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IPv6 Operations                                                 T. Chown
Internet-Draft                                 University of Southampton
Expires: January 17, 2005                                  July 19, 2004



              IPv6 Implications for TCP/UDP Port Scanning
            draft-chown-v6ops-port-scanning-implications-01


Status of this Memo


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   This Internet-Draft will expire on January 17, 2005.


Copyright Notice


   Copyright (C) The Internet Society (2004).  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 . . . . . . . . . . . .  3
     2.1   IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2   IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.3   Reducing the IPv6 Search Space . . . . . . . . . . . . . .  4
     2.4   DNS considerations . . . . . . . . . . . . . . . . . . . .  4
     2.5   Dual-stack networks  . . . . . . . . . . . . . . . . . . .  4
   3.  Alternatives for Attackers . . . . . . . . . . . . . . . . . .  5
   4.  Recommendations for Site Administrators  . . . . . . . . . . .  5
     4.1   Use of IPv6 Privacy Addresses  . . . . . . . . . . . . . .  5
     4.2   DHCPv6 Configuration . . . . . . . . . . . . . . . . . . .  5
     4.3   Defensive Scanning . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Informative References . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7

































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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.


   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.


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




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   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 [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.


   Even if the vendor code is not known, the search space can be reduced
   by using well-known, common vendor codes.


   Further reductions may be possible if the attacker knows the target
   is using 6over4, ISATAP, 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).


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.


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, with (more readily scannable) IPv4 connectivity




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   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.


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).


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.


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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   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.


4.3  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.


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 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.



Author's Address


   Tim Chown
   University of Southampton


   Southampton, Hampshire  SO17 1BJ
   United Kingdom


   EMail: tjc@ecs.soton.ac.uk




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Acknowledgment


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





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