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Internet Engineering Task Force (IETF)                      I. GashinskyRequest for Comments: 6583                                        Yahoo!Category: Informational                                       J. JaeggliISSN: 2070-1721                                                    Zynga                                                               W. Kumari                                                            Google, Inc.                                                              March 2012Operational Neighbor Discovery ProblemsAbstract   In IPv4, subnets are generally small, made just large enough to cover   the actual number of machines on the subnet.  In contrast, the   default IPv6 subnet size is a /64, a number so large it covers   trillions of addresses, the overwhelming number of which will be   unassigned.  Consequently, simplistic implementations of Neighbor   Discovery (ND) can be vulnerable to deliberate or accidental denial   of service (DoS), whereby they attempt to perform address resolution   for large numbers of unassigned addresses.  Such denial-of-service   attacks can be launched intentionally (by an attacker) or result from   legitimate operational tools or accident conditions.  As a result of   these vulnerabilities, new devices may not be able to "join" a   network, it may be impossible to establish new IPv6 flows, and   existing IPv6 transported flows may be interrupted.   This document describes the potential for DoS in detail and suggests   possible implementation improvements as well as operational   mitigation techniques that can, in some cases, be used to protect   against or at least alleviate the impact of such attacks.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/rfc6583.Gashinsky, et al.             Informational                     [Page 1]

RFC 6583                 Operational ND Problems              March 2012Copyright Notice   Copyright (c) 2012 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.Table of Contents1. Introduction ....................................................31.1. Applicability ..............................................32. The Problem .....................................................33. Terminology .....................................................44. Background ......................................................55. Neighbor Discovery Overview .....................................66. Operational Mitigation Options ..................................76.1. Filtering of Unused Address Space ..........................76.2. Minimal Subnet Sizing ......................................76.3. Routing Mitigation .........................................86.4. Tuning of the NDP Queue Rate Limit .........................87. Recommendations for Implementors ................................87.1. Prioritize NDP Activities ..................................97.2. Queue Tuning ..............................................108. Security Considerations ........................................119. Acknowledgements ...............................................1110. References ....................................................1110.1. Normative References .....................................1110.2. Informative References ...................................11Gashinsky, et al.             Informational                     [Page 2]

RFC 6583                 Operational ND Problems              March 20121.  Introduction   This document describes implementation issues with IPv6's Neighbor   Discovery protocol that can result in vulnerabilities when a network   is scanned, either by an intruder or through the use of scanning   tools that perform network inventory, security audits, etc. (e.g.,   "nmap").   This document describes the problem in detail, suggests possible   implementation improvements, as well as operational mitigation   techniques, that can, in some cases, protect against such attacks.   The RFCs generally describe the behavior of protocols, that is,   "what" is to be done by a protocol, but not exactly "how" it is to be   implemented.  The exact details of how best to implement a protocol   will depend on the overall hardware and software architecture of a   particular device.  The actual "how" decisions are (correctly) left   in the hands of implementors, so long as implementation differences   will generally produce proper on-the-wire behavior.   While reading this document, it is important to keep in mind that   discussions of how things have been implemented beyond basic   compliance with the specification is not within the scope of the   Neighbor Discovery RFCs.1.1.  Applicability   This document is primarily intended for operators of IPV6 networks   and implementors of [RFC4861].  The document provides some   operational considerations as well as recommendations to increase the   resilience of the Neighbor Discovery protocol.2.  The Problem   In IPv4, subnets are generally small, made just large enough to cover   the actual number of machines on the subnet.  For example, an IPv4   /20 contains only 4096 address.  In contrast, the default IPv6 subnet   size is a /64, a number so large it covers literally billions of   billions of addresses, the overwhelming majority of which will be   unassigned.  Consequently, simplistic implementations of Neighbor   Discovery may fail to perform as desired when they perform address   resolution of large numbers of unassigned addresses.  Such failures   can be triggered either intentionally by an attacker launching a   denial-of-service attack (DoS) [RFC4732] to exploit this   vulnerability or unintentionally due to the use of legitimate   operational tools that scan networks for inventory and otherGashinsky, et al.             Informational                     [Page 3]

RFC 6583                 Operational ND Problems              March 2012   purposes.  As a result of these failures, new devices may not be able   to "join" a network, it may be impossible to establish new IPv6   flows, and existing IPv6 transport flows may be interrupted.   Network scans attempt to find and probe devices on a network.   Typically, scans are performed on a range of target addresses, or all   the addresses on a particular subnet.  When such probes are directed   via a router, and the target addresses are on a directly attached   network, the router will attempt to perform address resolution on a   large number of destinations (i.e., some fraction of the 2^64   addresses on the subnet).  The router's process of testing for the   (non)existence of neighbors can induce a denial-of-service condition,   where the number of necessary Neighbor Discovery requests overwhelms   the implementation's capacity to process them, exhausts available   memory and replaces existing in-use mappings with incomplete entries   that will never be completed.  A directed DoS attack may seek to   intentionally create similar conditions to those created   unintentionally by a network scan.  The resulting network disruption   may impact existing traffic, and devices that join the network may   find that address resolution attempts fail.  The DoS as a consequence   of network scanning was previously described in [RFC5157].   In order to mitigate risk associated with this DoS threat, some   router implementations have taken steps to rate-limit the processing   rate of Neighbor Solicitations (NS).  While these mitigations do   help, they do not fully address the issue and may introduce their own   set of issues to the Neighbor Discovery process.3.  Terminology   Address Resolution:  Address resolution is the process through which      a node determines the link-layer address of a neighbor given only      its IP address.  In IPv6, address resolution is performed as part      of Neighbor Discovery[RFC4861], Section 7.2.   Forwarding Plane:  The part of a router responsible for forwarding      packets.  In higher-end routers, the forwarding plane is typically      implemented in specialized hardware optimized for performance.      Steps in the forwarding process include determining the correct      outgoing interface for a packet, decrementing its Time To Live      (TTL), verifying and updating the checksum, placing the correct      link-layer header on the packet, and forwarding it.   Control Plane:  The part of the router implementation that maintains      the data structures that determine where packets should be      forwarded.  The control plane is typically implemented as a      "slower" software process running on a general purpose processor      and is responsible for such functions as communicating networkGashinsky, et al.             Informational                     [Page 4]

RFC 6583                 Operational ND Problems              March 2012      status changes via routing protocols, maintaining the forwarding      table, performing management, and resolving the correct link-layer      address for adjacent neighbors.  The control plane "controls" the      forwarding plane by programming it with the information needed for      packet forwarding.   Neighbor Cache:  As described in [RFC4861], the data structure that      holds the cache of (amongst other things) IP address to link-layer      address mappings for connected nodes.  As the information in the      Neighbor Cache is needed by the forwarding plane every time it      forwards a packet, it is usually implemented in an Application-      specific Integrated Circuit (ASIC).   Neighbor Discovery Process:  The Neighbor Discovery Process (NDP) is      that part of the control plane that implements the Neighbor      Discovery protocol.  NDP is responsible for performing address      resolution and maintaining the Neighbor Cache.  When forwarding      packets, the forwarding plane accesses entries within the Neighbor      Cache.  When the forwarding plane processes a packet for which the      corresponding Neighbor Cache Entry (NCE) is missing or incomplete,      it notifies NDP to take appropriate action (typically via a shared      queue).  NDP picks up requests from the shared queue and performs      any necessary discovery action.  In many implementations, the NDP      is also responsible for responding to router solicitation      messages, Neighbor Unreachability Detection (NUD), etc.4.  Background   Modern router architectures separate the forwarding of packets   (forwarding plane) from the decisions needed to decide where the   packets should go (control plane).  In order to deal with the high   number of packets per second, the forwarding plane is generally   implemented in hardware and is highly optimized for the task of   forwarding packets.  In contrast, the NDP control plane is mostly   implemented in software processes running on a general purpose   processor.   When a router needs to forward an IP packet, the forwarding plane   logic performs the longest match lookup to determine where to send   the packet and what outgoing interface to use.  To deliver the packet   to an adjacent node, the forwarding plane encapsulates the packet in   a link-layer frame (which contains a header with the link-layer   destination address).  The forwarding plane logic checks the Neighbor   Cache to see if it already has a suitable link-layer destination, and   if not, places the request for the required information into a queue,   and signals the control plane (i.e., NDP) that it needs the link-   layer address resolved.Gashinsky, et al.             Informational                     [Page 5]

RFC 6583                 Operational ND Problems              March 2012   In order to protect NDP specifically and the control plane generally   from being overwhelmed with these requests, appropriate steps must be   taken.  For example, the size and fill rate of the queue might be   limited.  NDP running in the control plane of the router dequeues   requests and performs the address resolution function (by performing   a neighbor solicitation and listening for a neighbor advertisement).   This process is usually also responsible for other activities needed   to maintain link-layer information, such as Neighbor Unreachability   Detection (NUD).   By sending appropriate packets to addresses on a given subnet, an   attacker can cause the router to queue attempts to resolve so many   addresses that it crowds out attempts to resolve "legitimate"   addresses (and in many cases becomes unable to perform maintenance of   existing entries in the Neighbor Cache, and unable to answer Neighbor   Solicitation).  This condition can result in the inability to resolve   new neighbors and loss of reachability to neighbors with existing   NCEs.  During testing, it was concluded that four simultaneous nmap   sessions from a low-end computer were sufficient to make a router's   Neighbor Discovery process unusable; therefore, forwarding became   unavailable to the destination subnets.   The failure to maintain proper NDP behavior whilst under attack has   been observed across multiple platforms and implementations,   including the largest modern router platforms available (at the   inception of work on this document).5.  Neighbor Discovery Overview   When a packet arrives at (or is generated by) a router for a   destination on an attached link, the router needs to determine the   correct link-layer address to use in the destination field of the   Layer 2 encapsulation.  The router checks the Neighbor Cache for an   existing Neighbor Cache Entry for the neighbor, and if none exists,   invokes the address resolution portions of the IPv6 Neighbor   Discovery [RFC4861] protocol to determine the link-layer address of   the neighbor.[RFC4861], Section 5.2, outlines how this process works.  A very   high-level summary is that the device creates a new Neighbor Cache   Entry for the neighbor, sets the state to INCOMPLETE, queues the   packet, and initiates the actual address resolution process.  The   device then sends out one or more Neighbor Solicitations, and when it   receives a corresponding Neighbor Advertisement, completes the   Neighbor Cache Entry and sends the queued packet.Gashinsky, et al.             Informational                     [Page 6]

RFC 6583                 Operational ND Problems              March 20126.  Operational Mitigation Options   This section provides some feasible mitigation options that can be   employed today by network operators in order to protect network   availability while vendors implement more effective protection   measures.  It can be stated that some of these options are "kludges",   and can be operationally difficult to manage.  They are presented, as   they represent options we currently have.  It is each operator's   responsibility to evaluate and understand the impact of changes to   their network due to these measures.6.1.  Filtering of Unused Address Space   The DoS condition is induced by making a router try to resolve   addresses on the subnet at a high rate.  By carefully addressing   machines into a small portion of a subnet (such as the lowest   numbered addresses), it is possible to filter access to addresses not   in that assigned portion of address space using Access Control Lists   (ACLs), or by null routing, features which are available on most   existing platforms.  This will prevent the attacker from making the   router attempt to resolve unused addresses.  For example, if there   are only 50 hosts connected to an interface, you may be able to   filter any address above the first 64 addresses of that subnet by   null-routing the subnet carrying a more specific /122 route or by   applying ACLs on the WAN link to prevent the attack traffic reaching   the vulnerable device.   As mentioned at the beginning of this section, it is fully understood   that this is ugly (and difficult to manage); but failing other   options, it may be a useful technique especially when responding to   an attack.   This solution requires that the hosts be statically or statefully   addressed (as is often done in a datacenter), and they may not   interact well with networks using [RFC4862].6.2.  Minimal Subnet Sizing   By sizing subnets to reflect the number of addresses actually in use,   the problem can be avoided.  For example, [RFC6164] recommends sizing   the subnets for inter-router links so they only have two addresses (a   /127).  It is worth noting that this practice is common in IPv4   networks, in part to protect against the harmful effects of Address   Resolution Protocol (ARP) request flooding.   Subnet prefixes longer than a /64 are not able to use stateless auto-   configuration [RFC4862], so this approach is not suitable for use   with hosts that are not statically configured.Gashinsky, et al.             Informational                     [Page 7]

RFC 6583                 Operational ND Problems              March 20126.3.  Routing Mitigation   One very effective technique is to route the subnet to a discard   interface (most modern router platforms can discard traffic in   hardware / the forwarding plane) and then have individual hosts   announce routes for their IP addresses into the network (or use some   method to inject much more specific addresses into the local routing   domain).  For example, the network 2001:db8:1:2:3::/64 could be   routed to a discard interface on "border" routers, and then   individual hosts could announce 2001:db8:1:2:3::10/128, 2001:db8:1:2:   3::66/128 into the IGP.  This is typically done by having the IP   address bound to a virtual interface on the host (for example, the   loopback interface), enabling IP forwarding on the host and having it   run a routing daemon.  For obvious reasons, host participation in the   IGP makes many operators uncomfortable, but it can be a very powerful   technique if used in a disciplined and controlled manner.  One method   to help address these concerns is to have the hosts participate in a   different IGP (or difference instance of the same IGP) and carefully   redistribute into the main IGP.6.4.  Tuning of the NDP Queue Rate Limit   Many implementations provide a means to control the rate of   resolution of unknown addresses.  By tuning this rate, it may be   possible to ameliorate the issue, as with most tuning knobs   (especially those that deal with rate-limiting), the attack may be   completed more quickly due to the lower threshold.  By excessively   lowering this rate, you may negatively impact how long the device   takes to learn new addresses under normal conditions (for example,   after clearing the Neighbor Cache or when the router first boots).   Under attack conditions, you may be unable to resolve "legitimate"   addresses sooner than if you had just left the parameter untouched.   It is worth noting that this technique is worth investigating only if   the device has separate queues for resolution of unknown addresses   and the maintenance of existing entries.7.  Recommendations for Implementors   This section provides some recommendations to implementors of IPv6   Neighbor Discovery.   At a high-level, implementors should program defensively.  That is,   they should assume that attackers will attempt to exploit   implementation weaknesses, and they should ensure that   implementations are robust to various attacks.  In the case of   Neighbor Discovery, the following general considerations apply:Gashinsky, et al.             Informational                     [Page 8]

RFC 6583                 Operational ND Problems              March 2012   Manage Resources Explicitly:  Resources such as processor cycles,      memory, etc., are never infinite, yet with IPv6's large subnets,      it is easy to cause NDP to generate large numbers of address      resolution requests for nonexistent destinations.  Implementations      need to limit resources devoted to processing Neighbor Discovery      requests in a thoughtful manner.   Prioritize:  Some NDP requests are more important than others.  For      example, when resources are limited, responding to Neighbor      Solicitations for one's own address is more important than      initiating address resolution requests that create new entries.      Likewise, performing Neighbor Unreachability Detection, which by      definition is only invoked on destinations that are actively being      used, is more important than creating new entries for possibly      nonexistent neighbors.7.1.  Prioritize NDP Activities   Not all Neighbor Discovery activities are equally important.   Specifically, requests to perform large numbers of address   resolutions on non-existent Neighbor Cache Entries should not come at   the expense of servicing requests related to keeping existing, in-use   entries properly up to date.  Thus, implementations should divide   work activities into categories having different priorities.  The   following gives examples of different activities and their importance   in rough priority order.  If implemented, the operation and priority   of these should be configurable by the operator.   1.  It is critical to respond to Neighbor Solicitations for one's own       address, especially for a router.  Whether for address resolution       or Neighbor Unreachability Detection, failure to respond to       Neighbor Solicitations results in immediate problems.  Failure to       respond to NS requests that are part of NUD can cause neighbors       to delete the NCE for that address and will result in follow-up       NS messages using multicast.  Once an entry has been flushed,       existing traffic for destinations using that entry can no longer       be forwarded until address resolution completes successfully.  In       other words, not responding to NS messages further increases the       NDP load and causes ongoing communication to fail.   2.  It is critical to revalidate one's own existing NCEs in need of       refresh.  As part of NUD, ND is required to frequently revalidate       existing, in-use entries.  Failure to do so can result in the       entry being discarded.  For in-use entries, discarding the entry       will almost certainly result in a subsequent request to perform       address resolution on the entry, but this time using multicast.Gashinsky, et al.             Informational                     [Page 9]

RFC 6583                 Operational ND Problems              March 2012       As above, once the entry has been flushed, existing traffic for       destinations using that entry can no longer be forwarded until       address resolution completes successfully.   3.  To maintain the stability of the control plane, Neighbor       Discovery activity related to traffic sourced by the router (as       opposed to traffic being forwarded by the router) should be given       high priority.  Whenever network problems occur, debugging and       making other operational changes requires being able to query and       access the router.  In addition, routing protocols dependent on       Neighbor Discovery for connectivity may begin to react       (negatively) to perceived connectivity problems, causing       additional undesirable ripple effects.   4.  Traffic to unknown addresses should be given lowest priority.       Indeed, it may be useful to distinguish between "never seen"       addresses and those that have been seen before, but that do not       have a corresponding NCE.  Specifically, the conceptual       processing algorithm in IPv6 Neighbor Discovery [RFC4861] calls       for deleting NCEs under certain conditions.  Rather than delete       them completely, however, it might be useful to at least keep       track of the fact that an entry at one time existed, in order to       prioritize address resolution requests for such neighbors       compared with neighbors that have never been seen before.7.2.  Queue Tuning   On implementations in which requests to NDP are submitted via a   single queue, router vendors should provide operators with means to   control both the rate of link-layer address resolution requests   placed into the queue and the size of the queue.  This will allow   operators to tune Neighbor Discovery for their specific environment.   The ability to set, or have per-interface or per-prefix queue limits   at a rate below that of the global queue limit might restrict the   damage to the Neighbor Discovery processing to the network targeted   by the attack.   Setting those values must be a very careful balancing act -- the   lower the rate of entry into the queue, the less load there will be   on the ND process; however, it will take the router longer to learn   legitimate destinations as a result.  In a datacenter with 6,000   hosts attached to a single router, setting that value to be under   1000 would mean that resolving all of the addresses from an initial   state (or something that invalidates the address cache, such as a   Spanning Tree Protocol (STP) Topology Change Notification (TCN)) may   take over 6 seconds.  Similarly, the lower the size of the queue, the   higher the likelihood of an attack being able to knock out legitimate   traffic (but less memory utilization on the router).Gashinsky, et al.             Informational                    [Page 10]

RFC 6583                 Operational ND Problems              March 20128.  Security Considerations   This document outlines mitigation options that operators can use to   protect themselves from denial-of-service attacks.  Implementation   advice to router vendors aimed at ameliorating known problems carries   the risk of previously unforeseen consequences.  It is not believed   that these mitigation techniques or the implementation of finer-   grained queuing of NDP activity create additional security risks or   DoS exposure.9.  Acknowledgements   The authors would like to thank Ron Bonica, Troy Bonin, John Jason   Brzozowski, Randy Bush, Vint Cerf, Tassos Chatzithomaoglou, Jason   Fesler, Wes George, Erik Kline, Jared Mauch, Chris Morrow, and Suran   De Silva.  Special thanks to Thomas Narten and Ray Hunter for   detailed review and (even more so) for providing text!   Apologies for anyone we may have missed; it was not intentional.10.  References10.1.  Normative References   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,              "Neighbor Discovery for IP version 6 (IPv6)",RFC 4861,              September 2007.   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless              Address Autoconfiguration",RFC 4862, September 2007.   [RFC6164]  Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,              L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-              Router Links",RFC 6164, April 2011.10.2.  Informative References   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-              Service Considerations",RFC 4732, December 2006.   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",RFC 5157, March 2008.Gashinsky, et al.             Informational                    [Page 11]

RFC 6583                 Operational ND Problems              March 2012Authors' Addresses   Igor Gashinsky   Yahoo!   45 W 18th St   New York, NY   USA   EMail: igor@yahoo-inc.com   Joel Jaeggli   Zynga   111 Evelyn   Sunnyvale, CA   USA   EMail: jjaeggli@zynga.com   Warren Kumari   Google, Inc.   1600 Amphitheatre Parkway   Mountain View, CA   USA   EMail: warren@kumari.netGashinsky, et al.             Informational                    [Page 12]