Internet DRAFT - draft-gont-opsec-ipv6-implications-on-ipv4-nets
draft-gont-opsec-ipv6-implications-on-ipv4-nets
Operational Security Capabilities for F. Gont
IP Network Infrastructure (opsec) UK CPNI
Internet-Draft August 21, 2012
Intended status: Informational
Expires: February 22, 2013
Security Implications of IPv6 on IPv4 Networks
draft-gont-opsec-ipv6-implications-on-ipv4-nets-02
Abstract
This document discusses the security implications of native IPv6
support and IPv6 transition/co-existence technologies on "IPv4-only"
networks, and describes possible mitigations for the aforementioned
issues.
Status of this Memo
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This Internet-Draft will expire on February 22, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Implications of native IPv6 support . . . . . . . . . 4
2.1. Filtering Native IPv6 Traffic . . . . . . . . . . . . . . 4
3. Security Implications of tunneling Mechanisms . . . . . . . . 6
3.1. Filtering 6in4 . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Filtering 6over4 . . . . . . . . . . . . . . . . . . . . . 7
3.3. Filtering 6rd . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Filtering 6to4 . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Filtering ISATAP . . . . . . . . . . . . . . . . . . . . . 9
3.6. Filtering Teredo . . . . . . . . . . . . . . . . . . . . . 9
3.7. Filtering Tunnel Broker with Tunnel Setup Protocol
(TSP) . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Summary of filtering rules . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
Most general-purpose operating systems implement and enable by
default native IPv6 [RFC2460] support and a number of transition-co-
existence technologies. In those cases in which such devices are
deployed on networks that are assumed to be IPv4-only, the
aforementioned technologies could be leveraged by local or remote
attackers for a number of (illegitimate) purposes.
For example, a Network Intrusion Detection System (NIDS) might be
prepared to detect attack patterns for IPv4 traffic, but might be
unable to detect the same attack patterns when a transition/
co-existence technology is leveraged for that purpose. Additionally,
an IPv4 firewall might enforce a specific security policy in IPv4,
but might be unable to enforce the same policy in IPv6. Finally,
some transition/co-existence mechanisms (notably Teredo) are designed
to traverse Network Address Port Translation (NAPT) [RFC2663]
devices, which in many deployments provide a minimum level of
protection by only allowing those instances of communication that
have been initiated from the internal network. Thus, these
mechanisms might cause an internal host with otherwise limited IPv4
connectivity to become globally reachable over IPv6, therefore
resulting in increased (and possibly unexpected) host exposure. That
is, the aforementioned technologies might inadvertently allow
incoming IPv6 connections from the Internet to hosts behind the
organizational firewall.
In general, the aforementioned security implications can be mitigated
by enforcing security controls on native IPv6 traffic and on IPv4-
tunneled traffic. Among such controls is the enforcement of
filtering policies, such that undesirable traffic is blocked.
This document discusses the security implications of IPv6 and IPv6
transition/co-existence technologies on (allegedly) IPv4-only
networks, and provides guidance on how to mitigate the aforementioned
issues.
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2. Security Implications of native IPv6 support
Most popular operating systems include IPv6 support that is enabled
by default. This means that even if a network is expected to be
IPv4-only, much of its infrastructure is nevertheless likely to be
IPv6 enabled. For example, hosts are likely to have at least link-
local IPv6 connectivity which might be exploited by attackers with
access to the local network.
[CORE2007] is a security advisory about a buffer overflow which
could be remotely-exploited by leveraging link-local IPv6
connectivity that is enabled by default.
Additionally, unless appropriate measures are taken, an attacker with
access to an 'IPv4-only' local network could impersonate a local
router and cause local hosts to enable their 'non-link-local' IPv6
connectivity (e.g. by sending Router Advertisement messages),
possibly circumventing security controls that were enforced only on
IPv4 communications.
[THC-IPV6] is the first publicly-available toolkit that
implemented this attack vector (along with many others).
[Waters2011] provides an example of how this could be achieved
using publicly available tools (besides incorrectly claiming the
discovery of a "0day vulnerability").
In general, networks should enforce on native IPv6 traffic the same
security policies they currently enforce on IPv4 traffic. However,
in those networks in which IPv6 has not yet been deployed, and
enforcing the aforementioned policies is deemed as unfeasible, a
network administrator might mitigate IPv6-based attack vectors by
means of appropriate packet filtering.
2.1. Filtering Native IPv6 Traffic
Some layer-2 devices might have the ability to selectively filter
packets based on the type of layer-2 payload. When such
functionality is available, IPv6 traffic could be blocked at those
layer-2 devices by blocking e.g. Ethernet frames with the Protocol
Type field set to 0x86dd [IANA-ETHER].
SLAAC-based attacks [RFC3756] can be mitigated with technologies such
as RA-Guard [RFC6105] [I-D.ietf-v6ops-ra-guard-implementation]. In a
similar way, DHCPv6-based attacks can be mitigated with technologies
such as DHCPv6-Shield [I-D.gont-opsec-dhcpv6-shield]. However,
neither RA-Guard nor DHCPv6-Shield can mitigate attack vectors that
employ IPv6 link-local addresses, since configuration of such
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addresses does not rely on Router Advertisement messages or DCHPv6-
server messages.
In order to mitigate attacks based on native IPv6 traffic, IPv6
security controls should be enforced on both IPv4 and IPv6 networks.
The aforementioned controls might include: deploying IPv6-enabled
NIDS, implementing IPv6 firewalling, etc.
In some very specific scenarios (e.g., military operations
networks) in which only IPv4 service might be desired, a network
administrator might want to disable IPv6 support in all the
communicating devices.
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3. Security Implications of tunneling Mechanisms
Unless properly managed, tunneling mechanisms might result in
negative security implications ([RFC6169] describes the security
implications of tunneling mechanisms in detail). Therefore,
tunneling mechanisms should be a concern not only to network
administrators that have consciously deployed them, but also to
network and security administrators whose security policies might be
bypassed by exploiting these mechanisms.
[CERT2009] contains some examples of how tunnels can be leveraged
to bypass firewall rules.
The aforementioned issues could be mitigated by applying the common
security practice of only allowing traffic deemed as "necessary"
(i.e., the so-called "default deny" policy). Thus, when such policy
is enforced IPv6 transition/co-existence traffic would be blocked by
default, and would only be allowed as a result of an explicit
decision (rather than as a result of lack of awareness about such
traffic).
It should be noted that this type of policy is usually enforced at
a network that is the target of such traffic (such as an
enterprise network). IPv6 transition traffic should generally
never be filtered e.g. by an ISP when it is transit traffic.
In those scenarios in which transition/co-existence traffic is meant
to be blocked, it is highly recommended that, in addition to the
enforcement of filtering policies at the organizational perimeter,
the corresponding transition/co-existence mechanisms be disabled on
each node connected to the organizational network. This would not
only prevent security breaches resulting from accidental use of these
mechanisms, but would also disable this functionality altogether,
possibly mitigating vulnerabilities that might be present in the host
implementation of these transition/co-existence mechanisms.
IPv6-in-IPv4 tunnelling mechanisms (such as 6to4 or configured
tunnels) can generally be blocked by dropping IPv4 packets that
contain a Protocol field set to 41. Security devices such as NIDS
might also include signatures that detect such transition/
co-existence traffic.
3.1. Filtering 6in4
Probably the most basic type of tunnel employed for connecting IPv6
"islands" is the so-called "6in4", in which IPv6 packets are
encapsulated within IPv4 packets. These tunnels are typically result
from manual configuration at the two tunnel endpoints.
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6in4 tunnels can be blocked by blocking IPv4 packets with a Protocol
field of 41.
3.2. Filtering 6over4
[RFC2529] specifies a mechanism known as 6over4 or 'IPv6 over IPv4'
(or colloquially as 'virtual Ethernet'), which comprises a set of
mechanisms and policies to allow isolated IPv6 hosts located on
physical links with no directly-connected IPv6 router, to become
fully functional IPv6 hosts by using an IPv4 domain that supports
IPv4 multicast as their virtual local link.
This transition technology has never been widely deployed, because
of the low level of deployment of multicast in most networks.
6over4 encapsulates IPv6 packets in IPv4 packets with their Protocol
field set to 41. As a result, simply filtering all IPv4 packets that
have a Protocol field equal to 41 will filter 6over4 (along with many
other transition technologies).
A more selective filtering could be enforced such that 6over4 traffic
is filtered while other transition traffic is still allowed. Such a
filtering policy would block all IPv4 packets that have their
Protocol field set to 41, and that have a Destination Address that
belongs to the prefix 239.0.0.0/8.
This filtering policy basically blocks 6over4 Neighbor Discovery
traffic directed to multicast addresses, thus preventing Stateless
Address Auto-configuration (SLAAC), address resolution, etc.
Additionally, it would prevent the 6over multicast addresses from
being leveraged for the purpose of network reconnaissance.
3.3. Filtering 6rd
6rd builds upon the mechanisms of 6to4 to enable the rapid deployment
of IPv6 on IPv4 infrastructures, while avoiding some downsides of
6to4. Usage of 6rd was originally documented in [RFC5569], and the
mechanism was generalized to other access technologies and formally
standardized in [RFC5969].
6rd can be blocked by blocking IPv4 packets with the Protocol field
set to 41.
3.4. Filtering 6to4
6to4 [RFC3056] is an address assignment and router-to-router, host-
to-router, and router-to-host automatic tunnelling mechanism that is
meant to provide IPv6 connectivity between IPv6 sites and hosts
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across the IPv4 Internet.
The security considerations for 6to4 are discussed in detail in
[RFC3964].
As discussed in Section 3, all IPv6-in-IPv4 traffic, including 6to4,
could be easily blocked by filtering IPv4 that contain their Protocol
field set to 41. This is the most effective way of filtering such
traffic.
If 6to4 traffic is meant to be filtered while other IPv6-in-IPv4
traffic is allowed, then more finer-grained filtering rules could be
applied. For example, 6to4 traffic could be filtered by applying
filtering rules such as:
o Filter outgoing IPv4 packets that have the Destination Address set
to an address that belongs to the prefix 192.88.99.0/24.
o Filter incoming IPv4 packets that have the Source Address set to
an address that belongs to the prefix 192.88.99.0/24.
It has been suggested that 6to4 relays send their packets with
their IPv4 Source Address set to 192.88.99.1.
o Filter outgoing IPv4 packets that have the Destination Address set
to the IPv4 address of well-known 6to4 relays.
o Filter incoming IPv4 packets that have the Source Address set to
the IPv4 address of well-known 6to4 relays.
These last two filtering policies will generally be unnecessary,
and possibly unfeasible to enforce (given the number of potential
6to4 relays, and the fact that many relays might remain unknown to
the network administrator). If anything, they should be applied
with the additional requirement that such IPv4 packets have their
Protocol field set to 41, to avoid the case where other services
available at the same IPv4 address as a 6to4 relay are mistakenly
made inaccessible.
If the filtering device has capabilities to inspect the payload of
IPv4 packets, then the following filtering rules could be enforced:
o Filter outgoing IPv4 packets that have their Protocol field set to
41, and that have an IPv6 Source Address (embedded in the IPv4
payload) that belongs to the prefix 2002::/16.
o Filter incoming IPv4 packets that have their Protocol field set to
41, and that have an IPv6 Destination address (embedded in the
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IPv4 payload) that belongs to the prefix 2002::/16.
3.5. Filtering ISATAP
ISATAP [RFC5214] is an Intra-site tunnelling protocol, and thus it is
generally expected that such traffic will not traverse the
organizational firewall of an IPv4-only. Nevertheless, ISATAP can be
easily blocked by blocking IPv4 packets with a Protocol field of 41.
The most popular operating system that includes an implementation of
ISATAP in the default installation is Microsoft Windows. Microsoft
Windows obtains the ISATAP router address by resolving the domain
name isatap.<localdomain> DNS A resource records. Additionally, they
try to learn the ISATAP router address by employing Link-local
Multicast Name Resolution (LLMNR) [RFC4795] to resolve the name
"isatap". As a result, blocking ISATAP by preventing hosts from
successfully performing name resolution for the aforementioned names
and/or by filtering packets with specific IPv4 destination addresses
is both difficult and undesirable.
3.6. Filtering Teredo
Teredo [RFC4380] is an address assignment and automatic tunnelling
technology that provides IPv6 connectivity to dual-stack nodes that
are behind one or more Network Address Port Translation (NAPT)
[RFC2663] devices, by encapsulating IPv6 packets in IPv4-based UDP
datagrams. Teredo is meant to be a 'last resort' IPv6 connectivity
technology, to be used only when other technologies such as 6to4
cannot be deployed (e.g., because the edge device has not been
assigned a public IPv4 address).
As noted in [RFC4380], in order for a Teredo client to configure its
Teredo IPv6 address, it must contact a Teredo server, through the
Teredo service port (UDP port number 3544).
To prevent the Teredo initialization process from succeeding, and
hence prevent the use of Teredo, an organizational firewall could
filter outgoing UDP packets with a Destination Port of 3544.
It is clear that such a filtering policy does not prevent an
attacker from running its own Teredo server in the public
Internet, using a non-standard UDP port for the Teredo service
port (i.e., a port number other than 3544).
If the filtering device has capabilities to inspect the payload of
IPv4 packets, the following (additional) filtering policy could be
enforced:
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o Filter outgoing IPv4/UDP packets that have that embed an IPv6
packet with the "Version" field set to 6, and an IPv6 Source
Address that belongs to the prefix 2001::/32.
o Filter incoming IPv4/UDP packets that have that embed an IPv6
packet with the "Version" field set to 6, and an IPv6 Destination
Address that belongs to the prefix 2001::/32.
These two filtering rules could, at least in theory, result in
false positives. Additionally, they would generally require the
filtering device to reassemble fragments prior to enforcing
filtering rules, since the information required to enforce them
might be missing in the received fragments (which should be
expected if Teredo is being employed for malicious purposes).
The most popular operating system that includes an implementation of
Teredo in the default installation is Microsoft Windows. Microsoft
Windows obtains the Teredo server addresses (primary and secondary)
by resolving the domain name teredo.ipv6.microsoft.com into DNS A
records. A network administrator might want to prevent Microsoft
Windows hosts from obtaining Teredo service by filtering at the
organizational firewall outgoing UDP datagrams (i.e. IPv4 packets
with the Protocol field set to 17) that contain in the IPv4
Destination Address any of the IPv4 addresses that the domain name
teredo.ipv6.microsoft.com maps to. Additionally, the firewall would
filter incoming UDP datagrams from any of the IPv4 addresses to which
the domain names of well-known Teredo servers (such as
teredo.ipv6.microsoft.com) resolve.
As these IPv4 addresses might change over time, an administrator
should obtain these addresses when implementing the filtering
policy, and should also be prepared to keep this list up to date.
The corresponding addresses can be easily obtained from a UNIX
host by issuing the command 'dig teredo.ipv6.microsoft.com a'
(without quotes).
It should be noted that even with all these filtering policies in
place, a node in the internal network might still be able to
communicate with some Teredo clients. That is, it could configure an
IPv6 address itself (without even contacting a Teredo server), and
might send Teredo traffic to those peers for which intervention of
the host's Teredo server is not required (e.g., Teredo clients behind
a cone NAT).
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3.7. Filtering Tunnel Broker with Tunnel Setup Protocol (TSP)
The tunnel broker model enables dynamic configuration of tunnels
between a tunnel client and a tunnel server. The tunnel broker
provides a control channel for creating, deleting or updating a
tunnel between the tunnel client and the tunnel server.
Additionally, the tunnel broker may register the user IPv6 address
and name in the DNS. Once the tunnel is configured, data can flow
between the tunnel client and the tunnel server. [RFC3053] describes
the Tunnel Broker model, while [RFC5572] specifies the Tunnel Setup
Protocol (TSP), which can be used by clients to communicate with the
Tunnel Broker.
TSP can use either TCP or UDP as the transport protocol. In both
cases TSP uses port number 3653, which has been assigned by the IANA
for this purpose. As a result, TSP (the Tunnel Broker control
channel) can be blocked by blocking TCP and UDP packets originating
from the local network and destined to UDP port 3653 or TCP port
3653. Additionally, the data channel can be blocked by blocking UDP
packets originated from the local network and destined to UDP port
3653, and IPv4 packets with a Protocol field set to 41.
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4. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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5. Security Considerations
This document discusses the security implications of IPv6 on IPv4
networks, and describes a number of techniques to mitigate the
aforementioned issues. In general, the possible mitigations boil
down to enforcing on native IPv6 and IPv6 transition/co-existence
traffic the same security policies currently enforced for IPv4
traffic, and/or blocking the aforementioned traffic when it is deemed
as undesirable.
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6. Acknowledgements
The author would like to thank (in alphabetical order) Ran Atkinson,
Panos Kampanakis, David Malone, Arturo Servin, Donald Smith, Tina
Tsou, and Eric Vyncke, for providing valuable comments on earlier
versions of this document.
This document resulted from the project "Security Assessment of the
Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI).
Fernando Gont would like to thank the UK CPNI for their continued
support.
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7. References
7.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC3053] Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
Tunnel Broker", RFC 3053, January 2001.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
January 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
[RFC5572] Blanchet, M. and F. Parent, "IPv6 Tunnel Broker with the
Tunnel Setup Protocol (TSP)", RFC 5572, February 2010.
7.2. Informative References
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
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[RFC3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, December 2004.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)",
draft-ietf-v6ops-ra-guard-implementation-04 (work in
progress), May 2012.
[I-D.gont-opsec-dhcpv6-shield]
Gont, F., "DHCPv6-Shield: Protecting Against Rogue DHCPv6
Servers", draft-gont-opsec-dhcpv6-shield-00 (work in
progress), May 2012.
[IANA-ETHER]
IANA, "Ether Types", 2012,
<http://www.iana.org/assignments/ethernet-numbers>.
[CERT2009]
CERT, "Bypassing firewalls with IPv6 tunnels", 2009, <http
://www.cert.org/blogs/vuls/2009/04/
bypassing_firewalls_with_ipv6.html>.
[CORE2007]
CORE, "OpenBSD's IPv6 mbufs remote kernel buffer
overflow", 2007,
<http://www.coresecurity.com/content/open-bsd-advisorie>.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[THC-IPV6]
"The Hacker's Choice IPv6 Attack Toolkit",
<http://www.thc.org/thc-ipv6/>.
[Waters2011]
Waters, A., "SLAAC Attack - 0day Windows Network
Interception Configuration Vulnerability", 2011,
<http://resources.infosecinstitute.com/slaac-attack/>.
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Appendix A. Summary of filtering rules
+------------+------------------------------------------------------+
| Technology | Filtering rules |
+------------+------------------------------------------------------+
| Native | EtherType 0x86DD |
| IPv6 | |
+------------+------------------------------------------------------+
| 6in4 | IP proto 41 |
+------------+------------------------------------------------------+
| 6over4 | IP proto 41 |
+------------+------------------------------------------------------+
| 6rd | IP proto 41 |
+------------+------------------------------------------------------+
| 6to4 | IP proto 41 |
+------------+------------------------------------------------------+
| ISATAP | IP proto 41 |
+------------+------------------------------------------------------+
| Teredo | UDP Dest Port 3544 |
+------------+------------------------------------------------------+
| TB with | (IP proto 41) || (UDP Dest Port 3653 || TCP Dest |
| TSP | Port 3653) |
+------------+------------------------------------------------------+
Table 1: Summary of filtering rules
NOTE: the table above describes general and simple filtering rules
for blocking the corresponding traffic. More finer-grained rules
might be available in each of the corresponding sections of this
document.
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Author's Address
Fernando Gont
UK Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI: http://www.cpni.gov.uk
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