One document matched: draft-nakibly-v6ops-tunnel-loops-02.xml
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<rfc category="info" docName="draft-nakibly-v6ops-tunnel-loops-02.txt" ipr="trust200811">
<front>
<title abbrev="Routing Loop Attack">Routing Loop Attack using IPv6 Automatic Tunnels: Problem Statement and Proposed Mitigations</title>
<author fullname="Gabi Nakibly" initials="G." surname="Nakibly">
<organization abbrev="National EW Research & Simulation Center">National EW Research & Simulation Center</organization>
<address>
<postal>
<street>P.O. Box 2250 (630)</street>
<city>Haifa</city>
<code>31021</code>
<country>Israel</country>
</postal>
<email>gnakibly@yahoo.com</email>
</address>
</author>
<author fullname="Fred L. Templin" initials="F." surname="Templin">
<organization>Boeing Research & Technology</organization>
<address>
<postal>
<street>P.O. Box 3707 MC 7L-49</street>
<city>Seattle</city>
<region>WA</region>
<code>98124</code>
<country>USA</country>
</postal>
<email>fltemplin@acm.org</email>
</address>
</author>
<date day="12" month="May" year="2010"></date>
<keyword>I-D</keyword><keyword>Internet-Draft</keyword>
<abstract>
<t>This document is concerned with security vulnerabilities in IPv6-in-IPv4 automatic tunnels. These vulnerabilities allow an attacker to take advantage of inconsistencies between a tunnel's overlay IPv6 routing state and the native 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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>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 form a category of tunnels in which a packet's egress node's IPv4 address is derived from the destination IPv6 address of the packet. Recent research <xref target="USENIX09"></xref> has pointed out the existence of a vulnerability in the design of IPv6 automatic tunnels. All tunnel end points operate on the implicit assumption that once a packet arrives at the tunnel, its destination is indeed part of the tunnel. This assumption poses a security vulnerability since it may result in an inconsistency between a tunnel's overlay IPv6 routing state and the native IPv6 routing state there by allowing a routing loop to be formed.</t>
<t>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 zeroed out.</t>
<t>Unless proper security measures are in place all IPv6 automatic tunnels that are based on protocol-41 encapsulation are vulnerable to such an attack, in particular, ISATAP <xref target="RFC5214"></xref>, 6to4 <xref target="RFC3056"></xref> and 6rd <xref target="RFC5569"></xref>. The aim of this document is to shed light on the routing loop attack and present some possible mitigation measures that should be considered by operators of current IPv6 automatic tunnels and by designers of future ones.</t>
</section>
<section title="A Detailed Description of the Attack">
<t>In this section we shall denote an IPv6 address of a tunnel's node by the prefix of the tunnel and the IPv4 address of the node, 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.</t>
<t>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. </t>
<t>The attack is depicted in <xref target="figattack"></xref>. It is initiated by sending an IPv6 packet (packet 0 in <xref target="figattack"></xref>) destined to an end point in T2 having IP1 as its IPv4 address, 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. </t>
<t> 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 <xref target="figattack"></xref>.) 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. </t>
<t>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 <xref target="figattack"></xref>) is identical to the original attack packet. Hence, it is routed back to R2, in which the loop starts again.</t>
<figure anchor="figattack" title="Routing loop attack between two tunnels' routers">
<artwork><![CDATA[
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)
]]></artwork>
<postamble>Legend: ====> - tunneled IPv6, ---> - native IPv6</postamble>
</figure>
<t>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. Hence, the packet is repeatedly forwarded by both routers.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Proposed Mitigation Measures">
<t>This section presents some possible mitigation measures for the attack described above. For each measure we shall discuss its advantages and disadvantages.</t>
<t>The proposed measures fall under the following three categories:</t>
<t>
<list style="symbols">
<t>Destination and source addresses checks</t>
<t>Verification of end point existence</t>
<t>Architectural modifications</t>
</list>
</t>
<section anchor="address-check" title="Destination and Source Address Checks">
<t>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 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.</t>
<t>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.</t>
<t>Hence a tunnel router can avoid the attack by performing the following simple checks:</t>
<t>
<list style="symbols">
<t>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.</t>
<t>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.</t>
</list>
</t>
<t>This approach has the advantage that it is easy to implement and 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 inherit limitations:</t>
<t>
<list style="symbols">
<t>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 unacceptable.</t>
<t>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.</t>
<t>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.</t>
<t>The checks cannot be performed if the embedded IPv4 address is a private one <xref target="RFC1918"></xref> since it is ambiguous in scope. Namely, the private address may be legitimately allocated to another node in another routing region.</t>
</list>
</t>
<t>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.</t>
<section anchor="known-ipv6-check" title="Known IPv6 Prefix Check">
<t>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:</t>
<t>
<list style="symbols">
<t>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 <xref target="address-check"></xref>. Otherwise, the router forwards the packet.</t>
<t>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 accepted by performing the destination address check specified in <xref target="address-check"></xref>. Otherwise, the router accepts the packet.</t>
</list>
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.</t>
</section>
</section>
<section title="Verification of end point existence">
<t>The routing loop attack relies on the fact that a router does not know whether there is an end point in its tunnel having 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.</t>
<section anchor="neighbor-check" title="Neighbor Cache Check">
<t>One way to verify that an end point exists in a tunnel is by checking whether a valid entry exists for it in the Neighbor Cache of the corresponding tunnel interface. A valid entry may exist in the Neighbor Cache for legitimate end hosts if they generate traffic towards the router upon startup. For example, an initial RS/RA exchange to facilitate Stateless Address Auto configuration (as in the ISATAP case). This allows the router to keep valid Neighbor Cache entry for each legitimate end host in the tunnel.</t>
<t>By keeping track of the legitimate hosts in the tunnel via the Neighbor Cache, a router can perform the following simple checks:</t>
<t>
<list style="symbols">
<t>When the router forwards a packet into the tunnel with an IPv6 destination address that matches an on-link prefix and that embeds the IPv4 address IP1, it discards the packet if there is no corresponding neighbor cache entry.</t>
<t>When the router receives a packet on the tunnel's interface with an IPv6 source address that matches an on-link prefix and embeds the IPv4 address IP2, it discards the packet if there is no corresponding neighbor cache entry.</t>
</list>
</t>
<t>This approach is easy to implement, and naturally leverages the fact that an end host must successively send RSs in order to refresh configuration information as on-link prefix information. However, this requires the router to retain entries for a duration that is at least as long as the router's advertised prefix lifetimes. This may require an implementation to adjust its garbage-collection interval for stale neighbor cache entries.</t>
<t>Finally, 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 RS messages with a spoofed source address.</t>
</section>
<section anchor="known-ipv4-check" title="Known IPv4 Address Check">
<t>Another approach that enables a router to verify that an end host indeed exists in the tunnel is simply by pre-configuring the router with the set of IPv4 addresses that are authorized to use the tunnel's subnet. Upon this configuration the router can perform the following simple checks:</t>
<t>
<list style="symbols">
<t>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.</t>
<t>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.</t>
</list>
</t>
</section>
<section anchor="address-resolution" title="Neighbor Reachability Check">
<t>Yet another approach that allows a router to verify the existence of an end host in the tunnel is by performing an initial reachability confirmation, e.g., as specified in the second paragraph of Section 8.4 of <xref target="RFC5214"></xref>. This procedure parallels the address resolution specifications in Section 7.2 of <xref target="RFC4861"></xref>, 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 neighbor responds to the address and packets may be forwarded to it. Otherwise, the router returns ICMP destination unreachable indications as specified in Section 7.2.2 of <xref target="RFC4861"></xref>.</t>
</section>
</section>
<section anchor="arch" title="Architectural Modifications">
<t>This category includes a measure that change the architecture of the tunnel protocol specifications.</t>
<section title="IPv4 Protocol Values">
<t>The routing loop attack leverages the fact that a router can not determine whether a given packet originated from an end node in its own type of tunnel. If the router was able to determine that a packet originated from an end node which employs a tunnel of different type than the router's, the router would discard the packet. This approach proposes doing this by changing the semantics of the IPv4 Protocol field to denote the type of tunnel protocol and not just identify the IPv6 layer. Namely, the field will carry different values for different automatic tunnel types. Using this approach a router can drop packets in which the IPv4 Protocol field does not correspond to its own tunnel type.</t>
<t>This approach can eliminate routing loops between two routers that employ different types of tunneling protocols. However, it does not mitigate the attack when they are both of the same type.</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>This document has no IANA considerations.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>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.</t>
</section>
<section anchor="acknowledge" title="Acknowledgments">
<t>This work has benefited from discussions on the V6OPS, 6MAN and SECDIR mailing lists. Remi Despres, Christian Huitema and Dmitry Anipko are acknowledged for their contributions.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.3056"?>
<?rfc include="reference.RFC.5214"?>
<?rfc include="reference.RFC.5569"?>
<?rfc include="reference.RFC.1918"?>
<?rfc include="reference.RFC.4861"?>
</references>
<references title="Informative References">
<reference anchor="USENIX09">
<front>
<title>Routing Loop Attacks using IPv6 Tunnels</title>
<author fullname="Gabi Nakibly" initials="G." surname="Nakibly">
<organization />
</author>
<author fullname="Michael Arov" initials="M." surname="Arov">
<organization />
</author>
<date month="USENIX WOOT, August" year="2009" />
</front>
</reference>
</references>
</back>
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