Internet DRAFT - draft-chown-v6ops-port-scanning-implications
draft-chown-v6ops-port-scanning-implications
IPv6 Operations T. Chown
Internet-Draft University of Southampton
Expires: April 30, 2006 October 27, 2005
IPv6 Implications for TCP/UDP Port Scanning
draft-chown-v6ops-port-scanning-implications-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
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.
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 DNS considerations . . . . . . . . . . . . . . . . . . . . 5
2.5 Dual-stack networks . . . . . . . . . . . . . . . . . . . 5
2.6 Defensive Scanning . . . . . . . . . . . . . . . . . . . . 5
3. Alternatives for Attackers . . . . . . . . . . . . . . . . . . 5
4. Recommendations for Site Administrators . . . . . . . . . . . 6
4.1 Use of IPv6 Privacy Addresses . . . . . . . . . . . . . . 6
4.2 DHCPv6 Configuration . . . . . . . . . . . . . . . . . . . 6
4.3 Rolling Server Addresses . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. Informative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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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. 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.
It must be remembered that the defense 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 considerations should be given some weight where to
implement appropriate measures is of little cost to the
administrator.
Port scanning is quite a prevalent tactic from would-be attackers.
The author observes that a typical university firewall may generate
many tens of megabytes of log files on a daily basis purely from port
scanning activity.
It is also worth noting that worms that spread by scanning target
networks for hosts to re-attack have become more common in recent
times. Thus a much more sparsely address-populated IPv6 network will
have a more innate defense to such forms of worm infection, although
there may still be significant scanning traffic generated.
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2. Target Address Space for Port 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. At one probe per second, such a scan may 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.
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
upwards.
Second, in the case of statelessly autoconfiguring [1] hosts, the
host part of the address will take a well-known format that includes
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.
Further reductions may be possible if the attacker knows the target
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.
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2.4 DNS considerations
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 DNS registered. Where a site uses sequential
host numbering, publishing just one address may lead to a threat upon
the other hosts.
There is a relation between port scanning and DNS zone transfers. In
the IPv4 world, this relationship is very weak because the IPv4 space
is densely populated and a DNS zone transfer (usually) doesn't help
an attacker target a port scan significantly. In the IPv6 world, a
zone transfer is much more likely to narrow the number of targeted
hosts. This implies restricting zone transfers is (more) important
for IPv6, even if it is already good practice to restrict them in the
IPv4 world.
2.5 Dual-stack networks
Full advantage of the increased IPv6 address space in terms of
reslience 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 port scanning.
2.6 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 may have the advantage of being
on-link for scanning purposes though, or be able to deduce
information about on-link hosts through queries to managed Ethernet
switching equipment.
3. Alternatives for Attackers
If IPv6 port-scanning becomes infeasible, attackers will need to find
new methods to identify IPv6 addresses for subsequent port scanning.
One such method would be the harvesting of IPv6 addresses, either in
transit or from recorded logs such as web site logs. Another may be
to inspect the Received from: or other header lines in archived email
or Usenet news messages.
IPv6-enabled hosts on local subnets may still be discovered through
probing the "all hosts" link local multicast address. This implies
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that if an attacker can compromise one remote host, they may then
learn addresses of the hosts in the same subnet on the remote
network.
In IPv6 networks, attackers may also switch to using more aggressive
yet subtle methods of attack, e.g. by using worms or viruses that may
attach to or attack the new IPv6 applications (e.g. peer-to-peer
messaging).
4. Recommendations for Site Administrators
There are some methods that site administrators can apply to make the
task for IPv6 port scanning attackers harder. We describe such
methods in this 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 a presumably looking for other open ports on these hosts to
probe.
4.1 Use of IPv6 Privacy Addresses
By using the IPv6 Privacy Extensions [3] the hosts in the network may
be able to only ever connect to external sites using their
(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. An example implementation of RFC3041
already deployed has privacy addresses active for one day, but such
addresses reachable for seven days.
Note that an RFC3041 host may well 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 from it.
However, for client-only systems, RFC3041 offers some level of
defence.
4.2 DHCPv6 Configuration
The administrator could configure DHCPv6 so that the first addresses
allocated from the pool begin much higher in the address space than
[prefix]::1.
DHCPv6 also includes an option to use Privacy Extension [3]
addresses, i.e. temporary addresses, as described in Section 12 of
the DHCPv6 [4] specification.
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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 defense may
prove useful, especially as such IP lists may also be passed between
potential attackers for subsequent probing.
5. Security Considerations
There are no specific security considerations in this document
outside of the topic of discussion itself.
6. Acknowledgements
Thanks are due to people in the 6NET project for discussion of this
topic, including Pekka Savola (CSC/FUNET), Christian Strauf (JOIN
Project, University of Muenster) and Martin Dunmore (Lancaster), as
well as Tony Finch (Cambridge) and David Malone (TCD, Dublin).
7. Informative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[2] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[3] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[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-03 (work in
progress), October 2005.
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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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