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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Mills 3 Internet-Draft University of Deleware 4 Intended status: Informational B. Haberman, Ed. 5 Expires: November 19, 2017 JHU 6 May 18, 2017 8 Control Messages Protocol for Use with Network Time Protocol Version 4 9 draft-ietf-ntp-mode-6-cmds-01 11 Abstract 13 This document describes the structure of the control messages used 14 with the Network Time Protocol. These control messages can be used 15 to monitor and control the Network Time Protocol application running 16 on any IP network attached computer. The information in this 17 document was originally described in Appendix B of RFC 1305. The 18 goal of this document is to provide a historic description of the 19 control messages. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on November 19, 2017. 38 Copyright Notice 40 Copyright (c) 2017 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 This document may contain material from IETF Documents or IETF 54 Contributions published or made publicly available before November 55 10, 2008. The person(s) controlling the copyright in some of this 56 material may not have granted the IETF Trust the right to allow 57 modifications of such material outside the IETF Standards Process. 58 Without obtaining an adequate license from the person(s) controlling 59 the copyright in such materials, this document may not be modified 60 outside the IETF Standards Process, and derivative works of it may 61 not be created outside the IETF Standards Process, except to format 62 it for publication as an RFC or to translate it into languages other 63 than English. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 1.1. Control Message Overview . . . . . . . . . . . . . . . . 3 69 2. NTP Control Message Format . . . . . . . . . . . . . . . . . 4 70 3. Status Words . . . . . . . . . . . . . . . . . . . . . . . . 7 71 3.1. System Status Word . . . . . . . . . . . . . . . . . . . 8 72 3.2. Peer Status Word . . . . . . . . . . . . . . . . . . . . 10 73 3.3. Clock Status Word . . . . . . . . . . . . . . . . . . . . 12 74 3.4. Error Status Word . . . . . . . . . . . . . . . . . . . . 12 75 4. Commands . . . . . . . . . . . . . . . . . . . . . . . . . . 13 76 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 77 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 78 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 79 8. Normative References . . . . . . . . . . . . . . . . . . . . 18 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 82 1. Introduction 84 RFC 1305 [RFC1305] described a set of control messages for use within 85 the Network Time Protocol (NTP) when a comprehensive network 86 management solution was not available. The definitions of these 87 control messages were not promulgated to RFC 5905 [RFC5905] when NTP 88 version 4 was documented. These messages were intended for use only 89 in systems where no other management facilities were available or 90 appropriate, such as in dedicated-function bus peripherals. Support 91 for these messages is not required in order to conform to RFC 5905 92 [RFC5905]. The control messages are described here as a historical 93 record given their use within NTPv4. 95 1.1. Control Message Overview 97 The NTP Control Message has the value 6 specified in the mode field 98 of the first octet of the NTP header and is formatted as shown in 99 Figure 1. The format of the data field is specific to each command 100 or response; however, in most cases the format is designed to be 101 constructed and viewed by humans and so is coded in free-form ASCII. 102 This facilitates the specification and implementation of simple 103 management tools in the absence of fully evolved network-management 104 facilities. As in ordinary NTP messages, the authenticator field 105 follows the data field. If the authenticator is used the data field 106 is zero-padded to a 32-bit boundary, but the padding bits are not 107 considered part of the data field and are not included in the field 108 count. 110 IP hosts are not required to reassemble datagrams larger than 576 111 octets; however, some commands or responses may involve more data 112 than will fit into a single datagram. Accordingly, a simple 113 reassembly feature is included in which each octet of the message 114 data is numbered starting with zero. As each fragment is transmitted 115 the number of its first octet is inserted in the offset field and the 116 number of octets is inserted in the count field. The more-data (M) 117 bit is set in all fragments except the last. 119 Most control functions involve sending a command and receiving a 120 response, perhaps involving several fragments. The sender chooses a 121 distinct, nonzero sequence number and sets the status field and R and 122 E bits to zero. The responder interprets the opcode and additional 123 information in the data field, updates the status field, sets the R 124 bit to one and returns the three 32-bit words of the header along 125 with additional information in the data field. In case of invalid 126 message format or contents the responder inserts a code in the status 127 field, sets the R and E bits to one and, optionally, inserts a 128 diagnostic message in the data field. 130 Some commands read or write system variables and peer variables for 131 an association identified in the command. Others read or write 132 variables associated with a radio clock or other device directly 133 connected to a source of primary synchronization information. To 134 identify which type of variable and association a 16-bit association 135 identifier is used. System variables are indicated by the identifier 136 zero. As each association is mobilized a unique, nonzero identifier 137 is created for it. These identifiers are used in a cyclic fashion, 138 so that the chance of using an old identifier which matches a newly 139 created association is remote. A management entity can request a 140 list of current identifiers and subsequently use them to read and 141 write variables for each association. An attempt to use an expired 142 identifier results in an exception response, following which the list 143 can be requested again. 145 Some exception events, such as when a peer becomes reachable or 146 unreachable, occur spontaneously and are not necessarily associated 147 with a command. An implementation may elect to save the event 148 information for later retrieval or to send an asynchronous response 149 (called a trap) or both. In case of a trap the IP address and port 150 number is determined by a previous command and the sequence field is 151 set as described below. Current status and summary information for 152 the latest exception event is returned in all normal responses. Bits 153 in the status field indicate whether an exception has occurred since 154 the last response and whether more than one exception has occurred. 156 Commands need not necessarily be sent by an NTP peer, so ordinary 157 access-control procedures may not apply; however, the optional mask/ 158 match mechanism suggested elsewhere in this document provides the 159 capability to control access by mode number, so this could be used to 160 limit access for control messages (mode 6) to selected address 161 ranges. 163 2. NTP Control Message Format 165 The format of the NTP Control Message header, which immediately 166 follows the UDP header, is shown in Figure 1. Following is a 167 description of its fields. Bit positions marked as zero are reserved 168 and should always be transmitted as zero. 170 0 1 2 3 171 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 172 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 173 |LI | VN |Mode |R|E|M| OpCode | Sequence Number | 174 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 175 | Status | Association ID | 176 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 177 | Offset | Count | 178 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 179 | | 180 / Data (up to 468 bytes) / 181 | | 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 | Padding (optional) | 184 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 185 | | 186 / Authenticator (optional, 96 bytes) / 187 | | 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 190 Figure 1: NTP Control Message Header 192 Leap Indicator (LI): This is a two-bit integer that is set to b00 for 193 control message requests and responses. The Leap Indicator value 194 used as this position in mot NTP modes is in the System Status Word 195 provided in some control message responses. 197 Version Number (VN): This is a three-bit integer indicating a minimum 198 NTP version number. NTP servers do not respond to control messages 199 with an unrecognized version number. Requests may intentionally use 200 a lower version number to enable interoperability with earlier 201 versions of NTP. Responses carry the same version as the 202 corresponding request. 204 Mode: This is a three-bit integer indicating the mode. The value 6 205 indicates an NTP control message. 207 Response Bit (R): Set to zero for commands, one for responses. 209 Error Bit (E): Set to zero for normal response, one for error 210 response. 212 More Bit (M): Set to zero for last fragment, one for all others. 214 Operation Code (OpCode): This is a five-bit integer specifying the 215 command function. Values currently defined include the following: 217 +-------+--------------------------------------------------+ 218 | Code | Meaning | 219 +-------+--------------------------------------------------+ 220 | 0 | reserved | 221 | 1 | read status command/response | 222 | 2 | read variables command/response | 223 | 3 | write variables command/response | 224 | 4 | read clock variables command/response | 225 | 5 | write clock variables command/response | 226 | 6 | set trap address/port command/response | 227 | 7 | trap response | 228 | 8 | runtime configuration command/response | 229 | 9 | export configuration to file command/response | 230 | 10 | retrieve remote address stats command/response | 231 | 11 | retrieve ordered list command/response | 232 | 12 | request client-specific nonce command/response | 233 | 13-30 | reserved | 234 | 31 | unset trap address/port command/response | 235 +-------+--------------------------------------------------+ 237 Sequence Number: This is a 16-bit integer indicating the sequence 238 number of the command or response. Each request uses a different 239 sequence number. Each response carries the same sequence number as 240 its corresponding request. For asynchronous trap responses, the 241 responder increments the sequence number by one for each response, 242 allowing trap receivers to detect missing trap responses. The 243 sequence number of each fragment of a multiple-datagram response 244 carries the same sequence number, copied from the request. 246 Status: This is a 16-bit code indicating the current status of the 247 system, peer or clock, with values coded as described in following 248 sections. 250 Association ID: This is a 16-bit unsigned integer identifying a valid 251 association, or zero for the system clock. 253 Offset: This is a 16-bit unsigned integer indicating the offset, in 254 octets, of the first octet in the data area. The offset is set to 255 zero in requests. Responses spanning multiple datagrams use a 256 positive offset in all but the first datagram. 258 Count: This is a 16-bit unsigned integer indicating the length of the 259 data field, in octets. 261 Data: This contains the message data for the command or response. 262 The maximum number of data octets is 468. 264 Padding (optional): Conains zero to three octets with value zero, as 265 needed to ensure the overall control message size is a multiple of 4 266 octets. 268 Authenticator (optional): When the NTP authentication mechanism is 269 implemented, this contains the authenticator information defined in 270 Appendix C of RFC 1305. 272 3. Status Words 274 Status words indicate the present status of the system, associations 275 and clock. They are designed to be interpreted by network-monitoring 276 programs and are in one of four 16-bit formats shown in Figure 2 and 277 described in this section. System and peer status words are 278 associated with responses for all commands except the read clock 279 variables, write clock variables and set trap address/port commands. 280 The association identifier zero specifies the system status word, 281 while a nonzero identifier specifies a particular peer association. 282 The status word returned in response to read clock variables and 283 write clock variables commands indicates the state of the clock 284 hardware and decoding software. A special error status word is used 285 to report malformed command fields or invalid values. 287 0 1 288 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 289 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 290 | LI| Clock Src | Count | Code | 291 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 292 System Status Word 294 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 295 | Status | SEL | Count | Code | 296 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 297 Peer Status Word 299 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 300 | Clock Status | Code | 301 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 302 Radio Status Word 304 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 305 | Error Code | Reserved | 306 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 307 Error Status Word 309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 310 | Reserved | Count | Code | 311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 312 Clock Status Word 314 Figure 2: Status Word Formats 316 3.1. System Status Word 318 The system status word appears in the status field of the response to 319 a read status or read variables command with a zero association 320 identifier. The format of the system status word is as follows: 322 Leap Indicator (LI): This is a two-bit code warning of an impending 323 leap second to be inserted/deleted in the last minute of the current 324 day, with bit 0 and bit 1, respectively, coded as follows: 326 +------+------------------------------------------------------------+ 327 | LI | Meaning | 328 +------+------------------------------------------------------------+ 329 | 00 | no warning | 330 | 01 | insert second after 23:59:59 of the current day | 331 | 10 | delete second 23:59:59 of the current day | 332 | 11 | unsynchronized | 333 +------+------------------------------------------------------------+ 334 Clock Source (Clock Src): This is a six-bit integer indicating the 335 current synchronization source, with values coded as follows: 337 +-------+-----------------------------------------------------------+ 338 | Code | Meaning | 339 +-------+-----------------------------------------------------------+ 340 | 0 | unspecified or unknown | 341 | 1 | Calibrated atomic clock (e.g., PPS, HP 5061) | 342 | 2 | VLF (band 4) or LF (band 5) radio (e.g., OMEGA,, WWVB) | 343 | 3 | HF (band 7) radio (e.g., CHU, MSF, WWV/H) | 344 | 4 | UHF (band 9) satellite (e.g., GOES, GPS) | 345 | 5 | local net (e.g., DCN, TSP, DTS) | 346 | 6 | UDP/NTP | 347 | 7 | UDP/TIME | 348 | 8 | eyeball-and-wristwatch | 349 | 9 | telephone modem (e.g., NIST) | 350 | 10-63 | reserved | 351 +-------+-----------------------------------------------------------+ 353 System Event Counter (Count): This is a four-bit integer indicating 354 the number of system events occurring since the last time the System 355 Event Code changed. Upon reaching 15, subsequent events with the 356 same code are not counted. 358 System Event Code (Code): This is a four-bit integer identifying the 359 latest system exception event, with new values overwriting previous 360 values, and coded as follows: 362 +------+---------------------------------------------------------+ 363 | Code | Meaning | 364 +------+---------------------------------------------------------+ 365 | 0 | unspecified | 366 | 1 | frequency correction (drift) file not available | 367 | 2 | frequency correction started (frequency stepped) | 368 | 3 | spike detected and ignored, starting stepout timer | 369 | 4 | frequency training started | 370 | 5 | clock synchronized | 371 | 6 | system restart | 372 | 7 | panic stop (required step greater than panic threshold) | 373 | 8 | no system peer | 374 | 9 | leap second insertion/deletion armed for the | 375 | | of the current month | 376 | 10 | leap second disarmed | 377 | 11 | leap second inserted or deleted | 378 | 12 | clock stepped (stepout timer expired) | 379 | 13 | kernel loop discipline status changed | 380 | 14 | leapseconds table loaded from file | 381 | 15 | leapseconds table outdated, updated file needed | 382 +------+---------------------------------------------------------+ 384 3.2. Peer Status Word 386 A peer status word is returned in the status field of a response to a 387 read status, read variables or write variables command and appears 388 also in the list of association identifiers and status words returned 389 by a read status command with a zero association identifier. The 390 format of a peer status word is as follows: 392 Peer Status (Status): This is a five-bit code indicating the status 393 of the peer determined by the packet procedure, with bits assigned as 394 follows: 396 +-------------+---------------------------------------------------+ 397 | Peer Status | Meaning | 398 +-------------+---------------------------------------------------+ 399 | 0 | configured (peer.config) | 400 | 1 | authentication enabled (peer.authenable) | 401 | 2 | authentication okay (peer.authentic) | 402 | 3 | reachability okay (peer.reach != 0) | 403 | 4 | broadcast association | 404 +-------------+---------------------------------------------------+ 406 Peer Selection (SEL): This is a three-bit integer indicating the 407 status of the peer determined by the clock-selection procedure, with 408 values coded as follows: 410 +-----+-------------------------------------------------------------+ 411 | Sel | Meaning | 412 +-----+-------------------------------------------------------------+ 413 | 0 | rejected | 414 | 1 | discarded by intersection algorithm | 415 | 2 | discarded bu table overflow (not currently used) | 416 | 3 | discarded by the cluster algorithm | 417 | 4 | included by the combine algorithm | 418 | 5 | backup source (with more than sys.maxclock survivors) | 419 | 6 | system peer (synchronization source) | 420 | 7 | PPS (pulse per second) peer | 421 +-----+-------------------------------------------------------------+ 423 Peer Event Counter (Count): This is a four-bit integer indicating the 424 number of peer exception events that occurred since the last time the 425 peer event code changed. Upon reaching 15, subsequent events with 426 the same code are not counted. 428 Peer Event Code (Code): This is a four-bit integer identifying the 429 latest peer exception event, with new values overwriting previous 430 values, and coded as follows: 432 +-------+--------------------------------------------------------+ 433 | Peer | | 434 | Event | Meaning | 435 | Code | | 436 +-------+--------------------------------------------------------+ 437 | 0 | unspecified | 438 | 1 | association mobilized | 439 | 2 | association demobilized | 440 | 3 | peer unreachable (peer.reach was nonzero now zero) | 441 | 4 | peer reachable (peer.reach was zero now nonzero) | 442 | 5 | association restarted or timed out | 443 | 6 | no reply (only used with one-shot ntpd -q) | 444 | 7 | peer rate limit exceeded (kiss code RATE received) | 445 | 8 | access denied (kiss code DENY received) | 446 | 9 | leap second insertion/deletion at month's end armed | 447 | | by peer vote | 448 | 10 | became system peer (sys.peer) | 449 | 11 | reference clock event (see clock status word) | 450 | 12 | authentication failed | 451 | 13 | popcorn spike suppressed by peer clock filter register | 452 | 14 | entering interleaved mode | 453 | 15 | recovered from interleave error | 454 +-------+--------------------------------------------------------+ 456 3.3. Clock Status Word 458 There are two ways a reference clock can be attached to a NTP service 459 host, as an dedicated device managed by the operating system and as a 460 synthetic peer managed by NTP. As in the read status command, the 461 association identifier is used to identify which one, zero for the 462 system clock and nonzero for a peer clock. Only one system clock is 463 supported by the protocol, although many peer clocks can be 464 supported. A system or peer clock status word appears in the status 465 field of the response to a read clock variables or write clock 466 variables command. This word can be considered an extension of the 467 system status word or the peer status word as appropriate. The 468 format of the clock status word is as follows: 470 Reserved: An eight-bit integer that is ignored by requesters and 471 zeroed by responders. 473 Count: This is a four-bit integer indicating the number of clock 474 events that occurred since the last time the clock event code 475 changed. Upon reaching 15, subsequent events with the same code are 476 not counted. 478 Clock Code (Code): This is a four-bit integer indicating the current 479 clock status, with values coded as follows: 481 +--------------+--------------------------------------------------+ 482 | Clock Status | Meaning | 483 +--------------+--------------------------------------------------+ 484 | 0 | clock operating within nominals | 485 | 1 | reply timeout | 486 | 2 | bad reply format | 487 | 3 | hardware or software fault | 488 | 4 | propagation failure | 489 | 5 | bad date format or value | 490 | 6 | bad time format or value | 491 | 7-15 | reserved | 492 +--------------+--------------------------------------------------+ 494 3.4. Error Status Word 496 An error status word is returned in the status field of an error 497 response as the result of invalid message format or contents. Its 498 presence is indicated when the E (error) bit is set along with the 499 response (R) bit in the response. It consists of an eight-bit 500 integer coded as follows: 502 +--------------+--------------------------------------------------+ 503 | Error Status | Meaning | 504 +--------------+--------------------------------------------------+ 505 | 0 | unspecified | 506 | 1 | authentication failure | 507 | 2 | invalid message length or format | 508 | 3 | invalid opcode | 509 | 4 | unknown association identifier | 510 | 5 | unknown variable name | 511 | 6 | invalid variable value | 512 | 7 | administratively prohibited | 513 | 8-255 | reserved | 514 +--------------+--------------------------------------------------+ 516 4. Commands 518 Commands consist of the header and optional data field shown in 519 Figure 2. When present, the data field contains a list of 520 identifiers or assignments in the form 521 <>[=<>],<>[=<>],... where 522 <> is the ASCII name of a system or peer variable 523 specified in RFC 5905 and <> is expressed as a decimal, 524 hexadecimal or string constant in the syntax of the C programming 525 language. Where no ambiguity exists, the <169>sys.<170> or 526 <169>peer.<170> prefixes can be suppressed. Whitespace (ASCII 527 nonprinting format effectors) can be added to improve readability for 528 simple monitoring programs that do not reformat the data field. 529 Internet addresses are represented as four octets in the form 530 [n.n.n.n], where n is in decimal notation and the brackets are 531 optional. Timestamps, including reference, originate, receive and 532 transmit values, as well as the logical clock, are represented in 533 units of seconds and fractions, preferably in hexadecimal notation, 534 while delay, offset, dispersion and distance values are represented 535 in units of milliseconds and fractions, preferably in decimal 536 notation. All other values are represented as-is, preferably in 537 decimal notation. 539 Implementations may define variables other than those described in 540 RFC 5905. Called extramural variables, these are distinguished by 541 the inclusion of some character type other than alphanumeric or 542 <169>.<170> in the name. For those commands that return a list of 543 assignments in the response data field, if the command data field is 544 empty, it is expected that all available variables defined in RFC 545 5905 will be included in the response. For the read commands, if the 546 command data field is nonempty, an implementation may choose to 547 process this field to individually select which variables are to be 548 returned. 550 Commands are interpreted as follows: 552 Read Status (1): The command data field is empty or contains a list 553 of identifiers separated by commas. The command operates in two ways 554 depending on the value of the association identifier. If this 555 identifier is nonzero, the response includes the peer identifier and 556 status word. Optionally, the response data field may contain other 557 information, such as described in the Read Variables command. If the 558 association identifier is zero, the response includes the system 559 identifier (0) and status word, while the data field contains a list 560 of binary-coded pairs <> <>, one 561 for each currently defined association. 563 Read Variables (2): The command data field is empty or contains a 564 list of identifiers separated by commas. If the association 565 identifier is nonzero, the response includes the requested peer 566 identifier and status word, while the data field contains a list of 567 peer variables and values as described above. If the association 568 identifier is zero, the data field contains a list of system 569 variables and values. If a peer has been selected as the 570 synchronization source, the response includes the peer identifier and 571 status word; otherwise, the response includes the system identifier 572 (0) and status word. 574 Write Variables (3): The command data field contains a list of 575 assignments as described above. The variables are updated as 576 indicated. The response is as described for the Read Variables 577 command. 579 Read Clock Variables (4): The command data field is empty or contains 580 a list of identifiers separated by commas. The association 581 identifier selects the system clock variables or peer clock variables 582 in the same way as in the Read Variables command. The response 583 includes the requested clock identifier and status word and the data 584 field contains a list of clock variables and values, including the 585 last timecode message received from the clock. 587 Write Clock Variables (5): The command data field contains a list of 588 assignments as described above. The clock variables are updated as 589 indicated. The response is as described for the Read Clock Variables 590 command. 592 Set Trap Address/Port (6): The command association identifier, status 593 and data fields are ignored. The address and port number for 594 subsequent trap messages are taken from the source address and port 595 of the control message itself. The initial trap counter for trap 596 response messages is taken from the sequence field of the command. 597 The response association identifier, status and data fields are not 598 significant. Implementations should include sanity timeouts which 599 prevent trap transmissions if the monitoring program does not renew 600 this information after a lengthy interval. 602 Trap Response (7): This message is sent when a system, peer or clock 603 exception event occurs. The opcode field is 7 and the R bit is set. 604 The trap counter is incremented by one for each trap sent and the 605 sequence field set to that value. The trap message is sent using the 606 IP address and port fields established by the set trap address/port 607 command. If a system trap the association identifier field is set to 608 zero and the status field contains the system status word. If a peer 609 trap the association identifier field is set to that peer and the 610 status field contains the peer status word. Optional ASCII-coded 611 information can be included in the data field. 613 Configure (8): The command data is parsed and applied as if supplied 614 in the daemon configuration file. The reference implementation 615 daemon requires authentication for this command. 617 Save Configuration (9): Write a snapshot of the current configuration 618 to the file name supplied as the command data. The reference 619 implementation daemon requires authentication for this command. 620 Further, the command is refused unless a directory in which to store 621 the resulting files has been explicitly configured by the operator. 623 Read MRU (10): Retrieves records of recently seen remote addresses 624 and associated statistics. Command data consists of name=value pairs 625 controlling the selection of records, as well as a requestor-specific 626 nonce previously retrieved using this command or opcode 12, Request 627 Nonce. The response consists of name=value pairs where some names 628 can appear multiple times using a dot followed by a zero-based index 629 to distinguish them, and to associate elements of the same record 630 with the same index. A new nonce is provided with each successful 631 response. 633 Read ordered list (11): Retrieves an ordered list. If the command 634 data is empty or the seven characters "ifstats" the associated 635 statistics, status and counters for each local address are returned. 636 If the command data is the characters "addr_restrictions" then the 637 set of IPv4 remote address restrictions followed by the set of IPv6 638 remote address restrictions (access control lists) are returned. 639 Other command data returns error code 5 (unknown variable name). 640 Similar to Read MRU, response information uses zero-based indexes as 641 part of the variable name preceding the equals sign and value, where 642 each index relates information for a single address or network. This 643 opcode requires authentication. 645 Request Nonce (12): Retrieves a 96-bit nonce specific to the 646 requesting remote address, which is valid for a limited period. 647 Command data is not used in the request. The nonce consists of a 648 64-bit NTP timestamp and 32 bits of hash derived from that timestamp, 649 the remote address, and salt known only to the server which varies 650 between daemon runs. The reference implementation honors nonces 651 which were issued less than 16 seconds prior. Regurgitation of the 652 nonce by a managment agent demonstrates to the server that the agent 653 can receive datagrams sent to the source address of the request, 654 making source address "spoofing" more difficult in a similar way as 655 TCP's three-way handshake. 657 Unset Trap (31): Removes the requesting remote address and port from 658 the list of trap receivers. Command data is not used in the request. 659 If the address and port are not in the list of trap receivers, the 660 error code is 4, bad association. 662 5. IANA Considerations 664 This document makes no request of IANA. 666 Note to RFC Editor: this section may be removed on publication as an 667 RFC. 669 6. Security Considerations 671 A number of security vulnerabilities have been identified with these 672 control messages. 674 NTP's control query interface allows reading and writing of system, 675 peer, and clock variables remotely from arbitrary IP addresses using 676 commands mentioned in Section 4. Traditionally, overwriting these 677 variables, but not reading them, requires authentication by default. 678 However, this document argues that an NTP host must authenticate all 679 control queries and not just ones that overwrite these variables. 680 Alternatively, the host can use a whitelist to explicitly list IP 681 addresses that are allowed to control query the clients. These 682 access controls are required for the following reasons: 684 o NTP as a Distributed Denial-of-Service (DDoS) vector. NTP timing 685 query and response packets (modes 1-2, 3-4, 5) are usually short 686 in size. However, some NTP control queries generate a very long 687 packet in response to a short query. As such, there is a history 688 of use of NTP's control queries, which exhibit such behavior, to 689 perform DDoS attacks. These off-path attacks exploit the large 690 size of NTP control queries to cause UDP-based amplification 691 attacks (e.g., mode 7 monlist command generates a very long packet 692 in response to a small query (CVE-2013-5211)). These attacks only 693 use NTP as a vector for DoS atacks on other protocols, but do not 694 affect the time service on the NTP host itself. 696 o Time-shifting attacks through information leakage/overwriting. 697 NTP hosts save important system and peer state variables. An off- 698 path attacker who can read these variables remotely can leverage 699 the information leaked by these control queries to perform time- 700 shifting and DoS attacks on NTP clients. These attacks do affect 701 time synchronization on the NTP hosts. For instance, 703 * In the client/server mode, the client stores its local time 704 when it sends the query to the server in its xmt peer variable. 705 This variable is used to perform TEST2 to non-cryptographically 706 authenticate the server, i.e., if the origin timestamp field in 707 the corresponding server response packet matches the xmt peer 708 variable, then the client accepts the packet. An off-path 709 attacker, with the ability to read this variable can easily 710 spoof server response packets for the client, which will pass 711 TEST2, and can deny service or shift time on the NTP client. 712 CVE-2015-8139 describes the specific attack. 714 * The client also stores its local time when the server response 715 is received in its rec peer variable. This variable is used 716 for authentication in interleaved-pivot mode. An off-path 717 attacker with the ability to read this state variable can 718 easily shift time on the client by passing this test. CVE- 719 2016-1548 describes the attack. 721 o Fast-Scanning. NTP mode 6 control messages are usually small UDP 722 packets. Fast-scanning tools like ZMap can be used to spray the 723 entire (potentially reachable) Internet with these messages within 724 hours to identify vulnerable hosts. To make things worse, these 725 attacks can be extremely low-rate, only requiring a control query 726 for reconnaissance and a spoofed response to shift time on 727 vulnerable clients. CVE-2016-1548 is one such example. 729 NTP best practices recommend configuring ntpd with the no-query 730 parameter. The no-query parameter blocks access to all remote 731 control queries. However, sometimes the nosts do not want to block 732 all queries and want to give access for certain control queries 733 remotely. This could be for the purpose of remote management and 734 configuration of the hosts in certain scenarios. Such hosts tend to 735 use firewalls or other middleboxes to blacklist certain queries 736 within the network. 738 Recent work (reference needed) shows that significantly fewer hosts 739 respond to mode 7 monlist queries as compared to other control 740 queries because it is a well-known and exploited control query. 742 These queries are likely blocked using blacklists on firewalls and 743 middleboxes rather than the no-query option on NTP hosts. The 744 remaining control queries that can be exploited likely remain out of 745 the blacklist because they are undocumented in the current NTP 746 specification [RFC5905]. 748 This document describes all of the mode 6 control queries allowed by 749 NTP and can help administrators make informed decisions on security 750 measures to protect NTP devices from harmful queries and likely make 751 those systems less vulnerable. 753 7. Acknowledgements 755 Tim Plunkett created the original version of this document. Aanchal 756 Malhotra provided the initial version of the Security Considerations 757 section. 759 Karen O'Donoghue, David Hart, Harlan Stenn, and Philip Chimento 760 deserve credit for portions of this document due to their earlier 761 efforts to document these commands. 763 8. Normative References 765 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 766 Specification, Implementation and Analysis", RFC 1305, 767 DOI 10.17487/RFC1305, March 1992, 768 . 770 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 771 "Network Time Protocol Version 4: Protocol and Algorithms 772 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 773 . 775 Authors' Addresses 777 Dr. David L. Mills 778 University of Deleware 780 Email: mills@udel.edu 782 Brian Haberman (editor) 783 JHU 785 Email: brian@innovationslab.net