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Internet-Draft                                                 J. Jeong
                                                                S. Park
                                                    SAMSUNG Electronics
                                                             L. Beloeil
                                                     France Telecom R&D
                                                         S. Madanapalli
                                                            SAMSUNG ISO

Expires: January 2005                                      18 July 2004

             IPv6 DNS Discovery based on Router Advertisement

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which we become aware will be disclosed, in accordance
   with RFC3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


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Internet-Draft       IPv6 DNS Discovery based on RA          July 2004

   This document specifies the steps an IPv6 host takes in deciding how
   to configure the address of recursive DNS server for DNS name
   resolution.  The way of discovering recursive DNS server is based on
   Router Advertisement message.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

Table of Contents

   1. Introduction...................................................2
   2. Terminology....................................................3
   3. Overview.......................................................3
   4. Neighbor Discovery Extension...................................4
      4.1 Recursive DNS Server Option................................4
   5. Procedure of DNS Discovery.....................................5
   6. Configuration of DNS Information...............................6
      6.1 DNS Server Cache Management................................6
      6.2 Synchronization between DNS Server Cache and Resolver File.7
      6.3 DNS Resolution.............................................8
      6.4 Implementation Considerations..............................8
   7. Applicability Statements.......................................8
   8. Open Issues....................................................8
   9. Security Considerations........................................9
   10. Changes from Previous Version of the Draft....................9
   11. Acknowledgements..............................................9
   12. Normative References..........................................9
   13. Informative References.......................................10
   14. Authors' Addresses...........................................10
   Intellectual Property Statement..................................11
   Full Copyright Statement.........................................12

1. Introduction

   Neighbor Discovery (ND) for IP Version 6 and IPv6 Stateless
   Address Autoconfiguration provide ways to configure either fixed
   or mobile nodes with one or more IPv6 addresses, default routes
   and some other parameters [4][5].  To support access to additional
   services in the Internet that are identified by a DNS name, such
   as a web server, the configuration of at least one recursive DNS
   server is also needed for DNS name resolution.

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   This document defines the process of DNS discovery based on IPv6
   Router Advertisement (RA) to find out the addresses of recursive DNS
   servers within the local network.

2. Terminology

   This document uses the terminology described in [4][5].  In addition,
   three new terms are defined below:

   Recursive DNS Server (RDNSS)

     A Recursive DNS Server is a name server that offers the recursive
     service of DNS name resolution.

   DNS Server Cache

     DNS Server Cache is a data structure for managing DNS Server
     Information existing in IPv6 protocol stack in addition to
     Neighbor Cache and Destination Cache for Neighbor Discovery [4].

   Resolver File

     Resolver File is a configuration file which DNS resolver on the
     host uses for DNS name resolution, e.g., /etc/resolv.conf in UNIX.

3. Overview

   RA approach for RDNSS is to define a new ND option called RDNSS
   option that contains a recursive DNS server address.  Existing ND
   transport mechanisms (i.e., advertisements and solicitations) are
   used.  This works in the same way that nodes learn about routers
   and prefixes, etc.  An IPv6 host can configure the IPv6 addresses
   of one or more RDNSSes via RA message periodically sent by router
   or solicited by a Router Solicitation (RS).  This approach needs
   RDNSS information to be configured in the routers doing the
   advertisements.  The configuration of RDNSS address can be
   performed manually by operator or other ways, such as automatic
   configuration through DHCPv6 client running on the router [6]-[8].
   When advertising more than one RDNSS options, an RA message
   includes as many RDNSS options as RDNSSes.  Through ND protocol
   and RDNSS option along with prefix information option, an IPv6
   host can perform its network configuration of its IPv6 address and
   RDNSS simultaneously [4][5].  The RA option for RDNSS can be used
   on any network that supports the use of ND.  However, RA approach
   performs poorly in some wireless environments where RA message is
   used for IPv6 address autoconfiguration, such as WLAN networks.

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   The RA approach is useful in some non-WLAN mobile environments
   where the addresses of the RDNSSes are changing because the RA
   option includes a lifetime field.  This can be configured to a
   value that will require the client to time out the entry and
   switch over to another RDNSS address (see Section 6.1 and 6.2).

   The preference value of RDNSS, included in RDNSS option, allows
   IPv6 hosts to select primary RDNSS among several RDNSSes; this can
   be used for load balancing of RDNSSes (see Section 6.1 and 6.2).

4. Neighbor Discovery Extension

   The DNS discovery mechanism in this document needs a new ND option in
   Neighbor Discovery, Recursive DNS Server (RDNSS) option.

4.1 Recursive DNS Server Option

   RDNSS option contains the IPv6 address of the recursive DNS server.
   When advertising more than one RDNSS option, an RA message includes
   as many RDNSS options as DNS servers.  Figure 1 shows the format of
   RDNSS option.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |     Type      |     Length    |  Pref |        Reserved       |
    |                           Lifetime                            |
    |                                                               |
    :                     IPv6 Address of RDNSS                     :
    |                                                               |

      Figure 1. Recursive DNS Server (RDNSS) Option Format


      Type            8-bit identifier of the option type (TBD: IANA)

                               Option Name               Type

                               RDNSS option              (TBD)

      Length          8-bit unsigned integer.  The length of the
                      option (including the type and length fields)
                      in units of 8 octets SHOULD be 0x03 (3 x 8 = 24

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      Pref            The preference of an RDNSS.  A 4-bit unsigned
                      integer.  A decimal value of 15 indicates the
                      highest preference.  A value of zero means
                      unspecified.  The field can be used for load
                      balancing of DNS queries with multiple RDNSSes
                      according to local policy.

      Lifetime        32-bit unsigned integer.  The maximum time, in
                      seconds, over which this RDNSS is used for name
                      resolution.  Hosts should contact the source
                      of this information, router, before this time
                      interval expires.  A value of all one bits
                      (0xffffffff) represents infinity.  A value of
                      zero means that the RDNSS must no longer be used.

      IPv6 Address of RDNSS
                      128-bit IPv6 address of the recursive DNS server.

5. Procedure of DNS Discovery

   The processing of RDNSS option is handled like any other ND option
   and would happen when an RA is received.  No new procedure is needed.
   Figure 2 shows the procedure of DNS Discovery on the basis of IPv6 RA
   message in detail.

      IPv6 Host                                                Router
         |                                                       |
      (b)|<-----------------RA w/ RDNSS option(s)----------------|
      (c)|                    Processing of RA                   |
      (d)|          Stateless Address Autoconfiguration          |
      (e)|          DNS Configuration based on RA option         |
      (f)|        (DNS Configuration based on DHCP option)       |

      Figure 2. Procedure of DNS Discovery

   The procedure consists of the following steps:

   Step (a) : IPv6 Host sends RS (Router Solicitation) message to get
              RA message.  It is optional.

   Step (b) : For the RS message sent by IPv6 Host, Router sends RA
              message, which contains Prefix Information option for
              stateless address autoconfiguration and RDNSS options
              for DNS servers.

   Step (c) : If there are not any Prefix Information option and RDNSS

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              in RA message, IPv6 Host goes to Step (f).

   Step (d) : If there is Prefix Information option in RA message, IPv6
              Host performs stateless address autoconfiguration on the
              basis of the prefix included in the option.  If the auto-
              configuration fails, IPv6 Host goes to Step (f).

   Step (e) : If there is an RDNSS option in RA message, IPv6 Host
              stores the RDNSS address in both its DNS Server Cache and
              resolver configuration file.

   Step (f) : If M (Managed address configuration) flag is set on, IPv6
              Host MUST perform stateful address autoconfiguration
              through DHCPv6 [4]-[6].  If no RDNSS option is included in
              the RA message, IPv6 Host MAY perform DNS configuration
              through DHCPv6 [6]-[8] regardless of whether the O flag is
              set or not.

6. Configuration of DNS Information

   The addresses of RDNSSes are announced by RDNSS options in RA
   message.  The newly discovered RDNSS addresses are stored and managed
   in both DNS Server Cache and Resolver File.

6.1 DNS Server Cache Management

   DNS Discovery in this document needs a new DNS Server Cache in IPv6
   protocol stack in addition to Neighbor Cache and Destination Cache
   for Neighbor Discovery [4].  Each entry of DNS Server Cache consists
   of RDNSS address, Preference, Expire-time, and Onsite-flag as

   - RDNSS address:
     RDNSS address indicates the recursive DNS server in the site.

   - Preference:
     Preference, delivered in RDNSS option, is used to give the usage
     preference to the announced RDNSSes; e.g., the value of two of
     preference field may indicate a primary RDNSS and that of one a
     secondary one.  Like this, this field can be used for load
     balancing of DNS queries with multiple RDNSSes within an
     autonomous site.

   - Expire-time:
     Lifetime, delivered in RDNSS option, is used to give the time when
     this entry becomes invalid.  Expire-time is set to the value of
     Lifetime field of RDNSS option plus the current system time.

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     Whenever a new RDNSS option with the same address is received, it
     is updated.

   - Onsite-flag:
     Onsite-flag is set on while Expire-time is less than the current
     system time, namely this entry is valid.  When Expire-time becomes
     greater than the current system time, this flag is set off.  When
     Expire-time becomes less than the current system time again
     through a receipt of another RDNSS option, the flag is set on.
     The entry of which Onsite-flag is off is not deleted immediately,
     but used for DNS resolution in the site where IPv6 host is mobile
     node and RDNSS is not provided.  In such a site, IPv6 host MAY use
     the RDNSS of the previous site.  To limit the storage needed for
     the DNS Server Cache, a node may need to garbage-collect old
     entries.  However, care must be taken to insure that sufficient
     space is always present to hold the working set of active entries.
     Any LRU-based policy that only reclaims entries that have expired
     should be adequate for garbage-collecting unused entries [4].  For
     example, when the replacement is necessary, IPv6 host can choose
     one of which Onsite-flag is off and of which Expire-time is the

6.2 Synchronization between DNS Server Cache and Resolver File

   When an IPv6 host receives the information of multiple RDNSSes within
   a site through an RA message with RDNSS options, it stores the RDNSS
   addresses in order into both DNS Server Cache and Resolver File.  The
   processing of the RDNSS option included in RA message is as follows:

   Step (a) : Receive and parse RDNSS options.

   Step (b) : Arrange the addresses of RDNSSes in a descending order,
              starting with the biggest value of "Pref" field of the
              RDNSS option and store them in both DNS Server Cache
              and Resolver File.  In the case where there are several
              routers advertising RDNSS option(s) in a subnet, "Pref"
              field is used to arrange the information.

   Step (c) : For each RDNSS option, check the following; If the value
              of "Pref" and "Lifetime" fields is set to zero, delete the
              corresponding RDNSS entry from both DNS Server Cache and
              Resolver File in order to let the RDNSS not used any more
              for certain reason in network management, e.g., the
              breakdown of the RDNSS and a renumbering situation.

   Step (d) : Delete each entry of which Onsite-flag is set off from DNS
              Server Cache and the RDNSS address corresponding to the
              entry from Resolver File.  In mobile environments, in

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              order that a mobile node still uses an RDNSS of the
              previous site when the node moves into another site and no
              RDNSS is available there, it MAY be allowed to maintain
              the entry of which Onsite-flag is off, not to delete it
              from both DNS Server Cache and Resolver File.

6.3 DNS Resolution

   Whenever DNS resolver has to resolve a DNS name which is not cached
   in its local DNS cache, it can use DNS servers listed in Resolver
   File, which is synchronized with DNS Server Cache.  It refers to the
   address of RDNSS in order from the first RDNSS stored in Resolver

6.4 Implementation Considerations

   The current ND framework should be modified due to the
   synchronization between DNS Server Cache for RDNSSes in kernel
   space and DNS configuration file in user space.  Because it is
   unacceptable to write and rewrite the DNS configuration file (e.g.,
   resolv.conf) from the kernel, another approach is needed.  One
   simple approach to solve this is to have a daemon listening to
   what the kernel conveys, and to have the daemon do these steps,
   but such a daemon is not necessary with the current ND framework.

7. Applicability Statements

   RA-based DNS discovery is useful in a variety of networks where IPv6
   address is autoconfigured through IPv6 stateless address
   autoconfiguration, such as SOHO, home networks, celluar networks
   (e.g., 3GPP), MIPv6 (especially, HMIPv6), NEMO and MANET connected to
   the Internet.

8. Open Issues

   There might be some issues regarding RA-based DNS discovery as

   o How to optimize bandwidth on the link?

   o How to implement RA-based DNS discovery?
     Is the way of implementing RA option for RDNSS in the document

   o What about the use of "Pref" or "Lifetime" field?

   o What about several routers on the same link advertising distinct
     parameters, such as Prefix Information options and RDNSS options?

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     (Multihoming considerations)

   o What about advertising Well-known anycast addresses for RDNSSes
     through RA message? [9]

9. Security Considerations

   The security of RA option for RDNSS is the same as ND protocol
   security [4].  The RA option does not add any new vulnerability.

   It should be noted that the vulnerability of ND is not worse and is a
   subset of the attacks that any node attached to a LAN can do
   independently of ND.  The kind of attacks a malicious node on a LAN
   can include promiscuously receiving packets for any router's MAC
   address, sending packets with the router's MAC address as the source
   MAC addresses in the L2 header to tell the L2 switches to send
   packets addressed to the router to the malicious node instead of the
   router.  This attack can send redirects to tell the hosts to send
   their traffic somewhere else, send unsolicited RA or NA replies,
   answer RS or NS requests, etc.  All of this can be done independently
   of implementing ND.  The RA option for RDNSS does not add to this

   Security issues regarding the ND protocol are being discussed at IETF
   SEND (Securing Neighbor Discovery) Working Group [10].

10. Changes from Previous Version of the Draft

   This section briefly lists some of the major changes in this
   draft relative to the previous version of this same draft,

   - clarified the use of O flag in RA message to explain interworking
     of RA option and DHCP option for IPv6 DNS Configuration.

   - added Implementation Consideration subsection of RA option for
     RDNSS to the document.

   - rewrote Security Considerations section.

11. Acknowledgements

   This draft has greatly benefited from inputs by Robert Hinden,
   Pekka Savola, and Tim Chown.  The authors appreciate their

12. Normative References

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   [1]  S. Bradner, "Intellectual Property Rights in IETF Technology",
        RFC 3668, February 2004.

   [2]  S. Bradner, "IETF Rights in Contributions", RFC 3667,
        February 2004.

   [3]  S. Bradner, "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [4]  T. Narten, E. Nordmark and W. Simpson, "Neighbor Discovery for
        IP version 6 (IPv6)", RFC 2461, December 1998.

   [5]  S. Thomson and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

13. Informative References

   [6]  R. Droms et al., "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", RFC 3315, July 2003.

   [7]  R. Droms, "Stateless Dynamic Host Configuration Protocol (DHCP)
        Service for IPv6", RFC 3736, April 2004.

   [8]  R. Droms et al., "DNS Configuration options for Dynamic Host
        Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December

   [9]  M. Ohta, "Preconfigured DNS Server Addresses", draft-ohta-
        preconfigured-dns-01.txt, February 2004.

   [10] J. Arkko et al., "SEcure Neighbor Discovery (SEND)", draft-
        ietf-send-ndopt-05.txt, April 2004.

14. Authors' Addresses

   Jaehoon Paul Jeong
   161 Gajong-Dong, Yusong-Gu
   Daejeon 305-350

   Phone: +82 42 860 1664
   EMail: paul@etri.re.kr

   Soohong Daniel Park
   Mobile Platform Laboratory,
   SAMSUNG Electronics

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   Phone: +82 31 200 3728
   EMail: soohong.park@samsung.com

   Luc Beloeil
   France Telecom R&D
   42, rue des coutures
   BP 6243
   14066 CAEN Cedex 4

   Phone: +33 02 3175 9391
   EMail: luc.beloeil@francetelecom.com

   Syam Madanapalli
   Network Systems Division,
   SAMSUNG India Software Operations

   Phone: +91 80 555 0555
   EMail: syam@samsung.com

Intellectual Property Statement

   The following intellectual property notice is copied from RFC3668,
   Section 5.

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology
   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
   represent that it has made any independent effort to identify any
   such rights.  Information on the procedures with respect to rights
   in RFC documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to the
   IETF at ietf-ipr@ietf.org.

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Full Copyright Statement

   The following copyright notice is copied from RFC3667, Section 5.4.
   It describes the applicable copyright for this document.

   Copyright (C) The Internet Society (2004).  This document is
   subject to the rights, licenses and restrictions contained in BCP
   78, and except as set forth therein, the authors retain all their

   This document and the information contained herein are provided on


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

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