idnits 2.17.1 draft-gulbrandsen-dns-rr-srvcs-00.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** Cannot find the required boilerplate sections (Copyright, IPR, etc.) in this document. Expected boilerplate is as follows today (2024-04-26) according to https://trustee.ietf.org/license-info : IETF Trust Legal Provisions of 28-dec-2009, Section 6.a: This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. IETF Trust Legal Provisions of 28-dec-2009, Section 6.b(i), paragraph 2: Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. IETF Trust Legal Provisions of 28-dec-2009, Section 6.b(i), paragraph 3: This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- ** Missing expiration date. The document expiration date should appear on the first and last page. ** The document seems to lack a 1id_guidelines paragraph about Internet-Drafts being working documents. ** The document seems to lack a 1id_guidelines paragraph about 6 months document validity -- however, there's a paragraph with a matching beginning. Boilerplate error? ** The document seems to lack a 1id_guidelines paragraph about the list of current Internet-Drafts. ** The document seems to lack a 1id_guidelines paragraph about the list of Shadow Directories. ** Expected the document's filename to be given on the first page, but didn't find any == No 'Intended status' indicated for this document; assuming Proposed Standard Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The document seems to lack an IANA Considerations section. (See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) == There are 18 instances of lines with non-RFC2606-compliant FQDNs in the document. == There are 7 instances of lines with private range IPv4 addresses in the document. If these are generic example addresses, they should be changed to use any of the ranges defined in RFC 6890 (or successor): 192.0.2.x, 198.51.100.x or 203.0.113.x. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 88: '...ty of this target host. A client MUST...' RFC 2119 keyword, line 91: '... SHOULD be tried in pseudorando...' RFC 2119 keyword, line 99: '... one first SHOULD be proportiona...' RFC 2119 keyword, line 105: '...ame of the server host. There MUST be...' RFC 2119 keyword, line 110: '... Port numbers SHOULD NOT be used in ...' (5 more instances...) == The 'Updates: ' line in the draft header should list only the _numbers_ of the RFCs which will be updated by this document (if approved); it should not include the word 'RFC' in the list. Miscellaneous warnings: ---------------------------------------------------------------------------- -- The exact meaning of the all-uppercase expression 'MAY NOT' is not defined in RFC 2119. If it is intended as a requirements expression, it should be rewritten using one of the combinations defined in RFC 2119; otherwise it should not be all-uppercase. == The expression 'MAY NOT', while looking like RFC 2119 requirements text, is not defined in RFC 2119, and should not be used. Consider using 'MUST NOT' instead (if that is what you mean). Found 'MAY NOT' in this paragraph: - A client MAY NOT discard any of the answers returned. RFC974 allows clients to e.g. try to connect to just the 5 first MXes returned: Such behaviour is NOT legal with SRV lookups. == Couldn't figure out when the document was first submitted -- there may comments or warnings related to the use of a disclaimer for pre-RFC5378 work that could not be issued because of this. Please check the Legal Provisions document at https://trustee.ietf.org/license-info to determine if you need the pre-RFC5378 disclaimer. -- The document date (March 1995) is 10635 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 9 errors (**), 0 flaws (~~), 6 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Arnt Gulbrandsen 2 INTERNET-DRAFT Troll Technologies 3 Updates: RFC1035, RFC1183 Paul Vixie 4 Vixie Enterprises 5 March 1995 7 A DNS RR for specifying the location of services 9 Abstract 11 This document describes a DNS RR which specifies the location of the 12 server(s) for a specific protocol and domain (like a more general 13 form of MX). 15 Status of this memo 17 This document is an Internet-Draft. Internet-Drafts are working 18 documents of the Internet Engineering Task Force (IETF), its areas, 19 and its working groups. Note that other groups may also distribute 20 working documents as 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 To learn the current status of any Internet-Draft, please check the 28 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 29 Directories on ds.internic.net (US East Coast), nic.nordu.net 30 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific 31 Rim). 33 This draft has file name "draft-gulbrandsen-dns-rr-srvcs-00.txt" and 34 expires on September 6, 1995. 36 Overview and rationale 38 Currently, one must either know the exact address of a server to 39 contact it, or broadcast a question. This has led to e.g. 40 ftp.whatever.com aliases, the SMTP-specific MX RR, and using MAC- 41 level broadcasts to locate servers. 43 The SRV RR allows a client to ask for a specific service/protocol for 44 a specific domain (the word domain is used here in the strict RFC1034 45 sense), and get back the names of any available servers. This allows 46 domain adminstrators to use several servers for a single domain, to 47 move servers with little fuss, and to designate some servers as 48 primary and others as backups. 50 The format of the SRV RR 52 Here is the format of the SRV RR: 54 service.protocol.name [ttl] class SRV priority capacity target 56 (There is an example near the end of the draft.) 58 Service 59 The symbolic name of the desired service, as defined in Assigned 60 Numbers or locally. 62 Some widely-used services, notably POP, don't have a single 63 universal name. If Assigned Numbers names the service 64 indicated, that name is the only name which is legal for SRV 65 lookups. Only locally defined services may be named locally. 67 The Service is case insensitive (it has to be, it's part of the 68 DNS look-up key). 70 Protocol 71 The symbolic name of the desired protocol. TCP and UDP are at 72 present the most useful values for this field, though any name 73 defined by Assigned Numbers or locally may be used (as for 74 Service). Case insensitive. 76 Name 77 The domain this RR refers to. The SRV RR is unique in that the 78 name one searches for is not this name; the example near the end 79 shows this very clearly. 81 TTL 82 Standard DNS meaning 84 Class 85 Standard DNS meaning 87 Priority 88 As for MX, the priority of this target host. A client MUST 89 attempt to contact the target host with the lowest-numbered 90 priority it can reach; target hosts with the same priority 91 SHOULD be tried in pseudorandom order. The range is 0-65535. 92 Domain adminstrators are urged to use Priority 0 for the primary 93 server(s), to make the RR easier to read for humans using dig or 94 similar tools. 96 Weight 97 Load balancing mechanism. When selecting a target host among 98 the those that have the same priority, the chance of trying this 99 one first SHOULD be proportional to its weight. The range of 100 this number is 0-65535. Domain adminstrators are urged to use 101 Weight 0 when there isn't any load balancing to do, to make the 102 RR easier to read for humans (less noisy). 104 Target 105 As for MX, the domain name of the server host. There MUST be 106 one or more A records for this name. Implementors are urged, but 107 not required, to return the A record(s) in the Additional Data 108 section. Name compression is to be used for this field. 110 Port numbers SHOULD NOT be used in place of the symbolic service or 111 protocol names (for the same reason why variant names cannot be 112 allowed: Applications would have to do two or more lookups). 114 Domain adminstrator advice 116 Asking everyone to update their telnet (for example) clients when the 117 first internet site adds a SRV RR for SMTP/TCP is futile (even if 118 desirable). Therefore SRV will have to coexist with old-style A 119 record lookups for a long time, and DNS administrators should try to 120 provide A records to support old clients: 122 - Where the services for a single domain are spread over several 123 hosts, it seems advisable to have a list of A RRs at the same 124 DNS node as the SRV RR, listing reasonable (if perhaps 125 suboptimal) fallback hosts for telnet, nntp and other protocols 126 likely to be used with this name. Some programs only try the 127 first address they get back from e.g. gethostbyaddr(), and we 128 don't know how widespread this behaviour is. 130 - Where one service is provided by several hosts, one can either 131 provide A records for all the hosts (in which case the round- 132 robin mechanism, where available, will share the load equally) 133 or just for one (presumably the fastest). 135 - If a host is intended to provide a service only when the main 136 server(s) is/are down, it probably shouldn't be listed in A 137 records. 139 Currently there's a practical limit of 512 bytes for DNS replies. 140 Until all resolvers can handle larger responses, domain adminstrators 141 are strongly advised to keep their SRV replies below 512 bytes. 143 All round numbers, wrote Dr. Johnson, are false, and these numbers 144 are very round: A reply packet has a 30-byte overhead plus the name 145 of the service ("telnet.tcp.asdf.com" for instance); each SRV RR adds 146 20 bytes plus the name of the target host; each NS RR in the NS 147 section is 15 bytes plus the name of the name server host; and 148 finally each A RR in the additional data section is 20 bytes or so, 149 and there are A's for each SRV and NS RR mentioned in the answer. 150 This size estimate is extremely crude, but shouldn't underestimate 151 the actual answer size by much. If an answer may be close to the 152 limit, using "dig" or some similar program to look at the actual 153 answer is a good idea. 155 The "Weight" field 157 Weight, the load balancing field, is not quite satisfactory, but the 158 actual load on typical servers changes much too quickly to be kept 159 around in DNS caches. It seems to the authors that offering 160 administrators a way to say "this machine is three times as fast as 161 that one" is the best that can easily be done. 163 The only way the authors can see of getting a "better" load figure is 164 asking a separate server when the client selects a server and 165 contacts it. For short-lived services like SMTP an extra step in the 166 connection establishment seems too expensive, and for long-lived 167 services like telnet, the load figure may well be thrown off a minute 168 after the connection is established when someone else starts or 169 finishes a heavy job. 171 Usage rules 173 A SRV-cognizant client SHOULD use this procedure to locate a list of 174 servers and connect to the preferred one: 176 Do a lookup for QNAME=service.protocol.target, QCLASS=IN, 177 QTYPE=SRV. 179 If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV 180 RR which specifies the Service and Protocol in the reply: 182 for all such RR's, build a list of (Priority, Weight, 183 Target) tuples 185 Sort the list by priority (lowest first) 187 Create a new empty list 189 For each distinct priority level 190 While there are still elements left at this priority 191 level 192 Select an element randomly, with probability 193 Weight, and move it to the tail of the new list 195 For each element in the new list 197 query the DNS for A RR's for the Target or use any 198 RR's found in the Additional Data secion of the 199 earlier SRV query. 201 for each A RR found, try to connect to the (protocol, 202 address, service). 204 else if the service desired is SMTP 206 skip to RFC974 (MX). 208 else 210 Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A 212 for each A RR found, try to connect to the (protocol, 213 address, service) 215 Notes: 217 - If a truncated response comes back from an SRV query, and the 218 Additional Data section has at least one complete RR in it, the 219 answer MUST be considered complete and the client resolver 220 SHOULD NOT retry the query using TCP, but use normal UDP queries 221 for A RR's missing from the Additional Data section. 223 - A client MAY NOT discard any of the answers returned. RFC974 224 allows clients to e.g. try to connect to just the 5 first MXes 225 returned: Such behaviour is NOT legal with SRV lookups. 227 - If the Additional Data section doesn't contain A RR's for all 228 the SRV RR's, the client MUST look up the A RR(s). (This 229 happens quite often when the A RR has shorter TTL than the SRV 230 or NS RR's.) 232 - SRV RRs with Protocol TCP and Service SMTP override MX RR's. 233 This allows firewalled organizations with several SMTP relays to 234 control the load distribution using the Weight field. 236 - Designers of new protocols are urged to specify that SRV lookups 237 be mandatory for those protocols. 239 Fictional example 241 This is (part of) the zone file for asdf.com, a still-unused 242 domain: 244 $ORIGIN asdf.com. 245 @ SOA server.asdf.com. root.asdf.com. ( 246 1995032001 3600 3600 604800 86400 ) 247 NS server 248 NS ns1.ip-provider.net. 249 NS ns2.ip-provider.net. 250 ftp.tcp SRV 0 0 server.asdf.com. 251 finger.tcp SRV 0 0 server.asdf.com. 252 ; telnet - use old-slow-box or new-fast-box if either is 253 ; available, make three quarters of the logins go to 254 ; new-fast-box. 255 telnet.tcp SRV 0 1 old-slow-box.asdf.com. 256 SRV 0 3 new-fast-box.asdf.com. 257 ; if neither old-slow-box or new-fast-box is up, switch to 258 using the 259 ; sysdmin's box and the server 260 SRV 1 0 sysadmins-box.asdf.com. 261 SRV 1 0 server.asdf.com. 262 ; SMTP - mail goes to the server, and to the IP provider if 263 ; the net is down 264 smtp.tcp SRV 0 0 server.asdf.com. 265 SRV 1 0 mailhost.ip-provider.net. 266 MX 0 server.asdf.com. 267 MX 1 mailhost.ip-provider.net. 268 ; NNTP - use the IP providers's NNTP server 269 nntp.tcp SRV 0 0 nntphost.ip-provider.net. 270 ; for all other services, use the general-purpose server 271 *.tcp SRV 0 0 server.asdf.com. 272 *.udp SRV 0 0 server.asdf.com. 273 ; addresses 274 server A 172.30.79.10 275 old-slow-box A 172.30.79.11 276 sysadmins-box A 172.30.79.12 277 new-fast-box A 172.30.79.13 278 ; backup A records - new-fast-box and old-slow-box are 279 ; included, naturally, and server is too, but might go 280 ; if the load got too bad 281 @ A 172.30.79.10 282 A 172.30.79.11 283 A 172.30.79.13 285 In this example, a telnet connection to "asdf.com." needs an SRV 286 lookup of "telnet.tcp.asdf.com." and possibly A lookups of 287 "new-fast-box.asdf.com." and/or the other hosts named. The size 288 of the answer for this lookup for "telnet.tcp.asdf.com." is 289 approximately 385 bytes: 291 30 bytes general overhead 292 20 bytes for the query string, "telnet.tcp.asdf.com." 293 130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of 294 "new-fast-box", "old-slow-box", "server" and "sysadmins- 295 box" - "asdf.com." in the query section is quoted here and 296 doesn't need to be counted again. 297 75 bytes for 3 NS RRs, 15 bytes each plus the lengths of 298 "server", "ns1.ip-provider.net." and "ns2" - again, "ip- 299 provider.net." gets quoted and only needs to be counted 300 once. 301 140 bytes for the 7 A RR's mentioned by the SRV and NS RR's. 303 Refererences 305 RFC 1700: J. Reynolds, J. Postel, "ASSIGNED NUMBERS", 306 10/20/1994. 308 RFC 1536: A. Kumar, J. Postel, C. Neuman, P. Danzig, S. Miller, 309 "Common DNS Implementation Errors and Suggested Fixes.", 310 10/06/1993. 312 RFC 1348: B. Manning, "DNS NSAP RRs", 07/01/1992. 314 RFC 1183: R. Ullman, P. Mockapetris, L. Mamakos, C. Everhart, 315 "New DNS RR Definitions", 10/08/1990. 317 RFC 1101: P. Mockapetris, "DNS encoding of network names and 318 other types", 04/01/1989. 320 RFC 1035: P. Mockapetris, "Domain names - implementation and 321 specification", 11/01/1987. 323 RFC 1034: P. Mockapetris, "Domain names - concepts and 324 facilities", 11/01/1987. 326 RFC 1033: M. Lottor, "Domain administrators operations guide", 327 11/01/1987. 329 RFC 1032: M. Stahl, "Domain administrators guide", 11/01/1987. 331 RFC 974: C. Partridge, "Mail routing and the domain system", 332 01/01/1986. 334 Security Considerations 336 The authors believes this RR to be perfectly safe - or rather, 337 not to cause any new security problems. We assume that as the 338 DNS-security people invent new features, DNS servers will return 339 the relevant RRs in the Additional Data section when answering 340 an SRV query. 342 Authors' Addresses 344 Arnt Gulbrandsen 345 Troll Technologies 346 0600 Oslo 347 Norway 349 Phone: +47 22646966 351 Mail: agulbra@troll.no 353 Paul Vixie 354 Vixie Enterprises 355 Star Route 159A 356 Woodside, CA 94062 358 Phone: (415) 747-0204 360 Mail: paul@vix.com