Operational Neighbor Discovery Problems
RFC 6583

Internet Engineering Task Force (IETF)                      I. Gashinsky
Request for Comments: 6583                                        Yahoo!
Category: Informational                                       J. Jaeggli
ISSN: 2070-1721                                                    Zynga
                                                               W. Kumari
                                                            Google, Inc.
                                                              March 2012

                Operational Neighbor Discovery Problems

Abstract

   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; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6583.

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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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 Contents

   1. Introduction ....................................................3
      1.1. Applicability ..............................................3
   2. The Problem .....................................................3
   3. Terminology .....................................................4
   4. Background ......................................................5
   5. Neighbor Discovery Overview .....................................6
   6. Operational Mitigation Options ..................................7
      6.1. Filtering of Unused Address Space ..........................7
      6.2. Minimal Subnet Sizing ......................................7
      6.3. Routing Mitigation .........................................8
      6.4. Tuning of the NDP Queue Rate Limit .........................8
   7. Recommendations for Implementors ................................8
      7.1. Prioritize NDP Activities ..................................9
      7.2. Queue Tuning ..............................................10
   8. Security Considerations ........................................11
   9. Acknowledgements ...............................................11
   10. References ....................................................11
      10.1. Normative References .....................................11
      10.2. Informative References ...................................11

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1.  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 other

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   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 network

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      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.

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   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.

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6.  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.

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6.3.  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:

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   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.

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       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).

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8.  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.  References

10.1.  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.

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Authors' 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.net

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