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Versions: (draft-atwood-pim-sm-linklocal) 00 01 02 03 04 05 06 07 08 09 10 RFC 5796

PIM Working Group                                              W. Atwood
Internet-Draft                                                  S. Islam
Updates: 4601 (if approved)                     Concordia University/CSE
Intended status: Standards Track                        October 15, 2006
Expires: April 18, 2007


             Security Issues in PIM-SM Link-local Messages
                     draft-ietf-pim-sm-linklocal-00

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   This Internet-Draft will expire on April 18, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).













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Abstract

   This document outlines the security issues for the link-local
   messages in the Protocol Independent Multicast - Sparse Mode (PIM-SM)
   routing protocol.  It provides mechanisms to authenticate the PIM-SM
   link local messages using the IP security (IPsec) Authentication
   Header (AH).












































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1.  Introduction

   All the PIM-SM [1] control messages have IP protocol number 103.
   These messages are either unicast, or multicast with TTL = 1.  The
   source address used for unicast messages is a domain-wide reachable
   address.  For the multicast messages, a link-local address of the
   interface on which the message is being sent is used as the source
   address and a special multicast address, ALL_PIM_ROUTERS (224.0.0.13
   in IPv4 and ff02::d in IPv6) is used as the destination address.
   These messages are called link-local messages.  Hello, Join/Prune and
   Assert messages are included in this category.  A forged link-local
   message may be sent to the ALL_PIM_ROUTERS multicast address by an
   attacker.  This type of message affects the construction of the
   distribution tree [1].  The effects of these forged messages are
   outlined in section 6.1 of [1].  Some of the effects are very severe,
   whereas some are minor.

   PIM-SM version 2 was originally specified in RFC 2117, and revised in
   RFC 2362 and RFC 4601.  RFC 4601 obsoletes RFC 2362, and corrects a
   number of deficiencies.  The Security Considerations section of RFC
   4601 is based primarily on the new Authentication Header (AH)
   specification described in RFC 4302 [2].

   Securing the unicast messages can be achieved by the use of a normal
   unicast IPsec Security Association between the two communicants.
   Securing the user data exchanges is covered in RFC 3740 [5].  This
   document focuses on the security issues for link-local messages.  It
   provides some guidelines to take advantage of the new permitted AH
   functionality in RFC 4302, and to bring the PIM-SM specification into
   alignment with the new AH specification.  This document recommends
   manual key management as mandatory to implement, i.e., that all
   implementations MUST support, and discusses the need to develop a
   simple light-weight automated key management protocol that the PIM
   routers can use.

















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2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and
   indicate requirement levels for compliant PIM-SM implementations.













































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3.  Transport Mode vs. Tunnel Mode

   The transport mode Security Association (SA) is generally used
   between two hosts or routers/gateways when they are acting as hosts.
   The SA must be a tunnel mode SA if either end of the security
   association is a router/gateway.  Two hosts MAY establish a tunnel
   mode SA between themselves.  PIM-SM link-local messages are exchanged
   between routers.  However, since the packets are locally delivered,
   the routers assume the role of hosts in the context of the tunnel
   mode SA.  All implementations conforming to this specification MUST
   support transport mode SA to provide required IPsec security to
   PIM-SM link-local messages.  They MAY also support tunnel mode SA to
   provide required IPsec security to PIM-SM link-local messages.






































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4.  Authentication

   Implementations conforming to this specification MUST support
   authentication for PIM-SM link-local messages.  In order to provide
   authentication to PIM-SM link-local messages, implementations MUST
   support AH in transport mode.

   When authentication for PIM-SM link-local messages is enabled,

   o  PIM-SM link-local packets that are not protected with AH MUST be
      silently discarded.

   o  PIM-SM link-local packets that fail the authentication checks MUST
      be silently discarded.





































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5.   IPsec Requirements

   In order to implement this specification, the following IPsec
   capabilities are required.

   Transport mode
      IPsec in transport mode MUST be supported.

   Selectors
      The implementation MUST be able to use source address and SPI as
      selectors in the SPD.

   Manual key management
      Manually configured keys MUST be able to secure the specified
      traffic.




































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6.  Key Management

   All the implementations MUST support manual configuration of the SAs
   that will be used to authenticate PIM-SM link-local messages.  This
   does not preclude the use of a negotiation protocol such as the
   Internet Key Exchange (IKE) [9] or Group Secure Association Key
   Management Protocol (GSAKMP) [10]to establish SAs.

6.1.  Manual Key Management

   To establish the SAs at PIM-SM routers, manual key configuration will
   be feasible when the number of peers (directly connected routers) is
   small.  The Network Administrator will configure a router manually
   during its boot up process.  At that time, the authentication method
   and the choice of keys SHOULD be configured.  The SAD entry will be
   created.  The Network Administrator will also configure the Security
   Policy Database of a router to ensure the use of the associated SA
   while sending a link-local message.

6.2.  Automated Key Management

   All the link-local messages of the PIM-SM protocol are sent to the
   destination address, ALL_PIM_ROUTERS, which is a multicast address.
   By using the sender address in conjunction with the destination
   address for Security Association lookup, link-local communication
   turns to an SSM or "one to many" communication.  Since IKE is based
   on the Diffie-Hellman key agreement protocol and works only for two
   communicating parties, it is not possible to use IKE for providing
   the required "one to many" authentication.

   The other option is to use Group Domain Of Interpretation (GDOI)
   [11], which enables a group of users or devices to exchange encrypted
   data using IPsec data encryption.  GDOI has been developed to be used
   in multicast applications, where the number of end users or devices
   may be large and the end users or devices can dynamically join/leave
   a multicast group.  However, a PIM router is not expected to join/
   leave very frequently, and the number of routers is small when
   compared to the possible number of users of a multicast application.
   Moreover, most of the PIM routers will be located inside the same
   administrative domain and are considered as trusted parties.
   Probably, a GDOI-lite with a subset of GDOI functionalities should be
   designed by the PIM working group.









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7.  Number of Security Associations

   The number of Security Associations to be maintained by a PIM router
   depends on the required security level and available key management.
   This SHOULD be decided by the Network Administrator.  Two different
   ways are shown in Figure 1 and 2.  It is assumed that A, B and C are
   three PIM routers, where B and C are directly connected with A, and
   there is no direct link between B and C.

                  A                  |
                SAa     ------------>|
                SAb     <------------|
                SAc     <------------|
                                     |
                  B                  |
                SAb     ------------>|
                SAa     <------------|
                                     |
                  C                  |
                SAc     ------------>|
                SAa     <------------|
                                     |
                           Directly connected network

          Figure 1: Activate unique Security Association for each peer

   The first method, shown in Figure 1 is OPTIONAL to implement.  In
   this method, each node will use a unique SA for its outbound traffic.
   A, B, and C will use SAa, SAb, and SAc, respectively for sending any
   traffic.  Each node will look up the SA to be used based on the
   source address.  A will use SAb and SAc for packets received from B
   and C, respectively.  The number of SAs to be activated and
   maintained by a PIM router will be equal to the number of directly
   connected routers plus one, for sending its own traffic.  Also, the
   addition of a PIM router in the network will require the addition of
   another SA on every directly connected PIM router.  This solution
   will be scalable and practically feasible with an automated key
   management protocol.  However, it MAY be used with manual key
   management, if the number of directly connected router(s) is small.












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                 A                   |
                SAo     ------------>|
                SAi     <------------|
                                     |
                 B                   |
                SAo     ------------>|
                SAi     <------------|
                                     |
                 C                   |
                SAo     ------------>|
                SAi     <------------|
                                     |
                           Directly connected network

          Figure 2: Activate two Security Associations

   The second method, shown in Figure 2, MUST be supported by every
   implementation.  In this simple method, all the nodes will use two
   SAs, one for sending (SAo) and the other for receiving (SAi) traffic.
   Thus, the number of SAs is always two and will not be affected by
   addition of a PIM router.  This document RECOMMENDS the above method
   for manual key configuration.  However, it MAY also be used with
   automated key configuration.  When manually configured, the method
   suffers from impersonation attacks as mentioned in the Security
   Considerations section.


























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8.  Rekeying

   This section will provide the rekeying rules.  It will be written
   once is is decided whether or not to specify a re-keying protocol as
   part of this document.

8.1.  Rekeying Procedure

   TBD

8.2.  KeyRolloverInterval

   TBD

8.3.  Rekeying Interval

   TBD


































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9.  IPsec Protection Barrier and SPD

   This section will provide the SPD selection function rules.  It will
   be written once it is decided whether we need confidentiality in
   addition to authentication.














































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10.  Security Association Lookup

   For an SA that carries unicast traffic, three parameters (SPI,
   destination address and security protocol type (AH or ESP)) are used
   in the Security Association lookup process for inbound packets.  The
   SPI is sufficient to specify an SA.  However, an implementation may
   use the SPI in conjunction with the IPsec protocol type (AH or ESP)
   for the SA lookup process.  According to RFC 4301 [4] and the AH
   specification [2], for multicast SAs, in conjunction with the SPI,
   the destination address or the destination address plus the sender
   address may also be used in the SA lookup.  The security protocol
   field is not employed for a multicast SA lookup.

   The reason for the various prohibitions in the IPsec RFCs concerning
   multisender multicast SAs lies in the difficulty of coordinating the
   multiple senders.  However, if the use of multicast for link-local
   messages is examined, it may be seen that in fact the communication
   need not be coordinated---from the prospective of a receiving router,
   each peer router is an independent sender.  In effect, link-local
   communication is an SSM communication that happens to use an ASM
   address (which is shared among all the routers).  Two possibilities
   may be envisaged:

   1.  The address ALL_PIM_ROUTERS can be specified to operate as a set
       of SSM Security Associations, when IPsec is enabled;

   2.  Secure Link-local communication can be specified to occur on an
       SSM address, instead of ALL_PIM_ROUTERS.

   Given that the sender address of an incoming packet will be
   (globally) unique for a specific sender and in conjunction with the
   SPI it will be possible for a receiver to sort out the associated SA
   for that sender from all the SAD entries (even if a single SAD is
   maintained regardless of the number of interfaces), this document
   mandates that the SPI and the sender address MUST be used in the SA
   lookup process.















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11.  Activating the Anti-replay Mechanism

   Although link-level messages on a link constitute a multiple-sender,
   multiple-receiver group, the use of the sender address for SA lookup
   essentially resolves the communication into a separate SA for each
   sender/destination pair, even for the case where only two SAs are
   used for the entire administrative region.  Therefore, the statement
   in the AH RFC (section 2.5 of [2]) that "for a multi-sender SA, the
   anti-replay features are not available" becomes irrelevant to the
   PIM-SM link-local message exchange.

   To activate the anti-replay mechanism in a unicast communication, the
   receiver uses the sliding window protocol and it maintains a sequence
   number for this protocol.  This sequence number starts from zero.
   Each time the sender sends a new packet, it increments this number by
   one.  In a multi-sender multicast group communication, a single
   sequence number for the entire group would not be enough.

   The whole scenario is different for PIM link-local messages.  These
   messages are sent to local links with TTL = 1.  A link-local message
   never propagates through one router to another.  The use of the
   sender address for SA lookup converts the relationship from a
   multiple-sender group to multiple single-sender associations.  This
   specification RECOMMENDS activation of the anti-replay mechanism only
   if the SAs are assigned using an automated key management.  In manual
   key management, the anti-replay SHOULD NOT be activated.  If the
   number of router(s) to be assigned manually is small, the Network
   Administrator MAY consider to activate anti-replay.  If anti-replay
   is activated a PIM router MUST maintain a different sliding window
   for each directly connected sender.

   If the SAs are activated according to Figure 2, that is all the nodes
   use only two SAs, one SA for sending and the other is for receiving
   traffic, a PIM router MAY still activate the anti-replay mechanism.
   Instead of maintaining only two SAs, the router will maintain the
   same number of SAs as explained in the first method (see Figure 1)
   (because of the differentiation based on sender address).  For each
   active SA a corresponding sequence number MUST be maintained.  Thus,
   a PIM router will maintain a number of identical SAs, except that the
   sender address and the sequence number are different for each SA.  In
   this way a PIM router will be at least free from all the attacks that
   can be performed by replaying PIM-SM packets.

   Note that when activating anti-replay with manual key configuration,
   the following actions must be taken by the network administrator:






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   a.  If a new router is added, the Network Administrator MUST add a
       new SA entry in each peer router.

   b.  If an existing router has to restart, the Network Administrator
       MUST refresh the counter (ESN, see section 13) to zero for all
       the peer routers.  This implies deleting all the existing SAs and
       adding a new SA with the same configuration and a re-initialized
       counter.











































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12.  Implementing a Security Association Database per Interface

   RFC 4601 suggests that it may be desirable to implement a separate
   Security Association Database (SPD) for each router interface.  The
   use of the source address to resolve the SAs implies that the use of
   an SAD per interface is not necessary.  This is in conformance with
   RFC 4301, which explicitly removes the requirement for separate SPDs
   that was present in RFC 2401 [6].











































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13.  Extended Sequence Number

   In the [2], there is a provision for a 64-bit Extended Sequence
   Number (ESN) as the counter of the sliding window used in the anti-
   replay protocol.  Both the sender and the receiver maintain a 64-bit
   counter for the sequence number, although only the lower order 32
   bits is sent in the transmission.  In other words, it will not affect
   the present header format of AH.  If ESN is used, a sender router can
   send 2^64 -1 packets without any intervention.  This number is very
   large, and from a PIM router's point of view, a PIM router can never
   exceed this number in its lifetime.  This makes it reasonable to
   permit manual configuration for a small number of PIM routers, since
   the sequence number will never roll over.  For this reason, when
   manual configuration is used, ESN SHOULD be deployed as the sequence
   number for the sliding window protocol.




































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14.  Security Considerations

   The whole document considers the security issues of PIM link-local
   messages and proposes a mechanism to protect them.

   Limitations of manual keys:

   The following are some of the known limitations of the usage of
   manual keys.

   o  If the replay protection cannot be provided, the PIM routers will
      not be secured against all the attacks that can be performed by
      replaying PIM packets.

   o  Manual keys are usually long lived (changing them often is a
      tedious task).  This gives an attacker enough time to discover the
      keys.

   o  As the administrator is manually configuring the keys, there is a
      chance that the configured keys are weak (there are known weak
      keys for DES/3DES at least).

   Impersonation attacks:

   The usage of the same key on all the PIM routers connected to a link
   leaves them all insecure against impersonation attacks if any one of
   the PIM routers is compromised, malfunctioning, or misconfigured.

   Detailed analysis of various vulnerabilities of routing protocols is
   provided in [12].  For further discussion of PIM-SM and multicast
   security the reader is referred to [13], [14] and the Security
   Considerations section of RFC 4601.



















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15.  References

15.1.  Normative References

   [1]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
        "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
        Specification (Revised)", RFC 4601, August 2006.

   [2]  Kent, S., "IP Authentication Header", RFC 4302, December 2005.

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

   [4]  Kent, S. and K. Seo, "Security Architecture for the Internet
        Protocol", RFC 4301, December 2005.

   [5]  Hardjono, T. and B. Weis, "The Multicast Group Security
        Architecture", RFC 3740, March 2004.

15.2.  Informative References

   [6]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [7]   Islam, S., "Security Issues in PIM-SM Link-local Messages,
         Master's Thesis, Concordia University", December 2003.

   [8]   Islam, S., "Security Issues in PIM-SM Link-local Messages,
         Proceedings of LCN 2004", November 2004.

   [9]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         RFC 4306, December 2005.

   [10]  Harney, H., Meth, U., Colegrove, A., and G. Gross, "GSAKMP:
         Group Secure Association Key Management Protocol", RFC 4535,
         June 2006.

   [11]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group
         Domain of Interpretation", RFC 3547, July 2003.

   [12]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
         Routing Protocols", draft-ietf-rpsec-routing-threats-07 (work
         in progress), October 2004.

   [13]  Lingard, J. and P. Savola, "Last-hop Threats to Protocol
         Independent Multicast (PIM)",
         draft-savola-pim-lasthop-threats-02 (work in progress),
         June 2006.



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   [14]  Savola, P., Lehtonen, R., and D. Meyer, "PIM-SM Multicast
         Routing Security Issues and Enhancements",
         draft-ietf-mboned-mroutesec-04 (work in progress),
         October 2004.















































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Authors' Addresses

   J. William Atwood
   Concordia University/CSE
   1455 de Maisonneuve Blvd, West
   Montreal, QC  H3G 1M8
   Canada

   Phone: +1(514)848-2424 ext3046
   Email: bill@cse.concordia.ca
   URI:   http://www.cs.concordia.ca/~bill


   Salekul Islam
   Concordia University/CSE
   1455 de Maisonneuve Blvd, West
   Montreal, QC  H3G 1M8
   Canada

































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