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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

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on March 31, 2004.

Copyright Notice

   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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Intellectual Property Statement

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































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