





Internet Engineering Task Force (IETF)                         M. Bhatia
Request for Comments: 7492                                Ionos Networks
Category: Informational                                         D. Zhang
ISSN: 2070-1721                                                   Huawei
                                                         M. Jethanandani
                                                       Ciena Corporation
                                                              March 2015


     Analysis of Bidirectional Forwarding Detection (BFD) Security
According to the Keying and Authentication for Routing Protocols (KARP)
                           Design Guidelines

Abstract

   This document analyzes the Bidirectional Forwarding Detection (BFD)
   protocol according to the guidelines set forth in Section 4.2 of RFC
   6518, "Keying and Authentication for Routing Protocols (KARP) Design
   Guidelines".

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/rfc7492.
















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Copyright Notice

   Copyright (c) 2015 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements to Meet  . . . . . . . . . . . . . . . . . . . .   3
   3.  Current State of Security Methods . . . . . . . . . . . . . .   3
   4.  Impacts of BFD Replays  . . . . . . . . . . . . . . . . . . .   5
   5.  Impact of New Authentication Requirements . . . . . . . . . .   6
   6.  Considerations for Improvement  . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document performs a gap analysis of the current state of
   Bidirectional Forwarding Detection [RFC5880] according to the
   requirements of KARP Design Guidelines [RFC6518].  Previously, the
   OPSEC working group has provided an analysis of cryptographic issues
   with BFD in "Issues with Existing Cryptographic Protection Methods
   for Routing Protocols" [RFC6039].

   The existing BFD specifications provide a basic security solution.
   Key ID is provided so that the key used in securing a packet can be
   changed on demand.  Two cryptographic algorithms (MD5 and SHA-1) are
   supported for integrity protection of the control packets; the
   algorithms are both demonstrated to be subject to collision attacks.
   Routing protocols like "RIPv2 Cryptographic Authentication"
   [RFC4822], "IS-IS Generic Cryptographic Authentication" [RFC5310],
   and "OSPFv2 HMAC-SHA Cryptographic Authentication" [RFC5709] have
   started to use BFD for liveliness checks.  Moving the routing



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   protocols to a stronger algorithm while using a weaker algorithm for
   BFD would allow the attacker to bring down BFD in order to bring down
   the routing protocol.  BFD therefore needs to match the routing
   protocols in its strength of algorithm.

   While BFD uses a non-decreasing, per-packet sequence number to
   protect itself from intra-connection replay attacks, it still leaves
   the protocol vulnerable to the inter-session replay attacks.

2.  Requirements to Meet

   There are several requirements described in Section 4 of [RFC6862]
   that BFD, as defined in BFD [RFC5880], does not currently meet:

      Replay Protection: BFD provides an incomplete intra-session and no
      inter-session replay attack protection; this creates significant
      denial-of-service opportunities.

      Strong Algorithms: The cryptographic algorithms adopted for
      message authentication in BFD are MD5 or SHA-1 based.  However,
      both algorithms are known to be vulnerable to collision attacks.
      "BFD Generic Cryptographic Authentication" [BFD-CRYPTO] and
      "Authenticating BFD using HMAC-SHA-2 procedures" [BFD-HMAC]
      together propose a solution to support Hashed Message
      Authentication Code (HMAC) with the SHA-2 family of hash functions
      for BFD.

      Preventing DoS Attacks: BFD packets can be sent at millisecond
      intervals (the protocol uses timers at microsecond intervals).
      When malicious packets are sent at short intervals, with the
      authentication bit set, it can cause a DoS attack.  There is
      currently no lightweight mechanism within BFD to address this
      issue and is one of the reasons BFD authentication is still not
      widely deployed in the field.

   The remainder of this document explains the details of how these
   requirements fail to be met and proposes mechanisms for addressing
   them.

3.  Current State of Security Methods

   BFD [RFC5880] describes five authentication mechanisms for the
   integrity protection of BFD control packets: Simple Password, Keyed
   MD5 [RFC1321], Meticulous Keyed MD5, Keyed SHA-1, and Meticulous
   Keyed SHA-1.  In the simple password mechanism, every control packet
   is associated with a password transported in plain text; attacks
   eavesdropping the network traffic can easily learn the password and
   compromise the security of the corresponding BFD session.  In the



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   Keyed MD5 and the Meticulous Keyed MD5 mechanisms, BFD nodes use
   shared secret keys to generate Keyed MD5 digests for control packets.
   Similarly, in the Keyed SHA-1 and the Meticulous Keyed SHA-1
   mechanisms, BFD nodes use shared secret keys to generate Keyed SHA-1
   digests for control packets.  Note that in the keyed authentication
   mechanisms, every BFD control packet is associated with a non-
   decreasing, 32-bit sequence number to resist replay attacks.  In the
   Keyed MD5 and the Keyed SHA-1 mechanisms, the sequence member is only
   required to increase occasionally.  However, in the Meticulous Keyed
   MD5 and the Meticulous Keyed SHA-1 mechanisms, the sequence member is
   required to increase with each successive packet.

   Additionally, limited key updating functionality is provided.  There
   is a Key ID in every authenticated BFD control packet indicating the
   key used to hash the packet.  However, there is no mechanism
   described to provide a smooth key rollover that the BFD routers can
   use when moving from one key to the other.

   The BFD session timers are defined with the granularity of
   microseconds, and it is common in practice to send BFD packets at
   millisecond intervals.  Since the cryptographic sequence number space
   is only 32 bits, a sequence number used in a BFD session may reach
   its maximum value and roll over within a limited period.  For
   instance, if a sequence number is increased by one every 3.3
   milliseconds, then it will reach its maximum value in less than 24
   weeks.  This can result in potential inter-session replay attacks,
   especially when BFD uses the non-meticulous authentication modes.

   Note that when using authentication mechanisms, BFD drops all packets
   that fall outside the limited range (3 times the Detection Time
   multiplier).  Therefore, when meticulous authentication modes are
   used, a replayed BFD packet will be rejected if it cannot fit into a
   relatively short window (3 times the detection interval of the
   session).  This introduces some difficulties for replaying packets.
   However, in a non-meticulous authentication mode, such windows can be
   large (as sequence numbers are only increased occasionally), thus
   making it easier to perform replay attacks .

   In a BFD session, each node needs to select a 32-bit discriminator to
   identify itself.  Therefore, a BFD session is identified by two
   discriminators.  If a node randomly selects a new discriminator for a
   new session and uses authentication mechanisms to secure the control
   packets, inter-session replay attacks can be mitigated to some
   extent.  However, in existing BFD demultiplexing mechanisms, the
   discriminators used in a new BFD session may be predictable.  In some
   deployment scenarios, the discriminators of BFD routers may be
   decided by the destination and source addresses.  So, if the sequence
   number of a BFD router rolls over for some reason (e.g., reboot), the



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   discriminators used to identify the new session will be identical to
   the ones used in the previous session.  This makes performing a
   replay attack relatively simple.

   BFD allows a mode called the echo mode.  Echo packets are not defined
   in the BFD specification, though they can keep the BFD session up.
   The format of the echo packet is local to the sending side, and there
   are no guidelines on the properties of these packets beyond the
   choice of the source and destination addresses.  While the BFD
   specification recommends applying security mechanisms to prevent
   spoofing of these packets, there are no guidelines on what type of
   mechanisms are appropriate.

4.  Impacts of BFD Replays

   As discussed, BFD cannot meet the requirements of inter-session or
   intra-session replay protection.  This section discusses the impacts
   of BFD replays.

   When cryptographic authentication mechanisms are adopted for BFD, a
   non-decreasing, 32-bit-long sequence number is used.  In the Keyed
   MD5 and the Keyed SHA-1 mechanisms, the sequence member is not
   required to increase for every packet.  Therefore, an attacker can
   keep replaying the packets with the latest sequence number until the
   sequence number is updated.  This issue is eliminated in the
   Meticulous Keyed MD5 and the Meticulous Keyed SHA-1 mechanisms.
   However, note that a sequence number may reach its maximum and be
   rolled over in a session.  In this case, without the support from a
   automatic key management mechanism, the BFD session will be
   vulnerable to replay attacks performed by sending the packets before
   the roll over of the sequence number.  For instance, an attacker can
   replay a packet with a sequence number that is larger than the
   current one.  If the replayed packet is accepted, the victim will
   reject the legal packets whose sequence members are less than the one
   in the replayed packet.  Therefore, the attacker can get a good
   chance to bring down the BFD session.  This kind of attack assumes
   that the attacker has access to the link when the BFD session is on a
   point-to-point link or can inject packets for a BFD session with
   multiple hops.

   Additionally, the BFD specification allows for the change of
   authentication state based on the state of a received packet.  For
   instance, according to BFD [RFC5880], if the state of an accepted
   packet is down, the receiver of the packet needs to transfer its
   state to down as well.  Therefore, a carefully selected replayed
   packet can cause a serious denial-of-service attack.





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   BFD does not provide any solution to deal with inter-session replay
   attacks.  If two subsequent BFD sessions adopt an identical
   discriminator pair and use the same cryptographic key to secure the
   control packets, it is intuitive to use a malicious authenticated
   packet (stored from the past session) to perform interconnection
   replay attacks.

   Any security issues in the BFD echo mode will directly affect the BFD
   protocol and session states, and hence the network stability.  For
   instance, any replay attacks would be indistinguishable from normal
   forwarding of the tested router.  An attack would still cause a
   faulty link to be believed to be up, but there is little that can be
   done about it.  However, if the echo packets are guessable, it may be
   possible to spoof from an external source and cause BFD to believe
   that a one-way link is really bidirectional.  As a result, it is
   important that the echo packets contain random material that is also
   checked upon reception.

5.  Impact of New Authentication Requirements

   BFD can be run in software or hardware.  Hardware implementations run
   BFD at a much smaller timeout, typically in the order of few
   milliseconds.  For instance, with a timeout of 3.3 milliseconds, a
   BFD session is required to send or receive 3 packets every 10
   milliseconds.  Software implementations typically run with a timeout
   in hundreds of milliseconds.

   Additionally, it is not common to find hardware support for computing
   the authentication data for the BFD session in hardware or software.
   In the Keyed MD5 and Keyed SHA-1 implementation where the sequence
   number does not increase with every packet, software can be used to
   compute the authentication data.  This is true if the time between
   the increasing sequence number is long enough to compute the data in
   software.  The ability to compute the hash in software is difficult
   with Meticulous Keyed MD5 and Meticulous Keyed SHA-1 if the time
   interval between transmits or between receives is small.  The
   computation problem becomes worse if hundred or thousands of sessions
   require the hash to be recomputed every few milliseconds.

   Smaller and cheaper boxes that have to support a few hundred BFD
   sessions are boxes that also use a slower CPU.  The CPU is used for
   running the entire control plane software in addition to supporting
   the BFD sessions.  As a general rule, no more than 40-45% of the CPU
   can be dedicated towards supporting BFD.  Adding computation of the
   hash for every BFD session can easily cause the CPU to exceed the
   40-45% limit even with a few tens of sessions.  On higher-end boxes
   with faster and more CPU cores, the expectation is that the number of




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   sessions that need to be supported are in the thousands, but the
   number of BFD sessions with authentication that CPU can support is
   still in the hundreds.

   Implementors should assess the impact of authenticating BFD sessions
   on their platform.

6.  Considerations for Improvement

   This section suggests changes that can be adopted to improve the
   protection of BFD.

   The security risks brought by SHA-1 and MD5 have been well
   understood.  However, when using a stronger digest algorithm, e.g.,
   SHA-2, the imposed computing overhead will seriously affect the
   performance of BFD implementation.  In order to make the trade-off
   between the strong algorithm requirement and the imposed overhead,
   Galois Message Authentication Code (GMAC) can be a candidate option.
   This algorithm is relatively effective and has been supported by
   IPsec for data origin authentication.  More detailed information can
   be found in "The Use of Galois Message Authentication Code (GMAC) in
   IPsec ESP and AH" [RFC4543].

   There has been some hallway conversation around the idea of using BFD
   cryptographic authentication only when some data in the BFD payload
   changes.  The other BFD packets can be transmitted and received
   without authentication enabled.  The bulk of the BFD packets that are
   transmitted and received have no state change associated with them.
   Limiting authentication to BFD packets that affect a BFD session
   state allows for more sessions to be supported for authentication.
   This change can significantly help the routers since they don't have
   to compute and verify the authentication digest for the BFD packets
   coming at the millisecond intervals.  This proposal needs some more
   discussion in the BFD working group and is certainly a direction that
   BFD could look at.

7.  Security Considerations

   This document discusses vulnerabilities in the existing BFD protocol
   and suggests possible mitigations.

   In analyzing the improvements for BFD, the ability to repel a replay
   attack is discussed.  For example, increasing the sequence number to
   a 64-bit value makes the wrap-around time much longer, and a replay
   attack can be easily prevented.






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   Mindful of the impact that stronger algorithms can have on the
   performance of BFD, the document suggests GMAC as a possible
   candidate for MAC function.

8.  References

8.1.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992, <http://www.rfc-editor.org/info/rfc1321>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, October 2010,
              <http://www.rfc-editor.org/info/rfc6039>.

8.2.  Informative References

   [BFD-CRYPTO]
              Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
              "BFD Generic Cryptographic Authentication", Work in
              Progress, draft-ietf-bfd-generic-crypto-auth-06, April
              2014.

   [BFD-HMAC] Zhang, D., Bhatia, M., Manral, V., and M. Jethanandani,
              "Authenticating BFD using HMAC-SHA-2 procedures", Work in
              Progress, draft-ietf-bfd-hmac-sha-05, July 2014.

   [RFC4543]  McGrew, D. and J. Viega, "The Use of Galois Message
              Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
              May 2006, <http://www.rfc-editor.org/info/rfc4543>.

   [RFC4822]  Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007,
              <http://www.rfc-editor.org/info/rfc4822>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009,
              <http://www.rfc-editor.org/info/rfc5310>.







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   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009,
              <http://www.rfc-editor.org/info/rfc5709>.

   [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines", RFC 6518,
              February 2012, <http://www.rfc-editor.org/info/rfc6518>.

   [RFC6862]  Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
              Authentication for Routing Protocols (KARP) Overview,
              Threats, and Requirements", RFC 6862, March 2013,
              <http://www.rfc-editor.org/info/rfc6862>.

Acknowledgements

   We would like to thank Alexander Vainshtein for his comments on this
   document.

Authors' Addresses

   Manav Bhatia
   Ionos Networks
   Bangalore
   India

   EMail: manav@ionosnetworks.com


   Dacheng Zhang
   Huawei

   EMail: dacheng.zhang@gmail.com


   Mahesh Jethanandani
   Ciena Corporation
   3939 North 1st Street
   San Jose, CA  95134
   United States

   Phone: 408.904.2160
   Fax:   408.436.5582
   EMail: mjethanandani@gmail.com







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