Network Working Group G. Nakibly
Internet-Draft National EW Research &
Intended status: Informational Simulation Center
Expires: August 8, 2011 F. Templin
Boeing Research & Technology
February 04, 2011
Routing Loop Attack using IPv6 Automatic Tunnels: Problem Statement and
Proposed Mitigations
draft-ietf-v6ops-tunnel-loops-03.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 August 8, 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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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
3.3. Destination and Source Address Checks . . . . . . . . . . 9
3.3.1. Known IPv6 Prefix Check . . . . . . . . . . . . . . . 11
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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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 use stateless address mapping
(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.
#######
# R1 #
#######
// \
T1 // \
interface // \
_______________//_ __\________________
| | | |
| IPv4 Network | | IPv6 Network |
|__________________| |___________________|
\\ /
\\ /
T2 \\ /
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,
i.e., Addr(Prf2, IP1). The source address of the packet is a T1
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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
the routes.
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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,
administrative configuration, etc.
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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
subseqent 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.
3.3. 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
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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.
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
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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
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.
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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.
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.
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[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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