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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
Abstract
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
http://www.rfc-editor.org/info/rfc6419.
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 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
3.2.1.1. First-Hop Selection . . . . . . . . . . . . . . . 14
3.2.1.2. Outbound and Inbound Addresses . . . . . . . . . . 14
3.2.1.3. DNS Configuration . . . . . . . . . . . . . . . . 15
3.2.2. Linux and BSD-Based Operating Systems . . . . . . . . 16
3.2.2.1. First-Hop Selection . . . . . . . . . . . . . . . 16
3.2.2.2. Outbound and Inbound Addresses . . . . . . . . . . 16
3.2.2.3. 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
categories:
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
basis.
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
metric.
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"
interface.
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 |
+-------+ +-------+
| +--------+ |
+--------|Cellular|--------+
|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
making.
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
clears.
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
interface.
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 3.2.2.1 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
addresses.
The Android reference documentation describes the android.net 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
information.
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
technologies.
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
mechanism.
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].
3.2.1.1. 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.
3.2.1.2. 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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3.2.1.3. 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
suffix.
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
3.2.2.1. 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.
3.2.2.2. 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.
3.2.2.3. 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 android.net",
<http://developer.android.com/reference/android/net/
ConnectivityManager.html>.
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RFC 6419 MIF Current Practices November 2011
[ANDROID-RFC3484]
Gunderson, S., "RFC 3484 support for Android", 2010,
<http://gitorious.org/0xdroid/bionic/commit/
9ab75d4cc803e91b7f1b656ffbe2ad32c52a86f9>.
[BLACKBERRY] Research In Motion Limited, "BlackBerry Java
Development Environment - Fundamentals Guide: Wireless
gateways", <http://na.blackberry.com/eng/
deliverables/5827/Wireless_gateways_447132_11.jsp>.
[ISCDHCP] Internet Software Consortium, "ISC DHCP",
<http://www.isc.org/software/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, <http://technet.microsoft.com/en-
us/magazine/ff394369.aspx>.
[OPENBSDDHCLIENT]
OpenBSD, "OpenBSD dhclient", <http://www.openbsd.org/>.
[PHYSTECHDHCPC]
Phystech, "dhcpcd",
<http://www.phystech.com/download/dhcpcd.html>.
[PUMP] Red Hat, "PUMP", 2009, <http://redhat.com>.
[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, <http://www.forum.nokia.com/info/
sw.nokia.com/id/190358c8-7cb1-4be3-9321-f9d6788ecae5/
S60_Platform_IP_Bearer_Management_v1_0_en.pdf.html>.
[UDHCP] Busybox, "uDHCP",
<http://busybox.net/downloads/BusyBox.html>.
[WINDOWSMOBILE]
Microsoft Corporation, "SDK Documentation for Windows
Mobile-Based Smartphones: Connection Manager", 2005,
<http://msdn.microsoft.com/en-us/library/
aa457829.aspx>.
[WNDS-RFC3484]
Microsoft Corporation, "SDK Documentation for Windows
Mobile-Based Smartphones: Default Address Selection for
IPv6", April 2010, <http://msdn.microsoft.com/en-us/
library/aa925716.aspx>.
Authors' Addresses
Margaret Wasserman
Painless Security, LLC
356 Abbott Street
North Andover, MA 01845
USA
Phone: +1 781 405-7464
EMail: mrw@painless-security.com
URI: http://www.painless-security.com
Pierrick Seite
France Telecom - Orange
4, rue du clos courtel BP 91226
Cesson-Sevigne 35512
France
EMail: pierrick.seite@orange.com
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