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rfc:rfc6106

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]

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