IPv6 Operations T. Chown
Internet-Draft University of Southampton
Expires: April 26, 2007 October 23, 2006
IPv6 Implications for Network Scanning
draft-ietf-v6ops-scanning-implications-01
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Copyright (C) The Internet Society (2006).
Abstract
The 128 bits of IPv6 address space is considerably bigger than the 32
bits of address space of IPv4. In particular, the IPv6 subnets to
which hosts attach will by default have 64 bits of host address
space. As a result, traditional methods of remote TCP or UDP network
scanning to discover open or running services on a host will
potentially become far less feasible, due to the larger search space
in the subnet. In addition automated attacks, such as those
performed by network worms, may be hampered. This document discusses
this property of IPv6, and describes related issues for site
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administrators of IPv6 networks to consider, which may be of
importance when planning site address allocation and management
strategies. While traditional network scanning probes (whether by
individuals or automated via network worms) may become less common,
administrators should be aware of other methods attackers may use to
discover IPv6 addresses on a target network, and be aware of
appropriate measures to mitigate these.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Target Address Space for Network Scanning . . . . . . . . . . 4
2.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Reducing the IPv6 Search Space . . . . . . . . . . . . . . 4
2.4. Dual-stack Networks . . . . . . . . . . . . . . . . . . . 5
2.5. Defensive Scanning . . . . . . . . . . . . . . . . . . . . 5
3. Alternatives for Attackers . . . . . . . . . . . . . . . . . . 5
3.1. On-link Methods . . . . . . . . . . . . . . . . . . . . . 6
3.2. Multicast or Other Service Discovery . . . . . . . . . . . 6
3.3. Log File Analysis . . . . . . . . . . . . . . . . . . . . 6
3.4. DNS Advertised Hosts . . . . . . . . . . . . . . . . . . . 6
3.5. DNS Zone Transfers . . . . . . . . . . . . . . . . . . . . 7
3.6. Application Participation . . . . . . . . . . . . . . . . 7
3.7. Transition Methods . . . . . . . . . . . . . . . . . . . . 7
4. Site Administrator Tools . . . . . . . . . . . . . . . . . . . 7
4.1. IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . . 8
4.2. DHCP Service Configuration Options . . . . . . . . . . . . 8
4.3. Rolling Server Addresses . . . . . . . . . . . . . . . . . 8
4.4. Application-Specific Addresses . . . . . . . . . . . . . . 9
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. Informative References . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
One of the key differences between IPv4 and IPv6 is the much larger
address space for IPv6, which also goes hand-in-hand with much larger
subnet sizes. This change has a significant impact on the
feasibility of TCP and UDP network scanning, whereby an automated
process is run to detect open ports (services) on systems that may
then be subject of a subsequent attack. Today many IPv4 sites are
subjected to such probing on a recurring basis.
The 128 bits of IPv6 [1] address space is considerably bigger than
the 32 bits of address space in IPv4. In particular, the IPv6
subnets to which hosts attach will by default have 64 bits of host
address space [3]. As a result, traditional methods of remote TCP or
UDP network scanning to discover open or running services on a host
will potentially become far less feasible, due to the larger search
space in the subnet. This document discusses this property of IPv6,
and describes related issues for site administrators of IPv6 networks
to consider, which may be of importance when planning site address
allocation and management strategies.
This document complements the transition-centric discussion of the
issues that can be found in Appendix A of the IPv6 Transition/
Co-existence Security Considerations [7] text, which takes a broad
view of security issues for transitioning networks.
The reader is also referred to a recent paper by Bellovin on worm
propogation strategies in IPv6 networks [8]. This paper discusses
some of the issues included in this document, from a slightly
different perspective.
Network scanning is quite a prevalent tactic used by would-be
attackers. There are two general classes of such scanning. In one
case, the probes are from an attacker outside a site boundary who is
trying to find weaknesses on any system in that network which they
may then subsequently be able to compromise. The other case is
scanning by worms that spread through (site) networks, looking for
further hosts to compromise. Many worms, like Slammer, rely on such
address scanning methods to propagate, whether they pick subnets
numerically (and thus probably topologically) close to the current
victim, or subnets in random remote networks.
It must be remembered that the defence of a network must not rely
solely on the obscurity of the hosts on that network. Such a feature
or property is only one measure in a set of measures that may be
applied. However, with a growth in usage of IPv6 devices in open
networks likely, and security becoming more likely an issue for the
end devices, such obfuscation can be useful where its use is of
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little or no cost to the administrator to implement it. However, a
law of diminuishing returns does apply. An administrator who
undertakes an address hiding policy should be aware that while IPv6
host addresses may be picked that are likely to take significant time
to discover by traditional scanning methods, there are other means by
which such addresses may be discovered. Implementing all of them may
be deemed unwarranted effort. But it is up to the site administrator
to be aware of the context and the options available, and in
particular what new methods may attackers use to glean IPv6 address
information, and how these can potentially be mitigated against.
This document is intended to be informational; there is not yet
sufficient deployment experience for it to be considered BCP.
2. Target Address Space for Network Scanning
There are significantly different considerations for the feasibility
of plain, brute force IPv4 and IPv6 address scanning.
A typical IPv4 subnet may have 8 bits reserved for host addressing.
In such a case, a remote attacker need only probe at most 256
addresses to determine if a particular service is running publicly on
a host in that subnet. Even at only one probe per second, such a
scan would take under 5 minutes to complete.
A typical IPv6 subnet will have 64 bits reserved for host addressing.
In such a case, a remote attacker in principle needs to probe 2^64
addresses to determine if a particular open service is running on a
host in that subnet. At a very conservative one probe per second,
such a scan may take some 5 billion years to complete. A more rapid
probe will still be limited to (effectively) infinite time for the
whole address space. However, there are ways for the attacker to
reduce the address search space to scan against within the target
subnet, as we discuss below.
2.3. Reducing the IPv6 Search Space
The IPv6 host address space through which an attacker may search can
be reduced in at least two ways.
First, the attacker may rely on the administrator conveniently
numbering their hosts from [prefix]::1 upward. This makes scanning
trivial, and thus should be avoided unless the host's address is
readily obtainable from other sources (for example it is the site's
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primary DNS or email MX server). Alternatively if hosts are numbered
sequentially, or using any regular scheme, knowledge of one address
may expose other available addresses to scan.
Second, in the case of statelessly autoconfiguring [1] hosts, the
host part of the address will take a well-known format that includes
the Ethernet vendor prefix and the "fffe" stuffing. For such hosts,
the search space can be reduced to 48 bits. Further, if the Ethernet
vendor is also known, the search space may be reduced to 24 bits,
with a one probe per second scan then taking a less daunting 194
days. Even where the exact vendor is not known, using a set of
common vendor prefixes can reduce the search. In addition, many
nodes in a site network may be procured in batches, and thus have
sequential or near sequential MAC addresses; if one node's
autoconfigured address is known, scanning around that address may
yield results for the attacker. Again, any form of sequential host
addressing should be avoided if possible.
2.4. Dual-stack Networks
Full advantage of the increased IPv6 address space in terms of
resilience to network scanning may not be gained until IPv6-only
networks and devices become more commonplace, given that most IPv6
hosts are currently dual stack, with (more readily scannable) IPv4
connectivity. However, many applications or services (e.g. new peer-
to-peer applications) on the (dual stack) hosts may emerge that are
only accessible over IPv6, and that thus can only be discovered by
IPv6 address scanning.
2.5. Defensive Scanning
The problem faced by the attacker for an IPv6 network is also faced
by a site administrator looking for vulnerabilities in their own
network's systems. The administrator should have the advantage of
being on-link for scanning purposes though.
3. Alternatives for Attackers
If IPv6 hosts in subnets are allocated addresses 'randomly', and as a
result IPv6 network scanning becomes relatively infeasible, attackers
will need to find new methods to identify IPv6 addresses for
subsequent scanning. In this section, we discuss some possible paths
attackers may take. In these cases, the attacker will attempt to
identify specific IPv6 addresses for subsequent targeted probes.
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3.1. On-link Methods
If the attacker is on link, then traffic on the link, be it Neighbor
Discovery or application based traffic, can invariably be observed,
and target addresses learnt. In this document we are assuming the
attacker is off link, but traffic to or from other nodes (in
particular server systems) is likely to show up if an attacker can
gain a presence on any one subnet in a site's network.
IPv6-enabled hosts on local subnets may be discovered through probing
the "all hosts" link local multicast address. Likewise any routers
on link may be found via the "all routers" link local multicast
address.
Where a host has already been compromised, its Neighbor Discovery
cache is also likely to include information about active nodes on
link, just as an ARP cache would do for IPv4.
3.2. Multicast or Other Service Discovery
A site may also have site or organisational scope multicast
configured, in which case application traffic, or service discovery,
may be exposed site wide. An attacker may choose to use any other
service discovery methods supported by the site.
There are also issues with disclosure from multicast itself. Where
an Embedded RP [6] multicast group address is known, the unicast
address of the rendezvous point is implied by the group address.
Where unicast prefix based multicast group addresses [4] are used,
specific /64 link prefixes may also be disclosed.
3.3. Log File Analysis
IPv6 addresses may be harvested from recorded logs such as web site
logs. Anywhere else where IPv6 addresses are explicitly recorded may
prove a useful channel for an attacker, e.g. by inspection of the
(many) Received from: or other header lines in archived email or
Usenet news messages.
3.4. DNS Advertised Hosts
Any servers that are DNS listed, e.g. MX mail relays, or web
servers, will remain open to probing from the very fact that their
IPv6 addresses will be published in the DNS. Where a site uses
sequential host numbering, publishing just one address may lead to a
threat upon the other hosts.
Sites may use a two-faced DNS where internal system DNS information
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is only published in an internal DNS. It is also worth noting that
the reverse DNS tree may also expose address information.
3.5. DNS Zone Transfers
In the IPv6 world a DNS zone transfer is much more likely to narrow
the number of hosts an attacker needs to target. This implies
restricting zone transfers is (more) important for IPv6, even if it
is already good practice to restrict them in the IPv4 world.
3.6. Application Participation
More recent peer-to-peer applications often include some centralised
server which coordinates the transfer of data between peers. The
BitTorrent application 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 IP addresses to probe.
3.7. Transition Methods
Specific knowledge of the target network may be gleaned if that
attacker knows it is using 6to4, ISATAP, Teredo, or other techniques
that derive low-order bits from IPv4 addresses (though in this case,
unless they are using IPv4 NAT, the IPv4 addresses may be probed
anyway). For example, the current Microsoft 6to4 implementation uses
the address 2002:V4ADDR::V4ADDR while older Linux and FreeBSD
implementations default to 2002:V4ADDR::1. This leads to specific
knowledge of specific hosts in the network. Given one host in the
network is observed as using a given transition technique, it is
likely that there are more.
4. Site Administrator Tools
There are some tools that site administrators can apply to make the
task for IPv6 network scanning attackers harder. These methods arise
from the considerations in the previous section.
The author notes that at his current (university) site, there is no
evidence of general network scanning running across subnets.
However, there is network scanning over IPv6 connections to systems
whose IPv6 addresses are advertised (DNS servers, MX relays, web
servers, etc), which are presumably looking for other open ports on
these hosts to probe.
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4.1. IPv6 Privacy Addresses
By using the IPv6 Privacy Extensions [2] hosts in a network may only
be able to connect to external systems using their current
(temporary) privacy address. While an attacker may be able to port
scan that address if they do so quickly upon observing or otherwise
learning of the address, the threat or risk is reduced due to the
time-constrained value of the address. One implementation of RFC3041
already deployed has privacy addresses active for one day, with such
addresses reachable for seven days.
Note that an RFC3041 host will usually also have a separate static
global IPv6 address by which it can also be reached, and that may be
DNS-advertised if an externally reachable service is running on it.
The implication is that while Privacy Addresses can mitigate the
long-term value of harvested addresses, an attacker creating an IPv6
application server to which clients connect will still be able to
probe the clients by their Privacy Address as and when they visit
that server. In the general context of hiding the addresses exposed
from a site, an administrator may choose to use IPv6 Privacy
Addresses. The duration for which these are valid will impact on the
usefulness of such observed addresses to an external attacker. The
frequency with which such address get recycled could be increased,
though this will present the site administrator with more addresses
to track the usage of.
It may be worth exploring whether firewalls can be adapted to allow
the option to block traffic initiated to a known IPv6 Privacy Address
from outside a network boundary. While some applications may
genuinely require such capability, it may be useful to be able to
differentiate in some circumstances.
4.2. DHCP Service Configuration Options
The administrator should configure DHCPv6 so that the first addresses
allocated from the pool begins much higher in the address space than
at [prefix]::1. DHCPv6 also includes an option to use Privacy
Extension [2] addresses, i.e. temporary addresses, as described in
Section 12 of the DHCPv6 [5] specification. It is desirable that
allocated addresses are not sequential, nor have any predictable
pattern to them.
4.3. Rolling Server Addresses
Given the huge address space in an IPv6 subnet/link, and the support
for IPv6 multiaddressing, whereby a node or interface may have
multiple IPv6 valid addresses of which one is preferred for sending,
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it may be possible to periodically change the advertised addresses
that certain long standing services use (where 'short' exchanges to
those services are used).
For example, an MX server could be assigned a new primary address on
a weekly basis, and old addresses expired monthly. Where MX server
IP addresses are detected and cached by spammers, such a defence may
prove useful to reduce spam volumes, especially as such IP lists may
also be passed between potential attackers for subsequent probing.
4.4. Application-Specific Addresses
By a similar reasoning, it may be possible to consider using
application-specific addresses for systems, such that a given
application may have exclusive use of an address, meaning that
disclosure of the address should not expose other applications or
services running on the same system.
5. Conclusions
Due to the much larger size of IPv6 subnets in comparison to IPv4 it
will become less feasible for network scanning methods to detect open
services for subsequent attacks. If administrators number their IPv6
subnets in 'random', non-predictable ways, attackers, whether they be
in the form of automated network scanners or dynamic worm
propagation, will need to use new methods to determine IPv6 host
addresses to target. Of course, if those systems are dual-stack, and
have open IPv4 services running, they will remain exposed to
traditional probes over IPv4 transport.
This document has discussed the considerations a site administrator
should bear in mind when considering IPv6 address planning issues and
configuring various service elements. It highlights relevant issues
and offers some informational guidance for administrators. While
some suggestions are currently more practical than others, it is up
to individual administrators to determine how much effort they wish
to invest in 'address hiding' schemes, given that this is only one
aspect of network security, and certainly not one to rely solely on.
But by implementing the basic principle of allocating 'random', non
predictable addresses, some level of obfuscation can be cheaply
deployed.
6. Security Considerations
There are no specific security considerations in this document
outside of the topic of discussion itself.
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7. IANA Considerations
There are no IANA considerations for this document.
8. Acknowledgements
Thanks are due to people in the 6NET project for discussion of this
topic, including Pekka Savola, Christian Strauf and Martin Dunmore,
as well as other contributors from the IETF v6ops mailing list,
including Tony Finch, David Malone and Fred Baker.
9. Informative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[2] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[3] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[4] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast
Addresses", RFC 3306, August 2002.
[5] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[6] Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP)
Address in an IPv6 Multicast Address", RFC 3956, November 2004.
[7] Davies, E., "IPv6 Transition/Co-existence Security
Considerations", draft-ietf-v6ops-security-overview-05 (work in
progress), September 2006.
[8] Bellovin, S. et al, "Worm Propagation Strategies in an IPv6
Internet (http://www.cs.columbia.edu/~smb/papers/v6worms.pdf)",
;login:, February 2006.
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Author's Address
Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
Email: tjc@ecs.soton.ac.uk
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