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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Homenet Working Group M. Stenberg 3 Internet-Draft Independent 4 Intended status: Standards Track September 2, 2015 5 Expires: March 5, 2016 7 Auto-Configuration of a Network of Hybrid Unicast/Multicast DNS-Based 8 Service Discovery Proxy Nodes 9 draft-ietf-homenet-hybrid-proxy-zeroconf-01 11 Abstract 13 This document describes how a proxy functioning between Unicast DNS- 14 Based Service Discovery and Multicast DNS can be automatically 15 configured using an arbitrary network-level state sharing mechanism. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on March 5, 2016. 34 Copyright Notice 36 Copyright (c) 2015 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 2. Requirements language . . . . . . . . . . . . . . . . . . . . 3 53 3. Hybrid proxy - what to configure . . . . . . . . . . . . . . 3 54 3.1. Conflict resolution within network . . . . . . . . . . . 4 55 3.2. Per-link DNS-SD forward zone names . . . . . . . . . . . 4 56 3.3. Reasonable defaults . . . . . . . . . . . . . . . . . . . 5 57 3.3.1. Network-wide unique link name (scheme 1) . . . . . . 5 58 3.3.2. Node name (scheme 2) . . . . . . . . . . . . . . . . 5 59 3.3.3. Link name (scheme 2) . . . . . . . . . . . . . . . . 5 60 4. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 4.1. DNS Delegated Zone TLV . . . . . . . . . . . . . . . . . 6 62 4.2. Domain Name TLV . . . . . . . . . . . . . . . . . . . . . 7 63 4.3. Node Name TLV . . . . . . . . . . . . . . . . . . . . . . 7 64 5. Desirable behavior . . . . . . . . . . . . . . . . . . . . . 7 65 5.1. DNS search path in DHCP requests . . . . . . . . . . . . 8 66 5.2. Hybrid proxy . . . . . . . . . . . . . . . . . . . . . . 8 67 5.3. Hybrid proxy network zeroconf daemon . . . . . . . . . . 8 68 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 69 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 70 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 71 8.1. Normative references . . . . . . . . . . . . . . . . . . 9 72 8.2. Informative references . . . . . . . . . . . . . . . . . 9 73 Appendix A. Example configuration . . . . . . . . . . . . . . . 10 74 A.1. Used topology . . . . . . . . . . . . . . . . . . . . . . 10 75 A.2. Zero-configuration steps . . . . . . . . . . . . . . . . 10 76 A.3. TLV state . . . . . . . . . . . . . . . . . . . . . . . . 11 77 A.4. DNS zone . . . . . . . . . . . . . . . . . . . . . . . . 12 78 A.5. Interaction with hosts . . . . . . . . . . . . . . . . . 13 79 Appendix B. Implementation . . . . . . . . . . . . . . . . . . . 13 80 Appendix C. Why not just proxy Multicast DNS? . . . . . . . . . 13 81 C.1. General problems . . . . . . . . . . . . . . . . . . . . 14 82 C.2. Stateless proxying problems . . . . . . . . . . . . . . . 14 83 C.3. Stateful proxying problems . . . . . . . . . . . . . . . 15 84 Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 15 85 Appendix E. Changelog [RFC Editor: please remove] . . . . . . . 15 86 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 88 1. Introduction 90 Section 3 ("Hybrid Proxy Operation") of [I-D.ietf-dnssd-hybrid] 91 describes how to translate queries from Unicast DNS-Based Service 92 Discovery described in [RFC6763] to Multicast DNS described in 93 [RFC6762], and how to filter the responses and translate them back to 94 unicast DNS. 96 This document describes what sort of configuration the participating 97 hybrid proxy servers require, as well as how it can be provided using 98 any network-wide state sharing mechanism such as link-state routing 99 protocol or Home Networking Control Protocol [I-D.ietf-homenet-hncp]. 100 The document also describes a naming scheme which does not even need 101 to be same across the whole covered network to work as long as the 102 specified conflict resolution works. The scheme can be used to 103 provision both forward and reverse DNS zones which employ hybrid 104 proxy for heavy lifting. 106 This document does not go into low level encoding details of the 107 Type-Length-Value (TLV) data that we want synchronized across a 108 network. Instead, we just specify what needs to be available, and 109 assume every node that needs it has it available. 111 We go through the mandatory specification of the language used in 112 Section 2, then describe what needs to be configured in hybrid 113 proxies and participating DNS servers across the network in 114 Section 3. How the data is exchanged using arbitrary TLVs is 115 described in Section 4. Finally, some overall notes on desired 116 behavior of different software components is mentioned in Section 5. 118 2. Requirements language 120 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 121 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 122 document are to be interpreted as described in [RFC2119]. 124 3. Hybrid proxy - what to configure 126 Beyond the low-level translation mechanism between unicast and 127 multicast service discovery, the hybrid proxy draft 128 [I-D.ietf-dnssd-hybrid] describes just that there have to be NS 129 records pointing to hybrid proxy responsible for each link within the 130 covered network. 132 In zero-configuration case, choosing the links to be covered is also 133 non-trivial choice; we can use the border discovery functionality (if 134 available) to determine internal and external links. Or we can use 135 some other protocol's presence (or lack of it) on a link to determine 136 internal links within the covered network, and some other signs 137 (depending on the deployment) such as DHCPv6 Prefix Delegation (as 138 described in [RFC3633]) to determine external links that should not 139 be covered. 141 For each covered link we want forward DNS zone delegation to an 142 appropriate node which is connected to a link, and running hybrid 143 proxy. Therefore the links' forward DNS zone names should be unique 144 across the network. We also want to populate reverse DNS zone 145 similarly for each IPv4 or IPv6 prefix in use. 147 There should be DNS-SD browse domain list provided for the network's 148 domain which contains each physical link only once, regardless of how 149 many nodes and hybrid proxy implementations are connected to it. 151 Yet another case to consider is the list of DNS-SD domains that we 152 want hosts to enumerate for browse domain lists. Typically, it 153 contains only the local network's domain, but there may be also other 154 networks we may want to pretend to be local but are in different 155 scope, or controlled by different organization. For example, a home 156 user might see both home domain's services (TBD-TLD), as well as 157 ISP's services under isp.example.com. 159 3.1. Conflict resolution within network 161 Any naming-related choice on node may have conflicts in the network 162 given that we require only distributed loosely synchronized database. 163 We assume only that the underlying protocol used for synchronization 164 has some concept of precedence between nodes originating conflicting 165 information, and in case of conflict, the higher precedence node MUST 166 keep the name they have chosen. The one(s) with lower precedence 167 MUST either try different one (that is not in use at all according to 168 the current link state information), or choose not to publish the 169 name altogether. 171 If a node needs to pick a different name, any algorithm works, 172 although simple algorithm choice is just like the one described in 173 Multicast DNS[RFC6762]: append -2, -3, and so forth, until there are 174 no conflicts in the network for the given name. 176 3.2. Per-link DNS-SD forward zone names 178 How to name the links of a whole network in automated fashion? Two 179 different approaches seem obvious: 181 1. Unique link name based - (unique-link).(domain). 183 2. Node and link name - (link).(unique-node).(domain). 185 The first choice is appealing as it can be much more friendly 186 (especially given manual configuration). For example, it could mean 187 just lan.example.com and wlan.example.com for a simple home network. 188 The second choice, on the other hand, has a nice property of being 189 local choice as long as node name can be made unique. 191 The type of naming scheme to use can be left as implementation 192 option. And the actual names themselves SHOULD be also overridable, 193 if the end-user wants to customize them in some way. 195 3.3. Reasonable defaults 197 Note that any manual configuration, which SHOULD be possible, MUST 198 override the defaults provided here or chosen by the creator of the 199 implementation. 201 3.3.1. Network-wide unique link name (scheme 1) 203 It is not obvious how to produce network-wide unique link names for 204 the (unique-link).(domain) scheme. One option would be to base it on 205 type of physical network layer, and then hope that the number of the 206 networks won't be significant enough to confuse (e.g. "lan", or 207 "wlan"). 209 The network-wide unique link names should be only used in small 210 networks. Given a larger network, after conflict resolution, 211 identifying which link is 'lan-42.example.com' may be challenging. 213 3.3.2. Node name (scheme 2) 215 Our recommendation is to use some short form which indicates the type 216 of node it is, for example, "openwrt.example.com". As the name is 217 visible to users, it should be kept as short as possible. In theory 218 even more exact model could be helpful, for example, "openwrt- 219 buffalo-wzr-600-dhr.example.com". In practice providing some other 220 records indicating exact node information (and access to management 221 UI) is more sensible. 223 3.3.3. Link name (scheme 2) 225 Recommendation for (link) portion of (link).(node).(domain) is to use 226 physical network layer type as base, or possibly even just interface 227 name on the node if it's descriptive enough. For example, 228 "eth0.openwrt.example.com" and "wlan0.openwrt.example.com" may be 229 good enough. 231 4. TLVs 233 To implement this specification fully, support for following three 234 different TLVs is needed. However, only the DNS Delegated Zone TLVs 235 MUST be supported, and the other two SHOULD be supported. 237 4.1. DNS Delegated Zone TLV 239 This TLV is effectively a combined NS and A/AAAA record for a zone. 240 It MUST be supported by implementations conforming to this 241 specification. Implementations SHOULD provide forward zone per link 242 (or optimizing a bit, zone per link with Multicast DNS traffic). 243 Implementations MAY provide reverse zone per prefix using this same 244 mechanism. If multiple nodes advertise same reverse zone, it should 245 be assumed that they all have access to the link with that prefix. 246 However, as noted in Section 5.3, mainly only the node with highest 247 precedence on the link should publish this TLV. 249 Contents: 251 o Address field is IPv6 address (e.g. 2001:db8::3) or IPv4 address 252 mapped to IPv6 address (e.g. ::FFFF:192.0.2.1) where the 253 authoritative DNS server for Zone can be found. If the address 254 field is all zeros, the Zone is under global DNS hierarchy and can 255 be found using normal recursive name lookup starting at the 256 authoritative root servers (This is mostly relevant with the S bit 257 below). 259 o S-bit indicates that this delegated zone consists of a full DNS-SD 260 domain, which should be used as base for DNS-SD domain enumeration 261 (that is, (field)._dns-sd._udp.(zone) exists). Forward zones MAY 262 have this set. Reverse zones MUST NOT have this set. This can be 263 used to provision DNS search path to hosts for non-local services 264 (such as those provided by ISP, or other manually configured 265 service providers). 267 o B-bit indicates that this delegated zone should be included in 268 network's DNS-SD browse list of domains at b._dns- 269 sd._udp.(domain). Local forward zones SHOULD have this set. 270 Reverse zones SHOULD NOT have this set. 272 o L-bit indicates that this delegated zone should be included in the 273 network's DNS-SD legacy browse list of domains at lb._dns- 274 sd._udp.(DOMAIN-NAME). Local forward zones SHOULD have this bit 275 set, reverse zones SHOULD NOT. 277 o Zone is the label sequence of the zone, encoded according to 278 section 3.1. ("Name space definitions") of [RFC1035]. Note that 279 name compression is not required here (and would not have any 280 point in any case), as we encode the zones one by one. The zone 281 MUST end with an empty label. 283 In case of a conflict (same zone being advertised by multiple parties 284 with different address or bits), conflict should be addressed 285 according to Section 3.1. 287 4.2. Domain Name TLV 289 This TLV is used to indicate the base (domain) to be used for the 290 network. If multiple nodes advertise different ones, the conflict 291 resolution rules in Section 3.1 should result in only the one with 292 highest precedence advertising one, eventually. In case of such 293 conflict, user SHOULD be notified somehow about this, if possible, 294 using the configuration interface or some other notification 295 mechanism for the nodes. Like the Zone field in Section 4.1, the 296 Domain Name TLV's contents consist of a single DNS label sequence. 298 This TLV SHOULD be supported if at all possible. It may be derived 299 using some future DHCPv6 option, or be set by manual configuration. 300 Even on nodes without manual configuration options, being able to 301 read the domain name provided by a different node could make the user 302 experience better due to consistent naming of zones across the 303 network. 305 By default, if no node advertises domain name TLV, hard-coded default 306 (TBD) should be used. 308 4.3. Node Name TLV 310 This TLV is used to advertise a node's name. After the conflict 311 resolution procedure described in Section 3.1 finishes, there should 312 be exactly zero to one nodes publishing each node name. The contents 313 of the TLV should be a single DNS label. 315 This TLV SHOULD be supported if at all possible. If not supported, 316 and another node chooses to use the (link).(node) naming scheme with 317 this node's name, the contents of the network's domain may look 318 misleading (but due to conflict resolution of per-link zones, still 319 functional). 321 If the node name has been configured manually, and there is a 322 conflict, user SHOULD be notified somehow about this, if possible, 323 using the configuration interface or some other notification 324 mechanism for the nodes. 326 5. Desirable behavior 327 5.1. DNS search path in DHCP requests 329 The nodes following this specification SHOULD provide the used 330 (domain) as one item in the search path to it's hosts, so that DNS-SD 331 browsing will work correctly. They also SHOULD include any DNS 332 Delegated Zone TLVs' zones, that have S bit set. 334 5.2. Hybrid proxy 336 The hybrid proxy implementation SHOULD support both forward zones, 337 and IPv4 and IPv6 reverse zones. It SHOULD also detect whether or 338 not there are any Multicast DNS entities on a link, and make that 339 information available to the network zeroconf daemon (if implemented 340 separately). This can be done by (for example) passively monitoring 341 traffic on all covered links, and doing infrequent service 342 enumerations on links that seem to be up, but without any Multicast 343 DNS traffic (if so desired). 345 Hybrid proxy nodes MAY also publish it's own name via Multicast DNS 346 (both forward A/AAAA records, as well as reverse PTR records) to 347 facilitate applications that trace network topology. 349 5.3. Hybrid proxy network zeroconf daemon 351 The daemon should avoid publishing TLVs about links that have no 352 Multicast DNS traffic to keep the DNS-SD browse domain list as 353 concise as possible. It also SHOULD NOT publish delegated zones for 354 links for which zones already exist by another node with higher 355 precedence. 357 The daemon (or other entity with access to the TLVs) SHOULD generate 358 zone information for DNS implementation that will be used to serve 359 the (domain) zone to hosts. Domain Name TLV described in Section 4.2 360 should be used as base for the zone, and then all DNS Delegated Zones 361 described in Section 4.1 should be used to produce the rest of the 362 entries in zone (see Appendix A.4 for example interpretation of the 363 TLVs in Appendix A.3. 365 6. Security Considerations 367 There is a trade-off between security and zero-configuration in 368 general; if used network state synchronization protocol is not 369 authenticated (and in zero-configuration case, it most likely is 370 not), it is vulnerable to local spoofing attacks. We assume that 371 this scheme is used either within (lower layer) secured networks, or 372 with not-quite-zero-configuration initial set-up. 374 If some sort of dynamic inclusion of links to be covered using border 375 discovery or such is used, then effectively service discovery will 376 share fate with border discovery (and also security issues if any). 378 7. IANA Considerations 380 This document has no actions for IANA. 382 8. References 384 8.1. Normative references 386 [I-D.ietf-dnssd-hybrid] 387 Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service 388 Discovery", draft-ietf-dnssd-hybrid-00 (work in progress), 389 November 2014. 391 [RFC1035] Mockapetris, P., "Domain names - implementation and 392 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 393 November 1987, . 395 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 396 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 397 RFC2119, March 1997, 398 . 400 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 401 DOI 10.17487/RFC6762, February 2013, 402 . 404 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 405 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 406 . 408 8.2. Informative references 410 [I-D.ietf-homenet-hncp] 411 Stenberg, M., Barth, S., and P. Pfister, "Home Networking 412 Control Protocol", draft-ietf-homenet-hncp-09 (work in 413 progress), August 2015. 415 [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic 416 Host Configuration Protocol (DHCP) version 6", RFC 3633, 417 DOI 10.17487/RFC3633, December 2003, 418 . 420 [RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic 421 Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, 422 DOI 10.17487/RFC3646, December 2003, 423 . 425 8.3. URIs 427 [1] https://github.com/sbyx/hnetd/ 429 Appendix A. Example configuration 431 A.1. Used topology 433 Let's assume home network that looks like this: 435 |[0] 436 +-----+ 437 | CER | 438 +-----+ 439 [1]/ \[2] 440 / \ 441 +-----+ +-----+ 442 | IR1 |-| IR2 | 443 +-----+ +-----+ 444 |[3]| |[4]| 446 We're not really interested about links [0], [1] and [2], or the 447 links between IRs. Given the optimization described in Section 4.1, 448 they should not produce anything to network's Multicast DNS state 449 (and therefore to DNS either) as there isn't any Multicast DNS 450 traffic there. 452 The user-visible set of links are [3] and [4]; each consisting of a 453 LAN and WLAN link. We assume that ISP provides 2001:db8:1234::/48 454 prefix to be delegated in the home via [0]. 456 A.2. Zero-configuration steps 458 Given implementation that chooses to use the second naming scheme 459 (link).(node).(domain), and no configuration whatsoever, here's what 460 happens (the steps are interleaved in practice but illustrated here 461 in order): 463 1. Network-level state synchronization protocol runs, nodes get 464 effective precedences. For ease of illustration, CER winds up 465 with 2, IR1 with 3, and IR2 with 1. 467 2. Prefix delegation takes place. IR1 winds up with 468 2001:db8:1234:11::/64 for LAN and 2001:db8:1234:12::/64 for WLAN. 469 IR2 winds up with 2001:db8:1234:21::/64 for LAN and 470 2001:db8:1234:22::/64 for WLAN. 472 3. IR1 is assumed to be reachable at 2001:db8:1234:11::1 and IR2 at 473 2001:db8:1234:21::1. 475 4. Each node wants to be called 'node' due to lack of branding in 476 drafts. They announce that using the node name TLV defined in 477 Section 4.3. They also advertise their local zones, but as that 478 information may change, it's omitted here. 480 5. Conflict resolution ensues. As IR1 has precedence over the rest, 481 it becomes "node". CER and IR2 have to rename, and (depending on 482 timing) one of them becomes "node-2" and other one "node-3". Let 483 us assume IR2 is "node-2". During conflict resolution, each node 484 publishes TLVs for it's own set of delegated zones. 486 6. CER learns ISP-provided domain "isp.example.com" using DHCPv6 487 domain list option defined in [RFC3646]. The information is 488 passed along as S-bit enabled delegated zone TLV. 490 A.3. TLV state 492 Once there is no longer any conflict in the system, we wind up with 493 following TLVs (NN is used as abbreviation for Node Name, and DZ for 494 Delegated Zone TLVs): 496 (from CER) 497 DZ {s=1,zone="isp.example.com"} 499 (from IR1) 500 NN {name="node"} 502 DZ {address=2001:db8:1234:11::1, b=1, 503 zone="lan.node.example.com."} 504 DZ {address=2001:db8:1234:11::1, 505 zone="1.1.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa."} 507 DZ {address=2001:db8:1234:11::1, b=1, 508 zone="wlan.node.example.com."} 509 DZ {address=2001:db8:1234:11::1, 510 zone="2.1.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa."} 512 (from IR2) 513 NN {name="node-2"} 515 DZ {address=2001:db8:1234:21::1, b=1, 516 zone="lan.node-2.example.com."} 517 DZ {address=2001:db8:1234:21::1, 518 zone="1.2.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa."} 520 DZ {address=2001:db8:1234:21::1, b=1, 521 zone="wlan.node-2.example.com."} 522 DZ {address=2001:db8:1234:21::1, 523 zone="2.2.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa."} 525 A.4. DNS zone 527 In the end, we should wind up with following zone for (domain) which 528 is example.com in this case, available at all nodes, just based on 529 dumping the delegated zone TLVs as NS+AAAA records, and optionally 530 domain list browse entry for DNS-SD: 532 b._dns_sd._udp PTR lan.node 533 b._dns_sd._udp PTR wlan.node 535 b._dns_sd._udp PTR lan.node-2 536 b._dns_sd._udp PTR wlan.node-2 538 node AAAA 2001:db8:1234:11::1 539 node-2 AAAA 2001:db8:1234:21::1 541 node NS node 542 node-2 NS node-2 544 1.1.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa. NS node.example.com. 545 2.1.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa. NS node.example.com. 546 1.2.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa. NS node-2.example.com. 547 2.2.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa. NS node-2.example.com. 549 Internally, the node may interpret the TLVs as it chooses to, as long 550 as externally defined behavior follows semantics of what's given in 551 the above. 553 A.5. Interaction with hosts 555 So, what do the hosts receive from the nodes? Using e.g. DHCPv6 DNS 556 options defined in [RFC3646], DNS server address should be one (or 557 multiple) that point at DNS server that has the zone information 558 described in Appendix A.4. Domain list provided to hosts should 559 contain both "example.com" (the hybrid-enabled domain), as well as 560 the externally learned domain "isp.example.com". 562 When hosts start using DNS-SD, they should check both b._dns- 563 sd._udp.example.com, as well as b._dns-sd._udp.isp.example.com for 564 list of concrete domains to browse, and as a result services from two 565 different domains will seem to be available. 567 Appendix B. Implementation 569 There is an prototype implementation of this draft at hnetd github 570 repository [1] which contains variety of other homenet WG-related 571 things' implementation too. 573 Appendix C. Why not just proxy Multicast DNS? 575 Over the time number of people have asked me about how, why, and if 576 we should proxy (originally) link-local Multicast DNS over multiple 577 links. 579 At some point I meant to write a draft about this, but I think I'm 580 too lazy; so some notes left here for general amusement of people 581 (and to be removed if this ever moves beyond discussion piece). 583 C.1. General problems 585 There are two main reasons why Multicast DNS is not proxyable in the 586 general case. 588 First reason is the conflict resolution depends on the RRsets staying 589 constant. That is not possible across multiple links (due to e.g. 590 link-local addresses having to be filtered). Therefore, conflict 591 resolution breaks, or at least requires ugly hacks to work around. 593 A simple, but not really working workaround for this is to make sure 594 that in conflict resolution, propagated resources always loses. 595 Given that the proxy function only removes records, the result SHOULD 596 be consistently original set of records winning. Even with that, the 597 conflict resolution will effectively cease working, allowing for two 598 instances of same name to exist (as both think they 'own' the name 599 due to locally seen higher precedence). 601 Given some more extra logic, it is possible to make this work by 602 having proxies be aware of both the original record sets, and 603 effectively enforcing the correct conflict resolution results by (for 604 example) passing the unfiltered packets to the losing party just to 605 make sure they renumber, or by altering the RR sets so that they will 606 consistently win (by inserting some lower rrclass/rrtype records). 607 As the conflicts happen only in rrclass=1/rrtype=28, it is easy 608 enough to add e.g. extra TXT record (rrtype 16) to force precedence 609 even when removing the later rrtype 28 record. Obviously, this new 610 RRset must never wind up near the host with the higher precedence, or 611 it will cause spurious renaming loops. 613 Second reason is timing, which is relatively tight in the conflict 614 resolution phase, especially given lossy and/or high latency 615 networks. 617 C.2. Stateless proxying problems 619 In general, typical stateless proxy has to involve flooding, as 620 Multicast DNS assumes that most messages are received by every host. 621 And it won't scale very well, as a result. 623 The conflict resolution is also harder without state. It may result 624 in Multicast DNS responder being in constant probe-announce loop, 625 when it receives altered records, notes that it's the one that should 626 own the record. Given stateful proxying, this would be just a 627 transient problem but designing stateless proxy that won't cause this 628 is non-trivial exercise. 630 C.3. Stateful proxying problems 632 One option is to write proxy that learns state from one link, and 633 propagates it in some way to other links in the network. 635 A big problem with this case lies in the fact that due to conflict 636 resolution concerns above, it is easy to accidentally send packets 637 that will (possibly due to host mobility) wind up at the originator 638 of the service, who will then perform renaming. That can be 639 alleviated, though, given clever hacks with conflict resolution 640 order. 642 The stateful proxying may be also too slow to occur within the 643 timeframe allocated for announcing, leading to excessive later 644 renamings based on delayed finding of duplicate services with same 645 name 647 A work-around exists for this though; if the game doesn't work for 648 you, don't play it. One option would be simply not to propagate ANY 649 records for which conflict has seen even once. This would work, but 650 result in rather fragile, lossy service discovery infrastructure. 652 There are some other small nits too; for example, Passive Observation 653 Of Failure (POOF) will not work given stateful proxying. Therefore, 654 it leads to requiring somewhat shorter TTLs, perhaps. 656 Appendix D. Acknowledgements 658 Thanks to Stuart Cheshire for the original hybrid proxy draft and 659 interesting discussion in Orlando, where I was finally convinced that 660 stateful Multicast DNS proxying is a bad idea. 662 Also thanks to Mark Baugher, Ole Troan, Shwetha Bhandari and Gert 663 Doering for review comments. 665 Appendix E. Changelog [RFC Editor: please remove] 667 draft-ietf-homenet-hybrid-proxy-zeroconf-01: 669 o Refreshed the draft while waiting on progress of draft-ietf-dnssd- 670 hybrid. 672 Author's Address 674 Markus Stenberg 675 Independent 676 Helsinki 00930 677 Finland 679 Email: markus.stenberg@iki.fi