Internet Engineering Task Force P. Savola Internet Draft CSC/FUNET Expiration Date: March 2002 September 2001 Security Considerations and Enhancements for 6to4 draft-savola-ngtrans-6to4-security-00.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract The IPv6 interim mechanism 6to4 [6TO4] uses automatic IPv6-over-IPv4 tunneling to interconnect IPv6 networks. The architecture includes Relay Routers and Routers, which accept and decapsulate IPv4 traffic from anywhere. There aren't many constraints on the embedded IPv6 packets, or where IPv4 traffic will be automatically tunneled to. These could enable one to go around access controls, and more likely, being able to perform proxy Denial of Service attacks using Relays as reflectors. This document discusses these issues and tries to suggest enhancements to alleviate the problems. Savola [Expires March 2002] [Page 1] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 Table of Contents 1. Introduction ............................................... 3 2. Different 6to4 Forwarding Scenarios ........................ 3 2.1. From 6to4 to 6to4 ...................................... 4 2.2. From Native to 6to4 .................................... 4 2.3. From 6to4 to Native .................................... 5 3. Some Functionalities of 6to4 ............................... 5 3.1. Functions of Different 6to4 Network Components ......... 5 3.2. Non-functions of Different 6to4 Network Components ..... 7 4. Special Processing of 6to4 Packets ......................... 7 4.1. Encapsulating IPv6 Packets into IPv4 ................... 7 4.2. Decapsulating IPv4 Packets into IPv6 ................... 8 5. Solutions .................................................. 8 5.1. Generic Approach ....................................... 8 5.1.1. Encapsulating IPv6 into IPv4 ....................... 8 5.1.2. Decapsulating IPv4 into IPv6 ....................... 8 5.1.3. IPv4 and IPv6 Sanity Checks ........................ 9 5.1.3.1. IPv4 ........................................... 9 5.1.3.2. IPv6 ........................................... 9 5.1.3.3. Optional Ingress Filtering ..................... 10 5.1.3.4. Notes About the Checks ......................... 10 5.2. Simplified Approach .................................... 10 5.2.1. Encapsulating IPv6 into IPv4 ....................... 10 5.2.2. Decapsulating IPv4 into IPv6 ....................... 11 5.3. All Relays Must Be Anycast ............................. 11 6. Problems ................................................... 11 6.1. Implementation Considerations with Automatic Tunnels ... 11 6.2. Reduced Flexibility .................................... 12 7. Security Considerations .................................... 13 8. Acknowledgements ........................................... 13 9. References ................................................. 13 Author's Address ............................................... 13 A. Some Attack Scenarios Outlined ............................. 14 A.1. IPv6 Spoofing and Access Control Avoidance ............. 14 A.2. IPv4 Local Directed Broadcast Attacks .................. 15 A.3. DoS Reflector .......................................... 15 Savola [Expires March 2002] [Page 2] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 1. Introduction The IPv6 interim mechanism "6to4" [6TO4] specifies automatic IPv6-over-IPv4 tunneling to interconnect isolated IPv6 clouds without explicit tunnels by embedding the tunnel IPv4 address in the IPv6 6to4 prefix. One challenge with this mechanism is that all 6to4 routers must accept and decapsulate IPv4 packets from every other 6to4 router; there are no strict constraints on what the IPv6 packet may contain, which implies a trust relationship. Another, bigger challenge is that to interconnect native IPv6 networks and 6to4 clouds, relay routers are used as bridges between these two clouds. Relay routers can be tricked by malicious parties to send IPv4, or IPv6, traffic anywhere the attacker wants. With source address spoofing, this could be called traffic "laundering" or a "proxy" denial-of-service attack. The 6to4 specification outlined a few security considerations, but it has been shown that in practice some of these have been difficult to get implemented due to their abstract nature. This draft analyses the 6to4 security issues in more detail and outlines some enhancements and caveats. Sections 2-4 are more or less introductory in nature, rehashing how 6to4 should be used today based on the 6to4 specification, so that it is easier to understand how security could be affected. Appendix A outlines a few possible attack scenarios. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Different 6to4 Forwarding Scenarios It should be noted that when communicating between 6to4 and native domains, the relays that will be used in the two directions are very likely different; routing is highly asymmetric. Because of this, it is not feasible to limit relays you accept traffic from with e.g. access lists. Savola [Expires March 2002] [Page 3] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 2.1. From 6to4 to 6to4 6to4 domains always exchange 6to4 traffic directly via IPv4 tunneling; the endpoint address V4ADDR is derived from 6to4 prefix 2002:V4ADDR. .--------. _----_ .--------. | 6to4 | _( IPv4 )_ | 6to4 | | Router | <====> ( Internet ) <===> | Router | '--------' (_ _) '--------' ^ '----' ^ | Direct tunneling over IPv4 | V V .--------. .-------. | 6to4 | | 6to4 | | Client | | Client | '--------' '--------' It is required that every 6to4 router considers every other 6to4 router it wants to talk to to be "on-link" (with IPv4 as the link- layer). If this is restricted by increasing the prefix length from 2002::/16, some traffic will be sent to the 6to4 Relay Router, which would forward it to other 6to4 Routers. However, if the original destination does not have equally long prefix, the traffic it tries to send back will be tunneled directly, and will be dropped. Therefore, the restricted scenario is not globally workable and will not be considered here. 2.2. From Native to 6to4 Native domains send traffic to 6to4 address 2002:V4ADDR, and it will be routed to the topologically nearest, advertising 6to4 Relay Router. Relay router will tunnel the traffic over IPv4 to the corresponding IPv4 address V4ADDR. (Note that IPv4 address 9.0.0.1 here is just an example of a global IPv4 address.) Savola [Expires March 2002] [Page 4] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 2002::/16 Closest to 'Native Client' .--------. _----_ .------------. .--------. | Native | _( IPv6 )_ | 6to4 Relay | Tunneled | 6to4 | | Client | -> ( Internet ) --> | Router | =========> | Router | '--------' (_ _) '------------' 9.0.0.1 '--------' '----' dst=2002:0900:0000::1 | V .-------. | 6to4 | | Client | '--------' 2.3. From 6to4 to Native 6to4 domains send traffic to native domains by tunneling it over IPv4 to their configured 6to4 Relay Router, or the closest found using 6to4 IPv4 Anycast [6TO4ANY]. The relay will decapsulate the packet and forward it to native IPv6 Internet using normal routing table and mechanisms. Configured/found by IPv4 Anycast .--------. _----_ .------------. .--------. | Native | _( IPv6 )_ | 6to4 Relay | Tunneled | 6to4 | | Client | <- ( Internet ) <-- | Router | <========= | Router | '--------' (_ _) '------------' 192.88.99.1'--------' '----' (or configured) ^ dst=3ffe:ffff::1 | .-------. | 6to4 | | Client | '--------' 3. Some Functionalities of 6to4 3.1. Functions of Different 6to4 Network Components o Non-6to4 (Native) Node If native IPv6 nodes want to communicate with 6to4 nodes, they send the traffic along normally. The traffic will reach the topologically closest, advertising 6to4 Relay Router, and will be tunneled to the destination 6to4 Router, which will pass it to the 6to4 node via normal forwarding process. Savola [Expires March 2002] [Page 5] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 o 6to4 Host A host, usually autoconfigured and has an address from a 6to4 prefix, but doesn't have a 6to4 pseudo-interface. It doesn't need to know anything about 6to4, and it acts like a normal IPv6 Host in every manner. o 6to4 Router Acts on the border of a 6to4 domain. It does not have a native, global IPv6 address. More specifically: - provide "native-like" IPv6 connectivity to local clients and routers - if packets are sent to foreign 6to4 addresses, tunnel them to the destination 6to4 router using IPv4 - if packets are sent to locally configured 6to4 addresses, forward them normally - if packets are sent to non-6to4 addresses, tunnel them to the configured/closest-by-anycast 6to4 Relay Router, which will pass them on - if packets are received from 6to4 addresses, decapsulate the IPv4 packets received from 6to4 routers - if packets are received from non-6to4 addresses, decapsulate the IPv4 packets received from 6to4 Relay Router closest to the source. o 6to4 Relay Router Acts as a relay between all 6to4 domains and native IPv6; more specifically: - advertises the reachability of the 2002::/16 prefix to native IPv6 routing, thus receiving traffic to all 6to4 addresses from closest native IPv6 nodes - (if implements RFC3068) advertise the reachability of IPv4 '6to4 Relay anycast prefix' (192.88.99.0/24) to IPv4 routing, thus receiving some tunneled traffic to native IPv6 nodes from 6to4 Routers - if packets are received from 6to4 addresses through tunneling, decapsulate them and forwards them on using normal IPv6 routing - if packets are received through normal IPv6 routing from native addresses, and are destined for 2002::/16, tunnel them to the corresponding 6to4 Router Savola [Expires March 2002] [Page 6] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 3.2. Non-functions of Different 6to4 Network Components What should not happen; this forms a basis for the security checks. The lists are not exhaustive. o 6to4 Router or Relay - use private, broadcast or reserved IPv4 addresses in tunnels, or the matching 6to4 prefixes - receive traffic from 6to4 Routers where the IPv4 tunnel source address does not match the 6to4 prefix o 6to4 Router - send traffic to other 6to4 domains through 6to4 Relay Router or via some other 6to4 Router - receive traffic from other 6to4 domains via a 6to4 Relay Router - receive traffic to other than to your own 6to4 prefix(es) o 6to4 Relay Router - receive traffic from 6to4 to 6to4 4. Special Processing of 6to4 Packets One could summarize the special processing of 6to4 as follows: o Relay Router 1. incoming from native, tunneled to 6to4 2. tunneled from 6to4, going to native o Router 1. tunneled from relay, source is native 2. tunneled to relay, destination is native 3. tunneled directly, destination is 6to4 4.1. Encapsulating IPv6 Packets into IPv4 IPv6 packets are encapsulated into IPv4 in three scenarios: 1. 6to4 Router sends packets to other 6to4 Routers (2002::/16 destination) 2. 6to4 Router sends packets to its configured/nearest-by-anycast 6to4 Relay Router (non-2002::/16 destination) 3. 6to4 Relay Router sends packets from native IPv6 sources to 6to4 Routers (2002::/16 destination) Savola [Expires March 2002] [Page 7] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 4.2. Decapsulating IPv4 Packets into IPv6 IPv6 packets are decapsulated from IPv4 in three scenarios: 1. 6to4 Router receives packets from other 6to4 Routers (2002::/16 source) 2. 6to4 Router receives packets from its 6to4 Relay Router closest to the source (non-2002::/16 source) 3. 6to4 Relay Router receives packets from 6to4 Routers, to be sent to native IPv6 destinations (2002::/16 source) 5. Solutions 5.1. Generic Approach 5.1.1. Encapsulating IPv6 into IPv4 src and dst SHOULD pass ipv6-sanity checks, else drop (defined below) if src=2002 src MUST match src_v4 [ the scenario: 4.1. case 1. or 2. ] if dst=2002 dst_v4 SHOULD NOT be assigned to the router (avoid misconfigurations) [ the scenario: 4.1. case 1. ] fi elif dst=2002 src_v4 MAY have to match one of ipv4 equiv. of 6to4 prefixes masked by a user-specified prefix length (restricting who can use the relay) [ the scenario: 4.1. case 3. ] else drop [ the scenario: we somehow got a native-native ipv6 packet ] fi accept 5.1.2. Decapsulating IPv4 into IPv6 src_v4 and dst_v4 SHOULD pass ipv4-sanity checks, else drop (defined below) src and dst SHOULD pass ipv6-sanity checks, else drop (defined below) if dst=2002 dst MUST match dst_v4 src_v4 may be restricted to be 6to4_ipv4_anycast [not useful for a long time yet] [ the scenario: 4.2. case 1. or 2. ] if src=2002 src MUST match src_v4 dst_v4 SHOULD be assigned to the router (see notes below) [ the scenario: 4.2. case 1. ] fi Savola [Expires March 2002] [Page 8] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 elif src=2002 src MUST match src_v4 dst_v4 SHOULD be assigned to the router (see notes below) src_v4 MAY have to match one of ipv4 equiv. of 6to4 prefixes masked by a user-specified prefix length (restricting who can use the relay) [ the scenario: 4.2. case 3. ] else drop [ the scenario: we somehow got a native-native ipv6 packet ] fi accept 5.1.3. IPv4 and IPv6 Sanity Checks 5.1.3.1. IPv4 IPv4 address MUST be a global unicast address, as required by the 6to4 specification. The disallowed addresses include those defined in [RFC1812], and others widely used and known not to be global. These are: o 0.0.0.0/8 (the system has no address assigned yet) o 10.0.0.0/8 (private) o 127.0.0.0/8 (loopback) o 172.16.0.0/12 (private) o 192.168.0.0/16 (private) o 169.254.0.0/16 (IANA Assigned DHCP link-local) o 224.0.0.0/4 (multicast) o 255.0.0.0/8 (broadcast) In addition it it SHOULD be checked that the address is not any of the system's broadcast addresses. This is especially important if the implementation is made so that it can receive and process encapsulated IPv4 packets arriving at its broadcast addresses, or try to send encapsulated IPv4 packets to one of its broadcast addresses. 5.1.3.2. IPv6 IPv6 address MUST not be: o 0::/16 (compatible, mapped addresses, loopback, unspecified, ...) o fe80::/10 (link-local) o fec0::/10 (site-local) o ff02::/16 (link-local multicast) Other multicast could also be considered for filtering. In addition, it MUST be checked that equivalent 2002:V4ADDR checks, where V4ADDR is any of the above IPv4 addresses, will not be passed. Savola [Expires March 2002] [Page 9] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 5.1.3.3. Optional Ingress Filtering In addition, the implementation may perform some form of ingress filtering (e.g. Unicast Reverse Path Forwarding checks). For example, if a Router has multiple interfaces, of which some are "internal", receiving either IPv4 or IPv6 packets with source address belonging to any of these internal networks from the Internet might be disallowed. If these checks are implemented, it is RECOMMENDED that they default to disabled. 5.1.3.4. Notes About the Checks The rule 'dst_v4 SHOULD be assigned to the router' is not needed if the implementation is made in such a way that it only accepts and processes encapsulated IPv4 packets arriving on unicast IPv4 addresses, and that if destination address is known to be a local broadcast address, not try to encapsulate and send packets to it. Some checks, especially the IPv4/IPv6 Sanity Checks, could be at least partially implementable with system-level access lists, if one would like to avoid placing too many restrictions in the 6to4 implementation itself. This depends on how many hooks for ACL's are in place. In practice it seems like this could not be done effectively enough unless the access list mechanism is able to parse the encapsulated packets within IP-IP. 5.2. Simplified Approach This makes some assumptions about the implementation as pointed above to simplify the above rules. 5.2.1. Encapsulating IPv6 into IPv4 src and dst SHOULD pass ipv6-sanity checks, else drop if src=2002 src MUST match src_v4 elif dst=2002 (accept) else drop fi accept Savola [Expires March 2002] [Page 10] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 5.2.2. Decapsulating IPv4 into IPv6 src_v4 and dst_v4 SHOULD pass ipv4-sanity checks, else drop src and dst SHOULD pass ipv6-sanity checks, else drop if dst=2002 dst MUST match dst_v4 if src=2002 src MUST match src_v4 fi elif src=2002 src MUST match src_v4 else drop fi accept 5.3. All Relays Must Be Anycast In this model, all Relay Routers must always use 6to4 IPv4 Anycast address as the source address. In order for this to be implementable, every single 6to4 Relay Router must use only anycast. This may be too significant a deployment restriction. The checks would be: if src_v4 is 6to4 ipv4 anycast address accept [coming from relay] elif src_v4 is non-global ipv4 unicast address drop elif src = 2002 src MUST match src_v4 [coming from router] accept else drop fi 6. Problems 6.1. Implementation Considerations with Automatic Tunnels There is a problem with multiple transition mechanisms if security is implemented. This may vary a bit from implementation to implementation. Consider three mechanisms using automatic tunneling: 6to4, ISATAP [ISATAP] and Automatic Tunneling using Compatible Addresses [MECH]. Savola [Expires March 2002] [Page 11] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 All of these use IP-IP (protocol 41) [IPIP] IPv4 encapsulation with, more or less, a pseudo-interface. When a router, which has any two of these enabled, receives an IPv4 encapsulated IPv6 packet: src_v4 = 10.0.0.1 dst_v4 = 20.20.20.20 src = 3ffe:ffff::1 dst = 2002:1010:1010::2 what can it do? How should it decide which transitionary mechanism this belongs to; there is no "transitionary mechanism number" in IPv6 or IPv4 header to signify this. Without any kind of security checks (in any of implemented methods) these often just "work" as the mechanisms aren't differentiated but handled in "one big lump". Configured tunneling [MECH] does not suffer from this as it is point- to-point, and based on src/dst pairs of both IPv4 and IPv6 addresses it can be deduced which logical tunnel interface is in the question. Solutions for this include not using more than one automatic tunneling mechanisms in the same system or binding different mechanisms to different IPv4 addresses. 6.2. Reduced Flexibility There is a worry about too strict rules limiting the (future) flexibility of 6to4. If later, for some reason, one would want to introduce new revolutionary ways to use 6to4, strict checking in all relevant nodes might prevent it, as new updated version would have to be deployed everywhere before the new method could be used. On the other hand, one could argue that 6to4 has always been intended as an intermediate mechanism, and that future flexibility should not be critical. However, it is difficult to predict how long the intermediate period will be. Savola [Expires March 2002] [Page 12] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 7. Security Considerations This draft discusses security considerations. 8. Acknowledgements Alexey Kuznetsov brought up the implementation problem with IPv6 martian checks. Christian Huitema formulated the rules that rely on Relays using only anycast. Keith Moore brought up the point about reduced flexibility. 9. References [6TO4] Carpenter, B. and Moore K., "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [6TO4ANY] Huitema, C. "An Anycast Prefix for 6to4 Relay Routers", RFC 3068, June 2001. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [MECH] Gilligan, R., and Nordmark, E. "Transition Mechanisms for IPv6 Hosts and Routers", RFC 2893, August 2000. [IPIP] Simpson, W. "IP in IP Tunneling", RFC 1853, October 1995. [ISATAP] Templin, F. "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", draft-ietf-ngtrans-isatap-01.txt (work in progress). Author's Address Pekka Savola CSC/FUNET Espoo, Finland EMail: psavola@funet.fi Savola [Expires March 2002] [Page 13] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 A. Some Attack Scenarios Outlined The following addresses are used in these examples. IPv4 addresses are indeed just examples of global IPv4 addresses. __________________ __________________ | Native IPv6 N1 | | Native IPv6 N2 | | 3ffe:ffff:1::/48 | | 3ffe:ffff:2::/48 | '------------------' '------------------' _______________________ _______________________ | 6to4 Relay Router RR1 | | 6to4 Relay Router RR2 | | 3ffe:ffff:10::1/48 | | 3ffe:ffff:11:1::1/48 | | 2002:0900:0001::1/48 | | 2002:0900:0002::1/48 | | (IPv4) 9.0.0.1 | | (IPv4) 9.0.0.2 | '-----------------------' '-----------------------' ______________________ ______________________ | 6to4 Router RA | | 6to4 Router RB | | 2002:0800:0001::1/48 | | 2002:0800:0002::1/48 | | (IPv4) 8.0.0.1 | | (IPv4) 8.0.0.2 | '----------------------' '----------------------' When two 6to4 Routers send traffic to each others' domains, packet sent by RA to RB is like: src = 2002:0800:0001::aaaa dst = 2002:0800:0002::bbbb src_v4 = 8.0.0.1 (added when encapsulated to IPv4) dst_v4 = 8.0.0.2 (added when encapsulated to IPv4) When the packet is received by IPv4 stack on RB, it will be decapsulated so that only src and dst remain, as originally sent by RA: src = 2002:0800:0001::aaaa dst = 2002:0800:0002::bbbb As every other node is just "one hop away" and the "link-layer" addresses are lost, this may open a lot of possibilities for misuse. Some possible approaches are outlined below. A.1. IPv6 Spoofing and Access Control Avoidance Unidirectional IPv6 spoofing is made trivial because nobody can check (without delving into IP-IP packets) whether the encapsulated IPv6 addresses were authentic (With native IPv6, this can be done by e.g. RPF-like mechanisms or access lists in upstream routers). Savola [Expires March 2002] [Page 14] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 src = 2002:1234:5678::aaaa (forged) dst = 2002:0800:0002::bbbb src_v4 = 8.0.0.1 (added when encapsulated to IPv4) dst_v4 = 8.0.0.2 (added when encapsulated to IPv4) More worries come in to the picture if e.g. src = ::ffff:[some trusted IPv4 in a private network] src/dst = ::ffff:127.0.0.1 src/dst = ::1 src/dst = ... Some implementations might have been careful enough to have designed the stack to as to avoid these, but who can say for all ... A.2. IPv4 Local Directed Broadcast Attacks Relays could be targeted with local broadcast attacks: src = 3ffe:ffff:5678::aaaa (forged) dst = 2002:0900:00ff::bbbb Now, if the Relay doesn't check the destination address for broadcast, it would send the encapsulated IP-IP packet to 9.0.0.255 which might be a local broadcast address (packets sent to every IPv4 node in the subnet). Classic directed broadcast checks will not prevent this as the packet doesn't come from outside in IPv4 sense). A.3. DoS Reflector Denial-of-Service attacks could be reflected against either IPv4 nodes (not that useful, only thing that could be sent is IP-IP packets) or IPv6 nodes. 6to4 Relay Router could be abused as follows: src = X dst = Y src_v4 = 8.0.0.1 (added when encapsulated to IPv4) dst_v4 = 9.0.0.2 (added when encapsulated to IPv4) a) X=2002:1234:5678::1 or X=3ffe:1122:3344::1 (forged) Y=2002:0500:0001::1 or Y=::0500:0001 (the victim) Target IPv4 address 5.0.0.1 would be bombed with reflected packets from 6to4 Relay Router RR2. Source address in IPv6 header reveals nothing, or can be used to throw blame on someone else. Savola [Expires March 2002] [Page 15] Internet Draft draft-savola-ngtrans-6to4-security-00.txt September 2001 b) X=2002:1234:5678::1 or X=3ffe:1122:3344::1 (forged) Y=3ffe:aabb:ccdd::1 (the victim) As above, the target could be native IPv6 address too, not necessarily an IPv4 node. Improper 6to4 Router could be abused as follows: src = X dst = Y src_v4 = 8.0.0.1 (added when encapsulated to IPv4) dst_v4 = 8.0.0.2 (added when encapsulated to IPv4) Both issues from above may also apply, if not implemented properly. For example: a) X=2002:1234:5678::1 or X=3ffe:1122:3344::1 (forged) Y=3ffe:aabb:ccdd::1 (the victim) This might allow one to tunnel to the 6to4 Router RB, and place packets, with forged source addresses (be they 6to4 or not) to the router, and if it accepted them to be routed natively (ie. the forwarding process doesn't check that it shouldn't send them out the same way they came), be tunneled to Relay Router, and from there to Internet. Savola [Expires March 2002] [Page 16]