A DNS RR for specifying the location of services (DNS SRV)
RFC 2052
Document | Type |
RFC - Experimental
(October 1996; No errata)
Obsoleted by RFC 2782
Was draft-gulbrandsen-dns-rr-srvcs (individual)
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Authors | Paul Vixie , Arnt Gulbrandsen | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2052 (Experimental) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group A. Gulbrandsen Request for Comments: 2052 Troll Technologies Updates: 1035, 1183 P. Vixie Category: Experimental Vixie Enterprises October 1996 A DNS RR for specifying the location of services (DNS SRV) Status of this Memo This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Abstract This document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain (like a more general form of MX). Overview and rationale Currently, one must either know the exact address of a server to contact it, or broadcast a question. This has led to, for example, ftp.whatever.com aliases, the SMTP-specific MX RR, and using MAC- level broadcasts to locate servers. The SRV RR allows administrators to use several servers for a single domain, to move services from host to host with little fuss, and to designate some hosts as primary servers for a service and others as backups. Clients ask for a specific service/protocol for a specific domain (the word domain is used here in the strict RFC 1034 sense), and get back the names of any available servers. Introductory example When a SRV-cognizant web-browser wants to retrieve http://www.asdf.com/ it does a lookup of http.tcp.www.asdf.com Gulbrandsen & Vixie Experimental [Page 1] RFC 2052 DNS SRV RR October 1996 and retrieves the document from one of the servers in the reply. The example zone file near the end of the memo contains answering RRs for this query. The format of the SRV RR Here is the format of the SRV RR, whose DNS type code is 33: Service.Proto.Name TTL Class SRV Priority Weight Port Target (There is an example near the end of this document.) Service The symbolic name of the desired service, as defined in Assigned Numbers or locally. 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. Only locally defined services may be named locally. The Service is case insensitive. Proto 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. Class Standard DNS meaning. Priority As for MX, 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 pseudorandom order. The range is 0-65535. Gulbrandsen & Vixie Experimental [Page 2] RFC 2052 DNS SRV RR October 1996 Weight Load balancing mechanism. When selecting a target host among the those that have the same priority, the chance of trying this one first SHOULD be proportional to its weight. The range of this number is 1-65535. Domain administrators are urged to use Weight 0 when there isn't any load balancing to do, to make the RR easier to read for humans (less noisy). Port The port on this target host of this service. The range is 0-65535. This is often as specified in Assigned Numbers but need not be. Target As for MX, the domain name of the target host. There MUST be one or more A records for this name. Implementors are urged, but not required, to return the A record(s) in the Additional Data section. Name compression is to be used for this field. A Target of "." means that the service is decidedly not available at this domain. Domain administrator advice Asking everyone to update their telnet (for example) clients when the first internet site adds a SRV RR for Telnet/TCP is futile (even if desirable). Therefore SRV will have to coexist with A record lookups for a long time, and DNS administrators should try to provide A records to support old clients: - Where the services for a single domain are spread over severalShow full document text