Applications Area                                       Arnt Gulbrandsen
INTERNET-DRAFT                                        Troll Technologies
<draft-dnsind-rfc2052bis-00.txt>                              Paul Vixie
                                            Internet Software Consortium
                                                          September 1998

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

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as ``work in progress.''

   To view the entire list of current Internet-Drafts, please check the
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   Rim), (US East Coast), or (US West Coast).


   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, 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

   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.

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RFC 2052bis                    DNS SRV RR                 September 1998

Introductory example

   When a SRV-cognizant web-browser wants to retrieve

   it does a lookup of

   and retrieves the document from one of the servers in the reply.  The
   example zone file near the end of this 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.)

        The symbolic name of the desired service, as defined in Assigned
        Numbers or locally.  An underscore (_) is prepended to the
        service identifier to avoid collisions with DNS labels that
        occur in nature.

        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.

        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

        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.

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RFC 2052bis                    DNS SRV RR                 September 1998

        Standard DNS meaning.

        Standard DNS meaning.

        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.

        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).

        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.

        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

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RFC 2052bis                    DNS SRV RR                 September 1998

        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

      - 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 (``'' 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

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.

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RFC 2052bis                    DNS SRV RR                 September 1998

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.

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, QCLASS=IN,

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

             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
                       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).

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RFC 2052bis                    DNS SRV RR                 September 1998

        else if the service desired is SMTP

             skip to RFC 974 (MX).


             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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RFC 2052bis                    DNS SRV RR                 September 1998


      - Port numbers SHOULD NOT be used in place of the symbolic service
        or protocol names (for the same reason why variant names cannot
        be allowed: Applications would have to do two or more lookups).

      - If a truncated response comes back from an SRV query, and the
        Additional Data section has at least one complete RR in it, the
        answer MUST be considered complete and the client resolver
        SHOULD NOT retry the query using TCP, but use normal UDP queries
        for A RR's missing from the Additional Data section.

      - A client MAY use means other than Weight to choose among target
        hosts with equal Priority.

      - A client MUST parse all of the RR's in the reply.

      - If the Additional Data section doesn't contain A RR's 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 A RR(s).  (This
        happens quite often when the A RR has shorter TTL than the SRV
        or NS RR's.)

      - A future standard could specify that a SRV RR whose Protocol was
        _TCP and whose Service was _SMTP would override RFC 974's rules
        with regard to the use of an MX RR.  This would allow firewalled
        organizations with several SMTP relays to control the load
        distribution using the Weight field.

      - Future protocols could be designed to use SRV RR lookups as the
        means by which clients locate their servers.

Fictional example

   This is (part of) the zone file for, a still-unused domain:

        @               SOA (
                            1995032001 3600 3600 604800 86400 )
        _ftp._tcp       SRV 0 0 21
        _finger._tcp    SRV 0 0 79
        ; telnet - use old-slow-box or new-fast-box if either is
        ; available, make three quarters of the logins go to
        ; new-fast-box.

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RFC 2052bis                    DNS SRV RR                 September 1998

        _telnet._tcp    SRV 0 1 23
                        SRV 0 3 23
        ; if neither old-slow-box or new-fast-box is up, switch to
        ; using the sysdmin's box and the server
                        SRV 1 0 23
                        SRV 1 0 23
        ; HTTP - server is the main server, new-fast-box is the backup
        ; (On new-fast-box, the HTTP daemon runs on port 8000)
        _http._tcp      SRV 0 0 80
                        SRV 10 0 8000
        ; since we want to support both and
        ; we need the next two RRs as well
        _http._tcp.www  SRV 0 0 80
                        SRV 10 0 8000
        ; SMTP - mail goes to the server, and to the IP provider if
        ; the net is down
        _smtp._tcp      SRV 0 0 25
                        SRV 1 0 25
        @               MX  0
                        MX  1
        ; NNTP - use the IP providers's NNTP server
        _nntp._tcp      SRV 0 0 119
        ; IDB is an locally defined protocol
        _idb._tcp SRV  0 0 2025
        ; addresses
        server          A
        old-slow-box    A
        sysadmins-box   A
        new-fast-box    A
        ; backup A records - new-fast-box and old-slow-box are
        ; included, naturally, and server is too, but might go
        ; if the load got too bad
        @               A
        ; backup A RR for
        www             A
        ; NO other services are supported
        *._tcp         SRV  0 0 0 .
        *._udp         SRV  0 0 0 .

   In this example, a telnet connection to ``'' needs an SRV
   lookup of ``'' and possibly A lookups of ``new-'' 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, ``''

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RFC 2052bis                    DNS SRV RR                 September 1998

      130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of ``new-
        fast-box'', ``old-slow-box'', ``server'' and ``sysadmins-box'' -
        ``'' 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'', ``'' and ``ns2'' - again, ``ip-'' is quoted and only needs to be counted once.
      120 bytes for the 6 A RR's mentioned by the SRV and NS RR's.


   RFC 1034: Mockapetris, P., ``Domain names - concepts and
        facilities'', RFC 1034, November 1987.

   RFC 974: Partridge, C., ``Mail routing and the domain system'', RFC
        974, January 1986.

Security Considerations

   The authors believes this RR to not cause any new security problems.
   Some problems become more visible, though.

      - The ability to specify ports on a fine-grained basis obviously
        changes how a router can filter packets.  It becomes impossible
        to block internal clients from accessing specific external
        services, slightly harder to block internal users from running
        unautorised services, and more important for the router
        operations and DNS operations personnel to cooperate.

      - There is no way a site can keep its hosts from being referenced
        as servers (as, indeed, some sites become unwilling secondary
        MXes today).  This could lead to denial of service.

      - With SRV, DNS spoofers can supply false port numbers, as well as
        host names and addresses.  The authors do not see any practical
        effect of this.

Authors' Addresses

   Arnt Gulbrandsen              Paul Vixie
      Troll Tech                    Internet Software Consortium
      Postboks 6133 Etterstad            950 Charter Street
      N-0602 Oslo, Norway                Redwood City, CA 94063
      +47 22646966                       +1 650 779 7001
      <>                 <>

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