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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DPRIVE T. Reddy 3 Internet-Draft D. Wing 4 Intended status: Standards Track P. Patil 5 Expires: November 7, 2015 Cisco 6 May 6, 2015 8 DNS over DTLS (DNSoD) 9 draft-wing-dprive-dnsodtls-01 11 Abstract 13 DNS queries and responses are visible to network elements on the path 14 between the DNS client and its server. These queries and responses 15 can contain privacy-sensitive information which is valuable to 16 protect. An active attacker can send bogus responses causing 17 misdirection of the subsequent connection. 19 To counter passive listening and active attacks, this document 20 proposes the use of Datagram Transport Layer Security (DTLS) for DNS, 21 to protect against passive listeners and certain active attacks. As 22 DNS needs to remain fast, this proposal also discusses mechanisms to 23 reduce DTLS round trips and reduce DTLS handshake size. The proposed 24 mechanism runs over the default DNS port and can also run over an 25 alternate port. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on November 7, 2015. 44 Copyright Notice 46 Copyright (c) 2015 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. Relationship to TCP Queries and to DNSSEC . . . . . . . . . . 3 63 3. Common problems with DNS Privacy . . . . . . . . . . . . . . 3 64 3.1. Firewall Blocking Ports or DNS Privacy Protocol . . . . . 3 65 3.2. Authenticating the DNS Privacy Server . . . . . . . . . . 4 66 3.3. Downgrade attacks . . . . . . . . . . . . . . . . . . . . 5 67 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 68 5. Incremental Deployment . . . . . . . . . . . . . . . . . . . 5 69 6. Demultiplexing, Polling, Port Usage, and Discovery . . . . . 6 70 7. Performance Considerations . . . . . . . . . . . . . . . . . 7 71 8. Established sessions . . . . . . . . . . . . . . . . . . . . 8 72 9. DTLS Features and Cipher Suites . . . . . . . . . . . . . . . 9 73 10. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 74 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 75 12. Security Considerations . . . . . . . . . . . . . . . . . . . 10 76 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 77 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 78 14.1. Normative References . . . . . . . . . . . . . . . . . . 11 79 14.2. Informative References . . . . . . . . . . . . . . . . . 11 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 82 1. Introduction 84 The Domain Name System is specified in [RFC1034] and [RFC1035]. DNS 85 queries and responses are normally exchanged unencrypted and are thus 86 vulnerable to eavesdropping. Such eavesdropping can result in an 87 undesired entity learning domains that a host wishes to access, thus 88 resulting in privacy leakage. DNS privacy problem is further 89 discussed in [I-D.bortzmeyer-dnsop-dns-privacy]. 91 Active attackers have long been successful at injecting bogus 92 responses, causing cache poisoning and causing misdirection of the 93 subsequent connection (if attacking A or AAAA records). A popular 94 mitigation against that attack is to use ephemeral and random source 95 ports for DNS queries. 97 This document defines DNS over DTLS (DNSoD, pronounced "dee-enn-sod") 98 which provides confidential DNS communication for stub resolvers, 99 recursive resolvers, iterative resolvers and authoritative servers. 101 The motivations for proposing DNSoD are that 103 o TCP suffers from network head-of-line blocking, where the loss of 104 a packet causes all other TCP segments to not be delivered to the 105 application until the lost packet is re-transmitted. DNSoD, 106 because it uses UDP, does not suffer from network head-of-line 107 blocking. 109 o DTLS session resumption consumes 1 round trip whereas TLS session 110 resumption can start only after TCP handshake is complete. 111 Although TCP Fast Open [RFC7413] can reduce that handshake, TCP 112 Fast Open is not yet available in commercially-popular operating 113 systems. 115 2. Relationship to TCP Queries and to DNSSEC 117 DNS queries can be sent over UDP or TCP. The scope of this document, 118 however, is only UDP. DNS over TCP could be protected with TLS, as 119 described in [I-D.hzhwm-start-tls-for-dns]. Alternatively, a shim 120 protocol could be defined between DTLS and DNS, allowing large 121 responses to be sent over DTLS itself, see Section 7. 123 DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity 124 of DNS resource records, allowing end-users (or their resolver) to 125 verify legitimacy of responses. However, DNSSEC does not protect 126 privacy of DNS requests or responses. DNSoD works in conjunction 127 with DNSSEC, but DNSoD does not replace the need or value of DNSSEC. 129 3. Common problems with DNS Privacy 131 This section describes problems common to any DNS privacy solution. 132 To achieve DNS privacy an encrypted and integrity-protected channel 133 is needed between the client and server. This channel can be 134 blocked, and the client needs to react to such blockages. 136 3.1. Firewall Blocking Ports or DNS Privacy Protocol 138 When sending DNS over an encrypted channel, there are two choices: 139 send the encrypted traffic over the DNS ports (UDP 53, TCP 53) or 140 send the encrypted traffic over a different port. The encrypted 141 traffic is not normal DNS traffic, but rather is a cryptographic 142 handshake followed by encrypted payloads. There can be firewalls, 143 other security devices, or intercepting DNS proxies which block the 144 non-DNS traffic or otherwise react negatively (e.g., quarantining the 145 host for suspicious behavior). Alternatively, if a different port is 146 used for the encrypted traffic, a firewall or other security device 147 might block that port or otherwise react negatively. 149 There is no panacea, and only experiments on the Internet will 150 uncover which technique or combination of techniques will work best. 151 The authors believe a combination of techniques will be necessary, as 152 that has proven necessary with other protocols that desire to work on 153 existing networks. 155 3.2. Authenticating the DNS Privacy Server 157 DNS privacy requires encrypting the query (and response) from passive 158 attacks. Such encryption typically provides integrity protection as 159 a side-effect, which means on-path attackers cannot simply inject 160 bogus DNS responses. However, to provide stronger protection from 161 active attackers pretending to be the server, the server itself needs 162 to be authenticated. 164 To authenticate the server providing DNS privacy, the DNS client 165 needs to be configured with the names or IP addresses of those DNS 166 privacy servers. The server certificate MUST contain DNS-ID 167 (subjectAltName) as described in Section 4.1 of [RFC6125]. DNS names 168 and IP addresses can be contained in the subjectAltName entries. The 169 client MUST use the rules and guidelines given in section 6 of 170 [RFC6125] to validate the DNS server identity. 172 We imagine this could be implemented by adding the certificate name 173 to the /etc/resolv.conf file, such as below: 175 nameserver 8.8.8.8 176 certificate google-public-dns.google.com 177 nameserver 208.67.220.220 178 certificate resolver.opendns.com 180 For DNS privacy servers that don't have a certificate trust chain 181 (e.g., because they are on a home network or a corporate network), 182 the configured list of DNS privacy servers can contain the 183 certificate fingerprint of the DNS privacy server (i.e., a simple 184 whitelist of name and certificate fingerprint). 186 We imagine this could be implemented by adding the certificate 187 fingerprint to the /etc/resolv.conf file, such as below (line split 188 for Internet Draft formatting): 190 nameserver 192.168.1.1 191 certificate-fingerprint 192 01:56:D3:AC:CF:5B:3F:B8:8F:0F:B4:30:88:2D:F6:72:4E:8C:F2:EE 194 3.3. Downgrade attacks 196 Using DNS privacy with an authenticated server is most preferred, DNS 197 privacy with an unauthenticated server is next preferred, and plain 198 DNS is least preferred. An implementation will attempt to obtain DNS 199 privacy by contacting DNS servers on the local network (provided by 200 DHCP) and on the Internet, and will make those attempts in parallel 201 to reduce user impact. If DNS privacy cannot be successfully 202 negotiated for whatever reason, client can do three things: 204 1. refuse to send DNS queries on this network, which means the 205 client can not make effective use of this network, as modern 206 networks require DNS; or, 208 2. use DNS privacy with an un-authorized server, which means an 209 attacker could be spoofing the handshake with the DNS privacy 210 server; or, 212 3. send plain DNS queries on this network, which means no DNS 213 privacy is provided. 215 Heuristics can improve this situation, but only to a degree (e.g., 216 previous success of DNS privacy on this network may be reason to 217 alert the user about failure to establish DNS privacy on this network 218 now). Still, the client (in cooperation with the end user) has to 219 decide to use the network without the protection of DNS privacy. 221 4. Terminology 223 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 224 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 225 "OPTIONAL" in this document are to be interpreted as described in 226 [RFC2119]. 228 5. Incremental Deployment 230 DNSoD can be deployed incrementally by the Internet Service Provider 231 or as an Internet service. 233 If the ISP's DNS resolver supports DNSoD, then DNS queries are 234 protected from passive listening and from many active attacks along 235 that path. 237 DNSoD can be offered as an Internet service, and a stub resolver or 238 DNS resolver can be configured to point to that DNSoD server (rather 239 than to the ISP-provided DNS server). 241 6. Demultiplexing, Polling, Port Usage, and Discovery 243 [Note - This section requires further discussion] 245 Many modern operating systems already detect if a web proxy is 246 interfering with Internet communications, using proprietary 247 mechanisms that are out of scope of this document. After that 248 mechanism has run (and detected Internet connectivity is working), 249 the DNSoD procedure described in this document should commence. This 250 timing avoids delays in joining the network (and displaying an icon 251 indicating successful Internet connection), at the risk that those 252 initial DNS queries will be sent without protection afforded by 253 DNSoD. 255 DNSoD can run over standard UDP port 53 as defined in [RFC1035]. A 256 DNS client or server that does not implement this specification will 257 not respond to the incoming DTLS packets because they don't parse as 258 DNS packets (the DNS Opcode would be 15, which is undefined). A DNS 259 client or server that does implement this specification can 260 demultiplex DNS and DTLS packets by examining the third octet. For 261 TLS 1.2, which is what is defined by this specification, a DTLS 262 packet will contain 253 in the third octet, whereas a DNS packet will 263 never contain 253 in the third octet. 265 There has been some concern with sending DNSoD traffic over the same 266 port as normal, un-encrypted DNS traffic. The intent of this section 267 is to show that DNSoD could successfully be sent over port 53. 268 Further analysis and testing on the Internet may be valuable to 269 determine if multiplexing on port 53, using a separate port, or some 270 fallback between a separate port and port 53 brings the most success. 272 After performing the above steps, the host should determine if the 273 DNS server supports DNSoD by sending a DTLS ClientHello message. A 274 DNS server that does not support DNSoD will not respond to 275 ClientHello messages sent by the client, because they are not valid 276 DNS requests (specifically, the DNS Opcode is invalid). The client 277 MUST use timer values defined in Section 4.2.4.1 of [RFC6347] for 278 retransmission of ClientHello message and if no response is received 279 from the DNS server. After 15 seconds, it MUST cease attempts to re- 280 transmit its ClientHello. Thereafter, the client MAY repeat that 281 procedure in the event the DNS server has been upgraded to support 282 DNSoD, but such probing SHOULD NOT be done more frequently than every 283 24 hours and MUST NOT be done more frequently than every 15 minutes. 284 This mechanism requires no additional signaling between the client 285 and server. 287 7. Performance Considerations 289 To reduce number of octets of the DTLS handshake, especially the size 290 of the certificate in the ServerHello (which can be several 291 kilobytes), we should consider using plain public keys 292 [I-D.ietf-tls-oob-pubkey]. Considering that to authorize a certain 293 DNS server the client already needs explicit configuration of the DNS 294 servers it trusts, maybe the public key configuration problem is 295 really no worse than the configuration problem of those whitelisted 296 certificates? 298 Multiple DNS queries can be sent over a single DNSoD security 299 association. The existing QueryID allows multiple requests and 300 responses to be interleaved in whatever order they can be fulfilled 301 by the DNS server. This means DNSoD reduces the consumption of UDP 302 port numbers, and because DTLS protects the communication between the 303 DNS client and its server, the resolver SHOULD NOT use random 304 ephemeral source ports (Section 9.2 of [RFC5452]) because such source 305 port use would incur additional, unnecessary DTLS load on the DNSoD 306 server. 308 It is highly advantageous to avoid server-side DTLS state and reduce 309 the number of new DTLS security associations on the server which can 310 be done with [RFC5077]. This also eliminates a round-trip for 311 subsequent DNSoD queries, because with [RFC5077] the DTLS security 312 association does not need to be re-established. Note: with the shim 313 (described below) perhaps we could send the query and the restore 314 server-side state in the ClientHello packet. 316 Compared to normal DNS, DTLS adds at least 13 octets of header, plus 317 cipher and authentication overhead to every query and every response. 318 This reduces the size of the DNS payload that can be carried. 319 Certain DNS responses are large (e.g., many AAAA records, TXT, SRV) 320 and don't fit into a single UDP packet, causing a partial response 321 with the truncation (TC) bit set. The client is then expected to 322 repeat the query over TCP, which causes additional name resolution 323 delay. We have considered two ideas, one that reduces the need to 324 switch to TCP and another that eliminates the need to switch to TCP: 326 o Path MTU can be determined using Packetization Layer Path MTU 327 Discovery [RFC4821] using DTLS heartbeat. [RFC4821] does not rely 328 on ICMP or ICMPv6, and would not affect DNS state or 329 responsiveness on the client or server. However, it would be 330 additional chattiness. 332 o To avoid IP fragmentation, DTLS handshake messages incorporate 333 their own fragment offset and fragment length. We might utilize a 334 similar mechanism in a shim layer between DTLS and DNS, so that 335 large DNS messages could be carried without causing IP 336 fragmentation. 338 DNSoD puts an additional computational load on servers. The largest 339 gain for privacy is to protect the communication between the DNS 340 client (the end user's machine) and its caching resolver. Because of 341 the load imposed, and because of the infrequency of queries to root 342 servers means the DTLS overhead is unlikely to be amoritized over the 343 DNS queries sent over that DTLS connection, implementing DNSoD on 344 root servers is NOT RECOMMENDED. 346 8. Established sessions 348 In DTLS, all data is protected using the same record encoding and 349 mechanisms. When the mechanism described in this document is in 350 effect, DNS messages are encrypted using the standard DTLS record 351 encoding. When a user of DTLS wishes to send an DNS message, it 352 delivers it to the DTLS implementation as an ordinary application 353 data write (e.g., SSL_write()). A single DTLS session can be used to 354 receive multiple DNS requests and generate DNS multiple responses. 356 Client Server 357 ------ ------ 359 ClientHello --------> 361 <------- HelloVerifyRequest 362 (contains cookie) 364 ClientHello --------> 365 (contains cookie) 366 (empty SessionTicket extension) 367 ServerHello 368 (empty SessionTicket extension) 369 Certificate* 370 ServerKeyExchange* 371 CertificateRequest* 372 <-------- ServerHelloDone 374 Certificate* 375 ClientKeyExchange 376 CertificateVerify* 377 [ChangeCipherSpec] 378 Finished --------> 379 NewSessionTicket 380 [ChangeCipherSpec] 381 <-------- Finished 383 DNS Request ---------> 385 <--------- DNS Response 387 Message Flow for Full Handshake Issuing New Session Ticket 389 9. DTLS Features and Cipher Suites 391 To improve interoperability, the set of DTLS features and cipher 392 suites is restricted. The DTLS implementation MUST disable 393 compression. DTLS compression can lead to the exposure of 394 information that would not otherwise be revealed [RFC3749]. Generic 395 compression is unnecessary since DNS provides compression features 396 itself. DNS over DTLS MUST only be used with cipher suites that have 397 ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE) 398 [RFC5246] or the elliptic curve variant (ECDHE) [RFC4492]. Ephemeral 399 key exchange MUST have a minimum size of 2048 bits for DHE or 400 security level of 128 bits for ECDHE. Authenticated Encryption with 401 Additional Data (AEAD) modes, such as the Galois Counter Model (GCM) 402 mode for AES [RFC5288] are acceptable. 404 10. Anycast 406 DNS servers are often configured with anycast addresses. While the 407 network is stable, packets transmitted from a particular source to an 408 anycast address will reach the same server that has the cryptographic 409 context from the DNS over DTLS handshake. But when the network 410 configuration changes,a DNS over DTLS packet can be received by a 411 server that does not have the necessary cryptographic context. To 412 encourage the client to initiate a new DTLS handshake, DNS servers 413 SHOULD generate a DTLS Alert message in response to receiving a DTLS 414 packet for which the server does not have any cryptographic context. 416 11. IANA Considerations 418 If demultiplexing DTLS and DNS (using the third octet, Section 6) is 419 useful, we should reserve DNS Opcode 15 to ensure DNS always has a 0 420 bit where DTLS always has a 1 bit. 422 12. Security Considerations 424 The interaction between the DNS client and the DNS server requires 425 Datagram Transport Layer Security (DTLS) with a ciphersuite offering 426 confidentiality protection and the guidance given in [RFC7525] must 427 be followed to avoid attacks on DTLS. Once a DNSoD client has 428 established a security association with a particular DNS server, and 429 outstanding normal DNS queries with that server (if any) have been 430 received, the DNSoD client MUST ignore any subsequent normal DNS 431 responses from that server, as all subsequent responses should be 432 inside DNSoD. This behavior mitigates all (?) attacks described in 433 Measures for Making DNS More Resilient against Forged Answers 434 [RFC5452]. 436 Security considerations discussed in DTLS [RFC6347] also apply to 437 this document. 439 13. Acknowledgements 441 Thanks to Phil Hedrick for his review comments on TCP and to Josh 442 Littlefield for pointing out DNSoD load on busy servers (most notably 443 root servers). The authors would like to thank Simon Josefsson for 444 discussions and comments on the design of DNSoD. 446 14. References 448 14.1. Normative References 450 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 451 STD 13, RFC 1034, November 1987. 453 [RFC1035] Mockapetris, P., "Domain names - implementation and 454 specification", STD 13, RFC 1035, November 1987. 456 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 457 Requirement Levels", BCP 14, RFC 2119, March 1997. 459 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 460 Rose, "DNS Security Introduction and Requirements", RFC 461 4033, March 2005. 463 [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. 464 Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites 465 for Transport Layer Security (TLS)", RFC 4492, May 2006. 467 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 468 "Transport Layer Security (TLS) Session Resumption without 469 Server-Side State", RFC 5077, January 2008. 471 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 472 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 474 [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois 475 Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, 476 August 2008. 478 [RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More 479 Resilient against Forged Answers", RFC 5452, January 2009. 481 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 482 Security Version 1.2", RFC 6347, January 2012. 484 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 485 "Recommendations for Secure Use of Transport Layer 486 Security (TLS) and Datagram Transport Layer Security 487 (DTLS)", BCP 195, RFC 7525, May 2015. 489 14.2. Informative References 491 [I-D.bortzmeyer-dnsop-dns-privacy] 492 Bortzmeyer, S., "DNS privacy considerations", draft- 493 bortzmeyer-dnsop-dns-privacy-02 (work in progress), April 494 2014. 496 [I-D.hzhwm-start-tls-for-dns] 497 Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D. 498 Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls- 499 for-dns-01 (work in progress), July 2014. 501 [I-D.ietf-tls-oob-pubkey] 502 Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and 503 T. Kivinen, "Using Raw Public Keys in Transport Layer 504 Security (TLS) and Datagram Transport Layer Security 505 (DTLS)", draft-ietf-tls-oob-pubkey-11 (work in progress), 506 January 2014. 508 [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol 509 Compression Methods", RFC 3749, May 2004. 511 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 512 Discovery", RFC 4821, March 2007. 514 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 515 Verification of Domain-Based Application Service Identity 516 within Internet Public Key Infrastructure Using X.509 517 (PKIX) Certificates in the Context of Transport Layer 518 Security (TLS)", RFC 6125, March 2011. 520 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 521 Fast Open", RFC 7413, December 2014. 523 Authors' Addresses 525 Tirumaleswar Reddy 526 Cisco Systems, Inc. 527 Cessna Business Park, Varthur Hobli 528 Sarjapur Marathalli Outer Ring Road 529 Bangalore, Karnataka 560103 530 India 532 Email: tireddy@cisco.com 533 Dan Wing 534 Cisco Systems, Inc. 535 170 West Tasman Drive 536 San Jose, California 95134 537 USA 539 Email: dwing@cisco.com 541 Prashanth Patil 542 Cisco Systems, Inc. 543 Bangalore 544 India 546 Email: praspati@cisco.com