Network Working Group G. Nakibly
Internet-Draft National EW Research &
Intended status: Standards Track Simulation Center
Expires: September 10, 2011 F. Templin
Boeing Research & Technology
March 09, 2011
Routing Loop Attack using IPv6 Automatic Tunnels: Problem Statement and
Proposed Mitigations
draft-ietf-v6ops-tunnel-loops-04.txt
Abstract
This document is concerned with security vulnerabilities in IPv6-in-
IPv4 automatic tunnels. These vulnerabilities allow an attacker to
take advantage of inconsistencies between the IPv4 routing state and
the IPv6 routing state. The attack forms a routing loop which can be
abused as a vehicle for traffic amplification to facilitate DoS
attacks. The first aim of this document is to inform on this attack
and its root causes. The second aim is to present some possible
mitigation measures.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. A Detailed Description of the Attack . . . . . . . . . . . . . 4
3. Proposed Mitigation Measures . . . . . . . . . . . . . . . . . 6
3.1. Verification of end point existence . . . . . . . . . . . 6
3.1.1. Neighbor Cache Check . . . . . . . . . . . . . . . . . 6
3.1.2. Known IPv4 Address Check . . . . . . . . . . . . . . . 7
3.2. Operational Measures . . . . . . . . . . . . . . . . . . . 7
3.2.1. Avoiding a Shared IPv4 Link . . . . . . . . . . . . . 8
3.2.2. A Single Border Router . . . . . . . . . . . . . . . . 8
3.2.3. A Comprehensive List of Tunnel Routers . . . . . . . . 9
3.2.4. Avoidance of On-link Prefixes . . . . . . . . . . . . 9
3.3. Destination and Source Address Checks . . . . . . . . . . 21
3.3.1. Known IPv6 Prefix Check . . . . . . . . . . . . . . . 22
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 23
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
IPv6-in-IPv4 tunnels are an essential part of many migration plans
for IPv6. They allow two IPv6 nodes to communicate over an IPv4-only
network. Automatic tunnels that assign non-link-local IPv6 prefixes
with stateless address mapping properties (hereafter called
"automatic tunnels") are a category of tunnels in which a tunneled
packet's egress IPv4 address is embedded within the destination IPv6
address of the packet. An automatic tunnel's router is a router that
respectively encapsulates and decapsulates the IPv6 packets into and
out of the tunnel.
Ref. [USENIX09] pointed out the existence of a vulnerability in the
design of IPv6 automatic tunnels. Tunnel routers operate on the
implicit assumption that the destination address of an incoming IPv6
packet is always an address of a valid node that can be reached via
the tunnel. The assumption of path validity poses a denial of
service risk as inconsistency between the IPv4 routing state and the
IPv6 routing state allows a routing loop to be formed.
An attacker can exploit this vulnerability by crafting a packet which
is routed over a tunnel to a node that is not participating in that
tunnel. This node may forward the packet out of the tunnel to the
native IPv6 network. There the packet is routed back to the ingress
point that forwards it back into the tunnel. Consequently, the
packet loops in and out of the tunnel. The loop terminates only when
the Hop Limit field in the IPv6 header of the packet is decremented
to zero. This vulnerability can be abused as a vehicle for traffic
amplification to facilitate DoS attacks [RFC4732].
Without compensating security measures in place, all IPv6 automatic
tunnels that are based on protocol-41 encapsulation [RFC4213] are
vulnerable to such an attack including ISATAP [RFC5214], 6to4
[RFC3056] and 6rd [RFC5969]. It should be noted that this document
does not consider non-protocol-41 encapsulation attacks. In
particular, we do not address the Teredo [RFC4380] attacks described
in [USENIX09]. These attacks are considered in
[I-D.gont-6man-teredo-loops].
The aim of this document is to shed light on the routing loop attack
and describe possible mitigation measures that should be considered
by operators of current IPv6 automatic tunnels and by designers of
future ones. We note that tunnels may be deployed in various
operational environments, e.g. service provider network, enterprise
network, etc. Specific issues related to the attack which are
derived from the operational environment are not considered in this
document.
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2. A Detailed Description of the Attack
In this section we shall denote an IPv6 address of a node reached via
a given tunnel by the prefix of the tunnel and an IPv4 address of the
tunnel end point, i.e., Addr(Prefix, IPv4). Note that the IPv4
address may or may not be part of the prefix (depending on the
specification of the tunnel's protocol). The IPv6 address may be
dependent on additional bits in the interface ID, however for our
discussion their exact value is not important.
The two victims of this attack are routers - R1 and R2 - of two
different tunnels - T1 and T2. Both routers have the capability to
forward IPv6 packets in and out of their respective tunnels. The two
tunnels need not be based on the same tunnel protocol. The only
condition is that the two tunnel protocols be based on protocol-41
encapsulation. The IPv4 address of R1 is IP1, while the prefix of
its tunnel is Prf1. IP2 and Prf2 are the respective values for R2.
We assume that IP1 and IP2 belong to the same address realm, i.e.,
they are either both public, or both private and belong to the same
internal network. The following network diagram depicts the
locations of the two routers. The numbers indicate the packets of
the attack and the path they traverse as described below.
#######
# R1 #
#######
// \
T1 // 2 \ 1
interface // \
_______________//_ __\________________
| | | |
| IPv4 Network | | IPv6 Network |
|__________________| |___________________|
\\ /
\\ /
T2 \\ 2 / 0,1
interface \\ /
#######
# R2 #
#######
Figure 1: The network setting of the attack
The attack is depicted in Figure 2. It is initiated by sending an
IPv6 packet (packet 0 in Figure 2) destined to a fictitious end point
that appears to be reached via T2 and has IP1 as its IPv4 address,
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i.e., Addr(Prf2, IP1). The source address of the packet is a T1
address with Prf1 as the prefix and IP2 as the embedded IPv4 address,
i.e., Addr(Prf1, IP2). As the prefix of the destination address is
Prf2, the packet will be routed over the IPv6 network to T2.
We assume that R2 is the packet's entry point to T2. R2 receives the
packet through its IPv6 interface and forwards it over its T2
interface encapsulated with an IPv4 header having a destination
address derived from the IPv6 destination, i.e., IP1. The source
address is the address of R2, i.e., IP2. The packet (packet 1 in
Figure 2.) is routed over the IPv4 network to R1, which receives the
packet on its IPv4 interface. It processes the packet as a packet
that originates from one of the end nodes of T1.
Since the IPv4 source address corresponds to the IPv6 source address,
R1 will decapsulate the packet. Since the packet's IPv6 destination
is outside of T1, R1 will forward the packet onto a native IPv6
interface. The forwarded packet (packet 2 in Figure 2) is identical
to the original attack packet. Hence, it is routed back to R2, in
which the loop starts again. Note that the packet may not
necessarily be transported from R1 over native IPv6 network. R1 may
be connected to the IPv6 network through another tunnel.
R1 R2
| | 0
| 1 |<------
|<===============|
| 2 |
|--------------->|
| . |
| . |
1 - IPv4: IP2 --> IP1
IPv6: Addr(Prf1,IP2) --> Addr(Prf2,IP1)
0,2- IPv6: Addr(Prf1,IP2) --> Addr(Prf2,IP1)
Legend: ====> - tunneled IPv6, ---> - native IPv6
Figure 2: Routing loop attack between two tunnels' routers
The crux of the attack is as follows. The attacker exploits the fact
that R2 does not know that R1 does not take part of T2 and that R1
does not know that R2 does not take part of T1. The IPv4 network
acts as a shared link layer for the two tunnels. Hence, the packet
is repeatedly forwarded by both routers. It is noted that the attack
will fail when the IPv4 network can not transport packets between the
tunnels. For example, when the two routers belong to different IPv4
address realms or when ingress/egress filtering is exercised between
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the routes.
The loop will stop when the Hop Limit field of the packet reaches
zero. After a single loop the Hop Limit field is decreased by the
number of IPv6 routers on path from R1 and R2. Therefore, the number
of loops is inversely proportional to the number of IPv6 hops between
R1 and R2.
The tunnel pair T1 and T2 may be any combination of automatic tunnel
types, e.g., ISATAP, 6to4 and 6rd. This has the exception that both
tunnels can not be of type 6to4, since two 6to4 routers can not
belong to different tunnels (there is only one 6to4 tunnel in the
Internet). For example, if the attack were to be launched on an
ISATAP router (R1) and 6to4 relay (R2), then the destination and
source addresses of the attack packet would be 2002:IP1:* and Prf1::
0200:5EFE:IP2, respectively.
3. Proposed Mitigation Measures
This section presents some possible mitigation measures for the
attack described above. For each measure we shall discuss its
advantages and disadvantages.
The proposed measures fall under the following three categories:
o Verification of end point existence
o Operational measures
o Destination and source addresses checks
3.1. Verification of end point existence
The routing loop attack relies on the fact that a router does not
know whether there is an end point that can reached via its tunnel
that has the source or destination address of the packet. This
category includes mitigation measures which aim to verify that there
is a node which participate in the tunnel and its address corresponds
to the packet's destination or source addresses, as appropriate.
3.1.1. Neighbor Cache Check
One way that the router can verify that an end host exists and can be
reached via the tunnel is by checking whether a valid entry exists
for it in the neighbor cache of the corresponding tunnel interface.
The neighbor cache entry can be populated through, e.g., an initial
reachability check, receipt of neighbor discovery messages,
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administrative configuration, etc.
When the router has a packet to send to a potential tunnel host for
which there is no neighbor cache entry, it can perform an initial
reachability check on the packet's destination address, e.g., as
specified in the second paragraph of Section 8.4 of [RFC5214]. (The
router can similarly perform a "reverse reachability" check on the
packet's source address when it receives a packet from a potential
tunnel host for which there is no neighbor cache entry.) This
reachability check parallels the address resolution specifications in
Section 7.2 of [RFC4861], i.e., the router maintains a small queue of
packets waiting for reachability confirmation to complete. If
confirmation succeeds, the router discovers that a legitimate tunnel
host responds to the address. Otherwise, the router discards
subsequent packets and returns ICMP destination unreachable
indications as specified in Section 7.2.2 of [RFC4861].
Note that this approach assumes that the neighbor cache will remain
coherent and not subject to malicious attack, which must be confirmed
based on specific deployment scenarios. One possible way for an
attacker to subvert the neighbor cache is to send false neighbor
discovery messages with a spoofed source address.
3.1.2. Known IPv4 Address Check
Another approach that enables a router to verify that an end host
exists and can be reached via the tunnel is simply by pre-configuring
the router with the set of IPv4 addresses that are authorized to use
the tunnel. Upon this configuration the router can perform the
following simple checks:
o When the router forwards an IPv6 packet into the tunnel interface
with a destination address that matches an on-link prefix and that
embeds the IPv4 address IP1, it discards the packet if IP1 does
not belong to the configured list of IPv4 addresses.
o When the router receives an IPv6 packet on the tunnel's interface
with a source address that matches a on-link prefix and that
embeds the IPv4 address IP2, it discards the packet if IP2 does
not belong to the configured list of IPv4 addresses.
3.2. Operational Measures
The following measures can be taken by the network operator. Their
aim is to configure the network in such a way that the attacks can
not take place.
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3.2.1. Avoiding a Shared IPv4 Link
As noted above, the attack relies on having an IPv4 network as a
shared link-layer between more than one tunnel. From this the
following two mitigation measures arise:
3.2.1.1. Filtering IPv4 Protocol-41 Packets
In this measure a tunnel router may drop all IPv4 protocol-41 packets
received or sent over interfaces that are attached to an untrusted
IPv4 network. This will cut-off any IPv4 network as a shared link.
This measure has the advantage of simplicity. However, such a
measure may not always be suitable for scenarios where IPv4
connectivity is essential on all interfaces.
3.2.1.2. Operational Avoidance of Multiple Tunnels
This measure mitigates the attack by simply allowing for a single
IPv6 tunnel to operate in a bounded IPv4 network. For example, the
attack can not take place in broadband home networks. In such cases
there is a small home network having a single residential gateway
which serves as a tunnel router. A tunnel router is vulnerable to
the attack only if it has at least two interfaces with a path to the
Internet: a tunnel interface and a native IPv6 interface (as depicted
in Figure 1). However, a residential gateway usually has only a
single interface to the Internet, therefore the attack can not take
place. Moreover, if there are only one or a few tunnel routers in
the IPv4 network and all participate in the same tunnel then there is
no opportunity for perpetuating the loop.
This approach has the advantage that it avoids the attack profile
altogether without need for explicit mitigations. However, it
requires careful configuration management which may not be tenable in
large and/or unbounded IPv4 networks.
3.2.2. A Single Border Router
It is reasonable to assume that a tunnel router shall accept or
forward tunneled packets only over its tunnel interface. It is also
reasonable to assume that a tunnel router shall accept or forward
IPv6 packets only over its IPv6 interface. If these two interfaces
are physically different then the network operator can mitigate the
attack by ensuring that the following condition holds: there is no
path between these two interfaces that does not go through the tunnel
router.
The above condition ensures that an encapsulated packet which is
transmitted over the tunnel interface will not get to another tunnel
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router and from there to the IPv6 interface of the first router. The
condition also ensures the reverse direction, i.e., an IPv6 packet
which is transmitted over the IPv6 interface will not get to another
tunnel router and from there to the tunnel interface of the first
router. This condition is essentially translated to a scenario in
which the tunnel router is the only border router between the IPv6
network and the IPv4 network to which it is attached (as in broadband
home network scenario mentioned above).
3.2.3. A Comprehensive List of Tunnel Routers
If a tunnel router can be configured with a comprehensive list of
IPv4 addresses of all other tunnel routers in the network, then the
router can use the list as a filter to discard any tunneled packets
coming from other routers. For example, a tunnel router can use the
network's ISATAP Potential Router List (PRL) [RFC5214] as a filter as
long as there is operational assurance that all ISATAP routers are
listed and that no other types of tunnel routers are present in the
network.
This measure parallels the one proposed for 6rd in [RFC5969] where
the 6rd BR filters all known relay addresses of other tunnels inside
the ISP's network.
This measure is especially useful for intra-site tunneling
mechanisms, such as ISATAP and 6rd, since filtering can be exercised
on well-defined site borders.
3.2.4. Avoidance of On-link Prefixes
The looping attack exploits the fact that a router is permitted to
assign non-link-local IPv6 prefixes on its tunnel interfaces, which
could cause it to send tunneled packets to other routers that do not
configure an address from the prefix. Therefore, if the router does
not assign non-link-local IPv6 prefixes on its tunnel interfaces
there is no opportunity for it to initiate the loop. If the router
further ensures that the routing state is consistent for the packets
it receives on its tunnel interfaces there is no opportunity for it
to propagate a loop initiated by a different router.
This mitigation is available only to ISATAP routers, since the ISATAP
stateless address mapping operates only on the Interface Identifier
portion of the IPv6 address, and not on the IPv6 prefix. . The
mitigation is also only applicable on ISATAP links on which IPv4
source address spoofing is disabled. This section specifies new
operational procedures and mechanisms needed to implement the
mitigation; it therefore updates [RFC5214].
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3.2.4.1. ISATAP Router Interface Types
ISATAP provides a Potential Router List (PRL) to further ensure a
loop-free topology. Routers that are members of the provider network
PRL configure their provider network ISATAP interfaces as advertising
router interfaces (see: [RFC4861], Section 6.2.2), and therefore may
send Router Advertisement (RA) messages that include non-zero Router
Lifetimes. Routers that are not members of the provider network PRL
configure their provider network ISATAP interfaces as non-advertising
router interfaces.
3.2.4.2. ISATAP Source Address Verification
ISATAP nodes employ the source address verification checks specified
in Section 7.3 of [RFC5214] as a prerequisite for decapsulation of
packets received on an ISATAP interface. To enable the on-link
prefix avoidance procedures outlined in this section, ISATAP nodes
must employ an additional source address verification check; namely,
the node also considers the outer IPv4 source address correct for the
inner IPv6 source address if:
o a forwarding table entry exists that lists the packet's IPv4
source address as the link-layer address corresponding to the
inner IPv6 source address via the ISATAP interface.
3.2.4.3. ISATAP Host Behavior
ISATAP hosts send Router Solicitation (RS) messages to obtain RA
messages from an advertising ISATAP router. Whether or not non-link-
local IPv6 prefixes are advertised, the host can acquire IPv6
addresses, e.g., through the use of DHCPv6 stateful address
autoconfiguration [RFC3315]. To acquire addresses, the host performs
standard DHCPv6 exchanges while mapping the IPv6
"All_DHCP_Relay_Agents_and_Servers" link-scoped multicast address to
the IPv4 address of the advertising router (hence, the advertising
router must configure either a DHCPv6 relay or server function). The
host should also use DHCPv6 Authentication, and the DHCPv6 server
should refuse to process requests from hosts that cannot be
authenticated.
After the host receives IPv6 addresses, it assigns them to its ISATAP
interface and forwards any of its outbound IPv6 packets via the
advertising router as a default router. The advertising router in
turn maintains IPv6 forwarding table entries in the CURRENT state
that list the IPv4 address of the host as the link-layer address of
the delegated IPv6 addresses, and generates redirection messages to
inform the host of a better next hop when appropriate.
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3.2.4.4. ISATAP Router Behavior
In many use case scenarios (e.g., small enterprise networks, etc.),
advertising and non-advertising ISATAP routers can engage in a full-
or partial-topology dynamic IPv6 routing protocol, so that IPv6
routing/forwarding tables can be populated and standard IPv6
forwarding between ISATAP routers can be used. In other scenarios
(e.g., large ISP networks, etc.) this might be impractical dues to
scaling and security issues.
When a dynamic routing protocol cannot be used, non-advertising
ISATAP routers send RS messages to obtain RA messages from an
advertising ISATAP router, i.e., they act as "hosts" on their non-
advertising ISATAP interfaces. Non-advertising routers can also
acquire IPv6 prefixes, e.g., through the use of DHCPv6 Prefix
Delegation [RFC3633] via an advertising router in the same fashion as
described above for host-based DHCPv6 stateful address
autoconfiguration.
After the non-advertising router acquires IPv6 prefixes, it can sub-
delegate them to routers and links within its attached IPv6 edge
networks, then can forward any outbound IPv6 packets coming from its
edge networks via the advertising router as a default router. The
advertising router in turn maintains IPv6 forwarding table entries in
the CURRENT state that list the IPv4 address of the non-advertising
router as the link-layer address of the next hop toward the delegated
IPv6 prefixes, and generates redirection messages to inform the non-
advertising router of a better next hop when appropriate.
This implies that the advertising router considers the delegated
prefixes as identifying the non-advertising router as an on-link
neighbor for the purpose of generating redirection messages, and that
the non-advertising router accepts redirection messages coming from
the advertising router as though its ISATAP interface were configured
as a host interface.
3.2.4.5. Reference Operational Scenario
Figure 3 depicts a reference ISATAP network topology for operational
avoidance of on-link non-link-local IPv6 prefixes. The scenario
shows an advertising ISATAP router, a non-advertising ISATAP router,
an ISATAP host and a non-ISATAP IPv6 host in a typical deployment
configuration:
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.-(::::::::)
.-(::: IPv6 :::)-.
(:::: Internet ::::)
`-(::::::::::::)-'
`-(::::::)-'
,-.
,-----+-/-+--' \+------.
/ ,~~~~~~~~~~~~~~~~~, :
/ |companion gateway| |.
,-' '~~~~~~~~~~~~~~~~~' `.
; +--------------+ )
: | Router A | /
: | (isatap) | ;
+- +--------------+ -+
; fe80::5efe:192.0.2.1 :
| ;
: IPv4 Provider Network -+-'
`-. (PRL: 192.0.2.1) .)
\ _)
`-----+--------)----+'----'
fe80::5efe:192.0.2.2 fe80::5efe:192.0.2.3
2001:db8:0:1::1 +--------------+
+--------------+ | (isatap) |
| (isatap) | | Router C |
| Host B | +--------------+
+--------------+ 2001:db8:2::/48
.-.
,-( _)-. +------------+
.-(_ IPv6 )-. |(non-isatap)|
(__Edge Network )--| Host D |
`-(______)-' +------------+
2001:db8:2:1::1
Figure 3: Reference ISATAP Network Topology
In Figure 3, router 'A' within the IPv4 provider network connects to
the IPv6 Internet, either directly or via a companion gateway. 'A'
configures a provider network IPv4 interface with address 192.0.2.1
and arranges to add the address to the provider network PRL. 'A'
next configures an advertising ISATAP router interface with link-
local IPv6 address fe80::5efe:192.0.2.1 over the IPv4 interface.
Host 'B' connects to the provider network via an IPv4 interface with
address 192.0.2.2, and also configures an ISATAP host interface with
link-local address fe80::5efe:192.0.2.2 over the IPv4 interface. 'B'
next configures a default IPv6 route with next-hop address fe80::
5efe:192.0.2.1 via the ISATAP interface, then receives the IPv6
address 2001:db8:0:1::1 from a DHCPv6 address configuration exchange
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via 'A'. When 'B' receives the IPv6 address, it assigns the address
to the ISATAP interface but does not assign a non-link-local IPv6
prefix to the interface.
Router 'C' connects to one or more IPv6 edge networks and also
connects to the provider network via an IPv4 interface with address
192.0.2.3, but does not add the address to the provider network PRL.
'C' next configures a non-advertising ISATAP router interface with
link-local address fe80::5efe:192.0.2.3 over the IPv4 interface, but
does not engage in an IPv6 routing protocol over the interface. 'C'
therefore configures a default IPv6 route with next-hop address
fe80::5efe:192.0.2.1 via the ISATAP interface, and receives the IPv6
prefix 2001:db8:2::/48 through a DHCPv6 prefix delegation exchange
via 'A'. 'C' finally sub-delegates the prefix to its IPv6 edge
networks and configures its IPv6 edge network interfaces as
advertising router interfaces.
In this example, when 'B' has an IPv6 packet to send to host 'D'
within an IPv6 edge network connected by 'C', it prepares the IPv6
packet with source address 2001:db8:0:1::1 and destination address
2001:db8:2:1::1. 'B' then uses ISATAP encapsulation to forward the
packet to 'A' as its default router. 'A' forwards the packet to 'C',
and also sends redirection messages to inform 'B' that 'C' is a
better next hop toward 'D'. Future packets sent from 'B' to 'D'
therefore go directly to 'C' without involving 'A'. An analogous
redirection exchange occurs in the reverse direction when 'D' has a
packet to send to 'B' (via 'C'). Details of the redirection
exchanges are described in Section 3.2.4.6
3.2.4.6. ISATAP Predirection
With respect to the reference operational scenario depicted in
Figure 3, when ISATAP router 'A' receives an IPv6 packet on an
advertising ISATAP interface that it will forward back out the same
interface, 'A' must arrange to redirect the originating ISATAP node
'B' to a better next hop ISATAP node 'C' that is closer to the final
destination 'D'. First, however, 'A' must direct 'C' to establish a
forwarding table entry in order to satisfy the source address
verification check specified in Section 3.2.4.2. This process is
accommodated via a unidirectional reliable exchange in which 'A'
first informs 'C', then 'C' informs 'B' via 'A' as a trusted
intermediary. 'B' therefore knows that 'C' will accept the packets
it sends as long as 'C' retains the forwarding table entry. We call
this process "predirection", which stands in contrast to ordinary
IPv6 redirection.
Consider the alternative in which 'A' informs both 'B' and 'C'
separately via independent IPv6 Redirect messages (see: [RFC4861]).
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In that case, several conditions can occur that could result in
communications failures. First, if 'B' receives the Redirect message
but 'C' does not, subsequent packets sent by 'B' would disappear into
a black hole since 'C' would not have a forwarding table entry to
verify their source addresses. Second, if 'C' receives the Redirect
message but 'B' does not, subsequent packets sent in the reverse
direction by 'C' would be lost. Finally, timing issues surrounding
the establishment and garbage collection of forwarding table entries
at 'B' and 'C' could yield unpredictable behavior. For example,
unless the timing were carefully coordinated through some form of
synchronization loop, there would invariably be instances in which
one node has the correct forwarding table state and the other node
does not resulting in non-deterministic packet loss.
The following subsections discuss the predirection steps that support
the reference operational scenario:
3.2.4.6.1. 'A' Sends Predirect Forward To 'C'
When 'A' forwards an original IPv6 packet sent by 'B' out the same
ISATAP interface that it arrived on, it sends a "Predirect" message
forward toward 'C' instead of sending a Redirect message back to 'B'.
The Predirect message is simply an ISATAP-specific version of an
ordinary IPv6 Redirect message as depicted in Section 4.5 of
[RFC4861], and is identified by two new backward-compatible bits
taken from the Reserved field as shown in Figure 4:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (=137) | Code (=0) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I|P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: ISATAP-Specific IPv6 Redirect Message Format
Where the new bits are defined as:
I (1) the "ISATAP" bit. Set to 1 to indicate an ISATAP-specific
Redirect message, and set to 0 to indicate an ordinary IPv6
Redirect message.
P (1) the "Predirect" bit. Set to 1 to indicate a Predirect
message, and set to 0 to indicate a Redirect response to a
Predirect message. (This bit is valid only when the I bit is set
to 1.)
Using this new Predirect message format, 'A' prepares the message in
a similar fashion as for an ordinary ISATAP-encapsulated IPv6
Redirect message as follows:
o the outer IPv4 source address is set to 'A's IPv4 address.
o the outer IPv4 destination address is set to 'C's IPv4 address.
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o the inner IPv6 source address is set to 'A's ISATAP link-local
address.
o the inner IPv6 destination address is set to 'C's ISATAP link-
local address.
o the Predirect Target and Destination Addresses are both set to
'B's ISATAP link-local address.
o the Predirect message includes a Route Information Option (RIO)
[RFC4191] that encodes an IPv6 prefix taken from 'B's address/
prefix delegations that covers the IPv6 source address of the
originating IPv6 packet.
o the Predirect message includes a Redirected Header Option (RHO)
that contains at least the header of the originating IPv6 packet.
o the I and P bits in the Predirect message header are both set to
1.
'A' then sends the Predirect message forward to 'C'.
3.2.4.6.2. 'C' Processes the Predirect and Sends Redirect Back To 'A'
When 'C' receives the Predirect message, it decapsulates the message
according to Section 7.3 of [RFC5214] since the outer IPv4 source
address is a member of the PRL.
'C' then uses the message validation checks specified in Section 8.1
of [RFC4861], except that instead of verifying that the "IP source
address of the Redirect is the same as the current first-hop router
for the specified ICMP Destination Address" (i.e., the 6th
verification check), it accepts the message if the "outer IP source
address of the Predirect is the same as the current first-hop router
for the prefix specified in the RIO". (Note that this represents an
ISATAP-specific adaptation of the verification checks.) Finally, 'C'
only accepts the message if the destination address of the
originating IPv6 packet encapsulated in the RHO is covered by one of
its CURRENT delegated addresses/prefixes (see Section 3.2.4.9).
'C' then either creates or updates an IPv6 forwarding table entry
with the prefix encoded in the RIO option as the target prefix, and
the IPv6 Target Address of the Predirect message (i.e., 'B's ISATAP
link-local address) as the next hop. 'C' places the entry in the
FILTERING state, then sets/resets a filtering expiration timer value
of 40 seconds. If the filtering timer expires, the node clears the
FILTERING state and deletes the forwarding table entry if it is not
in the FORWARDING state. This suggests that 'C's ISATAP interface
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should maintain a private forwarding table separate from the common
IPv6 forwarding table, since the entry must be managed by the ISATAP
interface itself.
After processing the Predirect message and establishing the
forwarding table entry, 'C' prepares an ISATAP Redirect message in
response to the Predirect as follows:
o the outer IPv4 source address is set to 'C's IPv4 address.
o the outer IPv4 destination address is set to 'A's IPv4 address.
o the inner IPv6 source address, is set to 'C's ISATAP link-local
address.
o the inner IPv6 destination address is set to 'A's ISATAP link-
local address.
o the Redirect Target and the Redirect Destination Addresses are
both set to 'C's ISATAP link-local address.
o the Redirect message includes an RIO that encodes an IPv6 prefix
taken from 'C's address/prefix delegations that covers the IPv6
destination address of the originating IPv6 packet encapsulated in
the Redirected Header option of the Predirect.
o the Redirect message includes an RHO copied from the corresponding
Predirect message.
o the (I, P) bits in the Redirect message header are set to (1, 0).
'C' then sends the Redirect message to 'A'.
3.2.4.6.3. 'A' Processes the Redirect then Proxies it Back To 'B'
When 'A' receives the Predirect message, it decapsulates the message
according to Section 7.3 of [RFC5214] since the inner IPv6 source
address embeds the outer IPv4 source address.
'A' next accepts the message only if it satisfies the same message
validation checks specified for Predirects in Section 3.2.4.6.2.
'A' then locates a forwarding table entry that covers the IPv6 source
address of the packet segment in the RHO (i.e., a forwarding table
entry with next hop 'B'), then proxies the Redirect message back
toward 'B'. Without decrementing the IPv6 hop limit in the Redirect
message, 'A' next changes the IPv4 source address of the Redirect
message to its own IPv4 address, changes the IPv4 destination address
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to 'B's IPv4 address, changes the IPv6 source address to its own IPv6
link-local address, and changes the IPv6 destination address to 'B's
IPv6 link-local address. 'A' then sends the proxied Redirect message
to 'B'.
3.2.4.6.4. 'B' Processes The Redirect Message
When 'B' receives the Redirect message, it decapsulates the message
according to Section 7.3 of [RFC5214] since the outer IPv4 source
address is a member of the PRL.
'B' next accepts the message only if it satisfies the same message
validation checks specified for Predirects in Section 3.2.4.6.2.
'B' then either creates or updates an IPv6 forwarding table entry
with the prefix encoded in the RIO option as the target prefix, and
the IPv6 Target Address of the Redirect message (i.e., 'C's ISATAP
link-local address) as the next hop. 'B' places the entry in the
FORWARDING state, then sets/resets a forwarding expiration timer
value of 30 seconds. If the forwarding timer expires, the node
clears the FORWARDING state and deletes the forwarding table entry if
it is not in the FILTERING state. Again, this suggests that 'B's
ISATAP interface should maintain a private forwarding table separate
from the common IPv6 forwarding table, since the entry must be
managed by the ISATAP interface itself.
Now, 'B' has a forwarding table entry in the FORWARDING state, and
'C' has a forwarding table entry in the FILTERING state. Therefore,
'B' may send ordinary IPv6 data packets with destination addresses
covered by 'C's prefix directly to 'C' without involving 'A'. 'C'
will in turn accept the packets since they satisfy the source address
verification rule specified in Section 3.2.4.2.
To enable packet forwarding from 'C' directly to 'B', a reverse-
predirection operation is required which is the mirror-image of the
forward-predirection operation described above. Following the
reverse predirection, both 'B' and 'C' will have forwarding table
entries in the "(FORWARDING | FILTERING)" state, and IPv6 packets can
be exchanged bidirectionally without involving 'A'.
3.2.4.6.5. 'B' Sends Periodic Predirect Messages Forward to 'A'
In order to keep forwarding table entries alive while data packets
are actively flowing, 'B' can periodically send additional Predirect
messages via 'A' to solicit Redirect messages from 'C'. When 'B'
forwards an IPv6 packet via 'C', and the corresponding forwarding
table entry FORWARDING state timer is nearing expiration, 'B' sends
Predirect messages (subject to rate limiting) prepared as follows:
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o the outer IPv4 source address is set to 'B's IPv4 address.
o the outer IPv4 destination address is set to 'A's IPv4 address.
o the inner IPv6 source address is set to 'B's ISATAP link-local
address.
o the inner IPv6 destination address is set to 'A's ISATAP link-
local address.
o the Predirect Target and Destination Addresses are both set to
'B's ISATAP link-local address.
o the Predirect message includes an RIO that encodes an IPv6 prefix
taken from 'B's address/prefix delegations that covers the IPv6
source address of the originating IPv6 packet.
o the Predirect message includes an RHO that contains at least the
header of the originating IPv6 packet.
o the I and P bits in the Predirect message header are both set to
1.
When 'A' receives the Predirect message, it decapsulates the message
according to Section 7.3 of [RFC5214] since the inner IPv6 source
address embeds the outer IPv4 source address.
'A' next accepts the message only if it satisfies the same message
validation checks specified for Predirects in Section 3.2.4.6.2.
'A' then locates a forwarding table entry that covers the IPv6
destination address of the packet segment in the RHO (in this case, a
forwarding table entry with next hop 'C'). Without decrementing the
IPv6 hop limit in the Redirect message, 'A' next changes the IPv4
source address of the Predirect message to its own IPv4 address,
changes the IPv4 destination address to 'C's IPv4 address, changes
the IPv6 source address to its own IPv6 link-local address, and
changes the IPv6 destination address to 'C's IPv6 link-local address.
'A' then sends the proxied Predirect message to 'C'. When 'C'
receives the proxied message, it processes the message the same as if
it had originated from 'A' as described in Section 3.2.4.6.2.
3.2.4.7. Scaling Considerations
Figure 3 depicts an ISATAP network topology with only a single
advertising ISATAP router within the provider network. In order to
support larger numbers of non-advertising ISATAP routers and ISATAP
hosts, the provider network can deploy more advertising ISATAP
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routers to support load balancing and generally shortest-path
routing.
Such an arrangement requires that the advertising ISATAP routers
participate in an IPv6 routing protocol instance so that IPv6
address/prefix delegations can be mapped to the correct router. The
routing protocol instance can be configured as either a full mesh
topology involving all advertising ISATAP routers, or as a partial
mesh topology with each ISATAP router associating with one or more
companion gateways and a full mesh between companion gateways.
3.2.4.8. Proxy Chaining
In large ISATAP deployments, there may be many advertising ISATAP
routers, each serving many ISATAP clients (i.e., both non-advertising
routers and simple hosts). The advertising ISATAP routers then
either require full topology knowledge, or a default route to a
companion gateway that does have full topology knowledge. For
example, if Client 'A' connects to advertising ISATAP router 'B', and
Client 'E' connects to advertising ISATAP router 'D', then 'B' and
'D' must either have full topology knowledge or have a default route
to a companion gateway (e.g., 'C') that does.
In that case, when 'A' sends an initial packet to 'E', 'B' generates
a Predirect message toward 'C', which proxies the message toward 'D'
which finally proxies the message toward 'E'.
In the reverse direction, when 'E' sends a Redirect response message
to 'A', it first sends the message to 'D', which proxies the message
toward 'C', which proxies the message toward 'B', which finally
proxies the message toward 'A'.
3.2.4.9. Mobility
An ISATAP router 'A' can configure both a non-advertising ISATAP
interface on a provider network and an advertising ISATAP interface
on an edge network. In that case, 'A' can service ISATAP clients
(i.e. both non-advertising routers and simple hosts) within the edge
network by acting as a DHCPv6 relay. When a client 'B' in the edge
network that has obtained IPv6 addresses/prefixes moves to a
different edge network, however, 'B' can release its address/prefix
delegations via 'A' and re-establish them via a different ISATAP
router 'C' in the new edge network.
When 'B' releases its address/prefix delegations via 'A', 'A' marks
the IPv6 forwarding table entries that cover the addresses/prefixes
as DEPARTED (i.e., it clears the CURRENT state). 'A' therefore
ceases to respond to Predirect messages correlated with the DEPARTED
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entries, and also schedules a garbage-collection timer of 60 seconds,
after which it deletes the DEPARTED entries.
When 'A' receives IPv6 packets destined to an address covered by the
DEPARTED IPv6 forwarding table entries, it forwards them to the last-
known edge network link-layer address of 'B' as a means for avoiding
mobility-related packet loss during routing changes. Eventually,
correspondents will receive new Redirect messages from the network to
discover that 'B' is now associated with 'C'.
Note that this mobility management method works the same way when the
edge networks comprise native IPv6 links (i.e., and not just for
ISATAP links), however any IPv6 packets forwarded by 'A' via an IPv6
forwarding table entry in the DEPARTED state may be lost if the
mobile node moves off-link with respect to its previous edge network
point of attachment. This should not be a problem for large links
(e.g., large cellular network deployments, large ISP networks, etc.)
in which all/most mobility events are intra-link.
3.3. Destination and Source Address Checks
Tunnel routers can use a source address check mitigation when they
forward an IPv6 packet into a tunnel interface with an IPv6 source
address that embeds one of the router's configured IPv4 addresses.
Similarly, tunnel routers can use a destination address check
mitigation when they receive an IPv6 packet on a tunnel interface
with an IPv6 destination address that embeds one of the router's
configured IPv4 addresses. These checks should correspond to both
tunnels' IPv6 address formats, regardless of the type of tunnel the
router employs.
For example, if tunnel router R1 (of any tunnel protocol) forwards a
packet into a tunnel interface with an IPv6 source address that
matches the 6to4 prefix 2002:IP1::/48, the router discards the packet
if IP1 is one of its own IPv4 addresses. In a second example, if
tunnel router R2 receives an IPv6 packet on a tunnel interface with
an IPv6 destination address with an off-link prefix but with an
interface identifier that matches the ISATAP address suffix ::0200:
5EFE:IP2, the router discards the packet if IP2 is one of its own
IPv4 addresses.
Hence a tunnel router can avoid the attack by performing the
following checks:
o When the router forwards an IPv6 packet into a tunnel interface,
it discards the packet if the IPv6 source address has an off-link
prefix but embeds one of the router's configured IPv4 addresses.
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o When the router receives an IPv6 packet on a tunnel interface, it
discards the packet if the IPv6 destination address has an off-
link prefix but embeds one of the router's configured IPv4
addresses.
This approach has the advantage that that no ancillary state is
required, since checking is through static lookup in the lists of
IPv4 and IPv6 addresses belonging to the router. However, this
approach has some inherent limitations
o The checks incur an overhead which is proportional to the number
of IPv4 addresses assigned to the router. If a router is assigned
many addresses, the additional processing overhead for each packet
may be considerable. Note that an unmitigated attack packet would
be repetitively processed by the router until the Hop Limit
expires, which may require as many as 255 iterations. Hence, an
unmitigated attack will consume far more aggregate processing
overhead than per-packet address checks even if the router assigns
a large number of addresses.
o The checks should be performed for the IPv6 address formats of
every existing automatic IPv6 tunnel protocol (which uses
protocol-41 encapsulation). Hence, the checks must be updated as
new protocols are defined.
o Before the checks can be performed the format of the address must
be recognized. There is no guarantee that this can be generally
done. For example, one can not determine if an IPv6 address is a
6rd one, hence the router would need to be configured with a list
of all applicable 6rd prefixes (which may be prohibitively large)
in order to unambiguously apply the checks.
o The checks cannot be performed if the embedded IPv4 address is a
private one [RFC1918] since it is ambiguous in scope. Namely, the
private address may be legitimately allocated to another node in
another routing region.
The last limitation may be relieved if the router has some
information that allows it to unambiguously determine the scope of
the address. The check in the following subsection is one example
for this.
3.3.1. Known IPv6 Prefix Check
A router may be configured with the full list of IPv6 subnet prefixes
assigned to the tunnels attached to its current IPv4 routing region.
In such a case it can use the list to determine when static
destination and source address checks are possible. By keeping track
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of the list of IPv6 prefixes assigned to the tunnels in the IPv4
routing region, a router can perform the following checks on an
address which embeds a private IPv4 address:
o When the router forwards an IPv6 packet into its tunnel with a
source address that embeds a private IPv4 address and matches an
IPv6 prefix in the prefix list, it determines whether the packet
should be discarded or forwarded by performing the source address
check specified in Section 3.3. Otherwise, the router forwards
the packet.
o When the router receives an IPv6 packet on its tunnel interface
with a destination address that embeds a private IPv4 address and
matches an IPv6 prefix in the prefix list, it determines whether
the packet should be discarded or forwarded by performing the
destination address check specified in Section 3.3. Otherwise,
the router forwards the packet.
The disadvantage of this approach is the administrative overhead for
maintaining the list of IPv6 subnet prefixes associated with an IPv4
routing region may become unwieldy should that list be long and/or
frequently updated.
4. Recommendations
In light of the mitigation measures proposed above we make the
following recommendations in decreasing order:
1. When possible, it is recommended that the attacks are
operationally eliminated (as per one of the measures proposed in
Section 3.2).
2. For tunnel routers that keep a coherent and trusted neighbor
cache which includes all legitimate end-points of the tunnel, we
recommend exercising the Neighbor Cache Check.
3. For tunnel routers that can implement the Neighbor Reachability
Check, we recommend exercising it.
4. For tunnels having small and static list of end-points we
recommend exercising Known IPv4 Address Check.
5. We generally do not recommend using the Destination and Source
Address Checks since they can not mitigate routing loops with 6rd
routers. Therefore, these checks should not be used alone unless
there is operational assurance that other measures are exercised
to prevent routing loops with 6rd routers.
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As noted earlier, tunnels may be deployed in various operational
environments. There is a possibility that other mitigations may be
feasible in specific deployment scenarios. The above recommendations
are general and do not attempt to cover such scenarios.
5. IANA Considerations
This document has no IANA considerations.
6. Security Considerations
This document aims at presenting possible solutions to the routing
loop attack which involves automatic tunnels' routers. It contains
various checks that aim to recognize and drop specific packets that
have strong potential to cause a routing loop. These checks do not
introduce new security threats.
7. Acknowledgments
This work has benefited from discussions on the V6OPS, 6MAN and
SECDIR mailing lists. Remi Despres, Christian Huitema, Dmitry
Anipko, Dave Thaler and Fernando Gont are acknowledged for their
contributions.
8. References
8.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
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More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
8.2. Informative References
[I-D.gont-6man-teredo-loops]
Gont, F., "Mitigating Teredo Rooting Loop Attacks",
draft-gont-6man-teredo-loops-00 (work in progress),
September 2010.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[USENIX09]
Nakibly, G. and M. Arov, "Routing Loop Attacks using IPv6
Tunnels", USENIX WOOT, August 2009.
Authors' Addresses
Gabi Nakibly
National EW Research & Simulation Center
P.O. Box 2250 (630)
Haifa 31021
Israel
Email: gnakibly@yahoo.com
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Fred L. Templin
Boeing Research & Technology
P.O. Box 3707 MC 7L-49
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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