IPv6 Operations T. Chown
Internet-Draft University of Southampton
Expires: March 31, 2004 Oct 2003
IPv6 Implications for TCP/UDP Port Scanning
draft-chown-v6ops-port-scanning-implications-00
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Copyright (C) The Internet Society (2003). All Rights Reserved.
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.
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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 . . . . . . . . . . . . . . . . . . . . . 4
3. Alternatives for Attackers . . . . . . . . . . . . . . . . . . 5
4. Recommendations for Site Administrators . . . . . . . . . . . 6
4.1 Use of IPv6 Privacy Addresses . . . . . . . . . . . . . . . . 6
4.2 DHCPv6 Configuration . . . . . . . . . . . . . . . . . . . . . 6
5. Potential Standards Extensions . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
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.
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 growing 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 will generate
many Megabytes of log files on a daily basis purely from port
scanning activity.
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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 one probe per second, such a scan may take some 5
billion years to complete.
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 [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).
2.4 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, with (more readily scannable) IPv4 connectivity
also. 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.
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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
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 virii that may
attach to or attack the new IPv6 applications (e.g. peer-to-peer
messaging).
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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 decribe such
methods in this section.
4.1 Use of IPv6 Privacy Addresses
By using the IPv6 Privacy Extensions [3] the hosts in the network
would 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 have a
separate static global IPv6 address by which it can also be reached.
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.
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5. Potential Standards Extensions
It may be worth considering a standards extenstion to DHCPv6 that in
some way allows a "random" IPv6 host address part to be assigned to a
host, that will then be used for that host to receive incoming
communications (and upon which it would thus need to be port scanned
by an attacker).
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6. Security Considerations
There are no specific security considerations in this document
outside of the topic of discussion itself.
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7. Acknowledgements
Thanks are due to people in the 6NET project for discussion of this
topic, including Pekka Savola (CSS/FUNET) and Christian Strauf (JOIN
Project, University of Muenster).
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Normative 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.
Author's Address
Tim Chown
University of Southampton
Southampton, Hampshire SO17 1BJ
United Kingdom
EMail: tjc@ecs.soton.ac.uk
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