A DNS RR for specifying the location of services (DNS SRV)
RFC 2052

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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

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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.

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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 several