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'RFC2181' on line 282 looks like a reference Summary: 8 errors (**), 0 flaws (~~), 3 warnings (==), 3 comments (--). 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 Internet Software Consortium 5 November 1998 7 A DNS RR for specifying the location of services (DNS SRV) 9 Status of this Memo 11 This document is an Internet-Draft. Internet-Drafts are working 12 documents of the Internet Engineering Task Force (IETF), its areas, 13 and its working groups. Note that other groups may also distribute 14 working documents as Internet-Drafts. 16 Internet-Drafts are draft documents valid for a maximum of six months 17 and may be updated, replaced, or obsoleted by other documents at any 18 time. It is inappropriate to use Internet-Drafts as reference 19 material or to cite them other than as "work in progress." 21 To view the entire list of current Internet-Drafts, please check the 22 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 23 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern 24 Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 25 Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 27 Abstract 29 This document describes a DNS RR which specifies the location of the 30 server(s) for a specific protocol and domain (like a more general 31 form of MX). 33 Overview and rationale 35 Currently, one must either know the exact address of a server to 36 contact it, or broadcast a question. This has led to, for example, 37 ftp.whatever.com aliases, the SMTP-specific MX RR, and using MAC- 38 level broadcasts to locate servers. 40 The SRV RR allows administrators to use several servers for a single 41 domain, to move services from host to host with little fuss, and to 42 designate some hosts as primary servers for a service and others as 43 backups. 45 Clients ask for a specific service/protocol for a specific domain 46 (the word domain is used here in the strict RFC 1034 sense), and get 47 back the names of any available servers. 49 RFC 2052bis DNS SRV RR November 1998 51 Note that where this document refers to "address records", it means 52 either A RR's, AAAA RR's, or their most modern equivilent. 54 Introductory example 56 If a SRV-cognizant web-browser wants to retrieve 58 http://www.example.com/ 60 it does a lookup of 62 _http._tcp.www.example.com 64 and retrieves the document from one of the servers in the reply. The 65 example zone file near the end of this memo contains answering RRs 66 for this query. 68 The format of the SRV RR 70 Here is the format of the SRV RR, whose DNS type code is 33: 72 _Service._Proto.Name TTL Class SRV Priority Weight Port Target 74 (There is an example near the end of this document.) 76 Service 77 The symbolic name of the desired service, as defined in Assigned 78 Numbers or locally. An underscore (_) is prepended to the 79 service identifier to avoid collisions with DNS labels that 80 occur in nature. 82 Some widely used services, notably POP, don't have a single 83 universal name. If Assigned Numbers names the service 84 indicated, that name is the only name which is legal for SRV 85 lookups. Only locally defined services may be named locally. 86 The Service is case insensitive. 88 Proto 89 The symbolic name of the desired protocol, with an underscore 90 (_) prepended to prevent collisions with DNS labels that occur 91 in nature. _TCP and _UDP are at present the most useful values 92 for this field, though any name defined by Assigned Numbers or 93 locally may be used (as for Service). The Proto is case 94 insensitive. 96 Name 97 The domain this RR refers to. The SRV RR is unique in that the 99 RFC 2052bis DNS SRV RR November 1998 101 name one searches for is not this name; the example near the end 102 shows this clearly. 104 TTL 105 Standard DNS meaning. 107 Class 108 Standard DNS meaning. 110 Priority 111 As for MX, the priority of this target host. A client MUST 112 attempt to contact the target host with the lowest-numbered 113 priority it can reach; target hosts with the same priority 114 SHOULD be tried in pseudorandom order. The range is 0-65535. 115 This is a binary integer in network byte order. 117 Weight 118 Load balancing mechanism. When selecting a target host among 119 the those that have the same priority, the chance of trying this 120 one first SHOULD be proportional to its weight. The range of 121 this number is 1-65535. This is a binary integer in network 122 byte order. Domain administrators are urged to use Weight 0 123 when there isn't any load balancing to do, to make the RR easier 124 to read for humans (less noisy). 126 Port 127 The port on this target host of this service. The range is 128 0-65535. This is a binary integer in network byte order. This 129 is often as specified in Assigned Numbers but need not be. 131 Target 132 As for MX, the domain name of the target host. There MUST be 133 one or more address records for this name. Implementors are 134 urged, but not required, to return the address record(s) in the 135 Additional Data section. Name compression is to be used for 136 this field. 138 A Target of "." means that the service is decidedly not 139 available at this domain. 141 Domain administrator advice 143 Expecting everyone to update their client applications when the first 144 internet site adds a SRV RR for some server is futile (even if 145 desirable). Therefore SRV would have to coexist with address record 146 lookups for existing protocols, and DNS administrators should try to 147 provide address records to support old clients: 149 RFC 2052bis DNS SRV RR November 1998 151 - Where the services for a single domain are spread over several 152 hosts, it seems advisable to have a list of address records at 153 the same DNS node as the SRV RR, listing reasonable (if perhaps 154 suboptimal) fallback hosts for Telnet, NNTP and other protocols 155 likely to be used with this name. Note that some programs only 156 try the first address they get back from e.g. gethostbyname(), 157 and we don't know how widespread this behaviour is. 159 - Where one service is provided by several hosts, one can either 160 provide address records for all the hosts (in which case the 161 round-robin mechanism, where available, will share the load 162 equally) or just for one (presumably the fastest). 164 - If a host is intended to provide a service only when the main 165 server(s) is/are down, it probably shouldn't be listed in 166 address records. 168 - Hosts that are referenced by backup address records must use the 169 port number specified in Assigned Numbers for the service. 171 - Designers of future protocols for which "secondary servers" is 172 not useful (or meaningful) may choose to not use SRV's support 173 for secondary servers. Clients for such protocols may use or 174 ignore SRV RRs with Priority higher than the RR with the lowest 175 Priority for a domain. 177 Currently there's a practical limit of 512 bytes for DNS replies. 178 Until all resolvers can handle larger responses, domain 179 administrators are strongly advised to keep their SRV replies below 180 512 bytes. 182 All round numbers, wrote Dr. Johnson, are false, and these numbers 183 are very round: A reply packet has a 30-byte overhead plus the name 184 of the service ("_telnet._tcp.example.com" for instance); each SRV RR 185 adds 20 bytes plus the name of the target host; each NS RR in the NS 186 section is 15 bytes plus the name of the name server host; and 187 finally each A RR in the additional data section is 20 bytes or so, 188 and there are A's for each SRV and NS RR mentioned in the answer. 189 This size estimate is extremely crude, but shouldn't underestimate 190 the actual answer size by much. If an answer may be close to the 191 limit, using e.g. "dig" to look at the actual answer is a good idea. 193 The "Weight" field 195 Weight, the load balancing field, is not quite satisfactory, but the 196 actual load on typical servers changes much too quickly to be kept 197 around in DNS caches. It seems to the authors that offering 199 RFC 2052bis DNS SRV RR November 1998 201 administrators a way to say "this machine is three times as fast as 202 that one" is the best that can practically be done. 204 The only way the authors can see of getting a "better" load figure is 205 asking a separate server when the client selects a server and 206 contacts it. For short-lived services like SMTP an extra step in the 207 connection establishment seems too expensive, and for long-lived 208 services like telnet, the load figure may well be thrown off a minute 209 after the connection is established when someone else starts or 210 finishes a heavy job. 212 The Port number 214 Currently, the translation from service name to port number happens 215 at the client, often using a file such as /etc/services. 217 Moving this information to the DNS makes it less necessary to update 218 these files on every single computer of the net every time a new 219 service is added, and makes it possible to move standard services out 220 of the "root-only" port range on unix. 222 Usage rules 224 A SRV-cognizant client SHOULD use this procedure to locate a list of 225 servers and connect to the preferred one: 227 Do a lookup for QNAME=_service._protocol.target, QCLASS=IN, 228 QTYPE=SRV. 230 If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV 231 RR which specifies the requested Service and Protocol in the 232 reply: 234 If there is precisely one SRV RR, and its Target is "." 235 (the root domain), abort. 237 Else, for all such RR's, build a list of (Priority, Weight, 238 Target) tuples 240 Sort the list by priority (lowest number first) 242 Create a new empty list 244 For each distinct priority level 245 While there are still elements left at this priority 246 level 248 RFC 2052bis DNS SRV RR November 1998 250 Select an element randomly, with probability 251 Weight, and move it to the tail of the new list 253 For each element in the new list 255 query the DNS for address records for the Target or 256 use any such records found in the Additional Data 257 secion of the earlier SRV response. 259 for each address record found, try to connect to the 260 (protocol, address, service). 262 else if the service desired is SMTP 264 skip to RFC 974 (MX). 266 else 268 Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A 270 for each address record found, try to connect to the 271 (protocol, address, service) 273 RFC 2052bis DNS SRV RR November 1998 275 Notes: 277 - Port numbers SHOULD NOT be used in place of the symbolic service 278 or protocol names (for the same reason why variant names cannot 279 be allowed: Applications would have to do two or more lookups). 281 - If a truncated response comes back from an SRV query, the rules 282 described in [RFC2181] shall apply. 284 - A client MAY use means other than Weight to choose among target 285 hosts with equal Priority. 287 - A client MUST parse all of the RR's in the reply. 289 - If the Additional Data section doesn't contain address records 290 for all the SRV RR's and the client may want to connect to the 291 target host(s) involved, the client MUST look up the address 292 record(s). (This happens quite often when the address record 293 has shorter TTL than the SRV or NS RR's.) 295 - Future protocols could be designed to use SRV RR lookups as the 296 means by which clients locate their servers. 298 Fictional example 300 This is (part of) the zone file for example.com, a still-unused 301 domain: 303 $ORIGIN example.com. 304 @ SOA server.example.com. root.example.com. ( 305 1995032001 3600 3600 604800 86400 ) 306 NS server.example.com. 307 NS ns1.ip-provider.net. 308 NS ns2.ip-provider.net. 309 _ftp._tcp SRV 0 0 21 server.example.com. 310 _finger._tcp SRV 0 0 79 server.example.com. 311 ; telnet - use old-slow-box or new-fast-box if either is 312 ; available, make three quarters of the logins go to 313 ; new-fast-box. 314 _telnet._tcp SRV 0 1 23 old-slow-box.example.com. 315 SRV 0 3 23 new-fast-box.example.com. 316 ; if neither old-slow-box or new-fast-box is up, switch to 317 ; using the sysdmin's box and the server 318 SRV 1 0 23 sysadmins-box.example.com. 319 SRV 1 0 23 server.example.com. 320 ; HTTP - server is the main server, new-fast-box is the backup 321 ; (On new-fast-box, the HTTP daemon runs on port 8000) 323 RFC 2052bis DNS SRV RR November 1998 325 _http._tcp SRV 0 0 80 server.example.com. 326 SRV 10 0 8000 new-fast-box.example.com. 327 ; since we want to support both http://example.com/ and 328 ; http://www.example.com/ we need the next two RRs as well 329 _http._tcp.www SRV 0 0 80 server.example.com. 330 SRV 10 0 8000 new-fast-box.example.com. 331 ; SMTP - mail goes to the server, and to the IP provider if 332 ; the net is down 333 _smtp._tcp SRV 0 0 25 server.example.com. 334 SRV 1 0 25 mailhost.ip-provider.net. 335 @ MX 0 server.example.com. 336 MX 1 mailhost.ip-provider.net. 337 ; NNTP - use the IP providers's NNTP server 338 _nntp._tcp SRV 0 0 119 nntphost.ip-provider.net. 339 ; IDB is an locally defined protocol 340 _idb._tcp SRV 0 0 2025 new-fast-box.example.com. 341 ; addresses 342 server A 172.30.79.10 343 old-slow-box A 172.30.79.11 344 sysadmins-box A 172.30.79.12 345 new-fast-box A 172.30.79.13 346 ; backup address records - new-fast-box and old-slow-box are 347 ; included, naturally, and server is too, but might go 348 ; if the load got too bad 349 @ A 172.30.79.10 350 A 172.30.79.11 351 A 172.30.79.13 352 ; backup address record for www.example.com 353 www A 172.30.79.10 354 ; NO other services are supported 355 *._tcp SRV 0 0 0 . 356 *._udp SRV 0 0 0 . 358 In this example, a telnet connection to "example.com." needs an SRV 359 lookup of "_telnet._tcp.example.com." and possibly A lookups of "new- 360 fast-box.example.com." and/or the other hosts named. The size of the 361 SRV reply is approximately 365 bytes: 363 30 bytes general overhead 364 20 bytes for the query string, "_telnet._tcp.example.com." 365 130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new- 366 fast-box", "old-slow-box", "server" and "sysadmins-box" - 367 "example.com" in the query section is quoted here and doesn't 368 need to be counted again. 369 75 bytes for 3 NS RRs, 15 bytes each plus the lengths of "server", 370 "ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is 371 quoted and only needs to be counted once. 372 120 bytes for the 6 address records (assuming IPv4 only) mentioned 374 RFC 2052bis DNS SRV RR November 1998 376 by the SRV and NS RR's. 378 Refererences 380 RFC 1034: Mockapetris, P., "Domain names - concepts and facilities", 381 RFC 1034, November 1987. 383 RFC 974: Partridge, C., "Mail routing and the domain system", RFC 384 974, January 1986. 386 Security Considerations 388 The authors believes this RR to not cause any new security problems. 389 Some problems become more visible, though. 391 - The ability to specify ports on a fine-grained basis obviously 392 changes how a router can filter packets. It becomes impossible 393 to block internal clients from accessing specific external 394 services, slightly harder to block internal users from running 395 unauthorised services, and more important for the router 396 operations and DNS operations personnel to cooperate. 398 - There is no way a site can keep its hosts from being referenced 399 as servers (as, indeed, some sites become unwilling secondary 400 MXes today). This could lead to denial of service. 402 - With SRV, DNS spoofers can supply false port numbers, as well as 403 host names and addresses. The authors do not see any practical 404 effect of this. 406 Authors' Addresses 408 Arnt Gulbrandsen Paul Vixie 409 Troll Tech Internet Software Consortium 410 Postboks 6133 Etterstad 950 Charter Street 411 N-0602 Oslo, Norway Redwood City, CA 94063 412 +47 22646966 +1 650 779 7001 413