Network Working Group                                         G. Nakibly
Internet-Draft                                    National EW Research &
Intended status: Informational                         Simulation Center
Expires: July 28, August 8, 2011                                       F. Templin
                                            Boeing Research & Technology
                                                        January 24,
                                                       February 04, 2011

Routing Loop Attack using IPv6 Automatic Tunnels: Problem Statement and
                          Proposed Mitigations
                  draft-ietf-v6ops-tunnel-loops-02.txt
                  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
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 28, August 8, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  A Detailed Description of the Attack . . . . . . . . . . . . .  4
   3.  Proposed Mitigation Measures . . . . . . . . . . . . . . . . .  6
     3.1.  Destination and Source Address Checks  Verification of end point existence  . . . . . . . . . . .  6
       3.1.1.  Known IPv6 Prefix  Neighbor Cache Check . . . . . . . . . . . . . . .  8
     3.2.  Verification of end point existence . .  6
       3.1.2.  Known IPv4 Address Check . . . . . . . . .  8
       3.2.1.  Neighbor Cache Check . . . . . .  7
     3.2.  Operational Measures . . . . . . . . . . .  8
       3.2.2.  Known IPv4 Address Check . . . . . . . .  7
       3.2.1.  Avoiding a Shared IPv4 Link  . . . . . . .  9
     3.3.  Operational Measures . . . . . .  8
       3.2.2.  A Single Border Router . . . . . . . . . . . . .  9
       3.3.1.  Avoiding a Shared IPv4 Link . . .  8
       3.2.3.  A Comprehensive List of Tunnel Routers . . . . . . . .  9
     3.3.  Destination and Source Address Checks  . . . . . . . . . 10
       3.3.2.  A Single Border Router .  9
       3.3.1.  Known IPv6 Prefix Check  . . . . . . . . . . . . . . . 10 11
   4.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11 12
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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 [RFC5569]. [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.

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
   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.

   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  Destination and source addresses checks

   o  Verification of end point existence

   o  Operational measures

3.1.

   o  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 addresses checks

3.1.  Verification of end point existence

   The routing loop attack relies on the router's configured IPv4 addresses.
   Similarly, tunnel routers can use fact that a destination address check
   mitigation when they receive router does not
   know whether there is an IPv6 packet on a end point that can reached via its tunnel interface
   with an IPv6
   that has the source or destination address that embeds one of the router's
   configured IPv4 addresses.  These checks should correspond packet.  This
   category includes mitigation measures which aim to both
   tunnels' IPv6 address formats, regardless of verify that there
   is a node which participate in the type of 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 employs.

   For example, if tunnel router R1 (of any can verify that an end host exists and can be
   reached via the tunnel protocol) forwards a
   packet into is by checking whether a valid entry exists
   for it in the neighbor cache of the corresponding tunnel interface with interface.
   The neighbor cache entry can be populated through, e.g., an IPv6 source address that
   matches the 6to4 prefix 2002:IP1::/48, initial
   reachability check, receipt of neighbor discovery messages,
   administrative configuration, etc.

   When the router discards the packet
   if IP1 is one of its own IPv4 addresses.  In has a second example, if
   tunnel router R2 receives an IPv6 packet on to send to a potential tunnel interface with
   an IPv6 destination address with an off-link prefix but with host for
   which there is no neighbor cache entry, it can perform an
   interface identifier that matches the ISATAP address suffix ::0200:
   5EFE:IP2, initial
   reachability check on the router discards packet's destination address, e.g., as
   specified in the packet if IP2 is one second paragraph of its own
   IPv4 addresses.

   Hence a tunnel Section 8.4 of [RFC5214].  (The
   router can avoid the attack by performing the
   following checks:

   o  When the router forwards an IPv6 packet into similarly perform a tunnel interface,
      it discards the packet if "reverse reachability" check on the IPv6
   packet's source address has an off-link
      prefix but embeds one of the router's configured IPv4 addresses.

   o  When the router when it receives an IPv6 a packet on from a potential
   tunnel interface, it
      discards the packet if host for which there is no neighbor cache entry.)  This
   reachability check parallels 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 resolution specifications in
   Section 7.2 of [RFC4861], i.e., the lists router maintains a small queue of
   IPv4 and IPv6 addresses belonging
   packets waiting for reachability confirmation to complete.  If
   confirmation succeeds, the router discovers that a legitimate tunnel
   host responds to the router.  However, 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 has some inherent limitations

   o  The checks incur an overhead assumes that the neighbor cache will remain
   coherent and not subject to malicious attack, which is proportional must be confirmed
   based on specific deployment scenarios.  One possible way for an
   attacker to subvert the number
      of IPv4 addresses assigned neighbor cache is to the router.  If send false neighbor
   discovery messages with a spoofed source address.

3.1.2.  Known IPv4 Address Check

   Another approach that enables a router is assigned
      many addresses, the additional processing overhead for each packet
      may be considerable.  Note to verify that an unmitigated attack packet would end host
   exists and can be repetitively processed reached via the tunnel is simply by pre-configuring
   the router until with 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 set of IPv4 addresses that are authorized to use
   the tunnel.  Upon this configuration the router assigns
      a large number of addresses. can perform the
   following simple checks:

   o  The checks should be performed for  When the router forwards an 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 packet into the tunnel interface
      with a destination address must
      be recognized.  There is no guarantee that this can be generally
      done.  For example, one can not determine if matches an IPv6 on-link prefix and that
      embeds the IPv4 address is a
      6rd one, hence IP1, it discards the router would need packet if IP1 does
      not belong to be the configured with a list of all applicable 6rd prefixes (which may be prohibitively large)
      in order to unambiguously apply the checks. IPv4 addresses.

   o  The checks cannot be performed if  When the embedded IPv4 router receives an IPv6 packet on the tunnel's interface
      with a source address is that matches a
      private one [RFC1918] since it is ambiguous in scope.  Namely, on-link prefix and that
      embeds the
      private IPv4 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 IP2, it discards the packet if IP2 does
      not belong to unambiguously determine the scope configured list of
   the address. IPv4 addresses.

3.2.  Operational Measures

   The check in the following subsection is one example
   for this.

3.1.1.  Known IPv6 Prefix Check

   A router may measures can be configured with taken by the full list of IPv6 subnet prefixes
   assigned network operator.  Their
   aim is to configure the tunnels attached to its current IPv4 routing region.
   In network in such a case it can use way that 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 attacks can perform
   not take place.

3.2.1.  Avoiding a Shared IPv4 Link

   As noted above, the following checks attack relies on having an
   address which embeds a private IPv4 address:

   o  When 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 forwards may drop all IPv4 protocol-41 packets
   received or sent over interfaces that are attached to an IPv6 packet into its tunnel with untrusted
   IPv4 network.  This will cut-off any IPv4 network as a
      source address that embeds shared link.
   This measure has the advantage of simplicity.  However, such a private
   measure may not always be suitable for scenarios where IPv4 address and matches an
      IPv6 prefix in the prefix list, it determines whether
   connectivity is essential on all interfaces.

3.2.1.2.  Operational Avoidance of Multiple Tunnels

   This measure mitigates the packet
      should be discarded or forwarded attack by performing the source address
      check specified simply allowing for a single
   IPv6 tunnel to operate in Section 3.1.  Otherwise, 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 forwards is vulnerable to
   the packet.

   o  When attack only if it has at least two interfaces with a path to the router receives an IPv6 packet on its
   Internet: a tunnel interface
      with a destination address that embeds and a private IPv4 address and
      matches an native IPv6 prefix interface (as depicted
   in Figure 1).  However, a residential gateway usually has only a
   single interface to the prefix list, it determines whether Internet, therefore the packet should be discarded attack can not take
   place.  Moreover, if there are only one or forwarded by performing a few tunnel routers in
   the
      destination address check specified IPv4 network and all participate in Section 3.1.  Otherwise, the router forwards same tunnel then there is
   no opportunity for perpetuating the packet.

   The disadvantage of this loop.

   This approach is has the administrative overhead for
   maintaining advantage that it avoids the list of IPv6 subnet prefixes associated with an IPv4
   routing region attack profile
   altogether without need for explicit mitigations.  However, it
   requires careful configuration management which may become unwieldy should that list not be long tenable in
   large and/or
   frequently updated.

3.2.  Verification of end point existence

   The routing loop attack relies on the fact unbounded IPv4 networks.

3.2.2.  A Single Border Router

   It is reasonable to assume that a tunnel router does not
   know whether there is an end point that can reached via shall accept or
   forward tunneled packets only over its tunnel
   that has the source or destination address of the packet.  This
   category includes mitigation measures which aim interface.  It is also
   reasonable to verify assume that there
   is a node which participate in the tunnel and router shall accept or forward
   IPv6 packets only over its address corresponds
   to IPv6 interface.  If these two interfaces
   are physically different then the packet's destination or source addresses, as appropriate.

3.2.1.  Neighbor Cache Check

   One way network operator can mitigate the
   attack by ensuring that the router can verify following condition holds: there is no
   path between these two interfaces that an end host exists and can be
   reached via does not go through the tunnel
   router.

   The above condition ensures that an encapsulated packet which is by checking whether a valid entry exists
   for it in
   transmitted over the neighbor cache tunnel interface will not get to another tunnel
   router and from there to the IPv6 interface of the corresponding tunnel interface. first router.  The neighbor cache entry can be populated through, e.g.,
   condition also ensures the reverse direction, i.e., an initial
   reachability check, receipt of neighbor discovery messages,
   administrative configuration, etc.

   When IPv6 packet
   which is transmitted over the IPv6 interface will not get to another
   tunnel router has a packet and from there to send the tunnel interface of the first
   router.  This condition is essentially translated to a potential tunnel host for scenario in
   which there the tunnel router is no neighbor cache entry, it can perform an initial
   reachability check on the packet's destination address, e.g., as
   specified in only border router between the second paragraph of Section 8.4 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 [RFC5214].  (The Tunnel Routers

   If a tunnel router can similarly perform be configured with a "reverse reachability" check on comprehensive list of
   IPv4 addresses of all other tunnel routers in the
   packet's source address when it receives network, then the
   router can use the list as a packet filter to discard any tunneled packets
   coming from other routers.  For example, a potential tunnel host for which router can use the
   network's ISATAP Potential Router List (PRL) [RFC5214] as a filter as
   long as there is no neighbor cache entry.) operational assurance that all ISATAP routers are
   listed and that no other types of tunnel routers are present in the
   network.

   This
   reachability check measure parallels the address resolution specifications one proposed for 6rd in
   Section 7.2 of [RFC4861], i.e., [RFC5969] where
   the router maintains a small queue 6rd BR filters all known relay addresses of
   packets waiting for reachability confirmation to complete.  If
   confirmation succeeds, the router discovers that a legitimate tunnel
   host responds to the address.  Otherwise, other tunnels inside
   the router discards
   subseqent packets and returns ICMP destination unreachable
   indications ISP's network.

   This measure is especially useful for intra-site tunneling
   mechanisms, such as specified in Section 7.2.2 of [RFC4861].

   Note that this approach assumes that the neighbor cache will remain
   coherent ISATAP and not subject to malicious attack, which must 6rd, since filtering can be confirmed
   based exercised
   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.

3.2.2.  Known IPv4 well-defined site borders.

3.3.  Destination and Source Address Check

   Another approach that enables Checks

   Tunnel routers can use a router to verify that source address check mitigation when they
   forward an end host
   exists and can be reached via the IPv6 packet into a tunnel is simply by pre-configuring
   the router interface with the set of IPv4 addresses an IPv6 source
   address that are authorized to use
   the tunnel.  Upon this configuration embeds one of the router router's configured IPv4 addresses.
   Similarly, tunnel routers can perform the
   following simple checks:

   o  When the router forwards use a destination address check
   mitigation when they receive an IPv6 packet into the on a tunnel interface
   with a an IPv6 destination address that matches an on-link prefix and that embeds one of the router's
   configured IPv4 addresses.  These checks should correspond to both
   tunnels' IPv6 address IP1, it 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 does
      not belong to the configured list is one of its own IPv4 addresses.

   o  When the  In a second example, if
   tunnel router R2 receives an IPv6 packet on the tunnel's a tunnel interface with a source
   an IPv6 destination address that matches a on-link with an off-link prefix and but with an
   interface identifier that
      embeds matches the IPv4 ISATAP address IP2, it suffix ::0200:
   5EFE:IP2, the router discards the packet if IP2 does
      not belong to the configured list is one of its own
   IPv4 addresses.

3.3.  Operational Measures

   The following measures

   Hence a tunnel router can be taken avoid the attack by performing the network operator.  Their
   aim is to configure
   following checks:

   o  When the network in such router forwards an IPv6 packet into a way that tunnel interface,
      it discards the attacks can
   not take place.

3.3.1.  Avoiding a Shared IPv4 Link

   As noted above, packet if the attack relies on having IPv6 source address has an IPv4 network as a
   shared link-layer between more than off-link
      prefix but embeds one tunnel.  From this of the
   following two mitigation measures arise:

3.3.1.1.  Filtering router's configured IPv4 Protocol-41 Packets

   In this measure a tunnel addresses.

   o  When the router may drop all IPv4 protocol-41 packets
   received or sent over interfaces that are attached to receives an untrusted
   IPv4 network.  This will cut-off any IPv4 network as IPv6 packet on a shared link.
   This measure has the advantage tunnel interface, it
      discards the packet if the IPv6 destination address has an off-
      link prefix but embeds one of simplicity.  However, such a
   measure may not always be suitable for scenarios where the router's configured IPv4
   connectivity is essential on all interfaces.

3.3.1.2.  Operational Avoidance of Multiple Tunnels
      addresses.

   This measure mitigates approach has the attack by simply allowing for a single
   IPv6 tunnel to operate advantage that that no ancillary state is
   required, since checking is through static lookup in a bounded the lists of
   IPv4 network.  For example, and IPv6 addresses belonging to the
   attack can not take place in broadband home networks.  In such cases
   there is a small home network having a single residential gateway router.  However, this
   approach has some inherent limitations

   o  The checks incur an overhead which serves as a tunnel is proportional to the number
      of IPv4 addresses assigned to the router.  A tunnel  If a router is vulnerable to assigned
      many addresses, the additional processing overhead for each packet
      may be considerable.  Note that an unmitigated attack only 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 it has at least two interfaces with a path to the
   Internet: a tunnel interface and router assigns
      a native large number of addresses.

   o  The checks should be performed for the IPv6 interface (as depicted
   in Figure 1).  However, a residential gateway usually has only a
   single interface to address formats of
      every existing automatic IPv6 tunnel protocol (which uses
      protocol-41 encapsulation).  Hence, the Internet, therefore checks must be updated as
      new protocols are defined.

   o  Before the attack 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 take
   place.  Moreover, determine if there are only one or an IPv6 address is a few tunnel routers
      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 network and all participate address is a
      private one [RFC1918] since it is ambiguous in scope.  Namely, the same tunnel then there is
   no opportunity for perpetuating
      private address may be legitimately allocated to another node in
      another routing region.

   The last limitation may be relieved if the loop.

   This approach router has the advantage some
   information that allows it avoids to unambiguously determine the attack profile
   altogether without need scope of
   the address.  The check in the following subsection is one example
   for explicit mitigations.  However, it
   requires careful configuration management which this.

3.3.1.  Known IPv6 Prefix Check

   A router may not be tenable in
   large and/or unbounded IPv4 networks.

3.3.2.  A Single Border Router

   It is reasonable configured with the full list of IPv6 subnet prefixes
   assigned to assume that a tunnel router shall accept or
   forward tunneled packets only over its tunnel interface.  It is also
   reasonable the tunnels attached to assume that a tunnel router shall accept or forward
   IPv6 packets only over its IPv6 interface.  If these two interfaces current IPv4 routing region.
   In such a case it can use the list to determine when static
   destination and source address checks are physically different then possible.  By keeping track
   of the network operator can mitigate list of IPv6 prefixes assigned to the
   attack by ensuring that tunnels in the IPv4
   routing region, a router can perform the following condition holds: there is no
   path between these two interfaces that does not go through checks on an
   address which embeds a private IPv4 address:

   o  When the router forwards an IPv6 packet into its tunnel
   router.

   The above condition ensures with a
      source address that embeds a private IPv4 address and matches an encapsulated
      IPv6 prefix in the prefix list, it determines whether the packet which is
   transmitted over
      should be discarded or forwarded by performing the tunnel interface will not get to another tunnel
   router and from there to source address
      check specified in Section 3.3.  Otherwise, the IPv6 interface of router forwards
      the first router.  The
   condition also ensures packet.

   o  When the reverse direction, i.e., router receives an IPv6 packet
   which is transmitted over the IPv6 interface will not get to another on its tunnel router interface
      with a destination address that embeds a private IPv4 address and from there to
      matches an IPv6 prefix in the tunnel interface of prefix list, it determines whether
      the first
   router.  This condition is essentially translated to a scenario packet should be discarded or forwarded by performing the
      destination address check specified in
   which Section 3.3.  Otherwise,
      the tunnel router forwards the packet.

   The disadvantage of this approach is the only border router between administrative overhead for
   maintaining the list of IPv6
   network and the subnet prefixes associated with an IPv4 network to which it is attached (as in broadband
   home network scenario mentioned above).
   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 3.3). 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.  For all other cases we  We generally do not recommend using the Destination and Source
       Address Checks.  This is the least preferable measure Checks since it
       generally 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.

   [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.

   [RFC5569]  Despres, R.,

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", (6rd) -- Protocol Specification",
              RFC 5569, January 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
   Fred L. Templin
   Boeing Research & Technology
   P.O. Box 3707 MC 7L-49
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org