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rfc:rfc8106

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.

Jeong, et al. Standards Track [Page 1] RFC 8106 IPv6 DNS RA Options March 2017

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

Jeong, et al. Standards Track [Page 2] RFC 8106 IPv6 DNS RA Options March 2017

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.

Jeong, et al. Standards Track [Page 3] RFC 8106 IPv6 DNS RA Options March 2017

 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

Jeong, et al. Standards Track [Page 4] RFC 8106 IPv6 DNS RA Options March 2017

    [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.

Jeong, et al. Standards Track [Page 5] RFC 8106 IPv6 DNS RA Options March 2017

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.

Jeong, et al. Standards Track [Page 6] RFC 8106 IPv6 DNS RA Options March 2017

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

Jeong, et al. Standards Track [Page 7] RFC 8106 IPv6 DNS RA Options March 2017

                     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].

Jeong, et al. Standards Track [Page 8] RFC 8106 IPv6 DNS RA Options March 2017

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

Jeong, et al. Standards Track [Page 9] RFC 8106 IPv6 DNS RA Options March 2017

 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

Jeong, et al. Standards Track [Page 10] RFC 8106 IPv6 DNS RA Options March 2017

    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

Jeong, et al. Standards Track [Page 11] RFC 8106 IPv6 DNS RA Options March 2017

    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.

Jeong, et al. Standards Track [Page 12] RFC 8106 IPv6 DNS RA Options March 2017

 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].

Jeong, et al. Standards Track [Page 13] RFC 8106 IPv6 DNS RA Options March 2017

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>.

Jeong, et al. Standards Track [Page 14] RFC 8106 IPv6 DNS RA Options March 2017

 [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.

Jeong, et al. Standards Track [Page 15] RFC 8106 IPv6 DNS RA Options March 2017

 [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/>.

Jeong, et al. Standards Track [Page 16] RFC 8106 IPv6 DNS RA Options March 2017

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.

Jeong, et al. Standards Track [Page 17] RFC 8106 IPv6 DNS RA Options March 2017

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].

Jeong, et al. Standards Track [Page 18] RFC 8106 IPv6 DNS RA Options March 2017

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

Jeong, et al. Standards Track [Page 19]

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