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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Applications Area Arnt Gulbrandsen 3 INTERNET-DRAFT Troll Technologies 4 Paul Vixie 5 Internet Software Consortium 6 September 1998 8 A DNS RR for specifying the location of services (DNS SRV) 10 Status of this Memo 12 This document is an Internet-Draft. Internet-Drafts are working 13 documents of the Internet Engineering Task Force (IETF), its areas, 14 and its working groups. Note that other groups may also distribute 15 working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference 20 material or to cite them other than as ``work in progress.'' 22 To view the entire list of current Internet-Drafts, please check the 23 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 24 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern 25 Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 26 Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 28 Abstract 30 This document describes a DNS RR which specifies the location of the 31 server(s) for a specific protocol and domain (like a more general 32 form of MX). 34 Overview and rationale 36 Currently, one must either know the exact address of a server to 37 contact it, or broadcast a question. This has led to, for example, 38 ftp.whatever.com aliases, the SMTP-specific MX RR, and using MAC- 39 level broadcasts to locate servers. 41 The SRV RR allows administrators to use several servers for a single 42 domain, to move services from host to host with little fuss, and to 43 designate some hosts as primary servers for a service and others as 44 backups. 46 Clients ask for a specific service/protocol for a specific domain 47 (the word domain is used here in the strict RFC 1034 sense), and get 48 back the names of any available servers. 50 RFC 2052bis DNS SRV RR September 1998 52 Introductory example 54 When a SRV-cognizant web-browser wants to retrieve 56 http://www.asdf.com/ 58 it does a lookup of 60 _http._tcp.www.asdf.com 62 and retrieves the document from one of the servers in the reply. The 63 example zone file near the end of this memo contains answering RRs 64 for this query. 66 The format of the SRV RR 68 Here is the format of the SRV RR, whose DNS type code is 33: 70 _Service._Proto.Name TTL Class SRV Priority Weight Port Target 72 (There is an example near the end of this document.) 74 Service 75 The symbolic name of the desired service, as defined in Assigned 76 Numbers or locally. An underscore (_) is prepended to the 77 service identifier to avoid collisions with DNS labels that 78 occur in nature. 80 Some widely used services, notably POP, don't have a single 81 universal name. If Assigned Numbers names the service 82 indicated, that name is the only name which is legal for SRV 83 lookups. Only locally defined services may be named locally. 84 The Service is case insensitive. 86 Proto 87 The symbolic name of the desired protocol, with an underscore 88 (_) prepended to prevent collisions with DNS labels that occur 89 in nature. _TCP and _UDP are at present the most useful values 90 for this field, though any name defined by Assigned Numbers or 91 locally may be used (as for Service). The Proto is case 92 insensitive. 94 Name 95 The domain this RR refers to. The SRV RR is unique in that the 96 name one searches for is not this name; the example near the end 97 shows this clearly. 99 RFC 2052bis DNS SRV RR September 1998 101 TTL 102 Standard DNS meaning. 104 Class 105 Standard DNS meaning. 107 Priority 108 As for MX, the priority of this target host. A client MUST 109 attempt to contact the target host with the lowest-numbered 110 priority it can reach; target hosts with the same priority 111 SHOULD be tried in pseudorandom order. The range is 0-65535. 113 Weight 114 Load balancing mechanism. When selecting a target host among 115 the those that have the same priority, the chance of trying this 116 one first SHOULD be proportional to its weight. The range of 117 this number is 1-65535. Domain administrators are urged to use 118 Weight 0 when there isn't any load balancing to do, to make the 119 RR easier to read for humans (less noisy). 121 Port 122 The port on this target host of this service. The range is 123 0-65535. This is often as specified in Assigned Numbers but 124 need not be. 126 Target 127 As for MX, the domain name of the target host. There MUST be 128 one or more A records for this name. Implementors are urged, but 129 not required, to return the A record(s) in the Additional Data 130 section. Name compression is to be used for this field. 132 A Target of ``.'' means that the service is decidedly not 133 available at this domain. 135 Domain administrator advice 137 Asking everyone to update their telnet (for example) clients when the 138 first internet site adds a SRV RR for Telnet/TCP is futile (even if 139 desirable). Therefore SRV will have to coexist with A record lookups 140 for a long time, and DNS administrators should try to provide A 141 records to support old clients: 143 - Where the services for a single domain are spread over several 144 hosts, it seems advisable to have a list of A RRs at the same 145 DNS node as the SRV RR, listing reasonable (if perhaps 146 suboptimal) fallback hosts for Telnet, NNTP and other protocols 147 likely to be used with this name. Note that some programs only 149 RFC 2052bis DNS SRV RR September 1998 151 try the first address they get back from e.g. gethostbyname(), 152 and we don't know how widespread this behaviour is. 154 - Where one service is provided by several hosts, one can either 155 provide A records for all the hosts (in which case the round- 156 robin mechanism, where available, will share the load equally) 157 or just for one (presumably the fastest). 159 - If a host is intended to provide a service only when the main 160 server(s) is/are down, it probably shouldn't be listed in A 161 records. 163 - Hosts that are referenced by backup A records must use the port 164 number specified in Assigned Numbers for the service. 166 Currently there's a practical limit of 512 bytes for DNS replies. 167 Until all resolvers can handle larger responses, domain 168 administrators are strongly advised to keep their SRV replies below 169 512 bytes. 171 All round numbers, wrote Dr. Johnson, are false, and these numbers 172 are very round: A reply packet has a 30-byte overhead plus the name 173 of the service (``_telnet._tcp.asdf.com'' for instance); each SRV RR 174 adds 20 bytes plus the name of the target host; each NS RR in the NS 175 section is 15 bytes plus the name of the name server host; and 176 finally each A RR in the additional data section is 20 bytes or so, 177 and there are A's for each SRV and NS RR mentioned in the answer. 178 This size estimate is extremely crude, but shouldn't underestimate 179 the actual answer size by much. If an answer may be close to the 180 limit, using e.g. ``dig'' to look at the actual answer is a good 181 idea. 183 The ``Weight'' field 185 Weight, the load balancing field, is not quite satisfactory, but the 186 actual load on typical servers changes much too quickly to be kept 187 around in DNS caches. It seems to the authors that offering 188 administrators a way to say ``this machine is three times as fast as 189 that one'' is the best that can practically be done. 191 The only way the authors can see of getting a ``better'' load figure 192 is asking a separate server when the client selects a server and 193 contacts it. For short-lived services like SMTP an extra step in the 194 connection establishment seems too expensive, and for long-lived 195 services like telnet, the load figure may well be thrown off a minute 196 after the connection is established when someone else starts or 197 finishes a heavy job. 199 RFC 2052bis DNS SRV RR September 1998 201 The Port number 203 Currently, the translation from service name to port number happens 204 at the client, often using a file such as /etc/services. 206 Moving this information to the DNS makes it less necessary to update 207 these files on every single computer of the net every time a new 208 service is added, and makes it possible to move standard services out 209 of the ``root-only'' port range on unix. 211 Usage rules 213 A SRV-cognizant client SHOULD use this procedure to locate a list of 214 servers and connect to the preferred one: 216 Do a lookup for QNAME=_service._protocol.target, QCLASS=IN, 217 QTYPE=SRV. 219 If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV 220 RR which specifies the requested Service and Protocol in the 221 reply: 223 If there is precisely one SRV RR, and its Target is ``.'' 224 (the root domain), abort. 226 Else, for all such RR's, build a list of (Priority, Weight, 227 Target) tuples 229 Sort the list by priority (lowest number first) 231 Create a new empty list 233 For each distinct priority level 234 While there are still elements left at this priority 235 level 236 Select an element randomly, with probability 237 Weight, and move it to the tail of the new list 239 For each element in the new list 241 query the DNS for A RR's for the Target or use any 242 RR's found in the Additional Data secion of the 243 earlier SRV query. 245 for each A RR found, try to connect to the (protocol, 246 address, service). 248 RFC 2052bis DNS SRV RR September 1998 250 else if the service desired is SMTP 252 skip to RFC 974 (MX). 254 else 256 Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A 258 for each A RR found, try to connect to the (protocol, 259 address, service) 261 RFC 2052bis DNS SRV RR September 1998 263 Notes: 265 - Port numbers SHOULD NOT be used in place of the symbolic service 266 or protocol names (for the same reason why variant names cannot 267 be allowed: Applications would have to do two or more lookups). 269 - If a truncated response comes back from an SRV query, and the 270 Additional Data section has at least one complete RR in it, the 271 answer MUST be considered complete and the client resolver 272 SHOULD NOT retry the query using TCP, but use normal UDP queries 273 for A RR's missing from the Additional Data section. 275 - A client MAY use means other than Weight to choose among target 276 hosts with equal Priority. 278 - A client MUST parse all of the RR's in the reply. 280 - If the Additional Data section doesn't contain A RR's for all 281 the SRV RR's and the client may want to connect to the target 282 host(s) involved, the client MUST look up the A RR(s). (This 283 happens quite often when the A RR has shorter TTL than the SRV 284 or NS RR's.) 286 - A future standard could specify that a SRV RR whose Protocol was 287 _TCP and whose Service was _SMTP would override RFC 974's rules 288 with regard to the use of an MX RR. This would allow firewalled 289 organizations with several SMTP relays to control the load 290 distribution using the Weight field. 292 - Future protocols could be designed to use SRV RR lookups as the 293 means by which clients locate their servers. 295 Fictional example 297 This is (part of) the zone file for asdf.com, a still-unused domain: 299 $ORIGIN asdf.com. 300 @ SOA server.asdf.com. root.asdf.com. ( 301 1995032001 3600 3600 604800 86400 ) 302 NS server.asdf.com. 303 NS ns1.ip-provider.net. 304 NS ns2.ip-provider.net. 305 _ftp._tcp SRV 0 0 21 server.asdf.com. 306 _finger._tcp SRV 0 0 79 server.asdf.com. 307 ; telnet - use old-slow-box or new-fast-box if either is 308 ; available, make three quarters of the logins go to 309 ; new-fast-box. 311 RFC 2052bis DNS SRV RR September 1998 313 _telnet._tcp SRV 0 1 23 old-slow-box.asdf.com. 314 SRV 0 3 23 new-fast-box.asdf.com. 315 ; if neither old-slow-box or new-fast-box is up, switch to 316 ; using the sysdmin's box and the server 317 SRV 1 0 23 sysadmins-box.asdf.com. 318 SRV 1 0 23 server.asdf.com. 319 ; HTTP - server is the main server, new-fast-box is the backup 320 ; (On new-fast-box, the HTTP daemon runs on port 8000) 321 _http._tcp SRV 0 0 80 server.asdf.com. 322 SRV 10 0 8000 new-fast-box.asdf.com. 323 ; since we want to support both http://asdf.com/ and 324 ; http://www.asdf.com/ we need the next two RRs as well 325 _http._tcp.www SRV 0 0 80 server.asdf.com. 326 SRV 10 0 8000 new-fast-box.asdf.com. 327 ; SMTP - mail goes to the server, and to the IP provider if 328 ; the net is down 329 _smtp._tcp SRV 0 0 25 server.asdf.com. 330 SRV 1 0 25 mailhost.ip-provider.net. 331 @ MX 0 server.asdf.com. 332 MX 1 mailhost.ip-provider.net. 333 ; NNTP - use the IP providers's NNTP server 334 _nntp._tcp SRV 0 0 119 nntphost.ip-provider.net. 335 ; IDB is an locally defined protocol 336 _idb._tcp SRV 0 0 2025 new-fast-box.asdf.com. 337 ; addresses 338 server A 172.30.79.10 339 old-slow-box A 172.30.79.11 340 sysadmins-box A 172.30.79.12 341 new-fast-box A 172.30.79.13 342 ; backup A records - new-fast-box and old-slow-box are 343 ; included, naturally, and server is too, but might go 344 ; if the load got too bad 345 @ A 172.30.79.10 346 A 172.30.79.11 347 A 172.30.79.13 348 ; backup A RR for www.asdf.com 349 www A 172.30.79.10 350 ; NO other services are supported 351 *._tcp SRV 0 0 0 . 352 *._udp SRV 0 0 0 . 354 In this example, a telnet connection to ``asdf.com.'' needs an SRV 355 lookup of ``_telnet._tcp.asdf.com.'' and possibly A lookups of ``new- 356 fast-box.asdf.com.'' and/or the other hosts named. The size of the 357 SRV reply is approximately 365 bytes: 359 30 bytes general overhead 360 20 bytes for the query string, ``_telnet._tcp.asdf.com.'' 362 RFC 2052bis DNS SRV RR September 1998 364 130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of ``new- 365 fast-box'', ``old-slow-box'', ``server'' and ``sysadmins-box'' - 366 ``asdf.com'' in the query section is quoted here and doesn't 367 need to be counted again. 368 75 bytes for 3 NS RRs, 15 bytes each plus the lengths of 369 ``server'', ``ns1.ip-provider.net.'' and ``ns2'' - again, ``ip- 370 provider.net.'' is quoted and only needs to be counted once. 371 120 bytes for the 6 A RR's mentioned by the SRV and NS RR's. 373 Refererences 375 RFC 1034: Mockapetris, P., ``Domain names - concepts and 376 facilities'', RFC 1034, November 1987. 378 RFC 974: Partridge, C., ``Mail routing and the domain system'', RFC 379 974, January 1986. 381 Security Considerations 383 The authors believes this RR to not cause any new security problems. 384 Some problems become more visible, though. 386 - The ability to specify ports on a fine-grained basis obviously 387 changes how a router can filter packets. It becomes impossible 388 to block internal clients from accessing specific external 389 services, slightly harder to block internal users from running 390 unautorised services, and more important for the router 391 operations and DNS operations personnel to cooperate. 393 - There is no way a site can keep its hosts from being referenced 394 as servers (as, indeed, some sites become unwilling secondary 395 MXes today). This could lead to denial of service. 397 - With SRV, DNS spoofers can supply false port numbers, as well as 398 host names and addresses. The authors do not see any practical 399 effect of this. 401 Authors' Addresses 403 Arnt Gulbrandsen Paul Vixie 404 Troll Tech Internet Software Consortium 405 Postboks 6133 Etterstad 950 Charter Street 406 N-0602 Oslo, Norway Redwood City, CA 94063 407 +47 22646966 +1 650 779 7001 408