path: root/doc/rfc/rfc6731.txt

Internet Engineering Task Force (IETF)                     T. Savolainen
Request for Comments: 6731                                         Nokia
Category: Standards Track                                        J. Kato
ISSN: 2070-1721                                                      NTT
                                                                T. Lemon
                                                           Nominum, Inc.
                                                           December 2012


   Improved Recursive DNS Server Selection for Multi-Interfaced Nodes

Abstract

   A multi-interfaced node is connected to multiple networks, some of
   which might be utilizing private DNS namespaces.  A node commonly
   receives recursive DNS server configuration information from all
   connected networks.  Some of the recursive DNS servers might have
   information about namespaces other servers do not have.  When a
   multi-interfaced node needs to utilize DNS, the node has to choose
   which of the recursive DNS servers to use.  This document describes
   DHCPv4 and DHCPv6 options that can be used to configure nodes with
   information required to perform informed recursive DNS server
   selection decisions.

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/rfc6731.

Copyright Notice

   Copyright (c) 2012 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



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   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.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Private Namespaces and Problems for Multi-Interfaced Nodes . .  4
     2.1.  Fully Qualified Domain Names with Limited Scopes . . . . .  4
     2.2.  Network-Interface-Specific IP Addresses  . . . . . . . . .  5
     2.3.  A Problem Not Fully Solved by the Described Solution . . .  6
   3.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  CPE Deployment Scenario  . . . . . . . . . . . . . . . . .  7
     3.2.  Cellular Network Scenario  . . . . . . . . . . . . . . . .  7
     3.3.  VPN Scenario . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Dual-Stack Accesses  . . . . . . . . . . . . . . . . . . .  8
   4.  Improved RDNSS Selection . . . . . . . . . . . . . . . . . . .  8
     4.1.  Procedure for Prioritizing RDNSSes and Handling
           Responses  . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  RDNSS Selection DHCPv6 Option  . . . . . . . . . . . . . . 11
     4.3.  RDNSS Selection DHCPv4 Option  . . . . . . . . . . . . . . 13
     4.4.  Scalability Considerations . . . . . . . . . . . . . . . . 15
     4.5.  Limitations on Use . . . . . . . . . . . . . . . . . . . . 15
     4.6.  Coexistence of Various RDNSS Configuration Tools . . . . . 16
     4.7.  Considerations on Follow-Up Queries  . . . . . . . . . . . 17
     4.8.  Closing Network Interfaces and Local Caches  . . . . . . . 17
   5.  Example of a Node Behavior . . . . . . . . . . . . . . . . . . 17
   6.  Considerations for Network Administrators  . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
     8.1.  Attack Vectors . . . . . . . . . . . . . . . . . . . . . . 20
     8.2.  Trust Levels of Network Interfaces . . . . . . . . . . . . 21
     8.3.  Importance of Following the Algorithm  . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Possible Alternative Practices for RDNSS Selection  . 23
     A.1.  Sending Queries Out on Multiple Interfaces in Parallel . . 23
     A.2.  Search List Option for DNS Forward Lookup Decisions  . . . 23
     A.3.  More-Specific Routes for Reverse Lookup Decisions  . . . . 24
     A.4.  Longest Matching Prefix for Reverse Lookup Decisions . . . 24
   Appendix B.  DNSSEC and Multiple Answers Validating with
                Different Trust Anchors . . . . . . . . . . . . . . . 24
   Appendix C.  Pseudocode for RDNSS Selection  . . . . . . . . . . . 24
   Appendix D.  Acknowledgements  . . . . . . . . . . . . . . . . . . 29




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1.  Introduction

   A multi-interfaced node (MIF node) faces several problems a single-
   homed node does not encounter, as is described in [RFC6418].  This
   document studies in detail the problems private namespaces might
   cause for multi-interfaced nodes and provides a solution.  The node
   might be implemented as a host or as a router.

   We start from the premise that network operators sometimes include
   private, but still globally unique, namespaces in the answers they
   provide from Recursive DNS Servers (RDNSSes) and that those private
   namespaces are at least as useful to nodes as the answers from the
   public DNS.  When private namespaces are visible for a node, some
   RDNSSes have information other RDNSSes do not have.  The node ought
   to be able to query the RDNSS that can resolve the query regardless
   of whether the answer comes from the public DNS or a private
   namespace.

   An example of an application that benefits from multi-interfacing is
   a web browser that commonly accesses many different destinations,
   each of which is available on only one network.  The browser
   therefore needs to be able to communicate over different network
   interfaces, depending on the destination it is trying to reach.

   Selection of the correct interface and source address is often
   crucial in the networks using private namespaces.  In such
   deployments, the destination's IP addresses might only be reachable
   on the network interface over which the destination's name was
   resolved.  Henceforth, the solution described in this document is
   assumed to be commonly used in combination with tools for delivering
   additional routing and source and destination address selection
   policies (e.g., [RFC4191] and [RFC3442].

   This document is organized in the following manner.  Background
   information about problem descriptions and example deployment
   scenarios are included in Sections 2 and 3.  Section 4 contains all
   normative descriptions for DHCP options and node behavior.
   Informative Section 5 illustrates behavior of a node implementing
   functionality described in Section 4.  Section 6 contains normative
   guidelines related to creation of private namespaces.  The IANA
   considerations are in Section 7.  Informational Section 8 summarizes
   identified security considerations.

   Appendix A describes best current practices that are possible with
   tools preceding this document and that are possibilities on networks
   not supporting the solution described in this document.  Appendix B
   discusses a scenario where multiple answers are possible to validate,




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   but with different trust anchors.  Appendix C illustrates with
   pseudocode the functionality described in Section 4.

1.1.  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 RFC 2119 [RFC2119].

2.  Private Namespaces and Problems for Multi-Interfaced Nodes

   This section describes two private namespace scenarios related to
   node multi-interfacing for which the procedure described in Section 4
   provides a solution.  Additionally, Section 2.3 describes a problem
   for which this document provides only a partial solution.

2.1.  Fully Qualified Domain Names with Limited Scopes

   A multi-interfaced node can be connected to one or more networks that
   are using private namespaces.  As an example, the node can
   simultaneously open a Wireless LAN (WLAN) connection to the public
   Internet, a cellular connection to an operator network, and a Virtual
   Private Network (VPN) connection to an enterprise network.  When an
   application initiates a connection establishment to a Fully Qualified
   Domain Name (FQDN), the node needs to be able to choose the right
   RDNSS for making a successful DNS query.  This is illustrated in
   Figure 1.  An FQDN for a public name can be resolved with any RDNSS,
   but for an FQDN of the private name of an enterprise's or operator's
   service, the node needs to be able to correctly select the right
   RDNSS for the DNS resolution, i.e., do also network interface
   selection already before destination's IP address is known.




















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                            +---------------+
                            | RDNSS with    |    |   Enterprise
   +------+                 | public +      |----|   Intranet
   |      |                 | enterprise's  |    |
   |      |===== VPN =======| private names |    |
   |      |                 +---------------+  +----+
   | MIF  |                                    | FW |
   | node |                                    +----+
   |      |                 +---------------+    |
   |      |----- WLAN ------| RDNSS with    |----|   Public
   |      |                 | public names  |    |   Internet
   |      |                 +---------------+  +----+
   |      |                                    | FW |
   |      |                 +---------------+  +----+
   |      |---- cellular ---| RDNSS with    |    |
   +------+                 | public +      |    |   Operator
                            | operator's    |----|   Intranet
                            | private names |    |
                            +---------------+

               Figure 1: Private DNS Namespaces Illustrated

2.2.  Network-Interface-Specific IP Addresses

   In the second problem, an FQDN is valid and resolvable via different
   network interfaces, but to different and not necessarily globally
   reachable IP addresses, as is illustrated in Figure 2.  The node's
   routing, source, and destination address selection mechanism has to
   ensure the destination's IP address is only used in combination with
   source IP addresses of the network interface on which the name was
   resolved.

                            +--------------------|      |
   +------+   IPv6          | RDNSS A            |------| IPv6
   |      |-- interface 1 --| saying Peer is     |      |
   |      |                 | at: 2001:0db8:0::1 |      |
   | MIF  |                 +--------------------+   +------+
   | node |                                          | Peer |
   |      |                 +--------------------+   +------+
   |      |   IPv6          | RDNSS B            |      |
   |      |-- interface 2 --| saying Peer is     |      |
   +------+                 | at: 2001:0db8:1::1 |------| IPv6
                            +--------------------+      |

    Figure 2: Private DNS Namespaces and Different IP Addresses for an
                        FQDN on Interfaces 1 and 2





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   A similar situation can happen with IPv6 protocol translation and
   AAAA record synthesis [RFC6147].  A synthetic AAAA record is
   guaranteed to be valid only on the network on which it was
   synthesized.  Figure 3 illustrates a scenario where the peer's IPv4
   address is synthesized into different IPv6 addresses by RDNSSes A and
   B.

                            +-------------------|    +-------+
   +------+   IPv6          | RDNSS A           |----| NAT64 |
   |      |-- interface 1 --| saying Peer is    |    +-------+
   |      |                 | at: A_Pref96:IPv4 |       |
   | MIF  |                 +-------------------+       |   +------+
   | node |                                        IPv4 +---| Peer |
   |      |                 +-------------------+       |   +------+
   |      |   IPv6          | RDNSS B           |       |
   |      |-- interface 2 --| saying Peer is    |    +-------+
   +------+                 | at: B_Pref96:IPv4 |----| NAT64 |
                            +-------------------+    +-------+

                    Figure 3: AAAA Synthesis Results in
                 Network-Interface-Specific IPv6 Addresses

   It is worth noting that network-specific IP addresses can also cause
   problems for a single-homed node, if the node retains DNS cache
   during movement from one network to another.  After the network
   change, a node can have entries in its DNS cache that are no longer
   correct or appropriate for its new network position.

2.3.  A Problem Not Fully Solved by the Described Solution

   A more complex scenario is an FQDN, which in addition to possibly
   resolving into network-interface-specific IP addresses, identifies on
   different network interfaces completely different peer entities with
   potentially different sets of service offerings.  In an even more
   complex scenario, an FQDN identifies a unique peer entity, but one
   that provides different services on its different network interfaces.
   The solution described in this document is not able to tackle these
   higher-layer issues.  In fact, these problems might be solvable only
   by manual user intervention.

   However, when DNS Security (DNSSEC) is used, the DNSSEC validation
   procedure can provide assistance for selecting correct responses for
   some, but not all, use cases.  A node might prefer to use the DNS
   answer that validates with the preferred trust anchor.







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3.  Deployment Scenarios

   This document has been written with three particular deployment
   scenarios in mind.  The first is a Customer Premises Equipment (CPE)
   with two or more uplink Virtual Local Area Network (VLAN)
   connections.  The second scenario involves a cellular device with two
   uplink Internet connections: WLAN and cellular.  The third scenario
   is for VPNs, where use of a local RDNSS might be preferred for
   latency reasons, but the enterprise's RDNSS has to be used to resolve
   private names used by the enterprise.

   In this section, we are referring to the RDNSS preference values
   defined in Section 4.  The purpose of that is to illustrate when
   administrators might choose to utilize the different preference
   values.

3.1.  CPE Deployment Scenario

   A home gateway can have two uplink connections leading to different
   networks, as described in [WITHOUT-IPV6NAT].  In the two-uplink
   scenario, only one uplink connection leads to the Internet, while the
   other uplink connection leads to a private network utilizing private
   namespaces.

   It is desirable that the CPE does not have to send DNS queries over
   both uplink connections, but instead, CPE need only send default
   queries to the RDNSS of the interface leading to the Internet and
   queries related to the private namespace to the RDNSS of the private
   network.  This can be configured by setting the RDNSS of the private
   network to know about listed domains and networks, but not to be a
   default RDNSS.

   In this scenario, the legacy hosts can be supported by deploying DNS
   proxy on the CPE and configuring hosts in the LAN to talk to the DNS
   proxy.  However, updated hosts would be able to talk directly to the
   correct RDNSS of each uplink ISP's RDNSS.  It is a deployment
   decision whether the updated hosts would be pointed to a DNS proxy or
   to actual RDNSSes.

   Depending on actual deployments, all VLAN connections might be
   considered trusted.

3.2.  Cellular Network Scenario

   A cellular device can have both WLAN and cellular network interfaces
   up.  In such a case, it is often desirable to use WLAN by default,
   except for the connections that the cellular network operator wants
   to go over the cellular interface.  The use of WLAN for DNS queries



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   likely improves the power consumption of cellular devices and often
   provides lower latency.  The cellular network might utilize private
   names; hence, the cellular device needs to ask for those through the
   cellular interface.  This can be configured by setting the RDNSS of
   the cellular network to be of low preference and listing the domains
   and networks related to the cellular network's private namespaces as
   being available via the cellular network's RDNSS.  This will cause a
   node to send DNS queries by default to the RDNSS of the WLAN
   interface (that is, by default, considered to be of medium
   preference) and queries related to private namespaces to the RDNSS of
   the cellular interface.

   In this scenario, the cellular interface can be considered trusted
   and WLAN oftentimes untrusted.

3.3.  VPN Scenario

   Depending on a deployment, there might be interest in using VPN only
   for the traffic destined to a enterprise network.  The enterprise
   might be using private namespaces; hence, related DNS queries need to
   be sent over VPN to the enterprise's RDNSS, while by default, the
   RDNSS of a local access network might be used for all other traffic.
   This can be configured by setting the RDNSS of the VPN interface to
   be of low preference and listing the domains and networks related to
   an enterprise network's private namespaces being available via the
   RDNSS of the VPN interface.  This will cause a node to send DNS
   queries by default directly to the RDNSS of the WLAN interface (that
   is, by default, considered to be of medium preference) and queries
   related to private namespaces to the RDNSS of the VPN interface.

   In this scenario, the VPN interface can be considered trusted and the
   local access network untrusted.

3.4.  Dual-Stack Accesses

   In all three scenarios, one or more of the connected networks can
   support both IPv4 and IPv6.  In such a case, both or either of DHCPv4
   and DHCPv6 can be used to learn RDNSS selection information.

4.  Improved RDNSS Selection

   This section describes DHCP options and a procedure that a (stub/
   proxy) resolver can utilize for improved RDNSS selection in the face
   of private namespaces and multiple simultaneously active network
   interfaces.  The procedure is subject to limitations of use as
   described in Section 4.5.  The pseudocode in Appendix C illustrates
   how the improved RDNSS selection works.




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4.1.  Procedure for Prioritizing RDNSSes and Handling Responses

   A resolver SHALL build a preference list of RDNSSes it will contact
   depending on the query.  To build the list in an optimal way, a node
   SHALL request for RDNSS selection information with the DHCP options
   defined in Sections 4.2 and 4.3 before any DNS queries need to be
   made.  With help of the received RDNSS selection information, the
   node can determine if any of the available RDNSSes have special
   knowledge about specific domains needed for forward DNS lookups or
   network addresses (later referred as "network") needed for reverse
   DNS lookups.

   A resolver lacking more specific information can assume that all
   information is available from any RDNSS of any network interface.
   The RDNSSes learned by other RDNSS address configuration methods can
   be considered as default RDNSSes, but preference-wise, they MUST be
   handled as medium preference RDNSSes (see also Section 4.6).

   When a DNS query needs to be made, the resolver MUST give highest
   preference to the RDNSSes explicitly known to serve a matching domain
   or network.  The resolver MUST take into account differences in trust
   levels (see Section 8.2) of pieces of received RDNSS selection
   information.  The resolver MUST prefer RDNSSes of trusted interfaces.
   The RDNSSes of untrusted interfaces can be of highest preference only
   if the trusted interfaces specifically configures low preference
   RDNSSes.  The non-exhaustive list of cases in Figure 4 illustrates
   how the different trust levels of received RDNSS selection
   information influence the RDNSS selection logic.  In Figure 4,
   "Medium", "High", and "Low" indicate the explicitly configured
   RDNSS's preference over other RDNSSes.  The "Medium" preference is
   also used with RDNSSes for which no explicit preference configuration
   information is available.  The "Specific domains" in Figure 4
   indicate the explicitly configured "Domains and networks" private
   namespace information that a particular RDNSS has.

   A resolver MUST prioritize between equally trusted RDNSSes with the
   help of the DHCP option preference field.  The resolver MUST NOT
   prioritize less trusted RDNSSes higher than trusted, even in the case
   when a less trusted RDNSS would apparently have additional
   information.  In the case of all other things being equal, the
   resolver can make the prioritization decision based on its internal
   preferences.









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      Information from       | Information from       | Resulting RDNSS
      more trusted           | less trusted           | preference
      interface A            | interface B            | selection
   --------------------------+------------------------+-----------------
   1. Medium preference      | Medium preference      | Default:
      default                | default                | A, then B
   --------------------------+------------------------+-----------------
   2. Medium preference      | High preference default| Default:
      default                |                        | A, then B
                             | Specific domains       | Specific:
                             |                        | A, then B
   --------------------------+------------------------+-----------------
   3. Low preference default | Medium preference      | Default:
                             | default                | B, then A
   --------------------------+------------------------+-----------------
   4. Low preference default | Medium preference      | Default:
                             | default                | B, then A
      Specific domains       |                        | Specific:
                             |                        | A, then B
   --------------------------+------------------------+-----------------

      Figure 4: RDNSS Selection in the Case of Different Trust Levels

   Because DNSSEC provides cryptographic assurance of the integrity of
   DNS data, it is necessary to prefer data that can be validated under
   DNSSEC over data that cannot.  There are two ways that a node can
   determine that data is valid under DNSSEC.  The first is to perform
   DNSSEC validation itself.  The second is to have a secure connection
   to an authenticated RDNSS and to rely on that RDNSS to perform DNSSEC
   validation (signaling that it has done so using the AD bit).  DNSSEC
   is necessary to detect forged responses, and without it any DNS
   response could be forged or altered.  Unless the DNS responses have
   been validated with DNSSEC, a node cannot make a decision to prefer
   data from any interface with any great assurance.

   A node SHALL send requests to RDNSSes in the order defined by the
   preference list until an acceptable reply is received, all replies
   are received, or a timeout occurs.  In the case of a requested name
   matching to a specific domain or network rule accepted from any
   interface, a DNSSEC-aware resolver MUST NOT proceed with a reply that
   cannot be validated using DNSSEC until all RDNSSes on the preference
   list have been contacted or timed out.  This protects against
   possible redirection attacks.  In the case of the requested name not
   matching to any specific domain or network, the first received
   response from any RDNSS can be considered acceptable.  A DNSSEC-aware
   node MAY always contact all RDNSSes in an attempt to receive a
   response that can be validated, but contacting all RDNSSes is not




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   mandated for the default case as that would consume excess resources
   in some deployments.

   In the case of a validated NXDOMAIN response being received from an
   RDNSS that can provide answers for the queried name, a node MUST NOT
   accept non-validated replies from other RDNSSes (see Appendix B for
   considerations related to multiple trust anchors).

4.2.  RDNSS Selection DHCPv6 Option

   DHCPv6 option described below can be used to inform resolvers what
   RDNSS can be contacted when initiating forward or reverse DNS lookup
   procedures.  This option is DNS record type agnostic and applies, for
   example, equally to both A and AAAA queries.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    OPTION_RDNSS_SELECTION     |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |            DNS-recursive-name-server (IPv6 address)           |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved  |prf|                                               |
   +-+-+-+-+-+-+-+-+          Domains and networks                 |
   |                          (variable length)                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 5: DHCPv6 Option for Explicit Domain Configuration

   option-code:  OPTION_RDNSS_SELECTION (74)

   option-len:  Length of the option in octets

   DNS-recursive-name-server:  An IPv6 address of RDNSS

   Reserved:  Field reserved for the future.  MUST be set to zero and
              MUST be ignored on receipt.










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   prf:  RDNSS preference:

         01 High
         00 Medium
         11 Low
         10 Reserved

         Reserved preference value (10) MUST NOT be sent.  On receipt,
         the Reserved value MUST be treated as Medium preference (00).

   Domains and networks:  The list of domains for forward DNS lookup and
                          networks for reverse DNS lookup about which
                          the RDNSS has special knowledge.  Field MUST
                          be encoded as specified in Section 8 of
                          [RFC3315].  A special domain of "." is used to
                          indicate capability to resolve global names
                          and act as a default RDNSS.  Lack of a "."
                          domain on the list indicates that the RDNSS
                          only has information related to listed domains
                          and networks.  Networks for reverse mapping
                          are encoded as defined for IP6.ARPA [RFC3596]
                          or IN-ADDR.ARPA [RFC2317].

   A node SHOULD include the Option Request Option (OPTION_ORO
   [RFC3315]) in a DHCPv6 request with the OPTION_RDNSS_SELECTION option
   code to inform the DHCPv6 server about the support for the improved
   RDNSS selection logic.  The DHCPv6 server receiving this information
   can then choose to provision RDNSS addresses only with
   OPTION_RDNSS_SELECTION.

   OPTION_RDNSS_SELECTION contains one or more domains of which the
   related RDNSS has particular knowledge.  The option can occur
   multiple times in a single DHCPv6 message, if multiple RDNSSes are to
   be configured.  This can be the case, for example, if a network link
   has multiple RDNSSes for reliability purposes.

   The list of networks MUST cover all the domains configured in this
   option.  The length of the included networks SHOULD be as long as
   possible to avoid potential collision with information received on
   other option instances or with options received from DHCP servers of
   other network interfaces.  Overlapping networks are interpreted so
   that the resolver can use any of the RDNSSes for queries matching the
   networks.

   If OPTION_RDNSS_SELECTION contains an RDNSS address already learned
   from other DHCPv6 servers of the same network and contains new
   domains or networks, the node SHOULD append the information to the
   information received earlier.  The node MUST NOT remove previously



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   obtained information.  However, the node SHOULD NOT extend the
   lifetime of earlier information either.  When a conflicting RDNSS
   address is learned from a less trusted interface, the node MUST
   ignore the option.

   Like the RDNSS options of [RFC3646], OPTION_RDNSS_SELECTION MUST NOT
   appear in any other than the following DHCPv6 messages: Solicit,
   Advertise, Request, Renew, Rebind, Information-Request, and Reply.

   The client SHALL periodically refresh information learned with
   OPTION_RDNSS_SELECTION.  The information SHALL be refreshed on link-
   state changes, such as those caused by node mobility, and when
   renewing lifetimes of IPv6 addresses configured with DHCPv6.
   Additionally, the DHCPv6 Information Refresh Time Option, as
   specified in [RFC4242], can be used to control the update frequency.

4.3.  RDNSS Selection DHCPv4 Option

   The DHCPv4 option described below can be used to inform resolvers
   which RDNSS can be contacted when initiating forward or reverse DNS
   lookup procedures.  This option is DNS record type agnostic and
   applies, for example, equally to both A and AAAA queries.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     CODE      |     Len       | Reserved  |prf|    Primary .. |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | .. DNS-recursive-name-server's IPv4 address   |  Secondary .. |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | .. DNS-recursive-name-server's IPv4 address   |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   +                          Domains and networks                 |
   |                          (variable length)                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 6: DHCPv4 Option for Explicit Domain Configuration

   option-code:  RDNSS Selection (146)

   option-len:  Length of the option in octets

   Reserved:  Field reserved for the future.  MUST be set to zero and
              MUST be ignored on receipt.





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   prf:  RDNSS preference:

         01 High
         00 Medium
         11 Low
         10 Reserved

         Reserved preference value (10) MUST NOT be sent.  On receipt,
         the Reserved value MUST be treated as Medium preference (00).

   Primary DNS-recursive-name-server's IPv4 address:  Address of a
                                                      primary RDNSS

   Secondary DNS-recursive-name-server's IPv4 address:  Address of a
                                                        secondary RDNSS
                                                        or 0.0.0.0 if
                                                        not configured

   Domains and networks:  The list of domains for forward DNS lookup and
                          networks for reverse DNS lookup about which
                          the RDNSSes have special knowledge.  Field
                          MUST be encoded as specified in Section 8 of
                          [RFC3315].  A special domain of "." is used to
                          indicate capability to resolve global names
                          and act as the default RDNSS.  Lack of a "."
                          domain on the list indicates that RDNSSes only
                          have information related to listed domains and
                          networks.  Networks for reverse mapping are
                          encoded as defined for IP6.ARPA [RFC3596] or
                          IN-ADDR.ARPA [RFC2317].

   The RDNSS Selection option contains one or more domains of which the
   primary and secondary RDNSSes have particular knowledge.  If the
   length of the domains and networks field causes option length to
   exceed the maximum permissible for a single option (255 octets), then
   multiple options MAY be used, as described in "Encoding Long Options
   in the Dynamic Host Configuration Protocol (DHCPv4)" [RFC3396].  When
   multiple options are present, the data portions of all option
   instances are concatenated together.

   The list of networks MUST cover all the domains configured in this
   option.  The length of the included networks SHOULD be as long as
   possible to avoid potential collision with information received on
   other option instances or with options received from DHCP servers of
   other network interfaces.  Overlapping networks are interpreted so
   that the resolver can use any of the RDNSSes for queries matching the
   networks.




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   If the RDNSS Selection option contains an RDNSS address already
   learned from other DHCPv4 servers of the same network and contains
   new domains or networks, the node SHOULD append the information to
   the information received earlier.  The node MUST NOT remove
   previously obtained information.  However, the node SHOULD NOT extend
   the lifetime of earlier information either.  When a conflicting RDNSS
   address is learned from a less trusted interface, the node MUST
   ignore the option.

   The client SHALL periodically refresh information learned with the
   RDNSS Selection option.  The information SHALL be refreshed on link-
   state changes, such as those caused by node mobility, and when
   extending the lease of IPv4 addresses configured with DHCPv4.

4.4.  Scalability Considerations

   The general size limitations of the DHCP messages limit the number of
   domains and networks that can be carried inside of these RDNSS
   selection options.  The DHCP options for RDNSS selection are best
   suited for those deployments where relatively few and carefully
   selected domains and networks are enough.

4.5.  Limitations on Use

   The RDNSS selection option SHOULD NOT be enabled by default.  (In
   this section, "RDNSS selection option" refers to the DHCPv4 RDNSS
   Selection option and the DHCPv6 OPTION_RDNSS_SELECTION.)  The option
   can be used in the following environments:

   1.  The RDNSS selection option is delivered across a secure, trusted
       channel.

   2.  The RDNSS selection option is not secured, but the client on a
       node does DNSSEC validation.

   3.  The RDNSS selection option is not secured, the resolver does
       DNSSEC validation, and the client communicates with the resolver
       configured with the RDNSS selection option over a secure, trusted
       channel.

   4.  The IP address of the RDNSS that is being recommended in the
       RDNSS selection option is known and trusted by the client; that
       is, the RDNSS selection option serves not to introduce the client
       to a new RDNSS, but rather to inform it that the RDNSS it has
       already been configured to trust is available to it for resolving
       certain domains.





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   As the DHCP by itself cannot tell whether it is using a secure,
   trusted channel, or whether the client on a node is performing DNSSEC
   validation, this option cannot be used without being explicitly
   enabled.  The functionality can be enabled for an interface via
   administrative means, such as by provisioning tools or manual
   configuration.  Furthermore, the functionality can be automatically
   enabled by a client on a node that knows it is performing DNSSEC
   validation or by a node that is configured or hard-coded to trust
   certain interfaces (see Section 8.2).

4.6.  Coexistence of Various RDNSS Configuration Tools

   The DHCPv4 RDNSS Selection option and the DHCPv6
   OPTION_RDNSS_SELECTION are designed to coexist with each other and
   with other tools used for RDNSS address configuration.

   For RDNSS selection purposes, information received from all tools
   MUST be combined together into a single list, as discussed in
   Section 4.1.

   It can happen that DHCPv4 and DHCPv6 are providing conflicting RDNSS
   selection information on the same or on equally trusted interfaces.
   In such a case, DHCPv6 MUST be preferred unless DHCPv4 is utilizing
   additional security frameworks for protecting the messages.

   The RDNSSes learned via tools other than the DHCPv4 RDNSS Selection
   option and the DHCPv6 OPTION_RDNSS_SELECTION MUST be handled as
   default RDNSSes, with medium preference, when building a list of
   RDNSSes to talk to (see Section 4.1).

   The non-exhaustive list of possible other sources for RDNSS address
   configuration are:

   (1)  DHCPv6 OPTION_DNS_SERVERS defined in [RFC3646].

   (2)  DHCPv4 Domain Server option defined in [RFC2132].

   (3)  IPv6 Router Advertisement RDNSS Option defined in [RFC6106].

   When the RDNSS selection option contains a default RDNSS address and
   other sources are providing RNDSS addresses, the resolver MUST make
   the decision about which one to prefer based on the RDNSS preference
   field value.  If the RDNSS selection option defines medium
   preference, then the RDNSS from the RDNSS selection option SHALL be
   selected.

   If multiple sources are providing same RDNSS(es) IP address(es), each
   address MUST be added to the RDNSS list only once.



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   If a node had indicated support for OPTION_RDNSS_SELECTION in a
   DHCPv6 request, the DHCPv6 server MAY omit sending of
   OPTION_DNS_SERVERS.  This enables offloading use case where the
   network administrator wishes to only advertise low preference default
   RDNSSes.

4.7.  Considerations on Follow-Up Queries

   Any follow-up queries that are performed on the basis of an answer
   received on an interface MUST continue to use the same interface,
   irrespective of the RDNSS selection settings on any other interface.
   For example, if a node receives a reply with a canonical name (CNAME)
   or delegation name (DNAME), the follow-up queries MUST be sent to
   RDNSS(es) of the same interface, or to the same RDNSS, irrespectively
   of the FQDN received.  Otherwise, referrals can fail.

4.8.  Closing Network Interfaces and Local Caches

   Cached information related to private namespaces can become obsolete
   after the network interface over which the information was learned is
   closed (Section 2.2) or a new parallel network interface is opened
   that alters RDNSS selection preferences.  An implementation SHOULD
   ensure obsolete information is not retained in these events.  One
   implementation approach to avoid unwanted/obsolete responses from the
   local cache is to manage per-interface DNS caches or have interface
   information stored in the DNS cache.  An alternative approach is to
   perform, possibly selective, DNS cache flushing on interface change
   events.

5.  Example of a Node Behavior

   Figure 7 illustrates node behavior when it initializes two network
   interfaces for parallel usage and learns domain and network
   information from DHCPv6 servers.

















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    Application    Node      DHCPv6 server   DHCPv6 server
                             on interface 1  on interface 2
        |             |                |
        |         +-----------+        |
   (1)  |         | open      |        |
        |         | interface |        |
        |         +-----------+        |
        |             |                |
   (2)  |             |---option REQ-->|
        |             |<--option RESP--|
        |             |                |
        |         +-----------+        |
   (3)  |         | store     |        |
        |         | domains   |        |
        |         +-----------+        |
        |             |                |
        |         +-----------+        |
   (4)  |         | open      |        |
        |         | interface |        |
        |         +-----------+        |
        |             |                |                |
   (5)  |             |---option REQ------------------->|
        |             |<--option RESP-------------------|
        |             |                |                |
        |         +----------+         |                |
   (6)  |         | store    |         |                |
        |         | domains  |         |                |
        |         +----------+         |                |
        |             |                |                |

                Figure 7: Illustration of Learning Domains

   Flow explanations:

   1.  A node opens its first network interface.

   2.  The node obtains domain 'domain1.example.com' and IPv6 network
       '0.8.b.d.0.1.0.0.2.ip6.arpa' for the new interface 1 from the
       DHCPv6 server.

   3.  The node stores the learned domains and IPv6 networks for later
       use.

   4.  The node opens its second network interface 2.

   5.  The node obtains domain 'domain2.example.com' and IPv6 network
       information, say '1.8.b.d.0.1.0.0.2.ip6.arpa' for the new
       interface 2 from the DHCPv6 server.



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   6.  The node stores the learned domains and networks for later use.

   Figure 8 illustrates how a resolver uses the learned domain
   information.  Network information use for reverse lookups is not
   illustrated, but that would be similar to the example in Figure 8.

    Application     Node     RDNSS             RDNSS
                             on interface 1    on interface 2
        |             |                |                |
   (1)  |--Name REQ-->|                |                |
        |             |                |                |
        |      +----------------+      |                |
   (2)  |      | RDNSS          |      |                |
        |      | prioritization |      |                |
        |      +----------------+      |                |
        |             |                |                |
   (3)  |             |------------DNS resolution------>|
        |             |<--------------------------------|
        |             |                |                |
   (4)  |<--Name resp-|                |                |
        |             |                |                |

          Figure 8: Example on Choosing Interface Based on Domain

   Flow explanations:

   1.  An application makes a request for resolving an FQDN, e.g.,
       'private.domain2.example.com'.

   2.  A node creates list of RDNSSes to contact and uses configured
       RDNSS selection information and stored domain information on
       prioritization decisions.

   3.  The node has chosen interface 2, as it was learned earlier from
       DHCPv6 that the interface 2 has domain 'domain2.example.com'.
       The node then resolves the requested name using interface 2's
       RDNSS to an IPv6 address.

   4.  The node replies to the application with the resolved IPv6
       address.

6.  Considerations for Network Administrators

   Network administrators deploying private namespaces can assist
   advanced nodes in their RDNSS selection process by providing the
   information described within this document.





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   Private namespaces MUST be globally unique in order to keep DNS
   unambiguous and henceforth avoid caching-related issues and
   destination selection problems (see Section 2.3).  Exceptions to this
   rule are domains utilized for local name resolution (such as .local).

   Private namespaces MUST only consist of subdomains of domains for
   which the relevant operator provides authoritative name service.
   Thus, subdomains of example.com are permitted in the private
   namespace served by an operator's RDNSSes only if the same operator
   provides a SOA record for example.com.

   It is RECOMMENDED for administrators utilizing this tool to deploy
   DNSSEC for their zone in order to counter attacks against private
   namespaces.

7.  IANA Considerations

   Per this memo, IANA has assigned two new option codes.

   The first option code has been assigned for the DHCPv4 RDNSS
   Selection option (146) from the "BOOTP Vendor Extensions and DHCP
   Options" registry in the group "Dynamic Host Configuration Protocol
   (DHCP) and Bootstrap Protocol (BOOTP) Parameters".

   The second option code is requested to be assigned for the DHCPv6
   OPTION_RDNSS_SELECTION (74) from the "DHCP Option Codes" registry in
   the group "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)".

8.  Security Considerations

8.1.  Attack Vectors

   It is possible that attackers might try to utilize the DHCPv4 RDNSS
   Selection option or the DHCPv6 OPTION_RDNSS_SELECTION option to
   redirect some or all DNS queries sent by a resolver to undesired
   destinations.  The purpose of an attack might be denial of service,
   preparation for man-in-the-middle attack, or something akin.

   Attackers might try to lure specific traffic by advertising domains
   and networks from very small to very large scope or simply by trying
   to place the attacker's RDNSS as the highest preference default
   RDNSS.

   The best countermeasure for nodes is to implement validating DNSSEC-
   aware resolvers.  Trusting validation done by an RDNSS is a
   possibility only if a node trusts the RDNSS and can use a secure
   channel for DNS messages.




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8.2.  Trust Levels of Network Interfaces

   Trustworthiness of an interface and configuration information
   received over the interface is implementation and/or node deployment
   dependent, and the details of determining that trust are beyond the
   scope of this specification.  Trust might, for example, be based on
   the nature of the interface: an authenticated and encrypted VPN, or a
   layer 2 connection to a trusted home network or to a trusted cellular
   network, might be considered trusted, while an unauthenticated and
   unencrypted connection to an unknown visited network would likely be
   considered untrusted.

   In many cases, an implementation might not be able to determine trust
   levels without explicit configuration provided by the user or the
   node's administrator.  Therefore, for example, an implementation
   might not by default trust configuration received even over VPN
   interfaces.  In some occasions, standards defining organizations that
   are specific to access network technology might be able to define
   trust levels as part of the system design work.

8.3.  Importance of Following the Algorithm

   Section 4 uses normative language for describing a node's internal
   behavior in order to ensure that nodes will not open up new attack
   vectors by accidental use of RDNSS selection options.  During the
   standards work, consensus was that it is safer to not always enable
   this option by default, but only when deemed useful and safe.

9.  References

9.1.  Normative References

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

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
              ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.

   [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.

   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              November 2002.



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   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

9.2.  Informative References

   [RFC3397]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397,
              November 2002.

   [RFC3442]  Lemon, T., Cheshire, S., and B. Volz, "The Classless
              Static Route Option for Dynamic Host Configuration
              Protocol (DHCP) version 4", RFC 3442, December 2002.

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

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

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

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

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

   [WITHOUT-IPV6NAT]
              Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
              Wing, "IPv6 Multihoming without Network Address
              Translation", Work in Progress, February 2012.






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Appendix A.  Possible Alternative Practices for RDNSS Selection

   On some private namespace deployments, explicit policies for RDNSS
   selection are not available.  This section describes ways for nodes
   to mitigate the problem by sending wide-spread queries and by
   utilizing possibly existing indirect information elements as hints.

A.1.  Sending Queries Out on Multiple Interfaces in Parallel

   A possible current practice is to send DNS queries out of multiple
   interfaces and pick up the best out of the received responses.  A
   node can implement DNSSEC in order to be able to reject responses
   that cannot be validated.  Selection between legitimate answers is
   implementation specific, but replies from trusted RDNSSes are
   preferred.

   A downside of this approach is increased consumption of resources,
   namely, power consumption if an interface, e.g., wireless, has to be
   brought up just for the DNS query that could have been resolved via a
   cheaper interface.  Also, load on RDNSSes is increased.  However,
   local caching of results mitigates these problems, and a node might
   also learn interfaces that seem to be able to provide 'better'
   responses than others and prefer those, without forgetting that
   fallback is required for cases when the node is connected to more
   than one network using private namespaces.

A.2.  Search List Option for DNS Forward Lookup Decisions

   A node can learn the special domains of attached network interfaces
   from IPv6 Router Advertisement DNS Search List Option [RFC6106] or
   DHCP search list options -- DHCPv4 Domain Search Option number 119
   [RFC3397] and DHCPv6 Domain Search List Option number 24 [RFC3646].
   The node behavior is very similar to that illustrated in the example
   in Section 5.  While these options are not intended to be used in
   RDNSS selection, they can be used by the nodes as hints for smarter
   RDNSS prioritization purposes in order to increase likelihood of fast
   and successful DNS queries.

   Overloading of existing DNS search list options is not without
   problems: resolvers would obviously use the domains learned from
   search lists for name resolution purposes.  This might not be a
   problem in deployments where DNS search list options contain few
   domains like 'example.com, private.example.com' but can become a
   problem if many domains are configured.







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A.3.  More-Specific Routes for Reverse Lookup Decisions

   [RFC4191] defines how more-specific routes can be provisioned for
   nodes.  This information is not intended to be used in RDNSS
   selection, but nevertheless, a node can use this information as a
   hint about which interface would be best to try first for reverse
   lookup procedures.  An RDNSS configured via the same interface as
   more-specific routes is more likely capable to answer reverse lookup
   questions correctly than an RDNSS of another interface.  The
   likelihood of success is possibly higher if an RDNSS address is
   received in the same RA [RFC6106] as the more-specific route
   information.

A.4.  Longest Matching Prefix for Reverse Lookup Decisions

   A node can utilize the longest matching prefix approach when deciding
   which RDNSS to contact for reverse lookup purposes.  Namely, the node
   can send a DNS query to an RDNSS learned over an interface having a
   longest matching prefix to the address being queried.  This approach
   can help in cases where Unique Local Addressing (ULA) [RFC4193]
   addresses are used and when the queried address belongs to a node or
   server within the same network (for example, intranet).

Appendix B.  DNSSEC and Multiple Answers Validating with Different Trust
             Anchors

   When validating DNS answers with DNSSEC, a validator might order the
   list of trust anchors it uses to start validation chains, in the
   order of the node's preferences for those trust anchors.  A node
   could use this ability in order to select among alternative DNS
   results from different interfaces.  Suppose that a node has a trust
   anchor for the public DNS root and also has a special-purpose trust
   anchor for example.com.  An answer is received on interface i1 for
   www.example.com, and the validation for that succeeds by using the
   public trust anchor.  Also, an answer is received on interface i2 for
   www.example.com, and the validation for that succeeds by using the
   trust anchor for example.com.  In this case, the node has evidence
   for relying on i2 for answers in the example.com zone.

Appendix C.  Pseudocode for RDNSS Selection

   This section illustrates the RDNSS selection logic in C-style
   pseudocode.  The code is not intended to be usable as such; it is
   only here for illustration purposes.

   The beginning of the whole procedure is a call to "dns_query"
   function with a query and list of RDNSSes given as parameters.




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/* This is a structure that holds all information related to an RDNSS.*/
/* Here we include only the information related for this illustration.*/
struct rdnss
{
  int prf;        /* Preference of an RDNSS.                          */
  int interface;  /* Type of an interface RDNSS was learned over.     */
  struct d_and_n; /* Domains and networks information for this RDNSS. */
};

int has_special_knowledge( const struct rdnss *rdnss,
                           const char *query)
{
/* This function matches the query to the domains and networks
   information of the given RDNSS.  The function returns TRUE
   if the query matches the domains and networks; otherwise, FALSE.   */

/* The implementation of this matching function
   is left for reader, or rather writer.                              */

/* return TRUE if query matches rdnss->d_and_n, otherwise FALSE.      */
}

const struct rdnss* compare_rdnss_prf( const struct rdnss *rdnss_1,
                                       const struct rdnss *rdnss_2 )
{
/* This function compares preference values of two RDNSSes and
   returns the more preferred RDNSS.  The function prefers rdnss_1
   in the case of equal preference values.                            */

  if (rdnss_1->prf == HIGH_PRF) return rdnss_1;
  if (rdnss_2->prf == HIGH_PRF) return rdnss_2;
  if (rdnss_1->prf == MED_PRF) return rdnss_1;
  if (rdnss_2->prf == MED_PRF) return rdnss_2;
  return rdnss_1;
}

const struct rdnss* compare_rdnss_trust( const struct rdnss *rdnss_1,
                                         const struct rdnss *rdnss_2 )
{
/* This function compares trust of the two given RDNSSes.  The trust
   is based on the trust on the interface RDNSS was learned on.       */

/* If the interface is the same, the trust is also the same,
   and hence, function will return NULL to indicate lack of
   difference in trust.                                               */

  if (rdnss_1->interface == rdnss_2->interface) return NULL;




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/* Otherwise, implementation-specific rules define which interface
   is considered more secure than the other.  The rules shown here
   are only for illustrative purposes and must be overwritten by
   real implementations.                                              */

  if (rdnss_1->interface == IF_VPN) return rdnss_1;
  if (rdnss_2->interface == IF_VPN) return rdnss_2;
  if (rdnss_1->interface == IF_CELLULAR) return rdnss_1;
  if (rdnss_2->interface == IF_CELLULAR) return rdnss_2;
  if (rdnss_1->interface == IF_WLAN) return rdnss_1;
  if (rdnss_2->interface == IF_WLAN) return rdnss_2;

/* Both RDNSSes are from unknown interfaces, so return NULL as
   trust-based comparison is impossible.                              */
  return NULL;
}

int compare_rdnsses ( const struct rdnss *rdnss_1,
                      const struct rdnss *rdnss_2,
                      const char *query)
{
/* This function compares two RDNSSes and decides which one is more
   preferred for resolving the query.  If the rdnss_1 is more
   preferred, the function returns TRUE; otherwise, FALSE.            */

  const struct rdnss *more_trusted_rdnss = NULL;
  const struct rdnss *less_trusted_rdnss = NULL;

/* Find out if either RDNSS is more trusted.                          */
  more_trusted_rdnss = compare_rdnss_trust( rdnss_1, rdnss_2 );

/* Check if either was more trusted.                                  */
  if (more_trusted_rdnss)
    {

/* Check which RDNSS was less trusted.                                */
      less_trusted_rdnss =
          more_trusted_rdnss == rdnss_1 ? rdnss_2 : rdnss_1;

/* If the more trusted interface is not of low preference
   or has special knowledge about the query, or the more
   trusted is more preferred and the less trusted has no special
   information, prefer more trusted.  Otherwise, prefer less trusted. */
      if (more_trusted_rdnss->prf != LOW_PRF ||
          has_special_knowledge( more_trusted_rdnss, query ) ||
          (compare_rdnss_prf( more_trusted_rdnss, less_trusted_rdnss)
               == more_trusted_rdnss &&
           !has_special_knowledge( less_trusted_rdnss, query)))



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        {
/* If the more_trusted_rdnss was rdnss_1, return TRUE.                */
          return more_trusted_rdnss == rdnss_1 ? TRUE : FALSE;
        }
      else
        {
/* If the more_trusted_rdnss was rdnss_1, return TRUE.                */
          return less_trusted_rdnss == rdnss_1 ? TRUE : FALSE;
        }
    }
  else
    {
/* There is no trust difference between RDNSSes; therefore, prefer the
   RDNSS that has special knowledge.  If both have specific knowledge,
   then prefer the rdnss_1.                                           */
      if (has_special_knowledge( rdnss_1, query )) return TRUE;
      if (has_special_knowledge( rdnss_2, query )) return FALSE;

/* Neither had special knowledge.  Therefore, return TRUE if
   rdnss_1 is more preferred; otherwise, return FALSE                 */
      return compare_rdnss_prf( rdnss_1 , rdnss_2 )
          == rdnss_1 ? TRUE : FALSE;
    }
}

void bubble_sort_rdnsses( struct rdnss rdnss_list[],
                          const int rdnsses,
                          const char* query)
{
/* This function implements a bubble sort to arrange
   RDNSSes in rdnss_list into preference order.                       */

  int i;
  int swapped = 0;
  struct rdnss rdnss_swap;

  do
    {
/* Clear swapped-indicator.                                           */
      swapped = FALSE;

/* Go through the RDNSS list.                                         */
      for (i = 0; i < rdnsses-1; i++)
        {
/* Check if the next two items are in the right order, i.e.,
   more preferred before less preferred.                              */
          if (compare_rdnsses( &rdnss_list[i],
                               &rdnss_list[i+1], query) == FALSE)



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            {
/* The order between two was not right, so swap these two RDNSSes.    */
              rdnss_swap = rdnss_list[i];
              rdnss_list[i] = rdnss_list[i+1];
              rdnss_list[i+1] = rdnss_swap;
              swapped = TRUE;
            }
        }
    } while (swapped);

/* No more swaps, which means the rdnss_list is now sorted
   into preference order.                                             */
}

struct hostent *dns_query( struct rdnss rdnss_list[],
                           const int rdnsses,
                           const char* query )
{
/* Perform address resolution for the query.                          */
  int i;
  struct hostent response;

/* Sort the RDNSSes into preference order.                            */
/* This is the function with which this pseudocode starts.            */
  bubble_sort_rdnsses( &rdnss_list[0], rdnsses, query );

/* Go thourgh all RDNSSes or until valid response is found.           */
  for (i = 0; i < rdnsses; i++)
    {

/* Use the highest preference RDNSS first.                            */
      response = send_and_validate_dns_query( rndss_list[i], query);

/* Check if DNSSEC validation is in use, and if so, validate the
   received response.                                                 */
      if (dnssec_in_use)
        {
          response = dnssec_validate(response);

/* If response is validated, use that.  Otherwise, proceed to next
   RDNSS.                                                             */
          if (response) return response;
          else continue;
        }

/* If acceptable response has been found, return it.                  */
      if (response) return response;
    }



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  return NULL;
}

Appendix D.  Acknowledgements

   The authors would like to thank the following people for their
   valuable feedback and improvement ideas: Mark Andrews, Jari Arkko,
   Marcelo Bagnulo, Brian Carpenter, Stuart Cheshire, Lars Eggert,
   Stephan Farrell, Tomohiro Fujisaki, Brian Haberman, Peter Koch,
   Suresh Krishnan, Murray Kucherawy, Barry Leiba, Edward Lewis, Kurtis
   Lindqvist, Arifumi Matsumoto, Erik Nordmark, Steve Padgett, Fabien
   Rapin, Matthew Ryan, Robert Sparks, Dave Thaler, Sean Turner,
   Margaret Wasserman, Dan Wing, and Dec Wojciech.  Ted Lemon and Julien
   Laganier receive special thanks for their contributions to security
   considerations.

Authors' Addresses

   Teemu Savolainen
   Nokia
   Hermiankatu 12 D
   Tampere  FI-33720
   Finland

   EMail: teemu.savolainen@nokia.com


   Jun-ya Kato
   NTT
   9-11, Midori-Cho 3-Chome Musashino-Shi
   Tokyo  180-8585
   Japan

   EMail: kato@syce.net


   Ted Lemon
   Nominum, Inc.
   2000 Seaport Boulevard
   Redwood City, CA  94063
   USA

   Phone: +1 650 381 6000
   EMail: Ted.Lemon@nominum.com







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