IPv6 Operations T. Chown
Internet-Draft University of Southampton
Expires: December 21, 2006 June 19, 2006
IPv6 Implications for Network Scanning
draft-ietf-v6ops-scanning-implications-00
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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 in 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 port
scanning to discover open or running services on a host will
potentially become far less computationally feasible, due to the
larger search space in the subnet. This document discusses that
property of IPv6 subnets, and describes related issues for site
administrators of IPv6 networks to consider, which may be of
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importance when planning site address allocation and management
strategies. While traditional port 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 subnet, and take appropriate
measures to preempt these.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Target Address Space for Port 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 . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . 6
3.6. Application Participation . . . . . . . . . . . . . . . . 6
3.7. Transition Methods . . . . . . . . . . . . . . . . . . . . 6
4. Site Administrator Tools . . . . . . . . . . . . . . . . . . . 7
4.1. IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . . 7
4.2. DHCP Service Configuration Options . . . . . . . . . . . . 8
4.3. Rolling Server Addresses . . . . . . . . . . . . . . . . . 8
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. Informative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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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 based port scanning probing, which is
something that most of today's IPv4 sites are subjected to routinely
around the clock.
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 port scanning to discover open or running services on a host will
potentially become far less computationally feasible, due to the
larger search space in the subnet. This document discusses that
property of IPv6 subnets, 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 [5] text, which takes a broad
view of security issues for transitioning networks.
Port scanning is quite a prevalent tactic 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 then may
subsequently compromise. The author observes that a typical
university firewall may today generate many tens of megabytes of log
files on a daily basis purely from port scanning activity.
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 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
little or no cost to the administrator. That said, the administrator
must be aware of the context. What new methods may attackers use to
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glean IPv6 address information, and how can these be mitigated
against?
2. Target Address Space for Port Scanning
There are significantly different considerations for the feasibility
of plain, brute force IPv4 and IPv6 address scanning.
2.1. IPv4
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 open service is running on a
host in that subnet. Even at only one probe per second, such a scan
would take under 5 minutes to complete.
2.2. IPv6
A typical IPv6 subnet will have 64 bits reserved for host addressing.
In such a case, a remote attacker 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, unless the attacker can deduce ways to reduce the address
space to scan against within the target subnet.
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
primary DNS or email MX server).
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,
if the Ethernet vendor is known, the search space may be reduced to
24 bits (with a one probe per second scan then taking 194 days).
Even where the exact vendor is not known, using a set of common
vendor prefixes can reduce the search space. 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
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autoconfigured address is known, scanning around that address may
yield results for the attacker. 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 port scanning may not be gained until IPv6-only
networks and devices become more commonplace, given that most IPv6
hosts are currently dual stack, also 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 port-scanning becomes relatively infeasible, attackers will
need to find new methods to identify IPv6 addresses for subsequent
port 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.
3.1. On-link Methods
If the attacker is on link, then traffic on the link, be it Neighbour
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 Neighbour Discovery
cache is also likely to include information about active nodes on
link, just as an ARP cache would do for IPv4.
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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.
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.
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 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
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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 port 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 port scanning running across subnets. However,
there is port-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.
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 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, but 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.
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.
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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
[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 [4] specification. It is desirable that
allocated addresses are not sequential.
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,
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.
5. Conclusions
Due to the size of IPv6 subnets attackers, whether they be in the
form of automated port scanning or dynamic worm propagation, will
need to use new methods to determine IPv6 host addresses to target.
This document discusses 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 makes some informational recommendations for administrators.
6. Security Considerations
There are no specific security considerations in this document
outside of the topic of discussion itself.
7. IANA Considerations
There are no IANA considerations for this document.
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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] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[5] Davies, E., "IPv6 Transition/Co-existence Security
Considerations", draft-ietf-v6ops-security-overview-04 (work in
progress), March 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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