    Internet Engineering Task Force (IETF)                          J. Jeong
    Request for Comments: 8106                       Sungkyunkwan University
    Obsoletes: 6106                                                  S. Park
    Category: Standards Track                            Samsung Electronics
    ISSN: 2070-1721                                               L. Beloeil
                                                                      Orange
                                                              S. Madanapalli
                                                                    NTT Data
                                                                  March 2017

            IPv6 Router Advertisement Options for DNS Configuration

    Abstract

       This document specifies IPv6 Router Advertisement (RA) options
       (called "DNS RA options") to allow IPv6 routers to advertise a list
       of DNS Recursive Server Addresses and a DNS Search List to IPv6
       hosts.

       This document, which obsoletes RFC 6106, defines a higher default
       value of the lifetime of the DNS RA options to reduce the likelihood
       of expiry of the options on links with a relatively high rate of
       packet loss.

    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 7841.

       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/rfc8106.

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    Copyright Notice

       Copyright (c) 2017 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.

    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. DNS Configuration Procedure ................................8
               5.3.1. Procedure in IPv6 Hosts .............................9
               5.3.2. Warnings for DNS Options Configuration ..............9
       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 ........................................12
          7.1. Security Threats ..........................................12
          7.2. Recommendations ...........................................13
       8. IANA Considerations ............................................13
       9. References .....................................................14
          9.1. Normative References ......................................14
          9.2. Informative References ....................................14
       Appendix A. Changes from RFC 6106 .................................17
       Acknowledgements ..................................................18
       Authors' Addresses ................................................19

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    1.  Introduction

       The purpose of this document is to standardize IPv6 Router
       Advertisement (RA) options (DNS RA options) for DNS Recursive Server
       Addresses used for DNS name resolution in IPv6 hosts, and also for a
       DNS Search List (DNSSL) of domain suffixes.

       IPv6 Neighbor Discovery (ND) and IPv6 Stateless Address
       Autoconfiguration (SLAAC) provide ways to configure either fixed or
       mobile nodes with one or more IPv6 addresses, default routers, and
       some other parameters [RFC4861] [RFC4862].

       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
       namespaces would not be available to the host if it were to run its
       own recursive name server directly connected to the global DNS.

       The DNS information can also be provided through DHCPv6 [RFC3315]
       [RFC3736] [RFC3646].  However, access to DNS is a fundamental
       requirement for almost all hosts, so IPv6 SLAAC 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 DNS RA options to allow IPv6
       hosts to perform 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 SLAAC and where
       either (i) there is no DHCPv6 infrastructure at all or (ii) 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, for networks that need to distribute
       additional information, DHCPv6 is likely to be employed.  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 the DNSSL)
       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.

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       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
       RA options specified in this document and the DHCPv6 options
       specified 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 RA messages, unless static DNS
       configuration has been specified by the user.  If there is DNS
       information available from multiple RAs 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 defined in [RFC4861] and
       [RFC4862].  In addition, six new terms are defined below:

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

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

       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: An 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 the
          Neighbor Cache and Destination Cache for Neighbor Discovery

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          [RFC4861].  The first data structure is the DNS Server List for
          RDNSS addresses, and the second is the DNSSL for DNS search domain
          names.

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

    4.  Overview

       This document standardizes an ND option called the "RDNSS option",
       which contains the addresses of RDNSSes.  This document also
       standardizes an ND option called the "DNSSL option", which contains
       the DNSSL.  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., RA) 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]
       [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 networks that support the use of ND.

       This approach requires manual configuration or 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 DNSSL in routers
       is out of scope for this document.

    5.  Neighbor Discovery Extension

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

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    5.1.  Recursive DNS Server Option

       The RDNSS option contains one or more IPv6 addresses of RDNSSes.  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: RDNSS Option Format

       Fields:

       Type        8-bit identifier of the RDNSS option type as assigned by
                   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 received) over which
                   these RDNSS addresses MAY be used for name resolution.
                   The value of Lifetime SHOULD by default be at least
                   3 * MaxRtrAdvInterval, where MaxRtrAdvInterval is the
                   maximum RA interval as defined in [RFC4861].  A value of
                   all one bits (0xffffffff) represents infinity.  A value
                   of zero means that the RDNSS addresses MUST no longer
                   be used.

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

       Note: The addresses for RDNSSes in the RDNSS option MAY be link-local
             addresses.  Such link-local addresses SHOULD be registered in
             the Resolver Repository along with the corresponding link zone
             indices of the links that receive the RDNSS option(s) for them.
             The link-local addresses MAY be represented in the Resolver
             Repository with their link zone indices in the textual format
             for scoped addresses as described in [RFC4007].  When a
             resolver sends a DNS query message to an RDNSS identified by a
             link-local address, it MUST use the corresponding link.

             The rationale of the default value of the Lifetime field is as
             follows.  The Router Lifetime field, set by AdvDefaultLifetime,
             has the default of 3 * MaxRtrAdvInterval as specified in
             [RFC4861], so such a default or a larger default can allow for
             the reliability of DNS options even under the loss of RAs on
             links with a relatively high rate of packet loss.  Note that
             the ratio of AdvDefaultLifetime to MaxRtrAdvInterval is the
             number of unsolicited multicast RAs sent by the router.  Since
             the DNS option entries can survive for at most three
             consecutive losses of RAs containing DNS options, the default
             value of the Lifetime lets the DNS option entries be resilient
             to packet-loss environments.

    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                :
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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                           Figure 2: DNSSL Option Format

       Fields:

       Type        8-bit identifier of the DNSSL option type as assigned by
                   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 "Domain Names of DNS Search List" field.

       Lifetime    32-bit unsigned integer.  The maximum time in seconds
                   (relative to the time the packet is received) over which
                   these DNSSL domain names MAY be used for name resolution.
                   The Lifetime value has the same semantics as the
                   semantics for the RDNSS option.  That is, Lifetime SHOULD
                   by default be at least 3 * MaxRtrAdvInterval.  A value of
                   all one bits (0xffffffff) represents infinity.  A value
                   of zero means that the DNSSL domain names MUST no longer
                   be used.

       Domain Names of DNS Search List
                   One or more domain names of the DNSSL that MUST be
                   encoded as described in Section 3.1 of [RFC1035].  With
                   this technique, each domain name is represented as a
                   sequence of labels ending in a zero octet, defined as a
                   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 the
                   compressed form 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.

    5.3.  DNS Configuration Procedure

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

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    5.3.1.  Procedure in IPv6 Hosts

       When an IPv6 host receives DNS options (i.e., RDNSS and DNSSL
       options) 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 satisfies the
          requirement that (Length - 1) % 2 == 0.  The value of the Length
          field in the DNSSL option is greater than or equal to the minimum
          value (2).  Also, the validity of the RDNSS option is checked with
          the "Addresses of IPv6 Recursive DNS Servers" field; that is, the
          addresses should be unicast addresses.

       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.

       In the case where the DNS information of RDNSS and DNSSL can be
       obtained from multiple sources, such as RAs 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 as a local configuration option.  However, in
       the case of multiple sources, the ability to store a total of at
       least three RDNSS addresses (or DNSSL domain names) from the multiple
       sources is RECOMMENDED.  The DNS options from RAs and DHCP SHOULD be
       stored in 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 RAs for DNS queries.  On the other hand, for DNS
       options announced by RAs, 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.  Also, DNS options
       announced by RAs via SEND MUST be preferred over those announced by
       unauthenticated DHCP [RFC3118].  Refer to Section 7 for a detailed
       discussion of SEND for DNS RA options.

    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., RAs 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

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       information is provided by different mechanisms for autoconfiguration
       may lead to problems.  Therefore, the network administrator needs to
       carefully configure different DNS options in the multiple mechanisms
       for autoconfiguration in order to minimize the impact of such
       problems [DHCPv6-SLAAC].

       Second, if different DNS information is provided on different network
       interfaces, this can lead to inconsistent behavior.  The IETF worked
       on solving this problem for both DNS and other information obtained
       from multiple interfaces [RFC6418] [RFC6419] and standardized a
       DHCP-based solution for RDNSS selection for multi-interfaced nodes as
       described in [RFC6731].

    6.  Implementation Considerations

       The implementation considerations in this document include the
       following three: (i) DNS repository management, (ii) synchronization
       between the DNS Server List and the Resolver Repository, and
       (iii) synchronization between the DNSSL and the Resolver Repository.

       Note: The implementations that are updated according to this document
             will still interoperate with the existing implementations
             according to [RFC6106].  This is because the main change in
             this document is the increase of the default Lifetime of DNS
             options, considering lossy links.

    6.1.  DNS Repository Management

       For DNS repository management, the following two data structures
       SHOULD be synchronized with the Resolver Repository: (i) the DNS
       Server List, which keeps the list of RDNSS addresses and (ii) the
       DNSSL, which 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:

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

       o  DNSSL domain name for DNSSL: DNS suffix domain name that is used
          to perform DNS query searches for short, unqualified domain names.

       o  Expiration-time for DNS Server List or DNSSL: 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 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

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          field is updated to have a new Expiration-time.  When the current
          time becomes larger than Expiration-time, this entry is regarded
          as expired, so it should not be used any more.  Note that the DNS
          information for the RDNSS and DNSSL options need not be dropped if
          the expiry of the RA router lifetime happens.  This is because
          these options have their own lifetime values.

    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) in 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:

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

       o  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 scenario.  That is, the
          RDNSS can resign from its DNS service because the machine running
          the RDNSS is out of service intentionally or unintentionally.
          Also, in the renumbering scenario, 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).

       o  Step (c): For each RDNSS address, if it already exists in the DNS
          Server List and the RDNSS option's Lifetime field is not set to
          zero, 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).

       o  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 as the first one in
          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

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          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 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 through an RA message with DNSSL option(s), it
       stores the DNSSL domain names (in order) in both the DNSSL 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 the
       same as the processing of RDNSS option(s) as described in
       Section 6.2.

    7.  Security Considerations

       In this section, we analyze security threats related to DNS options
       and then make 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
       attacker can let IPv6 hosts resolve a hostname without a DNS suffix
       into an unintended host's IP address with a fraudulent DNSSL.  These
       attacks are similar to ND attacks specified in [RFC4861] that use
       Redirect or Neighbor Advertisement messages to redirect traffic to
       individual addresses of malicious parties.

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       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.

    7.2.  Recommendations

       The Secure Neighbor Discovery (SEND) protocol [RFC3971] is designed
       as a security mechanism for ND.  In this case, ND can use SEND to
       allow all the ND options, including the RDNSS and DNSSL options, to
       be automatically signed with digital signatures.

       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 DHCPv6 servers [RFC7610].  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 RA messages that include the RDNSS and
       DNSSL options from unauthorized nodes [RFC6104] [RFC6105].

       An attacker may provide a bogus DNSSL 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 assigned by IANA as follows:

          Option Name                    Type
          -----------------------------------
          Recursive DNS Server Option    25

       The DNSSL option defined in this document uses the IPv6 Neighbor
       Discovery Option type assigned by IANA as follows:

          Option Name                    Type
          -----------------------------------
          DNS Search List Option         31

       These options are registered in the "IPv6 Neighbor Discovery Option
       Formats" registry [ICMPv6].

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    9.  References

    9.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                  "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                  DOI 10.17487/RFC4861, September 2007,
                  <http://www.rfc-editor.org/info/rfc4861>.

       [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                  Address Autoconfiguration", RFC 4862,
                  DOI 10.17487/RFC4862, September 2007,
                  <http://www.rfc-editor.org/info/rfc4862>.

       [RFC1035]  Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
                  November 1987, <http://www.rfc-editor.org/info/rfc1035>.

       [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
                  B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
                  DOI 10.17487/RFC4007, March 2005,
                  <http://www.rfc-editor.org/info/rfc4007>.

    9.2.  Informative References

       [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
                  STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
                  <http://www.rfc-editor.org/info/rfc1034>.

       [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
                  C., and M. Carney, "Dynamic Host Configuration Protocol
                  for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315,
                  July 2003, <http://www.rfc-editor.org/info/rfc3315>.

       [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
                  (DHCP) Service for IPv6", RFC 3736, DOI 10.17487/RFC3736,
                  April 2004, <http://www.rfc-editor.org/info/rfc3736>.

       [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
                  Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
                  DOI 10.17487/RFC3646, December 2003,
                  <http://www.rfc-editor.org/info/rfc3646>.

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       [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
                  "IPv6 Router Advertisement Options for DNS Configuration",
                  RFC 6106, DOI 10.17487/RFC6106, November 2010,
                  <http://www.rfc-editor.org/info/rfc6106>.

       [RFC4339]  Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
                  Information Approaches", RFC 4339, DOI 10.17487/RFC4339,
                  February 2006, <http://www.rfc-editor.org/info/rfc4339>.

       [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
                  "SEcure Neighbor Discovery (SEND)", RFC 3971,
                  DOI 10.17487/RFC3971, March 2005,
                  <http://www.rfc-editor.org/info/rfc3971>.

       [RFC3118]  Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
                  DHCP Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001,
                  <http://www.rfc-editor.org/info/rfc3118>.

       [RFC6104]  Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
                  Problem Statement", RFC 6104, DOI 10.17487/RFC6104,
                  February 2011, <http://www.rfc-editor.org/info/rfc6104>.

       [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
                  Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
                  DOI 10.17487/RFC6105, February 2011,
                  <http://www.rfc-editor.org/info/rfc6105>.

       [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
                  Protecting against Rogue DHCPv6 Servers", BCP 199,
                  RFC 7610, DOI 10.17487/RFC7610, August 2015,
                  <http://www.rfc-editor.org/info/rfc7610>.

       [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction
                  With Widely Deployed DNS Software", RFC 1535,
                  DOI 10.17487/RFC1535, October 1993,
                  <http://www.rfc-editor.org/info/rfc1535>.

       [RFC1536]  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
                  Miller, "Common DNS Implementation Errors and Suggested
                  Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
                  <http://www.rfc-editor.org/info/rfc1536>.

       [DHCPv6-SLAAC]
                  Liu, B., Jiang, S., Gong, X., Wang, W., and E. Rey,
                  "DHCPv6/SLAAC Interaction Problems on Address and
                  DNS Configuration", Work in Progress,
                  draft-ietf-v6ops-dhcpv6-slaac-problem-07, August 2016.

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       [RFC6418]  Blanchet, M. and P. Seite, "Multiple Interfaces and
                  Provisioning Domains Problem Statement", RFC 6418,
                  DOI 10.17487/RFC6418, November 2011,
                  <http://www.rfc-editor.org/info/rfc6418>.

       [RFC6419]  Wasserman, M. and P. Seite, "Current Practices for
                  Multiple-Interface Hosts", RFC 6419, DOI 10.17487/RFC6419,
                  November 2011, <http://www.rfc-editor.org/info/rfc6419>.

       [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
                  Recursive DNS Server Selection for Multi-Interfaced
                  Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012,
                  <http://www.rfc-editor.org/info/rfc6731>.

       [ICMPv6]   IANA, "Internet Control Message Protocol version 6
                  (ICMPv6) Parameters",
                  <http://www.iana.org/assignments/icmpv6-parameters/>.

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    Appendix A.  Changes from RFC 6106

       The following changes were made from RFC 6106 ("IPv6 Router
       Advertisement Options for DNS Configuration"):

       o  This document allows a higher default value of the lifetime of the
          DNS RA options than RFC 6106 in order to avoid the frequent expiry
          of the options on links with a relatively high rate of packet
          loss; at the same time, this document also makes additional
          clarifications.  The lifetime's lower bound of
          2 * MaxRtrAdvInterval was shown to lead to the expiry of these
          options on links with a relatively high rate of packet loss.  To
          avoid this problem, this revision relaxes the lower bound and sets
          a higher default value of 3 * MaxRtrAdvInterval.

       o  The text regarding the generation of a Router Solicitation message
          to ensure that the RDNSS information is fresh before the expiry of
          the RDNSS option is removed in order to prevent multicast traffic
          on the link from increasing.

       o  The addresses for RDNSSes in the RDNSS option can be not only
          global addresses but also link-local addresses.  The link-local
          addresses for RDNSSes should be registered in the Resolver
          Repository along with the corresponding link zone indices.

       o  RFC 6106 recommended that the number of RDNSS addresses that
          should be learned and maintained through the RDNSS RA option
          should be limited to three.  This document removes that
          recommendation; thus, the number of RDNSS addresses to maintain is
          determined by an implementer's local policy.

       o  RFC 6106 recommended that the number of DNS search domains that
          should be learned and maintained through the DNSSL RA option
          should be limited to three.  This document removes that
          recommendation; thus, when the set of unique DNSSL values are not
          equivalent, none of them may be ignored for hostname lookups
          according to an implementer's local policy.

       o  The guidance of the specific implementation for the
          synchronization of the DNS Repository and Resolver Repository in
          the kernel space and user space is removed.

       o  The key words "SHOULD" and "RECOMMENDED" (RFC 2119) are removed in
          the recommendation of using SEND as a security mechanism for ND.
          Instead of using these key words, SEND is specified as only a
          possible security mechanism for ND.

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    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, Tony
       Cheneau, Fernando Gont, Jen Linkova, Ole Troan, Mark Smith, Tatuya
       Jinmei, Lorenzo Colitti, Tore Anderson, David Farmer, Bing Liu, and
       Tassos Chatzithomaoglou.  The authors sincerely appreciate their
       contributions.

       This document was supported by an Institute for Information &
       communications Technology Promotion (IITP) grant funded by the Korean
       government (MSIP) [10041244, Smart TV 2.0 Software Platform].

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    Authors' Addresses

       Jaehoon Paul Jeong
       Department of Software
       Sungkyunkwan University
       2066 Seobu-Ro, Jangan-Gu
       Suwon, Gyeonggi-Do  16419
       Republic of Korea

       Phone: +82 31 299 4957
       Fax:   +82 31 290 7996
       Email: pauljeong@skku.edu
       URI:   http://iotlab.skku.edu/people-jaehoon-jeong.php

       Soohong Daniel Park
       Software R&D Center
       Samsung Electronics
       Seoul R&D Campus D-Tower, 56, Seongchon-Gil, Seocho-Gu
       Seoul  06765
       Republic of Korea

       Email: soohong.park@samsung.com

       Luc Beloeil
       Orange
       5 rue Maurice Sibille
       BP 44211
       44042 Nantes Cedex 1
       France

       Phone: +33 2 28 56 11 84
       Email: luc.beloeil@orange.com

       Syam Madanapalli
       NTT Data
       #H304, Shriram Samruddhi, Thubarahalli
       Bangalore  560066
       India

       Phone: +91 959 175 7926
       Email: smadanapalli@gmail.com

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