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Versions: (draft-savolainen-mif-dns-server-selection) 00 01 02 03 04 05 06 07 08 09 10 RFC 6731

Internet Engineering Task Force                            T. Savolainen
Internet-Draft                                                     Nokia
Intended status: Standards Track                                 J. Kato
Expires: December 3, 2012                                            NTT
                                                                T. Lemon
                                                           Nominum, Inc.
                                                                Jun 2012


   Improved Recursive DNS Server Selection for Multi-Interfaced Nodes
                 draft-ietf-mif-dns-server-selection-10

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 3, 2012.

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



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   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.











































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Private Namespaces and Problems for Multi-Interfaced Nodes . .  5
     2.1.  Fully Qualified Domain Names With Limited Scopes . . . . .  5
     2.2.  Network Interface Specific IP Addresses  . . . . . . . . .  6
     2.3.  A Problem Not Fully Solved by the Described Solution . . .  8
   3.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  CPE Deployment Scenario  . . . . . . . . . . . . . . . . .  8
     3.2.  Cellular Network Scenario  . . . . . . . . . . . . . . . .  9
     3.3.  VPN Scenario . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Dual-Stack Accesses  . . . . . . . . . . . . . . . . . . . 10
   4.  Improved RDNSS Selection . . . . . . . . . . . . . . . . . . . 10
     4.1.  Procedure for Prioritizing RDNSSes and Handling
           Responses  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  RDNSS Selection DHCPv6 Option  . . . . . . . . . . . . . . 12
     4.3.  RDNSS Selection DHCPv4 Option  . . . . . . . . . . . . . . 15
     4.4.  Scalability Considerations . . . . . . . . . . . . . . . . 16
     4.5.  Limitations on Use . . . . . . . . . . . . . . . . . . . . 17
     4.6.  Coexistence of Various RDNSS Configuration Tools . . . . . 17
     4.7.  Considerations on Follow-Up Queries  . . . . . . . . . . . 18
     4.8.  Closing Network Interfaces and Local Caches  . . . . . . . 18
   5.  Example of a Node Behavior . . . . . . . . . . . . . . . . . . 18
   6.  Considerations for Network Administrators  . . . . . . . . . . 21
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
     9.1.  Attack vectors . . . . . . . . . . . . . . . . . . . . . . 22
     9.2.  Trust levels of Network Interfaces . . . . . . . . . . . . 22
     9.3.  Importance of Following the Algorithm  . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Possible Alternative Practices for RDNSS Selection  . 24
     A.1.  Sending Queries Out on Multiple Interfaces in Parallel . . 24
     A.2.  Search List Option for DNS Forward Lookup Decisions  . . . 25
     A.3.  More Specific Routes for Reverse Lookup Decisions  . . . . 25
     A.4.  Longest Matching Prefix for Reverse Lookup Decisions . . . 25
   Appendix B.  DNSSEC and Multiple Answers Validating with
                Different Trust Anchors . . . . . . . . . . . . . . . 26
   Appendix C.  Pseudo Code for RDNSS Selection . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30








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1.  Introduction

   A multi-interfaced 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 (RDNSS), 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 only on 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 on.  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 Section 2 and Section 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 the Section 4.  Section 4.4
   contains informational considerations about scalability.  Section 6
   contains normative guidelines related to creation of private
   namespaces.  Informational Section 9 summarizes identified security
   considerations.

   The Appendix A describes best current practices possible with tools
   preceding this document and that can be possibilities on networks not
   supporting the solution described in this document.




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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 node multi-interfacing related private
   namespace scenarios for which the procedure described in Section 4
   provides a solution for.  Additionally, Section 2.3 describes a
   problem for which this document provides only 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 have
   simultaneously open a Wireless LAN (WLAN) connection to the public
   Internet, 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 the
   figure 1.  An FQDN for a public name can be resolved with any RDNSS,
   but for an FQDN of enterprise's or operator's service's private name
   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 |    |
                            +---------------+

                    Private DNS namespaces illustrated

                                 Figure 1

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 the figure 2.  Node's
   routing and 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 the name was resolved
   on.


















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                            +--------------------|      |
   +------+   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
                            +--------------------+      |


     Private DNS namespaces and different IP addresses for an FQDN on
                            interfaces 1 and 2.

                                 Figure 2

   Similar situation can happen with IPv6 protocol translation and AAAA
   record synthesis [RFC6147].  A synthetic AAAA record is guaranteed to
   be valid only on a network it was synthesized on.  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 |
                            +-------------------+    +-------+


   AAAA synthesis results in network interface specific IPv6 addresses.

                                 Figure 3

   It is worth noting that network specific IP addresses can cause
   problems also 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.




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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 set of service offerings.  In even more complex
   scenario, an FQDN identifies 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 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.


3.  Deployment Scenarios

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

   In this section we are referring to the RDNSS reference values
   defined in the 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 is described in
   [I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat].  In the two
   uplinks scenario only one uplink connection leads to the Internet,
   while another 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 private namespace to the RDNSS of the private
   network.  This can be configured by setting the RDNSS of the private



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   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 deployment decision
   whether the updated hosts would be pointed to DNS proxy or to actual
   RDNSSes.

   Depending on actual deployments, all VLAN connections might be
   considered as 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 those connections cellular network operator wants to go
   over cellular interface.  The use of WLAN for DNS queries likely
   improves cellular devices power consumption and also often provides
   lower latency.  The cellular network might utilize private names and
   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 cellular network's private namespaces 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 cellular interface can be considered trusted and
   WLAN oftentimes untrusted.

3.3.  VPN Scenario

   Depending on a deployment, there might be interest to use VPN only
   for the traffic destined to a enterprise network.  The enterprise
   might be using private namespace, and hence related DNS queries need
   to be sent over VPN to the enterprise's RDNSS, while by default 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
   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.



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   In this scenario VPN interface can be considered trusted and 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 pseudo code at section Appendix C illustrates how
   the improved RDNSS selection works.

4.1.  Procedure for Prioritizing RDNSSes and Handling Responses

   A resolver SHALL build a preference list of RDNSSes it will contact
   to depending on the query.  To build the list in an optimal way, a
   node MUST ask with DHCP options defined in the Section 4.2 and the
   Section 4.3 which RDNSSes of each network interface are most likely
   to be able to successfully serve forward lookup requests matching to
   specific domain or reverse (PTR record) lookup requests matching to
   specific network addresses (later referred as "network").  For
   security reasons the RDNSS selection information MUST NOT be used
   unless it is safe to do so, see Section 4.5 for details.

   The node MUST create node specific routes for RDNSS addresses learned
   via DHCP.  The route MUST point to the interface each RDNSS address
   was learned on.  This is required to ensure DNS queries are sent out
   via the right network interface.

   A resolver lacking more specific information SHALL assume that all
   information is available from any RDNSS of any network interface.
   The RDNSSes learnt by other RDNSS address configuration methods MUST
   be handled as default, the medium, preference default 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 matching domain
   or network.  The resolver MUST take into account differences in trust
   levels (see Section 9.2) 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



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   RDNSSes.  The non-exhaustive list of cases on figure 4 illustrates
   how the different trust levels of received RDNSS selection
   information influences the RDNSS selection logic.  In the figure 4,
   "Medium", "High", and "Low" indicates the explicitly configured
   RDNSS's preference over other RDNSSes.  The "Medium" preference is
   also used with RDNSS for which no explicit preference configuration
   information is available.  The "Specific domains" on figure 4
   indicates the explicitly configured "Domains and networks" private
   namespace information that a particular RDNSS has.

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

   Information from       | Information from       | Resulting RDNSS
   more trusted           | less trusted           | preference
   interface A            | interface B            | selection
--------------------------+------------------------+--------------------
1. Medium preference      | Medium preference      | Default:  A, then B
   default                | default                |
--------------------------+------------------------+--------------------
2. Medium preference      | High preference default| Default:  A, then B
   default                | Specific domains       | Specific: A, then B
--------------------------+------------------------+--------------------
3. Low preference default | Medium preference      | Default:  B, then A
                          | default                |
--------------------------+------------------------+--------------------
4. Low preference default | Medium preference      | Default:  B, then A
   Specific domains       | default                | 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, data that can be validated under DNSSEC is necessarily to
   be preferred over data that cannot be.  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 (signalling 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.




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   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, first received response
   from any RDNSS MAY 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 mandated for the
   default case as in some deployments that would consume excess
   resources.

   The resolver SHOULD avoid sending queries over different network
   interfaces in parallel as that wastes resources such as power (in the
   case of battery powered and constrained environments).  The wasted
   power can be significant: consider starting multiple radio interfaces
   just for parallel DNS queries.

   In the case of validated NXDOMAIN response being received from a
   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.



















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

option-code:   OPTION_DNS_SERVER_SELECT (TBD)

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.

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 the RDNSS
               has special knowledge about. Field MUST be encoded as
               specified in Section "Representation and use of
               domain names" of [RFC3315].
               Special domain of "." is used to indicate
               capability to resolve global names and act as a
               default RDNSS. Lack of "."
               domain on the list indicates 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].




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              DHCPv6 option for explicit domain configuration

                                 Figure 5

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

   The OPTION_DNS_SERVER_SELECT contains one or more domains the related
   RDNSS has particular knowledge of.  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 the OPTION_DNS_SERVER_SELECT contains a 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 obtained information.  However, the node SHOULD NOT extend
   lifetime of earlier information either.  When a conflicting RDNSS
   address is learned from less trusted interface, the node MUST ignore
   the option.

   As the RDNSS options of [RFC3646], the OPTION_DNS_SERVER_SELECT
   option 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_DNS_SERVER_SELECT.  The refresh frequency is implementation
   specific, but the DHCPv6 Information Refresh Time Option, as
   specified in [RFC4242], can be used to control the frequency.  The
   information SHALL be refreshed at least on link-state changes, such
   as those caused by node mobility.






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4.3.  RDNSS Selection DHCPv4 Option

   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)                    |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code:   OPTION_DNS_SERVER_SELECT (TBD)

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.

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 the RDNSSes



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               have special knowledge about. Field MUST be encoded as
               specified in Section "Representation and use of
               domain names" of [RFC3315].
               Special domain of "." is used to indicate
               capability to resolve global names and act as
               default RDNSS. Lack of "."
               domain on the list indicates 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].

              DHCPv4 option for explicit domain configuration

                                 Figure 6

   The OPTION_DNS_SERVER_SELECT contains one or more domains the primary
   and secondary RDNSSes have particular knowledge of.  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.

   If the OPTION_DNS_SERVER_SELECT contains a 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
   lifetime of earlier information either.  When a conflicting RDNSS
   address is learned from less trusted interface, the node MUST ignore
   the option.

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.



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4.5.  Limitations on Use

   Use of OPTION_DNS_SERVER_SELECT is ideal in the following
   environments, but SHOULD NOT be enabled by default otherwise:

   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 RDNSS selection option over a secure, trusted
   channel.

   4.  The IP address of 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 RDNSS it has already been
   configured to trust is available to it for resolving certain domains.

4.6.  Coexistence of Various RDNSS Configuration Tools

   The DHCPv4 and DHCPv6 OPTION_DNS_SERVER_SELECT options are designed
   to coexist between 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 the DHCPv4 and the DHCPv6 are providing
   conflicting RDNSS selection information on the same or on the 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 other tools than OPTION_DNS_SERVER_SELECT
   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 Name Server Option defined in [RFC2132].




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

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

   If a node had indicated support for OPTION_DNS_SERVER_SELECT in
   DHCPv6 request, the DHCPv6 server MAY omit sending of
   OPTION_DNS_SERVERS.  This enables offloading use case where 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 same RDNSS, irrespectively of
   the FQDN received.  Otherwise referrals can fail.

4.8.  Closing Network Interfaces and Local Caches

   All information related to private namespaces can become obsolete
   after the network interface over which the information was learned on
   is closed (Section 2.2).  Therefore, during network interface
   closure, a node SHOULD flush its DNS cache at least from the entries
   that might relate to private namespaces: the names that were learned
   via RDNSS that had matching "Domains and Networks".


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  |         |                |
        |         +----------+         |                |
        |             |                |                |

                     Illustration of learning domains

                                 Figure 7

   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 DHCPv6
       server

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

   4.  The node opens its second network interface 2





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   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 DHCPv6 server

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

   Figure 8 below illustrates how a resolver uses the learned domain
   information.  Network information use for reverse lookups is not
   illustrated, but that would go as the figure 7 example.


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

               Example on choosing interface based on domain

                                 Figure 8

   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 to and uses configured
       RDNSS selection information and stored domain information on
       prioritization decisions.

   3.  The node has chosen interface 2, as from DHCPv6 it was learned
       earlier 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 application with the resolved IPv6 address





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6.  Considerations for Network Administrators

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

   Private namespaces MUST be globally unique in order to keep DNS
   unambiguous and henceforth avoiding 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 an 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.  Acknowledgements

   The author would like to thank 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.

   This document was prepared using xml2rfc template and the related
   web-tool.


8.  IANA Considerations

   This memo requests IANA to assign two new option codes.

   The first option code is requested to be assigned for the DHCPv4
   RDNSS Selection option (TBD) from the "BOOTP Vendor Extensions and
   DHCP Options" registry in the group "Dynamic Host Configuration
   Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters".




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


9.  Security Considerations

9.1.  Attack vectors

   It is possible that attackers might try to utilize
   OPTION_DNS_SERVER_SELECT 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 attacker's RDNSS as the highest preference default RDNSS.

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

9.2.  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 as trusted, while an unauthenticated and
   unencrypted connection to an unknown visited network would likely be
   considered as 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, access network technology specific
   standards defining organizations might be able to define trust levels
   as part of the system design work.

9.3.  Importance of Following the Algorithm

   The Section 4 uses normative language for describing node internal
   behavior in order to ensure nodes would not open up new attack



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   vectors by accidental use of RDNSS selection options.  During the
   standards work consensus was that it is safer to not to enable this
   option always by default, but only when deemed useful and safe.


10.  References

10.1.  Normative References

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

   [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.

   [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.

10.2.  Informative References

   [I-D.ietf-v6ops-ipv6-multihoming-without-ipv6nat]
              Matsushima, S., Okimoto, T., Troan, O., Miles, D., and D.
              Wing, "IPv6 Multihoming without Network Address
              Translation",
              draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-04 (work
              in progress), February 2012.

   [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



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              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.


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 is
   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 also
   via cheaper interface.  Also load on RDNSSes is increased.  However,
   local caching of results mitigates these problems, and a node might



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   also learn interfaces that seem to be able to provide 'better'
   responses than other and prefer those - without forgetting fallback
   required for cases when 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 as is illustrated in the example at
   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 query.

   Overloading of existing DNS search list options is not without
   problems: resolvers would obviously use the domains learned from
   search lists also 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.

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.  A RDNSS configured via the same interface as more
   specific routes is more likely capable to answer reverse lookup
   questions correctly than RDNSS of an another interface.  The
   likelihood of success is possibly higher if 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 a RDNSS learned over an interface having
   longest matching prefix to the address being queried.  This approach
   can help in cases where ULA [RFC4193] addresses are used and when the
   queried address belongs to a node or server within the same network
   (for example intranet).






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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 terms 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.  Pseudo Code for RDNSS Selection

   This section illustrates the RDNSS selection logic in C-style pseudo
   code.  The code is not intended to be usable as such, but 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.


/* This is a structure that holds all information related to a RDNSS. */
/* Here we include only the information related for this illustration.*/
struct rdnss
{
  int prf;        /* Preference of a 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.      */
}



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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 comparest 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;

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

  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



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   preferrer 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
   or it 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)))
        {
/* 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;



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    }
}

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 two next 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)
            {
/* 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;




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/* Sort the RDNSSes into preference order.                            */
/* This is the function this pseudo code starts with.                 */
  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_vaidate_dns_query( rndss_list[i], query);

/* If DNSSEC validation is used.                                      */
      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;
    }
  return NULL;
}


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




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   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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