IPv6 Operations E. Davies Internet-Draft Consultant Expires: January
6,19, 2006 S. Krishnan Ericsson P. Savola CSC/Funet July 5,18, 2005 IPv6 Transition/Co-existence Security Considerations draft-ietf-v6ops-security-overview-01.txtdraft-ietf-v6ops-security-overview-02.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 6,19, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract The transition from a pure IPv4 network to a network where IPv4 and IPv6 co-exist brings a number of extra security considerations that need to be taken into account when deploying IPv6 and operating the dual-protocol network and the associated transition mechanisms. This document attempts to give an overview of the various issues grouped into three categories: o issues due to the IPv6 protocol itself, o issues due to transition mechanisms, and o issues due to IPv6 deployment. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Issues Due to IPv6 Protocol . . . . . . . . . . . . . . . . 4 2.1 IPv6 Protocol-specific Issues . . . . . . . . . . . . . . 4 2.1.1 Routing Headers and Hosts . . . . . . . . . . . . . . 4 2.1.2 Routing Headers for Mobile IPv6 and Other Purposes . . 5 2.1.3 Site-scope Multicast Addresses . . . . . . . . . . . . 5 2.1.4 ICMPv6 and Multicast . . . . . . . . . . . . . . . . . 6 2.1.5 Anycast Traffic Identification and Security . . . . . 7 2.1.6 Address Privacy Extensions Interact with DDoS Defenses . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.7 Dynamic DNS: Stateless Address Auto-Configuration, Privacy Extensions and SEND . . . . . . . . . . . . . 8 2.1.8 Extension Headers . . . . . . . . . . . . . . . . . . 8 2.1.9 Fragmentation: Reassembly and Deep Packet Inspection . 10 2.1.10 Fragmentation Related DoS Attacks . . . . . . . . . 11 2.1.11 Link-Local Addresses and Securing Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . 12 2.1.12 Mobile IPv6 . . . . . . . . . . . . . . . . . . . . 13 2.2 IPv4-mapped IPv6 Addresses . . . . . . . . . . . . . . . . 14 2.3 Increased End-to-End Transparency . . . . . . . . . . . . 15 2.3.1 IPv6 Networks without NATs . . . . . . . . . . . . . . 15 2.3.2 Enterprise Network Security Model for IPv6 . . . . . . 15 3. Issues Due to Transition Mechanisms . . . . . . . . . . . . 17 3.1 IPv6 Transition/Co-existence Mechanism-specific Issues . . 17 3.2 Automatic Tunneling and Relays . . . . . . . . . . . . . . 17 3.3 Tunneling MayIPv6 Through IPv4 Networks may Break IPv4 Network Security Assumptions . . . . . . . . . . . . . . . 18 4. Issues Due to IPv6 Deployment . . . . . . . . . . . . . . . 19 4.1 IPv6 Service Piloting Done Insecurely . . . . . . . . . . 19 4.2 Enabling IPv6 by Default Reduces UsabilityDNS Server Problems . . . . . . . . . . . . . . . 20. . . . 21 4.3 Addressing Schemes and Securing Routers . . . . . . . . . 21 4.4 Consequences of Multiple Addresses in IPv6 . . . . . . . . 21 4.5 Deploying ICMPv6 . . . . . . . . . . . . . . . . . . . . . 22 4.5.1 Problems Resulting from ICMPv6 Transparency . . . . . 2322 4.6 IPsec Transport Mode . . . . . . . . . . . . . . . . . . . 23 4.7 Reduced Functionality Devices . . . . . . . . . . . . . . 23 4.8 Operational Factors when Enabling IPv6 in the Network . . 2423 4.9 Ingress Filtering Issues Due to Privacy Addresses . . . . 2524 4.10 Security Issues Due to ND Proxies . . . . . . . . . . . 25 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25 6. Security Considerations . . . . . . . . . . . . . . . . . . 25 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 2625 8.1 Normative References . . . . . . . . . . . . . . . . . . . 2625 8.2 Informative References . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 30 A. IPv6 Probing/Mapping Considerations . . . . . . . . . . . . 30 B. IPv6 Privacy Considerations . . . . . . . . . . . . . . . . 31 B.1 Exposing MAC Addresses . . . . . . . . . . . . . . . . . . 3231 B.2 Exposing Multiple Devices . . . . . . . . . . . . . . . . 32 B.3 Exposing the Site by a Stable Prefix . . . . . . . . . . . 32 Intellectual Property and Copyright Statements . . . . . . . 3433 1. Introduction The transition from a pure IPv4 network to a network where IPv4 and IPv6 co-exist brings a number of extra security considerations that need to be taken into account when deploying IPv6 and operating the dual-protocol network with its associated transition mechanisms. This document attempts to give an overview of the various issues grouped into three categories: o issues due to the IPv6 protocol itself, o issues due to transition mechanisms, and o issues due to IPv6 deployment. It is important to understand that we have to be concerned not about replacing IPv4 with IPv6 (in the short term), but with adding IPv6 to be operated in parallel with IPv4 [I-D.savola-v6ops-transarch]. This document also describes two matters which have been wrongly identified as potential security concerns for IPv6 in the past and explains why they are unlikely to cause problems: considerations about probing/mapping IPv6 addresses (Appendix A), and considerations with respect to privacy in IPv6 (Appendix B). 2. Issues Due to IPv6 Protocol 2.1 IPv6 Protocol-specific Issues There are significant differences between the features of IPv6 and IPv4: some of these specification changes may result in potential security issues. Several of these issues have been discussed in separate drafts but are summarized here to avoid normative references which may not become RFCs. The following specification-related problems have been identified, but this is not necessarily a complete list: 2.1.1 Routing Headers and Hosts All IPv6 nodes must be able to process Routing Headers [RFC2460]. This RFC can be interpreted, although it is not clearly stated, to mean that all nodes (including hosts) must have this processing enabled. This can result in hosts forwarding received traffic if there are segments left in the Routing Header when it arrives at the host. A number of potential security issues associated with this behavior were documented in [I-D.savola-ipv6-rh-hosts]. Some of these issues have been resolved (a separate routing header type is now used for Mobile IPv6 [RFC3775] and ICMP Traceback has not been standardized), but two issues remain: o Routing headers can be used to evade access controls based on destination addresses. This could be achieved by sending a packet ostensibly to a publicly accessible host address but with a routing header containing a 'forbidden' address. If the publicly accessible host is processing routing headers it will forward the packet to the destination address in the routing header which would have been forbidden by the packet filters if the address had been in the destination field when the packet was checked. o If the packet source address in the previous case can be spoofed, any host could be used to mediate an anonymous reflection denial- of-service attack by having any publicly accessible host redirect the attack packets. 2.1.2 Routing Headers for Mobile IPv6 and Other Purposes In addition to the basic Routing Header (Type 0), which is intended to influence the trajectory of a packet through a network by specifying a sequence of router 'waypoints', Routing Header (Type 2) has been defined as part of the Mobile IPv6 specifications in [RFC3775]. The Type 2 Routing Header is intended for use by hosts to handle 'interface local' forwarding needed when packets are sent to the care-of address of a mobile node which is away from its home address. It is important that nodes treat the different types of routing header appropriately. It should be possible to apply separate filtering rules to the different types of Routing Header. By design, hosts must process Type 2 Routing Headers to support Mobile IPv6 but routers should not: to avoid the issues in Section 2.1.1 it may be desirable to forbid or limit the processing of Type 0 Routing Headers in hosts and some routers. Routing Headers are an extremely powerful and general capability. Alternative future uses of Routing Headers need to be carefully assessed to ensure that they do not open new avenues of attack that can be exploited. 2.1.3 Site-scope Multicast Addresses IPv6 supports multicast addresses with site scope which can potentially allow an attacker to identify certain important resources on the site if misused. Particular examples are the 'all routers' (FF05::2) and 'all DHCP servers' (FF05::1:3) addresses defined in [RFC2375]: an attacker that is able to infiltrate a message destined for these addresses on to the site will potentially receive in return information identifying key resources on the site. This information can then be the target of directed attacks ranging from simple flooding to more specific mechanisms designed to subvert the device. Some of these addresses have current legitimate uses within a site. The risk can be minimized by ensuring that all firewalls and site boundary routers are configured to drop packets with site scope destination addresses. Also nodes should not join multicast groups for which there is no legitimate use on the site and site routers should be configured to drop packets directed to these unused addresses. 2.1.4 ICMPv6 and Multicast It is possible to launch a denial-of-service (DoS) attack using IPv6 which could be amplified by the multicast infrastructure. Unlike ICMP for IPv4, ICMPv6 [RFC2463] allows error notification responses to be sent when certain unprocessable packets are sent to multicast addresses. The cases in which responses are sent are: o The received packet is longer than the next link MTU: 'Packet Too Big' responses are needed to support Path MTU Discovery for multicast traffic. o The received packet contains an unrecognized option in a hop-by- hop or destination options extension header with the first two bits of the option type set to binary '10': 'Parameter Problem' responses are intended to inform the source that some or all of the recipients cannot handle the option in question. If an attacker can craft a suitable packet sent to a multicast destination, it may be possible to elicit multiple responses directed at the victim (the spoofed source of the multicast packet). On the other hand, the use of 'reverse path forwarding' checks to eliminate loops in multicast forwarding automatically limits the range of addresses which can be spoofed. In practice an attack using oversize packets is unlikely to cause much amplification unless the attacker is able to carefully tune the packet size to exploit a network with smaller MTU in the edge than the core. Similarly a packet with an hop-by-hop option would be dropped by the first router. However a packet with an destination option could generate multiple responses. In addition to amplification, this kind of attack would potentially consume large amounts of forwarding state resources in routers on multicast-enabled networks. These attacks are discussed in more detail in [I-D.savola-v6ops-firewalling]. 2.1.5 Anycast Traffic Identification and Security IPv6 introduces the notion of anycast addresses and services. Originally the IPv6 standards disallowed using an anycast address as the source address of a packet. Responses from an anycast server would therefore supply a unicast address for the responding server. To avoid exposing knowledge about the internal structure of the network, it is recommended that anycast servers now take advantage of the ability to return responses with the anycast address as the source address if possible. If the server needs to use a unicast address for any reason, it may be desirable to consider using specialized addresses for anycast servers which are not used for any other part of the network to restrict the information exposed. Alternatively operators may wish to restrict the use of anycast services from outside the domain, thus requiring firewalls to filter anycast requests. For this purpose, firewalls need to know which addresses are being used for anycast services: these addresses are arbitrary and not distinguishable from any other IPv6 unicast address by structure or pattern. One particular class of anycast addresses that should be given special attention is the set of Subnet-Router anycast addresses defined in The IPv6 Addressing Architecture [RFC3513]. All routers are required to support these addresses for all subnets for which they have interfaces. For most subnets using global unicast addresses, filtering anycast requests to these addresses can be achieved by dropping packets with the lower 64 bits (the Interface Identifier) set to all zeroes. 2.1.6 Address Privacy Extensions Interact with DDoS Defenses The purpose of the privacy extensions for stateless address auto- configuration [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] is to change the interface identifier (and hence the global scope addresses generated from it) from time to time. By varying the addresses used, eavesdroppers and other information collectors find it more difficult to identify which transactions actually relate to a specific node. A security issue may result from this if the frequency of node address change is sufficiently great to achieve the intended aim of the privacy extensions: with a relatively high rate of change, the observed behavior of the node could look very like that of a compromised node which was the source of a distributed denial of service (DDoS). It would thus be difficult to for any future defenses against DDoS attacks to distinguish between a high rate of change of addresses resulting from genuine use of the privacy extensions and a compromised node being used as the source of a DDoS with 'in-prefix' spoofed source addresses as described in [I-D.dupont-ipv6-rfc3041harmful]. Even if a node is well behaved, the change in the address could make it harder for a security administrator to define a policy rule (e.g. access control list) that takes into account a specific node. 2.1.7 Dynamic DNS: Stateless Address Auto-Configuration, Privacy Extensions and SEND The introduction of Stateless Address Auto-Configuration (SLAAC) [RFC2462] with IPv6 provides an additional challenge to the security of Dynamic DNS (DDNS). With manual addressing or the use of DHCP, the number of security associations that need to be maintained to secure access to the DNS server is limited, assuming any necessary updates are carried out by the DHCP server. This is true equally for IPv4 and IPv6. Since SLAAC does not make use of a single and potentially trusted DHCP server, but depends on the node obtaining the address, securing the insertion of updates into DDNS may need a security association between each node and the DDNS server. This is discussed further in [I-D.ietf-dnsop-ipv6-dns-issues]. Using the Privacy Extensions to SLAAC [RFC3041][I-D.ietf-ipv6- privacy-addrs-v2] may significantly increase the rate of updates of DDNS. Even if a node using the Privacy Extensions does not publish its address for 'forward' lookup (as that would effectively compromise the privacy which it is seeking), it may still need to update the reverse DNS records so that reverse routability checks can be carried out. If the rate of change needed to achieve real privacy has to be increased as is mentioned in Section 2.1.6 the update rate for DDNS may be excessive. Similarly, the cryptographically generated addresses used by SEND [RFC3971] are expected to be periodically regenerated in line with recommendations for maximum key lifetimes. This regeneration could also impose a significant extra load on DDNS. 2.1.8 Extension Headers A number of issues relating to the specification of IPv6 Extension headers have been identified. Several of these are discussed in [I-D.savola-v6ops-firewalling]. 22.214.171.124 Processing Extension Headers in Middleboxes In IPv4 deep packet inspection techniques are used to implement policing and filtering both as part of routers and in middleboxes such as firewalls. Fully extending these techniques to IPv6 would require inspection of all the extension headers in a packet. This is essential to ensure that policy constraints on the use of certain headers and options are enforced and to remove, at the earliest opportunity, packets containing potentially damaging unknown options. This requirement appears to conflict with Section 4 of the IPv6 specification in [RFC2460] which requires that destination options are not processed at all until the packet reaches the appropriate destination (either the final destination or a routing header waypoint). Also [RFC2460] forbids processing the headers other than in the order in which they appear in the packet. A further ambiguity relates to whether an intermediate node should discard a packet which contains a header or destination option which it does not recognize. If the rules above are followed slavishly, it is not (or may not be) legitimate for the intermediate node to discard the packet because it should not be processing those headers or options. [RFC2460] therefore does not appear to take account of the behavior of middleboxes and other non-final destinations which may be inspecting the packet, and thereby potentially limits the security protection of these boxes. 126.96.36.199 Processing Extension Header Chains There is a further problem for middleboxes that want to examine the transport headers which are located at the end of the IPv6 header chain. In order to locate the transport header or other protocol data unit, the node has to parse the header chain. The IPv6 specification [RFC2460] does not mandate the use of the Type-Length-Value format with a fixed layout for the start of each header although it is used for the majority of headers currently defined. (Only the Type field is guaranteed in size and offset). A middlebox cannot therefore guarantee to be able to process header chains which may contain headers defined after the box was manufactured. As noted in Section 188.8.131.52, middleboxes ought not to have to know about all header types in use but still need to be able to skip over such headers to find the transport PDU start. This either limits the security which can be applied in firewalls or makes it difficult to deploy new extension header types. At the time of writing, only the Fragment Header does not fully conform to the TLV format used for other extension headers. In practice, many firewalls reconstruct fragmented packets before performing deep packet inspection, so this divergence is less problematic than it might have been, and is at least partially justified because the full header chain is not present in all fragments. Destination Options may also contain unknown options. However, the options are encoded in TLV format so that intermediate nodes can skip over them during processing, unlike the enclosing extension headers. 184.108.40.206 Unknown Headers/Destination Options and Security Policy A strict security policy might dictate that packets containing either unknown headers or destination options are discarded by firewalls or other filters. This requires the firewall to process the whole extension header chain which may be currently in conflict with the IPv6 specification as discussed in Section 220.127.116.11. Even if the firewall does inspect the whole header chain, it may not be sensible to discard packets with items by the firewall: the intermediate node has no knowledge of which options and headers are implemented in the destination node. Hence it is highly desirable to make the discard policy configurable. This will avoid firewalls dropping packets with legitimate items that they do not recognize because their hardware or software is not aware of a new definition. 18.104.22.168 Excessive Hop-by-Hop Options IPv6 does not limit the number of hop by hop options which can be present in a hop-by-hop option header. The lack of a limit can be used to mount denial of service attacks affecting all nodes on a path as described in [I-D.krishnan-ipv6-hopbyhop]. 22.214.171.124 Overuse of Router Alert Option The IPv6 router alert option specifies a hop-by-hop option that, if present, signals the router to take a closer look at the packet. This can be used for denial of service attacks. By sending a large number of packets containing a router alert option an attacker can deplete the processor cycles on the routers available to legitimate traffic. 2.1.9 Fragmentation: Reassembly and Deep Packet Inspection The current specifications of IPv6 in [RFC2460] do not mandate any minimum packet size for the fragments of a packet before the last one, except for the need to carry the unfragmentable part in all fragments. The unfragmentable part does not include the transport port numbers so that it is possible that the first fragment does not contain sufficient information to carry out deep packet inspection involving the port numbers. Also the reassembly rules for fragmented packets in [RFC2460] do not mandate behavior which would minimize the effects of overlapping fragments. Depending on the implementation of packet reassembly and the treatment of packet fragments in firewalls and other nodes which use deep packet inspection for traffic filtering, this potentially leaves IPv6 open to the sort of attacks described in [RFC1858] and [RFC3128] for IPv4. There is no reason to allow overlapping packet fragments and overlaps could be prohibited in a future revision of the protocol specification. Some implementations already drop all packets with overlapped fragments. Specifying a minimum size for packet fragments does not help in the same way as it does for IPv4 because IPv6 extension headers can be made to appear very long: an attacker could insert one or more undefined destination options with long lengths and the 'ignore if unknown' bit set. Given the guaranteed minimum MTU of IPv6 it seems reasonable that hosts should be able to ensure that the transport port numbers are in the first fragment in almost all cases and that deep packet inspection should be very suspicious of first fragments that do not contain them. 2.1.10 Fragmentation Related DoS Attacks Packet reassembly in IPv6 hosts also opens up the possibility of various fragment-related security attacks. Some of these are analogous to attacks identified for IPv4. Of particular concern is a DoS attack based on sending large numbers of small fragments without a terminating last fragment which would potentially overload the reconstruction buffers and consume large amounts of CPU resources. Mandating the size of packet fragments could reduce the impact of this kind of attack by limiting the rate at which fragments could arrive and limiting the number of fragments which need to be processed. 2.1.11 Link-Local Addresses and Securing Neighbor Discovery All IPv6 nodes are required to configure a link-local address on each interface. This address is used to communicate with other nodes directly connected to the link accessed via the interface, especially during the neighbor discovery and auto-configuration processes. Link-local addresses are fundamental to the operation of the Neighbor Discovery Protocol (NDP) [RFC2461] and SLAAC [RFC2462]. NDP also provides the functionality of associating link layer and IP addresses provided by the Address Resolution Protocol (ARP) in IPv4 networks. The standard version of NDP is subject to a number of security threats related to ARP spoofing attacks on IPv4. These threats have been documented in [RFC3756] and mechanisms to combat them specified in SEcure Neighbor Discovery (SEND) [RFC3971]. SEND is an optional mechanism which is particularly applicable to wireless and other environments where it is difficult to physically secure the link. Because the link-local address can, by default, be acquired without external intervention or control, it allows an attacker to commence communication on the link without needing to acquire information about the address prefixes in use or communicate with any authorities on the link. This feature gives a malicious node the opportunity to mount an attack on any other node which is attached to this link; this vulnerability exists in addition to possible direct attacks on NDP. Link-local addresses may also facilitate the unauthorized use of the link bandwidth ('bandwidth theft') to communicate with another unauthorized node on the same link. Link-local addresses allocated from the prefix 169.254.0.0/16 are available in IPv4 as well and procedures for using them are described in [I-D.ietf-zeroconf-ipv4-linklocal] but the security issues were not as pronounced as for IPv6 for the following reasons: o link-local addresses are not mandatory in IPv4 and are primarily intended for isolated or ad hoc networks that cannot acquire a routable IPv4 address by other means, o IPv4 addresses are not universally supported across operating systems, and o the IPv4 link-local address should be removed when a non-link- local address is configured on the interface and will generally not be allocated unless other means of acquiring an address are not available. These vulnerabilities can be mitigated in several ways. A general solution will require o authenticating the link layer connectivity, for example by using IEEE 802.1x functionality, port-based MAC address security (locking), or physical security, and o using SEcure Neighbor Discovery (SEND) to create a cryptographically generated link-local address as described in [RFC3971] which is tied to the authenticated link layer address. This solution would be particularly appropriate in wireless LAN deployments where it is difficult to physically secure the infrastructure In wired environments, where the physical infrastructure is reasonably secure, it may be sufficient to ignore communication requests originating from a link-local address for other than local network management purposes. This requires that nodes should only accept packets with link-local addresses for a limited set of protocols including NDP, MLD and other functions of ICMPv6. 126.96.36.199 Securing Router Advertisements As part of the Neighbor Discovery process, routers on a link advertise their capabilities in Router Advertisement messages. The version of NDP defined in [RFC2461] does not protect the integrity of these messages or validate the assertions made in the messages with the result that any node which connects to the link can maliciously claim to offer routing services which it will not fulfill, and advertise inappropriate prefixes and parameters. These threats have been documented in [RFC3756]. SEND [RFC3971] can be used to provide verification that routers are authorized to provide the services they advertise through a certificate-based mechanism. This capability of SEND is also particularly appropriate for wireless environments where clients are reliant on the assertions of the routers rather than a physically secured connection. 2.1.12 Mobile IPv6 Mobile IPv6 offers significantly enhanced security compared with Mobile IPv4 especially when using optimized routing and care-of addresses. Return routability checks are used to provide relatively robust assurance that the different addresses which a mobile node uses as it moves through the network do indeed all refer to the same node. The threats and solutions are described in [RFC3775] and a more extensive discussion of the security aspects of the design can be found in [I-D.ietf-mip6-ro-sec]. 188.8.131.52 Obsolete Home Address Option in Mobile IPv6 The Home Address option specified in early drafts of Mobile IPv6 would have allowed a trivial source spoofing attack: hosts were required to substitute the source address of incoming packets with the address in the option, thereby potentially evading checks on the packet source address. This is discussed at greater length in [I-D.savola-ipv6-rh-ha-security]. The version of Mobile IPv6 as standardized in [RFC3775] has removed this issue by ensuring that the Home Address destination option is only processed if there is a corresponding binding cache entry and securing Binding Update messages. A number of pre-standard implementations of Mobile IPv6 were available which implemented this obsolete and insecure option: care should be taken to avoid running such obsolete systems. 2.2 IPv4-mapped IPv6 Addresses Overloaded functionality is always a double-edged sword: it may yield some deployment benefits, but often also incurs the price which comes with ambiguity. One example of such is IPv4-mapped IPv6 addresses: a representation of an IPv4 address as an IPv6 address inside an operating system. Since the original specification, the use of IPv4-mapped addresses has been extended to a transition mechanism, Stateless IP/ICMP Translation algorithm (SIIT) [RFC2765], where they are potentially used in the addresses of packets on the wire. Therefore, it becomes difficult to unambiguously discern whether an IPv4 mapped address is really an IPv4 address represented in the IPv6 address format *or* an IPv6 address received from the wire (which may be subject to address forgery, etc.). In addition, special cases like these, while giving deployment benefits in some areas, require a considerable amount of code complexity (e.g. in the implementations of bind() system calls and reverse DNS lookups) which is probably undesirable. Some of these issues are discussed in [I-D.cmetz-v6ops-v4mapped-api-harmful] and [I-D.itojun-v6ops-v4mapped-harmful]. In practice, although the packet translation mechanisms of SIIT are specified for use in the Network Address Translator - Protocol Translator (NAT-PT) [RFC2765], NAT-PT uses a mechanism different from IPv4-mapped IPv6 addresses for communicating embedded IPv4 addresses in IPv6 addresses. Also SIIT is not recommended for use as a standalone transition mechanism. Given the issues that have been identified, it seems appropriate that mapped addresses should not be used on the wire. However, changing application behavior by deprecating the use of mapped addresses in the operating system interface would have significant impact on application porting methods [RFC4038]and needs further study. 2.3 Increased End-to-End Transparency One of the major design aims of IPv6 has been to maintain the original IP architectural concept of end-to-end transparency. Transparency can help foster technological innovation in areas such as peer-to-peer communication but maintaining the security of the network at the same time requires some modifications in the network architecture. Ultimately, it is also likely to need changes in the security model as compared with the norms for IPv4 networks. 2.3.1 IPv6 Networks without NATs The necessity of introducing Network Address Translators (NATs) into IPv4 networks, resulting from a shortage of IPv4 addresses, has removed the end-to-end transparency of most IPv4 connections: the use of IPv6 would restore this transparency. However, the use of NATs, and the associated private addressing schemes, has become inappropriately linked to the provision of security in enterprise networks. The restored end-to-end transparency of IPv6 networks can therefore be seen as a threat by poorly informed enterprise network managers. Some seem to want to limit the end-to-end capabilities of IPv6, for example by deploying private, local addressing and translators, even when it is not necessary because of the abundance of IPv6 addresses. Recommendations for designing an IPv6 network to meet the perceived security and connectivity requirements implicit in the current usage of IPv4 NATs whilst maintaining the advantages of IPv6 end-to-end transparency are described in IPv6 Network Architecture Protection [I-D.ietf-v6ops-nap]. 2.3.2 Enterprise Network Security Model for IPv6 The favored model for enterprise network security in IPv4 stresses the use of a security perimeter policed by autonomous firewalls and incorporating the NATs. Both perimeter firewalls and NATs introduce asymmetry and reduce the transparency of communications through these perimeters. The symmetric bidirectionality and transparency which are extolled as virtues of IPv6 may seem to be at odds with this model. Consequently network managers may even see them as undesirable attributes, in conflict with their need to control threats to and attacks on the networks they administer. It is worth noting that IPv6 does not *require* end-to-end connectivity. It merely provides end-to-end addressability; the connectivity can still be controlled using firewalls (or other mechanisms), and it is indeed wise to do so. A number of matters indicate that IPv6 networks should migrate towards an improved security model, which will increase the overall security of the network but facilitate end-to-end communication: o Increased usage of end-to-end security especially at the network layer. IPv6 mandates the provision of IPsec capability in all nodes and increasing usage of end-to-end security is a challenge to current autonomous firewalls that are unable to perform deep packet inspection on encrypted packets. It is also incompatible with NATs because they modify the packets, even when packets are only authenticated rather than encrypted. o Acknowledgement that over-reliance on the perimeter model is potentially dangerous. An attacker who can penetrate today's perimeters will have free rein within the perimeter, in many cases. Also a successful attack will generally allow the attacker to capture information or resources and make use of them. o Development of mechanisms such as 'Trusted Computing' which will increase the level of trust which network managers are able to place on hosts. o Development of centralized security policy repositories and secure distribution mechanisms which, in conjunction with trusted hosts, will allow network managers to place more reliance on security mechanisms at the end points. The mechanisms are likely to include end-node firewalling and intrusion detection systems as well as secure protocols that allow end points to influence the behavior of perimeter security devices. o Review of the role of perimeter devices with increased emphasis on intrusion detection, network resource protection and coordination to thwart distributed denial of service attacks. Several of the technologies required to support an enhanced security model are still under development, including secure protocols to allow end points to control firewalls: the complete security model utilizing these technologies is now emerging but still requires some development. In the meantime, initial deployments will need to make use of similar firewalling and intrusion detection techniques to IPv4 which may limit end-to-end transparency temporarily, but should be prepared to use the new security model as it develops and avoid the use of NATs by the use of the architectural techniques described in [I-D.ietf- v6ops-nap]. In particular, using NAT-PT [RFC2766] as a general purpose transition mechanism should be avoided as it is likely to limit the exploitation of end-to-end security and other IPv6 capabilities in future as explained in [I-D.ietf-v6ops-natpt-to- exprmntl]. 3. Issues Due to Transition Mechanisms 3.1 IPv6 Transition/Co-existence Mechanism-specific Issues The more complicated the IPv6 transition/co-existence becomes, the greater the danger that security issues will be introduced either o in the mechanisms themselves, o in the interaction between mechanisms, or o by introducing unsecured paths through multiple mechanisms. These issues may or may not be readily apparent. Hence it would be desirable to keep the mechanisms simple, as few in number as possible and built from as small pieces as possible to simplify analysis. One case where such security issues have been analyzed in detail is the 6to4 tunneling mechanism [RFC3964]. As tunneling has been proposed as a model for several more cases than are currently being used, its security properties should be analyzed in more detail. There are some generic dangers to tunneling: o it may be easier to avoid ingress filtering checks o it is possible to attack the tunnel interface: several IPv6 security mechanisms depend on checking that Hop Limit equals 255 on receipt and that link-local addresses are used. Sending such packets to the tunnel interface is much easier than gaining access to a physical segment and sending them there. o automatic tunneling mechanisms are typically particularly dangerous as there is no pre-configured association between end points. Accordingly, at the receiving end of the tunnel packets have to be accepted and decapsulated from any source. Consequently, special care should be taken when specifying automatic tunneling techniques. 3.2 Automatic Tunneling and Relays Two mechanisms have been (or are being) specified which use automatic tunneling and are intended for use outside a single domain. These mechanisms encapsulate the IPv6 packet directly in an IPv4 packet in the case of 6to4 [RFC3056] or in an IPv4 UDP packet in the case of Teredo [I-D.huitema-v6ops-teredo]. In each case packets can be sent and received by any similarly equipped nodes in the IPv4 Internet. As mentioned in Section 3.1, a major vulnerability in such approaches is that receiving nodes must allow decapsulation of traffic sourced from anywhere in the Internet. This kind of decapsulation function must be extremely well secured because of the wide range of potential sources. An even more difficult problem is how these mechanisms are able to establish communication with native IPv6 nodes or between the automatic tunneling mechanisms: such connectivity requires the use of some kind of "relay". These relays could be deployed in various locations such as: o all native IPv6 nodes, o native IPv6 sites, o in IPv6-enabled ISPs, or o just somewhere in the Internet. Given that a relay needs to trust all the sources (e.g., in the 6to4 case, all 6to4 routers) which are sending it traffic, there are issues in achieving this trust and at the same time scaling the relay system to avoid overloading a small number of relays. As authentication of such a relay service is very difficult to achieve, and particularly so in some of the possible deployment models, relays provide a potential vehicle for address spoofing, (reflected) Denial-of-Service attacks, and other threats. Threats related to 6to4 and measures to combat them are discussed in [RFC3964]. [I-D.huitema-v6ops-teredo] incorporates extensive discussion of the threats to Teredo and measures to combat them. 3.3 Tunneling IPv6 Through IPv4 Networks May Break IPv4 Network Security Assumptions NATs and firewalls have been deployed extensively in the IPv4 Internet, as discussed in Section 2.3. Operators who deploy them typically have some security/operational requirements in mind (e.g. a desire to block inbound connection attempts), which may or may not be misguided. The addition of tunneling can change the security model which such deployments are seeking to enforce. IPv6-over-IPv4 tunneling using protocol 41 is typically either explicitly allowed, or disallowed implicitly. Tunneling IPv6 over IPv4 encapsulated in UDP constitutes a more difficult problem as UDP must usually be allowed to pass through NATs and firewalls. Consequently, using UDP implies the ability to punch holes in NAT's and firewalls although, depending on the implementation, this ability may be limited or only achieved in a stateful manner. In practice, the mechanisms have been explicitly designed to traverse both NATs and firewalls in a similar fashion. One possible view is that use of tunneling is especially questionable in home/SOHO environments where the level of expertise in network administration is typically not very high; in these environments the hosts may not be as tightly managed as in others (e.g., network services might be enabled unnecessarily), leading to possible security break-ins or other vulnerabilities. Holes can be punched both intentionally and unintentionally. In cases where the administrator or user makes an explicit decision to create the hole, this is less of a problem, although (for example) some enterprises might want to block IPv6 tunneling explicitly if employees were able to create such holes without reference to administrators. On the other hand, if a hole is punched transparently, it is likely that a proportion of users will not understand the consequences: this will very probably result in a serious threat sooner or later. When deploying tunneling solutions, especially tunneling solutions which are automatic and/or can be enabled easily by users who do not understand the consequences, care should be taken not to compromise the security assumptions held by the users. For example, NAT traversal should not be performed by default unless there is a firewall producing a similar by-default security policy to that provided by IPv4 NAT. IPv6-in-IPv4 (protocol 41) tunneling is less of a problem, as it is easier to block if necessary; however, if the host is protected in IPv4, the IPv6 side should be protected as well. As has been shown in Appendix A, it is relatively easy to determine the IPv6 address corresponding to an IPv4 address in tunneling deployments. It is therefore vital NOT to rely on "security by obscurity" i.e., assuming that nobody is able to guess or determine the IPv6 address of the host especially when using automatic tunneling transition mechanisms. 4. Issues Due to IPv6 Deployment 4.1 IPv6 Service Piloting Done Insecurely In many cases, IPv6 service piloting is done in a manner which is less secure than can be achieved for an IPv4 production service. For example, hosts and routers might not be protected by IPv6 firewalls, even if the corresponding IPv4 service is fully protected by firewalls.firewalls as described in [I-D.ietf-v6ops-v6onbydefault]. This is particularly critical where IPv6 capabilities are turned on by default in new equipment or new releases of operating systems: network managers may not be fully aware of the security exposure that this creates. The other possible alternative, in some instances, is that no service piloting is permitted because IPv6 firewalls and other security capabilities, such as intrusion detection systems may not be widely available. Consequently, IPv6 deployment suffers and expertise accumulates less rapidly. These problems may be partly due to the relatively slow development and deployment of IPv6-capable firewall equipment, but there is also a lack of information: actually, there are quite a few IPv6 packet filters and firewalls already in existence, which could be used for provide sufficient access controls, but network administrators may not be aware of them yet and there is a lack of documented operational practice. However, there appears to be a real lack in the area of 'personal firewalls'. Also enterprise firewalls are at an early stage of development and may not provide all the capabilities needed to implement the necessary IPv6 filtering rules. The same devices that support and are used for IPv4 today are often expected to also become IPv6-capable -- even though this is not really required and the equipment may not have the requisite hardware capabilities to support fast packet filtering for IPv6. That is, IPv4 access could be filtered by one firewall, and when IPv6 access is added, it could be protected by another firewall; they don't have to be the same box, and even their models don't have to be the same. A lesser factor may be that some design decisions in the IPv6 protocol make it more difficult for firewalls to be implemented and work in all cases and to be fully future proof (e.g. when new extension headers are used) as discussed in Section 2.1.8: it is significantly more difficult for intermediate nodes to process the IPv6 header chains than IPv4 packets. A similar argument, which is often quoted as hindering IPv6 deployment, has been the lack of Intrusion Detection Systems (IDS). It is not clear whether this is more of an excuse than a real reason. 4.2 Enabling IPv6 by Default Reduces Usability A practical disadvantage of enabling IPv6 at the time of writingAn additional problem is that it typically reducesthe observed service level by a small fraction; that is,limited implementation of high availability capabilities supporting IPv6. In particular, development of the usability suffers. There are at least three factors contributing to this effect: o globalIPv6 routing is still rather unstable, leading to packet loss, lower throughput, and higher delay (this was discussed with respect toversion of the experimental 6Bone network in [I-D.savola-v6ops- 6bone-mess]) o some applications cannot properly handle both IPv4 and IPv6 or may have problems handling allVirtual Router Redundancy Protocol (VRRP) [I-D.ietf-vrrp-ipv6-spec] has lagged the fallbacks and failure modes (and in some cases, e.g. ifdevelopment of the TCP timeout kicks in, thismain IPv6 protocol although alternatives may result in very poor performance) o some DNS server implementations have flaws that severely affect DNS queriesbe available for IPv6 addresses as discussed in [I-D.ietf-dnsop- misbehavior-against-aaaa]some environments. Actually, some providers are fully ready to offer IPv6 services (e.g. web) today, but because that would (or, at least, might) result in problems for many of their customers or users who are, by default, using active dual-stack systems the services are not turned on: as a compromise, the services are often published under a separate domain or subdomain, and are, in practice, not much used as a consequence. These issues are also described at some length4.2 DNS Server Problems Some DNS server implementations have flaws that severely affect DNS queries for IPv6 addresses as discussed in [I-D.ietf-v6ops- v6onbydefault]. An additional problem is the limited implementation of high availability capabilities supporting IPv6. In particular, development of the[RFC4074]. These flaws can be used for DoS attacks affecting both IPv4 and IPv6 version of the Virtual Router Redundancy Protocol (VRRP) [I-D.ietf-vrrp-ipv6-spec] has laggedby inducing caching DNS servers to believe that a domain is broken and causing the development ofserver to block access to all requests for the main IPv6 protocol although alternatives may be availabledomain for some environments.a precautionary period. 4.3 Addressing Schemes and Securing Routers Whilst in general terms brute force scanning of IPv6 subnets is essentially impossible due to the enormously larger address space of IPv6 and the 64 bit interface identifiers (see Appendix A), this will be obviated if administrators do not take advantage of the large space to use unguessable interface identifiers. Because the unmemorability of complete IPv6 addresses there is a temptation for administrators to use small integers as interface identifiers when manually configuring them, as might happen on point- to-point links. Such allocations make it easy for an attacker to find active nodes that they can then port scan. To make use of the larger address space properly, administrators should be very careful when entering IPv6 addresses in their configurations (e.g. Access Control List), since numerical IPv6 addresses are more prone to human error than IPv4 due to their length and unmemorability. It is also essential to ensure that the management interfaces of routers are well secured as the router will usually contain a significant cache of neighbor addresses in its neighbor cache. 4.4 Consequences of Multiple Addresses in IPv6 One positive consequence of IPv6 is that nodes which do not require global access can communicate locally just by the use of a link-local address (if very local access is sufficient) or across the site by using a Unique Local Address (ULA). In either case it is easy to ensure that access outside the assigned domain of activity can be controlled by simple filters (which may be the default for link- locals). However, the security hazards of using link-local addresses for non-management purposes as documented in Section 2.1.11 should be borne in mind. On the other hand, the possibility that a node or interface can have multiple global scope addresses makes access control filtering both on ingress and egress more complex and requires higher maintenance levels. The addresses could be from the same network prefix (for example, privacy mechanisms [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] will periodically create new addresses taken from the same prefix and two or more of these may be active at the same time), or from different prefixes (for example, when a network is multihomed or is implementing anycast services). In either case, it is possible that a single host could be using several different addresses with different prefixes. It would be desirable that the Security Administrator should be able to identify that the same host is behind all these addresses. 4.5 Deploying ICMPv6 In IPv4 it is commonly accepted that some filtering of ICMP packets by firewalls is essential to maintain security. Because of the extended use that is made of ICMPv6 [RFC2461] with a multitude of functions, the simple set of dropping rules that are usually applied in IPv4 need to be significantly developed for IPv6. The blanket dropping of all ICMP messages that is used in some very strict environments is simply not possible for IPv6. In an IPv6 firewall, policy needs to allow some messages through the firewall but also has to permit certain messages to and from the firewall, especially those with link-local sources on links to which the firewall is attached. These messages must be permitted to ensure that Neighbor Discovery [RFC2462], Multicast Listener Discovery [RFC2710], [RFC3810] and Stateless Address Configuration [RFC2463] work as expected. Recommendations for filtering ICMPv6 messages can be found in [I-D.davies-v6ops-icmpv6-filtering-bcp]. 4.5.1 Problems Resulting from ICMPv6 Transparency As described in Section 4.5, certain ICMPv6 error packets need to be passed through a firewall in both directions. This means that some ICMPv6 error packets can be exchanged between inside and outside without any filtering. Using this feature, malicious users can communicate between the inside and outside of a firewall bypassing the administrator's inspection (proxy, firewall etc.). For example in might be possible to carry out a covert conversation through the payload of ICMPv6 error messages or tunnel inappropriate encapsulated IP packets in ICMPv6 error messages. This problem can be alleviated by filtering ICMPv6 errors using a stateful packet inspection mechanism to ensure that the packet carried as a payload is associated with legitimate traffic to or from the protected network. 4.6 IPsec Transport Mode IPsec provides security to end-to-end communications at the network layer (layer 3). The security features available include access control, connectionless integrity, data origin authentication, protection against replay attacks, confidentiality, and limited traffic flow confidentiality (see [RFC2401] section 2.1). IPv6 mandates the implementation of IPsec in all conforming nodes, making the usage of IPsec to secure end-to-end communication possible in a way which is generally not available to IPv4. To secure IPv6 end-to-end communications, IPsec transport mode would generally be the solution of choice. However, use of these IPsec security features can result in novel problems for network administrators and decrease the effectiveness of perimeter firewalls because of the increased prevalence of encrypted packets on which the firewalls cannot perform deep packet inspection and filtering. One example of such problems is the lack of security solutions in the middlebox, including effective content-filtering, ability to provide DoS prevention based on the expected TCP protocol behavior, and intrusion detection. Future solutions to this problem are discussed in Section 2.3.2. Another example is an IPsec-based DoS (e.g., sending malformed ESP/AH packets) which can be especially detrimental to software-based IPsec implementations. 4.7 Reduced Functionality Devices With the deployment of IPv6 we can expect the attachment of a very large number of new IPv6-enabled devices with scarce resources and low computing capacity. The resource limitations are generally because of a market requirement for cost reduction. Some such devices may not be able even to perform the minimum set of functions required to protect themselves (e.g. 'personal' firewall, automatic firmware update, enough CPU power to endure DoS attacks). This means a different security scheme may be necessary for such embedded devices. 4.8 Operational Factors when Enabling IPv6 in the Network There are a number of reasons which make it essential to take particular care when enabling IPv6 in the network equipment: Initially, IPv6-enabled router software may be less stable than current IPv4-only implementations and there is less experience with configuring IPv6 routing, which can result in disruptions to the IPv6 routing environment and (IPv6) network outages. IPv6 processing may not happen at (near) line speed (or at a comparable performance level to IPv4 in the same equipment). A high level of IPv6 traffic (even legitimate, e.g. Network News Transport Protocol, NNTP) could easily overload IPv6 processing especially when it is software-based without the hardware support typical in high-end routers. This may potentially have deleterious knock-on effects on IPv4 processing, affecting availability of both services. Accordingly, if people don't feel confident enough in the IPv6 capabilities of their equipment, they will be reluctant to enable it in their "production" networks. Sometimes essential features may be missing from early releases of vendors' software; an example is provision of software enabling IPv6 telnet/SSH access (e.g., to the configuration application of a router), but without the ability to turn it off or limit access to it! Sometimes the default IPv6 configuration is insecure. For example, in one vendor's implementation, if you have restricted IPv4 telnet to only a few hosts in the configuration, you need to be aware that IPv6 telnet will be automatically enabled, that the configuration commands used previously do not block IPv6 telnet, IPv6 telnet is open to the world by default, and that you have to use a separate command to also lock down the IPv6 telnet access. Many operator networks have to run interior routing protocols for both IPv4 and IPv6. It is possible to run the both in one routing protocol, or have two separate routing protocols; either approach has its tradeoffs [RFC4029]. If multiple routing protocols are used, one should note that this causes double the amount of processing when links flap or recalculation is otherwise needed -- which might more easily overload the router's CPU, causing slightly slower convergence time. 4.9 Ingress Filtering Issues Due to Privacy Addresses [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] describes a method for creating temporary addresses on IPv6 nodes to address privacy issues created by the use of a constant identifier. In a network, which implements such a mechanism, with a large number of nodes, new temporary addresses may be created at a fairly high rate. This might make it hard for ingress filtering mechanisms to distinguish between legitimately changing temporary addresses and spoofed source addresses, which are "in-prefix" (They use a topologically correct prefix and non-existent interface ID). This can be addressed by using finer grained access control mechanisms on the network egress point. 4.10 Security Issues Due to ND Proxies In order to span a single subnet over multiple physical links, a new capability is being introduced in IPv6 to proxy Neighbor Discovery messages. This node will be called an NDProxy (see [I-D.ietf-ipv6- ndproxy]. NDProxies are susceptible to the same security issues as the ones faced by hosts using unsecured Neighbor Discovery or ARP. These proxies may process unsecured messages, and update the neighbor cache as a result of such processing, thus allowing a malicious node to divert or hijack traffic. This may undermine the advantages of using SEND [RFC3971]. To resolve the security issues introduced by NDProxies, SEND needs to be extended to be NDProxy aware. 5. IANA Considerations This memo does not contain any actions for IANA. 6. Security Considerations This memo attempts to give an overview of security considerations of the different aspects of IPv6.IPv6, particularly as they relate to the transition to a network in which IPv4- and IPv6-based communications need to coexist. 7. Acknowledgements Alain Durand, Alain Baudot, Luc Beloeil, Andras Kis-Szabo, Alvaro Vives, Janos Mohacsi and Mark Smith provided feedback to improve this memo. SUSZUKISatoshi Kondo, Shinsuke Suzuki and Alvaro Vives provided a number ofadditional issuesinputs in cooperation with the Deployment Working Group of the Japanese IPv6 Promotion Council.Council and the Euro6IX IST co-funded project, together with inputs from Jordi Palet, Brian Carpenter, and Peter Bieringer. Michael Wittsend and Michael Cole discussed issues relating to probing/mapping and privacy. 8. References 8.1 Normative References [I-D.huitema-v6ops-teredo] Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs", draft-huitema-v6ops-teredo-05 (work in progress), April 2005. [I-D.ietf-ipv6-privacy-addrs-v2] Narten, T., "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", draft-ietf-ipv6-privacy-addrs-v2-04 (work in progress), May 2005. [I-D.ietf-v6ops-natpt-to-exprmntl] Aoun, C. and E. Davies, "Reasons to Move NAT-PT to Experimental", draft-ietf-v6ops-natpt-to-exprmntl-00draft-ietf-v6ops-natpt-to-exprmntl-01 (work in progress), FebruaryJuly 2005. [I-D.ietf-vrrp-ipv6-spec] Hinden, R., "Virtual Router Redundancy Protocol for IPv6", draft-ietf-vrrp-ipv6-spec-07 (work in progress), October 2004. [RFC2375] Hinden, R. and S. Deering, "IPv6 Multicast Address Assignments", RFC 2375, July 1998. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [RFC2463] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998. [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999. [RFC3041] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001. [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, April 2003. [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. [RFC3964] Savola, P. and C. Patel, "Security Considerations for 6to4", RFC 3964, December 2004. 8.2 Informative References [FNAT] Bellovin, S., "Technique for Counting NATted Hosts", Proc. Second Internet Measurement Workshop , November 2002, <http://www.research.att.com/~smb/papers/fnat.pdf>. [I-D.chown-v6ops-port-scanning-implications] Chown, T., "IPv6 Implications for TCP/UDP Port Scanning", draft-chown-v6ops-port-scanning-implications-01 (work in progress), July 2004. [I-D.cmetz-v6ops-v4mapped-api-harmful] Metz, C. and J. Hagino, "IPv4-Mapped Address API Considered Harmful", draft-cmetz-v6ops-v4mapped-api-harmful-01 (work in progress), October 2003. [I-D.davies-v6ops-icmpv6-filtering-bcp] Davies, E.,E. and J. Mohacsi, "Best Current Practice for Filtering ICMPv6 Messages in Firewalls", draft-davies-v6ops-icmpv6-filtering-bcp-00 (work in progress), July 2005. [I-D.dupont-ipv6-rfc3041harmful] Dupont, F. and P. Savola, "RFC 3041 Considered Harmful", draft-dupont-ipv6-rfc3041harmful-05 (work in progress), June 2004. [I-D.ietf-dnsop-ipv6-dns-issues] Durand, A., Ihren, J., and P. Savola, "Operational Considerations and Issues with IPv6 DNS", draft-ietf-dnsop-ipv6-dns-issues-10 (work in progress), October 2004. [I-D.ietf-dnsop-misbehavior-against-aaaa] Morishita, Y. and T. Jinmei, "Common Misbehavior against DNS Queries for IPv6 Addresses", draft-ietf-dnsop-misbehavior-against-aaaa-02 (work in progress), October 2004.[I-D.ietf-ipv6-ndproxy] Thaler, D., "Bridge-like Neighbor"Neighbor Discovery Proxies (ND Proxy)", draft-ietf-ipv6-ndproxy-02draft-ietf-ipv6-ndproxy-03 (work in progress), MayJuly 2005. [I-D.ietf-mip6-ro-sec] Nikander, P., "Mobile IP version 6 Route Optimization Security Design Background", draft-ietf-mip6-ro-sec-03 (work in progress), May 2005. [I-D.ietf-v6ops-nap] Velde, G., "IPv6 Network Architecture Protection", draft-ietf-v6ops-nap-01 (work in progress), June 2005. [I-D.ietf-v6ops-v6onbydefault] Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03 (work in progress), July 2004. [I-D.ietf-zeroconf-ipv4-linklocal] Aboba, B., "Dynamic Configuration of Link-Local IPv4 Addresses", draft-ietf-zeroconf-ipv4-linklocal-17 (work in progress), July 2004. [I-D.itojun-v6ops-v4mapped-harmful] Metz, C. and J. Hagino, "IPv4-Mapped Addresses on the Wire Considered Harmful", draft-itojun-v6ops-v4mapped-harmful-02 (work in progress), October 2003. [I-D.krishnan-ipv6-hopbyhop] Krishnan, S., "Arrangement of Hop-by-Hop options", draft-krishnan-ipv6-hopbyhop-00 (work in progress), June 2004. [I-D.savola-ipv6-rh-ha-security] Savola, P., "Security of IPv6 Routing Header and Home Address Options", draft-savola-ipv6-rh-ha-security-02 (work in progress), March 2002. [I-D.savola-ipv6-rh-hosts] Savola, P., "Note about Routing Header Processing on IPv6 Hosts", draft-savola-ipv6-rh-hosts-00 (work in progress), February 2002. [I-D.savola-v6ops-6bone-mess] Savola, P., "Moving from 6bone to IPv6 Internet", draft-savola-v6ops-6bone-mess-01 (work in progress), November 2002.[I-D.savola-v6ops-firewalling] Savola, P., "Firewalling Considerations for IPv6", draft-savola-v6ops-firewalling-02 (work in progress), October 2003. [I-D.savola-v6ops-transarch] Savola, P., "A View on IPv6 Transition Architecture", draft-savola-v6ops-transarch-03 (work in progress), January 2004. [I-D.schild-v6ops-guide-v4mapping] Schild, C., "Guide to Mapping IPv4 to IPv6 Subnets", draft-schild-v6ops-guide-v4mapping-00 (work in progress), January 2004. [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security Considerations for IP Fragment Filtering", RFC 1858, October 1995. [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)", RFC 2765, February 2000. [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [RFC3128] Miller, I., "Protection Against a Variant of the Tiny Fragment Attack (RFC 1858)", RFC 3128, June 2001. [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC4029] Lind, M., Ksinant, V., Park, S., Baudot, A., and P. Savola, "Scenarios and Analysis for Introducing IPv6 into ISP Networks", RFC 4029, March 2005. [RFC4038] Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E. Castro, "Application Aspects of IPv6 Transition", RFC 4038, March 2005. [RFC4074] Morishita, Y. and T. Jinmei, "Common Misbehavior Against DNS Queries for IPv6 Addresses", RFC 4074, May 2005. Authors' Addresses Elwyn B. Davies Consultant Soham, Cambs UK Phone: +44 7889 488 335 Email: email@example.com Suresh Krishnan Ericsson 8400 Decarie Blvd. Town of Mount Royal, QC H4P 2N2 Canada Phone: +1 514-345-7900 Email: firstname.lastname@example.org Pekka Savola CSC/Funet Email: email@example.com Appendix A. IPv6 Probing/Mapping Considerations One school of thought wants the IPv6 numbering topology (either at network or node level) [I-D.schild-v6ops-guide-v4mapping] to match IPv4 as exactly as possible, whereas others see IPv6 as giving more flexibility to the address plans, not wanting to constrain the design of IPv6 addressing. Mirroring the address plans may also be seen as a security threat because an IPv6 deployment may have different security properties from IPv4. Given the relatively immature state of IPv6 network security, if an attacker knows the IPv4 address of the node and believes it to be dual-stacked with IPv4 and IPv6, he might want to try to probe the corresponding IPv6 address, based on the assumption that the security defenses might be lower. This might be the case particularly for nodes which are behind a NAT in IPv4, but globally addressable in IPv6. Naturally, this is not a concern if similar and adequate security policies are in place. On the other hand, brute-force scanning or probing of addresses is computationally infeasible due to the large search space of interface identifiers on most IPv6 subnets (somewhat less than 64 bits wide, depending on how identifiers are chosen), always provided that identifiers are chosen at random out of the available space, as discussed in [I-D.chown-v6ops-port-scanning-implications]. For example, automatic tunneling mechanisms typically use deterministic methods for generating IPv6 addresses, so probing/ port-scanning an IPv6 node is simplified. The IPv4 address is embedded at least in 6to4, Teredo and ISATAP addresses. Additionally, it is possible (in the case of 6to4 in particular) to learn the address behind the prefix; for example, Microsoft 6to4 implementation uses the address 2002:V4ADDR::V4ADDR while older Linux and FreeBSD implementations default to 2002:V4ADDR::1. This could also be used as one way to identify an implementation and hence target any specific weaknesses. One proposal has been to randomize the addresses or subnet identifier in the address of the 6to4 router. This does not really help, as the 6to4 router (whether a host or a router) will return an ICMPv6 Hop Limit Exceeded message, revealing the IP address. Hosts behind the 6to4 router can use methods such as RFC 3041 addresses to conceal themselves, though. To conclude, it seems that when an automatic tunneling mechanism is being used, given an IPv4 address, the corresponding IPv6 address could possibly be guessed with relative ease. This has significant implications if the IPv6 security policy is less adequate than that for IPv4. Appendix B. IPv6 Privacy Considerations The generation of IPv6 addresses of IPv6 addresses from MAC addresses potentially allows the behavior of users to be tracked in a way which may infringe their privacy. [RFC3041] specifies mechanisms which can be used to reduce the risk of infringement. It has also been claimed that IPv6 harms the privacy of the user, either by exposing the MAC address, or by exposing the number of nodes connected to a site. B.1 Exposing MAC Addresses Using stateless address autoconfiguration results in the MAC address being incorporated in an EUI64 that exposes the model of network card. The concern has been that a user might not want to expose the details of the system to outsiders, e.g., fearing a resulting burglary if a thief identifies expensive equipment from the vendor identifier embedded in MAC addresses. In most cases, this seems completely unfounded. First, such an address must be learned somehow -- this is a non-trivial process; the addresses are visible e.g., in web site access logs, but the chances that a random web site owner is collecting this kind of information (or whether it would be of any use) are quite slim. Being able to eavesdrop the traffic to learn such addresses (e.g., by the compromise of DSL or Cable modem physical media) seems also quite far-fetched. Further, using RFC 3041 addresses for such purposes is straightforward if worried about the risk. Second, the burglar would have to be able to map the IP address to the physical location; typically this would only be possible with information from the private customer database of the ISP and, for large sites, the administrative records of the site. B.2 Exposing Multiple Devices Another concern that has been aired involves the user wanting to conceal the presence of a large number of computers or other devices connected to a network; NAT can "hide" all this equipment behind a single address, but is not perfect either [FNAT]. One practical reason why some administrators may find this desirable is being able to thwart certain ISPs' business models. These models require payment based on the number of connected computers, rather than the connectivity as a whole. Similar feasibility issues as described above apply. To a degree, the number of machines present could be obscured by the sufficiently frequent re-use of RFC 3041 addresses -- that is, if during a short period, dozens of generated addresses seem to be in use, it's difficult to estimate whether they are generated by just one host or multiple hosts. B.3 Exposing the Site by a Stable Prefix When an ISP provides IPv6 connectivity to its customers, it delegates a fixed global routing prefix (usually a /48) to them. Due to this fixed allocation, it is easier to correlate the global routing prefix to a network site. In case of consumer users, this correlation leads to a privacy issue, since a site is often equivalent to an individual or a family in such a case. That is, some users might be concerned about being able to be tracked based on their /48 allocation if it is static [I-D.dupont-ipv6- rfc3041harmful]. This problem remains unsolved even when a user changes his/her interface ID or subnet ID, because malicious users can still discover this binding. This problem can be solved by untraceable IPv6 addresses as described in [I-D.ietf-v6ops-nap]. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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