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Network Working Group                                           T. ChownRequest for Comments: 5157                     University of SouthamptonCategory: Informational                                       March 2008IPv6 Implications for Network ScanningStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Abstract   The much larger default 64-bit subnet address space of IPv6 should in   principle make traditional network (port) scanning techniques used by   certain network worms or scanning tools less effective.  While   traditional network scanning probes (whether by individuals or   automated via network worms) may become less common, administrators   should be aware that attackers may use other techniques to discover   IPv6 addresses on a target network, and thus they should also be   aware of measures that are available to mitigate them.  This   informational document discusses approaches that administrators could   take when planning their site address allocation and management   strategies as part of a defence-in-depth approach to network   security.Chown                        Informational                      [Page 1]

RFC 5157                 IPv6 Network Scanning                March 2008Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Target Address Space for Network Scanning  . . . . . . . . . .42.1.  IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . .42.2.  IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . .42.3.  Reducing the IPv6 Search Space . . . . . . . . . . . . . .42.4.  Dual-Stack Networks  . . . . . . . . . . . . . . . . . . .52.5.  Defensive Scanning . . . . . . . . . . . . . . . . . . . .53.  Alternatives for Attackers: Off-Link . . . . . . . . . . . . .53.1.  Gleaning IPv6 Prefix Information . . . . . . . . . . . . .53.2.  DNS Advertised Hosts . . . . . . . . . . . . . . . . . . .63.3.  DNS Zone Transfers . . . . . . . . . . . . . . . . . . . .63.4.  Log File Analysis  . . . . . . . . . . . . . . . . . . . .63.5.  Application Participation  . . . . . . . . . . . . . . . .63.6.  Multicast Group Addresses  . . . . . . . . . . . . . . . .73.7.  Transition Methods . . . . . . . . . . . . . . . . . . . .74.  Alternatives for Attackers: On-Link  . . . . . . . . . . . . .74.1.  General On-Link Methods  . . . . . . . . . . . . . . . . .74.2.  Intra-Site Multicast or Other Service Discovery  . . . . .85.  Tools to Mitigate Scanning Attacks . . . . . . . . . . . . . .85.1.  IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . .95.2.  Cryptographically Generated Addresses (CGAs) . . . . . . .95.3.  Non-Use of MAC Addresses in EUI-64 Format  . . . . . . . .105.4.  DHCP Service Configuration Options . . . . . . . . . . . .106.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .107.  Security Considerations  . . . . . . . . . . . . . . . . . . .108.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .119.  Informative References . . . . . . . . . . . . . . . . . . . .11Chown                        Informational                      [Page 2]

RFC 5157                 IPv6 Network Scanning                March 20081.  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 to a subsequent attack.  Today many IPv4 sites are   subjected to such probing on a recurring basis.  Such probing is   common in part due to the relatively dense population of active hosts   in any given chunk of IPv4 address space.   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 [2].  As a result, traditional methods of remote TCP or   UDP network scanning to discover open or running services on a host   will potentially become less feasible, due to the larger search space   in the subnet.  Similarly, worms that rely on off-link network   scanning to propagate may also potentially be more limited in impact.   This document discusses this property of IPv6 and describes related   issues for IPv6 site network administrators to consider, which may be   useful when planning site address allocation and management   strategies.   For example, 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.  The nature of these worms may change, if   detection of target hosts between sites or subnets is harder to   achieve by traditional methods.  However, there are other worms that   propagate via methods such as email, for which the methods discussed   in this text are not relevant.   It must be remembered that the defence of a network must not rely   solely on the unpredictable sparseness of the host addresses on that   network.  Such a feature or property is only one measure in a set of   measures that may be applied.  This document discusses various   measures that can be used by a site to mitigate attacks as part of an   overall strategy.  Some of these have a lower cost to deploy than   others.  For example, if numbering hosts on a subnet, it may be as   cheap to number hosts without any predictable pattern as it is to   number them sequentially.  In contrast, use of IPv6 privacy   extensions [3] may complicate network management (identifying which   hosts use which addresses).Chown                        Informational                      [Page 3]

RFC 5157                 IPv6 Network Scanning                March 2008   This document complements the transition-centric discussion of the   issues that can be found inAppendix A of "IPv6 Transition/   Co-existence Security Considerations" [12], which takes a broad view   of security issues for transitioning networks.  The reader is also   referred to a recent paper by Bellovin on network worm propagation   strategies in IPv6 networks [13].  This paper discusses some of the   issues included in this document, from a slightly different   perspective.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   published primary DNS or email Mail Exchange (MX) server).   Alternatively, if hosts are numbered sequentially, or using any   regular scheme, knowledge of one address may expose other available   addresses to scan.Chown                        Informational                      [Page 4]

RFC 5157                 IPv6 Network Scanning                March 2008   Second, in the case of statelessly autoconfiguring [1] hosts, the   host part of the address will usually 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 Media Access Control (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: Off-Link   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.3.1.  Gleaning IPv6 Prefix Information   Note that in IPv6, an attacker would not be able to search across the   entire IPv6 address space as they might in IPv4.  An attacker may   learn general prefixes to focus their efforts on by observing route   view information (e.g., from public looking-glass services) or   information on allocated address space from Regional InternetChown                        Informational                      [Page 5]

RFC 5157                 IPv6 Network Scanning                March 2008   Registries (RIRs).  In general, this would only yield information at   most at the /48 prefix granularity, though some specific /64 prefixes   may be observed from route views on some parts of some networks.3.2.  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.   While servers are relatively easy to find because they are DNS-   published, any systems that are not DNS-published will be much harder   to locate via traditional scanning than is the case for IPv4   networks.  It is worth noting that where a site uses sequential host   numbering, publishing just one address may lead to a threat upon the   other hosts.3.3.  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 that   restricting zone transfers is (more) important for IPv6, even if it   is already good practice to restrict them in the IPv4 world.   There are some projects that provide Internet mapping data from   access to such transfers.  Administrators may of course agree to   provide such transfers where they choose to do so.3.4.  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.3.5.  Application Participation   More recent peer-to-peer applications often include some centralised   server that 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.Chown                        Informational                      [Page 6]

RFC 5157                 IPv6 Network Scanning                March 20083.6.  Multicast Group Addresses   Where an Embedded Rendezvous Point (RP) [7] multicast group address   is known, the unicast address of the RP is implied by the group   address.  Where unicast-prefix-based multicast group addresses [5]   are used, specific /64 link prefixes may also be disclosed in traffic   that goes off-site.  An administrator may thus choose to put aside   /64 bit prefixes for multicast group addresses that are not in use   for normal unicast routing and addressing.  Alternatively, a site may   simply use their non-specific /48 site prefix allocation to generateRFC3306 multicast group addresses.3.7.  Transition Methods   Specific knowledge of the target network may be gleaned if that   attacker knows it is using 6to4 [4], ISATAP [10], Teredo [11], 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.   In the case of Teredo, the 64-bit node identifier is generated from   the IPv4 address observed at a Teredo server along with a UDP port   number.  The Teredo specification also allows for discovery of other   Teredo clients on the same IPv4 subnet via a well-known IPv4   multicast address (seeSection 2.17 of RFC 4380 [11]).4.  Alternatives for Attackers: On-Link   The main thrust of this text is considerations for off-link attackers   or probing of a network.  In general, once one host on a link is   compromised, others on the link can be very readily discovered.4.1.  General On-Link Methods   If the attacker already has access to a system on the current subnet,   then traffic on that subnet, be it Neighbour Discovery or   application-based traffic, can invariably be observed, and active   node addresses within the local subnet learnt.Chown                        Informational                      [Page 7]

RFC 5157                 IPv6 Network Scanning                March 2008   In addition to making observations of traffic on the link, IPv6-   enabled hosts on local subnets may be discovered through probing the   "all hosts" link-local multicast address.  Likewise, any routers on   the subnet may be found via the "all routers" link-local multicast   address.  An attacker may choose to probe in a slightly more   obfuscated way by probing the solicited node multicast address of a   potential target host.   Where a host has already been compromised, its Neighbour Discovery   cache is also likely to include information about active nodes on the   current subnet, just as an ARP cache would do for IPv4.   Also, depending on the node, traffic to or from other nodes (in   particular, server systems) is likely to show up if an attacker can   gain a presence on a node in any one subnet in a site's network.4.2.  Intra-Site 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 also choose to use any   other service discovery methods supported by the site.5.  Tools to Mitigate Scanning Attacks   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 further.  At the time of writing, DHCPv6 [6] is   not yet in use at the author's site, and clients use stateless   autoconfiguration.  Therefore, the author's site does not yet have   sequentially numbered client hosts deployed as may typically be seen   in today's IPv4 DHCP-served networks.Chown                        Informational                      [Page 8]

RFC 5157                 IPv6 Network Scanning                March 20085.1.  IPv6 Privacy Addresses   Hosts in a network using IPv6 privacy extensions [3] will typically   only connect to external systems using their current (temporary)   privacy address.  The precise behaviour of a host with a stable   global address and one or more dynamic privacy address(es) when   selecting a source address to use may be operating-system-specific,   or configurable, but typical behaviour when initiating a connection   is use of a privacy address when available.   While an attacker may be able to port scan a privacy 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 ofRFC 4941 already deployed has   privacy addresses active (used by the node) for one day, with such   addresses reachable for seven days.   Note that anRFC 4941 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.   DHCPv6 can be used to serve normal global addresses and IPv6 privacy   addresses.   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 when they visit that   server.  The duration for which privacy addresses are valid will   impact the usefulness of such observed addresses to an external   attacker.  For example, a worm that may spread using such observed   addresses may be less effective if it relies on harvested privacy   addresses.  The frequency with which such address get recycled could   be increased, though this may increase the complexity of local   network management for the administrator, since doing so will cause   more addresses to be used over time in the site.   A further option here may be to consider using different addresses   for specific applications, or even each new application instance,   which may reduce exposure to other services running on the same host   when such an address is observed externally.5.2.  Cryptographically Generated Addresses (CGAs)   The use of Cryptographically Generated Addresses (CGAs) [9] may also   cause the search space to be increased from that presented by default   use of stateless autoconfiguration.  Such addresses would be seen   where Secure Neighbour Discovery (SEND) [8] is in use.Chown                        Informational                      [Page 9]

RFC 5157                 IPv6 Network Scanning                March 20085.3.  Non-Use of MAC Addresses in EUI-64 Format   The EUI-64 identifier format does not require the use of MAC   addresses for identifier construction.  At least one well known   operating system currently defaults to generation of the 64-bit   interface identifier by use of random bits, and thus does not embed   the MAC address.  Where such a method exists, an administrator may   wish to consider using that option.5.4.  DHCP Service Configuration Options   One option open to an administrator is to configure DHCPv6, if   possible, so that the first addresses allocated from the pool begins   much higher in the address space than at [prefix]::1.  Further, it is   desirable that allocated addresses are not sequential and do not have   any predictable pattern to them.  Unpredictable sparseness in the   allocated addresses is a desirable property.  DHCPv6 implementers   could reduce the cost for administrators to deploy such 'random'   addressing by supporting configuration options to allow such   behaviour.   DHCPv6 also includes an option to use privacy extension [3]   addresses, i.e., temporary addresses, as described inSection 12 of   the DHCPv6 [6] specification.6.  Conclusions   Due to the much larger size of IPv6 subnets in comparison to IPv4, it   will become less feasible for traditional network scanning methods to   detect open services for subsequent attacks, assuming the attackers   are off-site and services are not listed in the DNS.  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 make wider use of   new methods to determine IPv6 host addresses to target (e.g., looking   to obtain logs of activity from a site and scanning addresses around   the ones observed).  Such numbering schemes may be very low cost to   deploy in comparison to conventional sequential numbering, and thus,   a useful part of an overall defence-in-depth strategy.  Of course, if   those systems are dual-stack, and have open IPv4 services running,   they will remain exposed to traditional probes over IPv4 transport.7.  Security Considerations   There are no specific security considerations in this document   outside of the topic of discussion itself.  However, it must be noted   that the 'security through obscurity' discussions and commentary   within this text must be noted in their proper context.  RelyingChown                        Informational                     [Page 10]

RFC 5157                 IPv6 Network Scanning                March 2008   purely on obscurity of a node address is not prudent, rather the   advice here should be considered as part of a 'defence-in-depth'   approach to security for a site or network.  This also implies that   these measures require coordination between network administrators   and those who maintain DNS services, though this is common in most   scenarios.8.  Acknowledgements   Thanks are due to people in the 6NET project (www.6net.org) for   discussion of this topic, including Pekka Savola, Christian Strauf,   and Martin Dunmore, as well as other contributors from the IETF v6ops   and other mailing lists, including Tony Finch, David Malone, Bernie   Volz, Fred Baker, Andrew Sullivan, Tony Hain, Dave Thaler, and Alex   Petrescu.  Thanks are also due for editorial feedback from Brian   Carpenter, Lars Eggert, and Jonne Soininen amongst others.9.  Informative References   [1]   Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)         Specification",RFC 2460, December 1998.   [2]   Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address         Autoconfiguration",RFC 4862, September 2007.   [3]   Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions         for Stateless Address Autoconfiguration in IPv6",RFC 4941,         September 2007.   [4]   Carpenter, B. and K. Moore, "Connection of IPv6 Domains via         IPv4 Clouds",RFC 3056, February 2001.   [5]   Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6         Multicast Addresses",RFC 3306, August 2002.   [6]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.         Carney, "Dynamic Host Configuration Protocol for IPv6         (DHCPv6)",RFC 3315, July 2003.   [7]   Savola, P. and B. Haberman, "Embedding the Rendezvous Point         (RP) Address in an IPv6 Multicast Address",RFC 3956,         November 2004.   [8]   Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure         Neighbor Discovery (SEND)",RFC 3971, March 2005.   [9]   Aura, T., "Cryptographically Generated Addresses (CGA)",RFC 3972, March 2005.Chown                        Informational                     [Page 11]

RFC 5157                 IPv6 Network Scanning                March 2008   [10]  Templin, F., Gleeson, T., Talwar, M., and D. Thaler, "Intra-         Site Automatic Tunnel Addressing Protocol (ISATAP)",RFC 4214,         October 2005.   [11]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network         Address Translations (NATs)",RFC 4380, February 2006.   [12]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/         Co-existence Security Considerations",RFC 4942,         September 2007.   [13]  Bellovin, S., et al, "Worm Propagation Strategies in an IPv6         Internet", as published in ;login:, February 2006,         <http://www.cs.columbia.edu/~smb/papers/v6worms.pdf>.Author's Address   Tim Chown   University of Southampton   Southampton, Hampshire  SO17 1BJ   United Kingdom   EMail: tjc@ecs.soton.ac.ukChown                        Informational                     [Page 12]

RFC 5157                 IPv6 Network Scanning                March 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Chown                        Informational                     [Page 13]