IPv6 offers a much larger address space than that of its IPv4 counterpart. The standard /64 IPv6 subnets can (in theory) accommodate approximately 1.844 * 10^19 hosts, thus resulting in a much lower host density (#hosts/#addresses) than their IPv4 counterparts. As a result, it is widely assumed that it would take a tremendous effort to perform address scanning attacks against IPv6 networks, and therefore IPv6 address scanning attacks have long been considered unfeasible. This document analyzes how traditional address scanning techniques apply to IPv6 networks, and also explores a number of other techniques that can be employed for IPv6 network reconnaissance. Additionally, this document formally obsoletes RFC 5157.
048514499a17396a23d97600ebed59b44a15828ff936fd26e985822b271d5d5f
Operational Security Capabilities for F. Gont
IP Network Infrastructure (opsec) Huawei Technologies
Internet-Draft T. Chown
Obsoletes: 5157 (if approved) University of Southampton
Intended status: Informational December 12, 2012
Expires: June 15, 2013
Network Reconnaissance in IPv6 Networks
draft-ietf-opsec-ipv6-host-scanning-00
Abstract
IPv6 offers a much larger address space than that of its IPv4
counterpart. The standard /64 IPv6 subnets can (in theory)
accommodate approximately 1.844 * 10^19 hosts, thus resulting in a
much lower host density (#hosts/#addresses) than their IPv4
counterparts. As a result, it is widely assumed that it would take a
tremendous effort to perform address scanning attacks against IPv6
networks, and therefore IPv6 address scanning attacks have long been
considered unfeasible. This document analyzes how traditional
address scanning techniques apply to IPv6 networks, and also explores
a number of other techniques that can be employed for IPv6 network
reconnaissance. Additionally, this document formally obsoletes RFC
5157.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 15, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements for the Applicability of Network
Reconnaissance Techniques . . . . . . . . . . . . . . . . . . 4
3. IPv6 Address Scanning . . . . . . . . . . . . . . . . . . . . 5
3.1. Address Configuration in IPv6 . . . . . . . . . . . . . . 5
3.2. IPv6 Address Scanning of Remote Networks . . . . . . . . . 11
3.3. IPv6 Address Scanning of Local Networks . . . . . . . . . 11
3.4. Existing IPv6 Address Scanning Tools . . . . . . . . . . . 12
3.5. Mitigations . . . . . . . . . . . . . . . . . . . . . . . 13
4. Leveraging the Domain Name System (DNS) for Network
Reconnaissance . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. DNS Advertised Hosts . . . . . . . . . . . . . . . . . . . 15
4.2. DNS Zone Transfers . . . . . . . . . . . . . . . . . . . . 15
4.3. DNS Reverse Mappings . . . . . . . . . . . . . . . . . . . 15
5. Leveraging Local Name Resolution and Service Discovery
Services . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Public Archives . . . . . . . . . . . . . . . . . . . . . . . 18
7. Application Participation . . . . . . . . . . . . . . . . . . 19
8. Inspection of the IPv6 Neighbor Cache and Routing Table . . . 20
9. Inspection of System Configuration and Log Files . . . . . . . 21
10. Gleaning Information from Routing Protocols . . . . . . . . . 22
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
12. Security Considerations . . . . . . . . . . . . . . . . . . . 24
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
14.1. Normative References . . . . . . . . . . . . . . . . . . . 26
14.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Implementation of a full-fledged IPv6
address-scanning tool . . . . . . . . . . . . . . . . 29
A.1. Host-probing considerations . . . . . . . . . . . . . . . 29
A.2. Implementation of an IPv6 local address-scanning tool . . 30
A.3. Implementation of a IPv6 remote address-scanning tool . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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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 such changes is the reduced host density (Nr. of addresses/Nr. of
hosts) of typical IPv6 subnetworks: with default IPv6 subnets of /64,
each subnet comprises more than 1.844 * 10^19 addresses; however, the
actual number of nodes in each subnet is likely to remain similar to
that of IPv4 subnetworks (at most a few hundred nodes per subnet).
This lower host-density has lead to the widely-established myth that
IPv6 address-scanning attacks are unfeasible, since they would
require a ridiculously long time (along with a tremendous amount of
traffic) to be successfully performed.
This document analyzes the feasibility of "traditional" address-
scanning attacks in IPv6 networks. Namely, it performs a thorough
analysis of how IPv6 addresses are generated, and sheds some light on
the real size of the search space for IPv6 address scanning attacks
(e.g., "ping sweeps") thus dismantling the myth that such IPv6
address scanning attacks are unfeasible. Additionally, this document
explores a number of other techniques that can be employed for IPv6
network reconnaissance, and formally obsoletes [RFC5157].
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 an address-scanning tools (e.g. for penetration testing
purposes).
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2. Requirements for the Applicability of Network Reconnaissance
Techniques
Throughout this document, a number of network reconnaissance
techniques are discussed. Each of these techniques have different
requirements on the side of the practitioner, with respect to whether
they require local access to the target network, and whether they
require login access to the system on which the technique is applied.
The following table tries to summarize the aforementioned
requirements, and serve as a cross index to the corresponding
sections.
+---------------------------------------------+----------+----------+
| 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 4.3) | No | No |
+---------------------------------------------+----------+----------+
| Public archives (Section 6) | No | No |
+---------------------------------------------+----------+----------+
| Application Participation (Section 7) | No | No |
+---------------------------------------------+----------+----------+
| Inspection of the IPv6 Neighbor Cache and | No | Yes |
| Routing Table (Section 8) | | |
+---------------------------------------------+----------+----------+
| Inspecting System Configuration and Log | No | Yes |
| Files (Section 9) | | |
+---------------------------------------------+----------+----------+
| Gleaning information from Routing Protocols | Yes | No |
| (Section 10) | | |
+---------------------------------------------+----------+----------+
Table 1: Requirements for the Applicability of Network Reconnaissance
Techniques
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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 full-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 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.
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3.1.1.1. Interface-Identifiers Embedding IEEE Identifiers
Many network technologies generate the 64-bit interface identifier
based on the link-layer address of the corresponding network
interface card. For example, in the case of Ethernet addresses, the
IIDs are constructed as follows:
1. The "Universal" bit (bit 6, from left to right) of the address is
set to 1
2. The word 0xfffe is inserted between the OUI (Organizationally
Unique Identifier) and the rest of the Ethernet address
For example, the MAC address 00:1b:38:83:88:3c would lead to the IID
021b:38ff:fe83:883c.
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).
Another interesting factor arises from the use of virtualization
technologies, since they generally employ automatically-generated MAC
addresses, with very specific patterns. For example, all
automatically-generated MAC addresses in VirtualBox virtual machines
employ the OUI 08:00:27 [VBox2011]. This means that all SLAAC-
produced addresses will have an IID of the form a00:27ff:feXX:XXXX,
thus effectively reducing the IID search space from 64 bits to 24
bits.
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VMWare ESX server provides yet a more interesting example.
Automatically-generated MAC addresses have the following pattern
[vmesx2011]:
1. The OUI is set to 00:05:59
2. The next 16-bits of the MAC address are set to the same value as
the last 16 bits of the console operating system's primary IPv4
address
3. The final eight bits of the MAC address are set to a hash value
based on the name of the virtual machine's configuration file.
This means that, assuming the console operating system's primary IPv4
address is known, the IID search space is reduced from 64 bits to 8
bits.
On the other hand, manually-configured MAC addresses in VMWare ESX
server employ the OUI 00:50:56, with the low-order three bytes being
in the range 0x000000-0x3fffff (to avoid conflicts with other VMware
products). Therefore, even in the case of manually-configured MAC
addresses, the IID search space is reduced from 64-bits to 22 bits.
3.1.1.2. Privacy Addresses
Privacy concerns [CPNI-IPv6] [Gont-DEEPSEC2011] regarding interface
identifiers embedding IEEE identifiers led to the introduction of
"Privacy Extensions for Stateless Address Auto-configuration in IPv6"
[RFC4941], also known as "privacy addresses" or "temporary
addresses". Essentially, "privacy addresses" produce random
addresses by concatenating a random identifier to the auto-
configuration IPv6 prefix advertised in a Router Advertisement.
In addition to their unpredictability, these addresses are
typically short-lived, such that even if an attacker were to learn
one of these addresses, they would be of use for a reduced period
of time.
It is important to note that "privacy addresses" are generated in
addition to traditional SLAAC addresses (i.e., based on IEEE
identifiers): traditional SLAAC addresses are employed for incoming
(i.e. server-like) communications, while "privacy addresses" are
employed for outgoing (i.e., client-like) communications. This means
that implementation/use of "privacy addresses" does not prevent an
attacker from leveraging the predictability of traditional SLAAC
addresses, since "privacy addresses" are generated in addition to
(rather than in replacement of) the traditional SLAAC addresses
derived from e.g. IEEE identifiers.
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3.1.1.3. Randomized Stable Interface Identifiers
In order to mitigate the security implications arising from the
predictable IPv6 addresses derived from IEEE identifiers, Microsoft
Windows produced an alternative scheme for generating "stable
addresses" (in replacement of the ones embedding IEEE identifiers).
The aforementioned scheme is allegedly an implementation of RFC 4941
[RFC4941], but without regenerating the addresses over time. The
resulting interface IDs are constant across system bootstraps, and
also constant across networks.
Assuming no flaws in the aforementioned algorithm, this scheme would
remove any patterns from the SLAAC addresses.
However, since the resulting interface IDs are constant across
networks, these addresses may still be leveraged for host tracking
purposes [I-D.ietf-6man-stable-privacy-addresses].
3.1.1.4. Stable Privacy-Enhanced Addresses
In response to the predictability issues discussed in Section 3.1.1.1
and the privacy issues discussed in , the IETF is currently
standardizing (in [I-D.ietf-6man-stable-privacy-addresses]) a method
for generating IPv6 Interface Identifiers to be used with IPv6
Stateless Address Autoconfiguration (SLAAC), such that addresses
configured using this method are stable within each subnet, but the
Interface Identifier changes when hosts move from one network to
another. The aforementioned method is meant to be an alternative to
generating Interface Identifiers based on IEEE identifiers, such that
the benefits of stable addresses can be achieved without sacrificing
the privacy of users.
Implementation of this method (in replacement of Interface
Identifiers based on IEEE identifiers) would eliminate any patterns
from the Interface ID.
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 - 2001:
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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.
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.
While network administrators are mostly free to select the IID from
any value in the range 1 - 264 range, for the sake of simplicity
(i.e., ease of remembering) they tend to select addresses with one of
the following patterns:
o "low-byte" addresses: in which all bytes of the IID (except the
lowest one) are set to 0.
o IPv4-based addresses: in which the IID encodes the IPv4-address of
the network interface (as in 2001:db8::192.168.1.1)
o wordy addresses: which encode words (as in 2001:db8::dead:beef)
Clearly, the first two patterns reduce the search space from the
original 64 bits to roughly 8 bits (assuming the IPv4 address range
is known for the case of "IPv4-based" addresses). On the other hand,
the search space for IPv6 wordy-addresses is probably larger and more
complex, but still greatly reduced when compared to the original 64-
bit search space.
3.1.4. IPv6 Addresses Corresponding to Transition/Co-existence
Technologies
Some transition/co-existence technologies might be leveraged to
reduce the target search space of remote address-scanning attacks,
since they specify how the corresponding IPv6 address must be
generated. For example, in the case of Teredo [RFC4380], the 64-bit
interface identifier is generated from the IPv4 address observed at a
Teredo server along with a UDP port number.
3.1.5. IPv6 Address Assignment in Real-world Network Scenarios
Table 2 and Table 3 provide a rough summary of the results obtained
by [Malone2008] for IPv6 clients and IPv6 routers, respectively.
These results are provided mainly for completeness-sake, since they
are the most comprehensive address-measurement results that have so
far been made publicly available.
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We note, however, that evolution of IPv6 implementations, changes
in the IPv6 address selection policy, etc. since [Malone2008] was
published might limit (or even obsolete) the validity of these
results.
+--------------+------------+
| Address type | Percentage |
+--------------+------------+
| SLAAC | 50% |
+--------------+------------+
| IPv4-based | 20% |
+--------------+------------+
| Teredo | 10% |
+--------------+------------+
| Low-byte | 8% |
+--------------+------------+
| Privacy | 6% |
+--------------+------------+
| Wordy | <1% |
+--------------+------------+
| Other | <1% |
+--------------+------------+
Table 2: Measured client addresses
+--------------+------------+
| Address type | Percentage |
+--------------+------------+
| Low-byte | 70% |
+--------------+------------+
| IPv4-based | 5% |
+--------------+------------+
| SLAAC | 1% |
+--------------+------------+
| Wordy | <1% |
+--------------+------------+
| Privacy | <1% |
+--------------+------------+
| Teredo | <1% |
+--------------+------------+
| Other | <1% |
+--------------+------------+
Table 3: Measured router addresses
It should be clear from these measurements that a very high
percentage of host and router addresses follow very specific
patterns.
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3.2. IPv6 Address Scanning of Remote Networks
While in IPv4 networks attackers have been able to get away with
"brute force" scanning attacks (thanks to the reduced search space),
successfully performing a brute-force scan of an entire /64 network
would be infeasible. As a result, it is expected that attackers will
leverage the IPv6 address patterns discussed in Section 3.1 to reduce
the IPv6 address search space.
IPv6 address scanning of remote area networks should consider an
additional factor not present for the IPv4 case: since the typical
IPv6 subnet is a /64, scanning an entire /64 could, in theory, lead
to the creation of 2^^64 entries in the Neighbor Cache of the last-
hop router. Unfortunately, a number of IPv6 implementations have
been found to be unable to properly handle large number of entries in
the Neighbor Cache, and hence these address-scan attacks may have the
side effect of resulting in a Denial of Service (DoS) attack
[CPNI-IPv6] [RFC6583].
3.3. IPv6 Address Scanning of Local Networks
IPv6 address scanning in Local Area Networks could be considered, to
some extent, a completely different problem than that of scanning a
remote IPv6 network. The main difference is that use of link-local
multicast addresses can relieve the attacker of searching for unicast
addresses in a large IPv6 address space.
Obviously, a number of other network reconnaissance vectors (such
as network snooping, leveraging Neighbor Discovery traffic, etc.)
are available when scanning a local network. However, this
section focuses only on address-scanning attacks (a la "ping
sweep").
An attacker can simply send probe packets to the all-nodes link-local
multicast address (ff02::1), such that responses are elicited from
all local nodes.
Since Windows systems (Vista, 7, etc.) do not respond to ICMPv6 Echo
Request messages sent to multicast addresses, IPv6 address-scanning
tools typically employ a number of additional probe packets to elicit
responses from all the local nodes. For example, unrecognized IPv6
options of type 10xxxxxx elicit ICMPv6 Parameter Problem, code 2,
error messages.
Many address-scanning tools discover only IPv6 link-local addresses
(rather than e.g. the global addresses of the target systems): since
the probe packets are typically sent with the attacker's IPv6 link-
local address, the "victim" nodes send the response packets using the
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IPv6 link-local address of the corresponding network interface (as
specified by the IPv6 address selection rules [RFC6724]). However,
sending multiple probe packets, with each packet employing addresses
from different prefixes, typically helps to overcome this limitation.
This technique is employed by the scan6 tool of the IPv6 Toolkit
package [IPv6-Toolkit].
3.4. Existing IPv6 Address Scanning Tools
3.4.1. Remote IPv6 Network Scanners
IPv4 address scanning tools have traditionally carried out their task
for probing an entire address range (usually the entire range of a
target subnetwork). One might argue that the reason for which we
have been able to get away with such somewhat "rudimentary"
techniques is that the scale of the "problem" is so small in the IPv4
world, that a "brute-force" attack is "good enough". However, the
scale of the "address scanning" problem is so large in IPv6, that
attackers must be very creative to be "good enough".
Simply sweeping an entire /64 IPv6 subnet would just not be feasible.
For instance, that is one of the reasons for which address scanning
tools such as nmap [nmap2012] do not even support sweeping an IPv6
address range.
The nmap(1) manual page states "IPv6 addresses can only be
specified by their fully qualified IPv6 address or hostname. CIDR
and octet ranges aren't supported for IPv6 because they are rarely
useful.
On the other hand, the alive6 tool from [THC-IPV6] supports
sweeping address ranges, thus being able to leverage some patters
fond in IPv6 addresses, such as the incremental addresses
resulting from some DHCPv6 setups.
The most "advanced" IPv6 scanning technique that has been found in
the wild is that reported in [Ybema2010], in which the attacker
seemed to be scanning specific IPv6 addresses based on specific
patterns. However, the aforementioned attempt probably still falls
into the category of "rudimentary".
Clearly, a limitation of most currently-available tools is that they
lack of an "heuristics engine" that can help reduce the search space,
such that the problem of IPv6 address scanning becomes tractable.
However, we expect that this situation will change in the short term.
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3.4.2. Local IPv6 Network Scanners
There are a variety of publicly-available local IPv6 network
scanners:
Current versions of nmap [nmap2012] implement this functionality
THC's IPv6 Attack Toolkit [THC-IPV6] includes a tool that
implements this functionality
SI6 Network's IPv6 Toolkit [IPv6-Toolkit] includes a tool (scan6)
that implements this functionality
3.5. Mitigations
IPv6 address-scanning attacks can be mitigated in a number of ways.
A non-exhaustive list of the possible mitigations includes:
o Employing stable privacy-enhanced addresses
[I-D.ietf-6man-stable-privacy-addresses] in replacement of
addresses based on IEEE identifiers, such that any address
patterns are eliminated.
o Employing Intrusion Prevention Systems (IPS) at the perimeter,
such that address scanning attacks can be mitigated.
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.
It should be noted that some of the aforementioned mitigations are
operational, while others depend on the availability of specific
features (such as [I-D.ietf-6man-stable-privacy-addresses] on the
corresponding nodes.
Additionally, while some resistance to address scanning attacks is
generally desirable (particularly when lightweight mitigations are
available), there are scenarios in which mitigation of some address-
scanning vectors is unlikely to be a high-priority (if at all
possible).
Two of the techniques discussed in this document for local address-
scanning attacks are those that employ multicasted ICMPv6 Echo
Requests and multicasted IPv6 packets containing unsupported options
of type 10xxxxxx. These two vectors could be easily mitigated by
configuring nodes to not respond to multicasted ICMPv6 Echo Request
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(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
[draft-cheshire-dnsext-multicastdns]), or eventually rely on network
snooping as last resort for network reconnaissance.
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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
hosts.
Additionally, we note that publication of IPv6 addresses in the DNS
should not discourage the elimination of IPv6 address patterns: if
any address patterns are eliminated from addresses published in the
DNS, an attacker may have to rely on performing dictionary-based DNS
lookups in order to find all systems in a target network (which is
generally less reliable and more time/traffic consuming than mapping
nodes with predictable IPv6 addresses).
4.2. DNS Zone Transfers
A DNS zone transfer can readily provide information about potential
attack targets. Restricting zone transfers is thus probably more
important for IPv6, even if it is already good practice to restrict
them in the IPv4 world.
4.3. DNS Reverse Mappings
An interesting technique that employs DNS reverse mappings for
network reconnaissance has been recently disclosed [van-Dijk].
Essentially, the attacker walks through the "ip6.arpa" zone looking
up PTR records, in the hopes of learning the IPv6 addresses of hosts
in a given target network (assuming that the reverse mappings have
been configured, of course). What is most interesting about this
technique is that it can greatly reduce the IPv6 address search
space.
Basically, an attacker would walk the ip6.arpa zone corresponding to
a target network (e.g. "0.8.0.0.8.b.d.0.1.0.0.2.ip6.arpa." for "2001:
db8:80:/32"), issuing queries for PTR records corresponding to the
domain names "0.0.8.0.0.8.b.d.0.1.0.0.2.ip6.arpa.",
"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"
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search space.
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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)
[draft-cheshire-dnsext-multicastdns] and DNS Service Discovery
(DNS-SD) [I-D.cheshire-dnsext-dns-sd], or Link-Local Multicast Name
Resolution (LLNR) [RFC4795], are examples of such protocols.
Besides the Graphical User Interfaces (GUIs) included in products
supporting such protocols, command-line tools such as mdns-scan
[mdns-scan] and mzclient can help discover IPv6 hosts employing
mDNS/DNS-SD.
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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.
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7. Application Participation
Peer-to-peer applications often include some centralized server which
coordinates the transfer of data between peers. For example,
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.
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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.
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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].
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10. Gleaning Information from Routing Protocols
Some organizational IPv6 networks employ routing protocols to
dynamically maintain routing information. In such an environment, a
local attacker could become a passive listener of the routing
protocol, to determine other valid subnets within that organization
[V6-WORMS].
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11. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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12. Security Considerations
This document explores the topic of Network Reconnaissance in IPv6
networks. It analyzes the feasibility of address-scan attacks in
IPv6 networks, and showing that the search space for such attacks is
typically much smaller than the one traditionally assumed (64 bits).
Additionally, it explores a plethora of other network reconnaissance
techniques, ranging from inspecting the IPv6 Network Cache of an
attacker-controlled system, to gleaning information about IPv6
addresses from public mailing-list archives or Peer-To-Peer (P2P)
protocols.
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.
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13. Acknowledgements
The author would like to thank (in alphabetical order) Marc Heuse,
Ray Hunter, Libor Polcak, Jan Schaumann, and Arturo Servin, for
providing valuable comments on earlier versions of this document.
Part of the contents of this document are based on the results of the
project "Security Assessment of the Internet Protocol version 6
(IPv6)" [CPNI-IPv6], carried out by Fernando Gont on behalf of the UK
Centre for the Protection of National Infrastructure (CPNI).
Fernando Gont would like to thank the UK CPNI for their continued
support.
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14. References
14.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
January 2007.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[I-D.ietf-6man-stable-privacy-addresses]
Gont, F., "A method for Generating Stable Privacy-Enhanced
Addresses with IPv6 Stateless Address Autoconfiguration
(SLAAC)", draft-ietf-6man-stable-privacy-addresses-01
(work in progress), October 2012.
[draft-cheshire-dnsext-multicastdns]
Cheshire, S. and M. Krochmal, "Multicast DNS Trees in
PIM-SM", December 2011.
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
progress), December 2011.
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14.2. Informative References
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583, March 2012.
[I-D.gont-6man-ipv6-smurf-amplifier]
Gont, F., "Security Implications of IPv6 options of Type
10xxxxxx", draft-gont-6man-ipv6-smurf-amplifier-00 (work
in progress), December 2011.
[RFC5157] Chown, T., "IPv6 Implications for Network Scanning",
RFC 5157, March 2008.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[V6-WORMS]
Bellovin, S., Cheswick, B., and A. Keromytis, "Worm
propagation strategies in an IPv6 Internet", ;login:,
pages 70-76, February 2006,
<https://www.cs.columbia.edu/~smb/papers/v6worms.pdf>.
[Malone2008]
Malone, D., "Observations of IPv6 Addresses", Passive and
Active Measurement Conference (PAM 2008, LNCS 4979),
April 2008,
<http://www.maths.tcd.ie/~dwmalone/p/addr-pam08.pdf>.
[mdns-scan]
Poettering, L., "mdns-scan(1) manual page", 2012, <http://
manpages.ubuntu.com/manpages/precise/man1/
mdns-scan.1.html>.
[nmap2012]
Fyodor, "nmap - Network exploration tool and security /
port scanner", 2012, <http://insecure.org>.
[VBox2011]
VirtualBox, "Oracle VM VirtualBox User Manual, version
4.1.2", August 2011, <http://www.virtualbox.org>.
[vmesx2011]
vmware, "Setting a static MAC address for a virtual NIC",
vmware Knowledge Base, August 2011, <http://
kb.vmware.com/selfservice/microsites/
search.do?language=en_US&cmd=displayKC&externalId=219>.
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[Ybema2010]
Ybema, I., "just seen my first IPv6 network abuse scan, is
this the start for more?", Post to the NANOG mailing-
list, 2010, <http://mailman.nanog.org/pipermail/nanog/
2010-September/025049.html>.
[Gont-DEEPSEC2011]
Gont, "Results of a Security Assessment of the Internet
Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
Vienna, Austria, November 2011, <http://
www.si6networks.com/presentations/deepsec2011/
fgont-deepsec2011-ipv6-security.pdf>.
[THC-IPV6]
"THC-IPV6", <http://www.thc.org/thc-ipv6/>.
[IPv6-Toolkit]
"IPv6 Toolkit",
<http://www.si6networks.com/tools/ipv6toolkit>.
[van-Dijk]
van Dijk, P., "Finding v6 hosts by efficiently mapping
ip6.arpa", <http://7bits.nl/blog/2012/03/26/
finding-v6-hosts-by-efficiently-mapping-ip6-arpa>.
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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 ongoing work
on the implementation of a general (i.e., non-local) IPv6 host
scanner.
A.1. Host-probing considerations
A number of factors should be considered when selecting the probe
types and the probing-rate for an IPv6 address scanning tool.
Firstly, some hosts (or border firewalls) might be configured to
block or rate-limit some specific packet types. For example, it is
usual for host and router implementations to rate-limit ICMPv6 error
traffic. Additionally, some firewalls might be configured to block
or rate-limit incoming ICMPv6 echo request packets.
As noted earlier in this document, Windows systems simply do not
respond to ICMPv6 echo requests sent to multicast IPv6 addresses.
Among the possible probe types are:
o TCP segments meant to elicit SYN/ACK or RST segments,
o UDP segments meant to elicit a UDP application response or an
ICMPv6 Port Unreachable, an IPv6 packet containing any suitable
payload and an unrecognized extension header (such that a ICMPv6
Parameter Problem error message is elicited), or,
o an IPv6 packet containing any suitable payload and an unrecognized
option of type 10xxxxxx (such that a ICMPv6 Parameter Problem
error message is elicited)
Selecting an appropriate probe packet might help conceal the ongoing
attack, but may also be actually necessary if host or network
configuration causes certain probe packets to be dropped. In some
cases, it might be desirable to insert some IPv6 extension headers
before the actual payload, such that some filtering policies can be
circumvented.
Another factor to consider is the host-probing rate. Clearly, the
higher the rate, the smaller the amount of time required to perform
the attack. However, the probing-rate should not be too high, or
else:
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1. the attack might cause network congestion, thus resulting in
packet loss
2. the attack might hit rate-limiting, thus resulting in packet loss
3. the attack might reveal underlying problems in the Neighbor
Discovery implementation, thus leading to packet loss and
possibly even Denial of Service
Packet-loss is undesirable, since it would mean that an "alive" node
might remain undetected as a result of a lost probe or response.
Such losses could be the result of congestion (in case the attacker
is scanning a target network at a rate higher than the target network
can handle), or may be the result of rate-limiting as it would be
typically the case if ICMPv6 is employed for the probe packets.
Finally, as discussed in [CPNI-IPv6] and [RFC6583], some IPv6 router
implementations have been found to be unable to perform decent
resource management when faced with Neighbor Discovery traffic
involving a large number of local nodes. This essentially means that
regardless of the type of probe packets, a address scanning attack
might result in a Denial of Service (DoS) of the target network, with
the same (or worse) effects as that of network congestion or rate-
limiting.
The specific rates at which each of these issues may come into play
vary from one scenario to another, and depend on the type of deployed
routers/firewalls, configuration parameters, etc.
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).
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3. The same procedure of the previous bullet is performed, but this
time with ICMPv6 packets that contain an unrecognized option of
type 10xxxxxx, such that ICMPv6 Parameter Problem error messages
are elicited. This allows the tool to discover e.g. Windows
nodes, which otherwise do not respond to multicasted ICMPv6 Echo
Request messages.
4. Each time a new "alive" address is discovered, the corresponding
Interface-ID is combined with all the local prefixes, and the
resulting addresses are probed (with unicasted packets). This
can help to discover other addresses in use on the local network
segment, since the same Interface ID is typically used with all
the available prefixes for the local network.
The aforementioned scheme can fail to discover some addresses for
some implementation. For example, Mac OS X employs IPv6 addresses
embedding IEEE-identifiers (rather than "privacy addresses") when
responding to packets destined to a link-local multicast address,
sourced from an on-link prefix.
A.3. Implementation of a IPv6 remote address-scanning tool
An IPv6 remote address scanning tool, could be implemented with the
following features:
o The tool can be instructed to scan devices manufactured by a
specific vendor, such that only addresses resulting for the
corresponding OUIs are tried.
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 (as discussed earlier in this
document).
o The tool can be instructed to scan for low-byte or DHCPv6-like
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 specified an IPv4 address range in use at the
target network, such that only IPv4-based IPv6 addresses are
scanned.
In brute force mode, the tool can, at the very least:
o Skip addresses resulting from unassigned OUIs.
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o Skip addresses resulting from OUIs deemed as "legacy".
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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
University of Southampton
Highfield
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
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