Home Networking E. Kline
Internet-Draft Google Japan
Intended status: Informational November 6, 2012
Expires: May 10, 2013
Default Border Definition
draft-kline-default-perimeter-01
Abstract
Automatic, simple identification of when traffic is crossing a
perimeter is highly desirable for a variety of home network uses.
This document describes how to use homenet routing protocol
adjacencies as the primary signal of a common administrative domain
(e.g. "the home"). Classification of interfaces et cetera as
internal or external follow from this, as do various policy and
implementation implications. One fundamental implication is that the
active definition of a home network's interior is no more secure than
its policy for forming homenet routing protocol adjacencies.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 10, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Dynamically determining the border . . . . . . . . . . . . . . 4
3.1. Collect information about neighboring routers . . . . . . 5
3.2. Classify next hops . . . . . . . . . . . . . . . . . . . . 5
3.3. Classify interfaces . . . . . . . . . . . . . . . . . . . 6
3.3.1. Mixed mode . . . . . . . . . . . . . . . . . . . . . . 6
3.4. State changes and logging . . . . . . . . . . . . . . . . 6
4. Fixed-category interfaces . . . . . . . . . . . . . . . . . . 6
4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Disclamatory remarks . . . . . . . . . . . . . . . . . . . 7
5.2. Security of adjacency formation . . . . . . . . . . . . . 8
5.2.1. Secure links and authenticated adjacency formation . . 8
5.2.2. Unsecure links . . . . . . . . . . . . . . . . . . . . 8
5.2.3. Recommendations . . . . . . . . . . . . . . . . . . . 9
5.3. Example border-aware filtering policies . . . . . . . . . 9
5.3.1. Anti-spoofing on internal interfaces . . . . . . . . . 9
5.3.2. Stateful filtering on external interfaces . . . . . . 10
5.3.3. Mixed-mode interface filtering . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Automatic, simple identification of when traffic is crossing the
homenet perimeter is highly desirable for a variety of home network
uses. This is a non-trivial task as it is tantamount to automated
discovery of administrative boundaries.
Many architectures make it difficult or impossible (by design) to
detect administrative boundaries and rely on various mechanisms of
user or administrator invention to create or modify such boundaries.
Other hints about administrative boundaries can be insecure,
unreliable, operationally impractical, or may place arbitrary
requirements upon the architecture where previously no such
requirement existed.
This document describes how to use homenet routing protocol
adjacencies as the primary signal of a common administrative domain
(e.g. "the home"). Classification of interfaces et cetera as
internal or external follow from this, as do various policy and
implementation implications. One fundamental implication is that the
active definition of a home network's interior is no more secure than
its policy for forming homenet routing protocol adjacencies.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Terminology
In order to address border determination at a manageable scale the
scope has been limited to discussing concepts of "interior",
"exterior", and "border". Documents may reference any of the terms
"internal", "interior", "external", or "exterior" as required by
grammar (adjective versus noun use cases). Definitions in use by
this document are as follows.
Internal/Interior
The interior is broadly defined to include the collection of
interfaces (physical or virtual), nodes, and forwarding next hops
collectively under the control of a single logical administrative
domain. This document uses the homenet routing protocol
adjacencies as a indicator of membership in the same logical
administrative domain.
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External/Exterior
The exterior is broadly defined to include all interfaces
(physical or virtual), nodes, and forwarding next hops
collectively NOT under the control of any single logical
administrative domain and specifically NOT under the control of
the administrative domain which defines the interior.
Border/Perimeter
The border is taken to be the collection of all ephemeral
demarcations within the collection of interior nodes which
forward traffic such that any IP packet transiting that
demarcation can be said to be crossing either from the interior
toward the exterior or from the exterior toward the interior.
This is independent of whether or not such traffic is permitted
by policy to complete its transiting from one zone to the other.
Additionally, some implementations MAY choose to support handling
questionable home network configurations which result in an interface
qualifying for both interior and exterior classification
simultaneously. Requirements for this are discussed further below.
Expressly not considered by this document are architectures having
multiple interior networks, nor the relationships between them as
separate from their relationship to their common exterior or any
common border (e.g. a hierarchy of internal networks). This document
is solely concerned with a single interior, a single exterior, and a
single logical perimeter between the two.
3. Dynamically determining the border
Homenet routers may support interfaces which attempt to learn the
nature of their relationship to neighboring routers, and determine
where the border between the "interior" and the "exterior" lies.
Interfaces which have not yet determined their categorization may
consider themselves to be in a "learning" state, where no traffic is
routed but probing continues. Once nodes of any kind (e.g. either
routers or hosts) are detected, classification takes place and the
router exits the "learning" state.
The classification algorithm documented here is roughly:
1. Continously collect information on all interfaces about
neighboring routers (including manually configured routers).
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2. Classify next hops as either "internal" or "external" primarily
by homenet router protocol adjacency status.
3. Classify interfaces according to the nature of their next hops.
Manually configured next hops MUST also have their classification as
either "internal" or "external" explicitly specified. As such, they
can be considered to be of a fixed category, and on-going evaluation
is NOT required.
Policies of various sorts can be applied and updated as appropriate
based on these classifications, as and when they are determined.
3.1. Collect information about neighboring routers
An interface (physical or virtual) which is configured to dynamically
assess its internal or external classification MUST periodically
probe for routing information on the link. This includes sending
Router Solicitations, DHCPv6 Prefix Delegation requests, and probing
for homenet routing protocol-capable nodes.
Probing for routing information MUST be performed whenever the
interface is logically up. The periodicity of probes is protocol-
dependent (e.g. subject to Router Advertisement lifetimes, DHCPv6
lease timers, or homenet routing protocol timers). Wherever
possible, implementations SHOULD limit the impact of probing by
implementing mechanisms like exponential back-off.
Homenet routers MUST be able to identify loops, e.g. when two (or
more) of the router's own interfaces are connected on the same link.
Compliant routers MUST administratively log this misconfiguration,
and SHOULD implement a mechanism that permits maximum continued
homenet functionality if possible. For example, implementations MAY
administratively disable all but one of looped interfaces.
3.2. Classify next hops
Routing information is used to categorize next hops as either
"internal" or "external".
Routers with which a homenet routing protocol adjacency is
successfully established MUST be considered "internal".
Routers of which this homenet router has knowledge but with which no
homenet routing protocol adjacency is successfully establish AND from
which no routing information is learned SHOULD be considered
"internal". This includes "downstream" routers for which the homenet
router is acting as the Delegating Router via a DHCPv6 Prefix
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Delegation exchange.
All other routers with which no homenet routing protocol adjacency is
successfully established MUST be considered "external".
3.3. Classify interfaces
An interface with no "external" next hops SHOULD be categorized as
"internal". This includes interfaces serving leaf networks
consisting only of hosts, an interface which has "downstream" routers
for which this router is a Delegating Router, an interface with only
homenet routing protocol adjacent peers, or any combination thereof.
An interface with next hops all of which are categorized as
"external" MUST be categorized as "external".
3.3.1. Mixed mode
Some devices MAY choose to support handling questionable home network
configurations which result in an interface having both interior and
exterior next hops simultaneously. This can happen if, for example,
two homenet routers form an adjacency with each other over the same
interface they use for communicating to "upstream" ISP routers.
All homenet routers, whether this configuration is considered
supported or not, MUST administratively log and provide product-
relevant notification of this configuration, preferably with
recommendations for resolution.
3.4. State changes and logging
Home routers performing dynamic border discovery MUST continuously
evaluate the interior and exterior classifications of interfaces.
These may change at any time, for example if new devices are added
into the network or a power event causes all equipment to reset, and
specific ordering of device bring-up within a homenet network MAY NOT
be assumed.
Homenet routers performing dynamic border discovery SHOULD
administratively log the perimeter classification of all interfaces
(physical and virtual), the reason(s) for such classification, and
times at which such classifications are made or changed.
4. Fixed-category interfaces
Interfaces (physical or virtual) which have product-defined purposes
or are otherwise permanently categorized by the homenet router
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implementation as exclusively "internal" or exclusively "external" do
not require any algorithm to determine their categorization.
Homenet routers MUST restrict relevant traffic on fixed-category
interfaces according to their categorizations. Specifically, they
MUST NOT originate traffic which could result in re-categorizing the
interface IF the interface were dynamically attempting to learn its
categorization. For example, a fixed "external" interface MUST NOT
attempt to participate in the homenet routing protocol. Similarly,
fixed "internal" interfaces must not issue DHCPv6 Prefix Delegation
requests.
4.1. Examples
Examples of product-defined interfaces include home router interfaces
which are labeled for their intended use, e.g. RJ-45 ports labeled
"WAN" and "LAN" or symbols indicating "The Internet" and "inside the
home". Other examples include wireless routers with defined separate
WLAN "home" and "guest" ESSIDs.
Another set of examples of product-defined, fixed category interfaces
are those which require subscriber credentials in order for that
interface to successfully pass general purpose traffic. These
include authenticated PPPoE sessions and 3G/LTE PDP contexts (e.g.
requiring a SIM card associated with a valid customer account).
These SHOULD be classified as "exterior".
Similarly, an implementation may consider the interface a mobile
device uses to provide service to "tethering" clients to be a fixed-
category interface. Such interfaces SHOULD be classified as
"interior".
5. Security Considerations
A key motivation for border identification is the application of
security policies which can take into account classifications of
interior, exterior, and the transition from one the other. General
remarks, comments on the security of the adjacencies which form the
basis of border identification, and examples of potential policies
which might be applied follow.
5.1. Disclamatory remarks
By default all network nodes SHOULD follow best current security
practices. Any node may at times find itself in a hostile
environment. This is obviously true of mobile nodes, for example,
when connecting to any public "Wi-Fi" network. This is, of course,
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equally true of more traditionally "fixed" nodes. Any compromised
neighbor node ("fixed" or mobile) may be used as a conduit for
further compromise. Indeed, one study of a captured "botnet"
([TORPIG], section 5.2.4) found that roughly 78.9% of infected hosts
had RFC 1918 [RFC1918] addresses, commonly used in IPv4 NAT
deployments.
Though it goes without saying, at all times homenet implementers MUST
remain mindful of best current security practices, including but not
limited to RFC 4864 [RFC4864], RFC 4890 [RFC4890], RFC 6092
[RFC6092], and others.
5.2. Security of adjacency formation
The security of the border definition is limited by the security
applied to the formation of homenet routing protocol adjacencies: the
next hops with which a homenet router forms adjacencies are the next
hops the router trusts with the application of interior policies.
5.2.1. Secure links and authenticated adjacency formation
The trustworthiness of next hops during adjacency formation can be
improved if the security of the link connecting them can be trusted.
Using encrypted link technologies like 802.1x or secured "Wi-Fi"
ESSIDS when forming homenet adjacencies, or authenticating homenet
next hops by physical or cryptographic mechanisms limits the ability
of malicious nodes to join the homenet interior.
5.2.2. Unsecure links
In general IP over Ethernet connections, common to residential
Internet (and countless other places like some in-room hotel network)
service provider deployments, create the possibility of malicious
nodes attempting to join the homenet interior.
In a broad variety of circumstances users already implicitly trust
unsecured links. Residential subscribers generally trust that their
ISP has properly isolated their connection from any neighbors; few if
any subscribers validate the ISP's DHCP server in order to thwart a
malicious neighbor intervening.
In the event of a network with a single upstream where an interior
next hop is formed instead of an external next hop, the homenet
network as a whole would have detected no external next hops.
Homenet router networks in which there are no external next hops
SHOULD administratively log this configuration and SHOULD provide a
means to alert the user to this condition. Note that for isolated
networks of homenet routers (e.g. a lab network) this configuration
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is entirely valid.
User notification alone is not sufficient protection for the homenet
user, and will not correctly alert the user of a homenet with two
upstream connections, one of which has mistakenly not categorized a
next hop as external. To better serve the homenet user, homenet
routers are SHOULD follow one or more of the recommendations in
Section 5.2.3.
5.2.3. Recommendations
Homenet router implementations that support dynamic discovery of the
border (i.e. have interfaces on which the dynamic border detection
described in Section 3 can be configured to operate) SHOULD support
restricting homenet routing protocol adjacency formation to only next
hops which meet some user-defined or user-verified authentication
mechanism (including examples described in Section 5.2.1).
Alternatively, implementations MAY incorporate a mechanism (possibly
physical) whereby a homenet user can disable border detection on an
interface which the user wishes to force into either an interior or
exterior classification (e.g. a button to force an interface to be
"external" only).
5.3. Example border-aware filtering policies
As a homenet router forms adjacencies and learns internal aggregate
prefixes it could dynamically maintain a single logical entity
encompassing all current internal prefixes in use that can be treated
as a whole (e.g. an access list). Below are example filtering
policies that might be applied by homenet routers with knowledge of
both this prefix set and the interior or exterior classification of
all interfaces.
The examples below use the string "{interior_nets}" for refer to the
grouping of all internal aggregate prefixes. The sample filtering
policy rules are written in configuration pseudo-syntax that should
hopefully be intuitive.
5.3.1. Anti-spoofing on internal interfaces
Given knowledge of all interior network prefixes and the
categorization of interfaces, all interior interfaces could apply a
stateless filter designed to prevent devices in the home from
originating source-address-spoofed traffic.
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Using a filter configuration pseudo-syntax:
from !{interior_nets} to !{interior_nets} deny
... # permit or deny other kinds of traffic
5.3.2. Stateful filtering on external interfaces
Given knowledge of all interior network prefixes and the
categorization of interfaces, all exterior interfaces could apply a
stateful filter designed to discard traffic without matching state in
the homenet router.
Using a filter configuration pseudo-syntax:
... # permit other kinds of good traffic first
from {interior_nets} to !{interior_nets} permit
from !{interior_nets} to {interior_nets} established permit
from any to any deny
5.3.3. Mixed-mode interface filtering
Given knowledge of all interior network prefixes and the
categorization of interfaces, all mixed-node interfaces could apply a
stateful filter designed to discard exterior traffic without matching
state in the homenet router and still statelessly permit internal
traffic.
Using a filter configuration pseudo-syntax:
... # permit other kinds of good traffic first
from {interior_nets} to !{interior_nets} permit
from !{interior_nets} to {interior_nets} established permit
from {interior_nets} to {interior_nets} permit
from any to any deny
Because routing changes elsewhere in the home may cause traffic to
shift among interior next hops which may not have state, traffic
between interior routers may not be well-served by stateful
filtering. One consequence for this policy on mixed-mode interfaces
is that traffic from the exterior with spoofed source addresses from
the "{interior_nets}" set of prefixes may be mistakenly allowed into
the home.
Filter implementations which can incorporate knowledge of the
previous and next hops and their classifications can design much more
precise filters. Such implementations could deny traffic with
"{interior_nets}" source addresses arriving from an exterior next
hop, but permit them from an interior next hop on the same mixed-mode
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interface.
6. Acknowledgements
Many thanks for the constructive input and criticism of Shwetha
Bhandari, Lorenzo Colitti, Markus Stenberg, Mark Townsley, and Ole
Troan.
Thanks also must go to pleasant, peaceful and productive trips on the
Japan Rail (JR) Shinkansen ("bullet train").
7. IANA Considerations
This memo includes no request to IANA.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
May 2007.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, May 2007.
[RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment (CPE) for Providing
Residential IPv6 Internet Service", RFC 6092,
January 2011.
[TORPIG] Stone-Gross, B., "Your Botnet is My Botnet: Analysis of a
Botnet Takeover", 2009, <http://www.cs.ucsb.edu/~seclab/
projects/torpig/torpig.pdf>.
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Author's Address
Erik Kline
Google Japan
Roppongi 6-10-1, 26th Floor
Minato, Tokyo 106-6126
JP
Phone: +81 03 6384 9000
Email: ek@google.com
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