Internet Engineering Task Force (IETF)                      M. Wasserman
Request for Comments: 6419                        Painless Security, LLC
Category: Informational                                         P. Seite
ISSN: 2070-1721                                  France Telecom - Orange
                                                           November 2011

             Current Practices for Multiple-Interface Hosts


   An increasing number of hosts are operating in multiple-interface
   environments.  This document summarizes current practices in this
   area and describes in detail how some common operating systems cope
   with challenges that ensue from this context.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
   ( 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Summary of Current Approaches  . . . . . . . . . . . . . . . .  3
     2.1.  Centralized Connection Management  . . . . . . . . . . . .  3
     2.2.  Per-Application Connection Settings  . . . . . . . . . . .  4
     2.3.  Stack-Level Solutions to Specific Problems . . . . . . . .  4
       2.3.1.  DNS Resolution Issues  . . . . . . . . . . . . . . . .  5
       2.3.2.  First-Hop Selection  . . . . . . . . . . . . . . . . .  5
       2.3.3.  Address Selection Policy . . . . . . . . . . . . . . .  5
   3.  Current Practices in Some Operating Systems  . . . . . . . . .  6
     3.1.  Mobile Handset Operating Systems . . . . . . . . . . . . .  6
       3.1.1.  Nokia S60 3rd Edition, Feature Pack 2  . . . . . . . .  7
       3.1.2.  Microsoft Windows Mobile and Windows Phone 7 . . . . .  9
       3.1.3.  RIM BlackBerry . . . . . . . . . . . . . . . . . . . . 10
       3.1.4.  Google Android . . . . . . . . . . . . . . . . . . . . 11
       3.1.5.  Qualcomm Brew  . . . . . . . . . . . . . . . . . . . . 12
       3.1.6.  Leadcore Technology Arena  . . . . . . . . . . . . . . 13
     3.2.  Desktop Operating Systems  . . . . . . . . . . . . . . . . 14
       3.2.1.  Microsoft Windows  . . . . . . . . . . . . . . . . . . 14  First-Hop Selection  . . . . . . . . . . . . . . . 14  Outbound and Inbound Addresses . . . . . . . . . . 14  DNS Configuration  . . . . . . . . . . . . . . . . 15
       3.2.2.  Linux and BSD-Based Operating Systems  . . . . . . . . 16  First-Hop Selection  . . . . . . . . . . . . . . . 16  Outbound and Inbound Addresses . . . . . . . . . . 16  DNS Configuration  . . . . . . . . . . . . . . . . 17
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   Multiple-interface hosts face several challenges not faced by single-
   interface hosts, some of which are described in the multiple
   interfaces (MIF) problem statement [RFC6418].  This document
   summarizes how current implementations deal with the problems
   identified in the MIF problem statement.

   Publicly available information about the multiple-interface solutions
   implemented in some widely used operating systems, including both
   mobile handset and desktop operating systems, is collected in this
   document, including Nokia S60 [S60], Microsoft Windows Mobile
   [WINDOWSMOBILE], Blackberry [BLACKBERRY], Google Android [ANDROID],
   Microsoft Windows, Linux, and BSD-based operating systems.

2.  Summary of Current Approaches

   This section summarizes current approaches that are used to resolve
   the multiple-interface issues described in the MIF problem statement
   [RFC6418].  These approaches can be broken down into three major

   o  Centralized connection management

   o  Per-application connection settings

   o  Stack-level solutions to specific problems

2.1.  Centralized Connection Management

   It is a common practice for mobile handset operating systems to use a
   centralized connection manager that performs network interface
   selection based on application or user input.  However, connection
   managers usually restrict the problem to the selection of the
   interface and do not cope with selection of the provisioning domain,
   as defined in [RFC6418].  The information used by the connection
   manager may be programmed into an application or provisioned on a
   handset-wide basis.  When information is not available to make an
   interface selection, the connection manager will query the user to
   choose between available choices.

   Routing tables are not typically used for network interface selection
   when a connection manager is in use, as the criteria for network
   selection is not strictly IP-based but is also dependent on other
   properties of the interface (cost, type, etc.).  Furthermore,
   multiple overlapping private IPv4 address spaces are often exposed to
   a multiple-interface host, making it difficult to make interface
   selection decisions based on prefix matching.

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2.2.  Per-Application Connection Settings

   In mobile handsets, applications are often involved in choosing what
   interface and related configuration information should be used.  In
   some cases, the application selects the interface directly, and in
   other cases, the application provides more abstract information to a
   connection manager that makes the final interface choice.

2.3.  Stack-Level Solutions to Specific Problems

   In most desktop operating systems, multiple-interface problems are
   dealt with in the stack and related components, based on system-
   level configuration information, without the benefit of input from
   applications or users.  These solutions tend to map well to the
   problems listed in the problem statement:

   o  DNS resolution issues

   o  Routing

   o  Address selection policy

   The configuration information for desktop systems comes from one of
   the following sources: DHCP, router advertisements, proprietary
   configuration systems, or manual configuration.  While these systems
   universally accept IP address assignment on a per-interface basis,
   they differ in what set of information can be assigned on a per-
   interface basis and what can be configured only on a per-system

   When choosing between multiple sets of information provided, these
   systems will typically give preference to information received on the
   "primary" interface.  The mechanism for designating the "primary"
   interface differs by system.

   There is very little commonality in how desktop operating systems
   handle multiple sets of configuration information, with notable
   variations between different versions of the same operating system
   and/or within different software packages built for the same
   operating system.  Although these systems differ widely, it is not
   clear that any of them provide a completely satisfactory user
   experience in multiple-interface environments.

   The following sections discuss some of the solutions used in each of
   the areas raised in the MIF problem statement.

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2.3.1.  DNS Resolution Issues

   There is very little commonality in how desktop operating systems
   handle the DNS server list.  Some systems support per-interface DNS
   server lists, while others only support a single system-wide list.

   On hosts with per-interface DNS server lists, different mechanisms
   are used to determine which DNS server is contacted for a given
   query.  In most cases, the first DNS server listed on the "primary"
   interface is queried first, with back off to other servers if an
   answer is not received.

   Systems that support a single system-wide list differ in how they
   select which DNS server to use in cases where they receive more than
   one DNS server list to configure (e.g., from DHCP on multiple
   interfaces).  Some accept the information received on the "primary"
   interface, while others use either the first or last set DNS server
   list configured.

2.3.2.  First-Hop Selection

   Routing information is also handled differently on different desktop
   operating systems.  While all systems maintain some sort of routing
   cache, to handle redirects and/or statically configured routes, most
   packets are routed based on configured default gateway information.

   Some systems do allow the configuration of different default router
   lists for different interfaces.  These systems will always choose the
   default gateway on the interface with the lowest routing metric, with
   different behavior when two or more interfaces have the same routing

   Most systems do not allow the configuration of more than one default
   router list, choosing instead to use the first or last default router
   list configured and/or the router list configured on the "primary"

2.3.3.  Address Selection Policy

   There is somewhat more commonality in how desktop hosts handle
   address selection.  Applications typically provide the destination
   address for an outgoing packet, and the IP stack is responsible for
   picking the source address.

   IPv6 specifies a specific source address selection mechanism in
   [RFC3484], and several systems implement this mechanism with similar
   support for IPv4.  However, many systems do not provide any mechanism
   to update this default policy, and there is no standard way to do so.

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   In some cases, the routing decision (including which interface to
   use) is made before source address selection is performed, and a
   source address is chosen from the outbound interface.  In other
   cases, source address selection is performed before, or independently
   from, outbound interface selection.

3.  Current Practices in Some Operating Systems

   The material presented in this section is derived from contributions
   from people familiar with the operating systems described (see
   Section 6 a list of these individuals).  The authors and the IETF
   take no position about the operating systems described and understand
   that other operating systems also exist.  Furthermore, it should be
   understood that Section 3 describes particular behaviors that were
   believed to be current at the time this document was written: earlier
   and later versions of the operating systems described may exhibit
   different behaviors.  Please refer to the References section for
   pointers to original documentation, including further details.

3.1.  Mobile Handset Operating Systems

   Cellular devices typically run a variety of applications in parallel,
   each with different requirements for IP connectivity.  A typical
   scenario is shown in Figure 1, where a cellular device is utilizing
   Wireless Local Area Network (WLAN) access for web browsing and
   General Packet Radio Service (GPRS) access for transferring
   multimedia messages (MMS).  Another typical scenario would be a real-
   time Voice over IP (VoIP) session over one network interface in
   parallel with best-effort web browsing on another network interface.
   Yet another typical scenario would be global Internet access through
   one network interface and local (e.g., corporate VPN) network access
   through another.

        Web server                                       MMS Gateway
             |                                                |
            -+--Internet----            ----Operator network--+-
                    |                          |
                +-------+                  +-------+
                |WLAN AP|                  | GGSN  |
                +-------+                  +-------+
                    |        +--------+        |
                             |device  |

               A Cellular Device with Two Network Interfaces

                                 Figure 1

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   Different network access technologies require different settings.
   For example, WLAN requires the Service Set Identifier (SSID), and the
   GPRS network requires the Access Point Name (APN) of the Gateway GPRS
   Support Node (GGSN), among other parameters.  It is common that
   different accesses lead to different destination networks (e.g., to
   Internet, intranet, cellular network services, etc.).

3.1.1.  Nokia S60 3rd Edition, Feature Pack 2

   S60 is a software platform for mobile devices running on the Symbian
   operating system (OS).  S60 uses the concept of an Internet Access
   Point (IAP) [S60] that contains all information required for opening
   a network connection using a specific access technology.  A device
   may have several IAPs configured for different network technologies
   and settings (multiple WLAN SSIDs, GPRS APNs, dial-up numbers, and so
   forth).  There may also be 'virtual' IAPs that define parameters
   needed for tunnel establishment (e.g., for VPN).

   For each application, a correct IAP needs to be selected at the point
   when the application requires network connectivity.  This is
   essential, as the wrong IAP may not be able to support the
   application or reach the desired destination.  For example, an MMS
   application must use the correct IAP in order to reach the MMS
   Gateway, which typically is not accessible from the public Internet.
   As another example, an application might need to use the IAP
   associated with its corporate VPN in order to reach internal
   corporate servers.  Binding applications to IAPs avoids several
   problems, such as choosing the correct DNS server in the presence of
   split DNS (as an application will use the DNS server list from its
   bound IAP) and overlapping private IPv4 address spaces used for
   different interfaces (as each application will use the default routes
   from its bound IAP).

   If multiple applications utilize the same IAP, the underlying network
   connection can typically be shared.  This is often the case when
   multiple Internet-using applications are running in parallel.

   The IAP for an application can be selected in multiple ways:

   o  Statically: for example, from a configuration interface, via
      client provisioning/device management system, or at build-time.

   o  Manually by the user: for example, each time an application
      starts, the user may be asked to select the IAP to use.  This may
      be needed, for example, if a user sometimes wishes to access his
      corporate intranet and other times would prefer to access the
      Internet directly.

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   o  Automatically by the system: after the destination network has
      been selected statically or dynamically.

   The static approach is fine for certain applications, like MMS, for
   which configuration can be provisioned by the network operator and
   does not change often.  Manual selection works but may be seen as
   troublesome by the user.  An automatic selection mechanism needs to
   have some way of knowing which destination network the user, or an
   application, is trying access.

   S60 3rd Edition, Feature Pack 2 introduces the concept of Service
   Network Access Points (SNAPs) that group together IAPs that lead to
   the same destination.  This enables static or manual selection of the
   destination network for an application and leaves the problem of
   selecting the best of the available IAPs within a SNAP to the
   operating system.

   When SNAPs are used, the operating system can notify applications
   when a preferred IAP, leading to the same destination, becomes
   available (for example, when a user comes within range of his home
   WLAN access point) or when the currently used IAP is no longer
   available.  If so, applications have to reconnect via another IAP
   (for example, when a user goes out of range of his home WLAN and must
   move to the cellular network).

   S60 3.2 does not support RFC 3484 for source address selection
   mechanisms.  Applications are tightly bound to the network interface
   selected for them or by them.  For example, an application may be
   connected to an IPv6 3G connection, IPv4 3G connection, WLAN
   connection, or VPN connection.  The application can change between
   the connections but uses only one at a time.  If the interface
   happens to be dual-stack, then IPv4 is preferred over IPv6.

   DNS configuration is per-interface; an application bound to an
   interface will always use the DNS settings for that interface.
   Hence, the device itself remembers these pieces of information for
   each interface separately.

   S60 3.2 manages with totally overlapping addresses spaces.  Each
   interface can even have the same IPv4 address configured on it
   without issues because interfaces are kept totally separate from each
   other.  This implies that interface selection has to be done at the
   application layer, as from the network-layer point of view, a device
   is not multihomed in the IP-sense.

   Please see the S60 source documentation for more details and
   screenshots [S60].

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3.1.2.  Microsoft Windows Mobile and Windows Phone 7

   Microsoft Windows Mobile leverages a connection manager
   [WINDOWSMOBILE] to handle multiple network connections.  This
   architecture centralizes and automates network connection
   establishment and management and makes it possible to automatically
   select a connection, to dial-in automatically or by user initiation,
   and to optimize connection and shared resource usage.  The connection
   manager periodically re-evaluates the validity of the connection
   selection.  The connection manager uses various attributes such as
   cost, security, bandwidth, error rate, and latency in its decision

   The connection manager selects the best possible connection for the
   application based on the destination network the application wishes
   to reach.  The selection is made between available physical and
   virtual connections (e.g., VPN, GPRS, WLAN, and wired Ethernet) that
   are known to provide connectivity to the destination network, and the
   selection is based on the costs associated with each connection.
   Different applications are bundled to use the same network connection
   when possible, but in conflict situations when a connection cannot be
   shared, higher-priority applications take precedence, and the lower-
   priority applications lose connectivity until the conflict situation

   During operation, the connection manager opens new connections as
   needed and also disconnects unused or idle connections.

   To optimize resource use, such as battery power and bandwidth, the
   connection manager enables applications to synchronize network
   connection usage by allowing applications to register their
   requirements for periodic connectivity.  An application is notified
   when a suitable connection becomes available for its use.

   In comparison to Windows Mobile connection management, Windows Phone
   7 updates the routing functionality in the case where the terminal
   can be attached simultaneously to several interfaces.  Windows Phone
   7 selects the first hop corresponding to the interface that has a
   lower metric.  When there are multiple interfaces, the applications
   system will, by default, choose from an ordered list of available
   interfaces.  The default connection policy will prefer wired over
   wireless and WLAN over cellular.  Hence, if an application wants to
   use cellular 3G as the active interface when WLAN is available, the
   application needs to override the default connection mapping policy.
   An application-specific mapping policy can be set via a Microsoft API
   or provisioned by the Mobile Operator.  The application, in

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   compliance with the security model, can request connection type by
   interface (WLAN, cellular), by minimum interface speed (x kbit/s, y
   Mbit/s), or by name (Access Point Name).

   In dual-stack systems, Windows Mobile and Windows Phone 7 implement
   address selection rules per [WNDS-RFC3484].  An administrator can
   configure a policy table that can override the default behavior of
   the selection algorithms.  Note that the policy table specifies
   precedence values and preferred source prefixes for destination
   prefixes (see [RFC3484], Section 2.1 for details).  If the system has
   not been configured, then the default policy table specified in
   [RFC3484] is used.

3.1.3.  RIM BlackBerry

   Depending on the network configuration, applications in Research In
   Motion (RIM) BlackBerry devices [BLACKBERRY] can use direct TCP/IP
   connectivity or different application proxies to establish
   connections over the wireless network.  For instance, some wireless
   service providers provide an Internet gateway to offer direct TCP/IP
   connectivity to the Internet while some others can provide a Wireless
   Application Protocol (WAP) gateway that allows HTTP connections to
   occur over WAP.  It is also possible to use the BlackBerry Enterprise
   Server [BLACKBERRY] as a network gateway.  The BlackBerry Enterprise
   Server provides an HTTP and TCP/IP proxy service to allow the
   application to use it as a secure gateway for managing HTTP and
   TCP/IP connections to the intranet or the Internet.  An application
   connecting to the Internet can use either the BlackBerry Internet
   Service or the Internet gateway of the wireless server provider or
   direct Internet connectivity over WLAN to manage connections.  The
   problem of gateway selection is supposed to be managed independently
   by each application.  For instance, an application can be designed to
   always use the default Internet gateway, while another application
   can be designed to use a preferred proxy when available.

   A BlackBerry device [BLACKBERRY] can be attached to multiple networks
   simultaneously (wireless/wired).  In this case, multiple network
   interfaces can be associated to a single IP stack or multiple IP
   stacks.  The device, or the application, can select the network
   interface to be used in various ways.  For instance, the device can
   always map the applications to the default network interface (or the
   default access network).  When multiple IP stacks are associated to
   multiple interfaces, the application can select the source address
   corresponding to the preferred network interface.  Per-interface IP
   stacks also allow to manage overlapping address spaces.  When
   multiple network interfaces are aggregated into a single IP stack,

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   the device associates each application to the more appropriate
   network interface.  The selection can be based on cost, type of
   service (ToS), and/or user preference.

   The BlackBerry uses per-interface DNS configuration; applications
   bound to a specific interface will use the DNS settings for that

3.1.4.  Google Android

   Android is based on a Linux kernel and, in many situations, behaves
   like a Linux device as described in Section 3.2.2.  Per Linux,
   Android can manage multiple routing tables and relies on policy-based
   routing associated with packet-filtering capabilities (see
   Section for details).  Such a framework can be used to solve
   complex routing issue brought by multiple interfaces terminals, e.g.,
   address space overlapping.

   For incoming packets, Android implements the weak host model
   [RFC1122] on both IPv4 and IPv6.  However, Android can also be
   configured to support the strong host model.

   Regarding DNS configuration, Android does not list the DNS servers in
   the file /etc/resolv.conf, used by Linux.  However, per Linux, DNS
   configuration is node-scoped, even if DNS configuration can rely on
   the DHCP client.  For instance, the udhcp client [UDHCP], which is
   also available for Linux, can be used on Android.  Each time new
   configuration data is received by the host from a DHCP server,
   regardless of which interface it is received on, the DHCP client
   rewrites the global configuration data with the most recent
   information received.

   Actually, the main difference between Linux and Android is on the
   address selection mechanism.  Android versions prior to 2.2 simply
   prefer IPv6 connectivity over IPv4.  However, it should be noted
   that, at the time of this writing, IPv6 is available only on WiFi and
   virtual interfaces but not on the cellular interface (without IPv6 in
   IPv4 encapsulation).  Android 2.2 has been updated with
   [ANDROID-RFC3484], which implements some of the address selection
   rules defined in [RFC3484].  All [RFC3484] rules are supported,
   except rule 3 (avoid deprecated addresses), rule 4 (prefer home
   addresses), and rule 7 (prefer native transport).  Also, rule 9 (use
   longest matching prefix) has been modified so it does not sort IPv4

   The Android reference documentation describes the package
   [ANDROID] and the ConnectivityManager class that applications can use
   to request the first hop to a specified destination address via a

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   specified network interface (Third Generation Partnership Project
   (3GPP) or WLAN).  Applications also ask the connection manager for
   permission to start using a network feature.  The connection manager
   monitors changes in network connectivity and attempts to failover to
   another network if connectivity to an active network is lost.  When
   there are changes in network connectivity, applications are notified.
   Applications are also able to ask for information about all network
   interfaces, including their availability, type, and other

3.1.5.  Qualcomm Brew

   This section describes how multiple-interface support is handled by
   Advanced Mobile Station Software (AMSS) that comes with Brew OS for
   all Qualcomm chipsets (e.g., Mobile Station Modem (MSM), Snapdragon,
   etc.).  AMSS is a low-level connectivity platform, on top of which
   manufacturers can build to provide the necessary connectivity to
   applications.  The interaction model between AMSS, the operating
   system, and the applications is not unique and depends on the design
   chosen by the manufacturer.  The Mobile OS can let an application
   invoke the AMSS directly (via API) or provide its own connection
   manager that will request connectivity to the AMSS based on
   applications needs.  The interaction between the OS connection
   manager and the applications is OS dependent.

   AMSS supports a concept of netpolicy that allows each application to
   specify the type of network connectivity desired.  The netpolicy
   contains parameters such as access technology, IP version type, and
   network profile.  Access technology could be a specific technology
   type such as CDMA or WLAN or could be a group of technologies, such
   as ANY_Cellular or ANY_Wireless.  IP version could be one of IPv4,
   IPv6, or Default.  The network profile identifies a type of network
   domain or service within a certain network technology, such as 3GPP
   APN or Mobile IP Home Agent.  It also specifies all the mandatory
   parameters required to connect to the domain such authentication
   credentials and other optional parameters such as Quality of Service
   (QoS) attributes.  Network profile is technology specific, and the
   set of parameters contained in the profile could vary for different

   Two models of network usage are supported:

   o  Applications requiring network connectivity specify an appropriate
      netpolicy in order to select the desired network.  The netpolicy
      may match one or more network interfaces.  The AMSS system
      selection module selects the best interface out of the ones that
      match the netpolicy based on various criteria such as cost, speed,
      or other provisioned rules.  The application explicitly starts the

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      selected network interface and, as a result, the application also
      gets bound to the corresponding network interface.  All outbound
      packets from this application are always routed over this bound
      interface using the source address of the interface.

   o  Applications may rely on a separate connection manager to control
      (e.g., start/stop) the network interface.  In this model,
      applications are not necessarily bound to any one interface.  All
      outbound packets from such applications are routed on one of the
      interfaces that match its netpolicy.  The routing decision is made
      individually for each packet and selects the best interface based
      on the criteria described above and the destination address.
      Source address is always assigned to the interface used to
      transmit the packet.

   All of the routing/interface selection decisions are based on the
   netpolicy and not just on the destination address to avoid the issue
   of overlapping private IPv4 addresses.  This also allows multiple
   interfaces to be configured with the same IP address, for example, to
   handle certain tunneling scenarios.  Applications that do not specify
   a netpolicy are routed by AMSS to the best possible interface using
   the default netpolicy.  Default netpolicy could be pre-defined or
   provisioned by the administrator or operator.  Hence, the default
   interface could vary from device to device and also depends upon the
   available networks at any given time.

   AMSS allows each interface to be configured with its own set of DNS
   configuration parameters (e.g., list of DNS servers, domain names,
   etc.).  The interface selected to make a DNS resolution is the one to
   which the application making the DNS query is bound.  Applications
   can also specify a different netpolicy as part of the DNS request to
   select another interface for DNS resolution.  Regardless, all the DNS
   queries are sent only over this selected interface using the DNS
   configuration from the interface.  DNS resolution is first attempted
   with the primary server configured in the interface.  If a response
   is not received, the queries are sent to all the other servers
   configured in the interface in a sequential manner using a backoff

3.1.6.  Leadcore Technology Arena

   Arena, a mobile OS based on Linux, provides a connection manager,
   which is described in [MIF-ARENA] and [MIF-REQS].  The Arena
   connection manager provides a means for applications to register
   their connectivity requirement.  The connection manager can then
   choose an interface that matches the application's needs while

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   considering other factors such as availability, cost, and stability.
   Also, the connection manager can handle multiple-interface issues
   such as connection sharing.

3.2.  Desktop Operating Systems

   Multiple-interface issues also occur in desktop environments in those
   cases where a desktop host has multiple (logical or physical)
   interfaces connected to networks with different reachability
   properties, such as one interface connected to the global Internet,
   while another interface is connected to a corporate VPN.

3.2.1.  Microsoft Windows

   The multiple-interface functionality currently implemented in
   Microsoft Windows operation systems is described in more detail in
   [MULTIHOMING].  First-Hop Selection

   It is possible, although not often desirable, to configure default
   routers on more than one Windows interface.  In this configuration,
   Windows will use the default route on the interface with the lowest
   routing metric (i.e., the fastest interface).  If multiple interfaces
   share the same metric, the behavior will differ based on the version
   of Windows in use.  Prior to Windows Vista, the packet would be
   routed out of the first interface that was bound to the TCP/IP stack,
   the preferred interface.  In Windows Vista, host-to-router load
   sharing [RFC4311] is used for both IPv4 and IPv6.  Outbound and Inbound Addresses

   If the source address of the outgoing packet has not been determined
   by the application, Windows will choose from the addresses assigned
   to its interfaces.  Windows implements [RFC3484] for source address
   selection in IPv6 and, in Windows Vista, for IPv4.  Prior to Windows
   Vista, IPv4 simply chose the first address on the outgoing interface.

   For incoming packets, Windows will check if the destination address
   matches one of the addresses assigned to its interfaces.  Windows has
   implemented the weak host model [RFC1122] on IPv4 in Windows 2000,
   Windows XP, and Windows Server 2003.  The strong host model became
   the default for IPv4 in Windows Vista and Windows Server 2008;
   however, the weak host model is available via per-interface
   configuration.  IPv6 has always implemented the strong host model.

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RFC 6419                  MIF Current Practices            November 2011  DNS Configuration

   Windows largely relies on suffixes to solve DNS resolution issues.
   Suffixes are used for four different purposes:

   1.  DNS Suffix Search List (aka domain search list): suffix is added
       to non-FQDNs (Fully Qualified Domain Names).

   2.  Interface-specific suffix list: allows sending different DNS
       queries to different DNS servers.

   3.  Suffix to control Dynamic DNS Updates: determines which DNS
       server will receive a dynamic update for a name with a certain

   4.  Suffix in the Name Resolution Policy Table [NRPT]: aids in
       identifying a namespace that requires special handling (feature
       available only after Windows 7 and its server counterpart,
       Windows Server 2008 R2).

   However, this section focuses on the interface-specific suffix list
   since it is the only suffix usage in the scope of this document.

   DNS configuration information can be host-wide or interface specific.
   Host-wide DNS configuration is input via static configuration or, in
   sites that use Active Directory, Microsoft's Group Policy.
   Interface-specific DNS configuration can be input via static
   configuration or via DHCP.

   The host-wide configuration consists of a primary DNS suffix to be
   used for the local host, as well as a list of suffixes that can be
   appended to names being queried.  Before Windows Vista and Windows
   Server 2008, there was also a host-wide DNS server list that took
   precedence over per-interface DNS configuration.

   The interface-specific DNS configuration comprises an interface-
   specific suffix list and a list of DNS server IP addresses.

   Windows uses a host-wide "effective" server list for an actual query,
   where the effective server list may be different for different names.
   In the list of DNS server addresses, the first server is considered
   the "primary" server, with all other servers being secondary.

   When a DNS query is performed in Windows, the query is first sent to
   the primary DNS server on the preferred interface.  If no response is
   received in one second, the query is sent to the primary DNS servers
   on all interfaces under consideration.  If no response is received
   for 2 more seconds, the DNS server sends the query to all of the DNS

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   servers on the DNS server lists for all interfaces under
   consideration.  If the host still doesn't receive a response after 4
   seconds, it will send to all of the servers again and wait 8 seconds
   for a response.

3.2.2.  Linux and BSD-Based Operating Systems  First-Hop Selection

   In addition to the two commonly used routing tables (the local and
   main routing tables), the kernel can support up to 252 additional
   routing tables that can be added in the file /etc/iproute2/rt_tables.
   A routing table can contain an arbitrary number of routes; the
   selection of route is classically made according to the destination
   address of the packet.  Linux also provides more flexible routing
   selection based on the type of service, scope, and output interface.
   In addition, since kernel version 2.2, Linux supports policy-based
   routing using the multiple routing tables capability and a routing
   policy database.  This database contains routing rules used by the
   kernel.  Using policy-based routing, the source address, the ToS
   flags, the interface name, and an "fwmark" (a mark added in the data
   structure representing the packet) can be used as route selectors.

   Policy-based routing can be used in addition to Linux packet-
   filtering capabilities, e.g., provided by the "iptables" tool.  In a
   multiple-interface context, this tool can be used to mark the
   packets, i.e., assign a number to fwmark, in order to select the
   routing rule according to the type of traffic.  This mark can be
   assigned according to parameters like protocol, source and/or
   destination addresses, port number, and so on.

   Such a routing management framework allows management of complex
   situations such as address space overlapping.  In this situation, the
   administrator can use packet marking and policy-based routing to
   select the correct interface.  Outbound and Inbound Addresses

   By default, source address selection follows the following basics
   rules.  The initial source address for an outbound packet can be
   chosen by the application using the bind() call.  Without information
   from the application, the kernel chooses the first address configured
   on the interface that belongs to the same subnet as the destination
   address or the next-hop router.

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   Linux also implements [RFC3484] for source address selection for IPv6
   and dual-stack configurations.  However, the address-sorting rules
   from [RFC3484] are not always adequate.  For this reason, Linux
   allows the system administrator to dynamically change the sorting.
   This can be achieved with the /etc/gai.conf file.

   For incoming packets, Linux checks if the destination address matches
   one of the addresses assigned to its interfaces and then processes
   the packet according the configured host model.  By default, Linux
   implements the weak host model [RFC1122] on both IPv4 and IPv6.
   However, Linux can also be configured to support the strong host
   model.  DNS Configuration

   Most BSD and Linux distributions rely on their DHCP client to handle
   the configuration of interface-specific information (such as an IP
   address and netmask) and a set of system-wide configuration
   information (such a DNS server list, an NTP server list, and default
   routes).  Users of these operating systems have the choice of using
   any DHCP client available for their platform with an operating system
   default.  This section discusses the behavior of several DHCP clients
   that may be used with Linux and BSD distributions.

   The Internet Systems Consortium (ISC) DHCP Client [ISCDHCP] and its
   derivative for OpenBSD [OPENBSDDHCLIENT] can be configured with
   specific instructions for each interface.  However, each time new
   configuration data is received by the host from a DHCP server,
   regardless of which interface it is received on, the DHCP client
   rewrites the global configuration data, such as the default routes
   and the DNS server list (in /etc/resolv.conf) with the most recent
   information received.  Therefore, the last configured interface
   always become the primary one.  The ISC DHCPv6 client behaves
   similarly.  However, OpenBSD provides two mechanisms that prevent the
   configuration that the user made manually from being overwritten:

   o  OPTION MODIFIERS (default, supersede, prepend, and append): this
      mechanism allows the user to override the DHCP options.  For
      example, the supersede statement defines, for some options, the
      values the client should always use rather than any value supplied
      by the server.

   o  resolv.conf.tail: this allows the user to append anything to the
      resolv.conf file created by the DHCP client.

   The Phystech dhcpcd client [PHYSTECHDHCPC] behaves similarly to the
   ISC client.  It replaces the DNS server list in /etc/resolv.conf and
   the default routes each time new DHCP information is received on any

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   interface.  However, the -R flag can be used to instruct the client
   to not replace the DNS servers in /etc/resolv.conf.  However, this
   flag is a global flag for the DHCP server and is therefore applicable
   to all interfaces.  When dhcpd is called with the -R flag, the DNS
   servers are never replaced.

   The pump client [PUMP] also behaves similarly to the ISC client.  It
   replaces the DNS servers in /etc/resolv.conf and the default routes
   each time new DHCP information is received on any interface.
   However, the nodns and nogateway options can be specified on a per-
   interface basis, enabling the user to define which interface should
   be used to obtain the global configuration information.

   The udhcp client [UDHCP] is often used in embedded platforms based on
   busybox.  The udhcp client behaves similarly to the ISC client.  It
   rewrites default routes and the DNS server list each time new DHCP
   information is received.

   Red Hat-based distributions, such as Red Hat, Centos, and Fedora have
   a per-interface configuration option (PEERDNS) that indicates that
   the DNS server list should not be updated based on configuration
   received on that interface.

   Most configurable DHCP clients can be set to define a primary
   interface; only that interface is used for the global configuration
   data.  However, this is limited, since a mobile host might not always
   have the same set of interfaces available.  Connection managers may
   help in this situation.

   Some distributions also have a connection manager.  However, most
   connection managers serve as a GUI to the DHCP client and therefore
   do not change the functionality described above.

4.  Acknowledgements

   The authors of this document would like to thank following people for
   their input and feedback: Dan Wing, Hui Deng, Jari Arkko, Julien
   Laganier, and Steinar H. Gunderson.

5.  Security Considerations

   This document describes current operating system implementations and
   how they handle the issues raised in the MIF problem statement.
   While it is possible that the currently implemented mechanisms
   described in this document may affect the security of the systems
   described, this document merely reports on current practice.  It does
   not attempt to analyze the security properties (or any other
   architectural properties) of the currently implemented mechanisms.

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6.  Contributors

   The following people contributed most of the per-operating system
   information found in this document:

   o  Marc Blanchet, Viagenie

   o  Hua Chen, Leadcore Technology, Ltd.

   o  Yan Zhang, Leadcore Technology, Ltd.

   o  Shunan Fan, Huawei Technology

   o  Jian Yang, Huawei Technology

   o  Gabriel Montenegro, Microsoft Corporation

   o  Shyam Seshadri, Microsoft Corporation

   o  Dave Thaler, Microsoft Corporation

   o  Kevin Chin, Microsoft Corporation

   o  Teemu Savolainen, Nokia

   o  Tao Sun, China Mobile

   o  George Tsirtsis, Qualcomm

   o  David Freyermuth, France Telecom

   o  Aurelien Collet, Altran

   o  Giyeong Son, RIM

7.  References

7.1.  Normative References

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

7.2.  Informative References

   [ANDROID]     Google Inc., "Android developers: package",

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RFC 6419                  MIF Current Practices            November 2011

                 Gunderson, S., "RFC 3484 support for Android", 2010,

   [BLACKBERRY]  Research In Motion Limited, "BlackBerry Java
                 Development Environment - Fundamentals Guide: Wireless
                 gateways", <

   [ISCDHCP]     Internet Software Consortium, "ISC DHCP",

   [MIF-ARENA]   Zhang, Y., Sun, T., and H. Chen, "Multi-interface
                 Network Connection Manager in Arena Platform", Work
                 in Progress, February 2009.

   [MIF-REQS]    Yang, J., Sun, T., and S. Fan, "Multi-interface
                 Connection Manager Implementation and Requirements",
                 Work in Progress, March 2009.

   [MULTIHOMING] Montenegro, G., Thaler, D., and S. Seshadri, "Multiple
                 Interfaces on Windows", Work in Progress, March 2009.

   [NRPT]        Davies, J., "Name Resolution Policy Table",
                 February 2010, <

                 OpenBSD, "OpenBSD dhclient", <>.

                 Phystech, "dhcpcd",

   [PUMP]        Red Hat, "PUMP", 2009, <>.

   [RFC1122]     Braden, R., "Requirements for Internet Hosts -
                 Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC3484]     Draves, R., "Default Address Selection for Internet
                 Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC4311]     Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load
                 Sharing", RFC 4311, November 2005.

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   [S60]         Nokia Corporation, "S60 Platform: IP Bearer
                 Management", 2007, <

   [UDHCP]       Busybox, "uDHCP",

                 Microsoft Corporation, "SDK Documentation for Windows
                 Mobile-Based Smartphones: Connection Manager", 2005,

                 Microsoft Corporation, "SDK Documentation for Windows
                 Mobile-Based Smartphones: Default Address Selection for
                 IPv6", April 2010, <

Authors' Addresses

   Margaret Wasserman
   Painless Security, LLC
   356 Abbott Street
   North Andover, MA  01845

   Phone: +1 781 405-7464

   Pierrick Seite
   France Telecom - Orange
   4, rue du clos courtel BP 91226
   Cesson-Sevigne  35512


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