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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: October 14, 2011                                            NTT
                                                                T. Lemon
                                                           Nominum, Inc.
                                                          April 12, 2011


          Improved DNS Server Selection for Multi-Homed Nodes
                 draft-ietf-mif-dns-server-selection-02

Abstract

   A multi-homed node can be connected to multiple networks that may
   utilize different DNS namespaces.  The node commonly receives DNS
   server configuration information from all connected networks.  Some
   of the DNS servers may have information about namespaces other
   servers do not have.  When the multi-homed node needs to utilize DNS,
   it has to choose which of the servers to contact to.  This document
   describes a policy based method for helping on selection of DNS
   server, for both forward and reverse DNS lookup procedures, with help
   of DNS suffix and IPv6 prefix information received via DHCPv6.

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 October 14, 2011.

Copyright Notice

   Copyright (c) 2011 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Problem description for local namespaces with multi-homed
       nodes  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     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 . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  CPE deployment scenario  . . . . . . . . . . . . . . . . .  7
     3.2.  Cellular network scenario  . . . . . . . . . . . . . . . .  7
     3.3.  VPN scenario . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Improved DNS server selection  . . . . . . . . . . . . . . . .  8
     4.1.  Procedure for prioritizing DNS servers and handling
           responses  . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  DNS server selection option  . . . . . . . . . . . . . . .  9
     4.3.  Limitations on use . . . . . . . . . . . . . . . . . . . . 12
     4.4.  Coexistence with RFC3646 . . . . . . . . . . . . . . . . . 12
     4.5.  Interactions with OPTION_DOMAIN_LIST . . . . . . . . . . . 13
     4.6.  CNAME/DNAME record considerations  . . . . . . . . . . . . 13
   5.  Example of a node behavior . . . . . . . . . . . . . . . . . . 13
   6.  Scalability considerations . . . . . . . . . . . . . . . . . . 16
   7.  Considerations for network administrators  . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Best Current Practice for DNS server selection  . . . 19
     A.1.  Sending queries out on multiple interfaces in parallel . . 19
     A.2.  Search list option for DNS forward lookup decisions  . . . 19
     A.3.  More specific routes for reverse lookup decision . . . . . 20
     A.4.  Longest matching prefix for reverse lookup decision  . . . 20
   Appendix B.  DNSSEC and multiple answers validating with
                different trust anchors . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20




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1.  Introduction

   A multi-homed node faces several problems a single-homed node does
   not encounter, as is described in [I-D.ietf-mif-problem-statement].
   This document studies in detail the problems local namespaces may
   cause for multi-homed nodes and provides a solution for IPv6 domain.
   The node may be implemented as a host or as a router.

   When multiple namespaces are visible for a node, some DNS servers
   have information other servers do not have.  Because of that, a
   multi-homed node cannot assume every DNS server is able to properly
   answer for any query, but instead the node must be able to ask right
   server for the information it needs.

   An example of an application that benefits from multi-homing is a web
   browser that commonly accesses many different destinations and needs
   to be able to dynamically communicate over different network
   interfaces.

   In deployments where multiple namespaces are present, selection of
   correct route and destination and source addresses for the actual IP
   connection is crucial as well, as the resolved destination's IP
   addresses may be only usable on the network interface over which the
   name was resolved on.  Hence 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.

   The Appendix A describes best current practices possible with tools
   preceding this document and on networks not supporting the solution
   described in this document.  As it is possible to solve the problem
   with less efficient and less explicit manners, the new solution may
   be considered as an optimization.  However, in some environments this
   solution is considered essential.

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.  Problem description for local namespaces with multi-homed nodes

   This chapter describes two host multi-homing related local namespace
   scenarios for which the procedure described in chapter 3 provides a
   solution for.  Essentially the same challenges may be faced by
   Consumer Premises Equipment as is described in



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   [I-D.ietf-v6ops-multihoming-without-nat66].  This chapter
   additionally describes a related problem for which this document
   provides only partial solution.

2.1.  Fully qualified domain names with limited scopes

   A multi-homed node may be connected to one or more networks that are
   using local namespaces.  As an example, the node may 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 a corporate network.  When an
   application initiates a connection establishment to an FQDN, the host
   needs to be able to choose the right DNS server for making a
   successful DNS query.  This is illustrated in the figure 1.  An FQDN
   for a public name can be usually resolved with any DNS server, but
   for an FQDN of corporation's or operator's service's local name the
   node needs to be able to correctly select the right DNS server for
   the DNS resolution, i.e. do also network interface selection already
   before destination's IP address is known.


                            +---------------+
                            | DNS server w/ |    |   Corporate
   +------+                 | public +      |----|   Intranet
   |      |                 | corporation's |    |
   |      |===== VPN =======| local names   |    |
   |      |                 +---------------+  +----+
   | MIF  |                                    | FW |
   | node |                                    +----+
   |      |                 +---------------+    |
   |      |----- WLAN ------| DNS server w/ |----|   Public
   |      |                 | public names  |    |   Internet
   |      |                 +---------------+  +----+
   |      |                                    | FW |
   |      |                 +---------------+  +----+
   |      |---- cellular ---| DNS server w/ |    |
   +------+                 | public +      |    |   Operator
                            | operator's    |----|   Intranet
                            | local names   |    |
                            +---------------+

                     Local DNS namespaces illustrated

                                 Figure 1







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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 must
   ensure the destination's IP address is only used in combination with
   source IP addresses of the network interface the name was resolved
   on.


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


      Local 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 [I-D.ietf-behave-dns64].  A synthesised AAAA record
   is guaranteed to be valid only on a network interface it was
   synthesized on.  Figure 3 illustrates a scenario where the peer's
   IPv4 address is synthesized into different IPv6 addresses by DNS
   servers A and B.
















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


       AAAA synthesis results in interface specific IPv6 addresses.

                                 Figure 3

   A thing worth noting is that interface specific IP addresses can
   cause problems also for a single-homed host, if the host retains its
   DNS cache during movement from one network interface to another.
   After the interface change a host could have both positive and
   negative DNS cache entries no longer valid on the new network
   interface.  Because of this the cached DNS information should be
   considered network interface local instead of node global.

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 may be solvable only by
   manual user intervention.

   However, when DNSSEC is used, the DNSSEC validation procedure may
   provide assistance for selecting correct responses for some, but not
   all, use cases.  A node may 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 VLAN connections.  Second scenario involves a



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   cellular device with two uplink Internet connections: WLAN and
   cellular.  Third scenario is for VPNs, where use of local DNS server
   may be preferred for latency reasons, but corporate DNS server must
   be used to resolve private names used by the corporation.

3.1.  CPE deployment scenario

   A home gateway may have two uplink connections leading to different
   networks, as is described in
   [I-D.ietf-v6ops-multihoming-without-nat66].  In this scenario only
   first uplink connections lead to Internet, while second uplink
   connection leads to a private network utilizing private namespace.

   It is desirable that the CPE does not have to send DNS queries over
   both uplink connections, but instead CPE should send default queries
   to the DNS server of the interface leading to the Internet, and
   queries related to private namespace to the DNS server of the private
   network.

   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 DNS servers of each uplink ISP's DNS server.  It is
   deployment decision whether the updated hosts would be pointed to DNS
   proxy or to actual DNS servers.

   Depending on actual deployments, all VLAN connections may be
   considered secure.

3.2.  Cellular network scenario

   A cellular device may 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 cellular network may utilize private
   names and hence the cellular device needs to ask for those through
   the cellular interface.

   In this scenario cellular interface can be considered secure and WLAN
   often insecure.

3.3.  VPN scenario

   Depending on a deployment, there may be need to use VPN only for
   traffic destined to a corporate network.  The corporation may be
   using private namespace, and hence related DNS queries should be send
   over VPN to the corporate DNS server, while by default a DNS server
   of a local access network may be used.



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   In this scenario VPN interface can be considered secure and local
   access network insecure.


4.  Improved DNS server selection

   This chapter describes a procedure that a (stub / proxy) resolver may
   utilize for improved DNS server selection in face of multiple
   namespaces and multiple simultaneously active network interfaces.

4.1.  Procedure for prioritizing DNS servers and handling responses

   A resolver SHALL build a priority list of DNS servers it will contact
   to depending on the query.  To build the list in an optimal way, a
   node SHOULD ask with DHCPv6 which DNS servers of each network
   interface are most likely able to successfully serve forward lookup
   requests matching to specific DNS suffixes or reverse (PTR record)
   lookup requests matching to specific IPv6 prefixes.  For security
   reasons the DNS server selection information MUST be used only when
   it is safe to do so, see section 4.3 for details.

   A resolver lacking more explicit information shall assume that all
   information is available from any DNS server of any network
   interface.  The DNS servers learnt by other DNS server address
   configuration methods MUST be handled as medium priority default
   servers.

   When a DNS query needs to be made, the resolver SHOULD give highest
   precedence to the DNS servers explicitly known to serve matching
   suffixes or prefixes.  However, the resolver SHOULD take into account
   different trust levels of pieces of DNS server selection information
   the resolver may have received from node's network interfaces.  The
   resolver SHOULD prefer DNS servers of trusted interfaces.  The DNS
   servers of trusted interfaces may be of highest priority only if
   trusted interfaces specifically configure DNS servers to be of low
   priority.  The non-exhaustive list on figure 4 illustrates how the
   different trust levels of received DNS server selection information
   SHOULD influence the DNS server selection logic.













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   Information from       | Information from       | Resulting DNS
   from more trusted      | less trusted           | server priority
   interface A            | interface B            | selection
--------------------------+------------------------+--------------------
1. Medium priority        | Medium priority        | Default:  A, then B
   default                | default                |
--------------------------+------------------------+--------------------
2. Medium priority        | High priority default  | Default:  A, then B
   default                | High priority specific | Specific: A, then B
--------------------------+------------------------+--------------------
3. Low priority default   | Medium priority        | Default:  B, then A
                          | default                |
--------------------------+------------------------+--------------------
4. Low priority default   | Medium priority        | Default:  B, then A
   High priority specific | default                | Specific: A, then B
--------------------------+------------------------+--------------------

     Figure 4: DNS server selection in case of different trust levels

   The resolver SHOULD avoid sending queries to different interfaces in
   parallel as that may waste resources, sometimes significantly, and
   would also unnecessary reveal information about ongoing
   communications.  Independently of whether DNS queries are sent in
   series or parallel, replies for DNS queries MUST be waited until
   acceptable positive reply is received, all replies are received, or
   time out occurs.

   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.  It follows that, if
   validation is not performed by the host making the decision about
   whether to trust the DNS data from a given interface, it cannot make
   a decision to prefer data from any interface with any great
   assurance: any response could be forged, and there is no way to
   detect it without DNSSEC.  Specifically, the validating security
   aware host MUST NOT proceed with a reply that cannot be validated
   with DNSSEC if DNS queries sent to other servers are still pending.

   (DISCUSS: What about those DNS servers that instead of negative
   answer always return positive reply with an IP address of some
   captive portal?)

4.2.  DNS server selection option

   A DHCPv6 option described below is used to inform resolvers which DNS
   server should be contacted when initiating forward or reverse DNS
   lookup procedures.




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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)           |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|prf| Reserved  |                                               |
+-+-+-+-+-+-+-+-+          DNS suffixes and prefixes            |
|                          (variable length)                    |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code:   OPTION_DNS_SERVER_SELECT (TBD)

option-len:    Lenght of the option in octets

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

prf:           DNS server preference, for selecting between
               equally trusted DNS servers:
                   01 High
                   00 Medium
                   11 Low
                   10 Reserved

Reserved:      Flags reserved for the future. MUST be set to zero.

DNS suffixes and prefixes:  The list of DNS suffixes for forward DNS
               lookup and prefixes for reverse DNS lookup the DNS server
               has special knowledge about. Field MUST be encoded as
               specified in section "Representation and use of
               domain names" of [RFC3315].
               Special suffix of "." is used to indicate
               capability to resolve global names and act as a
               default name server. Lack of "."
               suffix on the list indicates DNS server only has
               information related to listed suffixes and prefixes.
               Prefixes for reverse mapping are encoded as
               defined for ip6.arpa [RFC3152].

            DHCPv6 option for explicit DNS suffix configuration

                                 Figure 5




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   A node SHOULD include an OPTION_ORO option in a DHCPv6 request with
   the OPTION_DNS_SERVER_SELECT option code to inform the DHCPv6 server
   about the support for the improved DNS server selection logic.
   DHCPv6 server receiving this information MAY then choose to provision
   DNS server addresses only with the OPTION_DNS_SERVER_SELECT.

   The OPTION_DNS_SERVER_SELECT contains one or more DNS suffixes the
   related DNS server has particular knowledge of.  The option can occur
   multiple times in a single DHCPv6 message, if multiple DNS servers
   are to be configured.

   A resolver SHOULD prioritize between equally trusted DNS servers with
   help of the preference field.  The resolver SHOULD NOT prioritize
   less trusted DNS servers higher than trusted, even in the case of
   less trusted server would apprently have additional information.  In
   the case of all other things being equal the resolver shall make the
   prioritization decision based on its internal preferences.

   IPv6 prefixes should cover all the DNS suffixes configured in this
   option.  Prefixes should be as long as possible to avoid potential
   collision with information received on other option instances or with
   options received from DHCPv6 servers of other network interfaces.
   Overlapping IPv6 prefixes are interpreted so that the resolver can
   use any of the DNS servers for queries mathing the prefixes.

   If the OPTION_DNS_SERVER_SELECT contains a DNS server address already
   learned from other DHCPv6 servers and possibly through other network
   interfaces, the node MAY append new prefixes and suffixes to the
   information received earlier.  The node MUST NOT remove previously
   obtained information.  However, the node SHOULD NOT extent lifetime
   of earlier information either.  In the case conflicting DNS server
   address and related information is learned from less trusted
   interface, the node MAY choose to ignore the option.

   As the DNS 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 node SHOULD create a host specific route for the DNS server
   address.  The route must point to the interface DNS server address
   was learned on.  This is required to ensure DNS queries are sent out
   via the right interface.

   In the case of a trusted DNS server replying negatively to a question
   having matching suffix, it will be for implementation to decide
   whether to consider that as a final response, or whether to ask also
   from other DNS servers.  The implementation decision may be based,



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   for example, on deployment or trust models.

   The information conveyed in OPTION_DNS_SERVER_SELECT is considered
   valid until changed or refreshed by general events that trigger
   DHCPv6 action.  In the event that it is desired for the client to
   request a refresh of the information, use of generic DHCPv6
   Information Refresh Time Option, as specified in [RFC4242] is
   envisaged.

4.3.  Limitations on use

   A node MAY use OPTION_DNS_SERVER_SELECT in any of the following four
   cases.  In other cases the node MUST NOT use OPTION_DNS_SERVER_SELECT
   unless the node is specifically configured to do so.

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

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

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

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

4.4.  Coexistence with RFC3646

   The OPTION_DNS_SERVER_SELECT is designed to coexist with
   OPTION_DNS_SERVERS defined in [RFC3646].  The DNS servers configured
   via OPTION_DNS_SERVERS MUST BE considered as default name servers
   with medium preference.  When both options are received from the same
   network interface and the OPTION_DNS_SERVER_SELECT contains default
   DNS server address, the resolver MUST make the decision which one to
   prefer based on preferences.  If OPTION_DNS_SERVER_SELECT defines
   medium preference then DNS server from OPTION_DNS_SERVER_SELECT SHALL
   be selected.  All default servers are assumed to be able to resolve
   queries for global names.

   If both OPTION_DNS_SERVERS and OPTION_DNS_SERVER_SELECT contain the
   same DNS server(s) IPv6 address(es), only one instance of each DNS
   servers' IPv6 addresses shall be added to the DNS server list.



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   If a node had indicated support for OPTION_DNS_SERVER_SELECT in
   DHCPv6 request, the DHCPv6 server may choose to omit sending of
   OPTION_DNS_SERVERS.  This enables offloading use case where network
   administrator wishes to only advertise low priority default DNS
   servers.

4.5.  Interactions with OPTION_DOMAIN_LIST

   A node may be configured with DNS search list with
   OPTION_DOMAIN_LIST.  Resolution for the name containing any dots
   SHOULD first be attempted with DNS servers of all interfaces as
   described earlier.  Only if the resolution fails the node SHOULD
   append the name with search list suffix(es) and then utilize improved
   DNS server selection algorith again to decide which DNS server(s) to
   contact next.  A name without any dots SHALL immediately be appended
   with suffix(es) and improved DNS server selection be utilized on
   resolution.

4.6.  CNAME/DNAME record considerations

   If a node receives a reply with a canonical name (CNAME) or
   delegation name (DNAME) the follow-up queries MUST be sent to the
   same DNS server irrespectively of the FQDN received.  Otherwise
   referrals may fail.


5.  Example of a node behavior

   Figure 6 illustrates node behavior when it initializes two network
   interfaces for parallel usage and learns DNS suffix and prefix
   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     |        |
        |         | suffixes  |        |
        |         +-----------+        |
        |             |                |
        |         +-----------+        |
   (4)  |         | open      |        |
        |         | interface |        |
        |         +-----------+        |
        |             |                |                |
   (5)  |             |---option REQ------------------->|
        |             |<--option RESP-------------------|
        |             |                |                |
        |         +----------+         |                |
   (6)  |         | store    |         |                |
        |         | suffixes |         |                |
        |         +----------+         |                |
        |             |                |                |

                   Illustration of learning DNS suffixes

                                 Figure 6

   Flow explanations:

   1.  A node opens its first network interface

   2.  The node obtains DNS suffix and IPv6 prefix information for the
       new interface 1 from DHCPv6 server

   3.  The node stores the learned DNS suffixes and IPv6 prefixes for
       later use

   4.  The node opens its seconds network interface 2

   5.  The node obtains DNS suffix, say 'example.com', and IPv6 prefix
       information, say '8.b.d.0.1.0.0.2.ip6.arpa' for the new interface



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       2 from DHCPv6 server

   6.  The node stores the learned DNS suffixes and prefixes for later
       use

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


    Application     Node     DHCPv6 server     DHCPv6 server
                             on interface 1    on interface 2
        |             |                |                |
   (1)  |--Name REQ-->|                |                |
        |             |                |                |
        |      +----------------+      |                |
   (2)  |      | DNS server     |      |                |
        |      | prioritization |      |                |
        |      +----------------+      |                |
        |             |                |                |
   (3)  |             |------------DNS resolution------>|
        |             |<--------------------------------|
        |             |                |                |
   (4)  |<--Name resp-|                |                |
        |             |                |                |

             Example on choosing interface based on DNS suffix

                                 Figure 7

   Flow explanations:

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

   2.  A node creates list of DNS servers to contact to and uses
       configured DNS server information and stored DNS suffix
       information on priorization decisions.

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

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






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6.  Scalability considerations

   The size limitations of DHCPv6 messages limit the number of suffixes
   and prefixes that can be carried in a configuration option.
   Including the suffixes and prefixes in a DHCPv6 option is best suited
   for deployments where relatively few carefully selected suffixes and
   prefixes are adequate.


7.  Considerations for network administrators

   Network administrators deploying private namespaces should assist
   advanced hosts in the DNS server selection by providing information
   described in this document for nodes.  To ensure nodes' routing and
   source and destination IP address selection also works correctly,
   network administrators should also deploy related technologies for
   that purpose.

   The solution described herein is best for selecting a DNS server
   having knowledge of some namespaces.  The solution is not able to
   make the right decision in a scenario where the same name points to
   different services on different network interfaces, as described in
   section 2.3.  Network administrators are recommended to avoid
   overloading of namespaces in such manner.

   To mitigate against attacks against local namespaces, administrators
   utilizing this tool should deploy DNSSEC for their zone.


8.  Acknowledgements

   The author would like to thank following people for their valuable
   feedback and improvement ideas: Mark Andrews, Jari Arkko, Marcelo
   Bagnulo, Stuart Cheshire, Lars Eggert, Tomohiro Fujisaki, Peter Koch,
   Suresh Krishnan, Edward Lewis, Kurtis Lindqvist, Arifumi Matsumoto,
   Erik Nordmark, Steve Padgett, Fabien Rapin, Dave Thaler, 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.


9.  IANA Considerations

   This memo includes a new DHCPv6 option that requires allocation of a
   new code point.



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10.  Security Considerations

   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 DNS
   suffixes and prefixes from very small to very large scope or simply
   by trying to place attacker's DNS server as the highest priority
   default server.

   The main countermeasure against these attacks is to use this option
   only when safe to do so, see section 4.3 for deatils.  The safest
   approach is for nodes to implement validating DNSSEC aware resolvers.
   Trusting on validation done by a DNS server is a possibility only if
   a host trusts the DNS server and can use a secure channel for DNS
   messages.

   Decision on trust levels of network interfaces depends very much on
   deployment scenario and types of network interfaces.  For example,
   unmanaged WLAN may be considered less trustworthy than managed
   cellular or VPN connections.

   A node that accepts DNS server selection rules from non-trusted
   interfaces and implements DNSSEC validation SHOULD send queries also
   to (all) other known DNS servers in case a non-validatable response
   is received from the preferred DNS server.  This protects against
   possible redirection attacks.


11.  References

11.1.  Normative References

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

   [RFC3152]  Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
              August 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.



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

11.2.  Informative References

   [I-D.ietf-behave-dns64]
              Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
              "DNS64: DNS extensions for Network Address Translation
              from IPv6 Clients to IPv4 Servers",
              draft-ietf-behave-dns64-11 (work in progress),
              October 2010.

   [I-D.ietf-mif-problem-statement]
              Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement",
              draft-ietf-mif-problem-statement-12 (work in progress),
              April 2011.

   [I-D.ietf-v6ops-multihoming-without-nat66]
              Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
              Wing, "IPv6 Multihoming without Network Address
              Translation",
              draft-ietf-v6ops-multihoming-without-nat66-00 (work in
              progress), December 2010.

   [I-D.wing-behave-dns64-config]
              Wing, D., "IPv6-only and Dual Stack Hosts on the Same
              Network with DNS64", draft-wing-behave-dns64-config-03
              (work in progress), February 2011.

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

   [RFC5006]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Option for DNS Configuration",
              RFC 5006, September 2007.



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Appendix A.  Best Current Practice for DNS server selection

   On some local namespace deployments explicit policies for DNS server
   selection are not available.  This section describes ways for hosts
   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
   host SHOULD 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 servers should be
   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 DNS servers 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 other and prefer those - without forgetting
   fallback required for cases when node is connected to more than one
   network using local namespaces.

   Another downside is revealing to all DNS servers the names a host is
   connecting to.  For example, a DNS server of a public hotspot could
   learn all the private names host is trying to connect on other
   interfaces.

A.2.  Search list option for DNS forward lookup decisions

   A host can learn the special DNS suffixes of attached network
   interfaces from DHCP search list options; DHCPv4 Domain Search Option
   number 119 [RFC3397] and DHCPv6 Domain Search List Option number 24
   [RFC3646].  The host behavior is very similar as is illustrated in
   the example at section 3.3.  While these DHCP options are not
   intented to be used in DNS server selection, they may be used by the
   host as hints for smarter DNS server prioritization purposes in order
   to increase likelyhood of fast and successful DNS query.

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



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A.3.  More specific routes for reverse lookup decision

   [RFC4191] defines how more specific routes can be provisioned for
   hosts.  This information is not intented to be used in DNS server
   selection, but nevertheless a host can use this information as a hint
   about which interface would be best to try first for reverse lookup
   procedures.  A DNS server configured via the same interface as more
   specific routes is more likely capable to answer reverse lookup
   questions correctly than DNS server of an another interface.  The
   likelyhood of success is possibly higher if DNS server address is
   received in the same RA [RFC5006] as the more specific route
   information.

A.4.  Longest matching prefix for reverse lookup decision

   A host may utilize the longest matching prefix approach when deciding
   which DNS server to contact for reverse lookup purposes.  Namely, the
   host may send a DNS query to a DNS server 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 host 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 terms of
   the host's preferences for those trust anchors.  A host could use
   this ability in order to select among alternative DNS results from
   different interfaces.  Suppose that a host 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 host has evidence
   for relying on i2 for answers in the example.com zone.












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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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