC 1034, ISI, November 1987. 505 [RFC1321] R. Rivest, ``The MD5 Message-Digest Algorithm,'' RFC 1321, 506 MIT LCS & RSA Data Security, Inc., April 1992. 508 [RFC1750] D. Eastlake, S. Crocker, J. Schiller, ``Randomness 509 Recommendations for Security,'' RFC 1750, DEC, CyberCash & 510 MIT, December 1995. 512 [RFC2065] D. Eastlake , C Kaufman, ``Domain Name System Security 513 Extensions,'' RFC 2065, CyberCash & Iris, January 1997. 515 [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, ``HMAC-MD5: Keyed-MD5 516 for Message Authentication,'' RFC 2104 , IBM, UCSD & IBM, 517 February 1997. 519 [RFC2136] P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ``Dynamic 520 Updates in the Domain Name System,'' RFC 2136, ISC & Bellcore 521 & Cisco & DEC, April 1997. 523 [RFC2137] D. Eastlake 3rd ``Secure Domain Name System Dynamic Update,'' 524 CyberCash, April 1997. 526 9 - Author's Addresses 528 Paul Vixie Olafur Gudmundsson 529 Internet Software Consortium TIS Labs at Network Associates 530 950 Charter Street 3060 Washington Road, Route 97 531 Redwood City, CA 94063 Glenwood, MD 21738 532 +1 650 779 7001 +1 301 854 6889 533 535 Donald E. Eastlake 3rd Brian Wellington 536 IBM TIS Labs at Network Associates 537 318 Acton Street 3060 Washington Road, Route 97 538 Carlisle, MA 01741 USA Glenwood, MD 21738 539 +1 978 287 4877 +1 301 854 6889 540