Internet Engineering Task Force (IETF)                           T. King
Request for Comments: 7999                                    C. Dietzel
Category: Informational                                           DE-CIX
ISSN: 2070-1721                                              J. Snijders
                                                              G. Doering
                                                             SpaceNet AG
                                                              G. Hankins
                                                            October 2016

                          BLACKHOLE Community


   This document describes the use of a well-known Border Gateway
   Protocol (BGP) community for destination-based blackholing in IP
   networks.  This well-known advisory transitive BGP community named
   "BLACKHOLE" allows an origin Autonomous System (AS) to specify that a
   neighboring network should discard any traffic destined towards the
   tagged IP prefix.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7999                   BLACKHOLE Community              October 2016

Copyright Notice

   Copyright (c) 2016 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
   ( 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. Requirements Language ......................................3
   2. BLACKHOLE Community .............................................4
   3. Operational Recommendations .....................................4
      3.1. IP Prefix Announcements with BLACKHOLE Community Attached ..4
      3.2. Local Scope of Blackholes ..................................4
      3.3. Accepting Blackholed IP Prefixes ...........................5
   4. Vendor Implementation Recommendations ...........................6
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   Acknowledgements ...................................................8
   Authors' Addresses .................................................9

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1.  Introduction

   Network infrastructures have been increasingly hampered by DDoS
   attacks.  In order to dampen the effects of these DDoS attacks, IP
   networks have offered blackholing with BGP [RFC4271] using various
   mechanisms such as those described in [RFC3882] and [RFC5635].

   DDoS attacks targeting a certain IP address may cause congestion of
   links used to connect to adjacent networks.  In order to limit the
   impact of such a scenario on legitimate traffic, networks adopted a
   mechanism called "BGP blackholing".  A network that wants to trigger
   blackholing needs to understand the triggering mechanism adopted by
   its neighboring networks.  Different networks provide different
   mechanisms to trigger blackholing, including but not limited to pre-
   defined blackhole next-hop IP addresses, specific BGP communities, or
   out-of-band BGP sessions with a special BGP speaker.

   Having several different mechanisms to trigger blackholing in
   different networks makes it an unnecessarily complex, error-prone,
   and cumbersome task for network operators.  Therefore, a well-known
   BGP community [RFC1997] is defined for operational ease.

   Having such a well-known BGP community for blackholing also further
   simplifies network operations because:

   o  Implementing and monitoring blackholing becomes easier when
      implementation and operational guides do not cover many variations
      to trigger blackholing.

   o  The number of support requests from customers about how to trigger
      blackholing in a particular neighboring network will be reduced as
      the codepoint for common blackholing mechanisms is unified and

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   be interpreted as described in [RFC2119] only when they appear in all
   upper case.  They may also appear in lower case or mixed case as
   English words, without normative meaning.

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2.  BLACKHOLE Community

   This document defines the use of a new well-known BGP transitive
   community, BLACKHOLE.

   The semantics of this community allow a network to interpret the
   presence of this community as an advisory qualification to drop any
   traffic being sent towards this prefix.

3.  Operational Recommendations

3.1.  IP Prefix Announcements with BLACKHOLE Community Attached

   Accepting and honoring the BLACKHOLE community, or ignoring it, is a
   choice that is made by each operator.  This community MAY be used in
   all bilateral and multilateral BGP deployment scenarios.  In a
   bilateral peering relationship, use of the BLACKHOLE community MUST
   be agreed upon by the two networks before advertising it.  In a
   multilateral peering relationship, the decision to honor or ignore
   the BLACKHOLE community is to be made according to the operator's
   routing policy.  The community SHOULD be ignored, if it is received
   by a network that it not using it.

   When a network is under DDoS duress, it MAY announce an IP prefix
   covering the victim's IP address(es) for the purpose of signaling to
   neighboring networks that any traffic destined for these IP
   address(es) should be discarded.  In such a scenario, the network
   operator SHOULD attach the BLACKHOLE community.

   The BLACKHOLE community MAY also be used as one of the trigger
   communities in a destination-based Remote Triggered Blackhole (RTBH)
   [RFC5635] configuration.

3.2.  Local Scope of Blackholes

   A BGP speaker receiving an announcement tagged with the BLACKHOLE
   community SHOULD add the NO_ADVERTISE or NO_EXPORT community as
   defined in [RFC1997], or a similar community, to prevent propagation
   of the prefix outside the local AS.  The community to prevent
   propagation SHOULD be chosen according to the operator's routing

   Unintentional leaking of more specific IP prefixes to neighboring
   networks can have adverse effects.  Extreme caution should be used
   when purposefully propagating IP prefixes tagged with the BLACKHOLE
   community outside the local routing domain, unless policy explicitly
   aims at doing just that.

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3.3.  Accepting Blackholed IP Prefixes

   It has been observed in provider networks running BGP that
   announcements of IP prefixes longer than /24 for IPv4 and /48 for
   IPv6 are usually not accepted on the Internet (see Section 6.1.3 of
   [RFC7454]).  However, blackhole prefix length should be as long as
   possible in order to limit the impact of discarding traffic for
   adjacent IP space that is not under DDoS duress.  The blackhole
   prefix length is typically as specific as possible, /32 for IPv4 or
   /128 for IPv6.

   BGP speakers in a bilateral peering relationship using the BLACKHOLE
   community MUST only accept and honor BGP announcements carrying the
   BLACKHOLE community under the two following conditions:

   o  The announced prefix is covered by an equal or shorter prefix that
      the neighboring network is authorized to advertise.

   o  The receiving party agreed to honor the BLACKHOLE community on the
      particular BGP session.

   In topologies with a route server or other multilateral peering
   relationships, BGP speakers SHOULD accept and honor BGP announcements
   under the same conditions.

   An operator MUST ensure that origin validation techniques (such as
   the one described in [RFC6811]) do not inadvertently block legitimate
   announcements carrying the BLACKHOLE community.

   The BLACKHOLE community is not intended to be used with Network Layer
   Reachability Information (NLRI) [RFC5575] to distribute traffic flow

   The error handling for this community follows the process in
   [RFC7606] that causes a malformed community to be treated as

   Operators are encouraged to store all BGP updates in their network
   carrying the BLACKHOLE community for long-term analysis or internal
   audit purposes.

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4.  Vendor Implementation Recommendations

   Without an explicit configuration directive set by the operator,
   network elements SHOULD NOT discard traffic destined towards IP
   prefixes that are tagged with the BLACKHOLE community.  The operator
   is expected to explicitly configure the network element to honor the
   BLACKHOLE community in a way that is compliant with the operator's
   routing policy.

   Vendors MAY provide a shorthand keyword in their configuration
   language to reference the well-known BLACKHOLE community attribute
   value.  The suggested string to be used is "blackhole".

5.  IANA Considerations

   The IANA has registered BLACKHOLE in the "BGP Well-known Communities"

      BLACKHOLE (= 0xFFFF029A)

   The low-order two octets in decimal are 666, a value commonly
   associated with BGP blackholing among network operators.

6.  Security Considerations

   BGP contains no specific mechanism to prevent the unauthorized
   modification of information by the forwarding agent.  This allows
   routing information to be modified or removed; it also allows false
   information to be added by forwarding agents.  Recipients of routing
   information are not able to detect this modification.  BGPsec
   [BGPSEC] does not resolve this situation.  Even when BGPsec is in
   place, a forwarding agent can alter, add, or remove BGP communities.

   The unauthorized addition of the BLACKHOLE community to an IP prefix
   by an adversary may cause a denial-of-service attack based on denial
   of reachability.

   In order to further limit the impact of unauthorized BGP
   announcements carrying the BLACKHOLE community, the receiving BGP
   speaker SHOULD verify by applying strict filtering (see
   Section of [RFC7454]) that the peer announcing the prefix
   is authorized to do so.  If not, the BGP announcement should be

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   BGP announcements carrying the BLACKHOLE community should only be
   accepted and honored if the neighboring network is authorized to
   advertise the prefix.  The method of validating announcements is to
   be chosen according to the operator's routing policy.

   It is RECOMMENDED that operators use best common practices to protect
   their BGP sessions, such as the ones in [RFC7454].

7.  References

7.1.  Normative References

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,

7.2.  Informative References

   [BGPSEC]   Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", Work in Progress, draft-ietf-sidr-bgpsec-
              protocol-18, August 2016.

   [RFC3882]  Turk, D., "Configuring BGP to Block Denial-of-Service
              Attacks", RFC 3882, DOI 10.17487/RFC3882, September 2004,

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,

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RFC 7999                   BLACKHOLE Community              October 2016

   [RFC5635]  Kumari, W. and D. McPherson, "Remote Triggered Black Hole
              Filtering with Unicast Reverse Path Forwarding (uRPF)",
              RFC 5635, DOI 10.17487/RFC5635, August 2009,

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,

   [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
              and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
              February 2015, <>.


   The authors would like to gratefully acknowledge many people who have
   contributed discussions and ideas to the development of this
   document.  They include Petr Jiran, Yordan Kritski, Christian Seitz,
   Nick Hilliard, Joel Jaeggli, Christopher Morrow, Thomas Mangin, Will
   Hargrave, Niels Bakker, David Farmer, Jared Mauch, John Heasley, and
   Terry Manderson.

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Authors' Addresses

   Thomas King
   DE-CIX Management GmbH
   Lichtstrasse 43i
   Cologne  50825


   Christoph Dietzel
   DE-CIX Management GmbH
   Lichtstrasse 43i
   Cologne  50825


   Job Snijders
   NTT Communications
   Theodorus Majofskistraat 100
   Amsterdam  1065 SZ
   The Netherlands


   Gert Doering
   SpaceNet AG
   Joseph-Dollinger-Bogen 14
   Munich  80807


   Greg Hankins
   777 E. Middlefield Road
   Mountain View, CA  94043
   United States of America


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