Network Working Group                                           J. Jeong
Internet-Draft                                   Sungkyunkwan University
Obsoletes: 6106 (if approved)                                    S. Park
Intended status: Standards Track                 Korean Bible University
Expires: September 7, 2016                                    L. Beloeil
                                                      France Telecom R&D
                                                          S. Madanapalli
                                                       iRam Technologies
                                                           March 6, 2016

        IPv6 Router Advertisement Options for DNS Configuration
                  draft-ietf-6man-rdnss-rfc6106bis-07
                  draft-ietf-6man-rdnss-rfc6106bis-08

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.

   This document obsoletes RFC 6106 and allows a higher default value of
   the lifetime of the RA DNS options to avoid the frequent expiry of
   the options on links with a relatively high rate of packet loss.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 7, 2016.

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   document authors.  All rights reserved.

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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 . . . . . . . .  9
   6.  Implementation Considerations  . . . . . . . . . . . . . . . .  9
     6.1.  DNS Repository Management  . . . . . . . . . . . . . . . . 10
     6.2.  Synchronization between DNS Server List and Resolver
           Repository . . . . . . . . . . . . . . . . . . . . . . . . 10 11
     6.3.  Synchronization between DNS Search List and Resolver
           Repository . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     7.1.  Security Threats . . . . . . . . . . . . . . . . . . . . . 12
     7.2.  Recommendations  . . . . . . . . . . . . . . . . . . . . . 12 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Changes from RFC 5006 . . . . . . . . . . . . . . . . 15
   Appendix B.  Changes from RFC 6106 . . . . . . . . . . . . . . . . 16

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 originally
   specified in an earlier Experimental specification [RFC5006]. [RFC5006] and was
   later published as a Standards Track in [RFC6106].  This document
   obsoletes [RFC6106], allowing a higher default value of the lifetime
   of the RA DNS options to avoid the frequent expiry of the options on
   links with a relatively high rate of packet loss, and also making
   additional clarifications, see Appendix B for details.

   Neighbor Discovery (ND) for IP version 6 and IPv6 stateless address
   autoconfiguration 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].  Most Internet services names 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. names.

   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
   recursive name server software directly connected to the global DNS.

   The DNS information can also be provided through DHCP DHCPv6 [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, 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 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.

   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 specified in this document and the
   DHCPv6 options described 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 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 defined 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
      or resolving PTR records, as defined in [RFC1034] and [RFC1035].

   o  RDNSS Option: 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: 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 [RFC6106] that contains the addresses of recursive DNS
   servers.  This document also standardizes the ND option called the
   DNSSL option defined in [RFC6106] that contains 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 the 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 ND
   options in Neighbor Discovery: (i) the Recursive DNS Server (RDNSS)
   option and (ii) the DNS Search List (DNSSL) option.

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 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 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.

     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.

   Note:  The addresses for recursive DNS servers in the RDNSS option
      MAY be link-local addresses.  Such link-local addresses SHOULD be
      registered into 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 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 with a link-local address, it MUST
      use the corresponding link.

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 received)
                   over which these DNSSL domain names MAY be used for
                   name resolution.  The Lifetime value has the same
                   semantics as with 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 DNS Search List that MUST
                   be encoded as 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.

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 satisfies 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 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. 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 at least two different the multiple 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.

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 different DNS options in
   the multiple sources in order to prevent minimize the impact of such problems from happening.
   [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
   by multiple interfaces [RFC6418][RFC6419], and standardized the
   solution for RDNSS selection for multi-interfaced nodes in [RFC6731],
   which is based on DHCP.

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.  In these
   environments, a user space application cannot receive RA via an
   ICMPv6 socket using the standard advanced socket application program
   interface Application Program
   Interface (API) in [RFC3542].  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:

   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 for the RDNSS address, which is
      advertised by the same RA message having the DNSSL option, 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. Expiration-time.  When Expiration-time becomes less than the
      current system
      time, time becomes larger than Expiration-time, this entry is
      regarded as expired.  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) 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).

      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 the
   same with that of RDNSS option(s) in Section 6.2 except Step (b).

   In Step (b), 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,
   e.g., the termination of the usage of the DNSSL domain name.  That
   is, the DNSSL domain name may not be used any more by the policy of
   the network.

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
   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 ND attacks specified in [RFC4861]
   that use Redirect or Neighbor Advertisement messages to redirect
   traffic to individual addresses of malicious parties.

   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. do.

7.2.  Recommendations

   The Secure Neighbor Discovery (SEND) protocol [RFC3971] MAY be used
   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 included in the signatures.  Other approaches specified
   in [RFC4861] can be used for securing the RA options for DNS
   configuration.

   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 Router Advertisement messages [RFC6104]
   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 6106 [RFC6106], which was
   assigned by the 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 defined in RFC 6106 [RFC6106], which was
   assigned by the IANA as follows:

                 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/assignments/icmpv6-parameters/
   icmpv6-parameters.xhtml#icmpv6-parameters-5).

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

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.

   [RFC4007]       Deering, S., Haberman, B., Jinmei, T., Nordmark, E.,
                   and B. Zill, "IPv6 Scoped Address Architecture",
                   RFC 4007, March 2005.

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.

   [RFC6106]       Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
                   "IPv6 Router Advertisement Options for DNS
                   Configuration", RFC 6106, November 2010.

   [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.

   [RFC6104]       Chown, T. and S. Venaas, "Rogue IPv6 Router
                   Advertisement Problem Statement", RFC 6104,
                   February 2011.

   [RFC7610]       Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield: "DHCPv6-
                   Shield: Protecting against Rogue DHCPv6 Servers",
                   RFC 7610, August 2015.

   [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.

   [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, February 2016.

   [RFC6418]       Blanchet, M. and P. Seite, "Multiple Interfaces and
                   Provisioning Domains Problem Statement", RFC 6418,
                   November 2011.

   [RFC6419]       Wasserman, M. and P. Seite, "Current Practices for
                   Multiple-Interface Hosts", RFC 6419, November 2011.

   [RFC6731]       Savolainen, T., Kato, J., and T. Lemon, "Improved
                   Recursive DNS Server Selection for Multi-Interfaced
                   Nodes", RFC 6731, December 2012.

   [RFC3542]       Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
                   "Advanced Sockets Application Program Interface (API)
                   for IPv6", RFC 3542, May 2003.

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.

Appendix B.  Changes from RFC 6106

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

   o  The generation of Router Solicitation 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 lifetime's upper bound of 2 * MaxRtrAdvInterval was shown to
      lead to the expiry of these options on links with a relatively
      high rate of packet loss.  This revision relaxes the upper bound
      and sets a higher default value to avoid this problem.

   o  The lifetimes of RDNSS and DNSSL options are decoupled from Router
      Lifetime.  An RA router lifetime of zero does not cause the RDNSS
      and DNSSL options to be considered invalid because these options
      have their own lifetime values.  Thus, due to the expiry of the RA
      router lifetime, the lists in the RDNSS and DNSSL options are not
      guaranteed to be reachable at any point in time.

   o  The addresses for recursive DNS servers in the RDNSS option can be
      not only global addresses, but also link-local addresses.  The
      link-local addresses for RDNSSes should be registered into the
      resolver repository along with the corresponding link zone
      indices.

   o  The recommendation that at most three RDNSS addresses to maintain
      by RDNSS options should be limited is removed.  By this removal,
      the number of RDNSSes to maintain is up to an implementer's local
      policy.

   o  The recommendation that at most three DNS domains to maintain by
      DNSSL options should be limited is removed.  By this removal, when
      the set of unique DNSSL values are not equivalent, none of them
      are ignored for hostname lookups.

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
   Department of Computer Software
   Korean Bible University
   205 SangGye7-Dong, Nowon-Gu
   Seoul  01757
   Republic of Korea

   Phone: +82 2 950 5494
   EMail: daniel@bible.ac.kr

   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