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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Applications Area Arnt Gulbrandsen 2 INTERNET-DRAFT Troll Technologies 3 Paul Vixie 4 Obsoletes: RFC 2052 Internet Software Consortium 5 Levon Esibov 6 Microsoft Corp. 7 November 1999 8 Expires May 2000 10 A DNS RR for specifying the location of services (DNS SRV) 12 Status of this Memo 14 This document is an Internet-Draft and is in full conformance with 15 all provisions of Section 10 of RFC2026. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as 20 Internet-Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 Abstract 35 This document describes a DNS RR which specifies the location of the 36 server(s) for a specific protocol and domain. 38 Overview and rationale 40 Currently, one must either know the exact address of a server to 41 contact it, or broadcast a question. 43 The SRV RR allows administrators to use several servers for a single 44 domain, to move services from host to host with little fuss, and to 45 designate some hosts as primary servers for a service and others as 46 backups. 48 Clients ask for a specific service/protocol for a specific domain 49 (the word domain is used here in the strict RFC 1034 sense), and get 50 back the names of any available servers. 52 Note that where this document refers to "address records", it means A 53 RR's, AAAA RR's, or their most modern equivalent. 55 RFC 2052bis DNS SRV RR November 1999 57 Definitions 59 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" 60 used in this document are to be interpreted as specified in [BCP 14]. 61 Other terms used in this document are defined in the DNS 62 specification, RFC 1034. 64 Applicability Statement 66 In general, it is expected that SRV records will be used by clients 67 for applications where the relevant protocol specification indicates 68 that clients should use the SRV record. Such specification MUST 69 define the symbolic name to be used in the Service field of the SRV 70 record as described below. It also MUST include security 71 considerations. Service SRV records SHOULD NOT be used in the absence 72 of such specification. 74 Introductory example 76 If a SRV-cognizant LDAP client wants to discover a LDAP server that 77 supports TCP protocol and provides LDAP service for the domain 78 example.com., it does a lookup of 80 _ldap._tcp.example.com 82 as described in [ARM]. The example zone file near the end of this 83 memo contains answering RRs for an SRV query. 85 The format of the SRV RR 87 Here is the format of the SRV RR, whose DNS type code is 33: 89 _Service._Proto.Name TTL Class SRV Priority Weight Port Target 91 (There is an example near the end of this document.) 93 Service 94 The symbolic name of the desired service, as defined in Assigned 95 Numbers [STD 2] or locally. An underscore (_) is prepended to 96 the service identifier to avoid collisions with DNS labels that 97 occur in nature. 99 Some widely used services, notably POP, don't have a single 100 universal name. If Assigned Numbers names the service 101 indicated, that name is the only name which is legal for SRV 102 lookups. The Service is case insensitive. 104 RFC 2052bis DNS SRV RR November 1999 106 Proto 107 The symbolic name of the desired protocol, with an underscore 108 (_) prepended to prevent collisions with DNS labels that occur 109 in nature. _TCP and _UDP are at present the most useful values 110 for this field, though any name defined by Assigned Numbers or 111 locally may be used (as for Service). The Proto is case 112 insensitive. 114 Name 115 The domain this RR refers to. The SRV RR is unique in that the 116 name one searches for is not this name; the example near the end 117 shows this clearly. 119 TTL 120 Standard DNS meaning [RFC 1035]. 122 Class 123 Standard DNS meaning [RFC 1035]. SRV records occur in the IN 124 Class. 126 Priority 127 The priority of this target host. A client MUST 128 attempt to contact the target host with the lowest-numbered 129 priority it can reach; target hosts with the same priority 130 SHOULD be tried in an order defined by the weight field. The 131 range is 0-65535. This is a 16 bit unsigned integer in network 132 byte order. 134 Weight 135 A server selection mechanism. The weight field specifies a 136 relative weight for entries with the same priority. Larger 137 weights SHOULD be given a proportionately higher probability of 138 being selected. The range of this number is 0-65535. This is a 139 16 bit unsigned integer in network byte order. Domain 140 administrators SHOULD use Weight 0 when there isn't any server 141 selection to do, to make the RR easier to read for humans (less 142 noisy). In the presence of records containing weights greater 143 than 0, records with weight 0 should have a very small chance of 144 being selected. 146 In the absence of a protocol whose specification calls for the 147 use of other weighting information, a client arranges the 148 SRV RRs of the same Priority in the order in which target hosts, 149 specified by the SRV RRs, will be contacted. The following 150 algorithm SHOULD be used to order the SRV RRs of the same 151 priority: 152 To select a target to be contacted next, arrange all SRV RRs 153 (that have not been ordered yet) in any order, except that all 154 those with weight 0 are placed at the beginning of the list. 156 RFC 2052bis DNS SRV RR November 1999 158 Compute the sum of the weights of those RRs, and with each RR 159 associate the running sum in the selected order. Then choose a 160 uniform random number between 0 and the sum computed 161 (inclusive), and select the RR whose running sum value is the 162 first in the selected order which is greater than or equal to 163 the random number selected. The target host specified in the 164 selected SRV RR is the next one to be contacted by the client. 165 Remove this SRV RR from the set of the unordered SRV RRs and 166 apply the described algorithm to the unordered SRV RRs to select 167 the next target host. 168 Continue the ordering process until there are no unordered SRV 169 RRs. 170 This process is repeated for each Priority. 172 Port 173 The port on this target host of this service. The range is 174 0-65535. This is a 16 bit unsigned integer in network byte 175 order. This is often as specified in Assigned Numbers but need 176 not be. 178 Target 179 The domain name of the target host. There MUST be 180 one or more address records for this name, the name MUST NOT be 181 an alias (in the sense of RFC 1034 or RFC 2181). Implementors 182 are urged, but not required, to return the address record(s) in 183 the Additional Data section. Unless and until permitted by 184 future standards action, name compression is not to be used for 185 this field. 187 A Target of "." means that the service is decidedly not 188 available at this domain. 190 Domain administrator advice 192 Expecting everyone to update their client applications when the first 193 server publishes a SRV RR is futile (even if desirable). Therefore 194 SRV would have to coexist with address record lookups for existing 195 protocols, and DNS administrators should try to provide address 196 records to support old clients: 198 - Where the services for a single domain are spread over several 199 hosts, it seems advisable to have a list of address records at 200 the same DNS node as the SRV RR, listing reasonable (if perhaps 201 suboptimal) fallback hosts for Telnet, NNTP and other protocols 202 likely to be used with this name. Note that some programs only 203 try the first address they get back from e.g. gethostbyname(), 204 and we don't know how widespread this behavior is. 206 RFC 2052bis DNS SRV RR November 1999 208 - Where one service is provided by several hosts, one can either 209 provide address records for all the hosts (in which case the 210 round-robin mechanism, where available, will share the load 211 equally) or just for one (presumably the fastest). 213 - If a host is intended to provide a service only when the main 214 server(s) is/are down, it probably shouldn't be listed in 215 address records. 217 - Hosts that are referenced by backup address records must use the 218 port number specified in Assigned Numbers for the service. 220 - Designers of future protocols for which "secondary servers" is 221 not useful (or meaningful) may choose to not use SRV's support 222 for secondary servers. Clients for such protocols may use or 223 ignore SRV RRs with Priority higher than the RR with the lowest 224 Priority for a domain. 226 Currently there's a practical limit of 512 bytes for DNS replies. 227 Until all resolvers can handle larger responses, domain 228 administrators are strongly advised to keep their SRV replies below 229 512 bytes. 231 All round numbers, wrote Dr. Johnson, are false, and these numbers 232 are very round: A reply packet has a 30-byte overhead plus the name 233 of the service ("_ldap._tcp.example.com" for instance); each SRV RR 234 adds 20 bytes plus the name of the target host; each NS RR in the NS 235 section is 15 bytes plus the name of the name server host; and 236 finally each A RR in the additional data section is 20 bytes or so, 237 and there are A's for each SRV and NS RR mentioned in the answer. 238 This size estimate is extremely crude, but shouldn't underestimate 239 the actual answer size by much. If an answer may be close to the 240 limit, using a DNS query tool (e.g. "dig") to look at the actual 241 answer is a good idea. 243 The "Weight" field 245 Weight, the server selection field, is not quite satisfactory, but 246 the actual load on typical servers changes much too quickly to be 247 kept around in DNS caches. It seems to the authors that offering 248 administrators a way to say "this machine is three times as fast as 249 that one" is the best that can practically be done. 251 The only way the authors can see of getting a "better" load figure is 252 asking a separate server when the client selects a server and 253 contacts it. For short-lived services an extra step in the 254 connection establishment seems too expensive, and for long-lived 255 services, the load figure may well be thrown off a minute 256 after the connection is established when someone else starts or 257 finishes a heavy job. 259 RFC 2052bis DNS SRV RR November 1999 261 Note: There are currently various experiments at providing relative 262 network proximity estimation, available bandwidth estimation, and 263 similar services. Use of the SRV record with such facilities, and in 264 particular the interpretation of the Weight field when these 265 facilities are used, is for further study. 266 Weight is only intended for static, not dynamic, server selection. 267 Using SRV weight for dynamic server selection would require assigning 268 unreasonably short TTLs to the SRV RRs, which would limit the 269 usefulness of the DNS caching mechanism, thus increasing overall 270 network load and decreasing overall reliability. Server selection 271 via SRV is only intended to express static information such as "this 272 server has a faster CPU than that one" or "this server has a much 273 better network connection than that one". 275 The Port number 277 Currently, the translation from service name to port number happens 278 at the client, often using a file such as /etc/services. 280 Moving this information to the DNS makes it less necessary to update 281 these files on every single computer of the net every time a new 282 service is added, and makes it possible to move standard services out 283 of the "root-only" port range on unix. 285 Usage rules 287 A SRV-cognizant client SHOULD use this procedure to locate a list of 288 servers and connect to the preferred one: 290 Do a lookup for QNAME=_service._protocol.target, QCLASS=IN, 291 QTYPE=SRV. 293 If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV 294 RR which specifies the requested Service and Protocol in the 295 reply: 297 If there is precisely one SRV RR, and its Target is "." 298 (the root domain), abort. 300 Else, for all such RR's, build a list of (Priority, Weight, 301 Target) tuples 303 Sort the list by priority (lowest number first) 305 Create a new empty list 307 For each distinct priority level 308 While there are still elements left at this priority 309 level 311 RFC 2052bis DNS SRV RR November 1999 313 Select an element as specified above, in the 314 description of Weight in "The format of the SRV 315 RR" Section, and move it to the tail of the new 316 list 318 For each element in the new list 320 query the DNS for address records for the Target or 321 use any such records found in the Additional Data 322 section of the earlier SRV response. 324 for each address record found, try to connect to the 325 (protocol, address, service). 327 else 329 Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A 331 for each address record found, try to connect to the 332 (protocol, address, service) 334 Notes: 336 - Port numbers SHOULD NOT be used in place of the symbolic service 337 or protocol names (for the same reason why variant names cannot 338 be allowed: Applications would have to do two or more lookups). 340 - If a truncated response comes back from an SRV query, the rules 341 described in [RFC2181] shall apply. 343 - A client MUST parse all of the RR's in the reply. 345 - If the Additional Data section doesn't contain address records 346 for all the SRV RR's and the client may want to connect to the 347 target host(s) involved, the client MUST look up the address 348 record(s). (This happens quite often when the address record 349 has shorter TTL than the SRV or NS RR's.) 351 - Future protocols could be designed to use SRV RR lookups as the 352 means by which clients locate their servers. 354 Fictional example 356 This example uses fictional service �foobar� as an aid in 357 understanding SRV records. If ever service �foobar� is implemented, 358 it is not intended that it will necessarily use SRV records. This 359 is (part of) the zone file for example.com, a still-unused domain: 361 RFC 2052bis DNS SRV RR November 1999 363 $ORIGIN example.com. 364 @ SOA server.example.com. root.example.com. ( 365 1995032001 3600 3600 604800 86400 ) 366 NS server.example.com. 367 NS ns1.ip-provider.net. 368 NS ns2.ip-provider.net. 369 ; foobar - use old-slow-box or new-fast-box if either is 370 ; available, make three quarters of the logins go to 371 ; new-fast-box. 372 _foobar._tcp SRV 0 1 9 old-slow-box.example.com. 373 SRV 0 3 9 new-fast-box.example.com. 374 ; if neither old-slow-box or new-fast-box is up, switch to 375 ; using the sysdmin's box and the server 376 SRV 1 0 9 sysadmins-box.example.com. 377 SRV 1 0 9 server.example.com. 378 server A 172.30.79.10 379 old-slow-box A 172.30.79.11 380 sysadmins-box A 172.30.79.12 381 new-fast-box A 172.30.79.13 382 ; NO other services are supported 383 *._tcp SRV 0 0 0 . 384 *._udp SRV 0 0 0 . 386 In this example, a client of the �foobar� service in the 387 "example.com." domain needs an SRV lookup of 388 "_foobar._tcp.example.com." and possibly A lookups of "new- 389 fast-box.example.com." and/or the other hosts named. The size of the 390 SRV reply is approximately 365 bytes: 392 30 bytes general overhead 393 20 bytes for the query string, "_foobar._tcp.example.com." 394 130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new- 395 fast-box", "old-slow-box", "server" and "sysadmins-box" - 396 "example.com" in the query section is quoted here and doesn't 397 need to be counted again. 398 75 bytes for 3 NS RRs, 15 bytes each plus the lengths of "server", 399 "ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is 400 quoted and only needs to be counted once. 401 120 bytes for the 6 address records (assuming IPv4 only) mentioned 402 by the SRV and NS RR's. 404 IANA Considerations 406 The IANA has assigned RR type value 33 to the SRV RR. No other IANA 407 services are required by this document. 409 RFC 2052bis DNS SRV RR November 1999 411 Changes from RFC 2052 413 This document obsoletes RFC 2052. The major change from that 414 previous, experimental, version of this specification is that now the 415 protocol and service labels are prepended with an underscore, to 416 lower the probability of an accidental clash with a similar name used 417 for unrelated purposes. Aside from that, changes are only intended 418 to increase the clarity and completeness of the document. This 419 document especially clarifies the use of the Weight field of the SRV 420 records. 422 Security Considerations 424 The authors believe this RR to not cause any new security problems. 425 Some problems become more visible, though. 427 - The ability to specify ports on a fine-grained basis obviously 428 changes how a router can filter packets. It becomes impossible 429 to block internal clients from accessing specific external 430 services, slightly harder to block internal users from running 431 unauthorized services, and more important for the router 432 operations and DNS operations personnel to cooperate. 434 - There is no way a site can keep its hosts from being referenced 435 as servers. This could lead to denial of service. 437 - With SRV, DNS spoofers can supply false port numbers, as well as 438 host names and addresses. Because this vunerability exists 439 already, with names and addresses, this is not a new 440 vunerability, merely a slightly extended one, with little 441 practical effect. 443 References 445 STD 2: Reynolds, J., Postel, J., "Assigned Numbers", STD 2, RFC 1700, 446 October 1994 (as currently updated by the IANA). 448 RFC 1034: Mockapetris, P., "Domain names - concepts and facilities", 449 STD 13, RFC 1034, November 1987. 451 RFC 1035: Mockapetris, P., "Domain names - Implementation and 452 Specification", STD 13, RFC 1035, November 1987. 454 RFC 974: Partridge, C., "Mail routing and the domain system", RFC 455 974, January 1986. 457 BCP 14: Bradner, S., "Key words for use in RFCs to Indicate 458 Requirement Levels", BCP 14, RFC 2119, March 1997. 460 RFC 2052bis DNS SRV RR November 1999 461 Expires May 2000 463 RFC 2181: Elz, R., Bush, R., "Clarifications to the DNS 464 Specification", RFC 2181, July 1997 466 RFC 2219: Hamilton, M., Wright, R., "Use of DNS Aliases for Network 467 Services", BCP 17, RFC 2219, October 1997 468 BCP 14: Bradner, S., �Key words for use in RFCs to Indicate 469 Requirement Levels�, RFC 2119, March 1997. 470 ARM: Armijo, M., Esibov, L., Leach, P., �Discovering LDAP Services 471 with DNS�, Internet Draft draft-armijo-ldap-locate-00.txt, 472 June 1999. 473 Hornstein, K., Altman, J., �Distributing Kerberos KDC and Realm 474 Information with DNS�, Internet Draft 475 draft-ietf-cat-krb-dns-locate-00.txt, June 1999. 477 Acknowledgements 479 The algorithm used to select from the weighted SRV RRs of equal 480 priority is adapted from one supplied by Dan Bernstein. 482 Authors' Addresses 484 Arnt Gulbrandsen Paul Vixie 485 Troll Tech Internet Software Consortium 486 Postboks 6133 Etterstad 950 Charter Street 487 N-0602 Oslo, Norway Redwood City, CA 94063 488 +47 22646966 +1 650 779 7001 490 Levon Esibov 491 Microsoft Corporation 492 One Microsoft Way 493 Redmond, WA 98052 494