Network Working Group A. Gulbrandsen Request for Comments: 2782 Troll Technologies Obsoletes: 2052 P. Vixie Category: Standards Track Internet Software Consortium L. Esibov Microsoft Corp. February 2000 A DNS RR for specifying the location of services (DNS SRV) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved.
AbstractThis document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain. RFC 1034 sense), and get back the names of any available servers. Note that where this document refers to "address records", it means A RR's, AAAA RR's, or their most modern equivalent.
Some widely used services, notably POP, don't have a single universal name. If Assigned Numbers names the service indicated, that name is the only name which is legal for SRV lookups. The Service is case insensitive. Proto The symbolic name of the desired protocol, with an underscore (_) prepended to prevent collisions with DNS labels that occur in nature. _TCP and _UDP are at present the most useful values for this field, though any name defined by Assigned Numbers or locally may be used (as for Service). The Proto is case insensitive. Name The domain this RR refers to. The SRV RR is unique in that the name one searches for is not this name; the example near the end shows this clearly. TTL Standard DNS meaning [RFC 1035]. Class Standard DNS meaning [RFC 1035]. SRV records occur in the IN Class. Priority The priority of this target host. A client MUST attempt to contact the target host with the lowest-numbered priority it can reach; target hosts with the same priority SHOULD be tried in an order defined by the weight field. The range is 0-65535. This is a 16 bit unsigned integer in network byte order. Weight A server selection mechanism. The weight field specifies a relative weight for entries with the same priority. Larger weights SHOULD be given a proportionately higher probability of being selected. The range of this number is 0-65535. This is a 16 bit unsigned integer in network byte order. Domain administrators SHOULD use Weight 0 when there isn't any server selection to do, to make the RR easier to read for humans (less noisy). In the presence of records containing weights greater than 0, records with weight 0 should have a very small chance of being selected. In the absence of a protocol whose specification calls for the use of other weighting information, a client arranges the SRV RRs of the same Priority in the order in which target hosts,
specified by the SRV RRs, will be contacted. The following algorithm SHOULD be used to order the SRV RRs of the same priority: To select a target to be contacted next, arrange all SRV RRs (that have not been ordered yet) in any order, except that all those with weight 0 are placed at the beginning of the list. Compute the sum of the weights of those RRs, and with each RR associate the running sum in the selected order. Then choose a uniform random number between 0 and the sum computed (inclusive), and select the RR whose running sum value is the first in the selected order which is greater than or equal to the random number selected. The target host specified in the selected SRV RR is the next one to be contacted by the client. Remove this SRV RR from the set of the unordered SRV RRs and apply the described algorithm to the unordered SRV RRs to select the next target host. Continue the ordering process until there are no unordered SRV RRs. This process is repeated for each Priority. Port The port on this target host of this service. The range is 0- 65535. This is a 16 bit unsigned integer in network byte order. This is often as specified in Assigned Numbers but need not be. Target The domain name of the target host. There MUST be one or more address records for this name, the name MUST NOT be an alias (in the sense of RFC 1034 or RFC 2181). Implementors are urged, but not required, to return the address record(s) in the Additional Data section. Unless and until permitted by future standards action, name compression is not to be used for this field. A Target of "." means that the service is decidedly not available at this domain.
suboptimal) fallback hosts for Telnet, NNTP and other protocols likely to be used with this name. Note that some programs only try the first address they get back from e.g. gethostbyname(), and we don't know how widespread this behavior is. - Where one service is provided by several hosts, one can either provide address records for all the hosts (in which case the round-robin mechanism, where available, will share the load equally) or just for one (presumably the fastest). - If a host is intended to provide a service only when the main server(s) is/are down, it probably shouldn't be listed in address records. - Hosts that are referenced by backup address records must use the port number specified in Assigned Numbers for the service. - Designers of future protocols for which "secondary servers" is not useful (or meaningful) may choose to not use SRV's support for secondary servers. Clients for such protocols may use or ignore SRV RRs with Priority higher than the RR with the lowest Priority for a domain. Currently there's a practical limit of 512 bytes for DNS replies. Until all resolvers can handle larger responses, domain administrators are strongly advised to keep their SRV replies below 512 bytes. All round numbers, wrote Dr. Johnson, are false, and these numbers are very round: A reply packet has a 30-byte overhead plus the name of the service ("_ldap._tcp.example.com" for instance); each SRV RR adds 20 bytes plus the name of the target host; each NS RR in the NS section is 15 bytes plus the name of the name server host; and finally each A RR in the additional data section is 20 bytes or so, and there are A's for each SRV and NS RR mentioned in the answer. This size estimate is extremely crude, but shouldn't underestimate the actual answer size by much. If an answer may be close to the limit, using a DNS query tool (e.g. "dig") to look at the actual answer is a good idea.
The only way the authors can see of getting a "better" load figure is asking a separate server when the client selects a server and contacts it. For short-lived services an extra step in the connection establishment seems too expensive, and for long-lived services, the load figure may well be thrown off a minute after the connection is established when someone else starts or finishes a heavy job. Note: There are currently various experiments at providing relative network proximity estimation, available bandwidth estimation, and similar services. Use of the SRV record with such facilities, and in particular the interpretation of the Weight field when these facilities are used, is for further study. Weight is only intended for static, not dynamic, server selection. Using SRV weight for dynamic server selection would require assigning unreasonably short TTLs to the SRV RRs, which would limit the usefulness of the DNS caching mechanism, thus increasing overall network load and decreasing overall reliability. Server selection via SRV is only intended to express static information such as "this server has a faster CPU than that one" or "this server has a much better network connection than that one".
Else, for all such RR's, build a list of (Priority, Weight, Target) tuples Sort the list by priority (lowest number first) Create a new empty list For each distinct priority level While there are still elements left at this priority level Select an element as specified above, in the description of Weight in "The format of the SRV RR" Section, and move it to the tail of the new list For each element in the new list query the DNS for address records for the Target or use any such records found in the Additional Data section of the earlier SRV response. for each address record found, try to connect to the (protocol, address, service). else Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A for each address record found, try to connect to the (protocol, address, service) RFC 2181] shall apply. - A client MUST parse all of the RR's in the reply. - If the Additional Data section doesn't contain address records for all the SRV RR's and the client may want to connect to the target host(s) involved, the client MUST look up the address record(s). (This happens quite often when the address record has shorter TTL than the SRV or NS RR's.)
- Future protocols could be designed to use SRV RR lookups as the means by which clients locate their servers.
In this example, a client of the "foobar" service in the "example.com." domain needs an SRV lookup of "_foobar._tcp.example.com." and possibly A lookups of "new-fast- box.example.com." and/or the other hosts named. The size of the SRV reply is approximately 365 bytes: 30 bytes general overhead 20 bytes for the query string, "_foobar._tcp.example.com." 130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new- fast-box", "old-slow-box", "server" and "sysadmins-box" - "example.com" in the query section is quoted here and doesn't need to be counted again. 75 bytes for 3 NS RRs, 15 bytes each plus the lengths of "server", "ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is quoted and only needs to be counted once. 120 bytes for the 6 address records (assuming IPv4 only) mentioned by the SRV and NS RR's. RFC 2052. The major change from that previous, experimental, version of this specification is that now the protocol and service labels are prepended with an underscore, to lower the probability of an accidental clash with a similar name used for unrelated purposes. Aside from that, changes are only intended to increase the clarity and completeness of the document. This document especially clarifies the use of the Weight field of the SRV records.
- With SRV, DNS spoofers can supply false port numbers, as well as host names and addresses. Because this vulnerability exists already, with names and addresses, this is not a new vulnerability, merely a slightly extended one, with little practical effect. RFC 1700, October 1994. RFC 1034: Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. RFC 1035: Mockapetris, P., "Domain names - Implementation and Specification", STD 13, RFC 1035, November 1987. RFC 974: Partridge, C., "Mail routing and the domain system", STD 14, RFC 974, January 1986. BCP 14: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. RFC 2181: Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. RFC 2219: Hamilton, M. and R. Wright, "Use of DNS Aliases for Network Services", BCP 17, RFC 2219, October 1997. BCP 14: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. ARM: Armijo, M., Esibov, L. and P. Leach, "Discovering LDAP Services with DNS", Work in Progress. KDC-DNS: Hornstein, K. and J. Altman, "Distributing Kerberos KDC and Realm Information with DNS", Work in Progress.
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