    Internet Engineering Task Force (IETF)                          J. Jeong
    Request for Comments: 6106                                  Brocade/ETRI
    Obsoletes: 5006                                                  S. Park
    Category: Standards Track                            SAMSUNG Electronics
    ISSN: 2070-1721                                               L. Beloeil
                                                          France Telecom R&D
                                                              S. Madanapalli
                                                           iRam Technologies
                                                               November 2010

            IPv6 Router Advertisement Options for DNS Configuration

    Abstract

       This document specifies IPv6 Router Advertisement options to allow
       IPv6 routers to advertise a list of DNS recursive server addresses
       and a DNS Search List to IPv6 hosts.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6106.

    Copyright Notice

       Copyright (c) 2010 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Jeong, et al.                Standards Track                    [Page 1]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    Table of Contents

       1. Introduction ....................................................3
          1.1. Applicability Statements ...................................3
          1.2. Coexistence of RA Options and DHCP Options for DNS
               Configuration ..............................................4
       2. Requirements Language ...........................................4
       3. Terminology .....................................................4
       4. Overview ........................................................5
       5. Neighbor Discovery Extension ....................................5
          5.1. Recursive DNS Server Option ................................6
          5.2. DNS Search List Option .....................................7
          5.3. Procedure of DNS Configuration .............................8
               5.3.1. Procedure in IPv6 Host ..............................8
               5.3.2. Warnings for DNS Options Configuration .............10
       6. Implementation Considerations ..................................10
          6.1. DNS Repository Management .................................10
          6.2. Synchronization between DNS Server List and
               Resolver Repository .......................................11
          6.3. Synchronization between DNS Search List and
               Resolver Repository .......................................12
       7. Security Considerations ........................................13
          7.1. Security Threats ..........................................13
          7.2. Recommendations ...........................................14
       8. IANA Considerations ............................................15
       9. Acknowledgements ...............................................15
       10. References ....................................................16
          10.1. Normative References .....................................16
          10.2. Informative References ...................................16
       Appendix A.  Changes from RFC 5006 ................................18

    Jeong, et al.                Standards Track                    [Page 2]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    1.  Introduction

       The purpose of this document is to standardize an IPv6 Router
       Advertisement (RA) option for DNS Recursive Server Addresses used for
       the DNS name resolution in IPv6 hosts.  This RA option was specified
       in an earlier Experimental specification [RFC5006].  This document is
       also to define a new RA option for Domain Name Search Lists for an
       enhanced DNS configuration.  Thus, this document obsoletes [RFC5006],
       which only defines the RA option for DNS Recursive Server Addresses.

       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 routers, and some
       other parameters [RFC4861][RFC4862].  Most Internet services are
       identified by using a DNS name.  The two RA options defined in this
       document provide the DNS information needed for an IPv6 host to reach
       Internet services.

       It is infeasible to manually configure nomadic hosts each time they
       connect to a different network.  While a one-time static
       configuration is possible, it is generally not desirable on general-
       purpose hosts such as laptops.  For instance, locally defined name
       spaces would not be available to the host if it were to run its own
       name server software directly connected to the global DNS.

       The DNS information can also be provided through DHCP
       [RFC3315][RFC3736][RFC3646].  However, the access to DNS is a
       fundamental requirement for almost all hosts, so IPv6 stateless
       autoconfiguration cannot stand on its own as an alternative
       deployment model in any practical network without any support for DNS
       configuration.

       These issues are not pressing in dual-stack networks as long as a DNS
       server is available on the IPv4 side, but they become more critical
       with the deployment of IPv6-only networks.  As a result, this
       document defines a mechanism based on IPv6 RA options to allow IPv6
       hosts to perform the automatic DNS configuration.

    1.1.  Applicability Statements

       RA-based DNS configuration is a useful alternative in networks where
       an IPv6 host's address is autoconfigured through IPv6 stateless
       address autoconfiguration and where there is either no DHCPv6
       infrastructure at all or some hosts do not have a DHCPv6 client.  The
       intention is to enable the full configuration of basic networking
       information for hosts without requiring DHCPv6.  However, when in

    Jeong, et al.                Standards Track                    [Page 3]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       many networks some additional information needs to be distributed,
       those networks are likely to employ DHCPv6.  In these networks, RA-
       based DNS configuration may not be needed.

       RA-based DNS configuration allows an IPv6 host to acquire the DNS
       configuration (i.e., DNS recursive server addresses and DNS Search
       List) for the link(s) to which the host is connected.  Furthermore,
       the host learns this DNS configuration from the same RA message that
       provides configuration information for the link, thereby avoiding
       also running DHCPv6.

       The advantages and disadvantages of the RA-based approach are
       discussed in [RFC4339] along with other approaches, such as the DHCP
       and well-known anycast address approaches.

    1.2.  Coexistence of RA Options and DHCP Options for DNS Configuration

       Two protocols exist to configure the DNS information on a host, the
       Router Advertisement options described in this document and the
       DHCPv6 options described in [RFC3646].  They can be used together.
       The rules governing the decision to use stateful configuration
       mechanisms are specified in [RFC4861].  Hosts conforming to this
       specification MUST extract DNS information from Router Advertisement
       messages, unless static DNS configuration has been specified by the
       user.  If there is DNS information available from multiple Router
       Advertisements and/or from DHCP, the host MUST maintain an ordered
       list of this information as specified in Section 5.3.1.

    2.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

    3.  Terminology

       This document uses the terminology described in [RFC4861] and
       [RFC4862].  In addition, four new terms are defined below:

       o  Recursive DNS Server (RDNSS): Server that provides a recursive DNS
          resolution service for translating domain names into IP addresses
          as defined in [RFC1034] and [RFC1035].

       o  RDNSS Option: IPv6 RA option to deliver the RDNSS information to
          IPv6 hosts [RFC4861].

    Jeong, et al.                Standards Track                    [Page 4]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       o  DNS Search List (DNSSL): The list of DNS suffix domain names used
          by IPv6 hosts when they perform DNS query searches for short,
          unqualified domain names.

       o  DNSSL Option: IPv6 RA option to deliver the DNSSL information to
          IPv6 hosts.

       o  DNS Repository: Two data structures for managing DNS Configuration
          Information in the IPv6 protocol stack in addition to Neighbor
          Cache and Destination Cache for Neighbor Discovery [RFC4861].  The
          first data structure is the DNS Server List for RDNSS addresses
          and the second is the DNS Search List for DNS search domain names.

       o  Resolver Repository: Configuration repository with RDNSS addresses
          and a DNS Search List that a DNS resolver on the host uses for DNS
          name resolution; for example, the Unix resolver file (i.e., /etc/
          resolv.conf) and Windows registry.

    4.  Overview

       This document standardizes the ND option called the RDNSS option
       defined in [RFC5006] that contains the addresses of recursive DNS
       servers.  This document also defines a new ND option called the DNSSL
       option for the Domain Search List.  This is to maintain parity with
       the DHCPv6 options and to ensure that there is necessary
       functionality to determine the search domains.

       The existing ND message (i.e., Router Advertisement) is used to carry
       this information.  An IPv6 host can configure the IPv6 addresses of
       one or more RDNSSes via RA messages.  Through the RDNSS and DNSSL
       options, along with the prefix information option based on the ND
       protocol ([RFC4861] and [RFC4862]), an IPv6 host can perform the
       network configuration of its IPv6 address and the DNS information
       simultaneously without needing DHCPv6 for the DNS configuration.  The
       RA options for RDNSS and DNSSL can be used on any network that
       supports the use of ND.

       This approach requires the manual configuration or other automatic
       mechanisms (e.g., DHCPv6 or vendor proprietary configuration
       mechanisms) to configure the DNS information in routers sending the
       advertisements.  The automatic configuration of RDNSS addresses and a
       DNS Search List in routers is out of scope for this document.

    5.  Neighbor Discovery Extension

       The IPv6 DNS configuration mechanism in this document needs two new
       ND options in Neighbor Discovery: (i) the Recursive DNS Server
       (RDNSS) option and (ii) the DNS Search List (DNSSL) option.

    Jeong, et al.                Standards Track                    [Page 5]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    5.1.  Recursive DNS Server Option

       The RDNSS option contains one or more IPv6 addresses of recursive DNS
       servers.  All of the addresses share the same Lifetime value.  If it
       is desirable to have different Lifetime values, multiple RDNSS
       options can be used.  Figure 1 shows the format of the 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    |           Reserved            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                           Lifetime                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         :            Addresses of IPv6 Recursive DNS Servers            :
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Recursive DNS Server (RDNSS) Option Format

       Fields:
         Type          8-bit identifier of the RDNSS option type as assigned
                       by the IANA: 25

         Length        8-bit unsigned integer.  The length of the option
                       (including the Type and Length fields) is in units of
                       8 octets.  The minimum value is 3 if one IPv6 address
                       is contained in the option.  Every additional RDNSS
                       address increases the length by 2.  The Length field
                       is used by the receiver to determine the number of
                       IPv6 addresses in the option.

         Lifetime      32-bit unsigned integer.  The maximum time, in
                       seconds (relative to the time the packet is sent),
                       over which this RDNSS address MAY be used for name
                       resolution.  Hosts MAY send a Router Solicitation to
                       ensure the RDNSS information is fresh before the
                       interval expires.  In order to provide fixed hosts
                       with stable DNS service and allow mobile hosts to
                       prefer local RDNSSes to remote RDNSSes, the value of
                       Lifetime SHOULD be bounded as
                       MaxRtrAdvInterval <= Lifetime <= 2*MaxRtrAdvInterval
                       where MaxRtrAdvInterval is the Maximum RA Interval
                       defined in [RFC4861].  A value of all one bits
                       (0xffffffff) represents infinity.  A value of zero
                       means that the RDNSS address MUST no longer be used.

    Jeong, et al.                Standards Track                    [Page 6]

    RFC 6106                   IPv6 RA DNS Options             November 2010

         Addresses of IPv6 Recursive DNS Servers
                       One or more 128-bit IPv6 addresses of the recursive
                       DNS servers.  The number of addresses is determined
                       by the Length field.  That is, the number of
                       addresses is equal to (Length - 1) / 2.

    5.2.  DNS Search List Option

       The DNSSL option contains one or more domain names of DNS suffixes.
       All of the domain names share the same Lifetime value.  If it is
       desirable to have different Lifetime values, multiple DNSSL options
       can be used.  Figure 2 shows the format of the DNSSL 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    |           Reserved            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                           Lifetime                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         :                Domain Names of DNS Search List                :
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: DNS Search List (DNSSL) Option Format

      Fields:
        Type          8-bit identifier of the DNSSL option type as assigned
                      by the IANA: 31

        Length        8-bit unsigned integer.  The length of the option
                      (including the Type and Length fields) is in units of
                      8 octets.  The minimum value is 2 if at least one
                      domain name is contained in the option.  The Length
                      field is set to a multiple of 8 octets to accommodate
                      all the domain names in the field of Domain Names of
                      DNS Search List.

        Lifetime      32-bit unsigned integer.  The maximum time, in
                      seconds (relative to the time the packet is sent),
                      over which this DNSSL domain name MAY be used for
                      name resolution.  The Lifetime value has the same
                      semantics as with the RDNSS option.  That is, Lifetime
                      SHOULD be bounded as follows:
                      MaxRtrAdvInterval <= Lifetime <= 2*MaxRtrAdvInterval.

    Jeong, et al.                Standards Track                    [Page 7]

    RFC 6106                   IPv6 RA DNS Options             November 2010

                      A value of all one bits (0xffffffff) represents
                      infinity.  A value of zero means that the DNSSL
                      domain name MUST no longer be used.

        Domain Names of DNS Search List
                      One or more domain names of DNS Search List that MUST
                      be encoded using the technique described in Section
                      3.1 of [RFC1035].  By this technique, each domain
                      name is represented as a sequence of labels ending in
                      a zero octet, defined as domain name representation.
                      For more than one domain name, the corresponding
                      domain name representations are concatenated as they
                      are.  Note that for the simple decoding, the domain
                      names MUST NOT be encoded in a compressed form, as
                      described in Section 4.1.4 of [RFC1035].  Because the
                      size of this field MUST be a multiple of 8 octets,
                      for the minimum multiple including the domain name
                      representations, the remaining octets other than the
                      encoding parts of the domain name representations
                      MUST be padded with zeros.

       Note:  An RDNSS address or a DNSSL domain name MUST be used only as
          long as both the RA router Lifetime (advertised by a Router
          Advertisement message [RFC4861]) and the corresponding option
          Lifetime have not expired.  The reason is that in the current
          network to which an IPv6 host is connected, the RDNSS may not be
          currently reachable, that the DNSSL domain name is not valid any
          more, or that these options do not provide service to the host's
          current address (e.g., due to network ingress filtering
          [RFC2827][RFC5358]).

    5.3.  Procedure of DNS Configuration

       The procedure of DNS configuration through the RDNSS and DNSSL
       options is the same as with any other ND option [RFC4861].

    5.3.1.  Procedure in IPv6 Host

       When an IPv6 host receives DNS options (i.e., RDNSS option and DNSSL
       option) through RA messages, it processes the options as follows:

       o  The validity of DNS options is checked with the Length field; that
          is, the value of the Length field in the RDNSS option is greater
          than or equal to the minimum value (3), and the value of the
          Length field in the DNSSL option is greater than or equal to the
          minimum value (2).

    Jeong, et al.                Standards Track                    [Page 8]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       o  If the DNS options are valid, the host SHOULD copy the values of
          the options into the DNS Repository and the Resolver Repository in
          order.  Otherwise, the host MUST discard the options.  Refer to
          Section 6 for the detailed procedure.

       When the IPv6 host has gathered a sufficient number (e.g., three) of
       RDNSS addresses (or DNS search domain names), it SHOULD maintain
       RDNSS addresses (or DNS search domain names) by the sufficient number
       such that the latest received RDNSS or DNSSL is more preferred to the
       old ones; that is, when the number of RDNSS addresses (or DNS search
       domain names) is already the sufficient number, the new one replaces
       the old one that will expire first in terms of Lifetime.  As an
       exceptional case, if the received RDNSS addresses (or DNS search
       domain names) already exist in the IPv6 host, their Lifetime fields
       update their Expiration-time, that is, when the corresponding DNS
       information expires in the IPv6 host; note that when the Lifetime
       field has zero, the corresponding RDNSS (or DNS search domain name)
       is deleted from the IPv6 host.  Except for this update, the IPv6 host
       SHOULD ignore other RDNSS addresses (or DNS search domain names)
       within an RDNSS (or a DNSSL) option and/or additional RDNSS (or
       DNSSL) options within an RA.  Refer to Section 6 for the detailed
       procedure.  Note that the sufficient number is a system parameter, so
       it can be determined by a local policy.  Also, separate parameters
       can be specified for the sufficient number of RDNSS addresses and
       that of DNS search domain names, respectively.  In this document,
       three is RECOMMENDED as a sufficient number considering both the
       robust DNS query and the reasonably time-bounded recognition of the
       unreachability of DNS servers.

       In the case where the DNS options of RDNSS and DNSSL can be obtained
       from multiple sources, such as RA and DHCP, the IPv6 host SHOULD keep
       some DNS options from all sources.  Unless explicitly specified for
       the discovery mechanism, the exact number of addresses and domain
       names to keep is a matter of local policy and implementation choice.
       However, the ability to store at least three RDNSS addresses (or
       DNSSL domain names) from at least two different sources is
       RECOMMENDED.  The DNS options from Router Advertisements and DHCP
       SHOULD be stored into the DNS Repository and Resolver Repository so
       that information from DHCP appears there first and therefore takes
       precedence.  Thus, the DNS information from DHCP takes precedence
       over that from RA for DNS queries.  On the other hand, for DNS
       options announced by RA, if some RAs use the Secure Neighbor
       Discovery (SEND) protocol [RFC3971] for RA security, they MUST be
       preferred over those that do not use SEND.  Refer to Section 7 for
       the detailed discussion on SEND for RA DNS options.

    Jeong, et al.                Standards Track                    [Page 9]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    5.3.2.  Warnings for DNS Options Configuration

       There are two warnings for DNS options configuration: (i) warning for
       multiple sources of DNS options and (ii) warning for multiple network
       interfaces.  First, in the case of multiple sources for DNS options
       (e.g., RA and DHCP), an IPv6 host can configure its IP addresses from
       these sources.  In this case, it is not possible to control how the
       host uses DNS information and what source addresses it uses to send
       DNS queries.  As a result, configurations where different information
       is provided by different sources may lead to problems.  Therefore,
       the network administrator needs to configure DNS options in multiple
       sources in order to prevent such problems from happening.

       Second, if different DNS information is provided on different network
       interfaces, this can lead to inconsistent behavior.  The IETF is
       working on solving this problem for both DNS and other information
       obtained by multiple interfaces [MIF-PROBLEM][MIF-PRACTICE].

    6.  Implementation Considerations

       Note:  This non-normative section gives some hints for implementing
          the processing of the RDNSS and DNSSL options in an IPv6 host.

       For the configuration and management of DNS information, the
       advertised DNS configuration information can be stored and managed in
       both the DNS Repository and the Resolver Repository.

       In environments where the DNS information is stored in user space and
       ND runs in the kernel, it is necessary to synchronize the DNS
       information (i.e., RDNSS addresses and DNS search domain names) in
       kernel space and the Resolver Repository in user space.  For the
       synchronization, an implementation where ND works in the kernel
       should provide a write operation for updating DNS information from
       the kernel to the Resolver Repository.  One simple approach is to
       have a daemon (or a program that is called at defined intervals) that
       keeps monitoring the Lifetimes of RDNSS addresses and DNS search
       domain names all the time.  Whenever there is an expired entry in the
       DNS Repository, the daemon can delete the corresponding entry from
       the Resolver Repository.

    6.1.  DNS Repository Management

       For DNS repository management, the kernel or user-space process
       (depending on where RAs are processed) should maintain two data
       structures: (i) DNS Server List that keeps the list of RDNSS
       addresses and (ii) DNS Search List that keeps the list of DNS search
       domain names.  Each entry in these two lists consists of a pair of an
       RDNSS address (or DNSSL domain name) and Expiration-time as follows:

    Jeong, et al.                Standards Track                   [Page 10]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       o  RDNSS address for DNS Server List: IPv6 address of the Recursive
          DNS Server, which is available for recursive DNS resolution
          service in the network advertising the RDNSS option.

       o  DNSSL domain name for DNS Search List: DNS suffix domain names,
          which are used to perform DNS query searches for short,
          unqualified domain names in the network advertising the DNSSL
          option.

       o  Expiration-time for DNS Server List or DNS Search List: The time
          when this entry becomes invalid.  Expiration-time is set to the
          value of the Lifetime field of the RDNSS option or DNSSL option
          plus the current system time.  Whenever a new RDNSS option with
          the same address (or DNSSL option with the same domain name) is
          received on the same interface as a previous RDNSS option (or
          DNSSL option), this field is updated to have a new Expiration-
          time.  When Expiration-time becomes less than the current system
          time, this entry is regarded as expired.

    6.2.  Synchronization between DNS Server List and Resolver Repository

       When an IPv6 host receives the information of multiple RDNSS
       addresses within a network (e.g., campus network and company network)
       through an RA message with RDNSS option(s), it stores the RDNSS
       addresses (in order) into both the DNS Server List and the Resolver
       Repository.  The processing of the RDNSS consists of (i) the
       processing of RDNSS option(s) included in an RA message and (ii) the
       handling of expired RDNSSes.  The processing of RDNSS option(s) is as
       follows:

          Step (a): Receive and parse the RDNSS option(s).  For the RDNSS
          addresses in each RDNSS option, perform Steps (b) through (d).

          Step (b): For each RDNSS address, check the following: If the
          RDNSS address already exists in the DNS Server List and the RDNSS
          option's Lifetime field is set to zero, delete the corresponding
          RDNSS entry from both the DNS Server List and the Resolver
          Repository in order to prevent the RDNSS address from being used
          any more for certain reasons in network management, e.g., the
          termination of the RDNSS or a renumbering situation.  That is, the
          RDNSS can resign from its DNS service because the machine running
          the RDNSS is out of service intentionally or unintentionally.
          Also, under the renumbering situation, the RDNSS's IPv6 address
          will be changed, so the previous RDNSS address should not be used
          any more.  The processing of this RDNSS address is finished here.
          Otherwise, go to Step (c).

    Jeong, et al.                Standards Track                   [Page 11]

    RFC 6106                   IPv6 RA DNS Options             November 2010

          Step (c): For each RDNSS address, if it already exists in the DNS
          Server List, then just update the value of the Expiration-time
          field according to the procedure specified in the third bullet of
          Section 6.1.  Otherwise, go to Step (d).

          Step (d): For each RDNSS address, if it does not exist in the DNS
          Server List, register the RDNSS address and Lifetime with the DNS
          Server List and then insert the RDNSS address in front of the
          Resolver Repository.  In the case where the data structure for the
          DNS Server List is full of RDNSS entries (that is, has more
          RDNSSes than the sufficient number discussed in Section 5.3.1),
          delete from the DNS Server List the entry with the shortest
          Expiration-time (i.e., the entry that will expire first).  The
          corresponding RDNSS address is also deleted from the Resolver
          Repository.  For the ordering of RDNSS addresses in an RDNSS
          option, position the first RDNSS address in the RDNSS option as
          the first one in the Resolver Repository, the second RDNSS address
          in the option as the second one in the repository, and so on.
          This ordering allows the RDNSS addresses in the RDNSS option to be
          preferred according to their order in the RDNSS option for the DNS
          name resolution.  The processing of these RDNSS addresses is
          finished here.

       The handling of expired RDNSSes is as follows: Whenever an entry
       expires in the DNS Server List, the expired entry is deleted from the
       DNS Server List, and also the RDNSS address corresponding to the
       entry is deleted from the Resolver Repository.

    6.3.  Synchronization between DNS Search List and Resolver Repository

       When an IPv6 host receives the information of multiple DNSSL domain
       names within a network (e.g., campus network and company network)
       through an RA message with DNSSL option(s), it stores the DNSSL
       domain names (in order) into both the DNS Search List and the
       Resolver Repository.  The processing of the DNSSL consists of (i) the
       processing of DNSSL option(s) included in an RA message and (ii) the
       handling of expired DNSSLs.  The processing of DNSSL option(s) is as
       follows:

          Step (a): Receive and parse the DNSSL option(s).  For the DNSSL
          domain names in each DNSSL option, perform Steps (b) through (d).

          Step (b): For each DNSSL domain name, check the following: If the
          DNSSL domain name already exists in the DNS Search List and the
          DNSSL option's Lifetime field is set to zero, delete the
          corresponding DNSSL entry from both the DNS Search List and the
          Resolver Repository in order to prevent the DNSSL domain name from
          being used any more for certain reasons in network management,

    Jeong, et al.                Standards Track                   [Page 12]

    RFC 6106                   IPv6 RA DNS Options             November 2010

          e.g., the termination of the RDNSS or a renaming situation.  That
          is, the RDNSS can resign from its DNS service because the machine
          running the RDNSS is out of service intentionally or
          unintentionally.  Also, under the renaming situation, the DNSSL
          domain names will be changed, so the previous domain names should
          not be used any more.  The processing of this DNSSL domain name is
          finished here.  Otherwise, go to Step (c).

          Step (c): For each DNSSL domain name, if it already exists in the
          DNS Server List, then just update the value of the Expiration-time
          field according to the procedure specified in the third bullet of
          Section 6.1.  Otherwise, go to Step (d).

          Step (d): For each DNSSL domain name, if it does not exist in the
          DNS Search List, register the DNSSL domain name and Lifetime with
          the DNS Search List and then insert the DNSSL domain name in front
          of the Resolver Repository.  In the case where the data structure
          for the DNS Search List is full of DNSSL domain name entries (that
          is, has more DNSSL domain names than the sufficient number
          discussed in Section 5.3.1), delete from the DNS Server List the
          entry with the shortest Expiration-time (i.e., the entry that will
          expire first).  The corresponding DNSSL domain name is also
          deleted from the Resolver Repository.  For the ordering of DNSSL
          domain names in a DNSSL option, position the first DNSSL domain
          name in the DNSSL option as the first one in the Resolver
          Repository, the second DNSSL domain name in the option as the
          second one in the repository, and so on.  This ordering allows the
          DNSSL domain names in the DNSSL option to be preferred according
          to their order in the DNSSL option for the DNS domain name used by
          the DNS query.  The processing of these DNSSL domain name is
          finished here.

          The handling of expired DNSSLs is as follows: Whenever an entry
          expires in the DNS Search List, the expired entry is deleted from
          the DNS Search List, and also the DNSSL domain name corresponding
          to the entry is deleted from the Resolver Repository.

    7.  Security Considerations

       In this section, we analyze security threats related to DNS options
       and then suggest recommendations to cope with such security threats.

    7.1.  Security Threats

       For the RDNSS option, an attacker could send an RA with a fraudulent
       RDNSS address, misleading IPv6 hosts into contacting an unintended
       DNS server for DNS name resolution.  Also, for the DNSSL option, an

    Jeong, et al.                Standards Track                   [Page 13]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       attacker can let IPv6 hosts resolve a host name without a DNS suffix
       into an unintended host's IP address with a fraudulent DNS Search
       List.

       These attacks are similar to Neighbor Discovery attacks that use
       Redirect or Neighbor Advertisement messages to redirect traffic to
       individual addresses of malicious parties.  That is, as a rogue
       router, a malicious node on a LAN can promiscuously receive packets
       for any router's Media Access Control (MAC) address and send packets
       with the router's MAC address as the source MAC address in the Layer
       2 (L2) header.  As a result, L2 switches send packets addressed to
       the router to the malicious node.  Also, this attack can send
       redirects that tell the hosts to send their traffic somewhere else.
       The malicious node can send unsolicited RA or Neighbor Advertisement
       (NA) replies, answer RS or Neighbor Solicitation (NS) requests, etc.
       Thus, the attacks related to RDNSS and DNSSL are similar to both
       Neighbor Discovery attacks and attacks against unauthenticated DHCP,
       as both can be used for both "wholesale" traffic redirection and more
       specific attacks.

       However, the security of these RA options for DNS configuration does
       not affect ND protocol security [RFC4861].  This is because learning
       DNS information via the RA options cannot be worse than learning bad
       router information via the RA options.  Therefore, 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.

    7.2.  Recommendations

       The Secure Neighbor Discovery (SEND) protocol [RFC3971] is used as a
       security mechanism for ND.  It is RECOMMENDED that ND use SEND to
       allow all the ND options including the RDNSS and DNSSL options to be
       automatically included in the signatures.  Through SEND, the
       transport for the RA options is integrity protected; that is, SEND
       can prevent the spoofing of these DNS options with signatures.  Also,
       SEND enables an IPv6 host to verify that the sender of an RA is
       actually a router authorized to act as a router.  However, since any
       valid SEND router can still insert RDNSS and DNSSL options, the
       current SEND cannot verify which one is or is not authorized to send
       the options.  Thus, this verification of the authorized routers for
       ND options will be required.  [CSI-SEND-CERT] specifies the usage of
       extended key for the certificate deployed in SEND.  This document
       defines the roles of routers (i.e., routers acting as proxy and
       address owner) and explains the authorization of the roles.  The
       mechanism in this document can be extended to verify which routers
       are authorized to insert RDNSS and DNSSL options.

    Jeong, et al.                Standards Track                   [Page 14]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       It is common for network devices such as switches to include
       mechanisms to block unauthorized ports from running a DHCPv6 server
       to provide protection from rogue DHCP servers.  That means that an
       attacker on other ports cannot insert bogus DNS servers using DHCPv6.
       The corresponding technique for network devices is RECOMMENDED to
       block rogue Router Advertisement messages including the RDNSS and
       DNSSL options from unauthorized nodes.

       An attacker may provide a bogus DNS Search List option in order to
       cause the victim to send DNS queries to a specific DNS server when
       the victim queries non-FQDNs (fully qualified domain names).  For
       this attack, the DNS resolver in IPv6 hosts can mitigate the
       vulnerability with the recommendations mentioned in [RFC1535],
       [RFC1536], and [RFC3646].

    8.  IANA Considerations

       The RDNSS option defined in this document uses the IPv6 Neighbor
       Discovery Option type defined in RFC 5006 [RFC5006], which was
       assigned by the IANA as follows:

                     Option Name                   Type
                     Recursive DNS Server Option   25

       The IANA has assigned a new IPv6 Neighbor Discovery Option type for
       the DNSSL option defined in this document:

                     Option Name                   Type
                     DNS Search List Option        31

       These options have been registered in the "Internet Control Message
       Protocol version 6 (ICMPv6) Parameters" registry
       (http://www.iana.org).

    9.  Acknowledgements

       This document has greatly benefited from inputs by Robert Hinden,
       Pekka Savola, Iljitsch van Beijnum, Brian Haberman, Tim Chown, Erik
       Nordmark, Dan Wing, Jari Arkko, Ben Campbell, Vincent Roca, and Tony
       Cheneau.  The authors sincerely appreciate their contributions.

    Jeong, et al.                Standards Track                   [Page 15]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    10.  References

    10.1.  Normative References

       [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                        Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC4861]        Narten, T., Nordmark, E., Simpson, W., and H.
                        Soliman, "Neighbor Discovery for IP version 6
                        (IPv6)", RFC 4861, September 2007.

       [RFC4862]        Thomson, S., Narten, T., and T. Jinmei, "IPv6
                        Stateless Address Autoconfiguration", RFC 4862,
                        September 2007.

       [RFC1035]        Mockapetris, P., "Domain names - implementation and
                        specification", STD 13, RFC 1035, November 1987.

    10.2.  Informative References

       [RFC1034]        Mockapetris, P., "Domain names - concepts and
                        facilities", STD 13, RFC 1034, November 1987.

       [RFC3315]        Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                        C., and M. Carney, "Dynamic Host Configuration
                        Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

       [RFC3736]        Droms, R., "Stateless Dynamic Host Configuration
                        Protocol (DHCP) Service for IPv6", RFC 3736,
                        April 2004.

       [RFC3646]        Droms, R., "DNS Configuration options for Dynamic
                        Host Configuration Protocol for IPv6 (DHCPv6)",
                        RFC 3646, December 2003.

       [RFC5006]        Jeong, J., Park, S., Beloeil, L., and S.
                        Madanapalli, "IPv6 Router Advertisement Option for
                        DNS Configuration", RFC 5006, September 2007.

       [RFC4339]        Jeong, J., "IPv6 Host Configuration of DNS Server
                        Information Approaches", RFC 4339, February 2006.

       [RFC3971]        Arkko, J., Kempf, J., Zill, B., and P. Nikander,
                        "SEcure Neighbor Discovery (SEND)", RFC 3971,
                        March 2005.

    Jeong, et al.                Standards Track                   [Page 16]

    RFC 6106                   IPv6 RA DNS Options             November 2010

       [RFC5358]        Damas, J. and F. Neves, "Preventing Use of Recursive
                        Nameservers in Reflector Attacks", BCP 140,
                        RFC 5358, October 2008.

       [RFC2827]        Ferguson, P. and D. Senie, "Network Ingress
                        Filtering: Defeating Denial of Service Attacks which
                        employ IP Source Address Spoofing", BCP 38,
                        RFC 2827, May 2000.

       [RFC1535]        Gavron, E., "A Security Problem and Proposed
                        Correction With Widely Deployed DNS Software",
                        RFC 1535, October 1993.

       [RFC1536]        Kumar, A., Postel, J., Neuman, C., Danzig, P., and
                        S. Miller, "Common DNS Implementation Errors and
                        Suggested Fixes", RFC 1536, October 1993.

       [MIF-PROBLEM]    Blanchet, M. and P. Seite, "Multiple Interfaces
                        Problem Statement", Work in Progress, August 2010.

       [MIF-PRACTICE]   Wasserman, M. and P. Seite, "Current Practices for
                        Multiple Interface Hosts", Work in Progress,
                        August 2010.

       [CSI-SEND-CERT]  Gagliano, R., Krishnan, S., and A. Kukec,
                        "Certificate profile and certificate management for
                        SEND", Work in Progress, October 2010.

    Jeong, et al.                Standards Track                   [Page 17]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    Appendix A.  Changes from RFC 5006

       The following changes were made from RFC 5006 "IPv6 Router
       Advertisement Option for DNS Configuration":

       o  Added the DNS Search List (DNSSL) option to support the
          advertisement of DNS suffixes used in the DNS search along with
          RDNSS option in RFC 5006.

       o  Clarified the coexistence of RA options and DHCP options for DNS
          configuration.

       o  Modified the procedure in IPv6 host:

          *  Clarified the procedure for DNS options in an IPv6 host.

          *  Specified a sufficient number of RDNSS addresses or DNS search
             domain names as three.

          *  Specified a way to deal with DNS options from multiple sources,
             such as RA and DHCP.

       o  Modified the implementation considerations for DNSSL option
          handling.

       o  Modified the security considerations to consider more attack
          scenarios and the corresponding possible solutions.

       o  Modified the IANA considerations to require another IPv6 Neighbor
          Discovery Option type for the DNSSL option.

    Jeong, et al.                Standards Track                   [Page 18]

    RFC 6106                   IPv6 RA DNS Options             November 2010

    Authors' Addresses

       Jaehoon Paul Jeong
       Brocade Communications Systems/ETRI
       6000 Nathan Ln N
       Plymouth, MN  55442
       USA

       Phone: +1 763 268 7173
       Fax:   +1 763 268 6800
       EMail: pjeong@brocade.com
       URI:   http://www.cs.umn.edu/~jjeong/

       Soohong Daniel Park
       Digital Media & Communications R&D Center
       SAMSUNG Electronics
       416 Maetan-3dong, Yeongtong-Gu
       Suwon, Gyeonggi-Do  443-742
       Korea

       Phone: +82 31 279 8876
       EMail: soohong.park@samsung.com

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

       Phone: +33 2 40 44 97 40
       EMail: luc.beloeil@orange-ftgroup.com

       Syam Madanapalli
       iRam Technologies
       #H304, Shriram Samruddhi, Thubarahalli
       Bangalore - 560066
       India

       EMail: smadanapalli@gmail.com

    Jeong, et al.                Standards Track                   [Page 19]