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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force D. Reilly, Ed. 3 Internet-Draft Orolia USA 4 Intended status: Best Current Practice H. Stenn 5 Expires: July 29, 2019 Network Time Foundation 6 D. Sibold 7 PTB 8 January 25, 2019 10 Network Time Protocol Best Current Practices 11 draft-ietf-ntp-bcp-12 13 Abstract 15 The Network Time Protocol (NTP) is one of the oldest protocols on the 16 Internet and has been widely used since its initial publication. 17 This document is a collection of Best Practices for general operation 18 of NTP servers and clients on the Internet. It includes 19 recommendations for stable, accurate and secure operation of NTP 20 infrastructure. This document is targeted at NTP version 4 as 21 described in RFC 5905. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on July 29, 2019. 40 Copyright Notice 42 Copyright (c) 2019 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 59 2. General Network Security Best Practices . . . . . . . . . . . 3 60 2.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 3. NTP Configuration Best Practices . . . . . . . . . . . . . . 4 62 3.1. Keeping NTP up to date . . . . . . . . . . . . . . . . . 4 63 3.2. Use enough time sources . . . . . . . . . . . . . . . . . 4 64 3.3. Use a diversity of Reference Clocks . . . . . . . . . . . 5 65 3.4. Control Messages . . . . . . . . . . . . . . . . . . . . 6 66 3.5. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 6 67 3.6. Using Pool Servers . . . . . . . . . . . . . . . . . . . 7 68 3.7. Leap Second Handling . . . . . . . . . . . . . . . . . . 7 69 3.7.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 8 70 4. NTP Security Mechanisms . . . . . . . . . . . . . . . . . . . 9 71 4.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . . . 10 72 4.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . . . 10 73 4.3. Network Time Security . . . . . . . . . . . . . . . . . . 11 74 4.4. External Security Protocols . . . . . . . . . . . . . . . 11 75 5. NTP Security Best Practices . . . . . . . . . . . . . . . . . 11 76 5.1. Minimizing Information Leakage . . . . . . . . . . . . . 11 77 5.2. Avoiding Daemon Restart Attacks . . . . . . . . . . . . . 12 78 5.3. Detection of Attacks Through Monitoring . . . . . . . . . 13 79 5.4. Kiss-o'-Death Packets . . . . . . . . . . . . . . . . . . 14 80 5.5. Broadcast Mode Should Only Be Used On Trusted Networks . 15 81 5.6. Symmetric Mode Should Only Be Used With Trusted Peers . . 15 82 6. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15 83 6.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 16 84 6.2. Server configuration . . . . . . . . . . . . . . . . . . 16 85 7. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 16 86 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 87 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 88 10. Security Considerations . . . . . . . . . . . . . . . . . . . 18 89 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 90 11.1. Normative References . . . . . . . . . . . . . . . . . . 18 91 11.2. Informative References . . . . . . . . . . . . . . . . . 19 92 11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21 93 Appendix A. Best Practices specific to the Network Time 94 Foundation implementation . . . . . . . . . . . . . 21 95 A.1. Use enough time sources . . . . . . . . . . . . . . . . . 21 96 A.2. NTP Control and Facility Messages . . . . . . . . . . . . 22 97 A.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22 98 A.4. Leap Second File . . . . . . . . . . . . . . . . . . . . 23 99 A.5. Leap Smearing . . . . . . . . . . . . . . . . . . . . . . 23 100 A.6. Configuring ntpd . . . . . . . . . . . . . . . . . . . . 23 101 A.7. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . . . 24 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 104 1. Introduction 106 NTP version 4 (NTPv4) has been widely used since its publication as 107 [RFC5905]. This document is a collection of best practices for the 108 operation of NTP clients and servers. 110 The recommendations in this document are intended to help operators 111 distribute time on their networks more accurately and more securely. 112 It is intended to apply generally to a broad range of networks. Some 113 specific networks may have higher accuracy requirements that require 114 additional techniques beyond what is documented here. 116 Among the best practices covered are recommendations for general 117 network security, time protocol specific security, and NTP server and 118 client configuration. NTP operation in embedded devices is also 119 covered. 121 This document also contains information for protocol implementors who 122 want to develop their own implementations that are compliant to RFC 123 5905. 125 1.1. Requirements Language 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 129 "OPTIONAL" in this document are to be interpreted as described in 130 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 131 capitals, as shown here. 133 2. General Network Security Best Practices 135 2.1. BCP 38 137 Many network attacks rely on modifying the IP source address of a 138 packet to point to a different IP address than the computer which 139 originated it. UDP-based protocols such as NTP are generally more 140 susceptible to spoofing attacks than connection-oriented protocols. 141 NTP control messages can generate a lot of data in response to a 142 small query, which makes it attractive as a vector for distributed 143 denial-of-service attacks. (NTP Control messages are discussed 144 further in Section 3.4). One documented instance of such an attack 145 can be found here [1], and further discussion in [IMC14] and 146 [NDSS14]. 148 Mitigating source address spoofing attacks should be a priority of 149 anyone administering NTP. BCP 38 [RFC2827] was published in 2000 to 150 to provide some level of remediation against address-spoofing 151 attacks. BCP 38 calls for filtering outgoing and incoming traffic to 152 make sure that the source and destination IP addresses are consistent 153 with the expected flow of traffic on each network interface. It is 154 RECOMMENDED that ISP's and large corporate networks implement ingress 155 and egress filtering. More information is available at the BCP38 156 Info Web page [2] . 158 3. NTP Configuration Best Practices 160 This section provides Best Practices for NTP configuration and 161 operation. Application of these best practices that are specific to 162 the Network Time Foundation implementation, including example 163 configuration directives valid at the time of this writing, are 164 compiled in Appendix A. 166 3.1. Keeping NTP up to date 168 There are multiple versions of the NTP protocol in use, and multiple 169 implementations, on many different platforms. The practices in this 170 document are meant to apply generally to any implementation of 171 [RFC5905]. NTP users should select an implementation that is 172 actively maintained. Users should keep up to date on any known 173 attacks on their selected implementation, and deploy updates 174 containing security fixes as soon as practical. 176 3.2. Use enough time sources 178 An NTP implementation that is compliant with [RFC5905] takes the 179 available sources of time and submits this timing data to 180 sophisticated intersection, clustering, and combining algorithms to 181 get the best estimate of the correct time. The description of these 182 algorithms is beyond the scope of this document. Interested readers 183 should read [RFC5905] or the detailed description of NTP in 184 [MILLS2006]. 186 o If there is only 1 source of time, the answer is obvious. It may 187 not be a good source of time, but it's the only source of time 188 that can be considered. Any issue with the time at the source 189 will be passed on to the client. 191 o If there are 2 sources of time and they agree well enough, then 192 the best time can be calculated easily. But if one source fails, 193 then the solution degrades to the single-source solution outlined 194 above. And if the two sources don't agree, then it's impossible 195 to know which one is correct by simply looking at the time. 197 o If there are 3 sources of time, there is more data available to 198 converge on the best calculated time, and this time is more likely 199 to be accurate. And the loss of one of the sources (by becoming 200 unreachable or unusable) can be tolerated. But at that point, the 201 solution degrades to the 2 source solution. 203 o 4 or more sources of time is better, as long as the sources are 204 diverse (Section 3.3). If one of these sources develops a problem 205 there are still at least 3 other time sources. 207 Operators who are concerned with maintaining accurate time SHOULD use 208 at least 4 independent, diverse sources of time. Four sources will 209 provide sufficient backup in case one source goes down. If four 210 sources are not available, operators MAY use fewer sources, subject 211 to the risks outlined above. 213 But even with 4 or more sources of time, systemic problems can 214 happen. One example involves the leap smearing concept detailed in 215 Section 3.7.1. For several hours before and after the June 2015 leap 216 second, several operators configured their NTP servers with leap 217 smearing while others did not. Many NTP end nodes could not 218 determine an accurate time source because 2 of their 4 sources of 219 time gave them consistent UTC/POSIX time, while the other 2 gave them 220 consistent leap-smeared time. This is just one of many potential 221 causes of disagreement among time sources. 223 Operators are advised to monitor all time sources that are in use. 224 If time sources do not generally agree, operators are encouraged to 225 investigate the cause of this and either correct the problems or stop 226 using defective servers. See Section 3.5 for more information. 228 3.3. Use a diversity of Reference Clocks 230 When using servers with attached hardware reference clocks, it is 231 suggested that different types of reference clocks be used. Having a 232 diversity of sources with independent implementations means that any 233 one issue is less likely to cause a service interruption. 235 Are all clocks on a network from the same vendor? They may have the 236 same bugs. Even devices from different vendors may not be truly 237 independent if they share common elements. Are they using the same 238 base chipset? Are they all running the same version of firmware? 239 Chipset and firmware bugs can happen, but they can be more difficult 240 to diagnose than application software bugs. When having the correct 241 time is of critical importance, it's ultimately up to operators to 242 ensure that their sources are sufficiently independent, even if they 243 are not under the operator's control. 245 A systemic problem with time from any satellite navigation service is 246 possible and has happened. Sunspot activity can render satellite or 247 radio-based time source unusable. Depending on the application 248 requirements, operators may need to consider backup scenarios in the 249 rare circumstance when the satellite system is faulty or unavailable. 251 3.4. Control Messages 253 Some implementations of NTPv4 provide the NTP Control Messages (also 254 known as Mode 6 messages) that were originally specified in 255 Appendix B of [RFC1305] which defined NTPv3. These messages were 256 never included the NTPv4 specification, but they are still used. At 257 the time of this writing, work is being done to formally document the 258 structure of these control messages in [I-D.ietf-ntp-mode-6-cmds]. 260 The NTP Control Messages are designed to permit monitoring and 261 optionally authenticated control of NTP and its configuration. Used 262 properly, these facilities provide vital debugging and performance 263 information and control. But these facilities can be a vector for 264 amplification attacks when abused. For this reason, it is 265 RECOMMENDED that publicly-facing NTP servers should block NTP Control 266 Message queries from outside their organization. 268 The ability to use NTP Control Messages beyond their basic monitoring 269 capabilities SHOULD be limited to authenticated sessions that provide 270 a 'controlkey'. It can also be limited through mechanisms outside of 271 the NTP specification, such as Access Control Lists, that only allow 272 access from approved IP addresses. 274 The NTP Control Messages responses are much larger than the 275 corresponding queries. Thus, they can be abused in high-bandwidth 276 DDoS attacks. Section 2.1 gives more information on how to provide 277 protection for this abuse by implementing BCP 38. 279 3.5. Monitoring 281 Operators SHOULD use their NTP implementation's remote monitoring 282 capabilities to quickly identify servers which are out of sync, and 283 ensure correctness of the service. Operators SHOULD also monitor 284 system logs for messages so problems and abuse attempts can be 285 quickly identified. 287 If a system starts to receive NTP Reply packets from a time server 288 that do not correspond to any requests sent by the system, that can 289 be an indication that an attacker is forging that system's IP address 290 in requests to the remote time server. The goal of this attack would 291 be to convince the time server to stop serving time to the system 292 whose address is being forged. 294 If a system is a broadcast client and its system log shows that it is 295 receiving early time messages from its server, that is an indication 296 that somebody may be forging packets from a broadcast server. 297 (Broadcast client and server modes are defined in Section 3 of 298 [RFC5905]) 300 If a server's system log shows messages that indicates it is 301 receiving NTP timestamps that are much earlier than the current 302 system time, then either the system clock is unusually fast or 303 somebody is trying to launch a replay attack against that server. 305 3.6. Using Pool Servers 307 It only takes a small amount of bandwidth and system resources to 308 synchronize one NTP client, but NTP servers that can service tens of 309 thousands of clients take more resources to run. Network operators 310 and advanced users who want to synchronize their computers MUST only 311 synchronize to servers that they have permission to use. 313 The NTP Pool Project is a group of volunteers who have donated their 314 computing and bandwidth resources to freely distribute time from 315 primary time sources to others on the Internet. The time is 316 generally of good quality but comes with no guarantee whatsoever. If 317 you are interested in using this pool, please review their 318 instructions at http://www.pool.ntp.org/en/use.html [3]. 320 Vendors can obtain their own subdomain that is part of the NTP Pool 321 Project. This offers vendors the ability to safely make use of the 322 time distributed by the pool for their devices. Details are 323 available at http://www.pool.ntp.org/en/vendors.html [4] . 325 If there is a need to synchronize many computers, an operator may 326 want to run local NTP servers that are synchronized to the NTP Pool 327 Project. NTP users on that operator's networks can then be 328 synchronized to local NTP servers. 330 3.7. Leap Second Handling 332 UTC is kept in agreement with the astronomical time UT1 [5] to within 333 +/- 0.9 seconds by the insertion (or possibly a deletion) of a leap 334 second. UTC is an atomic time scale whereas UT1 is based on the 335 rotational rate of the earth. Leap seconds are not introduced at a 336 fixed rate. They are announced by the International Earth Rotation 337 and Reference Systems Service (IERS) in its Bulletin C [6] when 338 necessary to keep UTC and UT1 aligned. 340 NTP time is based on the UTC timescale, and the protocol has the 341 capability to broadcast leap second information. Some Global 342 Navigation Satellite Systems (like GPS) or radio transmitters (like 343 DCF77) broadcast leap second information. If an NTP client is synced 344 to an NTP server that provides leap second notification, the client 345 will get advance notification of impending leap seconds 346 automatically. 348 Since the length of the UT1 day is generally slowly increasing [7], 349 all leap seconds that have been introduced since the practice started 350 in 1972 have been positive leap seconds, where a second is added to 351 UTC. NTP also supports a negative leap second, where a second is 352 removed from UTC, if that ever becomes necessary. 354 While earlier versions of NTP contained some ambiguity regarding when 355 a leap second that is broadcast by a server should be applied by a 356 client, RFC 5905 is clear that leap seconds are only applied on the 357 last day of a month. However, because some older clients may apply 358 it at the end of the current day, it is RECOMMENDED that NTP servers 359 wait until the last day of the month before broadcasting leap 360 seconds. Doing this will prevent older clients from applying a leap 361 second at the wrong time. When implementing this recommendation, 362 operators should ensure that clients are not configured to use 363 polling intervals greater than 24 hours, so the leap second 364 notification is not missed. 366 In circumstances where an NTP server is not receiving leap second 367 information from an automated source, certain organizations maintain 368 files which are updated every time a new leap second is announced: 370 NIST: ftp://time.nist.gov/pub/leap-seconds.list 372 US Navy (maintains GPS Time): ftp://tycho.usno.navy.mil/pub/ntp/leap- 373 seconds.list 375 IERS (announces leap seconds): 376 https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list 378 3.7.1. Leap Smearing 380 Some NTP installations make use of a technique called Leap Smearing. 381 With this method, instead of introducing an extra second (or 382 eliminating a second) on a leap second event, NTP time will be slewed 383 in small increments over a comparably large window of time (called 384 the smear interval) around the leap second event. The smear interval 385 should be large enough to make the rate that the time is slewed 386 small, so that clients will follow the smeared time without 387 objecting. Periods ranging from 2 to 24 hours have been used 388 successfully. During the adjustment window, all the NTP clients' 389 times may be offset from UTC by as much as a full second, depending 390 on the implementation. But at least all clients will generally agree 391 on what time they think it is. 393 The purpose of Leap Smearing is to enable systems that don't deal 394 with the leap second event properly to function consistently, at the 395 expense of fidelity to UTC during the smear window. During a 396 standard leap second event, that minute will have 61 (or possibly 59) 397 seconds in it, and some applications (and even some OS's) are known 398 to have problems with that. 400 Operators who have legal obligations or other strong requirements to 401 be synchronized with UTC or civil time SHOULD NOT use leap smearing, 402 because the distributed time cannot be guaranteed to be traceable to 403 UTC during the smear interval. 405 Clients that are connected to leap smearing servers MUST NOT apply 406 the standard NTP leap second handling. These clients must never have 407 a leap second file loaded, and the smearing servers must never 408 advertise to clients that a leap second is pending. 410 Any use of leap smearing servers should be limited to within a 411 single, well-controlled environment. Leap Smearing MUST NOT be used 412 for public-facing NTP servers, as they will disagree with non- 413 smearing servers (as well as UTC) during the leap smear interval, and 414 there is no standardized way for a client to detect that a server is 415 using leap smearing. However, be aware that some public-facing 416 servers may be configured this way anyway in spite of this guidance. 418 System Administrators are advised to be aware of impending leap 419 seconds and how the servers (inside and outside their organization) 420 they are using deal with them. Individual clients MUST NOT be 421 configured to use a mixture of smeared and non-smeared servers. If a 422 client uses smeared servers, the servers it uses must all have the 423 same leap smear configuration. 425 4. NTP Security Mechanisms 427 In the standard configuration NTP packets are exchanged unprotected 428 between client and server. An adversary that is able to become a 429 Man-In-The-Middle is therefore able to drop, replay or modify the 430 content of the NTP packet, which leads to degradation of the time 431 synchronization or the transmission of false time information. A 432 threat analysis for time synchronization protocols is given in 434 [RFC7384]. NTP provides two internal security mechanisms to protect 435 authenticity and integrity of the NTP packets. Both measures protect 436 the NTP packet by means of a Message Authentication Code (MAC). 437 Neither of them encrypts the NTP's payload, because this payload 438 information is not considered to be confidential. 440 4.1. Pre-Shared Key Approach 442 This approach applies a symmetric key for the calculation of the MAC, 443 which protects authenticity and integrity of the exchanged packets 444 for an association. NTP does not provide a mechanism for the 445 exchange of the keys between the associated nodes. Therefore, for 446 each association, keys MUST be exchanged securely by external means, 447 and they MUST be protected from disclosure. It is RECOMMENDED that 448 each association be protected by its own unique key. It is 449 RECOMMENDED that participants agree to refresh keys periodically. 450 However, NTP does not provide a mechanism to assist in doing so. 451 Each communication partner will need to keep track of its keys in its 452 own local key storage. 454 [RFC5905] specifies using the MD5 hash algorithm for calculation of 455 the MAC, but other algorithms may be supported as well. The MD5 hash 456 is now considered to be too weak and unsuitable for cryptographic 457 usage. [RFC6151] has more information on the algorithm's weaknesses. 458 Implementations will soon be available based on AES-128-CMAC 459 [I-D.ietf-ntp-mac], and users SHOULD use that when it is available. 461 Some implementations store the key in clear text. Therefore it MUST 462 only be readable by the NTP process. 464 An NTP client has to be able to link a key to a particular server in 465 order to establish a protected association. This linkage is 466 implementation specific. Once applied, a key will be trusted until 467 the link is removed. 469 4.2. Autokey 471 [RFC5906] specifies the Autokey protocol. It was published in 2010 472 to provide automated key management and authentication of NTP 473 servers. However, security researchers have identified 474 vulnerabilities [8] in the Autokey protocol. 476 Autokey SHOULD NOT be used. 478 4.3. Network Time Security 480 Work is in progress on an enhanced replacement for Autokey. Refer to 481 [I-D.ietf-ntp-using-nts-for-ntp] for more information. 483 4.4. External Security Protocols 485 If applicable, external security protocols such as IPsec and MACsec 486 can be applied to enhance integrity and authenticity protection of 487 NTP time synchronization packets. Usage of such external security 488 protocols can decrease time synchronization performance [RFC7384]. 489 Therefore, operators are advised to carefully evaluate if the 490 decreased time synchronization performance meets their specific 491 timing requirements. 493 Note that none of the security measures described in Section 4 can 494 prevent packet delay manipulation attacks on NTP. Such delay attacks 495 can target time synchronization packets sent as clear-text or even 496 within an encrypted tunnel. These attacks are described further in 497 Section 3.2.6 of [RFC7384]. 499 5. NTP Security Best Practices 501 This section lists some general NTP security practices, but these 502 issues may (or may not) have been mitigated in particular versions of 503 particular implementations. Contact the maintainers of the relevant 504 implementation for more information. 506 5.1. Minimizing Information Leakage 508 The base NTP packet leaks important information (including reference 509 ID and reference time) that may be used in attacks [NDSS16], 510 [CVE-2015-8138], [CVE-2016-1548]. A remote attacker can learn this 511 information by sending mode 3 queries to a target system and 512 inspecting the fields in the mode 4 response packet. NTP control 513 queries also leak important information (including reference ID, 514 expected origin timestamp, etc.) that may be used in attacks 515 [CVE-2015-8139]. A remote attacker can learn this information by 516 sending control queries to a target system and inspecting the leaked 517 information in the response. 519 As such, mechanisms outside of the NTP protocol, such as Access 520 Control Lists, SHOULD be used to limit the exposure of this 521 information to allowed IP addresses, and keep it from remote 522 attackers not on the list. Hosts SHOULD only respond to NTP control 523 queries from authorized parties. 525 An NTP client that does not provide time on the network can 526 additionally log and drop incoming mode 3 timing queries from 527 unexpected sources. Note well that the easiest way to monitor the 528 status of an NTP instance is to send it a mode 3 query, so it may not 529 be desirable to drop all mode 3 queries. As an alternative, 530 operators SHOULD either filter mode 3 queries from outside their 531 networks, or make sure mode 3 queries are allowed only from trusted 532 systems or networks. 534 A "leaf-node host" is a host that is using NTP solely for the purpose 535 of adjusting its own system time. Such a host is not expected to 536 provide time to other hosts, and relies exclusively on NTP's basic 537 mode to take time from a set of servers. (That is, the host sends 538 mode 3 queries to its servers and receives mode 4 responses from 539 these servers containing timing information.) To minimize 540 information leakage, leaf-node hosts SHOULD drop all incoming NTP 541 packets except mode 4 response packets that come from known sources. 542 An exception to this can be made if a leaf-node host is being 543 actively monitored, in which case incoming packets from the 544 monitoring server can be allowed. 546 Please refer to [I-D.ietf-ntp-data-minimization] for more 547 information. 549 5.2. Avoiding Daemon Restart Attacks 551 [RFC5905] says NTP clients should not accept time shifts greater than 552 the panic threshold. Specifically, RFC 5905 says "PANIC means the 553 offset is greater than the panic threshold PANICT (1000 s) and SHOULD 554 cause the program to exit with a diagnostic message to the system 555 log." 557 However, this behavior can be exploited by attackers as described in 558 [NDSS16], when the following two conditions hold: 560 1. The operating system automatically restarts the NTP client when 561 it quits. (Modern *NIX operating systems are replacing 562 traditional init systems with process supervisors, such as 563 systemd, which can be configured to automatically restart any 564 daemons that quit. This behavior is the default in CoreOS and 565 Arch Linux. As of the time of this writing, it appears likely to 566 become the default behavior in other systems as they migrate 567 legacy init scripts to process supervisors such as systemd.) 569 2. The NTP client is configured to ignore the panic threshold on all 570 restarts. 572 In such cases, if the attacker can send the target an offset that 573 exceeds the panic threshold, the client will quit. Then, when it 574 restarts, it ignores the panic threshold and accepts the attacker's 575 large offset. 577 Operators need to be aware that when operating with the above two 578 conditions, the panic threshold offers no protection from attacks. 579 The natural solution is not to run hosts with these conditions. 580 Specifically, operators SHOULD NOT ignore the panic threshold in all 581 cold-start situations unless sufficient oversight and checking is in 582 place to make sure that this type of attack cannot happen. 584 As an alternative, the following steps MAY be taken by operators to 585 mitigate the risk of attack: 587 o Monitor the NTP system log to detect when the NTP daemon has quit 588 due to a panic event, as this could be a sign of an attack. 590 o Request manual intervention when a timestep larger than the panic 591 threshold is detected. 593 o Configure the ntp client to only ignore the panic threshold in a 594 cold start situation. 596 o Increase the minimum number of servers required before the NTP 597 client adjusts the system clock. This will make the NTP client 598 wait until enough trusted sources of time agree before declaring 599 the time to be correct. 601 In addition, the following steps SHOULD be taken by those who 602 implement the NTP protocol: 604 o Prevent the NTP daemon from taking time steps that set the clock 605 to a time earlier than the compile date of the NTP daemon. 607 o Prevent the NTP daemon from putting 'INIT' in the reference ID of 608 its NTP packets upon initializing. This will make it more 609 difficult for attackers to know when the daemon reboots. 611 5.3. Detection of Attacks Through Monitoring 613 Operators SHOULD monitor their NTP instances to detect attacks. Many 614 known attacks on NTP have particular signatures. Common attack 615 signatures include: 617 1. Bogus packets - A packet whose origin timestamp does not match 618 the value that expected by the client. 620 2. Zero origin packet - A packet with an origin timestamp set to 621 zero [CVE-2015-8138]. 623 3. A packet with an invalid cryptographic MAC [CCR16]. 625 The observation of many such packets could indicate that the client 626 is under attack. 628 5.4. Kiss-o'-Death Packets 630 The "Kiss-o'-Death" (KoD) packet includes a rate management mechanism 631 where a server can tell a misbehaving client to reduce its query 632 rate. KoD packets in general (and the RATE packet in particular) are 633 defined in Section 7.4 of [RFC5905]. It is RECOMMENDED that all NTP 634 devices respect these packets and back off when asked to do so by a 635 server. It is even more important for an embedded device, which may 636 not have an exposed control interface for NTP. 638 That said, a client MUST only accept a KoD packet if it has a valid 639 origin timestamp. Once a RATE packet is accepted, the client should 640 increase its poll interval value (thus decreasing its polling rate) 641 up to a reasonable maximum. This maximum can vary by implementation 642 but should not exceed a poll interval value of 13 (2 hours). The 643 mechanism to determine how much to increase the poll interval value 644 is undefined in [RFC5905]. If the client uses the poll interval 645 value sent by the server in the RATE packet, it MUST NOT simply 646 accept any value. Using large interval values may open a vector for 647 a denial-of-service attack that causes the client to stop querying 648 its server [NDSS16]. 650 The KoD rate management mechanism relies on clients behaving properly 651 in order to be effective. Some clients ignore the RATE packet 652 entirely, and other poorly-implemented clients might unintentionally 653 increase their poll rate and simulate a denial of service attack. 654 Server administrators are advised to be prepared for this and take 655 measures outside of the NTP protocol to drop packets from misbehaving 656 clients when these clients are detected. 658 Kiss-o'-Death (KoD) packets can be used in denial of service attacks. 659 Thus, the observation of even just one RATE packet with a high poll 660 value could be sign that the client is under attack. And KoD packets 661 are commonly accepted even when not cryptographically authenticated, 662 which increases the risk of denial of service attacks. 664 5.5. Broadcast Mode Should Only Be Used On Trusted Networks 666 Per [RFC5905], NTP's broadcast mode is authenticated using symmetric 667 key cryptography. The broadcast server and all its broadcast clients 668 share a symmetric cryptographic key, and the broadcast server uses 669 this key to append a message authentication code (MAC) to the 670 broadcast packets it sends. 672 Importantly, all broadcast clients that listen to this server have to 673 know the cryptographic key. This mean that any client can use this 674 key to send valid broadcast messages that look like they come from 675 the broadcast server. Thus, a rogue broadcast client can use its 676 knowledge of this key to attack the other broadcast clients. 678 For this reason, an NTP broadcast server and all its clients have to 679 trust each other. Broadcast mode SHOULD only be run from within a 680 trusted network. 682 5.6. Symmetric Mode Should Only Be Used With Trusted Peers 684 In symmetric mode, two peers Alice and Bob can both push and pull 685 synchronization to and from each other using either ephemeral 686 symmetric passive (mode 2) or persistent symmetric active (NTP mode 687 1) packets. The persistent association is preconfigured and 688 initiated at the active peer but not preconfigured at the passive 689 peer (Bob). Upon receipt of a mode 1 NTP packet from Alice, Bob 690 mobilizes a new ephemeral association if he does not have one 691 already. This is a security risk for Bob because an arbitrary 692 attacker can attempt to change Bob's time by asking Bob to become its 693 symmetric passive peer. 695 For this reason, a host SHOULD only allow symmetric passive 696 associations to be established with trusted peers. Specifically, a 697 host SHOULD require each of its symmetric passive association to be 698 cryptographically authenticated. Each symmetric passive association 699 SHOULD be authenticated under a different cryptographic key. 701 6. NTP in Embedded Devices 703 As computing becomes more ubiquitous, there will be many small 704 embedded devices that require accurate time. These devices may not 705 have a persistent battery-backed clock, so using NTP to set the 706 correct time on power-up may be critical for proper operation. These 707 devices may not have a traditional user interface, but if they 708 connect to the Internet they will be subject to the same security 709 threats as traditional deployments. 711 6.1. Updating Embedded Devices 713 Vendors of embedded devices are advised to pay attention to the 714 current state of protocol security issues and bugs in their chosen 715 implementation, because their customers don't have the ability to 716 update their NTP implementation on their own. Those devices may have 717 a single firmware upgrade, provided by the manufacturer, that updates 718 all capabilities at once. This means that the vendor assumes the 719 responsibility of making sure their devices have an up-to-date and 720 secure NTP implementation. 722 Vendors of embedded devices SHOULD include the ability to update the 723 list of NTP servers used by the device. 725 There is a catalog of NTP server abuse incidents, some of which 726 involve embedded devices, on the Wikipedia page for NTP Server Misuse 727 and Abuse [9]. 729 6.2. Server configuration 731 Vendors of embedded devices with preconfigured NTP servers need to 732 carefully consider which servers to use. There are several public- 733 facing NTP servers available, but they may not be prepared to service 734 requests from thousands of new devices on the Internet. Vendors MUST 735 only preconfigure NTP servers that they have permission to use. 737 Vendors are encouraged to invest resources into providing their own 738 time servers for their devices to connect to. This may be done 739 through the NTP Pool Project, as documented in Section 3.6. 741 Vendors should read [RFC4085], which advises against embedding 742 globally-routable IP addresses in products, and offers several better 743 alternatives. 745 7. NTP over Anycast 747 Anycast is described in BCP 126 [RFC4786]. (Also see [RFC7094]). 748 With anycast, a single IP address is assigned to multiple servers, 749 and routers direct packets to the closest active server. 751 Anycast is often used for Internet services at known IP addresses, 752 such as DNS. Anycast can also be used in large organizations to 753 simplify configuration of many NTP clients. Each client can be 754 configured with the same NTP server IP address, and a pool of anycast 755 servers can be deployed to service those requests. New servers can 756 be added to or taken from the pool, and other than a temporary loss 757 of service while a server is taken down, these additions can be 758 transparent to the clients. 760 Note well that using a single anycast address for NTP presents its 761 own potential issues. It means each client will likely use a single 762 time server source. A key element of a robust NTP deployment is each 763 client using multiple sources of time. With multiple time sources, a 764 client will analyze the various time sources, selecting good ones, 765 and disregarding poor ones. If a single Anycast address is used, 766 this analysis will not happen. This can be mitigated by creating 767 multiple, separate anycast pools so clients can have multiple sources 768 of time while still gaining the configuration benefits of the anycast 769 pools. 771 If clients are connected to an NTP server via anycast, the client 772 does not know which particular server they are connected to. As 773 anycast servers enter and leave the network, or the network topology 774 changes, the server a particular client is connected to may change. 775 This may cause a small shift in time from the perspective of the 776 client when the server it is connected to changes. In extreme cases 777 where the network topology is changing rapidly, this could cause the 778 server seen by a client to rapidly change as well, which can lead to 779 larger time inaccuracies. It is RECOMMENDED that anycast only be 780 deployed in environments where this behavior can be tolerated 782 Configuration of an anycast interface is independent of NTP. Clients 783 will always connect to the closest server, even if that server is 784 having NTP issues. It is RECOMMENDED that anycast NTP 785 implementations have an independent method of monitoring the 786 performance of NTP on a server. If the server is not performing to 787 specification, it should remove itself from the Anycast network. It 788 is also RECOMMENDED that each Anycast NTP server have an alternative 789 method of access, such as an alternate Unicast IP address, so its 790 performance can be checked independently of the anycast routing 791 scheme. 793 One useful application in large networks is to use a hybrid unicast/ 794 anycast approach. Stratum 1 NTP servers can be deployed with unicast 795 interfaces at several sites. Each site may have several Stratum 2 796 servers with two ethernet interfaces, or a single interface which can 797 support multiple addresses. One interface has a unique unicast IP 798 address. The second has an anycast IP interface (with a shared IP 799 address per location). The unicast interfaces can be used to obtain 800 time from the Stratum 1 servers globally (and perhaps peer with the 801 other Stratum 2 servers at their site). Clients at each site can be 802 configured to use the shared anycast address for their site, 803 simplifying their configuration. Keeping the anycast routing 804 restricted on a per-site basis will minimize the disruption at the 805 client if its closest anycast server changes. Each Stratum 2 server 806 can be uniquely identified on their unicast interface, to make 807 monitoring easier. 809 8. Acknowledgments 811 The authors wish to acknowledge the contributions of Sue Graves, 812 Samuel Weiler, Lisa Perdue, Karen O'Donoghue, David Malone, Sharon 813 Goldberg, Martin Burnicki, Miroslav Lichvar, Daniel Fox Franke, 814 Robert Nagy, and Brian Haberman. 816 9. IANA Considerations 818 This memo includes no request to IANA. 820 10. Security Considerations 822 Time is a fundamental component of security on the internet. The 823 absence of a reliable source of current time subverts many common web 824 authentication schemes, e.g., by allowing the use of expired 825 credentials or by allowing for replay of messages only intended to be 826 processed once. 828 Much of this document directly addresses how to secure NTP servers. 829 In particular, see Section 2, Section 4, and Section 5. 831 There are several general threats to time synchronization protocols 832 which are discussed in [RFC7384]. 834 [I-D.ietf-ntp-using-nts-for-ntp] specifies the Network Time Security 835 (NTS) mechanism and applies it to NTP. Readers are encouraged to 836 check the status of the draft, and make use of the methods it 837 describes. 839 11. References 841 11.1. Normative References 843 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 844 Requirement Levels", BCP 14, RFC 2119, 845 DOI 10.17487/RFC2119, March 1997, 846 . 848 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 849 Defeating Denial of Service Attacks which employ IP Source 850 Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, 851 May 2000, . 853 [RFC4085] Plonka, D., "Embedding Globally-Routable Internet 854 Addresses Considered Harmful", BCP 105, RFC 4085, 855 DOI 10.17487/RFC4085, June 2005, 856 . 858 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast 859 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, 860 December 2006, . 862 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 863 "Network Time Protocol Version 4: Protocol and Algorithms 864 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 865 . 867 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 868 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 869 May 2017, . 871 11.2. Informative References 873 [CCR16] Malhotra, A. and S. Goldberg, "Attacking NTP's 874 Authenticated Broadcast Mode", SIGCOMM Computer 875 Communications Review (CCR) , 2016. 877 [CVE-2015-8138] 878 Van Gundy, M. and J. Gardner, "NETWORK TIME PROTOCOL 879 ORIGIN TIMESTAMP CHECK IMPERSONATION VULNERABILITY", 2016, 880 . 882 [CVE-2015-8139] 883 Van Gundy, M., "NETWORK TIME PROTOCOL NTPQ AND NTPDC 884 ORIGIN TIMESTAMP DISCLOSURE VULNERABILITY", 2016, 885 . 887 [CVE-2016-1548] 888 Gardner, J. and M. Lichvar, "Xleave Pivot: NTP Basic Mode 889 to Interleaved", 2016, 890 . 893 [I-D.ietf-ntp-data-minimization] 894 Franke, D. and A. Malhotra, "NTP Client Data 895 Minimization", draft-ietf-ntp-data-minimization-03 (work 896 in progress), September 2018. 898 [I-D.ietf-ntp-mac] 899 Malhotra, A. and S. Goldberg, "Message Authentication Code 900 for the Network Time Protocol", draft-ietf-ntp-mac-06 901 (work in progress), January 2019. 903 [I-D.ietf-ntp-mode-6-cmds] 904 Haberman, B., "Control Messages Protocol for Use with 905 Network Time Protocol Version 4", draft-ietf-ntp-mode- 906 6-cmds-06 (work in progress), September 2018. 908 [I-D.ietf-ntp-using-nts-for-ntp] 909 Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. 910 Sundblad, "Network Time Security for the Network Time 911 Protocol", draft-ietf-ntp-using-nts-for-ntp-15 (work in 912 progress), December 2018. 914 [IMC14] Czyz, J., Kallitsis, M., Gharaibeh, M., Papadopoulos, C., 915 Bailey, M., and M. Karir, "Taming the 800 Pound Gorilla: 916 The Rise and Decline of NTP DDoS Attacks", Internet 917 Measurement Conference , 2014. 919 [MILLS2006] 920 Mills, D., "Computer network time synchronization: the 921 Network Time Protocol", CRC Press , 2006. 923 [NDSS14] Rossow, C., "Amplification Hell: Revisiting Network 924 Protocols for DDoS Abuse", NDSS'14, San Diego, CA. , 2014. 926 [NDSS16] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg, 927 "Attacking the Network Time Protocol", NDSS'16, San Diego, 928 CA. , 2016, . 930 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 931 Specification, Implementation and Analysis", RFC 1305, 932 DOI 10.17487/RFC1305, March 1992, 933 . 935 [RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol 936 Version 4: Autokey Specification", RFC 5906, 937 DOI 10.17487/RFC5906, June 2010, 938 . 940 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 941 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 942 RFC 6151, DOI 10.17487/RFC6151, March 2011, 943 . 945 [RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil, 946 "Architectural Considerations of IP Anycast", RFC 7094, 947 DOI 10.17487/RFC7094, January 2014, 948 . 950 [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in 951 Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, 952 October 2014, . 954 11.3. URIs 956 [1] https://blog.cloudflare.com/technical-details-behind-a-400gbps- 957 ntp-amplification-ddos-attack/ 959 [2] http://www.bcp38.info 961 [3] http://www.pool.ntp.org/en/use.html 963 [4] http://www.pool.ntp.org/en/vendors.html 965 [5] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time 967 [6] https://www.iers.org/IERS/EN/Publications/Bulletins/ 968 bulletins.html 970 [7] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time 972 [8] https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html 974 [9] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse 976 [10] http://www.ntp.org/downloads.html 978 [11] http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno 980 [12] https://support.ntp.org/bin/view/Support/ConfiguringNTP 982 Appendix A. Best Practices specific to the Network Time Foundation 983 implementation 985 The Network Time Foundation (NTF) provides a widely used 986 implementation of NTP, known as ntpd [10]. It is an evolution of the 987 first NTP implementations developed by David Mills at the University 988 of Delaware. This appendix contains additional recommendations 989 specific to this implementation that are valid at the time of this 990 writing. 992 A.1. Use enough time sources 994 In addition to the recommendation given in Section 3.2 the ntpd 995 implementation provides the 'pool' directive. Starting with ntp- 996 4.2.6, using this directive in the ntp.conf file will spin up enough 997 associations to provide robust time service, and will disconnect poor 998 servers and add in new servers as-needed. The 'minclock' and 999 'maxclock' options of the 'tos' command may be used to override the 1000 default values of how many servers are discovered through the 'pool' 1001 directive. 1003 A.2. NTP Control and Facility Messages 1005 In addition to NTP Control Messages the ntpd implementation also 1006 offers the Mode 7 commands for monitoring and configuration. 1008 If Mode 7 has been explicitly enabled to be used for more than basic 1009 monitoring it should be limited to authenticated sessions that 1010 provide a 'requestkey'. 1012 As mentioned above, there are two general ways to use Mode 6 and Mode 1013 7 requests. One way is to query ntpd for information, and this mode 1014 can be disabled with: 1016 restrict ... noquery 1018 The second way to use Mode 6 and Mode 7 requests is to modify ntpd's 1019 behavior. Modification of ntpd's configuration requires an 1020 authenticated session by default. If no authentication keys have 1021 been specified no modifications can be made. For additional 1022 protection, the ability to perform these modifications can be 1023 controlled with: 1025 restrict ... nomodify 1027 Users can prevent their NTP servers from considering query/ 1028 configuration traffic by default by adding the following to their 1029 ntp.conf file: 1031 restrict default -4 nomodify notrap nopeer noquery 1033 restrict default -6 nomodify notrap nopeer noquery 1035 restrict source nomodify notrap noquery 1037 A.3. Monitoring 1039 The ntpd implementation allows remote monitoring. Access to this 1040 service is generally controlled by the "noquery" directive in NTP's 1041 configuration file (ntp.conf) via a "restrict" statement. The syntax 1042 reads: 1044 restrict address mask address_mask noquery 1045 If a system is using broadcast mode and is running ntp-4.2.8p6 or 1046 later, use the 4th field of the ntp.keys file to specify the IPs of 1047 machines that are allowed to serve time to the group. 1049 A.4. Leap Second File 1051 The use of leap second files requires ntpd 4.2.6 or later. After 1052 fetching the leap seconds file onto the server, add this line to 1053 ntpd.conf to apply and use the file, substituting the proper path: 1055 leapfile "/path/to/leap-file" 1057 There may need to restart ntpd to apply this change. 1059 ntpd servers with a manually configured leap second file will ignore 1060 leap second information broadcast from upstream NTP servers until the 1061 leap second file expires. If no valid leap second file is available 1062 then a leap second notification from an attached reference clock is 1063 always accepted by ntpd. 1065 If no valid leap second file is available, a leap second notification 1066 may be accepted from upstream NTP servers. As of ntp-4.2.6, a 1067 majority of servers must provide the notification before it is 1068 accepted. Before 4.2.6, a leap second notification would be accepted 1069 if a single upstream server of a group of configured servers provided 1070 a leap second notification. This would lead to misbehavior if single 1071 NTP servers sent an invalid leap second warning, e.g. due to a faulty 1072 GPS receiver in one server, but this behavior was once chosen because 1073 in the "early days" there was a greater chance that leap second 1074 information would be available from a very limited number of sources. 1076 A.5. Leap Smearing 1078 Leap Smearing was introduced in ntpd versions 4.2.8.p3 and 4.3.47, in 1079 response to client requests. Support for leap smearing is not 1080 configured by default and must be added at compile time. In 1081 addition, no leap smearing will occur unless a leap smear interval is 1082 specified in ntpd.conf . For more information, refer to 1083 http://bk.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [11]. 1085 A.6. Configuring ntpd 1087 See https://support.ntp.org/bin/view/Support/ConfiguringNTP [12] for 1088 additional information on configuring ntpd. 1090 A.7. Pre-Shared Keys 1092 Each communication partner must add the key information to their key 1093 file in the form: 1095 keyid type key 1097 where "keyid" is a number between 1 and 65534, inclusive, "type" is 1098 an ASCII character which defines the key format, and "key" is the key 1099 itself. 1101 An ntpd client establishes a protected association by appending the 1102 option "key keyid" to the server statement in ntp.conf: 1104 server address key keyid 1106 substituting the server address in the "address" field and the 1107 numerical keyid to use with that server in the "keyid" field. 1109 A key is deemed trusted when its keyid is added to the list of 1110 trusted keys by the "trustedkey" statement in ntp.conf. 1112 trustedkey keyid_1 keyid_2 ... keyid_n 1114 Starting with ntp-4.2.8p7 the ntp.keys file accepts an optional 4th 1115 column, a comma-separated list of IPs that are allowed to serve time. 1116 Use this feature. Note, however, that an adversarial client that 1117 knows the symmetric broadcast key could still easily spoof its source 1118 IP to an IP that is allowed to serve time. (This is easy to do 1119 because the origin timestamp on broadcast mode packets is not 1120 validated by the client. By contrast, client/server and symmetric 1121 modes do require origin timestamp validation, making it more 1122 difficult to spoof packets [CCR16]). 1124 Authors' Addresses 1126 Denis Reilly (editor) 1127 Orolia USA 1128 1565 Jefferson Road, Suite 460 1129 Rochester, NY 14623 1130 US 1132 Email: denis.reilly@orolia.com 1133 Harlan Stenn 1134 Network Time Foundation 1135 P.O. Box 918 1136 Talent, OR 97540 1137 US 1139 Email: stenn@nwtime.org 1141 Dieter Sibold 1142 Physikalisch-Technische Bundesanstalt 1143 Bundesallee 100 1144 Braunschweig D-38116 1145 Germany 1147 Phone: +49-(0)531-592-8420 1148 Fax: +49-531-592-698420 1149 Email: dieter.sibold@ptb.de