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rfc:rfc5157

Network Working Group T. Chown Request for Comments: 5157 University of Southampton Category: Informational March 2008

               IPv6 Implications for Network Scanning

Status 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 2008

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: Off-Link . . . . . . . . . . . . .  5
   3.1.  Gleaning IPv6 Prefix Information . . . . . . . . . . . . .  5
   3.2.  DNS Advertised Hosts . . . . . . . . . . . . . . . . . . .  6
   3.3.  DNS Zone Transfers . . . . . . . . . . . . . . . . . . . .  6
   3.4.  Log File Analysis  . . . . . . . . . . . . . . . . . . . .  6
   3.5.  Application Participation  . . . . . . . . . . . . . . . .  6
   3.6.  Multicast Group Addresses  . . . . . . . . . . . . . . . .  7
   3.7.  Transition Methods . . . . . . . . . . . . . . . . . . . .  7
 4.  Alternatives for Attackers: On-Link  . . . . . . . . . . . . .  7
   4.1.  General On-Link Methods  . . . . . . . . . . . . . . . . .  7
   4.2.  Intra-Site Multicast or Other Service Discovery  . . . . .  8
 5.  Tools to Mitigate Scanning Attacks . . . . . . . . . . . . . .  8
   5.1.  IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . .  9
   5.2.  Cryptographically Generated Addresses (CGAs) . . . . . . .  9
   5.3.  Non-Use of MAC Addresses in EUI-64 Format  . . . . . . . . 10
   5.4.  DHCP Service Configuration Options . . . . . . . . . . . . 10
 6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 10
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
 9.  Informative References . . . . . . . . . . . . . . . . . . . . 11

Chown Informational [Page 2] RFC 5157 IPv6 Network Scanning March 2008

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 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 in Appendix 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 Internet

Chown 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 2008

3.6. 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 generate
 RFC3306 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 (see Section 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 2008

5.1. 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 of RFC 4941 already deployed has
 privacy addresses active (used by the node) for one day, with such
 addresses reachable for seven days.
 Note that an RFC 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 2008

5.3. 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 in Section 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.  Relying

Chown 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.uk

Chown Informational [Page 12] RFC 5157 IPv6 Network Scanning March 2008

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Chown Informational [Page 13]

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