[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00 01 02

Network Working Group                                          W. Kumari
Internet-Draft                                                    Google
Intended status: Informational                              I. Gashinsky
Expires: July 11, 2012                                            Yahoo!
                                                              J. Jaeggli
                                                         January 8, 2012

          Neighbor Discovery Enhancements for DOS mititgation


   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 can be vulnerable to denial of service attacks whereby they
   attempt to perform address resolution for large numbers of unassigned
   addresses.  Such denial of attacks can be launched intentionally (by
   an attacker), or result from legitimate operational tools that scan
   networks for inventory and other purposes.  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 possible modifications to the traditional
   [RFC4861] neighbor discovery protocol for improving the resilience of
   the neighbor discovery process as well as an alternative method for
   maintaining ND caches.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

Kumari, et al.            Expires July 11, 2012                 [Page 1]

Internet-Draft                 ND Enhance                   January 2012

   This Internet-Draft will expire on July 11, 2012.

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.

Kumari, et al.            Expires July 11, 2012                 [Page 2]

Internet-Draft                 ND Enhance                   January 2012

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The Problem  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Neighbor Discovery Overview  . . . . . . . . . . . . . . . . .  7
   6.  Recommendations for Implementors.  . . . . . . . . . . . . . .  7
     6.1.  Priortize NDP Activities . . . . . . . . . . . . . . . . .  8
     6.2.  Queue Tuning.  . . . . . . . . . . . . . . . . . . . . . .  9
     6.3.  ND cache priming and refresh . . . . . . . . . . . . . . .  9
     6.4.  NDP Protocol Gratuitous NA . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Text goes here. . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

Kumari, et al.            Expires July 11, 2012                 [Page 3]

Internet-Draft                 ND Enhance                   January 2012

1.  Introduction

   This document describes modifications to the IPv6 Neighbor Discovery
   protocol [RFC4861] in order to reduce exposure to vulnerabilities
   when a network is scanned, either by an intruder, as part of a
   deliberate dos attempt, or through the use of scanning tools that
   peform network inventory, security audits, etc. (e.g., "nmap").

1.1.  Applicability

   This document is primarily intended for implementors of [RFC4861].

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 number of which will be
   unassigned.  Consequently, simplistic implementations of Neighbor
   Discovery can be vulnerable to denial of service attacks whereby they
   perform address resolution for large numbers of unassigned addresses.
   Such denial of attacks can be launched intentionally (by an
   attacker), or result from legitimate operational tools that scan
   networks for inventory and other purposes.  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
   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 to attempt to perform address resolution on
   a large number of destinations (i.e., some fraction of the 2^64
   addresses on the subnet).  The process of testing for the
   (non)existance of neighbors can induce a denial of service condition,
   where the number of Neighbor Discovery requests overwhelms the
   implementation's capacity to process them, exhausts available memory,
   replaces existing in-use mappings with incomplete entries that will
   never be completed, etc.  The result can be network disruption, where
   existing traffic may be impacted, and devices that join the net find
   that address resolutions fails.

   In order to alleviate 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

Kumari, et al.            Expires July 11, 2012                 [Page 4]

Internet-Draft                 ND Enhance                   January 2012

   help, they do not fully address the issue and may introduce their own
   set of potential liabilities 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], p60

   Forwarding Plane  That part of a router responsible for forwarding
      packets.  In higher-end routers, the forwarding plane is typically
      implemented in specialized hardware optimized for performance.
      Forwarding steps 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  That 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 the routing protocols,
      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.  The forwarding plane
      accesses the Neighbor Cache on every forwarded packet.  Thus it is
      usually implemented in an 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.  Whenever the forwarding plane processes a packet for which
      the corresponding Neighbor Cache Entry 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 actions.  In many implementations it is also
      responsible for responding to router solicitation messages,
      Neighbor Unreachability Detection (NUD), etc.

Kumari, et al.            Expires July 11, 2012                 [Page 5]

Internet-Draft                 ND Enhance                   January 2012

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

   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, It 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

   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 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).

   An attacker sending the appropriate packets to addresses on a given
   subnet 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
   Solicitiation).  This condition can result the inability to resolve
   new neighbors and loss of reachability to neighbors with existing ND-
   Cache entries.  During testing it was concluded that 4 simultaneous
   nmap sessions from a low-end computer was sufficient to make a
   router's neighbor discovery process unhappy and therefore forwarding

   This behavior has been observed across multiple platforms and

Kumari, et al.            Expires July 11, 2012                 [Page 6]

Internet-Draft                 ND Enhance                   January 2012

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 send the packet to.  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.

   RFC4861 Section 5.2 (Conceptual Sending Algorithm) 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
   Solicitiations, and when it receives a correpsonding Neighbor
   Advertisement, completes the Neighbor Cache Entry and sends the
   queued packet.

6.  Recommendations for Implementors.

   The section provides some recommendations to implementors of IPv4
   Neighbor Discovery.

   At a high-level, implementors should program defensively.  That is,
   they should assume that intruders will attempt to exploit
   implementation weaknesses, and should ensure that implementations are
   robust to various attacks.  In the case of Neighbor Discovery, the
   following general considerations apply:

   Manage Resources Explicitely  - 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 non-existant 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
      non-existant neighbors.

Kumari, et al.            Expires July 11, 2012                 [Page 7]

Internet-Draft                 ND Enhance                   January 2012

6.1.  Priortize NDP Activities

   Not all Neighbor Discovery activies are equally important.
   Specifically, requests to perform large numbers of address
   resolutions on non-existant 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.

   1.  It is critical to respond to Neighbor Solicitations for one's own
   address, especially when 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 followup 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 succesfully.  In other words, not
   responding to NS messages further increases the NDP load, and causes
   on-going 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 subsquent request to perform address
   resolution on the entry, but this time using multicast.  As above,
   once the entry has been flushed, existing traffic for destinations
   using that entry can no longer be forwarded until address resolution
   completes succesfully.

   3.  To maintin 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 may begin to react
   (negatively) to perceived connectivity problems, causing addition
   undesirable ripple effects.

   4.  Activities related to the sending and recieving of Router
   Advertisements also impact address resolutions.  [XXX say more?]

   5.  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

Kumari, et al.            Expires July 11, 2012                 [Page 8]

Internet-Draft                 ND Enhance                   January 2012

   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.

6.2.  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 Neighbour Discovery for their specific environment.
   The ability to set or have per interface or subnet queue limits at a
   rate below that of the global queue limit might limit the damage to
   the neighbor discovery process to the taret network.

   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 also means that it will take the router
   longer to learn legitimate destinations.  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 STP
   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).

6.3.  ND cache priming and refresh

   With all of the above recommendations implemented, it should be
   possible to survive a "scan attack" with very little impact to the
   network, however, adding new hosts to the network (and the sending of
   traffic to them) may still be negatively impacted.  Traffic to those
   new hosts would have to go through the unknown Neighbor Resolution
   queue, which is where the attack traffic would end up as well.  A
   solution to this would be that any new host that joins the network
   would "announce" itself, and be added to the cache, therefore not
   requiring packets destined to it to go through the unknown NDP queue.
   This could be done by sending a ping packet to the all-routers
   multicast address, which would then trigger the router's own neighbor
   resolution process, which should be in a different queue then other

   All attempts should be made to keep these addresses in cache, since
   any eviction of legitimate hosts from the cache could potentially

Kumari, et al.            Expires July 11, 2012                 [Page 9]

Internet-Draft                 ND Enhance                   January 2012

   place resolutions for them into the same queue as the attack traffic.
   At present, [RFC4861] states that there should be MAX_UNICAST_SOLICIT
   (3) attempts, RETRANS_TIMER 1 second apart, so if there is an
   interruption in the network or control plane processing for longer
   then 3 seconds during the refresh, the entry would be evicted from
   the ND Cache.  Any network event which takes longer then 3 seconds to
   converge (UDLD, STP, etc may take 30+ seconds) while under an attack,
   would result in ND cache eviction.  If an entry is evicted during a
   scan, connectivity could be lost for an extended period of time.

   NDP refresh timers could be revised as suggested in
   draft-nordmark-6man-impatient-nud-00 [1] and SHOULD have a
   configurable value for MAX_UNICAST_SOLICIT and RETRANS_TIMER, and
   include capabilities for binary/exponential backoff.

   A suggested algorithm, which retains backward compatiblity with
   [RFC4861] is: operator configurable values for MAX_UNICAST_SOLICIT,
   RETRANS_TIMER, and a way to set adaptive back-of multiple, simmilar
   to ipv4 -- call it BACKOFF_MULTIPLE), so that we could implement:

   next_retrans =
   ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RETRANS_TIMER + jittered

   The recommended behavior is to have 5 attempts, with timing spacing
   of 0 (initial request), 1 second later, 3 seconds later, then 9, and
   then 27, which represents:


   RETRANS_TIMER=1 (default)


   If BACKOFF_MULTIPLE=1 (which should be the default value), and
   MAX_UNICAST_SOLICIT=3, you would get the backwards-compatible RFC
   behavior, but operators should be able to adjust the values as
   necessary to insure that they are sufficiently aggressive about
   retaining ND entries in cache.

   An Implementation following this algorithm would if the request was
   not answered at first due for example to a transitory condition,
   retry immediately, and then back off for progressively longer
   periods.  This would allow for a reasonably fast resolution time when
   the transitory condition clears.

Kumari, et al.            Expires July 11, 2012                [Page 10]

Internet-Draft                 ND Enhance                   January 2012

6.4.  NDP Protocol Gratuitous NA

   Per RFC 4861, section 7.2.5 and 7.2.6 [RFC4861] requires that
   unsolicited neighbor advertisements result in the receiver setting
   it's neighbor cache entry to STALE, kicking off the resolution of the
   neighbor using neighbor solicitation.  If the link layer address in
   an unsolicited neighbor advertisement matches that of the existing ND
   cache entry, routers SHOULD retain the existing entry updating it's
   status with regards to LRU retention policy.

   Hosts MAY be configured to send unsolicited Neighbor advertisement at
   a rate set at the discretion of the operators.  The rate SHOULD be
   appropriate to the sizing of ND cache parameters and the host count
   on the subnet.  An unsolicited NA rate parameter MUST NOT be enabled
   by default.  The unsolicted rate interval as interpreted by hosts
   must jitter the value for the interval between transmissions.  Hosts
   receiving a neighbor solicitation requests from a router following
   each of three subsequent gratuitous NA intervals MUST revert to RFC
   4861 behavior.

   Implementation of new behavior for unsolicited neighbor advertisement
   would make it possible under appropriate circumstances to greatly
   reduce the dependence on the neighbor solicitation process for
   retaining existing ND cache entries.

   This may impact the detection of one-way reachability.

   It is understood that this section may need to be moved into a
   separate document -- it is (currently) provided here for discussion

7.  IANA Considerations

   No IANA resources or consideration are requested in this draft.

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 techniques create additional security or DOS exposure

Kumari, et al.            Expires July 11, 2012                [Page 11]

Internet-Draft                 ND Enhance                   January 2012

9.  Acknowledgements

   The authors would like to thank Ron Bonica, Troy Bonin, John Jason
   Brzozowski, Randy Bush, Vint Cerf, Jason Fesler Erik Kline, Jared
   Mauch, Chris Morrow and Suran De Silva.  Special thanks to Thomas
   Narten 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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4398]  Josefsson, S., "Storing Certificates in the Domain Name
              System (DNS)", RFC 4398, March 2006.

   [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

   [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
              Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
              January 2006.


   [1]  <http://tools.ietf.org/html/

Appendix A.  Text goes here.


Kumari, et al.            Expires July 11, 2012                [Page 12]

Internet-Draft                 ND Enhance                   January 2012

Authors' Addresses

   Warren Kumari

   Email: warren@kumari.net

   45 W 18th St
   New York, NY

   Email: igor@yahoo-inc.com

   111 Evelyn
   Sunnyvale, CA

   Email: jjaeggli@zynga.com

Kumari, et al.            Expires July 11, 2012                [Page 13]

Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/