opsec                                                            F. Gont
Internet-Draft                                       Huawei Technologies
Obsoletes: 5157 (if approved)                                   T. Chown
Intended status: Informational                 University of Southampton
Expires: July 27, December 16, 2014                                  January 23,                                 June 14, 2014

                Network Reconnaissance in IPv6 Networks


   IPv6 offers a much larger address space than that of its IPv4
   counterpart.  The standard /64  An IPv6 subnets subnet of size /64 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 5157, which first
   discussed this assumption, 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

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   This Internet-Draft will expire on July 27, December 16, 2014.

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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  10
       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
       3.2.1.  Reducing the subnet ID search space . . . . . . . . .  16
     3.3.  IPv6 Address Scanning of Local Networks . . . . . . . . .  16
     3.4.  Existing IPv6 Address Scanning Tools  . . . . . . . . . .  16  17
       3.4.1.  Remote IPv6 Network Scanners  . . . . . . . . . . . .  16  17
       3.4.2.  Local IPv6 Network Scanners . . . . . . . . . . . . .  17  18
     3.5.  Mitigations . . . . . . . . . . . . . . . . . . . . . . .  17  19
   4.  Leveraging the Domain Name System (DNS) for Network
       Reconnaissance  . . . . . . . . . . . . . . . . . . . . . . .  18  20
     4.1.  DNS Advertised Hosts  . . . . . . . . . . . . . . . . . .  18  20
     4.2.  DNS Zone Transfers  . . . . . . . . . . . . . . . . . . .  19  20
     4.3.  DNS Brute Forcing . . . . . . . . . . . . . . . . . . . .  19  20
     4.4.  DNS Reverse Mappings  . . . . . . . . . . . . . . . . . .  19  21
   5.  Leveraging Local Name Resolution and Service Discovery
       Services  . . . . . . . . . . . . . . . . . . . . . . . . . .  20  21
   6.  Public Archives . . . . . . . . . . . . . . . . . . . . . . .  20  21
   7.  Application Participation . . . . . . . . . . . . . . . . . .  20  22
   8.  Inspection of the IPv6 Neighbor Cache and Routing Table . . .  20  22
   9.  Inspection of System Configuration and Log Files  . . . . . .  21  22
   10. Gleaning Information from Routing Protocols . . . . . . . . .  21  23
   11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  23
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   12.  23
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   13.  23
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   14.  24
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     14.1.  24
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     14.2.  24
     15.2.  Informative References . . . . . . . . . . . . . . . . .  23  25
   Appendix A.  Implementation of a full-fledged IPv6 address-
                scanning tool  . . . . . . . . . . . . . . . . . . .  25  27
     A.1.  Host-probing considerations . . . . . . . . . . . . . . .  25  27
     A.2.  Implementation of an IPv6 local address-scanning tool . .  27  29
     A.3.  Implementation of a IPv6 remote address-scanning tool . .  27  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28  31

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 hosts
   divided by the number of hosts) addresses) 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 and obsoletes RFC 5157). 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
   the addresses of discovered servers; since then, evidence that some
   classic address scanning is occurring 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. included in
   the Appendices.

   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.

   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

2.  Requirements for the Applicability of Network Reconnaissance

   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

   |                  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

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
   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 hosts, typically servers, 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.  Interface-Identifiers Embedding IEEE Identifiers

   Many network technologies generate the 64-bit

   The traditional SLAAC interface identifier identifiers are based on the link-layer 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

      [RFC7136] notes that all bits of an IID should be treated as
      "opaque" bits.  Furthermore, [I-D.ietf-6man-default-iids] is
      currently in the process of changing the default IID generation
      scheme to [RFC7217].  Therefore, the traditional IIDs based on
      link-layer addresses are expected to become less common over time.

   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

   VMWare ESX server provides yet a more interesting example.
   Automatically-generated MAC addresses have the following pattern

   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

   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

   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.  Privacy  Temporary 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 "temporary addresses" or "temporary "privacy
   addresses".  Essentially, "privacy "temporary 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 limited period
      of time.  A typical implementation may keep a temporary address
      preferred for 24 hours, and configured but deprecated for seven

   It is important to note that "privacy "temporary addresses" are generated in
   addition to traditional SLAAC addresses (i.e., based on IEEE
   identifiers): traditional SLAAC addresses are meant to be employed
   for incoming
   (i.e. server-like) "server-like" inbound communications, while "privacy "temporary addresses"
   are meant to be employed for outgoing (i.e., client-like) "client-like" outbound communications.
   This means that implementation/use of "privacy "temporary addresses" does not
   prevent an attacker from leveraging the predictability of traditional
   SLAAC addresses, since "privacy "temporary addresses" are generated in
   addition to (rather than in as a replacement of) the traditional SLAAC
   addresses derived from e.g.  IEEE identifiers.

   The benefit that privacy temporary 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. traffic sent from a host where temporary addresses
   are enabled and used by default.  But, in the absence of firewall
   protection for the host, the its SLAAC address remains liable to be
   scanned from offsite.  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 believed to be 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]. [RFC7217]

   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 temporary addresses, the host is liable to be
   tracked across visited networks.  Stable Privacy-Enhanced Addresses

   In response to the predictability issues discussed in Section
   and the privacy issues discussed in
   [I-D.ietf-6man-ipv6-address-generation-privacy], the IETF is currently
   standardizing has
   standardized (in [I-D.ietf-6man-stable-privacy-addresses]) [RFC7217]) 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 subnet 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
   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 -

   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, 2^64, 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::

   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.  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
   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 2^24
   (although most systems can be found by searching 2**16 2^16 or even
   2**8 2^8
   addresses).  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::  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).  Service-port Addresses

   Address following this pattern include the service port, e.g., port (e.g. 80 for
   HTTP, or perhaps 80a, 80b, etc where multiple HTTP servers live in
   one subnet)
   HTTP) 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

   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.

   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. 2^10.  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

   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. respectively.  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.

                      |  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
                      |  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
                      |  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

   Table 6 shows that while around 70% of clients observed in this
   measurement appear to be using privacy temporary 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 host subnet is a /64, scanning an entire /64 could, in theory,
   lead to the creation of 2^^64 2^64 entries in the Neighbor Cache of the last-
   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].

   [I-D.ietf-6man-why64] discusses the "default" /64 boundary for host
   subnets, and the assumptions surrounding it.  While there are reports
   of a handful of sites implementing host subnets of size /112 or
   smaller to reduce concerns about the above attack, such smaller
   subnets are likely to make address-based scanning more feasible, in
   addition to encountering the issues with non-/64 host subnets
   discussed in the above draft.

3.2.1.  Reducing the subnet ID search space

   When scanning a remote network, consideration is required to select
   which subnet IDs to choose.  A typical site might have a /48
   allocation, which would mean up to 65,000 or so host /64 subnets to
   be scanned.

   However, just as with the search space within a host IID being able
   to be reduced, we may also be able to reduce the subnet ID space in a
   number of ways, by guessing likely address plan schemes, or using any
   complementary clues that might exist from other sources or

   Address plans might include use of subnets which:

   o  Run from low ID upwards, e.g. 2001:db8:0::/64, 2001:db8:1::/64,

   o  Use building numbers, in hex or decimal form.

   o  Use VLAN numbers.

   o  Use IPv4 subnet number in a dual-stack target, e.g. a site with a
      /16 for IPv4 might use /24 subnets, and the IPv6 address plan may
      re-use the third byte as the IPv6 subnet ID.

   o  Use the service "colour", as defined for service-based prefix
      colouring, or semantic prefixes.  For example, a site using a
      specific colouring for a specific service such as VoIP may reduce
      the subnet ID search space for those devices.

   In general, any subnet ID address plan may convey information, or be
   based on known information, which may in turn be of advantage to an

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

   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 or challenge of the "problem" task is so small in
   the IPv4 world, that a "brute-force" attack is "good enough".
   However, the scale of the "address scanning" problem task is so large in
   IPv6, that 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 appropriately tuned
   "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".

   It should be noted that IPv6 network monitoring and management tools
   also need to build and maintain information about the hosts in their
   network.  Such systems can no longer scan internal systems in a
   reasonable time to build a database of connected systems.  Rather,
   such systems will need more efficient approaches, e.g. by polling
   network devices for data held about observed IP addresses, MAC
   addresses, physical ports used, etc.  Such an approach can also
   enhance address accountability, by mapping IPv4 and IPv6 addresses to
   observed MAC addresses.  This of course implies that any access
   control mechanisms for querying such network devices, e.g.  community
   strings for SNMP, should be set appropriately to avoid an attacker
   being able to gather address information remotely.

3.4.2.  Local IPv6 Network Scanners

   There are a variety of publicly-available local IPv6 network

   o  Current versions of nmap [nmap2012] implement this functionality functionality.

   o  THC's IPv6 Attack Toolkit [THC-IPV6] includes a tool (alive6) that
      implements this functionality functionality.

   o  SI6 Network's IPv6 Toolkit [IPv6-Toolkit] includes a tool (scan6)
      that implements this functionality 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] [RFC7217] 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.

   o  Enforce IPv6 packet filtering where applicable (see e.g.

   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

   o  Use the "default" /64 size IPv6 subnet prefixes.

   o  In general, avoid being predictable in the way addresses are

   It should be noted that some of the aforementioned mitigations are
   operational, while others depend on the availability of specific
   protocol features (such as [I-D.ietf-6man-stable-privacy-addresses] [RFC7217]) 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).  And one should always remember that security by obscurity
   is not a reasonable defence in itself; it may only be one (relatively
   small) layer in a broader security environment.

   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.  There is ongoing work in the IETF on
   extending mDNS, or at least DNS-based service discovery, to work
   across a whole site, rather than in just a single subnet, which will
   have associated security implications.

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
   where the addresses used at a site follow specific patterns,
   publishing just one address may lead to a threat upon the other

   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


   Attackers 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

   Basically, an attacker would walk the ip6.arpa zone corresponding to
   a target network (e.g. "" for
   "2001:db8:80::/32"), issuing queries for PTR records corresponding to
   the domain names "",
   "", etc.  If, say, there were PTR
   records for any hosts "starting" with the domain name
   "" (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.

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

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.

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, environment, a
   local attacker could become a passive listener of the routing
   protocol, to determine other valid subnets within that organization

11.  Conclusions

   In this document we have discussed issues around host-based scanning
   of IPv6 networks.  We have shown why a /64 host subnet may be more
   vulnerable to address-based scanning that might intuitively be
   thought, and how an attacker might reduce the target search space
   when scanning.

   We have described a number of mitigations against host-based
   scanning, including the replacement of traditional SLAAC with stable
   privacy-enhanced IIDs (which will require support from system
   vendors).  We have also offered some practical guidance, around the
   principle of avoiding having predictability in host addressing
   schemes.  Finally, examples of scanning approaches and tools are
   discussed in the Appendices.

   While most early IPv6-enabled networks remain dual-stack, they are
   more likely to be scanned and attacked over IPv4 transport, and one
   may argue that the IPv6-specific considerations discussed here are
   not of an immediate concern.  However, an early IPv6 deployment
   within a
   local dual-stack network may be seen by an attacker could become as a passive listener
   potentially "easier" target, if the implementation of security
   policies are not as strict for IPv6 (for whatever reason).  As and
   when IPv6-only networks become more common, the routing
   protocol, to determine other valid subnets within that organization

11. considerations in
   this document will be of much greater importance.

12.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an


13.  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)

   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.


14.  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


15.  References


15.1.  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

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795, January

   [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.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC6762]  Cheshire, S.

   [RFC7136]  Carpenter, B. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", Jiang, "Significance of IPv6
              Interface Identifiers", RFC 6763, 7136, February 2013.

   [I-D.ietf-6man-stable-privacy-addresses] 2014.

   [RFC7217]  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. RFC 7217, April 2014.

15.2.  Informative References

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795, January

   [RFC4890]  Davies, E. and J. Mohacsi, "Recommendations for Filtering
              ICMPv6 Messages in Firewalls", RFC 4890, May 2007.

   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning", RFC
              5157, March 2008.

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583, March 2012.

   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC
              5157, March 2008. 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

              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.

              Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu,
              A., and A. Yourtchenko, "Analysis of the 64-bit Boundary
              in IPv6 Addressing", draft-ietf-6man-why64-01 (work in
              progress), May 2014.

              Gont, F., Cooper, A., Thaler, D., and W. Will,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-00 (work in progress),
              January 2014.

              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-ietf-6man-ipv6-address-generation-privacy-01 (work
              in progress), February 2014.

              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)", UK Centre for the Protection of
              National Infrastructure, (available on request).

              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/

              Malone, D., "Observations of IPv6 Addresses", Passive and
              Active Measurement Conference (PAM 2008, LNCS 4979), April

              Poettering, L., "mdns-scan(1) manual page", 2012,

              Fyodor, , "nmap - Network exploration tool and security /
              port scanner", 2012, <http://insecure.org>.

              VirtualBox, , "Oracle VM VirtualBox User Manual, version
              4.1.2", August 2011, <http://www.virtualbox.org>.

              vmware, , "Setting a static MAC address for a virtual
              NIC", vmware Knowledge Base, August 2011,

              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>. <http://mailman.nanog.org/pipermail/

              Gont, F., "Results of a Security Assessment of the
              Internet Protocol version 6 (IPv6)", DEEPSEC 2011
              Conference, Vienna, Austria, November 2011, 2011,

              Gont, F., "IPv6 Network Reconnaissance: Theory &
              Practice", LACSEC 2013 Conference, Medellin, Colombia, May
              2013, 2013, <http://www.si6networks.com/presentations/

              Ford, M., "IPv6 Address Analysis - Privacy In, Transition
              Out", 2013, <http://www.internetsociety.org/blog/2013/05/

              "THC-IPV6", <http://www.thc.org/thc-ipv6/>.

              "SI6 Networks' IPv6 Toolkit",

              "BitTorrent", <http://en.wikipedia.org/wiki/BitTorrent>.

              van Dijk, P., "Finding v6 hosts by efficiently mapping
              ip6.arpa", 2012, <http://7bits.nl/blog/2012/03/26/

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

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. packets (see e.g.

      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),

   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

   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

   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 an 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-

   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.

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 "temporary 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.

   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

   The scan6 tool of [IPv6-Toolkit] implements all these techniques/

Authors' Addresses

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Tim Chown
   University of Southampton
   Southampton , Hampshire   SO17 1BJ
   United Kingdom

   Email: tjc@ecs.soton.ac.uk