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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 csi Working Group G. Daley 3 Internet-Draft 4 Intended status: Informational J-M. Combes 5 Expires: November 28, 2008 Orange Labs R&D 6 May 27, 2008 8 Securing Neighbour Discovery Proxy Problem Statement 9 draft-daley-csi-sndp-prob-00.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on November 28, 2008. 36 Abstract 38 Neighbour Discovery Proxy is used to provide an address presence on a 39 link from nodes which are no themselves present. It allows a node to 40 receive packets directed at its address by allowing another device to 41 neighbour advertise on its behalf. 43 Neighbour Discovery Proxy is used in Mobile IPv6 and related 44 protocols to provide reachability from nodes on the home network when 45 a Mobile Node is not at home, by allowing the Home Agent to act as 46 proxy. It is also used as a mechanism to allow a global prefix to 47 span multiple links, where proxies act as relays for neighbour 48 discovery messages. 50 Neighbour Discovery Proxy currently cannot be secured using SEND. 51 Today, SEND assumes that a node advertising an address is the address 52 owner and in possession of appropriate public and private keys for 53 that node. This document describes how existing practice for proxy 54 Neighbour Discovery relates to Secured Neighbour Discovery. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2.1. IPv6 Mobile Nodes and Neighbour Discovery Proxy . . . . . 3 61 2.2. IPv6 Fixed Nodes and Neighbor Discovery Proxy . . . . . . 5 62 2.3. Bridge-like ND proxies . . . . . . . . . . . . . . . . . . 5 63 3. Proxy ND and Mobility . . . . . . . . . . . . . . . . . . . . 7 64 4. Proxy Neighbour Discovery and SEND . . . . . . . . . . . . . . 10 65 4.1. CGA signatures and Proxy Neighbour Discovery . . . . . . . 11 66 4.2. Non-CGA signatures and Proxy Neighbour Discovery . . . . . 11 67 4.3. Securing proxy DAD . . . . . . . . . . . . . . . . . . . . 12 68 5. Potential Approaches to Securing Proxy ND . . . . . . . . . . 13 69 5.1. Secured Proxy ND and Mobile IPv6 . . . . . . . . . . . . . 14 70 5.1.1. Mobile IPv6 and Router-based authorization . . . . . . 14 71 5.1.2. Mobile IPv6 and per-address authorization . . . . . . 14 72 5.1.3. Cryptographic based solutions . . . . . . . . . . . . 15 73 5.1.4. 'Point-to-Point' link model based solution . . . . . . 15 74 5.2. Secured Proxy ND and Bridge-like proxies . . . . . . . . . 15 75 5.2.1. Authorization Delegation . . . . . . . . . . . . . . . 15 76 5.2.2. Unauthorized routers and proxies . . . . . . . . . . . 16 77 5.2.3. Multiple proxy spans . . . . . . . . . . . . . . . . . 16 78 5.2.4. Routing Infrastructure Delegation . . . . . . . . . . 17 79 5.2.5. Local Delegation . . . . . . . . . . . . . . . . . . . 17 80 5.2.6. Host delegation of trust to proxies . . . . . . . . . 18 81 5.3. Proxying unsecured addresses . . . . . . . . . . . . . . . 19 82 6. Two or more nodes defending a same address . . . . . . . . . . 19 83 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 84 8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 85 8.1. Router Trust Assumption . . . . . . . . . . . . . . . . . 20 86 8.2. Certificate Transport . . . . . . . . . . . . . . . . . . 20 87 8.3. Timekeeping . . . . . . . . . . . . . . . . . . . . . . . 21 88 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 89 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 90 10.1. Normative References . . . . . . . . . . . . . . . . . . . 22 91 10.2. Informative References . . . . . . . . . . . . . . . . . . 23 92 Appendix A. Changes from the previous versions . . . . . . . . . 23 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 94 Intellectual Property and Copyright Statements . . . . . . . . . . 25 96 1. Introduction 98 Neighbour Discovery Proxy is defined in IPv6 Neighbour Discovery 99 [RFC4861]. It is used in networks where a prefix has to span 100 multiple links [RFC4389] but also in Mobile IPv6 [RFC3775] (and so in 101 Mobile IPv6 based protocols like NEMO [RFC3963], FMIPv6 [RFC4068] or 102 HMIPv6 [RFC4140]) and in IKEv2 [RFC4306]. It allows a device which 103 is not physically present on a link to have another advertise its 104 presence, and forward on packets to the off-link device. 106 Neighbour Discovery Proxy relies upon another device, the proxy, to 107 monitor for Neighbour Solicitations (NS), and answer with Neighbour 108 Advertisements (NA). These proxy Neighbour Advertisements direct 109 data traffic through the proxy. Proxied traffic is then forwarded on 110 to the end destination. 112 2. Scenarios 114 This section describes the different scenarios where the interaction 115 between SEND and ND-Proxy raises issues. 117 2.1. IPv6 Mobile Nodes and Neighbour Discovery Proxy 119 When moving in the Internet, the aim of IPv6 mobility is to allow a 120 device continued packet delivery, whether present on its home network 121 or not. The following text is focused on Mobile IPv6 but the issue 122 is the same with Mobile IPv6 based protocols (e.g. NEMO, HMIPv6). 124 For Mobile IPv6 Mobile Nodes (MN), it is necessary to keep existing 125 sessions going even when one leaves the home network. If a neighbour 126 is actively delivering packets to a Mobile Node which is at home, 127 this neighbour will have a valid neighbour cache entry pointed at the 128 MN's link-layer address on the Home link. 130 As seen in Figure 1, solicitors send a multicast solicitation to the 131 solicited nodes address of the absent node (based on the unicast 132 address). 134 Absent Mobile Proxy Solicitor 136 NS:SL3=S,DL3=Sol(A),TA=A 137 +-----+ SL2=s,DL2=sol(a),SLL=s 138 | |<================ 139 | | 140 | |================> 141 +-----+ NA:SL3=P,DL3=S,TA=A, 142 SL2=p,DL2=s,TLL=p 144 Legend: 145 SL3: Source IPv6 Address NS: Neighbour Solicitation 146 DL3: Destination IPv6 Address NA: Neighbour Advertisement 147 SL2: Source Link-Layer Address RS: Router Solicitation 148 DL2: Destination Link-Layer Address RA: Router Advertisement 149 TA: Target Address 150 SLL/TLL: Source/Target Link-Layer Address Option 152 Figure 1 154 The Proxy, which listens to this address responds with a Neighbour 155 Advertisement which originates at its own IPv6 address and has the 156 proxy's address as the Target Link-Layer Address, but contains the 157 absent mobile in the Target Address field of the Neighbour 158 Advertisement. In this case, no solicitations are proxied, as the 159 advertisements originate within the proxy itself. 161 If Cryptographically Generated Addressing (CGA) [RFC3972] is 162 available, the MN may be able to secure its neighbour cache bindings 163 while at home using Secured Neighbour Discovery (SEND) [RFC3971]. 164 SEND assumes that the address owner is the advertiser and therefore 165 has access to the keys required to sign advertisements about the 166 address. Movement away from the home link requires that a proxy 167 undertake Neighbour Discovery. 169 In Mobile IPv6, the role of the proxy is undertaken by the Home 170 Agent. While the Home agent has a security association with the MN, 171 it as proxy will not have access to the public-private key pair used 172 to generate the MN's cryptographic address. This prevents Proxy 173 Neighbour Discovery from using SEND as defined [RFC3971]. 175 Where a host moves from the home network to a visited network, the 176 proxy needs to override existing valid neighbour cache entries which 177 may have SEND protection. This is needed in order to redirect 178 traffic to use the proxy's link-layer address, allowing packets to 179 flow onto the tunnel connecting the Home Agent/Proxy and the MN. 180 With current specifications, any solicitation or advertisement sent 181 by the proxy will not be able to update the MN's home address if the 182 existing NC entry is protected by SEND. Such existing neighbour 183 cache entries will time-out after Neighbour Unreachability Detection 184 [RFC4861]. 186 Where secured proxy services are not able to be provided, a proxy's 187 advertisement may be overridden by a bogus proxy without it even 188 knowing the attack has occurred. 190 2.2. IPv6 Fixed Nodes and Neighbor Discovery Proxy 192 This scenario is a sub-case from the previous one. The IPv6 node 193 will never be on the link where the ND messages are proxied. This is 194 case with IKEv2 [RFC4306] when a node needs an IP address in the 195 network protected by a security gateway and this latest assigns it 196 dynamically using Configuration Payload during IKEv2 exchanges. The 197 security gateway will have to proxy ND messages to be able to 198 intercept messages, sent to the node, to tunnel them to this latest. 200 2.3. Bridge-like ND proxies 202 Where proxies exist between two segments, messages may be sent by the 203 proxy on the far link, in order to gain or pass on neighbour 204 information. The proxy in this case forwards messages while 205 modifying their source and destination MAC addresses, and rewrites 206 their Link-Layer Address Options solicited and override flags. This 207 is defined in Bridge Like ND Proxy (ND Proxy) [RFC4389]. 209 This rewriting is incompatible with SEND signed messages for a number 210 of reasons: 212 o Rewriting elements within the message will break the digital 213 signature. 215 o The source IP address of the packets is the packet's origin, not 216 the proxy's address. The proxy is unable to generate another 217 signature for this address, as it doesn't have the CGA private key 218 [RFC3971]. 220 Proxy modification of SEND solicitations and advertisements require 221 removal of (at least) CGA and Signature options, and may also need 222 new options with proxy capabilities if non-CGA signatures are added 223 to SEND. 225 While bridge-like ND proxies aim to provide as little interference 226 with ND mechanisms as possible, SEND has been designed to prevent 227 modification or spoofing of advertisements by devices on the link. 229 Of particular note is the fact that ND Proxy performs a different 230 kind of proxy neighbour discovery to Mobile IPv6 [RFC3775] [RFC4389]. 231 The Mobile IPv6 RFC specifies that the Home Agent as proxy sends 232 Neighbour Advertisements from its own address with the Target Address 233 set to the absent Mobile Node's address. The Home Agent's own link- 234 layer address is placed in the Target Link-Layer address option 235 [RFC3775]. 237 ND Proxy resends messages containing their original address, even 238 after modification [RFC4389]. Figure 2 describes packet formats for 239 proxy neighbour solicitation and advertisement as specified by the 240 specification. 242 Advertisor Proxy Solicitor 244 NS:SL3=S,DL3=Sol(A),TA=A, NS:SL3=S,DL3=Sol(A),TA=A, 245 SL2=p,DL2=sol(A),SLL=p +-----+ SL2=s,DL2=sol(a),SLL=s 246 <==================| |<================ 247 | | 248 ==================>| |================> 249 NA:SL3=A,DL3=S,TA=A, +-----+ NA:SL3=A,DL3=S,TA=A 250 SL2=a,DL2=p,TLL=a SL2=p,DL2=s,TLL=p 252 Figure 2 254 In order to use the same security procedures for both ND Proxy and 255 Mobile IPv6, changes may be needed to the proxying procedures in 256 [RFC4389], as well as changes to SEND. 258 An additional (and undocumented) requirement for bridge-like proxying 259 is the operation of router discovery. Router Discovery packets may 260 similarly modify neighbour cache state, and require protection from 261 SEND. 263 In Figure 3, the router discovery messages propagate without 264 modification to the router address, but elements within the message 265 change. This is consistent with the description of Neighbour 266 Discovery above. 268 Advertisor Proxy Solicitor 270 RS:SL3=S,DL3=AllR, RS:SL3=S,DL3=AllR, 271 SL2=p,DL2=sol(A),SLL=p +-----+ SL2=s,DL2=allr,SLL=s 272 <==================| |<================ 273 | | 274 ==================>| |================> 275 RA:SL3=A,DL3=S, +-----+ RA:SL3=A,DL3=S, 276 SL2=a,DL2=p,SLL=a SL2=p,DL2=s,SLL=p 277 Figure 3 279 Once again, these messages may not be signed with a CGA signature by 280 the re-advertisor, because it does not own the source address. 282 Additionally, multicast Authorization Delegation Discovery ICMPv6 283 messages need to be exchanged for bridge-like ND proxies to prove 284 their authority to forward. Unless the proxy receives explicit 285 authority to act as a router, or the router knows of its presence, no 286 authorization may be made. This explicit authorization requirement 287 may be at odds with zero configuration goal of ND proxying [RFC4389]. 289 An alternative (alluded to in an appendix of ND Proxy) suggests that 290 the proxy send Router Advertisements (RA) from its own address. As 291 described by ND Proxy, this is insufficient for providing proxied 292 Neighbour Advertisement service, but may be matched with neighbour 293 solicitation and advertisement services using the proxy's source 294 address in the same way as Mobile IPv6 [RFC4389] [RFC3775]. This 295 means that all router and neighbour advertisements would come from 296 the proxied address, but may contain a target address which allows 297 proxied neighbour presence to be established with peers on other 298 segments. Router Discovery in this case has the identity of the 299 original (non-proxy) router completely obscured in router discovery 300 messages. 302 The resultant proxy messages would have no identifying information 303 indicating their origin, which means that proxying between multiple 304 links would require state to be stored on outstanding solicitations 305 (effectively a ND only NAT). This level of state storage may be 306 undesirable. 308 Mobile IPv6 does not experience this issue when supplying its own 309 address, since ND messages are never forwarded on to the absent node 310 (the Home Agent having sufficient information to respond itself). 312 Authorization from a router may still be required for Router 313 Advertisement, and will be discussed in Section 5.2. 315 3. Proxy ND and Mobility 317 Whenever a mobile device moves off a link and requires another device 318 to forward packets from that address to the MN's new location, proxy 319 Neighbour Discovery is required. 321 In the Mobile IPv6 case, where the Mobile Node moves away from home, 322 a Home Agent needs to be able to override existing neighbour cache 323 entries in order to redirect packet flow over a tunnel to the Mobile 324 Node's Care-of-Address (CoA) [RFC3775]. 326 In Fast Handovers for Mobile IPv6, local neighbours or routers with 327 existing valid neighbour cache states need to be told the PAR's link- 328 layer address when the MN is departing for a new link, or after 329 arrival on the new link when tunnel forwarding begins [RFC4068]. 330 This allows the MN to maintain reachability to the hosts on that link 331 until it is able to send Mobile IPv6 Binding signalling subsequent to 332 address configuration on the new network. 334 As shown in Figure 4, after the mobile node departs, the Home Agent 335 or Proxy sends an overriding Neighbour advertisement, in order to 336 update existing neighbour cache entries. 338 Absent Mobile Proxy Solicitor 340 +-----+ 341 Binding Update | | 342 ---------------->| | 343 or Fast BU | |================> 344 +-----+ NA:SL3=P,DL3=AllN,TA=A, 345 SL2=p,DL2=alln,TLL=p 347 Figure 4 349 Where the proxy forwards between segments of a network, nodes may 350 move between segments [RFC4389]. For this scenario, the proxy is 351 responsible for updating neighbour cache entries as incorrect state 352 is left in them after the move. 354 Devices which were on the same segment as the moving node, 355 subsequently have incorrect neighbour cache state, as they now need 356 to traverse the proxy to get to the other node. Devices which were 357 previously being proxied may now be on the same segment as the mobile 358 node, and may go direct. 360 As illustrated in Figure 5, the nodes may have incorrect neighbour 361 cache state, even if the proxy knows of the departure to another 362 segment. 364 Mobile Node Proxy Mobile Node - M 365 (Departed) P (New Location) 367 + - - + +-----+ NC: 368 ' ' NC: NC: | | N -> n 369 + - - + N -> n+-----+M -> m +-----+ 370 | | | | 371 ------------------| |-------------------- 372 | | | 373 +-----+NC: +-----+ 374 | |M -> m 375 +-----+ 377 Existing 378 Neighbour - N 380 Figure 5 382 While neighbour cache state times out, and causes devices to probe 383 for the location of a peer, long delays may occur before timeouts of 384 neighbour cache state [RFC4861]. In cases where these delays are too 385 long, the proxy may have to override the neighbour cache entries of 386 hosts which were previously on the same segment as the moving node. 388 Those devices now resident on the same segment as the mobile node 389 will have the proxy's link-layer address in its neighbour cache. In 390 ND Proxy, it is indicated that packets are never forwarded back to 391 the same segment upon which they arrived (potentially to prevent 392 forwarding loops) [RFC4389]. 394 Similarly, if the mobile node is unaware of its movement, it too may 395 have incorrect neighbour cache entries for devices which it is now on 396 the same segment as. This is shown below in Figure 6. 398 Mobile Node Proxy Mobile Node - M 399 (Departed) P (New Location) 401 + - - + +-----+ NC: 402 ' ' NC: NC: | | N -> p2 403 + - - + +-----+M -> m +-----+ 404 | | |N -> n | 405 ------------------| |-------------------- 406 | | | 407 P2+-----+P1 +-----+ NC: 408 | | M -> p1 409 +-----+ 411 Existing 412 Neighbour - N 414 Figure 6 416 For the remaining duration of their incorrect neighbour cache entry 417 (up to around 35 seconds), all packets will be dropped. Therefore, 418 these devices may need to be updated with the present node's link- 419 layer address. 421 Procedures regarding updating caches rely upon Section 7.2.6 of IPv6 422 Neighbour Discovery [RFC4861], which allows proxies to neighbour 423 advertise to all-nodes with the override flag set when becoming a 424 proxy or addresses change. 426 For either environment, updates are required to neighbour cache 427 entries which may be for SEND nodes. These advertisements must 428 therefore have enough authority to override neighbour cache entries 429 even though they are secured. 431 4. Proxy Neighbour Discovery and SEND 433 There are currently no existing secured Neighbour Discovery 434 procedures for proxied addresses, and all Neighbour Advertisements 435 from SEND nodes are required to have equal source and target 436 addresses, and be signed by the transmitter (section 7.4 of 437 [RFC3971]). 439 Signatures over SEND messages are required to be applied on the CGA 440 source address of the message, and there is no way of indicating that 441 a message is proxied. 443 Even if the message is able to be transmitted from the original 444 owner, differences in link-layer addressing and options require 445 modification by a proxy. If a message is signed with a CGA-based 446 signature, the proxy is unable to regenerate a signature over the 447 changed message as it lacks the keying material. 449 Therefore, a router wishing to provide proxy Neighbour Advertisement 450 service can not use existing SEND procedures on those messages. 452 A host may wish to establish a session with a device which is not on- 453 link but is proxied. As a SEND host, it prefers to create neighbour 454 cache entries using secured procedures. Since SEND signatures cannot 455 be applied to an existing proxy Neighbour Advertisement, it must 456 accept non-SEND advertisements in order to receive proxy Neighbour 457 Advertisements. 459 Neighbour Cache spoofing of another node therefore becomes trivial, 460 as any address may be proxy advertised to the SEND node, and 461 overridden only if the node is there to protect itself. When a node 462 is present to defend itself, it may also be difficult for the 463 solicitor determine the difference between a proxy-spoofing attack, 464 and a situation where a proxied device returns to a link and 465 overrides other proxy advertisers [RFC4861]. 467 4.1. CGA signatures and Proxy Neighbour Discovery 469 SEND defines one public-key and signature format for use with 470 Cryptographically Generated Addresses (CGAs) [RFC3971]. CGAs are 471 intended to tie address ownership to a particular Public/Private key 472 pair. 474 In SEND as defined today, Neighbour Discovery Messages (including the 475 IP Addresses from the IPv6 header) are signed with the same key used 476 to generate the CGA. This means that message recipients have proof 477 that the signer of the message owned the address. 479 Where a proxy replaces the message source with its own CGA, the 480 existing CGA option and RSA signature option need to be replaced with 481 the proxy's. Such a message will validate using SEND, except that 482 the Target Address field will not match the IPv6 Source Address in 483 Neighbour Advertisements [RFC3971]. 485 Additional authorization information may be needed to prove that the 486 proxy is indeed allowed to advertise for the target address, as is 487 described in Section 5. 489 4.2. Non-CGA signatures and Proxy Neighbour Discovery 491 Where a proxy retains the original source address in a proxied 492 message, existing SEND-CGA checks will fail, since fields within the 493 message will be changed. In order to achieve secured proxy neighbour 494 discovery in this case, new signature authentication mechanisms may 495 be needed for SEND. 497 SEND provides interfaces for extension of SEND to non-CGA based 498 authorization. Messages are available for Authorization Delegation 499 Discovery, which is able to carry arbitrary PKIX/X.509 certificates 500 [RFC5280]. 502 There is no specification though of keying information option formats 503 analogous to the SEND CGA Option [RFC3971]. The existing option 504 allows a host to verify message integrity by specifying a key and 505 algorithm for digital signature, without providing authorization for 506 functions other than CGA ownership. 508 The digital signature in SEND is transported in the RSA Signature 509 Option. As currently specified, the signature operation is performed 510 over a CGA Message type, and infers support for CGA verification. 511 Clarification or changing of this issue for non-CGA operations may be 512 necessary. 514 Within SEND, more advanced functions such as routing may be 515 authorized by certificate path verification using Authorization 516 Delegation Discovery. 518 With non-CGA signatures and authentication, certificate contents for 519 authorization may need to be determined, as outlined in Section 5. 521 While SEND provides for extensions to new non-CGA methods, existing 522 SEND hosts may silently discard messages with unverifiable RSA 523 signature options (Section 5.2.2 of [RFC3971]), if configured only to 524 accept SEND messages. In cases where unsecured neighbour cache 525 entries are still accepted, messages from new algorithms will be 526 treated as unsecured. 528 4.3. Securing proxy DAD 530 Initiation of Proxy Neighbour Discovery also requires Duplicate 531 Address Detection (DAD) checks of the address [RFC4862]. These DAD 532 checks need to be performed by sending Neighbour Solicitations, from 533 the unspecified source address, with the target being the proxied 534 address. 536 In existing SEND procedures, the address which is used for CGA tests 537 on DAD NS is the target address. A Proxy which originates this 538 message while the proxied address owner is absent is unable to 539 generate a CGA-based signature for this address and must undertake 540 DAD with an unsecured NS. It may be possible that the proxy can 541 ensure that responding NA's are secured though. 543 Where bridge-like ND proxy operations are being performed, DAD NS's 544 may be copied from the original source, without modification 545 (considering they have an unspecified source address and contain no 546 link-layer address options) [RFC4389] 548 If non-CGA based signatures are available, then the signature over 549 the DAD NS doesn't need to have a CGA relationship to the Target 550 Address, but authorization for address configuration needs to be 551 shown using certificates. Where SEND-only nodes do not understand 552 the signature format. 554 5. Potential Approaches to Securing Proxy ND 556 SEND nodes already have the concept of delegated authority through 557 requiring external authorization of routers to perform their routing 558 and advertisement roles. The authorization of these routers takes 559 the form of delegation certificates. 561 Proxy Neighbour Discovery requires a delegation of authority on 562 behalf of the absent address owner, to the proxier. Without this 563 authority, other devices on the link have no reason to trust an 564 advertiser. 566 For bridge-like proxies, it is assumed that there is no preexisting 567 trust between the host owning the address and the proxy. Therefore, 568 authority may necessarily be dynamic or based on topological roles 569 within the network [RFC4389]. 571 Existing trust relationships lend themselves to providing authority 572 for proxying in two alternative ways. 574 First, the SEND router authorization mechanisms described above 575 provide delegation from the organization responsible for routing in 576 an address domain, to the certified routers. It may be argued that 577 routers so certified may be trusted to provide service for nodes 578 which form part of a link's address range, but are themselves absent. 579 Devices which are proxies could either be granted the right to proxy 580 by the network's router, or be implicitly allowed to proxy by virtue 581 of being an authorized router. 583 Second, where the proxied address is itself a CGA, the holder of the 584 public and private keys is seen to be authoritative about the 585 address' use. If this address owner was able to sign the proxier's 586 address and public key information, it would be possible to identify 587 that the proxy is known and trusted by the CGA address owner for 588 proxy service. This method requires that the proxied address know or 589 learn the proxy's address and public key, and that the certificate 590 signed by the proxied node's is passed to the proxy, either while 591 they share the same link, or at a later stage. 593 In both methods, the original address owner's advertisements need to 594 override the proxy if it suddenly returns, and therefore timing and 595 replay protection from such messages need to be carefully considered. 597 5.1. Secured Proxy ND and Mobile IPv6 599 Mobile IPv6 has a security association between the Mobile Node and 600 Home Agent. The Mobile Node sends a Binding Update to the Home 601 Agent, to indicate that it is not at home. This implies that the 602 Mobile Node wishes the Home Agent to begin proxy Neighbour Discovery 603 operations for its home address(es). 605 5.1.1. Mobile IPv6 and Router-based authorization 607 A secured Proxy Neighbour Advertisements proposal based on existing 608 router trust would require no explicit authorization signalling 609 between HA and MN to allow proxying. Hosts on the home link will 610 believe proxied advertisements solely because they come from a 611 trusted router. 613 Where the home agent operates as a router without explicit trust to 614 route from the advertising routing infrastructure (such as in a home, 615 with a router managed by an ISP), more explicit proxying 616 authorization may be required, as described in Section 5.2. 618 5.1.2. Mobile IPv6 and per-address authorization 620 Where proxy Neighbour Discovery is delegated by the MN to the home 621 agent, the MN needs to learn the public key for the Home Agent, so 622 that it can generate a certificate authorizing the public-private 623 key-pair to be used in proxying. It may conceivably either do this 624 using Certificate Path Solicitations over a home tunnel, over the 625 Internet, or Router Discovery while still at home [RFC3971] 626 [RFC3775]. 628 When sending its Binding Update to the HA, the MN would need to 629 provide a certificate containing the subject(proxy-HA)'s public key 630 and address, the issuer(MN)'s CGA and public key, and timestamps 631 indicating when the authority began and when it ends. This 632 certificate would need to be passed near to binding time, possibly in 633 a Certificate Path Advertisement [RFC3971]. Messaging or such an 634 exchange mechanism would have to be developed. 636 5.1.3. Cryptographic based solutions 638 Specific cryptographic algorithms may help to allow trust between 639 entities of a same group. 641 This is the case, for example, with ring signature algorithms, a type 642 of signature generated using the private key of any entity from the 643 same group but to check the signature, the public keys of all group 644 members are required. Applied to SEND, the addresses are 645 cryptographically generated using multiple public keys and the 646 Neighbor Discovery messages are signed with an RSA ring signature. 648 5.1.4. 'Point-to-Point' link model based solution 650 Another approach is to use the 'Point-to-Point' link model. 652 In this model, one prefix is provided per MN and only a MN and the HA 653 are on a same link. The consequence is the HA no more needs to act 654 as ND Proxy. 656 One way to design such a solution is to use virtual interfaces, on 657 the MN and the HA, and a virtual link between them. Addresses 658 generated on the virtual interfaces will only be advertised on the 659 virtual link. For Mobile IPv6, this results to use a virtual Home 660 Network where the MN will never come back. 662 5.2. Secured Proxy ND and Bridge-like proxies 664 In link-extension environments, the role of a proxy is more 665 explicitly separated from that of a router. In SEND, routers may 666 expect to be authorized by the routing infrastructure to advertise, 667 and provide this authority to hosts in order to allow them to change 668 forwarding state. 670 Proxies are not part of the traditional infrastructure of the 671 Internet, and hosts or routers may not have an explicit reason to 672 trust them, except that they can forward packets to regions where 673 otherwise they could not reach. 675 5.2.1. Authorization Delegation 677 If a proxy can convince a device that it should be trusted to perform 678 proxying function, it may require that device to vouch for its 679 operation in dealing with other devices. It may do this by receiving 680 a certificate, signed by the originating device that the proxy is 681 believed capable of proxying under certain circumstances. 683 This allows nodes receiving proxied neighbour discovery packets to 684 quickly check if the proxy is authorized for the operation. There 685 are several bases for such trust, and requirements in proxied 686 environments, which are discussed below. 688 5.2.2. Unauthorized routers and proxies 690 Routers advertising on networks without routers may have to operate 691 with no explicit authorization, and SEND hosts will configure these 692 if there's no other option [RFC3971]. While proxies may similarly 693 attempt to advertise without authority, this provides no security for 694 the routing infrastructure. Any device can set up to be a SEND 695 proxy/router so long as it signs its own ND messages from its CGA. 697 This may not help in the case that a proxy attempts to update 698 neighbour cache entries for SEND node which moves between links, 699 since the SEND node's authority to advertise its own CGA address 700 would not be superceded by a proxy with no credentials. 702 5.2.3. Multiple proxy spans 704 Proxies may have multiple levels of nesting, which allow the network 705 to connect between non-adjacent segments. 707 In this case, authority delegated at one point will have to be 708 redelegated (possibly in a diluted form) to proxies further away from 709 the origin of the trust. 711 Trust ProxyA ProxyB Distant 712 Origin - T Node - D 714 +-----+ +-----+ 715 | | | | 716 +-----+ +-----+ +-----+ +-----+ 717 | | | | | | 718 ------------| |------------| |---------- 719 | | | | 720 +-----+ +-----+ 721 ==========> ==============> ==========> 722 Deleg(A,T) Deleg(B,Deleg(A,T)) Advertise(D, Deleg(B, 723 Deleg(A,T)) 725 Figure 7 727 As shown in Figure 7, the Proxy A needs to redelegate authority to 728 proxy for T to B, this allows it to proxy advertisements back to D, 729 which target T. 731 5.2.4. Routing Infrastructure Delegation 733 Where it is possible for the proxy to pre-establish trust with the 734 routing infrastructure, or at least to the local router, it may be 735 possible to authorize proxying as a function of routing within the 736 subnet. The router or CA may then be able to certify proxying for 737 only a subset of the prefixes which is itself certified for. 739 If a router or CA provides certification for a particular prefix, it 740 may be able to indicate that only proxying is supported, so that 741 neighbour cache entries of routers connected to internet 742 infrastructure are never overridden by the proxy, if the router is 743 present on a segment. 745 Hosts understanding such certificates may allow authorized proxies 746 and routers to override host SEND/CGA when assuming proxy roles, if 747 the host is absent. 749 Proxy certificate signing could be done either dynamically (requiring 750 exchanges of identity and authorization information), or statically 751 when the network is set up. 753 5.2.5. Local Delegation 755 Where no trust tie exists between the authority which provides the 756 routing infrastructure and the provider of bridging and proxying 757 services, it may still be possible for SEND hosts to trust the 758 bridging provider to authorize proxying operations. 760 SEND itself requires that routers be able to show authorization, but 761 doesn't require routers to have a single trusted root. 763 A local bridging/proxying authority trust delegation may be possible. 764 It would be possible for this authority to pass out local use 765 certificates, allowing proxying on a specific subnet or subnets, with 766 a separate authorization chain to that for the routers with Internet 767 access. 769 This would require little modification to SEND, other than addition 770 of router based proxy authority (as in Section 5.2.4), and proxies 771 would in effect be treated as routers by SEND hosts [RFC3971]. 772 Distribution of keying and trust material for the initial bootstrap 773 of proxies would not be provided though (and may be static). 775 Within small domains, key management and distribution may be a 776 tractable problem, so long as these operations are are simple enough 777 to perform. 779 Since these domains may be small, it may be necessary to provide 780 certificate chains for trust anchors which weren't requested in 781 Certificate Path Solicitations, if the proxy doesn't have a trust 782 chain to any requested trust anchor. 784 This is akin to 'suggesting' an appropriate trusted root. It may 785 allow for user action in allowing trust extension when visiting 786 domains without ties to a global keying infrastructure. In this 787 case, the trust chain would have to start with a self-signed 788 certificate from the original CA. 790 5.2.6. Host delegation of trust to proxies 792 Unlike Mobile IPv6, for bridge-like proxied networks, there is no 793 existing security association upon which to transport proxying 794 authorization credentials. 796 Proxies need then to convince neighbours to delegate proxy authority 797 to them, in order to proxy-advertise to nodes on different segments. 798 It will be difficult without additional information to distinguish 799 between legitimate proxies, and devices which have no need or right 800 to proxy (and may wish two network segments to appear to be 801 connected). 803 When proxy advertising, proxies must not only identify that proxying 804 needs to occur, but provide proof that they are allowed to do so, so 805 that SEND Neighbour Cache entries may be updated. Unless the 806 authorization to update such entries is tied to address ownership 807 proofs from the proxied host or the verifiable routing 808 infrastructure, spoofing may occur. 810 When a host received a proxied neighbour advertisement, it would be 811 necessary to check authorization in the same way that authorization 812 delegation discovery is performed in SEND. 814 Otherwise, certificate transport will be required to exchange 815 authorization between proxied nodes and proxies. 817 Proxies would have to be able to delegate this authorization to 818 downstream proxies, as described in Section 5.2.3. 820 Movement between segments could be controlled with increasing 821 certificate sequence numbers and timestamps. The timestamp of the 822 root authority (in this case, the CGA address owner) would be most 823 significant. Where ties exist, the shortest chain would supercede, 824 as this would indicate a proxy closer to the proxied node. 826 5.3. Proxying unsecured addresses 828 Where the original neighbour discovery message is unsecured, there is 829 an argument for not providing secured proxy service for that node. 831 In both the Mobile IPv6 and extended networks cases, the node may 832 arrive back at the network and require other hosts to map their 833 existing neighbour cache entry to the node's link-layer address. The 834 re-arriving node's overriding of link-layer address mappings will 835 occur without SEND in this case. 837 It is notable that without SEND protection any node may spoof the 838 arrival, and effectively steal service across an extended network. 839 This is the same as in the non-proxy case, and is not made 840 significantly worse by the proxy's presence (although the identity of 841 the attacker may be masked if source addresses are being replaced). 843 If signatures over the proxied messages were to be used, re-arrival 844 and override of the neighbour cache entries would have to be allowed, 845 so the signatures would indicate that at least the proxy wasn't 846 spoofing (even if the original sender was). 848 For non-SEND/CGA routers, though, it may be possible for secured 849 proxies to send signed router advertisement messages, in order to 850 ensure that routers aren't spoofed, and subsequently switched to 851 being on different parts of the extended network. 853 This has problems in that the origin is again unsecured, and any node 854 on the network could spoof router advertisement for an unsecured 855 address. These spoofed messages may become almost indistinguishable 856 (except for the non-CGA origin address) from unspoofed messages from 857 SEND routers. 859 Given these complexities, the simplest method is to allow unsecured 860 devices to be spoofed from any port on the network, as is the case 861 today. 863 6. Two or more nodes defending a same address 865 The previous part of this document focused on the case where two 866 nodes defend a same address (i.e. the node and the proxy). But, 867 there are scenarios where two or more nodes are defending a same 868 address. This is at least the case for: 870 o Nodes having the same address, as the MAG's ingress link-local 871 address in PMIPv6 [I-D.ietf-netlmm-mn-ar-if]. 873 o Nodes having a common anycast address [RFC4291]. 875 The problem statement, described previously in this document, applies 876 for these cases and the issues are the same from a signalling point 877 of view. 879 In the first case, [I-D.ietf-netlmm-mn-ar-if] assumes that the 880 security material used by SEND (i.e. public-private key pair) is 881 shared between all the MAGs. For the second case, there is no 882 solution today. But, in a same way, it should be possible to assume 883 that the nodes having a common anycast address could also share the 884 security material. 886 It is important to notice that when many nodes defending a same 887 address are not in the same administrative domain (e.g. MAGs in 888 different administrative domains but in a same PMIPv6 domain 889 [I-D.ietf-netlmm-proxymip6]), sharing the security material used by 890 SEND may raise a security issue. 892 7. IANA Considerations 894 No new options or messages are defined in this document. 896 8. Security Considerations 898 8.1. Router Trust Assumption 900 Router based authorization for Secured Proxy ND may occur without the 901 knowledge or consent of a device. It is susceptible to the 'Good 902 Router Goes Bad' attack described in [RFC3756]. 904 8.2. Certificate Transport 906 The certification delegation relies upon transfer of the new 907 credentials to the proxying HA in order to undertake ND proxy on its 908 behalf. Since the Binding cannot come into effect until DAD has 909 taken place, the delegation of the proxying authority necessarily 910 predates the return of the Binding Ack, as described in [RFC3775]. 911 In the above described case, the home tunnel which comes into 912 creation as part of the binding process may be required for 913 Certificate Path Solicitation or Advertisement transport [RFC3971]. 914 This constitutes a potential chicken-and-egg problem. Either 915 modifications to initial home binding semantics or certificate 916 transport are required. This may be trivial if signed, non- 917 repudiable certificates are sent in the clear between the MN's CoA 918 and the HA without being tunneled. 920 8.3. Timekeeping 922 All of the presented methods rely on accurate timekeeping on the 923 receiver nodes of Neighbour Discovery Timestamp Options and 924 certificates. 926 For router-authorized proxy ND, a neighbour may not know that a 927 particular ND message is replayed from the time when the proxied host 928 was still on-link, since the message's timestamp falls within the 929 valid timing window. Where the router advertises its secured proxy 930 NA, a subsequent replay of the old message will override the NC entry 931 created by the proxy. 933 Creating the neighbour cache entry in this way, without reference to 934 accurate subsequent timing, may only be done once. Otherwise the 935 receiver will notice that the timestamp of the advertisement is old 936 or doesn't match. 938 One way of creating a sequence of replayable messages which have 939 timestamps likely to be accepted is to pretend to do an unsecured DAD 940 on the address each second while the MN is at home. The attacker 941 saves each DAD defence in a sequence. The granularity of SEND 942 timestamp matching is around 1 second, so the attacker has a set of 943 SEND NA's to advertise, starting at a particular timestamp, and valid 944 for as many seconds as the original NA gathering occurred. 946 This sequence may then be played against any host which doesn't have 947 a timestamp history for that MN, by tracking the number of seconds 948 elapsed since the initial transmission of the replayed NA to that 949 victim, and replaying the appropriate cached NA. 951 Where certificate based authorization of ND proxy is in use, the 952 origination/starting timestamp of the delegated authority may be used 953 to override a replayed (non-proxy) SEND NA, while also ensuring that 954 the Proxy NA's timestamp (provided by the proxy) is fresh. A 955 returning MN would advertise a more recent timestamp than the 956 delegated authority and thus override it. This method is therefore 957 not subject to the above attack, since the proxy advertisement's 958 certificate will have a timestamp greater than any replayed messages, 959 preventing it from being overridden. 961 9. Acknowledgments 963 James Kempf and Dave Thaler particularly contributed to work on this 964 document. Contributions to discussion on this topic helped to 965 develop this document. Thanks go to Jari Arkko, Vijay Devarapalli, 966 and Mohan Parthasarathy. 968 Jean-Michel Combes is partly funded by MobiSEND, a research project 969 supported by the French 'National Research Agency' (ANR). 971 10. References 973 10.1. Normative References 975 [I-D.ietf-netlmm-mn-ar-if] 976 Laganier, J., Narayanan, S., and P. McCann, "Interface 977 between a Proxy MIPv6 Mobility Access Gateway and a Mobile 978 Node", draft-ietf-netlmm-mn-ar-if-03 (work in progress), 979 February 2008. 981 [I-D.ietf-netlmm-proxymip6] 982 Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K., 983 and B. Patil, "Proxy Mobile IPv6", 984 draft-ietf-netlmm-proxymip6-16 (work in progress), 985 May 2008. 987 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 988 Requirement Levels", BCP 14, RFC 2119, March 1997. 990 [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support 991 in IPv6", RFC 3775, June 2004. 993 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 994 Neighbor Discovery (SEND)", RFC 3971, March 2005. 996 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 997 RFC 3972, March 2005. 999 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1000 Architecture", RFC 4291, February 2006. 1002 [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 1003 RFC 4306, December 2005. 1005 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 1006 Proxies (ND Proxy)", RFC 4389, April 2006. 1008 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1009 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1010 September 2007. 1012 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1013 Address Autoconfiguration", RFC 4862, September 2007. 1015 10.2. Informative References 1017 [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor 1018 Discovery (ND) Trust Models and Threats", RFC 3756, 1019 May 2004. 1021 [RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. 1022 Thubert, "Network Mobility (NEMO) Basic Support Protocol", 1023 RFC 3963, January 2005. 1025 [RFC4068] Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068, 1026 July 2005. 1028 [RFC4140] Soliman, H., Castelluccia, C., El Malki, K., and L. 1029 Bellier, "Hierarchical Mobile IPv6 Mobility Management 1030 (HMIPv6)", RFC 4140, August 2005. 1032 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1033 Housley, R., and W. Polk, "Internet X.509 Public Key 1034 Infrastructure Certificate and Certificate Revocation List 1035 (CRL) Profile", RFC 5280, May 2008. 1037 Appendix A. Changes from the previous versions 1039 To be removed prior to publication as an RFC. 1041 Previous version: draft-daley-send-spnd-prob-02 1043 o Integration of the "Two or more nodes defending a same address" 1044 section in the core document. 1046 o Addition of the "Cryptographic based solutions" section. 1048 o Addition of the "'Point-to-Point' link model based solution" 1049 section. 1051 o Update of the references. 1053 Previous version: draft-daley-send-spnd-prob-01 1055 o Reorganisation of the draft structure. 1057 o Addition of the "Fixed Nodes and Neighbor Discovery Proxy" 1058 section. 1060 o Update of the references. 1062 o Addition of the "Two or more nodes defending a same address" 1063 Appendix 1065 o Addition of the "Changes from the previous version" Appendix. 1067 Authors' Addresses 1069 Greg Daley 1070 55 Pakington St 1071 Kew, Victoria 3101 1072 Australia 1074 Phone: +61 405 494849 1075 Email: hoskuld@hotmail.com 1077 Jean-Michel Combes 1078 Orange Labs R&D 1079 38 rue du General Leclerc 1080 92794 Issy-les-Moulineaux Cedex 9 1081 France 1083 Email: jeanmichel.combes@gmail.com 1085 Full Copyright Statement 1087 Copyright (C) The IETF Trust (2008). 1089 This document is subject to the rights, licenses and restrictions 1090 contained in BCP 78, and except as set forth therein, the authors 1091 retain all their rights. 1093 This document and the information contained herein are provided on an 1094 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1095 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1096 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1097 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1098 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1099 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1101 Intellectual Property 1103 The IETF takes no position regarding the validity or scope of any 1104 Intellectual Property Rights or other rights that might be claimed to 1105 pertain to the implementation or use of the technology described in 1106 this document or the extent to which any license under such rights 1107 might or might not be available; nor does it represent that it has 1108 made any independent effort to identify any such rights. 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