rfc2052.txt

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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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   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
        hosts, it seems advisable to have a list of A RRs at the same
        DNS node as the SRV RR, listing reasonable (if perhaps
        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 behaviour is.

      - Where one service is provided by several hosts, one can either
        provide A 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 A
        records.





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      - Hosts that are referenced by backup A records must use the port
        number specified in Assigned Numbers for the service.

   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 ("telnet.tcp.asdf.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 e.g. "dig" to look at the actual answer is a good idea.

The "Weight" field

   Weight, the load balancing field, is not quite satisfactory, but the
   actual load on typical servers changes much too quickly to be kept
   around in DNS caches.  It seems to the authors that offering
   administrators a way to say "this machine is three times as fast as
   that one" is the best that can practically be done.

   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 like SMTP an extra step in the
   connection establishment seems too expensive, and for long-lived
   services like telnet, the load figure may well be thrown off a minute
   after the connection is established when someone else starts or
   finishes a heavy job.

The Port number

   Currently, the translation from service name to port number happens
   at the client, often using a file such as /etc/services.

   Moving this information to the DNS makes it less necessary to update
   these files on every single computer of the net every time a new
   service is added, and makes it possible to move standard services out
   of the "root-only" port range on unix.







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RFC 2052                       DNS SRV RR                   October 1996


Usage rules

   A SRV-cognizant client SHOULD use this procedure to locate a list of
   servers and connect to the preferred one:

        Do a lookup for QNAME=service.protocol.target, QCLASS=IN,
        QTYPE=SRV.

        If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV
        RR which specifies the requested Service and Protocol in the
        reply:

             If there is precisely one SRV RR, and its Target is "."
             (the root domain), abort.

             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 randomly, with probability
                       Weight, and move it to the tail of the new list

             For each element in the new list

                  query the DNS for A RR's for the Target or use any
                  RR's found in the Additional Data secion of the
                  earlier SRV query.

                  for each A RR found, try to connect to the (protocol,
                  address, service).

        else if the service desired is SMTP

             skip to RFC 974 (MX).

        else

             Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A

             for each A RR found, try to connect to the (protocol,
             address, service)




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