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Versions: 00 01 02 03 04 rfc5157                                        
IPv6 Operations                                                 T. Chown
Internet-Draft                                 University of Southampton
Expires: April 26, 2007                                 October 23, 2006

                 IPv6 Implications for Network Scanning

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The 128 bits of IPv6 address space is considerably bigger than the 32
   bits of address space of 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 network
   scanning to discover open or running services on a host will
   potentially become far less feasible, due to the larger search space
   in the subnet.  In addition automated attacks, such as those
   performed by network worms, may be hampered.  This document discusses
   this property of IPv6, and describes related issues for site

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   administrators of IPv6 networks to consider, which may be of
   importance when planning site address allocation and management
   strategies.  While traditional network 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 network, and be aware of
   appropriate measures to mitigate these.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Target Address Space for Network 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  . . . . . . . . . . . . . . . . . . . . .  6
     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 . . . . . . . . . . . . . . . . . . . .  7
     3.6.  Application Participation  . . . . . . . . . . . . . . . .  7
     3.7.  Transition Methods . . . . . . . . . . . . . . . . . . . .  7
   4.  Site Administrator Tools . . . . . . . . . . . . . . . . . . .  7
     4.1.  IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . .  8
     4.2.  DHCP Service Configuration Options . . . . . . . . . . . .  8
     4.3.  Rolling Server Addresses . . . . . . . . . . . . . . . . .  8
     4.4.  Application-Specific Addresses . . . . . . . . . . . . . .  9
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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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 network scanning, whereby an automated
   process is run to detect open ports (services) on systems that may
   then be subject of a subsequent attack.  Today many IPv4 sites are
   subjected to such probing on a recurring basis.

   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 network scanning to discover open or running services on a host
   will potentially become far less feasible, due to the larger search
   space in the subnet.  This document discusses this property of IPv6,
   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 [7] text, which takes a broad
   view of security issues for transitioning networks.

   The reader is also referred to a recent paper by Bellovin on worm
   propogation strategies in IPv6 networks [8].  This paper discusses
   some of the issues included in this document, from a slightly
   different perspective.

   Network scanning is quite a prevalent tactic used 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
   may then subsequently be able to compromise.  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
   solely 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

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   little or no cost to the administrator to implement it.  However, a
   law of diminuishing returns does apply.  An administrator who
   undertakes an address hiding policy should be aware that while IPv6
   host addresses may be picked that are likely to take significant time
   to discover by traditional scanning methods, there are other means by
   which such addresses may be discovered.  Implementing all of them may
   be deemed unwarranted effort.  But it is up to the site administrator
   to be aware of the context and the options available, and in
   particular what new methods may attackers use to glean IPv6 address
   information, and how these can potentially be mitigated against.
   This document is intended to be informational; there is not yet
   sufficient deployment experience for it to be considered BCP.

2.  Target Address Space for Network 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 service is running publicly 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 in principle 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.  However, there are ways for the attacker to
   reduce the address search space to scan against within the target
   subnet, as we discuss below.

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

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   primary DNS or email MX server).  Alternatively if hosts are numbered
   sequentially, or using any regular scheme, knowledge of one address
   may expose other available addresses to scan.

   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,
   the search space can be reduced to 48 bits.  Further, if the Ethernet
   vendor is also known, the search space may be reduced to 24 bits,
   with a one probe per second scan then taking a less daunting 194
   days.  Even where the exact vendor is not known, using a set of
   common vendor prefixes can reduce the search.  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
   autoconfigured address is known, scanning around that address may
   yield results for the attacker.  Again, 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 network 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.  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 hosts in subnets are allocated addresses 'randomly', and as a
   result IPv6 network scanning becomes relatively infeasible, attackers
   will need to find new methods to identify IPv6 addresses for
   subsequent 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.

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3.1.  On-link Methods

   If the attacker is on link, then traffic on the link, be it Neighbor
   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

   Where a host has already been compromised, its Neighbor Discovery
   cache is also likely to include information about active nodes on
   link, just as an ARP cache would do for IPv4.

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.

   There are also issues with disclosure from multicast itself.  Where
   an Embedded RP [6] multicast group address is known, the unicast
   address of the rendezvous point is implied by the group address.
   Where unicast prefix based multicast group addresses [4] are used,
   specific /64 link prefixes may also be disclosed.

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.

   Sites may use a two-faced DNS where internal system DNS information

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   is only published in an internal DNS.  It is also worth noting that
   the reverse DNS tree may also expose address information.

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 may 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
   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 network 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 network scanning running across subnets.
   However, there is network 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.

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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 or otherwise
   learning of 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, with 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.  The
   frequency with which such address get recycled could be increased,
   though this will present the site administrator with more addresses
   to track the usage of.

   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.

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
   at [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 [5] specification.  It is desirable that
   allocated addresses are not sequential, nor have any predictable
   pattern to them.

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,

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   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.

4.4.  Application-Specific Addresses

   By a similar reasoning, it may be possible to consider using
   application-specific addresses for systems, such that a given
   application may have exclusive use of an address, meaning that
   disclosure of the address should not expose other applications or
   services running on the same system.

5.  Conclusions

   Due to the much larger size of IPv6 subnets in comparison to IPv4 it
   will become less feasible for network scanning methods to detect open
   services for subsequent attacks.  If administrators number their IPv6
   subnets in 'random', non-predictable ways, attackers, whether they be
   in the form of automated network scanners or dynamic worm
   propagation, will need to use new methods to determine IPv6 host
   addresses to target.  Of course, if those systems are dual-stack, and
   have open IPv4 services running, they will remain exposed to
   traditional probes over IPv4 transport.

   This document has discussed 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 offers some informational guidance for administrators.  While
   some suggestions are currently more practical than others, it is up
   to individual administrators to determine how much effort they wish
   to invest in 'address hiding' schemes, given that this is only one
   aspect of network security, and certainly not one to rely solely on.
   But by implementing the basic principle of allocating 'random', non
   predictable addresses, some level of obfuscation can be cheaply

6.  Security Considerations

   There are no specific security considerations in this document
   outside of the topic of discussion itself.

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7.  IANA Considerations

   There are no IANA considerations for this document.

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]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast
        Addresses", RFC 3306, August 2002.

   [5]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
        Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
        RFC 3315, July 2003.

   [6]  Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP)
        Address in an IPv6 Multicast Address", RFC 3956, November 2004.

   [7]  Davies, E., "IPv6 Transition/Co-existence Security
        Considerations", draft-ietf-v6ops-security-overview-05 (work in
        progress), September 2006.

   [8]  Bellovin, S. et al, "Worm Propagation Strategies in an IPv6
        Internet (http://www.cs.columbia.edu/~smb/papers/v6worms.pdf)",
        ;login:, February 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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