RA Guard: IPv6 Router Advertisement Guard

Internet Engineering Task Force (IETF)                  E. Levy-Abegnoli
Request for Comments: 6105                               G. Van de Velde
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                             C. Popoviciu
                                                              Technodyne
                                                              J. Mohacsi
                                                          NIIF/Hungarnet
                                                           February 2011


                    IPv6 Router Advertisement Guard

Abstract

   Routed protocols are often susceptible to spoof attacks.  The
   canonical solution for IPv6 is Secure Neighbor Discovery (SEND), a
   solution that is non-trivial to deploy.  This document proposes a
   light-weight alternative and complement to SEND based on filtering in
   the layer-2 network fabric, using a variety of filtering criteria,
   including, for example, SEND status.

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/rfc6105.

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Table of Contents

   1. Introduction ....................................................3
   2. Model and Applicability .........................................3
   3. Stateless RA-Guard ..............................................5
   4. Stateful RA-Guard ...............................................6
      4.1. State Machine ..............................................6
      4.2. SEND-Based RA-Guard ........................................8
   5. RA-Guard Use Considerations .....................................8
   6. Security Considerations .........................................9
   7. Acknowledgements ................................................9
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References .....................................9

1.  Introduction

   When operating IPv6 in a shared layer-2 (L2) network segment without
   complete SEcure Neighbor Discovery (SEND) support by all devices
   connected or without the availability of the infrastructure necessary
   to support SEND [RFC3971], there is always the risk of facing
   operational problems due to rogue Router Advertisements (RAs)
   generated maliciously or unintentionally by unauthorized or
   improperly configured routers connecting to the segment.

   There are several examples of work done on this topic that resulted
   in related studies and code, including [NDPMON] [KAME]
   [IPv6-SAMURAIS].  This document describes a solution framework for
   the rogue-RA problem [RFC6104] where network segments are designed
   around a single L2-switching device or a set of L2-switching devices
   capable of identifying invalid RAs and blocking them.  The solutions
   developed within this framework can span the spectrum from basic
   (where the port of the L2 device is statically instructed to forward
   or not to forward RAs received from the connected device) to advanced
   (where a criterion is used by the L2 device to dynamically validate
   or invalidate a received RA, this criterion can even be based on SEND
   mechanisms).

2.  Model and Applicability

   RA-Guard applies to an environment where all messages between IPv6
   end-devices traverse the controlled L2 networking devices.  It does
   not apply to shared media, when devices can communicate directly
   without going through an RA-Guard-capable L2 networking device.

   Figure 1 illustrates a deployment scenario for RA-Guard.

                      Block                Allow
       +------+       incoming +---------+ incoming     +--------+
       |Host  |       RA       |    L2   | RA           | Router |
       |      |----------------|  device |--------------|        |
       +------+                +----+----+              +--------+
                                    |
                                    |Block
                                    |incoming
                                    |RA
                                    |
                                    |
                                    |
                                +---+---+
                                |  Host |
                                |       |
                                +-------+

                                Figure 1

   RA-Guard does not intend to provide a substitute for SEND-based
   solutions.  It actually intends to provide complementary solutions in
   those environments where SEND might not be suitable or fully
   supported by all devices involved.  It may take time until SEND is
   ubiquitous in IPv6 networks and some of its large-scale deployment
   aspects are sorted out, such as provisioning hosts with trust
   anchors.  It is also reasonable to expect that some devices, such as
   IPv6-enabled sensors, might not consider implementing SEND at all.
   An RA-Guard implementation that SEND-validates RAs on behalf of hosts
   would potentially simplify some of these challenges.

   RA-Guard can be seen as a superset of SEND with regard to router
   authorization.  Its purpose is to filter Router Advertisements based
   on a set of criteria, from a simplistic "RA disallowed on a given
   interface" to "RA allowed from pre-defined sources" and up to a full-
   fledged SEND "RA allowed from authorized sources only".

   In addition to this granularity on the criteria for filtering out
   Router Advertisements, RA-Guard introduces the concept of router
   authorization proxy.  Instead of each node on the link analyzing RAs
   and making an individual decision, a legitimate "node-in-the-middle"
   performs the analysis on behalf of all other nodes on the link.  The
   analysis itself is not different from what each node would do: if
   SEND is enabled, the RA is checked against X.509 certificates
   [RFC4861].  If any other criterion is in use, such as known L3
   (addresses) or L2 (link-layer address, port number) legitimate
   sources of RAs, the node-in-the middle can use this criterion and
   filter out any RA that does not comply.  If this node-in-the-middle
   is an L2 device, it will not change the content of the validated RA
   and will avoid any of the ND-proxy pitfalls.

   RA-Guard intends to provide simple solutions to the rogue-RA problem
   in contexts where simplicity is required while leveraging SEND in a
   context environment consisting of a mix of SEND-capable devices (L2
   switches and routers) and devices that do not consistently use SEND.
   Furthermore, RA-Guard is useful to simplify SEND deployments, as only
   the L2 switch and the routers are required to carry certificates
   (their own and the trust anchor certificates).

3.  Stateless RA-Guard

   Stateless RA-Guard examines incoming RAs and decides whether to
   forward or block them based solely on information found in the
   message or in the L2-device configuration.  Typical information
   available in the frames received, useful for RA validation, is as
   follows:

   o  Link-layer address of the sender

   o  Port on which the frame was received

   o  IP source address

   o  Prefix list

   The following configuration information created on the L2 device can
   be made available to RA-Guard, to validate against the information
   found in the received RA frame:

   o  Allowed/Disallowed link-layer address of the RA sender

   o  Allowed/Disallowed ports for receiving RAs

   o  Allowed/Disallowed IP source addresses of the RA sender

   o  Allowed Prefix list and Prefix ranges

   o  Router Priority

   Once the L2 device has validated the content of the RA frame against
   the configuration, it forwards the RA to its destination, whether
   unicast or multicast.  Otherwise, the RA is dropped.

   An example of a very simple stateless RA-Guard implementation could
   be a small L2 switch for which there is one interface "statically
   configured" as the interface connecting to a router, while all other
   interfaces are for non-router devices.  With this small static setup,
   the only interface forwarding RAs will be the pre-assigned router
   interface, while the non-router interfaces block all RAs.

4.  Stateful RA-Guard

4.1.  State Machine

   Stateful RA-Guard learns dynamically about legitimate RA senders and
   stores this information for allowing subsequent RAs.  A simple
   stateful scheme would be for the L2 device to listen to RAs during a
   certain manually configured period of time, where the start of the
   listening period and the duration of the listening period for a
   single instance are controlled by the manual intervention.  As a
   result, the L2 device can then allow subsequent RAs only on those
   ports on which valid RAs were received during this period.  Often,
   the "LEARNING" state will only be activated by manual configuration
   when a new IPv6 router is provisioned on the L2 network.

   A more sophisticated stateful scheme is based on SEND and is
   described in Section 4.2.

   The state machine for stateful RA-Guard can be global, per-interface,
   or per-peer, depending on the scheme used for authorizing RAs.

   When RA-Guard is SEND-based, the state machine is per-peer and
   defined in [RFC3971].

   When RA-Guard is using a discovery method, the state machine of the
   RA-Guard capability consists of four different states:

   o  State 1: OFF

      A device or interface in the RA-Guard "OFF" state operates as if
      the RA-Guard capability is not available.

   o  State 2: LEARNING

      A device or interface in the RA-Guard "LEARNING" state is actively
      acquiring information about the IPv6 routing devices connected to
      its interfaces.  The learning process takes place over a
      pre-defined unique period of time, as set by manual configuration;

      or it can be event-triggered.  The information gathered is
      compared against pre-defined criteria (criteria similar to the
      stateless RA-Guard rules) to qualify the validity of the RAs.

      In this state, the RA-Guard-enabled device or interface is either
      blocking "all" RAs until their validity is verified or,
      alternatively, it can temporarily forward "all" of the RAs until
      their validity is verified.

      When the L2 device reaches the end of the LEARNING state, it has a
      record of which interfaces are attached to links with valid IPv6
      routers.  The L2 device transitions each interface from the
      LEARNING state into either the BLOCKING state if there was no
      valid IPv6 router discovered at the interface, or into the
      FORWARDING state if there was a valid IPv6 router discovered.

   o  State 3: BLOCKING

      A device or interface running RA-Guard and in the BLOCKING state
      will block ingress RA messages.

      An interface can transition from the BLOCKING state into the
      FORWARDING state directly if explicitly instructed by the
      L2-device operator.

      An interface can transition from the BLOCKING state into the
      LEARNING state if either explicitly instructed by the L2-device
      operator or prompted by a triggered event.

   o  State 4: FORWARDING

      A device or interface running RA-Guard and in the FORWARDING state
      will accept valid ingress RAs and forward them to their
      destination.

      An interface can transition from the FORWARDING state into the
      BLOCKING state directly if explicitly instructed by the L2-device
      operator.

      An interface can transition from the FORWARDING state into the
      LEARNING state if either explicitly instructed by the L2-device
      operator or prompted by a triggered event.

   The transition between these states can be triggered by manual
   configuration or by meeting a pre-defined criterion.

4.2.  SEND-Based RA-Guard

   In this scenario, the L2 device is blocking or forwarding RAs based
   on SEND considerations.  Upon capturing an RA on the interface, the
   L2 device will first verify the Cryptographically Generated Address
   (CGA) [RFC3971] and the RSA (Rivest, Shamir, and Adleman algorithm
   for public-key cryptography) signature, as specified in Section 5 of
   [RFC3971].  The RA should be dropped in case of failure of this
   verification.  It will then apply host behavior as described in
   Section 6.4.6 of [RFC3971].  In particular, the L2 device will
   attempt to retrieve a valid certificate from its cache for the public
   key referred to in the RA.  If such a certificate is found, the L2
   device will forward the RA to its destination.  If not, the L2 device
   will generate a Certification Path Solicitation (CPS) [RFC3971] with
   an unspecified source address, to query the router certificate(s).
   It will then capture the Certification Path Advertisement (CPA)
   [RFC3971] and attempt to validate the certificate chain.  Failure to
   validate the chain will result in dropping the RA.  Upon validation
   success, the L2 device will forward the RA to its destination and
   store the router certificate in its cache.

   In order to operate in this scenario, the L2 device should be
   provisioned with a trust anchor certificate, as specified in
   Section 6 of [RFC3971].  It may also establish layer-3 connectivity
   with a Certificate Revocation List (CRL) Certification Path
   Advertisement server and/or with an NTP server.  A bootstrapping
   issue in this case can be resolved by using the configuration method
   to specify a trusted port to a first router, and the SEND-based
   RA-Guard method on all other ports.  The first router can then be
   used for Network Time Protocol (NTP) [RFC5905] and CRL connectivity.

5.  RA-Guard Use Considerations

   The RA-Guard mechanism is effective only when all messages between
   IPv6 devices in the target environment traverse controlled L2
   networking devices.  In the case of environments such as Ethernet
   hubs, devices can communicate directly without going through an
   RA-Guard-capable L2 networking device, and the RA-Guard feature
   cannot protect against rogue RAs.

   RA-Guard mechanisms do not offer protection in environments where
   IPv6 traffic is tunneled.

6.  Security Considerations

   Once RA-Guard has set up the proper criteria (for example, it
   specified that a port is allowed to receive RAs, or it identified
   legitimate sources of RAs or certificate bases [RFC4861]), then there
   are no possible instances of accidentally filtered legitimate Router
   Advertisements, assuming the RA-Guard filter enforcement strictly
   follows the RA-Guard set criteria.

   In Section 4.1, a simple mechanism to dynamically learn the valid
   IPv6 routers connected to an L2 device is explained.  It is important
   that this LEARNING state is only entered intentionally by manual
   configuration.  The list of learned IPv6 routers should be verified
   by the network manager to make sure that it corresponds with the
   expected valid RA list.  This procedure will make sure that either
   accidentally or intentionally generated rogue RAs are blocked by
   RA-Guard.

7.  Acknowledgements

   The authors dedicate this document to the memory of Jun-ichiro Hagino
   (itojun) for his contributions to the development and deployment of
   IPv6.

8.  References

8.1.  Normative References

   [RFC3971]   Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
               "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4861]   Narten, T., Nordmark, E., Simpson, W., and H.  Soliman,
               "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
               September 2007.

   [RFC5905]   Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
               "Network Time Protocol Version 4: Protocol and Algorithms
               Specification", RFC 5905, June 2010.

8.2.  Informative References

   [NDPMON]    LORIA/INRIA, "NDPMon - IPv6 Neighbor Discovery Protocol
               Monitor", November 2007,
               <http://ndpmon.sourceforge.net/>.

   [KAME]      KAME Project, "rafixd - developed at KAME - An active
               rogue RA nullifier", November 2007,
               <http://www.kame.net/>.

   [IPv6-SAMURAIS]
               Hagino (itojun), J., "IPv6 demystified: I have a problem
               with rogue RAs in my IPv6 network", 2007,
               <http://ipv6samurais.com/>.

   [RFC6104]   Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
               Problem Statement", RFC 6104, February 2011.

Authors' Addresses

   Eric Levy-Abegnoli
   Cisco Systems
   Village d'Entreprises Green Side - 400, Avenue Roumanille
   Biot - Sophia Antipolis, PROVENCE-ALPES-COTE D'AZUR  06410
   France

   Phone: +33 49 723 2620
   EMail: elevyabe@cisco.com


   Gunter Van de Velde
   Cisco Systems
   De Kleetlaan 6a
   Diegem  1831
   Belgium

   Phone: +32 2704 5473
   EMail: gunter@cisco.com


   Ciprian Popoviciu
   Technodyne
   111 Wood Ave. S.
   Iselin, NJ  08830
   USA

   Phone: +1 1 919 599-5666
   EMail: chip@technodyne.com


   Janos Mohacsi
   NIIF/Hungarnet
   18-22 Victor Hugo
   Budapest  H-1132
   Hungary

   EMail: mohacsi@niif.hu