Network Reconnaissance in IPv6 Networks
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This is an older version of an Internet-Draft that was ultimately published as RFC 7707.
|Authors||Fernando Gont , Tim Chown|
|RFC stream||Internet Engineering Task Force (IETF)|
SECDIR Last Call review (of -07) Has Nits
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opsec F. Gont Internet-Draft Huawei Technologies Obsoletes: 5157 (if approved) T. Chown Intended status: Informational University of Southampton Expires: July 27, 2014 January 23, 2014 Network Reconnaissance in IPv6 Networks draft-ietf-opsec-ipv6-host-scanning-03 Abstract IPv6 offers a much larger address space than that of its IPv4 counterpart. The standard /64 IPv6 subnets can (in theory) accommodate approximately 1.844 * 10^19 hosts, thus resulting in a much lower host density (#hosts/#addresses) than is typical in IPv4 networks, where a site typically has 65,000 or less unique addresses. As a result, it is widely assumed that it would take a tremendous effort to perform address scanning attacks against IPv6 networks, and therefore brute-force IPv6 address scanning attacks have been considered unfeasible. This document updates RFC 5157 by providing further analysis on how traditional address scanning techniques apply to IPv6 networks, and exploring some additional techniques that can be employed for IPv6 network reconnaissance. In doing so, this document formally obsoletes RFC 5157. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 27, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. Gont & Chown Expires July 27, 2014 [Page 1] Internet-Draft IPv6 Reconnaissance January 2014 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements for the Applicability of Network Reconnaissance Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. IPv6 Address Scanning . . . . . . . . . . . . . . . . . . . . 5 3.1. Address Configuration in IPv6 . . . . . . . . . . . . . . 5 3.1.1. StateLess Address Auto-Configuration (SLAAC) . . . . 6 3.1.2. Dynamic Host Configuration Protocol version 6 (DHCPv6) . . . . . . . . . . . . . . . . . . . . . . 9 3.1.3. Manually-configured Addresses . . . . . . . . . . . . 10 3.1.4. IPv6 Addresses Corresponding to Transition/Co- existence Technologies . . . . . . . . . . . . . . . 12 3.1.5. IPv6 Address Assignment in Real-world Network Scenarios . . . . . . . . . . . . . . . . . . . . . . 12 3.2. IPv6 Address Scanning of Remote Networks . . . . . . . . 15 3.3. IPv6 Address Scanning of Local Networks . . . . . . . . . 16 3.4. Existing IPv6 Address Scanning Tools . . . . . . . . . . 16 3.4.1. Remote IPv6 Network Scanners . . . . . . . . . . . . 16 3.4.2. Local IPv6 Network Scanners . . . . . . . . . . . . . 17 3.5. Mitigations . . . . . . . . . . . . . . . . . . . . . . . 17 4. Leveraging the Domain Name System (DNS) for Network Reconnaissance . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. DNS Advertised Hosts . . . . . . . . . . . . . . . . . . 18 4.2. DNS Zone Transfers . . . . . . . . . . . . . . . . . . . 19 4.3. DNS Brute Forcing . . . . . . . . . . . . . . . . . . . . 19 4.4. DNS Reverse Mappings . . . . . . . . . . . . . . . . . . 19 5. Leveraging Local Name Resolution and Service Discovery Services . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6. Public Archives . . . . . . . . . . . . . . . . . . . . . . . 20 7. Application Participation . . . . . . . . . . . . . . . . . . 20 8. Inspection of the IPv6 Neighbor Cache and Routing Table . . . 20 9. Inspection of System Configuration and Log Files . . . . . . 21 10. Gleaning Information from Routing Protocols . . . . . . . . . 21 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 12. Security Considerations . . . . . . . . . . . . . . . . . . . 21 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 Gont & Chown Expires July 27, 2014 [Page 2] Internet-Draft IPv6 Reconnaissance January 2014 14.1. Normative References . . . . . . . . . . . . . . . . . . 22 14.2. Informative References . . . . . . . . . . . . . . . . . 23 Appendix A. Implementation of a full-fledged IPv6 address- scanning tool . . . . . . . . . . . . . . . . . . . 25 A.1. Host-probing considerations . . . . . . . . . . . . . . . 25 A.2. Implementation of an IPv6 local address-scanning tool . . 27 A.3. Implementation of a IPv6 remote address-scanning tool . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 1. Introduction The main driver for IPv6 [RFC2460] deployment is its larger address space [CPNI-IPv6]. This larger address space not only allows for an increased number of connected devices, but also introduces a number of subtle changes in several aspects of the resulting networks. One of these changes is the reduced host density (the number of addresses divided by the number of hosts) of typical IPv6 subnetworks: with default IPv6 subnets of /64, each subnet comprises more than 1.844 * 10^19 available addresses; however, the actual number of nodes in each subnet is likely to remain similar to that of IPv4 subnetworks (typically a few hundred nodes per subnet). [RFC5157] describes how this significantly lower IPv6 host-density is likely to make classic network address scans less feasible, since even by applying various heuristics, the address space to be scanned remains very large. RFC 5157 goes on to describe some alternative methods for attackers to glean active IPv6 addresses, and provides some guidance for administrators and implementors, e.g. not using sequential addresses with DHCPv6. With the benefit of five years of additional IPv6 deployment experience, this document formally updates (and obsoletes RFC 5157). It emphasises that while scanning attacks are less feasible, they may, with appropriate heuristics, remain possible. At the time that RFC 5157 was written, observed scans were typically across ports on discovered servers; since then, evidence that some classic address scanning is being witnessed. This text thus updates the analysis on the feasibility of "traditional" address-scanning attacks in IPv6 networks, and it explores a number of additional techniques that can be employed for IPv6 network reconnaissance. Practical examples and guidance are also included. On one hand, raising awareness about IPv6 network reconnaissance techniques may allow (in some cases) network and security administrators to prevent or detect such attempts. On the other hand, network reconnaissance is essential for the so-called "penetration tests" typically performed to assess the security of production networks. As a result, we believe the benefits of a thorough discussion of IPv6 network reconnaissance are two-fold. Gont & Chown Expires July 27, 2014 [Page 3] Internet-Draft IPv6 Reconnaissance January 2014 Section 3 analyzes the feasibility of traditional address-scanning attacks (e.g. ping sweeps) in IPv6 networks, and explores a number of possible improvements to such techniques. [van-Dijk] describes a recently-disclosed technique for leveraging DNS reverse mappings for discovering IPv6 nodes. Finally, Appendix A describes how the analysis carried out throughout this document can be leveraged to produce address-scanning tools (e.g. for penetration testing purposes). 2. Requirements for the Applicability of Network Reconnaissance Techniques Throughout this document, a number of network reconnaissance techniques are discussed. Each of these techniques have different requirements on the side of the practitioner, with respect to whether they require local access to the target network, and whether they require login access to the system on which the technique is applied. The following table tries to summarize the aforementioned requirements, and serves as a cross index to the corresponding sections. Gont & Chown Expires July 27, 2014 [Page 4] Internet-Draft IPv6 Reconnaissance January 2014 +---------------------------------------------+----------+----------+ | Technique | Local | Login | | | access | access | +---------------------------------------------+----------+----------+ | Local address scans (Section 3.3) | Yes | No | +---------------------------------------------+----------+----------+ | Remote Address scans (Section 3.2) | No | No | +---------------------------------------------+----------+----------+ | DNS Advertised Hosts (Section 4.1) | No | No | +---------------------------------------------+----------+----------+ | DNS Zone Transfers (Section 4.2) | No | No | +---------------------------------------------+----------+----------+ | DNS reverse mappings (Section 4.4) | No | No | +---------------------------------------------+----------+----------+ | Public archives (Section 6) | No | No | +---------------------------------------------+----------+----------+ | Application Participation (Section 7) | No | No | +---------------------------------------------+----------+----------+ | Inspection of the IPv6 Neighbor Cache and | No | Yes | | Routing Table (Section 8) | | | +---------------------------------------------+----------+----------+ | Inspecting System Configuration and Log | No | Yes | | Files (Section 9) | | | +---------------------------------------------+----------+----------+ | Gleaning information from Routing Protocols | Yes | No | | (Section 10) | | | +---------------------------------------------+----------+----------+ Table 1: Requirements for the Applicability of Network Reconnaissance Techniques 3. IPv6 Address Scanning This section discusses how traditional address scanning techniques (e.g. "ping sweeps") apply to IPv6 networks. Section 3.1 provides an essential analysis of how address configuration is performed in IPv6, identifying patterns in IPv6 addresses that can be leveraged to reduce the IPv6 address search space when performing IPv6 address scans. Appendix A discusses how the insights obtained in the previous sub-sections can be incorporated into into a fully-fledged IPv6 address scanning tool. Section 3.5 provides advice on how to mitigate IPv6 address scans. 3.1. Address Configuration in IPv6 IPv6 incorporates two automatic address-configuration mechanisms: SLAAC (StateLess Address Auto-Configuration) [RFC4862] and DHCPv6 (Dynamic Host Configuration Protocol version 6) [RFC3315]. SLAAC is Gont & Chown Expires July 27, 2014 [Page 5] Internet-Draft IPv6 Reconnaissance January 2014 the mandatory mechanism for automatic address configuration, while DHCPv6 is optional - however, most current versions of general- purpose operating systems support both. In addition to automatic address configuration, hosts may employ manual configuration, in which all the necessary information is manually entered by the host or network administrator into configuration files at the host. The following subsections describe each of the possible configuration mechanisms/approaches in more detail. 3.1.1. StateLess Address Auto-Configuration (SLAAC) The basic idea behind SLAAC is that every host joining a network will send a multicasted solicitation requesting network configuration information, and local routers will respond to the request providing the necessary information. SLAAC employs two different ICMPv6 message types: ICMPv6 Router Solicitation and ICMPv6 Router Advertisement messages. Router Solicitation messages are employed by hosts to query local routers for configuration information, while Router Advertisement messages are employed by local routers to convey the requested information. Router Advertisement messages convey a plethora of network configuration information, including the IPv6 prefix that should be used for configuring IPv6 addresses on the local network. For each local prefix learned from a Router Advertisement message, an IPv6 address is configured by appending a locally-generated Interface Identifier (IID) to the corresponding IPv6 prefix. The following subsections describe currently-deployed policies for generating the IIDs used with SLAAC. 18.104.22.168. Interface-Identifiers Embedding IEEE Identifiers Many network technologies generate the 64-bit interface identifier based on the link-layer address of the corresponding network interface card. For example, in the case of Ethernet addresses, the IIDs are constructed as follows: 1. The "Universal" bit (bit 6, from left to right) of the address is set to 1 2. The word 0xfffe is inserted between the OUI (Organizationally Unique Identifier) and the rest of the Ethernet address For example, the MAC address 00:1b:38:83:88:3c would lead to the IID 021b:38ff:fe83:883c. Gont & Chown Expires July 27, 2014 [Page 6] Internet-Draft IPv6 Reconnaissance January 2014 A number of considerations should be made about these identifiers. Firstly, as it should be obvious from the algorithm described above, two bytes (bytes 4-5) of the resulting address always have a fixed value (0xff, 0xfe), thus reducing the search space for the IID. Secondly, the first three bytes of these identifiers correspond to the OUI of the network interface card vendor. Since not all possible OUIs have been assigned, this further reduces the IID search space. Furthermore, of the assigned OUIs, many could be regarded as corresponding to legacy devices, and thus unlikely to be used for Internet-connected IPv6-enabled systems, yet further reducing the IID search space. Finally, in some scenarios it could be possible to infer the OUI in use by the target network devices, yet narrowing down the possible IIDs even more. For example, an organization known for being provisioned by vendor X is likely to have most of the nodes in its organizational network with OUIs corresponding to vendor X. These considerations mean that in some scenarios, the original IID search space of 64 bits may be effectively reduced to 2^24 , or n * 2^24 (where "n" is the number of different OUIs assigned to the target vendor). Further, if just one host address is detected or known within a subnet, it is not unlikely that, if systems were ordered in a batch, that they may have sequential MAC addresses. Additionally, given a MAC address observed in one subnet, sequential or nearby MAC addresses may be seen in other subnets in the same site. Another interesting factor arises from the use of virtualization technologies, since they generally employ automatically-generated MAC addresses, with very specific patterns. For example, all automatically-generated MAC addresses in VirtualBox virtual machines employ the OUI 08:00:27 [VBox2011]. This means that all SLAAC- produced addresses will have an IID of the form a00:27ff:feXX:XXXX, thus effectively reducing the IID search space from 64 bits to 24 bits. VMWare ESX server provides yet a more interesting example. Automatically-generated MAC addresses have the following pattern [vmesx2011]: 1. The OUI is set to 00:05:59 2. The next 16-bits of the MAC address are set to the same value as the last 16 bits of the console operating system's primary IPv4 address Gont & Chown Expires July 27, 2014 [Page 7] Internet-Draft IPv6 Reconnaissance January 2014 3. The final eight bits of the MAC address are set to a hash value based on the name of the virtual machine's configuration file. This means that, assuming the console operating system's primary IPv4 address is known, the IID search space is reduced from 64 bits to 8 bits. On the other hand, manually-configured MAC addresses in VMWare ESX server employ the OUI 00:50:56, with the low-order three bytes being in the range 0x000000-0x3fffff (to avoid conflicts with other VMware products). Therefore, even in the case of manually-configured MAC addresses, the IID search space is reduced from 64-bits to 22 bits. 22.214.171.124. Privacy Addresses Privacy concerns [CPNI-IPv6] [Gont-DEEPSEC2011] regarding interface identifiers embedding IEEE identifiers led to the introduction of "Privacy Extensions for Stateless Address Auto-configuration in IPv6" [RFC4941], also known as "privacy addresses" or "temporary addresses". Essentially, "privacy addresses" produce random addresses by concatenating a random identifier to the auto- configuration IPv6 prefix advertised in a Router Advertisement. In addition to their unpredictability, these addresses are typically short-lived, such that even if an attacker were to learn one of these addresses, they would be of use for a reduced period of time. It is important to note that "privacy addresses" are generated in addition to traditional SLAAC addresses (i.e., based on IEEE identifiers): traditional SLAAC addresses are employed for incoming (i.e. server-like) communications, while "privacy addresses" are employed for outgoing (i.e., client-like) communications. This means that implementation/use of "privacy addresses" does not prevent an attacker from leveraging the predictability of traditional SLAAC addresses, since "privacy addresses" are generated in addition to (rather than in replacement of) the traditional SLAAC addresses derived from e.g. IEEE identifiers. The benefit that privacy addresses offer in this context is that they reduce the exposure of the SLAAC address to any third parties that may observe that address in use. But, in the absence of firewall protection for the host, the SLAAC address remains liable to be scanned from offsite. Gont & Chown Expires July 27, 2014 [Page 8] Internet-Draft IPv6 Reconnaissance January 2014 126.96.36.199. Randomized Stable Interface Identifiers In order to mitigate the security implications arising from the predictable IPv6 addresses derived from IEEE identifiers, Microsoft Windows produced an alternative scheme for generating "stable addresses" (in replacement of the ones embedding IEEE identifiers). The aforementioned scheme is allegedly an implementation of RFC 4941 [RFC4941], but without regenerating the addresses over time. The resulting interface IDs are constant across system bootstraps, and also constant across networks. Assuming no flaws in the aforementioned algorithm, this scheme would remove any patterns from the SLAAC addresses. However, since the resulting interface IDs are constant across networks, these addresses may still be leveraged for host tracking purposes [I-D.ietf-6man-stable-privacy-addresses]. The benefit of this scheme is thus that the host may be less readily detected by applying heuristics to a scan, but, in the absence of concurrent use of privacy addresses, the host is liable to be tracked. 188.8.131.52. Stable Privacy-Enhanced Addresses In response to the predictability issues discussed in Section 184.108.40.206 and the privacy issues discussed in [I-D.ietf-6man-stable-privacy-addresses], the IETF is currently standardizing (in [I-D.ietf-6man-stable-privacy-addresses]) a method for generating IPv6 Interface Identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), such that addresses configured using this method are stable within each subnet, but the Interface Identifier changes when hosts move from one network to another. The aforementioned method is meant to be an alternative to generating Interface Identifiers based on IEEE identifiers, such that the benefits of stable addresses can be achieved without sacrificing the privacy of users. Implementation of this method (in replacement of Interface Identifiers based on IEEE identifiers) would eliminate any patterns from the Interface ID, thus benefiting user privacy and reducing the ease with which addresses can be scanned. 3.1.2. Dynamic Host Configuration Protocol version 6 (DHCPv6) DHCPv6 can be employed as a stateful address configuration mechanism, in which a server (the DHCPv6 server) leases IPv6 addresses to IPv6 hosts. As with the IPv4 counterpart, addresses are assigned Gont & Chown Expires July 27, 2014 [Page 9] Internet-Draft IPv6 Reconnaissance January 2014 according to a configuration-defined address range and policy, with some DHCPv6 servers assigned addresses sequentially, from a specific range. In such cases, addresses tend to be predictable. For example, if the prefix 2001:db8::/64 is used for assigning addresses on the local network, the DHCPv6 server might (sequentially) assign addresses from the range 2001:db8::1 - 2001:db8::100. In most common scenarios, this means that the IID search space will be reduced from the original 64 bits, to 8 or 16 bits. RFC 5157 recommended that DHCPv6 instead issue addresses randomly from a large pool; that advice is repeated here. 3.1.3. Manually-configured Addresses In some scenarios, node addresses may be manually configured. This is typically the case for IPv6 addresses assigned to routers (since routers do not employ automatic address configuration) but also for servers (since having a stable address that does not depend on the underlying link-layer address is generally desirable). While network administrators are mostly free to select the IID from any value in the range 1 - 264 range, for the sake of simplicity (i.e., ease of remembering) they tend to select addresses with one of the following patterns: o "low-byte" addresses: in which most of the bytes of the IID are set to 0 (except for the least significant byte). o IPv4-based addresses: in which the IID embeds the IPv4 address of the network interface (as in 2001:db8::192.0.2.1) o "service port" addresses: in which the IID embeds the TCP/UDP service port of the main service running on that node (as in 2001:db8::80 or 2001:db8::25) o wordy addresses: which encode words (as in 2001:db8::dead:beef) Each of these patterns is discussed in detail in the following subsections. 220.127.116.11. Low-byte Addresses The most common form of low-byte addresses is that in which all the the bytes of the IID (except the least significant bytes) are set to zero (as in 2001:db8::1, 2001:db8::2, etc.). However, it is also common to find similar addresses in which the two lowest order 16-bit Gont & Chown Expires July 27, 2014 [Page 10] Internet-Draft IPv6 Reconnaissance January 2014 words are set to small numbers (as in 2001::db8::1:10, 2001:db8::2:10, etc.). Yet it is not uncommon to find IPv6 addresses in which the second lowest-order 16-bit word is set to a small value in the range 0-255, while the lowest-order 16-bit word varies in the range 0-65535. It should be noted that all of these address patterns are generally referred to as "low-byte addresses", even when, strictly speaking, it is not not only the lowest-order byte of the IPv6 address that varies from one address to another. In the worst-case scenario, the search space for this pattern is 2**24 (although most systems can be found by searching 2**16 or even 2**8 addresses). 18.104.22.168. IPv4-based Addresses The most common form of these addresses is that in which an IPv4 address is encoded in the lowest-order 32 bits of the IPv6 address (usually as a result of the notation of addresses in the form 2001:db8::192.0.2.1). However, it is also common for administrators to encode one byte of the IPv4 address in each of the 16-bit words of the IID (as in e.g. 2001:db8::192:0:2:1). For obvious reasons, the search space for addresses following this pattern is that of the corresponding IPv4 prefix (or twice the size of that search space if both forms of "IPv4-based addresses" are to be searched). 22.214.171.124. Service-port Addresses Address following this pattern include the service port, e.g., 80 for HTTP, or perhaps 80a, 80b, etc where multiple HTTP servers live in one subnet) in the lowest-order byte of the IID, and set the rest of the IID to zero. There are a number of variants for this address pattern: o The lowest-order 16-bit word may contain the service port, and the second lowest-order 16-bit word may be set to a number in the range 0-255 (as in e.g. 2001:db8::1:80). o The lowest-order 16-bit word may be set to a value in the range 0-255, while the second lowest-order 16-bit word may contain the service port (as in e.g. 2001:db8::80:1). o The service port itself might be encoded in decimal or in hexadecimal notation (e.g., an address embedding the HTTP port might be 2001:db8::80 or 2001:db8::50) -- with addresses encoding the service port as a decimal number being more common. Gont & Chown Expires July 27, 2014 [Page 11] Internet-Draft IPv6 Reconnaissance January 2014 Considering a maximum of 20 popular service ports, the search space for addresses following in this pattern is, in the worst-case scenario, 20 * 2**10. 126.96.36.199. Wordy Addresses Since IPv6 address notation allows for a number of hexadecimal digits, it is not difficult to encode words into IPv6 addresses (as in, e.g., 2001:db8::dead:beef). Addresses following this pattern are likely to be explored by means of "dictionary attacks", and therefore computing the corresponding search-space is not straight-forward. 3.1.4. IPv6 Addresses Corresponding to Transition/Co-existence Technologies Some transition/co-existence technologies might be leveraged to reduce the target search space of remote address-scanning attacks, since they specify how the corresponding IPv6 address must be generated. For example, in the case of Teredo [RFC4380], the 64-bit interface identifier is generated from the IPv4 address observed at a Teredo server along with a UDP port number. 3.1.5. IPv6 Address Assignment in Real-world Network Scenarios Table 2, Table 3 and Table 4 provide a summary of the results obtained by [Gont-LACSEC2013] for web servers, nameservers, and mailservers, resectively. Table 5 provides a rough summary of the results obtained by [Malone2008] for IPv6 routers. Table 6 provides a summary of the results obtained by [Ford2013] for clients. Gont & Chown Expires July 27, 2014 [Page 12] Internet-Draft IPv6 Reconnaissance January 2014 +---------------+------------+ | Address type | Percentage | +---------------+------------+ | IEEE-based | 1.44% | +---------------+------------+ | Embedded-IPv4 | 25.41% | +---------------+------------+ | Embedded-Port | 3.06% | +---------------+------------+ | ISATAP | 0% | +---------------+------------+ | Low-byte | 56.88% | +---------------+------------+ | Byte-pattern | 6.97% | +---------------+------------+ | Randomized | 6.24% | +---------------+------------+ Table 2: Measured webserver addresses +---------------+------------+ | Address type | Percentage | +---------------+------------+ | IEEE-based | 0.67% | +---------------+------------+ | Embedded-IPv4 | 22.11% | +---------------+------------+ | Embedded-Port | 6.48% | +---------------+------------+ | ISATAP | 0% | +---------------+------------+ | Low-byte | 56.58% | +---------------+------------+ | Byte-pattern | 11.07% | +---------------+------------+ | Randomized | 3.09% | +---------------+------------+ Table 3: Measured nameserver addresses Gont & Chown Expires July 27, 2014 [Page 13] Internet-Draft IPv6 Reconnaissance January 2014 +---------------+------------+ | Address type | Percentage | +---------------+------------+ | IEEE-based | 0.48% | +---------------+------------+ | Embedded-IPv4 | 4.02% | +---------------+------------+ | Embedded-Port | 1.07% | +---------------+------------+ | ISATAP | 0% | +---------------+------------+ | Low-byte | 92.65% | +---------------+------------+ | Byte-pattern | 1.20% | +---------------+------------+ | Randomized | 0.59% | +---------------+------------+ Table 4: Measured mailserver addresses +--------------+------------+ | Address type | Percentage | +--------------+------------+ | Low-byte | 70% | +--------------+------------+ | IPv4-based | 5% | +--------------+------------+ | SLAAC | 1% | +--------------+------------+ | Wordy | <1% | +--------------+------------+ | Randomized | <1% | +--------------+------------+ | Teredo | <1% | +--------------+------------+ | Other | <1% | +--------------+------------+ Table 5: Measured router addresses Gont & Chown Expires July 27, 2014 [Page 14] Internet-Draft IPv6 Reconnaissance January 2014 +---------------+------------+ | Address type | Percentage | +---------------+------------+ | IEEE-based | 7.72% | +---------------+------------+ | Embedded-IPv4 | 14.31% | +---------------+------------+ | Embedded-Port | 0.21% | +---------------+------------+ | ISATAP | 1.06% | +---------------+------------+ | Randomized | 69.73% | +---------------+------------+ | Low-byte | 6.23% | +---------------+------------+ | Byte-pattern | 0.74% | +---------------+------------+ Table 6: Measured client addresses It should be clear from these measurements that a very high percentage of host and router addresses follow very specific patterns. Table 6 shows that while around 70% of clients observed in this measurement appear to be using privacy addresses, there are still a significant amount exposing IEEE-based addresses, and addresses using embedded IPv4 (thus also revealing IPv4 addresses). 3.2. IPv6 Address Scanning of Remote Networks While in IPv4 networks attackers have been able to get away with "brute force" scanning attacks (thanks to the reduced search space), successfully performing a brute-force scan of an entire /64 network would be infeasible. As a result, it is expected that attackers will leverage the IPv6 address patterns discussed in Section 3.1 to reduce the IPv6 address search space. IPv6 address scanning of remote area networks should consider an additional factor not present for the IPv4 case: since the typical IPv6 subnet is a /64, scanning an entire /64 could, in theory, lead to the creation of 2^^64 entries in the Neighbor Cache of the last- hop router. Unfortunately, a number of IPv6 implementations have been found to be unable to properly handle large number of entries in the Neighbor Cache, and hence these address-scan attacks may have the side effect of resulting in a Denial of Service (DoS) attack [CPNI-IPv6] [RFC6583]. Gont & Chown Expires July 27, 2014 [Page 15] Internet-Draft IPv6 Reconnaissance January 2014 3.3. IPv6 Address Scanning of Local Networks IPv6 address scanning in Local Area Networks could be considered, to some extent, a completely different problem than that of scanning a remote IPv6 network. The main difference is that use of link-local multicast addresses can relieve the attacker of searching for unicast addresses in a large IPv6 address space. Obviously, a number of other network reconnaissance vectors (such as network snooping, leveraging Neighbor Discovery traffic, etc.) are available when scanning a local network. However, this section focuses only on address-scanning attacks (a la "ping sweep"). An attacker can simply send probe packets to the all-nodes link-local multicast address (ff02::1), such that responses are elicited from all local nodes. Since Windows systems (Vista, 7, etc.) do not respond to ICMPv6 Echo Request messages sent to multicast addresses, IPv6 address-scanning tools typically employ a number of additional probe packets to elicit responses from all the local nodes. For example, unrecognized IPv6 options of type 10xxxxxx elicit ICMPv6 Parameter Problem, code 2, error messages. Many address-scanning tools discover only IPv6 link-local addresses (rather than e.g. the global addresses of the target systems): since the probe packets are typically sent with the attacker's IPv6 link- local address, the "victim" nodes send the response packets using the IPv6 link-local address of the corresponding network interface (as specified by the IPv6 address selection rules [RFC6724]). However, sending multiple probe packets, with each packet employing addresses from different prefixes, typically helps to overcome this limitation. This technique is employed by the scan6 tool of the IPv6 Toolkit package [IPv6-Toolkit]. 3.4. Existing IPv6 Address Scanning Tools 3.4.1. Remote IPv6 Network Scanners IPv4 address scanning tools have traditionally carried out their task for probing an entire address range (usually the entire range of a target subnetwork). One might argue that the reason for which we have been able to get away with such somewhat "rudimentary" techniques is that the scale of the "problem" is so small in the IPv4 world, that a "brute-force" attack is "good enough". However, the scale of the "address scanning" problem is so large in IPv6, that Gont & Chown Expires July 27, 2014 [Page 16] Internet-Draft IPv6 Reconnaissance January 2014 attackers must be very creative to be "good enough". Simply sweeping an entire /64 IPv6 subnet would just not be feasible. Many address scanning tools such as nmap [nmap2012] do not even support sweeping an IPv6 address range. On the other hand, the alive6 tool from [THC-IPV6] supports sweeping address ranges, thus being able to leverage some patters found in IPv6 addresses, such as the incremental addresses resulting from some DHCPv6 setups. Finally, the scan6 tool from [IPv6-Toolkit] supports sweeping address ranges, and can also leverage all the address patterns described in Section 3.1 of this document. Clearly, a limitation of many of the currently-available tools for IPv6 address scanning is that they lack of an "heuristics engine" that can help reduce the search space, such that the problem of IPv6 address scanning becomes tractable. The most "advanced" IPv6 scanning technique that has been found in the wild (and publicly reported) is described in [Ybema2010] (the attacker seemed to be scanning specific IPv6 addresses based on some specific patterns). However, the aforementioned attempt probably still falls into the category of "rudimentary". 3.4.2. Local IPv6 Network Scanners There are a variety of publicly-available local IPv6 network scanners: Current versions of nmap [nmap2012] implement this functionality THC's IPv6 Attack Toolkit [THC-IPV6] includes a tool (alive6) that implements this functionality SI6 Network's IPv6 Toolkit [IPv6-Toolkit] includes a tool (scan6) that implements this functionality 3.5. Mitigations IPv6 address-scanning attacks can be mitigated in a number of ways. A non-exhaustive list of the possible mitigations includes: o Employing stable privacy-enhanced addresses [I-D.ietf-6man-stable-privacy-addresses] in replacement of addresses based on IEEE identifiers, such that any address patterns are eliminated. o Employing Intrusion Prevention Systems (IPS) at the perimeter, such that address scanning attacks can be mitigated. Gont & Chown Expires July 27, 2014 [Page 17] Internet-Draft IPv6 Reconnaissance January 2014 o If virtual machines are employed, and "resistance" to address scanning attacks is deemed as desirable, manually-configured MAC addresses can be employed, such that even if the virtual machines employ IEEE-derived IIDs, they are generated from non-predictable MAC addresses. o When using DHCPv6, avoid use of sequential addresses. Ideally, the DHCPv6 server would allocate random addresses from a large pool. It should be noted that some of the aforementioned mitigations are operational, while others depend on the availability of specific features (such as [I-D.ietf-6man-stable-privacy-addresses] on the corresponding nodes. Additionally, while some resistance to address scanning attacks is generally desirable (particularly when lightweight mitigations are available), there are scenarios in which mitigation of some address- scanning vectors is unlikely to be a high-priority (if at all possible). Two of the techniques discussed in this document for local address- scanning attacks are those that employ multicasted ICMPv6 Echo Requests and multicasted IPv6 packets containing unsupported options of type 10xxxxxx. These two vectors could be easily mitigated by configuring nodes to not respond to multicasted ICMPv6 Echo Request (default on Windows systems), and by updating the IPv6 specifications (and/or possibly configuring local nodes) such that multicasted packets never elicit ICMPv6 error messages (even if they contain unsupported options of type 10xxxxxx). [I-D.gont-6man-ipv6-smurf-amplifier] proposes such update to the IPv6 specifications. In any case, when it comes to local networks, there are a variety of network reconnaissance vectors. Therefore, even if address-scanning vectors are mitigated, an attacker could still rely on e.g. protocols employed for the so-called "opportunistic networking" (such as mDNS [RFC6762]), or eventually rely on network snooping as last resort for network reconnaissance. 4. Leveraging the Domain Name System (DNS) for Network Reconnaissance 4.1. DNS Advertised Hosts Any systems that are "published" in the DNS, e.g. MX mail relays, or web servers, will remain open to probing from the very fact that their IPv6 addresses are publicly available. It is worth noting that Gont & Chown Expires July 27, 2014 [Page 18] Internet-Draft IPv6 Reconnaissance January 2014 where the addresses used at a site follow specific patterns, publishing just one address may lead to a threat upon the other hosts. Additionally, we note that publication of IPv6 addresses in the DNS should not discourage the elimination of IPv6 address patterns: if any address patterns are eliminated from addresses published in the DNS, an attacker may have to rely on performing dictionary-based DNS lookups in order to find all systems in a target network (which is generally less reliable and more time/traffic consuming than mapping nodes with predictable IPv6 addresses). 4.2. DNS Zone Transfers A DNS zone transfer can readily provide information about potential attack targets. Restricting zone transfers is thus probably more important for IPv6, even if it is already good practice to restrict them in the IPv4 world. 4.3. DNS Brute Forcing Attakers may employ DNS brute-forcing techniques by testing for the presence of DNS AAAA records against commonly used host names. 4.4. DNS Reverse Mappings An interesting technique that employs DNS reverse mappings for network reconnaissance has been recently disclosed [van-Dijk]. Essentially, the attacker walks through the "ip6.arpa" zone looking up PTR records, in the hopes of learning the IPv6 addresses of hosts in a given target network (assuming that the reverse mappings have been configured, of course). What is most interesting about this technique is that it can greatly reduce the IPv6 address search space. Basically, an attacker would walk the ip6.arpa zone corresponding to a target network (e.g. "0.8.0.0.8.b.d.0.1.0.0.2.ip6.arpa." for "2001:db8:80::/32"), issuing queries for PTR records corresponding to the domain names "0.0.8.0.0.8.b.d.0.1.0.0.2.ip6.arpa.", "188.8.131.52.0.8.b.d.0.1.0.0.2.ip6.arpa.", etc. If, say, there were PTR records for any hosts "starting" with the domain name "0.0.8.0.0.8.b.d.0.1.0.0.2.ip6.arpa." (e.g., the ip6.arpa domain name corresponding to the IPv6 address 2001:db8:80::1), the response would contain an RCODE of 0 (no error). Otherwise, the response would contain an RCODE of 4 (NXDOMAIN). As noted in [van-Dijk], this technique allows for a tremendous reduction in the "IPv6 address" search space. Gont & Chown Expires July 27, 2014 [Page 19] Internet-Draft IPv6 Reconnaissance January 2014 5. Leveraging Local Name Resolution and Service Discovery Services A number of protocols allow for unmanaged local name resolution and service. For example, multicast DNS (mDNS) [RFC6762] and DNS Service Discovery (DNS-SD) [RFC6763], or Link-Local Multicast Name Resolution (LLMNR) [RFC4795], are examples of such protocols. Besides the Graphical User Interfaces (GUIs) included in products supporting such protocols, command-line tools such as mdns-scan [mdns-scan] and mzclient can help discover IPv6 hosts employing mDNS/DNS-SD. 6. Public Archives Public mailing-list archives or Usenet news messages archives may prove a useful channel for an attacker, since hostnames and/or IPv6 addresses could be easily obtained by inspection of the (many) "Received from:" or other header lines in the archived email or Usenet news messages. 7. Application Participation Peer-to-peer applications often include some centralized server which coordinates the transfer of data between peers. For example, BitTorrent [BitTorrent] builds swarms of nodes that exchange chunks of files, with a tracker passing information about peers with available chunks of data between the peers. Such applications may offer an attacker a source of peer addresses to probe. 8. Inspection of the IPv6 Neighbor Cache and Routing Table Information about other systems connected to the local network might be readily available from the Neighbor Cache [RFC4861] and/or the routing table of any system connected to such network. While the requirement of having "login" access to a system in the target network may limit the applicability of this technique, there are a number of scenarios in which this technique might be of use. For example, security audit tools might be provided with the necessary credentials such that the Neighbor Cache and the routing table of all systems for which the tool has "login" access can be automatically gleaned. On the other hand, IPv6 worms [V6-WORMS] could leverage this technique for the purpose of spreading on the local network, since they will typically have access to the Neighbor Cache and routing table of an infected system. Gont & Chown Expires July 27, 2014 [Page 20] Internet-Draft IPv6 Reconnaissance January 2014 9. Inspection of System Configuration and Log Files Nodes are generally configured with the addresses of other important local computers, such as email servers, local file servers, web proxy servers, recursive DNS servers, etc. The /etc/hosts file in UNIX, SSH known_hosts files, or the Microsoft Windows registry are just some examples of places where interesting information about such systems might be found. Additionally, system log files (including web server logs, etc.) may also prove a useful channel for an attacker. While the required credentials to access the aforementioned configuration and log files may limit the applicability of this technique, there are a number of scenarios in which this technique might be of use. For example, security audit tools might be provided with the necessary credentials such that these files can be automatically accessed. On the other hand, IPv6 worms could leverage this technique for the purpose of spreading on the local network, since they will typically have access to these files on an infected system [V6-WORMS]. 10. Gleaning Information from Routing Protocols Some organizational IPv6 networks employ routing protocols to dynamically maintain routing information. In such an environment, a local attacker could become a passive listener of the routing protocol, to determine other valid subnets within that organization [V6-WORMS]. 11. IANA Considerations There are no IANA registries within this document. The RFC-Editor can remove this section before publication of this document as an RFC. 12. Security Considerations This document explores the topic of Network Reconnaissance in IPv6 networks. It analyzes the feasibility of address-scan attacks in IPv6 networks, and showing that the search space for such attacks is typically much smaller than the one traditionally assumed (64 bits). Additionally, it explores a plethora of other network reconnaissance techniques, ranging from inspecting the IPv6 Network Cache of an attacker-controlled system, to gleaning information about IPv6 addresses from public mailing-list archives or Peer-To-Peer (P2P) protocols. Gont & Chown Expires July 27, 2014 [Page 21] Internet-Draft IPv6 Reconnaissance January 2014 We expect traditional address-scanning attacks to become more and more elaborated (i.e., less "brute force"), and other network reconnaissance techniques to be actively explored, as global deployment of IPv6 increases and. more specifically, as more IPv6-only devices are deployed. 13. Acknowledgements The authors would like to thank (in alphabetical order) Marc Heuse, Ray Hunter, Libor Polcak, Jan Schaumann, and Arturo Servin, for providing valuable comments on earlier versions of this document. Part of the contents of this document are based on the results of the project "Security Assessment of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by Fernando Gont on behalf of the UK Centre for the Protection of National Infrastructure (CPNI). Fernando Gont would like to thank the UK CPNI for their continued support. 14. References 14.1. Normative References [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, September 2012. [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, February 2006. [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local Multicast Name Resolution (LLMNR)", RFC 4795, January 2007. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. Gont & Chown Expires July 27, 2014 [Page 22] Internet-Draft IPv6 Reconnaissance January 2014 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007. [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, February 2013. [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, February 2013. [I-D.ietf-6man-stable-privacy-addresses] Gont, F., "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)", draft-ietf-6man-stable- privacy-addresses-16 (work in progress), December 2013. 14.2. Informative References [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational Neighbor Discovery Problems", RFC 6583, March 2012. [RFC5157] Chown, T., "IPv6 Implications for Network Scanning", RFC 5157, March 2008. [I-D.gont-6man-ipv6-smurf-amplifier] Gont, F. and W. Liu, "Security Implications of IPv6 Options of Type 10xxxxxx", draft-gont-6man-ipv6-smurf- amplifier-03 (work in progress), March 2013. [CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). [V6-WORMS] Bellovin, S., Cheswick, B., and A. Keromytis, "Worm propagation strategies in an IPv6 Internet", ;login:, pages 70-76, February 2006, <https://www.cs.columbia.edu/ ~smb/papers/v6worms.pdf>. [Malone2008] Malone, D., "Observations of IPv6 Addresses", Passive and Active Measurement Conference (PAM 2008, LNCS 4979), April 2008, <http://www.maths.tcd.ie/~dwmalone/p/addr-pam08.pdf>. Gont & Chown Expires July 27, 2014 [Page 23] Internet-Draft IPv6 Reconnaissance January 2014 [mdns-scan] Poettering, L., "mdns-scan(1) manual page", 2012, <http://manpages.ubuntu.com/manpages/precise/man1/ mdns-scan.1.html>. [nmap2012] Fyodor, , "nmap - Network exploration tool and security / port scanner", 2012, <http://insecure.org>. [VBox2011] VirtualBox, , "Oracle VM VirtualBox User Manual, version 4.1.2", August 2011, <http://www.virtualbox.org>. [vmesx2011] vmware, , "Setting a static MAC address for a virtual NIC", vmware Knowledge Base, August 2011, <http://kb.vmware.com/selfservice/microsites/ search.do?language=en_US&cmd=displayKC&externalId=219>. [Ybema2010] Ybema, I., "just seen my first IPv6 network abuse scan, is this the start for more?", Post to the NANOG mailing-list, 2010, <http://mailman.nanog.org/pipermail/nanog/ 2010-September/025049.html>. [Gont-DEEPSEC2011] Gont, F., "Results of a Security Assessment of the Internet Protocol version 6 (IPv6)", DEEPSEC 2011 Conference, Vienna, Austria, November 2011, 2011, <http://www.si6networks.com/presentations/deepsec2011/ fgont-deepsec2011-ipv6-security.pdf>. [Gont-LACSEC2013] Gont, F., "IPv6 Network Reconnaissance: Theory & Practice", LACSEC 2013 Conference, Medellin, Colombia, May 2013, 2013, <http://www.si6networks.com/presentations/ lacnic19/ lacsec2013-fgont-ipv6-network-reconnaissance.pdf>. [Ford2013] Ford, M., "IPv6 Address Analysis - Privacy In, Transition Out", 2013, <http://www.internetsociety.org/blog/2013/05/ ipv6-address-analysis-privacy-transition-out>. [THC-IPV6] "THC-IPV6", <http://www.thc.org/thc-ipv6/>. Gont & Chown Expires July 27, 2014 [Page 24] Internet-Draft IPv6 Reconnaissance January 2014 [IPv6-Toolkit] "SI6 Networks' IPv6 Toolkit", <http://www.si6networks.com/tools/ipv6toolkit>. [BitTorrent] "BitTorrent", <http://en.wikipedia.org/wiki/BitTorrent>. [van-Dijk] van Dijk, P., "Finding v6 hosts by efficiently mapping ip6.arpa", 2012, <http://7bits.nl/blog/2012/03/26/ finding-v6-hosts-by-efficiently-mapping-ip6-arpa>. Appendix A. Implementation of a full-fledged IPv6 address-scanning tool This section describes the implementation of a full-fledged IPv6 address scanning tool. Appendix A.1 discusses the selection of host probes. Appendix A.2 describes the implementation of an IPv6 address scanner for local area networks. Appendix A.3 outlines ongoing work on the implementation of a general (i.e., non-local) IPv6 host scanner. A.1. Host-probing considerations A number of factors should be considered when selecting the probe types and the probing-rate for an IPv6 address scanning tool. Firstly, some hosts (or border firewalls) might be configured to block or rate-limit some specific packet types. For example, it is usual for host and router implementations to rate-limit ICMPv6 error traffic. Additionally, some firewalls might be configured to block or rate-limit incoming ICMPv6 echo request packets. As noted earlier in this document, Windows systems simply do not respond to ICMPv6 echo requests sent to multicast IPv6 addresses. Among the possible probe types are: o ICMPv6 Echo Request packets (meant to elicit ICMPv6 Echo Replies), o TCP SYN segments (meant to elicit SYN/ACK or RST segments), o TCP segments that do not contain the ACK bit set (meant to elicit RST segments), o UDP datagrams (meant to elicit a UDP application response or an ICMPv6 Port Unreachable), Gont & Chown Expires July 27, 2014 [Page 25] Internet-Draft IPv6 Reconnaissance January 2014 o IPv6 packets containing any suitable payload and an unrecognized extension header (meant to elicit ICMPv6 Parameter Problem error messages), or, o IPv6 packets containing any suitable payload and an unrecognized option of type 10xxxxxx (such that a ICMPv6 Parameter Problem error message is elicited) Selecting an appropriate probe packet might help conceal the ongoing attack, but may also be actually necessary if host or network configuration causes certain probe packets to be dropped. In some cases, it might be desirable to insert some IPv6 extension headers before the actual payload, such that some filtering policies can be circumvented. Another factor to consider is the host-probing rate. Clearly, the higher the rate, the smaller the amount of time required to perform the attack. However, the probing-rate should not be too high, or else: 1. the attack might cause network congestion, thus resulting in packet loss 2. the attack might hit rate-limiting, thus resulting in packet loss 3. the attack might reveal underlying problems in the Neighbor Discovery implementation, thus leading to packet loss and possibly even Denial of Service Packet-loss is undesirable, since it would mean that an "alive" node might remain undetected as a result of a lost probe or response. Such losses could be the result of congestion (in case the attacker is scanning a target network at a rate higher than the target network can handle), or may be the result of rate-limiting as it would be typically the case if ICMPv6 is employed for the probe packets. Finally, as discussed in [CPNI-IPv6] and [RFC6583], some IPv6 router implementations have been found to be unable to perform decent resource management when faced with Neighbor Discovery traffic involving a large number of local nodes. This essentially means that regardless of the type of probe packets, a address scanning attack might result in a Denial of Service (DoS) of the target network, with the same (or worse) effects as that of network congestion or rate- limiting. The specific rates at which each of these issues may come into play vary from one scenario to another, and depend on the type of deployed routers/firewalls, configuration parameters, etc. Gont & Chown Expires July 27, 2014 [Page 26] Internet-Draft IPv6 Reconnaissance January 2014 A.2. Implementation of an IPv6 local address-scanning tool scan6 [IPv6-Toolkit] is prototype IPv6 local address scanning tool, which has proven to be effective and efficient for the discovery of IPv6 hosts on a local network. The scan6 tool operates (roughly) as follows: 1. The tool learns the local prefixes used for auto-configuration, an generates/configures one address for each local prefix (in addition to a link-local address). 2. An ICMPv6 Echo Request message destined to the all-nodes on-link multicast address (ff02::1) is sent with each of the addresses "configured" in the previous step. Because of the different Source Addresses, each probe causes the victim nodes to use different Source Addresses for the response packets (this allows the tool to learn virtually all the addresses in use in the local network segment). 3. The same procedure of the previous bullet is performed, but this time with ICMPv6 packets that contain an unrecognized option of type 10xxxxxx, such that ICMPv6 Parameter Problem error messages are elicited. This allows the tool to discover e.g. Windows nodes, which otherwise do not respond to multicasted ICMPv6 Echo Request messages. 4. Each time a new "alive" address is discovered, the corresponding Interface-ID is combined with all the local prefixes, and the resulting addresses are probed (with unicasted packets). This can help to discover other addresses in use on the local network segment, since the same Interface ID is typically used with all the available prefixes for the local network. The aforementioned scheme can fail to discover some addresses for some implementation. For example, Mac OS X employs IPv6 addresses embedding IEEE-identifiers (rather than "privacy addresses") when responding to packets destined to a link-local multicast address, sourced from an on-link prefix. A.3. Implementation of a IPv6 remote address-scanning tool An IPv6 remote address scanning tool, could be implemented with the following features: o The tool can be instructed to target specific address ranges (e.g. 2001:db8::0-10:0-1000) Gont & Chown Expires July 27, 2014 [Page 27] Internet-Draft IPv6 Reconnaissance January 2014 o The tool can be instructed to scan for SLAAC addresses of a specific vendor, such that only addresses embedding the corresponding IEEE OUIs are probed. o The tool can be instructed to scan for SLAAC addresses that employ a specific IEEE OUI. o The tool can be instructed to discover virtual machines, such that a given IPv6 prefix is only scanned for the address patterns resulting from virtual machines. o The tool can be instructed to scan for low-byte addresses. o The tool can be instructed to scan for wordy-addresses, in which case the tool selects addresses based on a local dictionary. o The tool can be instructed to scan for IPv6 addresses embedding TCP/UDP service ports, in which case the tool selects addresses based on a list of well-known service ports. o The tool can be specified an IPv4 address range in use at the target network, such that only IPv4-based IPv6 addresses are scanned. The scan6 tool of [IPv6-Toolkit] implements all these techniques/ features. Authors' Addresses Fernando Gont Huawei Technologies Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Phone: +54 11 4650 8472 Email: firstname.lastname@example.org URI: http://www.si6networks.com Tim Chown University of Southampton Highfield Southampton , Hampshire SO17 1BJ United Kingdom Email: email@example.com Gont & Chown Expires July 27, 2014 [Page 28]