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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Fujiwara 3 Internet-Draft JPRS 4 Intended status: Best Current Practice P. Vixie 5 Expires: 27 May 2021 Farsight 6 23 November 2020 8 Fragmentation Avoidance in DNS 9 draft-ietf-dnsop-avoid-fragmentation-03 11 Abstract 13 EDNS0 enables a DNS server to send large responses using UDP and is 14 widely deployed. Path MTU discovery remains widely undeployed due to 15 security issues, and IP fragmentation has exposed weaknesses in 16 application protocols. Currently, DNS is known to be the largest 17 user of IP fragmentation. It is possible to avoid IP fragmentation 18 in DNS by limiting response size where possible, and signaling the 19 need to upgrade from UDP to TCP transport where necessary. This 20 document proposes to avoid IP fragmentation in DNS. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on 27 May 2021. 39 Copyright Notice 41 Copyright (c) 2020 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 46 license-info) in effect on the date of publication of this document. 47 Please review these documents carefully, as they describe your rights 48 and restrictions with respect to this document. Code Components 49 extracted from this document must include Simplified BSD License text 50 as described in Section 4.e of the Trust Legal Provisions and are 51 provided without warranty as described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 3. Proposal to avoid IP fragmentation in DNS . . . . . . . . . . 4 58 3.1. Recommendations for UDP responders . . . . . . . . . . . 4 59 3.2. Recommendations for UDP requestors . . . . . . . . . . . 5 60 4. Maximum DNS/UDP payload size . . . . . . . . . . . . . . . . 5 61 5. Incremental deployment . . . . . . . . . . . . . . . . . . . 6 62 6. Request to zone operators and DNS server operators . . . . . 7 63 7. Considerations . . . . . . . . . . . . . . . . . . . . . . . 7 64 7.1. Protocol compliance . . . . . . . . . . . . . . . . . . . 7 65 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 66 9. Security Considerations . . . . . . . . . . . . . . . . . . . 7 67 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 68 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 69 11.1. Normative References . . . . . . . . . . . . . . . . . . 7 70 11.2. Informative References . . . . . . . . . . . . . . . . . 9 71 Appendix A. How to retrieve path MTU value to a destination from 72 applications . . . . . . . . . . . . . . . . . . . . . . 10 73 Appendix B. Minimal-responses . . . . . . . . . . . . . . . . . 10 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 76 1. Introduction 78 DNS has EDNS0 [RFC6891] mechanism. It enables a DNS server to send 79 large responses using UDP. EDNS0 is now widely deployed, and DNS 80 (over UDP) is said to be the biggest user of IP fragmentation. 82 However, "Fragmentation Considered Poisonous" [Herzberg2013] proposed 83 effective off-path DNS cache poisoning attack vectors using IP 84 fragmentation. "IP fragmentation attack on DNS" [Hlavacek2013] and 85 "Domain Validation++ For MitM-Resilient PKI" [Brandt2018] proposed 86 that off-path attackers can intervene in path MTU discovery [RFC1191] 87 to perform intentionally fragmented responses from authoritative 88 servers. [RFC7739] stated the security implications of predictable 89 fragment identification values. 91 DNSSEC is a countermeasure against cache poisoning attacks that use 92 IP fragmentation. However, DNS delegation responses are not signed 93 with DNSSEC, and DNSSEC does not have a mechanism to get the correct 94 response if an incorrect delegation is injected. This is a denial- 95 of-service vulnerability that can yield failed name resolutions. If 96 cache poisoning attacks can be avoided, DNSSEC validation failures 97 will be avoided. 99 In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines 100 [RFC8085] we are told that an application SHOULD NOT send UDP 101 datagrams that result in IP packets that exceed the Maximum 102 Transmission Unit (MTU) along the path to the destination. 104 A DNS message receiver cannot trust fragmented UDP datagrams 105 primarily due to the small amount of entropy provided by UDP port 106 numbers and DNS message identifiers, each of which being only 16 bits 107 in size, and both likely being in the first fragment of a packet, if 108 fragmentation occurs. By comparison, TCP protocol stack controls 109 packet size and avoid IP fragmentation under ICMP NEEDFRAG attacks. 110 In TCP, fragmentation should be avoided for performance reasons, 111 whereas for UDP, fragmentation should be avoided for resiliency and 112 authenticity reasons. 114 [RFC8900] summarized that IP fragmentation introduces fragility to 115 Internet communication. The transport of DNS messages over UDP 116 should take account of the observations stated in that document. 118 TCP avoids fragmentation using its Maximum Segment Size (MSS) 119 parameter, but each transmitted segment is header-size aware such 120 that the size of the IP and TCP headers is known, as well as the far 121 end's MSS parameter and the interface or path MTU, so that the 122 segment size can be chosen so as to keep the each IP datagram below a 123 target size. This takes advantage of the elasticity of TCP's 124 packetizing process as to how much queued data will fit into the next 125 segment. In contrast, DNS over UDP has little datagram size 126 elasticity and lacks insight into IP header and option size, and so 127 must make more conservative estimates about available UDP payload 128 space. 130 This document proposes to set IP_DONTFRAG / IPV6_DONTFRAG in DNS/UDP 131 responses in order to avoid IP fragmentation, and describes how to 132 avoid packet losses due to IP_DONTFRAG / IPV6_DONTFRAG. 134 2. Terminology 136 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 137 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 138 "OPTIONAL" in this document are to be interpreted as described in 139 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all 140 capitals, as shown here. 142 "Requestor" refers to the side that sends a request. "Responder" 143 refers to an authoritative, recursive resolver or other DNS component 144 that responds to questions. (Quoted from EDNS0 [RFC6891]) 146 "Path MTU" is the minimum link MTU of all the links in a path between 147 a source node and a destination node. (Quoted from [RFC8201]) 149 "Path MTU discovery" is defined by [RFC1191], [RFC8201] and 150 [RFC8899]. 152 Many of the specialized terms used in this document are defined in 153 DNS Terminology [RFC8499]. 155 3. Proposal to avoid IP fragmentation in DNS 157 The methods to avoid IP fragmentation in DNS are described below: 159 3.1. Recommendations for UDP responders 161 * UDP responders SHOULD send DNS responses with IP_DONTFRAG / 162 IPV6_DONTFRAG [RFC3542] options. 164 * If the UDP responder detects immediate error that the UDP packet 165 cannot be sent beyond the path MTU size (EMSGSIZE), the UDP 166 responder MAY recreate response packets fit in path MTU size, or 167 TC bit set. 169 * UDP responders MAY probe to discover the real MTU value per 170 destination. If the path MTU discovery failed or is impossible, 171 use the default path MTU described in Section 4. 173 * UDP responders SHOULD compose UDP responses that result in IP 174 packets that do not exceed the path MTU to the requestor. Of 175 course, as in the conventional case, a specified value (1220 or 176 1232) as the DNS packet size limit may be used. 178 The cause and effect of the TC bit is unchanged from EDNS0 179 [RFC6891]. 181 3.2. Recommendations for UDP requestors 183 * UDP requestors SHOULD send DNS responses with IP_DONTFRAG / 184 IPV6_DONTFRAG [RFC3542] options. 186 * UDP requestors MAY probe to discover the real MTU value per 187 destination. If the path MTU discovery failed or is impossible, 188 use the default path MTU described in Section 4. 190 * UDP reqoestors SHOULD use the requestor's payload size to limit 191 the path MTU value minus the IP header length and UDP header 192 length. Of course, as in the conventional case, a specified value 193 (1220 or 1232) as the requestor's payload size may be used. 195 * UDP requestors MAY drop fragmented DNS/UDP responses without IP 196 reassembly to avoid cache poisoning attacks. 198 * DNS responses may be dropped by IP fragmentation. Upon a timeout, 199 UDP requestors may retry using TCP or UDP, per local policy. 201 4. Maximum DNS/UDP payload size 203 Default path MTU value for IPv6 is XXXX. Default path MTU value for 204 IPv4 is XXXX. 206 Discussions under here will be deleted when the discussion is over. 207 There are many discussions for default path MTU values. 209 * The minimum MTU for an IPv6 interface is 1280 octets (see 210 Section 5 of [RFC8200]). Then, we can use it as default path MTU 211 value for IPv6. 213 * Most of the Internet and especially the inner core has an MTU of 214 at least 1500 octets. An operator of a full resolver would be 215 well advised to measure their path MTU to several authority name 216 servers and to a random sample of their expected stub resolver 217 client networks, to find the upper boundary on IP/UDP packet size 218 in the average case. This limit should not be exceeded by most 219 messages received or transmitted by a full resolver, or else 220 fallback to TCP will occur too often. An operator of 221 authoritative servers would also be well advised to measure their 222 path MTU to several full-service resolvers. The Linux tool 223 "tracepath" can be used to measure the path MTU to well known 224 authority name servers such as [a-m].root-servers.net or [a- 225 m].gtld-servers.net. If the reported path MTU is for example no 226 smaller than 1460, then the maximum DNS/UDP payload would be 1432 227 for IP4 (which is 1460 - IP4 header(20) - UDP header(8)) and 1412 228 for IP6 (which is 1460 - IP6 header(40) - UDP header(8)). To 229 allow for possible IP options and distant tunnel overhead, a 230 useful default for maximum DNS/UDP payload size would be 1400. 232 * [RFC4035] defines that "A security-aware name server MUST support 233 the EDNS0 message size extension, MUST support a message size of 234 at least 1220 octets". Then, the smallest number of the maximum 235 DNS/UDP payload size is 1220. 237 * In order to avoid IP fragmentation, [DNSFlagDay2020] proposed that 238 the UDP requestors set the requestor's payload size to 1232, and 239 the UDP responders compose UDP responses fit in 1232 octets. The 240 size 1232 is based on an MTU of 1280, which is required by the 241 IPv6 specification [RFC8200], minus 48 octets for the IPv6 and UDP 242 headers. 244 By the above reasoning, this proposal is either too small or too 245 large. 247 5. Incremental deployment 249 The proposed method supports incremental deployment. 251 When a full-service resolver implements the proposed method, its stub 252 resolvers (clients) and the authority server network will no longer 253 observe IP fragmentation or reassembly from that server, and will 254 fall back to TCP when necessary. 256 When an authoritative server implements the proposed method, its full 257 service resolvers (clients) will no longer observe IP fragmentation 258 or reassembly from that server, and will fall back to TCP when 259 necessary. 261 6. Request to zone operators and DNS server operators 263 Large DNS responses are the result of zone configuration. Zone 264 operators SHOULD seek configurations resulting in small responses. 265 For example, 267 * Use smaller number of name servers (13 may be too large) 269 * Use smaller number of A/AAAA RRs for a domain name 271 * Use 'minimal-responses' configuration: Some implementations have 272 'minimal responses' configuration that causes DNS servers to make 273 response packets smaller, containing only mandatory and required 274 data (Appendix B). 276 * Use smaller signature / public key size algorithm for DNSSEC. 277 Notably, the signature size of ECDSA or EdDSA is smaller than RSA. 279 7. Considerations 281 7.1. Protocol compliance 283 In prior research ([Fujiwara2018] and dns-operations mailing list 284 discussions), there are some authoritative servers that ignore EDNS0 285 requestor's UDP payload size, and return large UDP responses. 287 It is also well known that there are some authoritative servers that 288 do not support TCP transport. 290 Such non-compliant behavior cannot become implementation or 291 configuration constraints for the rest of the DNS. If failure is the 292 result, then that failure must be localized to the non-compliant 293 servers. 295 8. IANA Considerations 297 This document has no IANA actions. 299 9. Security Considerations 301 10. Acknowledgments 303 The author would like to specifically thank Paul Wouters, Mukund 304 Sivaraman and Tony Finch for extensive review and comments. 306 11. References 308 11.1. Normative References 310 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 311 DOI 10.17487/RFC1191, November 1990, 312 . 314 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 315 Requirement Levels", BCP 14, RFC 2119, 316 DOI 10.17487/RFC2119, March 1997, 317 . 319 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 320 Rose, "Protocol Modifications for the DNS Security 321 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 322 . 324 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 325 Security (DNSSEC) Hashed Authenticated Denial of 326 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, 327 . 329 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 330 for DNS (EDNS(0))", STD 75, RFC 6891, 331 DOI 10.17487/RFC6891, April 2013, 332 . 334 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage 335 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, 336 March 2017, . 338 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 339 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 340 May 2017, . 342 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 343 (IPv6) Specification", STD 86, RFC 8200, 344 DOI 10.17487/RFC8200, July 2017, 345 . 347 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 348 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 349 DOI 10.17487/RFC8201, July 2017, 350 . 352 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 353 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 354 January 2019, . 356 [RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T. 357 Völker, "Packetization Layer Path MTU Discovery for 358 Datagram Transports", RFC 8899, DOI 10.17487/RFC8899, 359 September 2020, . 361 [RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., 362 and F. Gont, "IP Fragmentation Considered Fragile", 363 BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020, 364 . 366 11.2. Informative References 368 [Brandt2018] 369 Brandt, M., Dai, T., Klein, A., Shulman, H., and M. 370 Waidner, "Domain Validation++ For MitM-Resilient PKI", 371 Proceedings of the 2018 ACM SIGSAC Conference on Computer 372 and Communications Security , 2018. 374 [DNSFlagDay2020] 375 "DNS flag day 2020", n.d., . 377 [Fujiwara2018] 378 Fujiwara, K., "Measures against cache poisoning attacks 379 using IP fragmentation in DNS", OARC 30 Workshop , 2019. 381 [Herzberg2013] 382 Herzberg, A. and H. Shulman, "Fragmentation Considered 383 Poisonous", IEEE Conference on Communications and Network 384 Security , 2013. 386 [Hlavacek2013] 387 Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67 388 Meeting , 2013, . 391 [RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, 392 "Advanced Sockets Application Program Interface (API) for 393 IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003, 394 . 396 [RFC7739] Gont, F., "Security Implications of Predictable Fragment 397 Identification Values", RFC 7739, DOI 10.17487/RFC7739, 398 February 2016, . 400 Appendix A. How to retrieve path MTU value to a destination from 401 applications 403 Socket options: "IP_MTU (since Linux 2.2) Retrieve the current known 404 path MTU of the current socket. Valid only when the socket has been 405 connected. Returns an integer. Only valid as a getsockopt(2)." 406 (Quoted from Debian GNU Linux manual: ip(7)) 408 "IPV6_MTU getsockopt(): Retrieve the current known path MTU of the 409 current socket. Only valid when the socket has been connected. 410 Returns an integer." (Quoted from Debian GNU Linux manual: ipv6(7)) 412 Appendix B. Minimal-responses 414 Some implementations have 'minimal responses' configuration that 415 causes a DNS server to make response packets smaller, containing only 416 mandatory and required data. 418 Under the minimal-responses configuration, DNS servers compose 419 response messages using only RRSets corresponding to queries. In 420 case of delegation, DNS servers compose response packets with 421 delegation NS RRSet in authority section and in-domain (in-zone and 422 below-zone) glue in the additional data section. In case of non- 423 existent domain name or non-existent type, the start of authority 424 (SOA RR) will be placed in the Authority Section. 426 In addition, if the zone is DNSSEC signed and a query has the DNSSEC 427 OK bit, signatures are added in answer section, or the corresponding 428 DS RRSet and signatures are added in authority section. Details are 429 defined in [RFC4035] and [RFC5155]. 431 Authors' Addresses 433 Kazunori Fujiwara 434 Japan Registry Services Co., Ltd. 435 Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda, Chiyoda-ku, Tokyo 436 101-0065 437 Japan 439 Phone: +81 3 5215 8451 440 Email: fujiwara@jprs.co.jp 441 Paul Vixie 442 Farsight Security Inc 443 177 Bovet Road, Suite 180 444 San Mateo, CA, 94402 445 United States of America 447 Phone: +1 650 393 3994 448 Email: vixie@fsi.io