Network Reconnaissance in IPv6 Networks
RFC 7707
| Document | Type |
RFC - Informational
(March 2016; Errata)
Obsoletes RFC 5157
|
|
|---|---|---|---|
| Authors | Fernando Gont , Tim Chown | ||
| Last updated | 2020-01-21 | ||
| Replaces | draft-gont-opsec-ipv6-host-scanning | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized with errata bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Gunter Van de Velde | ||
| Shepherd write-up | Show (last changed 2015-05-30) | ||
| IESG | IESG state | RFC 7707 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Joel Jaeggli | ||
| Send notices to | (None) | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | No IANA Actions | ||
Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 7707 Huawei Technologies
Obsoletes: 5157 T. Chown
Category: Informational Jisc
ISSN: 2070-1721 March 2016
Network Reconnaissance in IPv6 Networks
Abstract
IPv6 offers a much larger address space than that of its IPv4
counterpart. An IPv6 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 fewer unique addresses. As a
result, it is widely assumed that it would take a tremendous effort
to perform address-scanning attacks against IPv6 networks; therefore,
IPv6 address-scanning attacks have been considered unfeasible. This
document formally obsoletes RFC 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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7707.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements for the Applicability of Network Reconnaissance
Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IPv6 Address Scanning . . . . . . . . . . . . . . . . . . . . 6
4.1. Address Configuration in IPv6 . . . . . . . . . . . . . . 6
4.1.1. Stateless Address Autoconfiguration (SLAAC) . . . . . 6
4.1.2. Dynamic Host Configuration Protocol for IPv6 (DHCPv6) 11
4.1.3. Manually Configured Addresses . . . . . . . . . . . . 12
4.1.4. IPv6 Addresses Corresponding to
Transition/Coexistence Technologies . . . . . . . . . 14
4.1.5. IPv6 Address Assignment in Real-World Network
Scenarios . . . . . . . . . . . . . . . . . . . . . . 14
4.2. IPv6 Address Scanning of Remote Networks . . . . . . . . 17
4.2.1. Reducing the Subnet ID Search Space . . . . . . . . . 18
4.3. IPv6 Address Scanning of Local Networks . . . . . . . . . 19
4.4. Existing IPv6 Address-Scanning Tools . . . . . . . . . . 20
4.4.1. Remote IPv6 Network Address Scanners . . . . . . . . 20
4.4.2. Local IPv6 Network Address Scanners . . . . . . . . . 21
4.5. Mitigations . . . . . . . . . . . . . . . . . . . . . . . 21
4.6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 22
5. Alternative Methods to Glean IPv6 Addresses . . . . . . . . . 23
5.1. Leveraging the Domain Name System (DNS) for Network
Reconnaissance . . . . . . . . . . . . . . . . . . . . . 23
5.1.1. DNS Advertised Hosts . . . . . . . . . . . . . . . . 23
5.1.2. DNS Zone Transfers . . . . . . . . . . . . . . . . . 23
5.1.3. DNS Brute Forcing . . . . . . . . . . . . . . . . . . 23
5.1.4. DNS Reverse Mappings . . . . . . . . . . . . . . . . 24
5.2. Leveraging Local Name Resolution and Service Discovery
Services . . . . . . . . . . . . . . . . . . . . . . . . 24
5.3. Public Archives . . . . . . . . . . . . . . . . . . . . . 25
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5.4. Application Participation . . . . . . . . . . . . . . . . 25
5.5. Inspection of the IPv6 Neighbor Cache and Routing Table . 25
5.6. Inspection of System Configuration and Log Files . . . . 26
5.7. Gleaning Information from Routing Protocols . . . . . . . 26
5.8. Gleaning Information from IP Flow Information Export
(IPFIX) . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.9. Obtaining Network Information with traceroute6 . . . . . 26
5.10. Gleaning Information from Network Devices Using SNMP . . 27
5.11. Obtaining Network Information via Traffic Snooping . . . 27
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. Implementation of a Full-Fledged IPv6 Address-
Scanning Tool . . . . . . . . . . . . . . . . . . . 34
A.1. Host-Probing Considerations . . . . . . . . . . . . . . . 34
A.2. Implementation of an IPv6 Local Address-Scanning Tool . . 35
A.3. Implementation of an IPv6 Remote Address-Scanning Tool . 36
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
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 hosts
divided by the number of addresses) of typical IPv6 subnetworks, when
compared to their IPv4 counterparts. [RFC5157] describes how this
significantly lower IPv6 host density is likely to make classic
network address-scanning attacks 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 more than five years of additional IPv6
deployment experience, this document formally obsoletes RFC 5157. It
emphasizes that while address-scanning attacks are less feasible,
they may, with appropriate heuristics, remain possible. At the time
that RFC 5157 was written, observed address-scanning attacks 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 address-scanning attacks in IPv6 networks, and it
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explores a number of additional techniques that can be employed for
IPv6 network reconnaissance. Practical examples and guidance are
also 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 twofold.
Section 4 analyzes the feasibility of address-scanning attacks (e.g.,
ping sweeps) in IPv6 networks and explores a number of possible
improvements to such techniques. Appendix A describes how the
aforementioned analysis can be leveraged to produce address-scanning
tools (e.g., for penetration testing purposes). Finally, the rest of
this document discusses a number of miscellaneous techniques that
could be leveraged for IPv6 network reconnaissance.
2. Conventions
Throughout this document, we consider that bits are numbered from
left to right, starting at 0, and that bytes are numbered from left
to right, starting at 0.
3. Requirements for the Applicability of Network Reconnaissance
Techniques
Throughout this document, a number of network reconnaissance
techniques are discussed. Each of these techniques has 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 (or similar access credentials) 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.
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+---------------------------------------------+----------+----------+
| Technique | Local | Login |
| | access | access |
+---------------------------------------------+----------+----------+
| Remote Address Scanning (Section 4.2) | No | No |
+---------------------------------------------+----------+----------+
| Local Address Scanning (Section 4.3) | Yes | No |
+---------------------------------------------+----------+----------+
| DNS Advertised Hosts (Section 5.1.1) | No | No |
+---------------------------------------------+----------+----------+
| DNS Zone Transfers (Section 5.1.2) | No | No |
+---------------------------------------------+----------+----------+
| DNS Brute Forcing (Section 5.1.3) | No | No |
+---------------------------------------------+----------+----------+
| DNS Reverse Mappings (Section 5.1.4) | No | No |
+---------------------------------------------+----------+----------+
| Leveraging Local Name Resolution and | Yes | No |
| Service Discovery Services (Section 5.2) | | |
+---------------------------------------------+----------+----------+
| Public Archives (Section 5.3) | No | No |
+---------------------------------------------+----------+----------+
| Application Participation (Section 5.4) | No | No |
+---------------------------------------------+----------+----------+
| Inspection of the IPv6 Neighbor Cache and | No | Yes |
| Routing Table (Section 5.5) | | |
+---------------------------------------------+----------+----------+
| Inspecting System Configuration and Log | No | Yes |
| Files (Section 5.6) | | |
+---------------------------------------------+----------+----------+
| Gleaning Information from Routing Protocols | Yes | No |
| (Section 5.7) | | |
+---------------------------------------------+----------+----------+
| Gleaning Information from IP Flow | No | Yes |
| Information Export (IPFIX) (Section 5.8) | | |
+---------------------------------------------+----------+----------+
| Obtaining Network Information with | No | No |
| traceroute6 (Section 5.9) | | |
+---------------------------------------------+----------+----------+
| Gleaning Information from Network Devices | No | Yes |
| Using SNMP (Section 5.10) | | |
+---------------------------------------------+----------+----------+
| Obtaining Network Information via Traffic | Yes | No |
| Snooping (Section 5.11) | | |
+---------------------------------------------+----------+----------+
Table 1: Requirements for the Applicability of
Network Reconnaissance Techniques
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4. IPv6 Address Scanning
This section discusses how traditional address-scanning techniques
(e.g., "ping sweeps") apply to IPv6 networks. Section 4.1 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-
scanning attacks. Section 4.2 discusses IPv6 address scanning of
remote networks. Section 4.3 discusses IPv6 address scanning of
local networks. Section 4.4 discusses existing IPv6 address-scanning
tools. Section 4.5 provides advice on how to mitigate IPv6 address-
scanning attacks. Finally, Appendix A discusses how the insights
obtained in the following subsections can be incorporated into a
fully fledged IPv6 address-scanning tool.
4.1. Address Configuration in IPv6
IPv6 incorporates two automatic address-configuration mechanisms:
Stateless Address Autoconfiguration (SLAAC) [RFC4862] and Dynamic
Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315]. Support for
SLAAC for automatic address configuration is mandatory, while support
for DHCPv6 is optional -- however, most current versions of general-
purpose operating systems support both. In addition to automatic
address configuration, 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.
4.1.1. Stateless Address Autoconfiguration (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.
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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.
4.1.1.1. Interface Identifiers Embedding IEEE Identifiers
The traditional SLAAC IIDs are 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 Organizationally Unique
Identifier (OUI) and the rest of the Ethernet address.
For example, the Media Access Control (MAC) address 00:1b:38:83:88:3c
would lead to the IID 021b:38ff:fe83:883c.
A number of considerations should be made about these identifiers.
Firstly, one 16-bit word (bytes 3-4) of the resulting address always
has a fixed value (0xfffe), thus reducing the search space for the
IID. Secondly, the high-order three bytes of the IID 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 are 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.
NOTE:
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).
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Furthermore, if just one host address is detected or known within a
subnet, it is not unlikely that, if systems were ordered in a batch,
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.
NOTE:
[RFC7136] notes that all bits of an IID should be treated as
"opaque" bits. Furthermore, [DEFAULT-IIDS] is currently in the
process of changing the default IID generation scheme to align
with [RFC7217] (as described below in Section 4.1.1.5), such that
IIDs are semantically opaque and do not follow any patterns.
Therefore, the traditional IIDs based on link-layer addresses are
expected to become less common over time.
4.1.1.2. Interface Identifiers of Virtualization Technologies
IIDs resulting from virtualization technologies can be considered a
specific subcase of IIDs embedding IEEE identifiers (please see
Section 4.1.1.1): they employ IEEE identifiers, but part of the IID
has specific patterns. The following subsections describe IIDs of
some popular virtualization technologies.
4.1.1.2.1. VirtualBox
All automatically generated MAC addresses in VirtualBox virtual
machines employ the OUI 08:00:27 [VBox2011]. This means that all
addresses resulting from traditional SLAAC will have an IID of the
form a00:27ff:feXX:XXXX, thus effectively reducing the IID search
space from 64 bits to 24 bits.
4.1.1.2.2. VMware ESX Server
The VMware ESX server (versions 1.0 to 2.5) provides yet a more
interesting example. Automatically generated MAC addresses have the
following pattern [vmesx2011]:
1. The OUI is set to 00:05:69.
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.
3. The final 8 bits of the MAC address are set to a hash value based
on the name of the virtual machine's configuration file.
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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 the VMware
ESX server employ the OUI 00:50:56, with the low-order three bytes of
the MAC address being in the range 00:00:00-3F:FF:FF (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.
4.1.1.2.3. VMware vSphere
VMware vSphere [vSphere] supports these default MAC address
generation algorithms:
o Generated addresses
* Assigned by the vCenter server
* Assigned by the ESXi host
o Manually configured addresses
By default, MAC addresses assigned by the vCenter server use the OUI
00:50:56 and have the format 00:50:56:XX:YY:ZZ, where XX is
calculated as (0x80 + vCenter Server ID (in the range 0x00-0x3F)),
and XX and YY are random two-digit hexadecimal numbers. Thus, the
possible IID range is 00:50:56:80:00:00-00:50:56:BF:FF:FF; therefore,
the search space for the resulting SLAAC addresses will be 22 bits.
MAC addresses generated by the ESXi host use the OUI 00:0C:29 and
have the format 00:0C:29:XX:YY:ZZ, where XX, YY, and ZZ are the last
three octets in hexadecimal format of the virtual machine Universally
Unique Identifier (UUID) (based on a hash calculated with the UUID of
the ESXi physical machine and the path to a configuration file).
Thus, the MAC addresses will be in the range
00:0C:29:00:00:00-00:0C:29:FF:FF:FF; therefore, the search space for
the resulting SLAAC addresses will be 24 bits.
Finally, manually configured MAC addresses employ the OUI 00:50:56,
with the low-order three bytes being in the range 00:00:00-3F:FF:FF
(to avoid conflicts with other VMware products). Therefore, the
resulting MAC addresses will be in the range
00:50:56:00:00:00-00:50:56:3F:FF:FF, and the search space for the
corresponding SLAAC addresses will be 22 bits.
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4.1.1.3. Temporary Addresses
Privacy concerns [Gont-DEEPSEC2011] [RFC7721] regarding IIDs
embedding IEEE identifiers led to the introduction of "Privacy
Extensions for Stateless Address Autoconfiguration in IPv6"
[RFC4941], also known as "temporary addresses" or "privacy
addresses". Essentially, "temporary addresses" produce random
addresses by concatenating a random identifier to the
autoconfiguration IPv6 prefix advertised in a Router Advertisement
message.
NOTE:
In addition to their unpredictability, these addresses are
typically short-lived, such that even if an attacker were to learn
of one of these addresses, they would be of use for a limited
period of time. A typical implementation may keep a temporary
address preferred for 24 hours, and configured but deprecated for
seven days.
It is important to note that "temporary addresses" are generated in
addition to the stable addresses [RFC7721] (such as the traditional
SLAAC addresses based on IEEE identifiers): stable SLAAC addresses
are meant to be employed for "server-like" inbound communications,
while "temporary addresses" are meant to be employed for "client-
like" outbound communications. This means that implementation/use of
"temporary addresses" does not prevent an attacker from leveraging
the predictability of stable SLAAC addresses, since "temporary
addresses" are generated in addition to (rather than as a replacement
of) the stable SLAAC addresses (such as those derived from IEEE
identifiers).
The benefit that temporary addresses offer in this context is that
they reduce the exposure of the host addresses to any third parties
that may observe traffic sent from a host where temporary addresses
are enabled and used by default. But, in the absence of firewall
protection for the host, its stable SLAAC address remains liable to
be scanned from off-site.
4.1.1.4. Constant, Semantically Opaque IIDs
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 believed to be an implementation of RFC
4941 [RFC4941], but without regenerating the addresses over time.
The resulting IIDs are constant across system bootstraps, and also
constant across networks.
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Assuming no flaws in the aforementioned algorithm, this scheme would
remove any patterns from the SLAAC addresses.
NOTE:
However, since the resulting IIDs are constant across networks,
these addresses may still be leveraged for host-tracking purposes
[RFC7217] [RFC7721].
The benefit of this scheme is thus that the host may be less readily
detected by applying heuristics to an address-scanning attack, but,
in the absence of concurrent use of temporary addresses, the host is
liable to be tracked across visited networks.
4.1.1.5. Stable, Semantically Opaque IIDs
In response to the predictability issues discussed in Section 4.1.1.1
and the privacy issues discussed in [RFC7721], the IETF has
standardized (in [RFC7217]) a method for generating IPv6 IIDs to be
used with IPv6 SLAAC, such that addresses configured using this
method are stable within each subnet, but the IIDs change when hosts
move from one subnet to another. The aforementioned method is meant
to be an alternative to generating IIDs 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 IIDs based on IEEE
identifiers) eliminates any patterns from the IID, thus benefiting
user privacy and reducing the ease with which addresses can be
scanned.
4.1.2. Dynamic Host Configuration Protocol for IPv6 (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 assigning addresses sequentially, from a specific
range. In such cases, addresses tend to be predictable.
NOTE:
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. [RFC5157]
recommended that DHCPv6 instead issue addresses randomly from a large
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pool; that advice is repeated here. [IIDS-DHCPv6] specifies an
algorithm that can be employed by DHCPv6 servers to produce stable
addresses that do not follow any specific pattern, thus resulting in
an IID search space of 64 bits.
4.1.3. 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 - 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::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::bad:cafe)
Each of these patterns is discussed in detail in the following
subsections.
4.1.3.1. Low-Byte Addresses
The most common form of low-byte addresses is that in which all 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
(from the right to left) 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
(from right to left) is set to a small value in the range
0x0000:0x00ff, while the lowest-order 16-bit word (from right to
left) varies in the range 0x0000:0xffff. It should be noted that all
of these address patterns are generally referred to as "low-byte
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addresses", even when, strictly speaking, it is 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).
4.1.3.2. 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 address notation of the form
2001:db8::192.0.2.1). However, it is also common for administrators
to encode each of the bytes of the IPv4 address in each of the 16-bit
words of the IID (as in, e.g., 2001:db8::192:0:2:1).
Therefore, 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).
4.1.3.3. Service-Port Addresses
Addresses following this pattern include the service port (e.g., 80
for HTTP) in the lowest-order byte of the IID and have the rest of
the bytes of the IID set to zero. There are a number of variants for
this address pattern:
o The lowest-order 16-bit word (from right to left) may contain the
service port, and the second lowest-order 16-bit word (from right
to left) may be set to a number in the range 0x0000-0x00ff (as in,
e.g., 2001:db8::1:80).
o The lowest-order 16-bit word (from right to left) may be set to a
value in the range 0x0000-0x00ff, while the second lowest-order
16-bit word (from right to left) 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 this pattern is, in the worst-case scenario,
10 * 2^11.
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4.1.3.4. Wordy Addresses
Since the 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::bad:cafe).
Addresses following this pattern are likely to be explored by means
of "dictionary attacks"; therefore, computing the corresponding
search space is not straightforward.
4.1.4. IPv6 Addresses Corresponding to Transition/Coexistence
Technologies
Some transition/coexistence 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 IID is
generated from the IPv4 address observed at a Teredo server along
with a UDP port number.
For obvious reasons, the search space for these addresses will depend
on the specific transition/coexistence technology being employed.
4.1.5. IPv6 Address Assignment in Real-World Network Scenarios
Figures 1, 2, and 3 provide a summary of the results obtained by
[Gont-LACSEC2013] when measuring the address patterns employed by web
servers, name servers, and mail servers, respectively. Figure 4
provides a rough summary of the results obtained by [Malone2008] for
IPv6 routers. Figure 5 provides a summary of the results obtained by
[Ford2013] for clients.
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+---------------+------------+
| Address type | Percentage |
+---------------+------------+
| IEEE-based | 1.44% |
+---------------+------------+
| Embedded-IPv4 | 25.41% |
+---------------+------------+
| Embedded-Port | 3.06% |
+---------------+------------+
| ISATAP | 0.00% |
+---------------+------------+
| Low-byte | 56.88% |
+---------------+------------+
| Byte-pattern | 6.97% |
+---------------+------------+
| Randomized | 6.24% |
+---------------+------------+
Figure 1: Measured Web Server Addresses
+---------------+------------+
| Address type | Percentage |
+---------------+------------+
| IEEE-based | 0.67% |
+---------------+------------+
| Embedded-IPv4 | 22.11% |
+---------------+------------+
| Embedded-Port | 6.48% |
+---------------+------------+
| ISATAP | 0.00% |
+---------------+------------+
| Low-byte | 56.58% |
+---------------+------------+
| Byte-pattern | 11.07% |
+---------------+------------+
| Randomized | 3.09% |
+---------------+------------+
Figure 2: Measured Name Server Addresses
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+---------------+------------+
| Address type | Percentage |
+---------------+------------+
| IEEE-based | 0.48% |
+---------------+------------+
| Embedded-IPv4 | 4.02% |
+---------------+------------+
| Embedded-Port | 1.07% |
+---------------+------------+
| ISATAP | 0.00% |
+---------------+------------+
| Low-byte | 92.65% |
+---------------+------------+
| Byte-pattern | 1.20% |
+---------------+------------+
| Randomized | 0.59% |
+---------------+------------+
Figure 3: Measured Mail Server Addresses
+--------------+------------+
| Address type | Percentage |
+--------------+------------+
| Low-byte | 70.00% |
+--------------+------------+
| IPv4-based | 5.00% |
+--------------+------------+
| SLAAC | 1.00% |
+--------------+------------+
| Wordy | <1.00% |
+--------------+------------+
| Randomized | <1.00% |
+--------------+------------+
| Teredo | <1.00% |
+--------------+------------+
| Other | <1.00% |
+--------------+------------+
Figure 4: Measured Router Addresses
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+---------------+------------+
| 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% |
+---------------+------------+
Figure 5: Measured Client Addresses
NOTE:
"ISATAP" stands for "Intra-Site Automatic Tunnel Addressing
Protocol" [RFC5214].
It should be clear from these measurements that a very high
percentage of host and router addresses follow very specific
patterns.
Figure 5 shows that while around 70% of clients observed in this
measurement appear to be using temporary addresses, a significant
number of clients still expose IEEE-based addresses and addresses
using embedded IPv4 (thus also revealing IPv4 addresses). Besides,
as noted in Section 4.1.1.3, temporary addresses are employed along
with stable IPv6 addresses; thus, hosts employing a temporary address
may still be the subject of address-scanning attacks that target
their stable address(es).
[ADDR-ANALYSIS] contains a spatial and temporal analysis of IPv6
addresses corresponding to clients and routers.
4.2. IPv6 Address Scanning of Remote Networks
Although attackers have been able to get away with "brute-force"
address-scanning attacks in IPv4 networks (thanks to the lesser
search space), successfully performing a brute-force address-scanning
attack 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 4.1 to reduce the IPv6 address search space.
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IPv6 address scanning of remote 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 a large number of entries in
the Neighbor Cache; hence, these address-scanning attacks may have
the side effect of resulting in a Denial-of-Service (DoS) attack
[CPNI-IPv6] [RFC6583].
[RFC7421] discusses the "default" /64 boundary for host subnets and
the assumptions surrounding it. While there are reports of sites
implementing IPv6 subnets of size /112 or smaller to reduce concerns
about the above attack, such smaller subnets are likely to make
address-scanning attacks more feasible, in addition to encountering
the issues with non-/64 host subnets discussed in [RFC7421].
4.2.1. Reducing the Subnet ID Search Space
When address 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 IPv6 /64 subnets to
be scanned.
However, in the same way the search space for the IID can be reduced,
we may also be able to reduce the subnet ID search space in a number
of ways, by guessing likely address plan schemes or using any
complementary clues that might exist from other sources or
observations. For example, there are a number of documents available
online (e.g., [RFC5375]) that provide recommendations for the
allocation of address space, which address various operational
considerations, including Regional Internet Registry (RIR) assignment
policy, ability to delegate reverse DNS zones to different servers,
ability to aggregate routes efficiently, address space preservation,
ability to delegate address assignment within the organization,
ability to add/allocate new sites/prefixes to existing entities
without updating Access Control Lists (ACLs), and ability to
de-aggregate and advertise subspaces via various Autonomous System
(AS) interfaces.
Address plans might include use of subnets that:
o Run from low ID upwards, e.g., 2001:db8:0::/64, 2001:db8:1::/64,
etc.
o Use building numbers, in hexadecimal or decimal form.
o Use Virtual Local Area Network (VLAN) numbers.
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o Use an 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 reuse the third byte of the IPv4 address as the IPv6
subnet ID.
o Use the service "color", as defined for service-based prefix
coloring, or semantic prefixes. For example, a site using a
specific coloring for a specific service such as Voice over IP
(VoIP) may reduce the subnet ID search space for those devices.
The net effect is that the address space of an organization may be
highly structured, and allocations of individual elements within this
structure may be predictable once other elements are known.
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
attacker.
4.3. IPv6 Address Scanning of Local Networks
IPv6 address scanning in Local Area Networks (LANs) 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.
NOTE:
While a number of other network reconnaissance vectors (such as
network snooping, leveraging Neighbor Discovery traffic, etc.) are
available when scanning a local network, 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 Internet Control Message Protocol
version 6 (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
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interface (as specified by the IPv6 address-selection rules
[RFC6724]). However, sending multiple probe packets, with each
packet employing source addresses from different prefixes, typically
helps to overcome this limitation.
4.4. Existing IPv6 Address-Scanning Tools
4.4.1. Remote IPv6 Network Address Scanners
IPv4 address-scanning tools have traditionally carried out their task
by probing an entire address range (usually the entire address range
comprised by the target subnetwork). One might argue that the reason
for which they have been able to get away with such somewhat
"rudimentary" techniques is that the scale or challenge of the task
is so small in the IPv4 world that a "brute-force" attack is "good
enough". However, the scale of the "address-scanning" 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 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
patterns 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 4.1 of this document.
Clearly, a limitation of many of the currently available tools for
IPv6 address scanning is that they lack an appropriately tuned
"heuristics engine" that can help reduce the search space, such that
the problem of IPv6 address scanning becomes tractable.
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.
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4.4.2. Local IPv6 Network Address Scanners
There are a variety of publicly available local IPv6 network address-
scanners:
o Current versions of nmap [nmap2015] implement this functionality.
o The Hacker's Choice (THC) IPv6 Attack Toolkit [THC-IPV6] includes
a tool (alive6) that implements this functionality.
o SI6 Network's IPv6 Toolkit [IPv6-Toolkit] includes a tool (scan6)
that implements this functionality.
4.5. Mitigations
IPv6 address-scanning attacks can be mitigated in a number of ways.
A non-exhaustive list of the possible mitigations includes:
o Employing [RFC7217] (stable, semantically opaque IIDs) in
replacement of addresses based on IEEE identifiers, such that any
address patterns are eliminated.
o Employing Intrusion Prevention Systems (IPSs) at the perimeter.
o Enforcing IPv6 packet filtering where applicable (see, e.g.,
[RFC4890]).
o Employing manually configured MAC addresses if virtual machines
are employed and "resistance" to address-scanning attacks is
deemed desirable, such that even if the virtual machines employ
IEEE-derived IIDs, they are generated from non-predictable MAC
addresses.
o Avoiding use of sequential addresses when using DHCPv6. Ideally,
the DHCPv6 server would allocate random addresses from a large
pool (see, e.g., [IIDS-DHCPv6]).
o Using the "default" /64 size IPv6 subnet prefixes.
o In general, avoiding being predictable in the way addresses are
assigned.
It should be noted that some of the aforementioned mitigations are
operational, while others depend on the availability of specific
protocol features (such as [RFC7217]) on the corresponding nodes.
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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 defense 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 Requests
(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).
NOTE:
[SMURF-AMPLIFIER] proposed such an 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 were mitigated, an attacker could still rely on, e.g.,
protocols employed for the so-called "service discovery protocols"
(see Section 5.2) or eventually rely on network snooping as a 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.
4.6. Conclusions
In the previous subsections, we have shown why a /64 host subnet may
be more vulnerable to address-based scanning than might intuitively
be thought and how an attacker might reduce the target search space
when performing an address-scanning attack.
We have described a number of mitigations against address-scanning
attacks, including the replacement of traditional SLAAC with stable
semantically opaque IIDs (which requires support from system
vendors). We have also offered some practical guidance in regard to
the principle of avoiding predictability in host addressing schemes.
Finally, examples of address-scanning approaches and tools are
discussed in the appendices.
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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 dual-stack network may be seen by an attacker as a
potentially "easier" target if the implementation of security
policies is not as strict for IPv6 (for whatever reason). As
IPv6-only networks become more common, the above considerations will
be of much greater importance.
5. Alternative Methods to Glean IPv6 Addresses
The following subsections describe alternative methods by which an
attacker might attempt to glean IPv6 addresses for subsequent
probing.
5.1. Leveraging the Domain Name System (DNS) for Network Reconnaissance
5.1.1. DNS Advertised Hosts
Any systems that are "published" in the DNS, e.g., Mail Exchange (MX)
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 an attack upon the other
nodes.
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).
5.1.2. DNS Zone Transfers
A DNS zone transfer (DNS query type "AXFR") [RFC1034] [RFC1035] 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.
5.1.3. DNS Brute Forcing
Attackers may employ DNS brute-forcing techniques by testing for the
presence of DNS AAAA records against commonly used host names.
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5.1.4. DNS Reverse Mappings
[van-Dijk] describes an interesting technique that employs DNS
reverse mappings for network reconnaissance. 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::/48"), 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.",
"1.0.8.0.0.8.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.
NOTE:
Some name servers, incorrectly implementing the DNS protocol,
reply NXDOMAIN instead of NODATA (NOERROR=0 and ANSWER=0) when
encountering a domain without any resource records but that has
child domains, something that is very common in ip6.arpa (these
domains are called ENT for Empty Non-Terminals; see [RFC7719]).
When scanning ip6.arpa, this behavior may slow down or completely
prevent the exploration of ip6.arpa. Nevertheless, since such
behavior is wrong (see [NXDOMAIN-DEF]), one cannot rely on it to
"secure" ip6.arpa against tree walking.
[IPv6-RDNS] analyzes different approaches and considerations for
ISPs in managing the ip6.arpa zone for IPv6 address space assigned
to many customers, which may affect the technique described in
this section.
5.2. Leveraging Local Name Resolution and Service Discovery Services
A number of protocols allow for unmanaged local name resolution and
service. For example, mDNS [RFC6762] and DNS Service Discovery (DNS-
SD) [RFC6763], or Link-Local Multicast Name Resolution (LLMNR)
[RFC4795], are examples of such protocols.
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NOTE:
Besides the Graphical User Interfaces (GUIs) included in products
supporting such protocols, command-line tools such as mdns-scan
[mdns-scan] and mzclient [mzclient] can help discover IPv6 hosts
employing mDNS/DNS-SD.
5.3. Public Archives
Public mailing-list archives or Usenet news messages archives may
prove to be 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.
5.4. Application Participation
Peer-to-peer applications often include some centralized server that
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.
5.5. 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. Source
Address Validation Improvement (SAVI) [RFC6620] also builds a cache
of IPv6 and link-layer addresses (without actively participating in
the Neighbor Discovery packet exchange) and hence is another source
of similar information.
These data structures could be inspected via either "login" access or
SNMP. While this requirement 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" or SNMP
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.
Section 2.5.1.4 of [OPSEC-IPv6] discusses additional considerations
for the inspection of the IPv6 Neighbor Cache.
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5.6. 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-
like systems, Secure Shell (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 to be a useful source 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].
5.7. 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/prefixes and some router
addresses within that organization [V6-WORMS].
5.8. Gleaning Information from IP Flow Information Export (IPFIX)
IPFIX [RFC7012] can aggregate the flows by source addresses and hence
may be leveraged for obtaining a list of "active" IPv6 addresses.
Additional discussion of IPFIX can be found in Section 2.5.1.2 of
[OPSEC-IPv6].
5.9. Obtaining Network Information with traceroute6
IPv6 traceroute [traceroute6] and similar tools (such as path6 from
[IPv6-Toolkit]) can be employed to find router addresses and valid
network prefixes.
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5.10. Gleaning Information from Network Devices Using SNMP
SNMP can be leveraged to obtain information from a number of data
structures such as the Neighbor Cache [RFC4861], the routing table,
and the SAVI [RFC6620] cache of IPv6 and link-layer addresses. SNMP
access should be secured, such that unauthorized access to the
aforementioned information is prevented.
5.11. Obtaining Network Information via Traffic Snooping
Snooping network traffic can help in discovering active nodes in a
number of ways. Firstly, each captured packet will reveal the source
and destination of the packet. Secondly, the captured traffic may
correspond to network protocols that transfer information such as
host or router addresses, network topology information, etc.
6. Conclusions
This document explores the topic of network reconnaissance in IPv6
networks. It analyzes the feasibility of address-scanning attacks in
IPv6 networks and shows that the search space for such attacks is
typically much smaller than the one traditionally assumed (64 bits).
Additionally, this document 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.
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.
7. Security Considerations
This document reviews methods by which addresses of hosts within IPv6
subnets can be determined. As such, it raises no new security
concerns.
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8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
DOI 10.17487/RFC4380, February 2006,
<http://www.rfc-editor.org/info/rfc4380>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
DOI 10.17487/RFC5214, March 2008,
<http://www.rfc-editor.org/info/rfc5214>.
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[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, DOI 10.17487/RFC6620, May 2012,
<http://www.rfc-editor.org/info/rfc6620>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.
[RFC7012] Claise, B., Ed. and B. Trammell, Ed., "Information Model
for IP Flow Information Export (IPFIX)", RFC 7012,
DOI 10.17487/RFC7012, September 2013,
<http://www.rfc-editor.org/info/rfc7012>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <http://www.rfc-editor.org/info/rfc7136>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<http://www.rfc-editor.org/info/rfc7217>.
8.2. Informative References
[ADDR-ANALYSIS]
Plonka, D. and A. Berger, "Temporal and Spatial
Classification of Active IPv6 Addresses", ACM Internet
Measurement Conference (IMC), Tokyo, Japan, Pages 509-522,
DOI 10.1145/2815675.2815678, October 2015,
<http://conferences2.sigcomm.org/imc/2015/papers/
p509.pdf>.
[BitTorrent]
Wikipedia, "BitTorrent", November 2015,
<https://en.wikipedia.org/w/
index.php?title=BitTorrent&oldid=690381343>.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
Gont & Chown Informational [Page 29]
RFC 7707 IPv6 Reconnaissance March 2016
[DEFAULT-IIDS]
Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
Work in Progress, draft-ietf-6man-default-iids-10,
February 2016.
[Ford2013] Ford, M., "IPv6 Address Analysis - Privacy In, Transition
Out", May 2013,
<http://www.internetsociety.org/blog/2013/05/
ipv6-address-analysis-privacy-transition-out>.
[Gont-DEEPSEC2011]
Gont, F., "Results of a Security Assessment of the
Internet Protocol version 6 (IPv6)", DEEPSEC
Conference, Vienna, Austria, November 2011,
<http://www.si6networks.com/presentations/deepsec2011/
fgont-deepsec2011-ipv6-security.pdf>.
[Gont-LACSEC2013]
Gont, F., "IPv6 Network Reconnaissance: Theory &
Practice", LACSEC Conference, Medellin, Colombia, May
2013, <http://www.si6networks.com/presentations/lacnic19/
lacsec2013-fgont-ipv6-network-reconnaissance.pdf>.
[IIDS-DHCPv6]
Gont, F. and W. Liu, "A Method for Generating Semantically
Opaque Interface Identifiers with Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", Work in
Progress, draft-ietf-dhc-stable-privacy-addresses-02,
April 2015.
[IPV6-EXT-HEADERS]
Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", Work in Progress,
draft-ietf-v6ops-ipv6-ehs-in-real-world-02, December 2015.
[IPv6-RDNS]
Howard, L., "Reverse DNS in IPv6 for Internet Service
Providers", Work in Progress, draft-ietf-dnsop-isp-
ip6rdns-00, October 2015.
[IPv6-Toolkit]
SI6 Networks, "SI6 Networks' IPv6 Toolkit",
<http://www.si6networks.com/tools/ipv6toolkit>.
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RFC 7707 IPv6 Reconnaissance March 2016
[Malone2008]
Malone, D., "Observations of IPv6 Addresses", Passive and
Active Network Measurement (PAM 2008, LNCS 4979),
DOI 10.1007/978-3-540-79232-1_3, April 2008,
<http://www.maths.tcd.ie/~dwmalone/p/addr-pam08.pdf>.
[mdns-scan]
Poettering, L., "mdns-scan(1) Manual Page",
<http://manpages.ubuntu.com/manpages/precise/man1/
mdns-scan.1.html>.
[mzclient] Bockover, A., "Mono Zeroconf Project -- mzclient command-
line tool",
<http://www.mono-project.com/archived/monozeroconf/>.
[nmap2015] Lyon, Gordon "Fyodor", "Nmap 7.00", November 2015,
<http://insecure.org>.
[NXDOMAIN-DEF]
Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there
is nothing underneath", Work in Progress, draft-ietf-
dnsop-nxdomain-cut-00, December 2015.
[OPSEC-IPv6]
Chittimaneni, K., Kaeo, M., and E. Vyncke, "Operational
Security Considerations for IPv6 Networks", Work in
Progress, draft-ietf-opsec-v6-07, September 2015.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
DOI 10.17487/RFC4795, January 2007,
<http://www.rfc-editor.org/info/rfc4795>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<http://www.rfc-editor.org/info/rfc4890>.
[RFC5157] Chown, T., "IPv6 Implications for Network Scanning",
RFC 5157, DOI 10.17487/RFC5157, March 2008,
<http://www.rfc-editor.org/info/rfc5157>.
[RFC5375] Van de Velde, G., Popoviciu, C., Chown, T., Bonness, O.,
and C. Hahn, "IPv6 Unicast Address Assignment
Considerations", RFC 5375, DOI 10.17487/RFC5375, December
2008, <http://www.rfc-editor.org/info/rfc5375>.
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RFC 7707 IPv6 Reconnaissance March 2016
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583,
DOI 10.17487/RFC6583, March 2012,
<http://www.rfc-editor.org/info/rfc6583>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<http://www.rfc-editor.org/info/rfc7421>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.
[SMURF-AMPLIFIER]
Gont, F. and W. Liu, "Security Implications of IPv6
Options of Type 10xxxxxx", Work in Progress, draft-gont-
6man-ipv6-smurf-amplifier-03, March 2013.
[THC-IPV6] "THC-IPV6", <http://www.thc.org/thc-ipv6/>.
[traceroute6]
FreeBSD, "FreeBSD System Manager's Manual: traceroute6(8)
manual page", August 2009, <https://www.freebsd.org/cgi/
man.cgi?query=traceroute6>.
[V6-WORMS] Bellovin, S., Cheswick, B., and A. Keromytis, "Worm
propagation strategies in an IPv6 Internet", Vol. 31, No.
1, pp. 70-76, February 2006,
<https://www.cs.columbia.edu/~smb/papers/v6worms.pdf>.
[van-Dijk] van Dijk, P., "Finding v6 hosts by efficiently mapping
ip6.arpa", March 2012, <http://7bits.nl/blog/2012/03/26/
finding-v6-hosts-by-efficiently-mapping-ip6-arpa>.
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[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
(219)", VMware Knowledge Base, August 2011,
<http://kb.vmware.com/selfservice/microsites/
search.do?language=en_US&cmd=displayKC&externalId=219>.
[vSphere] VMware, "vSphere Networking", vSphere 5.5, Update 2,
September 2014, <http://pubs.vmware.com/
vsphere-55/topic/com.vmware.ICbase/PDF/
vsphere-esxi-vcenter-server-552-networking-guide.pdf>.
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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 the
implementation of a general (i.e., non-local) IPv6 address scanner.
A.1. Host-Probing Considerations
A number of factors should be considered when selecting the probe
packet 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 (see, e.g.,
[RFC4890]).
NOTE:
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 (meant to elicit an ICMPv6 Parameter
Problem error message).
Selecting an appropriate probe packet might help conceal the ongoing
attack, but it may also be actually necessary if host or network
configuration causes certain probe packets to be dropped.
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Some address-scanning tools (such as scan6 of [IPv6-Toolkit])
incorporate support for IPv6 extension headers. In some cases,
inserting some IPv6 extension headers in the probe packet may allow
some filtering policies or monitoring devices to be circumvented.
However, it may also result in the probe packets being dropped, as a
result of the widespread dropping of IPv6 packets that employ IPv6
extension headers (see [IPV6-EXT-HEADERS]).
Another factor to consider is the address-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 Neighbor Discovery
implementations, 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, an address-scanning attack
might result in a 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.
A.2. Implementation of an IPv6 Local Address-Scanning Tool
scan6 [IPv6-Toolkit] is a full-fledged IPv6 local address-scanning
tool, which has proven to be effective and efficient for the
discovery of IPv6 hosts on a local network.
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The scan6 tool operates (roughly) as follows:
1. The tool learns the local prefixes used for autoconfiguration and
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 from each of the addresses
"configured" in the previous step. Because of the different
source addresses, each probe packet 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
IID 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 IID is typically used with all the available
prefixes for the local network.
NOTE:
The aforementioned scheme can fail to discover some addresses for
some implementations. For example, Mac OS X employs IPv6
addresses embedding IEEE identifiers (rather than "temporary
addresses") when responding to packets destined to a link-local
multicast address, sourced from an on-link prefix.
A.3. Implementation of an 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).
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.
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o The tool can be instructed to scan for SLAAC addresses that employ
a specific IEEE OUI or set of OUIs corresponding to a specific
vector.
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 to scan 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. Furthermore, when given a target domain name or sample
IPv6 address for a given prefix, the tool will try to infer the
address pattern in use at the target network, and reduce the address
search space accordingly.
Acknowledgements
The authors would like to thank Ray Hunter, who provided valuable
text that was readily incorporated into Section 4.2.1 of this
document.
The authors would like to thank (in alphabetical order) Ivan Arce,
Alissa Cooper, Spencer Dawkins, Stephen Farrell, Wesley George, Marc
Heuse, Ray Hunter, Barry Leiba, Libor Polcak, Alvaro Retana, Tomoyuki
Sahara, Jan Schaumann, Arturo Servin, and Eric Vyncke for providing
valuable comments on earlier draft versions of this document.
Fernando Gont would like to thank Jan Zorz of Go6 Lab
<http://go6lab.si/> and Jared Mauch of NTT America for providing
access to systems and networks that were employed to perform
experiments and measurements that helped to improve this document.
Additionally, he would like to thank SixXS <https://www.sixxs.net>
for providing IPv6 connectivity.
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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 Daniel Bellomo (UNRC) for his
continued support.
Authors' Addresses
Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
Tim Chown
Jisc
Lumen House, Library Avenue
Harwell Oxford, Didcot. OX11 0SG
United Kingdom
Email: tim.chown@jisc.ac.uk
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