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Internet Engineering Task Force (IETF)                          F. BakerRequest for Comments: 6018                                 Cisco SystemsCategory: Informational                                        W. HarropISSN: 2070-1721                                              G. Armitage                                      Swinburne University of Technology                                                          September 2010IPv4 and IPv6 GreynetsAbstract   This note discusses a feature to support building Greynets for IPv4   and IPv6.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6018.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Baker, et al.                 Informational                     [Page 1]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  History and Experience  . . . . . . . . . . . . . . . . . .32.  Deploying Greynets  . . . . . . . . . . . . . . . . . . . . . .42.1.  Deployment Using Routing - Darknets . . . . . . . . . . . .42.2.  Deployment Using Sparse Address Space - Greynets  . . . . .42.3.  Other Filters . . . . . . . . . . . . . . . . . . . . . . .63.  Implications for Router Design  . . . . . . . . . . . . . . . .64.  Security Considerations . . . . . . . . . . . . . . . . . . . .65.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .76.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .86.1.  Normative References  . . . . . . . . . . . . . . . . . . .86.2.  Informative References  . . . . . . . . . . . . . . . . . .81.  Introduction   Darknets, also called "Network Telescopes" among other things, have   been deployed by several organizations (including CAIDA, Team Cymru,   and the University of Michigan) to look at traffic directed to   addresses in blocks that are not in actual use.  Such traffic becomes   visible by either direct capture (it is routed to a collector) or by   virtue of its backscatter (its resulting in ICMP traffic or   transport-layer resets).   Darknets, of course, have two problems.  As their address spaces   become known, attackers stop probing them, so they are less   effective.  Also, the administrators of those prefixes are pressured   by Regional Internet Registry (RIR) policy and business requirements   to deploy them in active networks.   [Harrop] defines a 'Greynet' by extension, in these words:      Darknets are often proposed to monitor for anomalous, externally      sourced traffic, and require large, contiguous blocks of unused IP      addresses - not always feasible for enterprise network operators.      We introduce and evaluate the Greynet - a region of IP address      space that is sparsely populated with "darknet" addresses      interspersed with active (or "lit") IP addresses.  Based on a      small sample of traffic collected within a university campus      network we saw that relatively sparse greynets can achieve useful      levels of network scan detection.   In other words, instead of setting aside prefixes that an attacker   might attempt to probe and in so doing court discovery, Harrop   proposed that individual (or small groups of adjacent) addresses in   subnets be set aside for the purpose, using different host   identifiers in each subnet to make it more difficult for an addressBaker, et al.                 Informational                     [Page 2]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   scan to detect them.  The concept has value in the sense that it is   harder to map the addresses or prefixes out of an attacker's search   pattern, as their presence is more obscure.  Harrop's research was   carried out using IPv4 [RFC0791] and yielded interesting information.1.1.  History and Experience   The research supporting this proposal includes two prototypes, one   with IPv4 [RFC0791] and one with IPv6 [RFC2460].  Both have   limitations, being research experiments as opposed to deployment of a   finished product.   The original research was done by Warren Harrop and documented in   [Harrop].  This was IPv4-only.  His premise was that one would put a   virtual or physical machine on a LAN that one was not otherwise   using, and use it to identify scans of various kinds.  As reported in   his paper, the concept worked effectively in a prototype deployment   at the Centre for Advanced Internet Architectures (CAIA), Swinburne   University of Technology.  The basic reason was that there was a   reasonable expectation on the part of a potential attacker that a   given address might be represented, and there was no pattern that   would enable the attacker to predict which addresses were being used   in this way.  CAIA developed and released a prototype FreeBSD-based   Greynet system in 2008 built around this premise [Armitage].   Baker's addition to his concept started from the router, the idea   that the router would be highly likely to encounter any such scan if   it came from off-LAN, and the fact that the router would have to use   Address Resolution Protocol (ARP) or Neighbor Discovery (ND) to   identify -- or fail to identify -- the machine in question.  In   effect, any address that is not currently instantiated in the subnet   acts as a Greynet trigger address.  This clearly also works for any   system that would implement ARP or ND, but the router is an obvious   focal point in any subnet.   Tim Chown, of the School of Electronics and Computer Science,   University of Southampton, offered privately to do some research on   it, and had Owen Stephens do a Linux prototype in spring 2010.  They   demonstrated that the technology was straightforward to implement and   in fact worked in a prototype IPv6 implementation.   The question that remains with IPv6 address scanning is the   likelihood that the attack would occur at all.  Chown originally   argued in [RFC5157] that address scans were impossible due to the   sheer number of possibilities.  However, in September 2010 a report   was made to NANOG of an IPv6 address scan.  Additionally, there are   ways to limit the field; for example, one can observe that a company   buys a certain kind of machine or network interface card (NIC), andBaker, et al.                 Informational                     [Page 3]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   therefore its probable EUI-64 addresses are limited to a much smaller   range than 2^64 -- more like 2^24 addresses on a given subnet -- or   one can observe DNS, SMTP envelopes, Extensible Messaging and   Presence Protocol (XMPP) messages, FTP, HTTP, etc., that carry IP   addresses in other ways.  Such attacks can be limited by the use of   Privacy Addresses [RFC4941], which periodically change, rendering   historical information less useful, but the fact is that such   analytic methods exist.2.  Deploying Greynets   Corporate IT departments and other network operators frequently run   collectors or other kinds of sensors.  A collector is a computer   system on the Internet that is expressly set up to attract and "trap"   nefarious attempts to penetrate computer systems.  Such systems may   simply record the attempt or the datagram that initiated the attempt   (darknets/Greynets), or they may act as a decoy, luring in potential   attacks in order to study their activities and study their methods   (honeypots).   To accomplish this, we separate nefarious traffic from that which is   likely normal and important, studying one and facilitating the other.2.1.  Deployment Using Routing - Darknets   One obvious way to isolate and identify nefarious traffic is to   realize that it is sent to a prefix or address that is not   instantiated.  If a campus uses an IPv4 /24 prefix or an IPv6 /56   prefix but contains less than 100 actual subnets, for example, we   might use only odd numbered subnets (128 of the 256 available in that   prefix), and not quite all of those.  Knowing that the active   prefixes are more specific and therefore attract appropriate traffic,   we might also advertise the default prefix from the collector,   attracting traffic directed to the uninstantiated prefixes in that   routing domain.   A second question involves mimicking a host under attack; the   collector may simply record this uninvited traffic, or may reply as a   honeypot system.2.2.  Deployment Using Sparse Address Space - Greynets   IPv4 subnets usually have some unallocated space in them, if only   because Classless Inter-Domain Routing (CIDR) allocates O(2^n)   addresses to an IP subnet and there are not exactly that many systems   there.Baker, et al.                 Informational                     [Page 4]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   Similarly, with active IPv6 prefixes, even a very large switched LAN   is likely to use a small fraction of the available addresses.  This   is by design, as discussed inSection 2.5.1 of [RFC4291].  If the   addresses are distributed reasonably randomly among the possible   values, the likelihood of an attacker guessing what addresses are in   actual use is limited.  This gives us an opportunity with respect to   unused addresses within an IP prefix.   Routers use IPv4 ARP [RFC0826] and IPv6 Neighbor Discovery [RFC4861]   to determine the MAC (Media Access Control) address of a neighbor to   which a datagram needs to be sent.  Both specifications intend that   when a datagram arrives at a router that serves the target prefix,   but that doesn't know the MAC address of the intended destination, it   should:   o  Enqueue the datagram,   o  Emit a Neighbor Solicitation or ARP Request,   o  Await a Neighbor Advertisement or ARP Response, and   o  On receipt, dequeue and forward the datagram.   Once the host's MAC address is in the router's tables (and in so   doing the address proven valid), the matter is not an issue.   In [Harrop], the Greynet is described as being instantiated on an   end-host that replies to ARP Requests for all 'dark' IP addresses.   However, a small modification to router behavior can augment this   model.  As well as queuing or dropping a datagram that has triggered   an ARP Request or Neighbor Solicitation, the router forwards a copy   of this datagram over an independent link to the Greynet's analytic   equipment.  This independent link may be a different physical   interface, a circuit, VLAN, tunnel, UDP, or other encapsulation, or   in fact any place such a datagram could be handled.  Depending on the   requirements of the receiving collector, one could also imagine   summarizing information in a form similar to IP Flow Information   Export (IPFIX) [RFC5101] [RFC5610].   The analytic equipment will now receive two types of datagrams.  Of   most interest will be those destined for 'dark' IP addresses.  Of   less interest will be the irregular case where a datagram arrives for   a legitimate local neighbor who has, for some temporary reason, no   MAC address in the router's tables.  Datagrams arriving for an IP   destination for which an ARP reply (or Neighbor Advertisement) has   not yet received might also be forwarded to the analytical equipment   over the independent link -- or might not, if they are considered to   be unlikely to provide new analytic information.Baker, et al.                 Informational                     [Page 5]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   Analytic equipment, depending on the router to recognize 'dark' IP   addresses in this manner, can easily track arrival patterns of   datagrams destined to unused parts of the network.  It may also   optionally choose to respond to such datagrams, acting as a honeypot   to elicit further datagrams from the remote source.   If the collector replies directly, the attacker may be able to   identify the fact through information in or about the datagram -   datagrams sent to the same IP subnet may come back with different TTL   values, for example.  Hence, it may be advisable for the collector to   send the reply back through the tunnel and therefore as if from the   same IP subnet.  Naturally, the collector in this scenario should not   respond to datagrams destined for 'lit' IP addresses -- the intended   destination will eventually respond to the router's ARP or Neighbor   Solicitation anyway.   One implication of this model is that distributed denial-of-service   (DDoS) attacks terminate on router subnets within a network, as   opposed to stopping on inter-router links.2.3.  Other Filters   An obvious extension of the concept would include traffic identified   by other filters as appropriate to send to the collector.  For   example, one might configure the system to forward traffic that fail   a unicast Reverse Path Forwarding (uRPF) check [RFC2827] to the   collector via the same tunnel.3.  Implications for Router Design   The implication for router design applies to the IPv4 ARP and IPv6   Neighbor Discovery algorithms.  It might be interesting to provide,   under configuration control, the ability to forward to an analytic   system the arriving datagrams that trigger an ARP Request or Neighbor   Solicit, and then fail to receive the intended response, to an   interface, circuit, VLAN, or tunnel.4.  Security Considerations   This note describes a tool for managing IPv4 and IPv6 network   security.  Like any tool, it has limitations and possible attacks.   If discarding traffic under overload is a good thing, then holding   and subsequently forwarding the traffic instead places a potential   load on the network and the router in question, and as such   represents a possible attack.  Such an attack has obvious   mitigations, however; one simply selects (in a manner the operator   deems appropriate) a subset of the traffic to forward and discards   the rest.  In addition, this attack is not new; it is only changed inBaker, et al.                 Informational                     [Page 6]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   character.  A stream that would instantiate the attack today results   in a load of ARP or Neighbor Solicit messages that all listening   hosts must intelligently discard.  The new attack additionally   consumes bandwidth that is presumably set aside specifically for that   purpose.   The question of exactly what subset of traffic is interesting and   economical to forward is intentionally left open.  Key questions in   algorithm design include what can be learned from a given sample (Are   bursts happening?  If so, with what data?), what the impact on the   router and other equipment in question is, how that might be   mitigated, etc.  Possible selection algorithms dependent only on   state and algorithms typically available in a router include:   o  Select all datagrams that trigger an ARP Request or Neighbor      Solicit.   o  Select the subset of those that are not responded to within some      stated interval and are therefore likely dark.   o  Select the subset of those that are new; if the address is      currently being solicited, forwarding redundant data may not be      useful.   o  Select all datagrams up to some rate.   o  Select all datagrams matching (or not matching) a specified filter      rule.5.  Acknowledgements   Algorithms for learning about Internet attack behavior by observing   backscatter traffic have been used by CAIDA, University of Michigan,   Team Cymru, and others.  Harrop extended them in his research.  This   formulation of the notion originated in a discussion among the   authors in 2005.  This note grew out of a conversation with Paul   Vixie and Rhette Marsh on Internet traffic sensors; they also made   useful comments on it.  Albert Manfredi commented on the distinction   between a LAN (as defined by IEEE 802) and an IP subnet.   Tim Chown [RFC5157] has observed that, at least at the time of   writing that RFC, address scanning attacks in IPv6 have not been   reported in the wild.  However, as mentioned inSection 1.1 above, a   (partial) scanning attack was recently reported on the NANOG mailing   list.  Rhette Marsh has suggested the structure of such an attack,   however, and Fred Baker has suggested approaches based on addressingBaker, et al.                 Informational                     [Page 7]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   information exchanged by applications.  Hence, we believe that such   issues may be relevant to IPv6 in the future, when IPv6 is a more   interesting target.   Tim Chown and Owen Stephens tested the proposal, and made useful   comments that have been incorporated in this text.  His fundamental   comment was, however, that "it works".6.  References6.1.  Normative References   [Harrop]   Harrop, W. and G. Armitage, "Greynets: a definition and              evaluation of sparsely populated darknets", IEEE LCN IEEE              30th Conference on Local Computer Networks, 2005.   [RFC0791]  Postel, J., "Internet Protocol", STD 5,RFC 791,              September 1981.   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or              converting network protocol addresses to 48.bit Ethernet              address for transmission on Ethernet hardware", STD 37,RFC 826, November 1982.   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, December 1998.   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing              Architecture",RFC 4291, February 2006.   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,              "Neighbor Discovery for IP version 6 (IPv6)",RFC 4861,              September 2007.   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy              Extensions for Stateless Address Autoconfiguration in              IPv6",RFC 4941, September 2007.6.2.  Informative References   [Armitage] Armitage, G., Harrop, W., Heyde, A., Parry, L., "Greynets:              Passive Detection of Unsolicited Network Scans in Small              ISP and Enterprise networks", CAIA, Swinburne University              of Technology, December 2008,http://caia.swin.edu.au/greynets/.Baker, et al.                 Informational                     [Page 8]

RFC 6018                 IPv4 and IPv6 Greynets           September 2010   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:              Defeating Denial of Service Attacks which employ IP Source              Address Spoofing",BCP 38,RFC 2827, May 2000.   [RFC5101]  Claise, B., "Specification of the IP Flow Information              Export (IPFIX) Protocol for the Exchange of IP Traffic              Flow Information",RFC 5101, January 2008.   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",RFC 5157, March 2008.   [RFC5610]  Boschi, E., Trammell, B., Mark, L., and T. Zseby,              "Exporting Type Information for IP Flow Information Export              (IPFIX) Information Elements",RFC 5610, July 2009.Authors' Addresses   Fred Baker   Cisco Systems   Santa Barbara, California  93117   USA   EMail: fred@cisco.com   Warren Harrop   Centre for Advanced Internet Architectures   Swinburne University of Technology   PO Box 218   John Street, Hawthorn,   Victoria, 3122   Australia   EMail: wazz@bud.cc.swin.edu.au   Grenville Armitage   Centre for Advanced Internet Architectures   Swinburne University of Technology   PO Box 218   John Street, Hawthorn,   Victoria, 3122   Australia   EMail: garmitage@swin.edu.auBaker, et al.                 Informational                     [Page 9]