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 host scanning attacks against IPv6 networks, and therefore IPv6 host scanning attacks have long been considered unfeasible. This document analyzes the IPv6 address configuration policies implemented in most popular IPv6 stacks, and identifies a number of patterns in the resulting addresses lead to a tremendous reduction in the host address search space, thus dismantling the myth that IPv6 host scanning attacks are unfeasible.
3e402c5d8f47be6b853bd514ed35744c8ab3f764907fb96603770a5396359be0
Operational Security Capabilities for F. Gont
IP Network Infrastructure (opsec) UK CPNI
Internet-Draft April 20, 2012
Intended status: Informational
Expires: October 22, 2012
Host Scanning in IPv6 Networks
draft-gont-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 host scanning attacks against IPv6
networks, and therefore IPv6 host scanning attacks have long been
considered unfeasible. This document analyzes the IPv6 address
configuration policies implemented in most popular IPv6 stacks, and
identifies a number of patterns in the resulting addresses lead to a
tremendous reduction in the host address search space, thus
dismantling the myth that IPv6 host scanning attacks are unfeasible.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.
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 October 22, 2012.
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
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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. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Address configuration in IPv6 . . . . . . . . . . . . . . . . 5
3.1. StateLess Address Auto-Configuration (SLAAC) . . . . . . . 5
3.1.1. Interface-Identifiers embedding IEEE Identifiers . . . 5
3.1.2. Privacy Addresses . . . . . . . . . . . . . . . . . . 7
3.1.3. Stable and random Interface Identifiers . . . . . . . 7
3.2. Dynamic Host Configuration Protocol version 6 (DHCPv6) . . 8
3.3. Manually-configured addresses . . . . . . . . . . . . . . 8
4. IPv6 address assignment in real-world network scenarios . . . 10
5. Previous work in the area of IPv6 host scanning . . . . . . . 12
5.1. IPv6 host scanning of remote networks . . . . . . . . . . 12
6. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Disclaimer
This document is a "stripped down" version of a document we have
authored on IPv6 host scanning. This version is ssentially meant to
provide some numbers as to how feasible IPv6 host scanning attacks
are. Future revisions will cover the topic more thoroughly.
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2. Introduction
The main driver for IPv6 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 a typical IPv6 subnet: with default IPv6 subnets of /64,
each subnet comprises for 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 host-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 performs a careful analysis of how IPv6 addresses are
generated, and sheds some light on the real size of the search space
when performing an IPv6 host scanning attack, dismantling the myth
that such IPv6 ahost scanning attacks are unfeasible. Section 3
discusses how address configuration is performed in IPv6, describes
the IPv6 address generation algorithms currently implemented in
popular IPv6 stacks, and identifies patterns in IPv6 addresses that
can be leveraged to reduce the IPv6 address search space when
performing host scanning attacks. Section 5 describes previous work
in the area of IPv6 host scanning, and explains its limitations. .
Section 6 provides advice on how to mitigate IPv6 host scanning
attacks.
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3. 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. 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.
3.1.1. 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
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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
addressses, with very specific patterns. For example, all
automatically-generated MAC addresses in VirtualBox virtual machines
employ the OUI 08:00:27 [VBox2011]. This means that all SLAAC-
produced addresses will have an IID of the form a00:27ff:feXX:XXXX,
thus effectively reducing the IID search space from 64 bits to 24
bits.
VMWare ESX server provides yet a more interesting example.
Automatically-generated MAC addresses have the following pattern
[vmesx2011]:
1. The OUI is set to 00:05:59
2. The next 16-bits of the MAC address are set to the same value as
the last 16 bits of the console operating system's primary IPv4
address
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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.
3.1.2. 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.
3.1.3. Stable and random 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 repalcement 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
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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.gont-6man-stable-privacy-addresses]
3.2. Dynamic Host Configuration Protocol version 6 (DHCPv6)
DHCPv6 is 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 acording to a
configuration-defined address range and policy, with some DHCPv6
servers assigned addresses sequentially, from a specific range. In
such cases, addresses tend to be predictable.
For example, if the prefix 2001:db8::/64 is used for assigning
addresses on the local network, the DHCPv6 server might
(sequentially) assign addresses from the range 2001:db8::1 - 2001:
db8::100.
In most common scenarios, this means that the IID search space will
be reduced from the origina 64 bits, to 8 or 16 bits.
3.3. Manually-configured addresses
In some scenarios, node addresses may be manually configured. This
is typically the case for IPv6 addresses assigned to routers, since
router 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,
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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.
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4. IPv6 address assignment in real-world network scenarios
Table 1 and Table 2 provide a rough summary of the results obtained
by [Malone2008] for IPv6 clients and IPv6 routers, respectively.
These results are provided manly for completeness-sake, since they
are the most comprehensive address-measurement results that have so
far been made publicly available.
We note, however, that evolution of IPv6 implementations, changes
in the IPv6 address selection policy, etc., 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 1: Measured client addresses
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+--------------+------------+
| Address type | Percentage |
+--------------+------------+
| Low-byte | 70% |
+--------------+------------+
| IPv4-based | 5% |
+--------------+------------+
| SLAAC | 1% |
+--------------+------------+
| Wordy | <1% |
+--------------+------------+
| Privacy | <1% |
+--------------+------------+
| Teredo | <1% |
+--------------+------------+
| Other | <1% |
+--------------+------------+
Table 2: Measured router addresses
It should be clear from these measurements that a very high
percentage of the follow very specific patterns.
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5. Previous work in the area of IPv6 host scanning
5.1. IPv6 host scanning of remote networks
IPv4 host 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 they
we have been able to get off with such somewhat "rudimentary" tools
is that the scale of the "problem" is so small in the IPv4 world,
that even such a "poor" job is "good enough". However, the scale of
the "host scanning" problem is so large in IPv6, that we must be very
creative to be "good enough".
Simply sweeping an entire /64 IPv6 subnet would just not be feasible.
For instance, that is probably one of the reasons for which host
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.
The most "advanced" IPv6 scanning technique that we are aware of is
that reported in [Ybema2010], in which the attacker seemed to be
scanning specific IPv6 addressesbased on specific patterns. However,
it probably still falls into the category of "rudimentary".
Clearly, the limitation of currently-available tools is that they
lack of an "heuristics engine" that can help reduce the search space,
such that the problem of IPv6 host scanning becomes tractable.
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6. Mitigations
IPv6 host scanning attacks can be mitigated in a number of ways. A
non-exhaustive of the possible mitigations follows:
o Employ stable privacy-enhanced addresses
[I-D.gont-6man-stable-privacy-addresses] in replacement of
addresses based on IEEE identifiers, such that any address
patterns are eliminated
o Employ Intrusion Prevention Systems (IPS) at the perimeter, such
that host scanning attacks are mitigated
o If virtual machines are employed, and "resistance" to host
scanning attacks is deemed as desirable, employ manually-
configured MAC addresses
It should be noted that some of the aforementioned mitigations are
operational, while others depend on the availability of corresponding
"patches".
Additionally, while some resistance to host scanning attacks is
generally desirable (particularly when lightweight mitigations are
available), there are scenarios in which such mitigation is unlikely
to be a high-priority (if at all possible). Such scenarios include:
o Mitigation of IPv6 local host-scanning scans
o Mitigation of IPv6 host-scanning attacks in public-facing servers
In general, it is only possible to mitigate some vectors for IPv6
local host scanning attacks, but virtually impossible to completely
mitigate them, particularly when a local attacker can rely on network
snooping, and protocols employed for the so-called "opportunistic
networking" (such as mDNS). On the other hand, public-facing servers
generally contain corresponding entries in the DNS, in which case an
attacker can rely on the DNS for network reconnaissance.
o We note, however, that if any address patterns are eliminated from
servers with entries in the DNS, an attacker may have to rely on
using dictionaries with the DNS, which is generally less reliable
and more time/traffic consuming than mapping nodes with
predictable IPv6 addresses.
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7. Security Considerations
This document demonstrates that the widely-established myth of IPv6
host-scanning attacks being unfeasible is more based on "hope" than
on careful analysis or facts. We expect host scanning attacks to
become more and more elaborated (i.e., less "brute force") as general
deployment of IPv6 increases, and more specifically, as more IPv6-
only devices are deployed.
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8. Acknowledgements
This document resulted from 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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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[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.
9.2. Informative References
[I-D.gont-6man-stable-privacy-addresses]
Gont, F., "A method for Generating Stable Privacy-Enhanced
Addresses with IPv6 Stateless Address Autoconfiguration
(SLAAC)", draft-gont-6man-stable-privacy-addresses-01
(work in progress), March 2012.
[I-D.ietf-v6ops-v6nd-problems]
Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems",
draft-ietf-v6ops-v6nd-problems-05 (work in progress),
March 2012.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[Malone2008]
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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>.
[nmap2012]
Fyodor, F., "nmap - Network exploration tool and security
/ port scanner", 2011, <http://insecure.org>.
[VBox2011]
VirtualBox, V., "Oracle VM VirtualBox User Manual, version
4.1.2", August 2011, <http://www.virtualbox.org>.
[vmesx2011]
vmware, vmware., "Setting a static MAC address for a
virtual NIC", vmware Knowledge Base, August 2011, <http:/
/kb.vmware.com/selfservice/microsites/
search.do?language=en_US&cmd=displayKC&externalId=219>.
[Ybema2010]
Ybema, I., "just seen my first IPv6 network abuse scan, is
this the start for more?", Post to the NANOG mailing-
list, August 2011, <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/>.
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Author's Address
Fernando Gont
UK Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI: http://www.cpni.gov.uk
Gont Expires October 22, 2012 [Page 18]
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