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rfc:rfc2782

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.

Abstract

 This document describes a DNS RR which specifies the location of the
 server(s) for a specific protocol and domain.

Overview and rationale

 Currently, one must either know the exact address of a server to
 contact it, or broadcast a question.
 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.
 Note that where this document refers to "address records", it means A
 RR's, AAAA RR's, or their most modern equivalent.

Gulbrandsen, et al. Standards Track [Page 1] RFC 2782 DNS SRV RR February 2000

Definitions

 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
 used in this document are to be interpreted as specified in [BCP 14].
 Other terms used in this document are defined in the DNS
 specification, RFC 1034.

Applicability Statement

 In general, it is expected that SRV records will be used by clients
 for applications where the relevant protocol specification indicates
 that clients should use the SRV record. Such specification MUST
 define the symbolic name to be used in the Service field of the SRV
 record as described below. It also MUST include security
 considerations. Service SRV records SHOULD NOT be used in the absence
 of such specification.

Introductory example

 If a SRV-cognizant LDAP client wants to discover a LDAP server that
 supports TCP protocol and provides LDAP service for the domain
 example.com., it does a lookup of
    _ldap._tcp.example.com
 as described in [ARM].  The example zone file near the end of this
 memo contains answering RRs for an SRV query.
 Note: LDAP is chosen as an example for illustrative purposes only,
 and the LDAP examples used in this document should not be considered
 a definitive statement on the recommended way for LDAP to use SRV
 records. As described in the earlier applicability section, consult
 the appropriate LDAP documents for the recommended procedures.

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 [STD 2] or locally.  An underscore (_) is prepended to
      the service identifier to avoid collisions with DNS labels that
      occur in nature.

Gulbrandsen, et al. Standards Track [Page 2] RFC 2782 DNS SRV RR February 2000

      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,

Gulbrandsen, et al. Standards Track [Page 3] RFC 2782 DNS SRV RR February 2000

      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.

Domain administrator advice

 Expecting everyone to update their client applications when the first
 server publishes a SRV RR is futile (even if desirable).  Therefore
 SRV would have to coexist with address record lookups for existing
 protocols, and DNS administrators should try to provide address
 records to support old clients:
  1. Where the services for a single domain are spread over several

hosts, it seems advisable to have a list of address records at

      the same DNS node as the SRV RR, listing reasonable (if perhaps

Gulbrandsen, et al. Standards Track [Page 4] RFC 2782 DNS SRV RR February 2000

      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.
  1. 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).
  1. 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.
  1. Hosts that are referenced by backup address records must use the

port number specified in Assigned Numbers for the service.

  1. 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 "Weight" field

 Weight, the server selection 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.

Gulbrandsen, et al. Standards Track [Page 5] RFC 2782 DNS SRV RR February 2000

 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".

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 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.

Gulbrandsen, et al. Standards Track [Page 6] RFC 2782 DNS SRV RR February 2000

          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)

Notes:

  1. 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).
  1. If a truncated response comes back from an SRV query, the rules

described in [RFC 2181] shall apply.

  1. A client MUST parse all of the RR's in the reply.
  1. 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.)

Gulbrandsen, et al. Standards Track [Page 7] RFC 2782 DNS SRV RR February 2000

  1. Future protocols could be designed to use SRV RR lookups as the

means by which clients locate their servers.

Fictional example

 This example uses fictional service "foobar" as an aid in
 understanding SRV records. If ever service "foobar" is implemented,
 it is not intended that it will necessarily use SRV records.  This is
 (part of) the zone file for example.com, a still-unused domain:
    $ORIGIN example.com.
    @               SOA server.example.com. root.example.com. (
                        1995032001 3600 3600 604800 86400 )
                    NS  server.example.com.
                    NS  ns1.ip-provider.net.
                    NS  ns2.ip-provider.net.
    ; foobar - use old-slow-box or new-fast-box if either is
    ; available, make three quarters of the logins go to
    ; new-fast-box.
    _foobar._tcp    SRV 0 1 9 old-slow-box.example.com.
                     SRV 0 3 9 new-fast-box.example.com.
    ; if neither old-slow-box or new-fast-box is up, switch to
    ; using the sysdmin's box and the server
                     SRV 1 0 9 sysadmins-box.example.com.
                     SRV 1 0 9 server.example.com.
    server           A   172.30.79.10
    old-slow-box     A   172.30.79.11
    sysadmins-box    A   172.30.79.12
    new-fast-box     A   172.30.79.13
    ; NO other services are supported
    *._tcp          SRV  0 0 0 .
    *._udp          SRV  0 0 0 .

Gulbrandsen, et al. Standards Track [Page 8] RFC 2782 DNS SRV RR February 2000

 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.

IANA Considerations

 The IANA has assigned RR type value 33 to the SRV RR.  No other IANA
 services are required by this document.

Changes from RFC 2052

 This document obsoletes 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.

Security Considerations

 The authors believe this RR to not cause any new security problems.
 Some problems become more visible, though.
  1. 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
   unauthorized services, and more important for the router
   operations and DNS operations personnel to cooperate.
  1. There is no way a site can keep its hosts from being referenced

as servers. This could lead to denial of service.

Gulbrandsen, et al. Standards Track [Page 9] RFC 2782 DNS SRV RR February 2000

  1. 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.

References

 STD 2:    Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, 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.

Gulbrandsen, et al. Standards Track [Page 10] RFC 2782 DNS SRV RR February 2000

Acknowledgements

 The algorithm used to select from the weighted SRV RRs of equal
 priority is adapted from one supplied by Dan Bernstein.

Authors' Addresses

 Arnt Gulbrandsen
 Troll Tech
 Waldemar Thranes gate 98B
 N-0175 Oslo, Norway
 Fax:   +47 22806380
 Phone: +47 22806390
 EMail: arnt@troll.no
 Paul Vixie
 Internet Software Consortium
 950 Charter Street
 Redwood City, CA 94063
 Phone: +1 650 779 7001
 Levon Esibov
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 EMail: levone@microsoft.com

Gulbrandsen, et al. Standards Track [Page 11] RFC 2782 DNS SRV RR February 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
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 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
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 The limited permissions granted above are perpetual and will not be
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Gulbrandsen, et al. Standards Track [Page 12]

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