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7 Network Working Group P. Vixie
8 Request for Comments: 2845 ISC
9 Category: Standards Track O. Gudmundsson
10 Updates: 1035 NAI Labs
11 D. Eastlake 3rd
12 Motorola
13 B. Wellington
14 Nominum
15 May 2000
18 Secret Key Transaction Authentication for DNS (TSIG)
20 Status of this Memo
22 This document specifies an Internet standards track protocol for the
23 Internet community, and requests discussion and suggestions for
24 improvements. Please refer to the current edition of the "Internet
25 Official Protocol Standards" (STD 1) for the standardization state
26 and status of this protocol. Distribution of this memo is unlimited.
28 Copyright Notice
30 Copyright (C) The Internet Society (2000). All Rights Reserved.
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
50 translation and mail handling information. DNS has recently been
51 extended [RFC2535] to provide for data origin authentication, and
52 public key distribution, all based on public key cryptography and
53 public key based digital signatures. To be practical, this form of
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60 RFC 2845 DNS TSIG May 2000
63 security generally requires extensive local caching of keys and
64 tracing of authentication through multiple keys and signatures to a
65 pre-trusted locally configured key.
67 1.2. One difficulty with the [RFC2535] scheme is that common DNS
68 implementations include simple "stub" resolvers which do not have
69 caches. Such resolvers typically rely on a caching DNS server on
70 another host. It is impractical for these stub resolvers to perform
71 general [RFC2535] authentication and they would naturally depend on
72 their caching DNS server to perform such services for them. To do so
73 securely requires secure communication of queries and responses.
74 [RFC2535] provides public key transaction signatures to support this,
75 but such signatures are very expensive computationally to generate.
76 In general, these require the same complex public key logic that is
77 impractical for stubs. This document specifies use of a message
78 authentication code (MAC), specifically HMAC-MD5 (a keyed hash
79 function), to provide an efficient means of point-to-point
80 authentication and integrity checking for transactions.
82 1.3. A second area where use of straight [RFC2535] public key based
83 mechanisms may be impractical is authenticating dynamic update
84 [RFC2136] requests. [RFC2535] provides for request signatures but
85 with [RFC2535] they, like transaction signatures, require
86 computationally expensive public key cryptography and complex
87 authentication logic. Secure Domain Name System Dynamic Update
88 ([RFC2137]) describes how different keys are used in dynamically
89 updated zones. This document's secret key based MACs can be used to
90 authenticate DNS update requests as well as transaction responses,
91 providing a lightweight alternative to the protocol described by
92 [RFC2137].
94 1.4. A further use of this mechanism is to protect zone transfers.
95 In this case the data covered would be the whole zone transfer
96 including any glue records sent. The protocol described by [RFC2535]
97 does not protect glue records and unsigned records unless SIG(0)
98 (transaction signature) is used.
100 1.5. The authentication mechanism proposed in this document uses
101 shared secret keys to establish a trust relationship between two
102 entities. Such keys must be protected in a fashion similar to
103 private keys, lest a third party masquerade as one of the intended
104 parties (forge MACs). There is an urgent need to provide simple and
105 efficient authentication between clients and local servers and this
106 proposal addresses that need. This proposal is unsuitable for
107 general server to server authentication for servers which speak with
108 many other servers, since key management would become unwieldy with
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116 RFC 2845 DNS TSIG May 2000
119 the number of shared keys going up quadratically. But it is suitable
120 for many resolvers on hosts that only talk to a few recursive
121 servers.
123 1.6. A server acting as an indirect caching resolver -- a "forwarder"
124 in common usage -- might use transaction-based authentication when
125 communicating with its small number of preconfigured "upstream"
126 servers. Other uses of DNS secret key authentication and possible
127 systems for automatic secret key distribution may be proposed in
128 separate future documents.
130 1.7. New Assigned Numbers
132 RRTYPE = TSIG (250)
133 ERROR = 0..15 (a DNS RCODE)
134 ERROR = 16 (BADSIG)
135 ERROR = 17 (BADKEY)
136 ERROR = 18 (BADTIME)
138 1.8. The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and
139 "MAY" in this document are to be interpreted as described in [RFC
140 2119].
142 2 - TSIG RR Format
144 2.1 TSIG RR Type
146 To provide secret key authentication, we use a new RR type whose
147 mnemonic is TSIG and whose type code is 250. TSIG is a meta-RR and
148 MUST not be cached. TSIG RRs are used for authentication between DNS
149 entities that have established a shared secret key. TSIG RRs are
150 dynamically computed to cover a particular DNS transaction and are
151 not DNS RRs in the usual sense.
153 2.2 TSIG Calculation
155 As the TSIG RRs are related to one DNS request/response, there is no
156 value in storing or retransmitting them, thus the TSIG RR is
157 discarded once it has been used to authenticate a DNS message. The
158 only message digest algorithm specified in this document is "HMAC-
159 MD5" (see [RFC1321], [RFC2104]). The "HMAC-MD5" algorithm is
160 mandatory to implement for interoperability. Other algorithms can be
161 specified at a later date. Names and definitions of new algorithms
162 MUST be registered with IANA. All multi-octet integers in the TSIG
163 record are sent in network byte order (see [RFC1035 2.3.2]).
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175 2.3. Record Format
177 NAME The name of the key used in domain name syntax. The name
178 should reflect the names of the hosts and uniquely identify
179 the key among a set of keys these two hosts may share at any
180 given time. If hosts A.site.example and B.example.net share a
181 key, possibilities for the key name include
182 <id>.A.site.example, <id>.B.example.net, and
183 <id>.A.site.example.B.example.net. It should be possible for
184 more than one key to be in simultaneous use among a set of
185 interacting hosts. The name only needs to be meaningful to
186 the communicating hosts but a meaningful mnemonic name as
187 above is strongly recommended.
189 The name may be used as a local index to the key involved and
190 it is recommended that it be globally unique. Where a key is
191 just shared between two hosts, its name actually only need
192 only be meaningful to them but it is recommended that the key
193 name be mnemonic and incorporate the resolver and server host
194 names in that order.
196 TYPE TSIG (250: Transaction SIGnature)
198 CLASS ANY
200 TTL 0
202 RdLen (variable)
204 RDATA
206 Field Name Data Type Notes
207 --------------------------------------------------------------
208 Algorithm Name domain-name Name of the algorithm
209 in domain name syntax.
210 Time Signed u_int48_t seconds since 1-Jan-70 UTC.
211 Fudge u_int16_t seconds of error permitted
212 in Time Signed.
213 MAC Size u_int16_t number of octets in MAC.
214 MAC octet stream defined by Algorithm Name.
215 Original ID u_int16_t original message ID
216 Error u_int16_t expanded RCODE covering
217 TSIG processing.
218 Other Len u_int16_t length, in octets, of
219 Other Data.
220 Other Data octet stream empty unless Error == BADTIME
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228 RFC 2845 DNS TSIG May 2000
231 2.4. Example
233 NAME HOST.EXAMPLE.
235 TYPE TSIG
237 CLASS ANY
239 TTL 0
241 RdLen as appropriate
243 RDATA
245 Field Name Contents
246 -------------------------------------
247 Algorithm Name SAMPLE-ALG.EXAMPLE.
248 Time Signed 853804800
249 Fudge 300
250 MAC Size as appropriate
251 MAC as appropriate
252 Original ID as appropriate
253 Error 0 (NOERROR)
254 Other Len 0
255 Other Data empty
257 3 - Protocol Operation
259 3.1. Effects of adding TSIG to outgoing message
261 Once the outgoing message has been constructed, the keyed message
262 digest operation can be performed. The resulting message digest will
263 then be stored in a TSIG which is appended to the additional data
264 section (the ARCOUNT is incremented to reflect this). If the TSIG
265 record cannot be added without causing the message to be truncated,
266 the server MUST alter the response so that a TSIG can be included.
267 This response consists of only the question and a TSIG record, and
268 has the TC bit set and RCODE 0 (NOERROR). The client SHOULD at this
269 point retry the request using TCP (per [RFC1035 4.2.2]).
271 3.2. TSIG processing on incoming messages
273 If an incoming message contains a TSIG record, it MUST be the last
274 record in the additional section. Multiple TSIG records are not
275 allowed. If a TSIG record is present in any other position, the
276 packet is dropped and a response with RCODE 1 (FORMERR) MUST be
277 returned. Upon receipt of a message with a correctly placed TSIG RR,
278 the TSIG RR is copied to a safe location, removed from the DNS
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284 RFC 2845 DNS TSIG May 2000
287 Message, and decremented out of the DNS message header's ARCOUNT. At
288 this point the keyed message digest operation is performed. If the
289 algorithm name or key name is unknown to the recipient, or if the
290 message digests do not match, the whole DNS message MUST be
291 discarded. If the message is a query, a response with RCODE 9
292 (NOTAUTH) MUST be sent back to the originator with TSIG ERROR 17
293 (BADKEY) or TSIG ERROR 16 (BADSIG). If no key is available to sign
294 this message it MUST be sent unsigned (MAC size == 0 and empty MAC).
295 A message to the system operations log SHOULD be generated, to warn
296 the operations staff of a possible security incident in progress.
297 Care should be taken to ensure that logging of this type of event
298 does not open the system to a denial of service attack.
300 3.3. Time values used in TSIG calculations
302 The data digested includes the two timer values in the TSIG header in
303 order to defend against replay attacks. If this were not done, an
304 attacker could replay old messages but update the "Time Signed" and
305 "Fudge" fields to make the message look new. This data is named
306 "TSIG Timers", and for the purpose of digest calculation they are
307 invoked in their "on the wire" format, in the following order: first
308 Time Signed, then Fudge. For example:
310 Field Name Value Wire Format Meaning
311 ----------------------------------------------------------------------
312 Time Signed 853804800 00 00 32 e4 07 00 Tue Jan 21 00:00:00 1997
313 Fudge 300 01 2C 5 minutes
315 3.4. TSIG Variables and Coverage
317 When generating or verifying the contents of a TSIG record, the
318 following data are digested, in network byte order or wire format, as
319 appropriate:
321 3.4.1. DNS Message
323 A whole and complete DNS message in wire format, before the TSIG RR
324 has been added to the additional data section and before the DNS
325 Message Header's ARCOUNT field has been incremented to contain the
326 TSIG RR. If the message ID differs from the original message ID, the
327 original message ID is substituted for the message ID. This could
328 happen when forwarding a dynamic update request, for example.
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343 3.4.2. TSIG Variables
345 Source Field Name Notes
346 -----------------------------------------------------------------------
347 TSIG RR NAME Key name, in canonical wire format
348 TSIG RR CLASS (Always ANY in the current specification)
349 TSIG RR TTL (Always 0 in the current specification)
350 TSIG RDATA Algorithm Name in canonical wire format
351 TSIG RDATA Time Signed in network byte order
352 TSIG RDATA Fudge in network byte order
353 TSIG RDATA Error in network byte order
354 TSIG RDATA Other Len in network byte order
355 TSIG RDATA Other Data exactly as transmitted
357 The RR RDLEN and RDATA MAC Length are not included in the hash since
358 they are not guaranteed to be knowable before the MAC is generated.
360 The Original ID field is not included in this section, as it has
361 already been substituted for the message ID in the DNS header and
362 hashed.
364 For each label type, there must be a defined "Canonical wire format"
365 that specifies how to express a label in an unambiguous way. For
366 label type 00, this is defined in [RFC2535], for label type 01, this
367 is defined in [RFC2673]. The use of label types other than 00 and 01
368 is not defined for this specification.
370 3.4.3. Request MAC
372 When generating the MAC to be included in a response, the request MAC
373 must be included in the digest. The request's MAC is digested in
374 wire format, including the following fields:
376 Field Type Description
377 ---------------------------------------------------
378 MAC Length u_int16_t in network byte order
379 MAC Data octet stream exactly as transmitted
381 3.5. Padding
383 Digested components are fed into the hashing function as a continuous
384 octet stream with no interfield padding.
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399 4 - Protocol Details
401 4.1. TSIG generation on requests
403 Client performs the message digest operation and appends a TSIG
404 record to the additional data section and transmits the request to
405 the server. The client MUST store the message digest from the
406 request while awaiting an answer. The digest components for a
407 request are:
409 DNS Message (request)
410 TSIG Variables (request)
412 Note that some older name servers will not accept requests with a
413 nonempty additional data section. Clients SHOULD only attempt signed
414 transactions with servers who are known to support TSIG and share
415 some secret key with the client -- so, this is not a problem in
416 practice.
418 4.2. TSIG on Answers
420 When a server has generated a response to a signed request, it signs
421 the response using the same algorithm and key. The server MUST not
422 generate a signed response to an unsigned request. The digest
423 components are:
425 Request MAC
426 DNS Message (response)
427 TSIG Variables (response)
429 4.3. TSIG on TSIG Error returns
431 When a server detects an error relating to the key or MAC, the server
432 SHOULD send back an unsigned error message (MAC size == 0 and empty
433 MAC). If an error is detected relating to the TSIG validity period,
434 the server SHOULD send back a signed error message. The digest
435 components are:
437 Request MAC (if the request MAC validated)
438 DNS Message (response)
439 TSIG Variables (response)
441 The reason that the request is not included in this digest in some
442 cases is to make it possible for the client to verify the error. If
443 the error is not a TSIG error the response MUST be generated as
444 specified in [4.2].
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455 4.4. TSIG on TCP connection
457 A DNS TCP session can include multiple DNS envelopes. This is, for
458 example, commonly used by zone transfer. Using TSIG on such a
459 connection can protect the connection from hijacking and provide data
460 integrity. The TSIG MUST be included on the first and last DNS
461 envelopes. It can be optionally placed on any intermediary
462 envelopes. It is expensive to include it on every envelopes, but it
463 MUST be placed on at least every 100'th envelope. The first envelope
464 is processed as a standard answer, and subsequent messages have the
465 following digest components:
467 Prior Digest (running)
468 DNS Messages (any unsigned messages since the last TSIG)
469 TSIG Timers (current message)
471 This allows the client to rapidly detect when the session has been
472 altered; at which point it can close the connection and retry. If a
473 client TSIG verification fails, the client MUST close the connection.
474 If the client does not receive TSIG records frequently enough (as
475 specified above) it SHOULD assume the connection has been hijacked
476 and it SHOULD close the connection. The client SHOULD treat this the
477 same way as they would any other interrupted transfer (although the
478 exact behavior is not specified).
480 4.5. Server TSIG checks
482 Upon receipt of a message, server will check if there is a TSIG RR.
483 If one exists, the server is REQUIRED to return a TSIG RR in the
484 response. The server MUST perform the following checks in the
485 following order, check KEY, check TIME values, check MAC.
487 4.5.1. KEY check and error handling
489 If a non-forwarding server does not recognize the key used by the
490 client, the server MUST generate an error response with RCODE 9
491 (NOTAUTH) and TSIG ERROR 17 (BADKEY). This response MUST be unsigned
492 as specified in [4.3]. The server SHOULD log the error.
494 4.5.2. TIME check and error handling
496 If the server time is outside the time interval specified by the
497 request (which is: Time Signed, plus/minus Fudge), the server MUST
498 generate an error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18
499 (BADTIME). The server SHOULD also cache the most recent time signed
500 value in a message generated by a key, and SHOULD return BADTIME if a
501 message received later has an earlier time signed value. A response
502 indicating a BADTIME error MUST be signed by the same key as the
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508 RFC 2845 DNS TSIG May 2000
511 request. It MUST include the client's current time in the time
512 signed field, the server's current time (a u_int48_t) in the other
513 data field, and 6 in the other data length field. This is done so
514 that the client can verify a message with a BADTIME error without the
515 verification failing due to another BADTIME error. The data signed
516 is specified in [4.3]. The server SHOULD log the error.
518 4.5.3. MAC check and error handling
520 If a TSIG fails to verify, the server MUST generate an error response
521 as specified in [4.3] with RCODE 9 (NOTAUTH) and TSIG ERROR 16
522 (BADSIG). This response MUST be unsigned as specified in [4.3]. The
523 server SHOULD log the error.
525 4.6. Client processing of answer
527 When a client receives a response from a server and expects to see a
528 TSIG, it first checks if the TSIG RR is present in the response.
529 Otherwise, the response is treated as having a format error and
530 discarded. The client then extracts the TSIG, adjusts the ARCOUNT,
531 and calculates the keyed digest in the same way as the server. If
532 the TSIG does not validate, that response MUST be discarded, unless
533 the RCODE is 9 (NOTAUTH), in which case the client SHOULD attempt to
534 verify the response as if it were a TSIG Error response, as specified
535 in [4.3]. A message containing an unsigned TSIG record or a TSIG
536 record which fails verification SHOULD not be considered an
537 acceptable response; the client SHOULD log an error and continue to
538 wait for a signed response until the request times out.
540 4.6.1. Key error handling
542 If an RCODE on a response is 9 (NOTAUTH), and the response TSIG
543 validates, and the TSIG key is different from the key used on the
544 request, then this is a KEY error. The client MAY retry the request
545 using the key specified by the server. This should never occur, as a
546 server MUST NOT sign a response with a different key than signed the
547 request.
549 4.6.2. Time error handling
551 If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 18
552 (BADTIME), or the current time does not fall in the range specified
553 in the TSIG record, then this is a TIME error. This is an indication
554 that the client and server clocks are not synchronized. In this case
555 the client SHOULD log the event. DNS resolvers MUST NOT adjust any
556 clocks in the client based on BADTIME errors, but the server's time
557 in the other data field SHOULD be logged.
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567 4.6.3. MAC error handling
569 If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG),
570 this is a MAC error, and client MAY retry the request with a new
571 request ID but it would be better to try a different shared key if
572 one is available. Client SHOULD keep track of how many MAC errors
573 are associated with each key. Clients SHOULD log this event.
575 4.7. Special considerations for forwarding servers
577 A server acting as a forwarding server of a DNS message SHOULD check
578 for the existence of a TSIG record. If the name on the TSIG is not
579 of a secret that the server shares with the originator the server
580 MUST forward the message unchanged including the TSIG. If the name
581 of the TSIG is of a key this server shares with the originator, it
582 MUST process the TSIG. If the TSIG passes all checks, the forwarding
583 server MUST, if possible, include a TSIG of his own, to the
584 destination or the next forwarder. If no transaction security is
585 available to the destination and the response has the AD flag (see
586 [RFC2535]), the forwarder MUST unset the AD flag before adding the
587 TSIG to the answer.
589 5 - Shared Secrets
591 5.1. Secret keys are very sensitive information and all available
592 steps should be taken to protect them on every host on which they are
593 stored. Generally such hosts need to be physically protected. If
594 they are multi-user machines, great care should be taken that
595 unprivileged users have no access to keying material. Resolvers
596 often run unprivileged, which means all users of a host would be able
597 to see whatever configuration data is used by the resolver.
599 5.2. A name server usually runs privileged, which means its
600 configuration data need not be visible to all users of the host. For
601 this reason, a host that implements transaction-based authentication
602 should probably be configured with a "stub resolver" and a local
603 caching and forwarding name server. This presents a special problem
604 for [RFC2136] which otherwise depends on clients to communicate only
605 with a zone's authoritative name servers.
607 5.3. Use of strong random shared secrets is essential to the security
608 of TSIG. See [RFC1750] for a discussion of this issue. The secret
609 should be at least as long as the keyed message digest, i.e. 16 bytes
610 for HMAC-MD5 or 20 bytes for HMAC-SHA1.
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620 RFC 2845 DNS TSIG May 2000
623 6 - Security Considerations
625 6.1. The approach specified here is computationally much less
626 expensive than the signatures specified in [RFC2535]. As long as the
627 shared secret key is not compromised, strong authentication is
628 provided for the last hop from a local name server to the user
629 resolver.
631 6.2. Secret keys should be changed periodically. If the client host
632 has been compromised, the server should suspend the use of all
633 secrets known to that client. If possible, secrets should be stored
634 in encrypted form. Secrets should never be transmitted in the clear
635 over any network. This document does not address the issue on how to
636 distribute secrets. Secrets should never be shared by more than two
637 entities.
639 6.3. This mechanism does not authenticate source data, only its
640 transmission between two parties who share some secret. The original
641 source data can come from a compromised zone master or can be
642 corrupted during transit from an authentic zone master to some
643 "caching forwarder." However, if the server is faithfully performing
644 the full [RFC2535] security checks, then only security checked data
645 will be available to the client.
647 6.4. A fudge value that is too large may leave the server open to
648 replay attacks. A fudge value that is too small may cause failures
649 if machines are not time synchronized or there are unexpected network
650 delays. The recommended value in most situation is 300 seconds.
652 7 - IANA Considerations
654 IANA is expected to create and maintain a registry of algorithm names
655 to be used as "Algorithm Names" as defined in Section 2.3. The
656 initial value should be "HMAC-MD5.SIG-ALG.REG.INT". Algorithm names
657 are text strings encoded using the syntax of a domain name. There is
658 no structure required other than names for different algorithms must
659 be unique when compared as DNS names, i.e., comparison is case
660 insensitive. Note that the initial value mentioned above is not a
661 domain name, and therefore need not be a registered name within the
662 DNS. New algorithms are assigned using the IETF Consensus policy
663 defined in RFC 2434. The algorithm name HMAC-MD5.SIG-ALG.REG.INT
664 looks like a FQDN for historical reasons; future algorithm names are
665 expected to be simple (i.e., single-component) names.
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679 IANA is expected to create and maintain a registry of "TSIG Error
680 values" to be used for "Error" values as defined in section 2.3.
681 Initial values should be those defined in section 1.7. New TSIG
682 error codes for the TSIG error field are assigned using the IETF
683 Consensus policy defined in RFC 2434.
685 8 - References
687 [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
688 STD 13, RFC 1034, November 1987.
690 [RFC1035] Mockapetris, P., "Domain Names - Implementation and
691 Specification", STD 13, RFC 1034, November 1987.
693 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
694 April 1992.
696 [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
697 Recommendations for Security", RFC 1750, December 1995.
699 [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC-MD5:
700 Keyed-MD5 for Message Authentication", RFC 2104, February
701 1997.
703 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
704 Requirement Levels", BCP 14, RFC 2119, March 1997.
706 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound "Dynamic
707 Updates in the Domain Name System", RFC 2136, April 1997.
709 [RFC2137] Eastlake 3rd, D., "Secure Domain Name System Dynamic
710 Update", RFC 2137, April 1997.
712 [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
713 RFC 2535, March 1999.
715 [RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
716 RFC 2673, August 1999.
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735 9 - Authors' Addresses
737 Paul Vixie
738 Internet Software Consortium
739 950 Charter Street
740 Redwood City, CA 94063
742 Phone: +1 650 779 7001
743 EMail: vixie@isc.org
746 Olafur Gudmundsson
747 NAI Labs
748 3060 Washington Road, Route 97
749 Glenwood, MD 21738
751 Phone: +1 443 259 2389
752 EMail: ogud@tislabs.com
755 Donald E. Eastlake 3rd
756 Motorola
757 140 Forest Avenue
758 Hudson, MA 01749 USA
760 Phone: +1 508 261 5434
761 EMail: dee3@torque.pothole.com
764 Brian Wellington
765 Nominum, Inc.
766 950 Charter Street
767 Redwood City, CA 94063
769 Phone: +1 650 779 6022
770 EMail: Brian.Wellington@nominum.com
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791 10 Full Copyright Statement
793 Copyright (C) The Internet Society (2000). All Rights Reserved.
795 This document and translations of it may be copied and furnished to
796 others, and derivative works that comment on or otherwise explain it
797 or assist in its implementation may be prepared, copied, published
798 and distributed, in whole or in part, without restriction of any
799 kind, provided that the above copyright notice and this paragraph are
800 included on all such copies and derivative works. However, this
801 document itself may not be modified in any way, such as by removing
802 the copyright notice or references to the Internet Society or other
803 Internet organizations, except as needed for the purpose of
804 developing Internet standards in which case the procedures for
805 copyrights defined in the Internet Standards process must be
806 followed, or as required to translate it into languages other than
807 English.
809 The limited permissions granted above are perpetual and will not be
810 revoked by the Internet Society or its successors or assigns.
812 This document and the information contained herein is provided on an
813 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
814 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
815 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
816 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
817 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
819 Acknowledgement
821 Funding for the RFC Editor function is currently provided by the
822 Internet Society.
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