BLACKHOLE Community
A standard BGP community to signal that traffic to an IP prefix should be discarded.
King, T., Dietzel, C., Snijders, J., Doering, G., and G. Hankins, "BLACKHOLE Community", RFC 7999, DOI 10.17487/RFC7999, October 2016, <https://www.rfc-editor.org/info/rfc7999>.
@misc{rfc7999,
series = {Request for Comments},
number = 7999,
howpublished = {RFC 7999},
publisher = {RFC Editor},
doi = {10.17487/RFC7999},
url = {https://www.rfc-editor.org/info/rfc7999},
author = {T. King and C. Dietzel and J. Snijders and G. Doering and G. Hankins},
title = {{BLACKHOLE Community}},
pagetotal = 9,
year = 2016,
month = oct,
} RFC text
Contents
Internet Engineering Task Force (IETF) T. King
Request for Comments: 7999 C. Dietzel
Category: Informational DE-CIX
ISSN: 2070-1721 J. Snijders
NTT
G. Doering
SpaceNet AG
G. Hankins
Nokia
October 2016
BLACKHOLE Community
Abstract
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
http://www.rfc-editor.org/info/rfc7999.
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
(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.
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
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
well-known.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
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.
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
policy.
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.
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
specifications.
The error handling for this community follows the process in
[RFC7606] that causes a malformed community to be treated as
withdrawn.
Operators are encouraged to store all BGP updates in their network
carrying the BLACKHOLE community for long-term analysis or internal
audit purposes.
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"
registry.
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 6.2.1.1.2 of [RFC7454]) that the peer announcing the prefix
is authorized to do so. If not, the BGP announcement should be
filtered.
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,
<http://www.rfc-editor.org/info/rfc1997>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc4271>.
[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,
<http://www.rfc-editor.org/info/rfc7606>.
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,
<http://www.rfc-editor.org/info/rfc3882>.
[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,
<http://www.rfc-editor.org/info/rfc5575>.
[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,
<http://www.rfc-editor.org/info/rfc5635>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<http://www.rfc-editor.org/info/rfc6811>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <http://www.rfc-editor.org/info/rfc7454>.
Acknowledgements
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.
Authors' Addresses
Thomas King
DE-CIX Management GmbH
Lichtstrasse 43i
Cologne 50825
Germany
Email: thomas.king@de-cix.net
Christoph Dietzel
DE-CIX Management GmbH
Lichtstrasse 43i
Cologne 50825
Germany
Email: christoph.dietzel@de-cix.net
Job Snijders
NTT Communications
Theodorus Majofskistraat 100
Amsterdam 1065 SZ
The Netherlands
Email: job@ntt.net
Gert Doering
SpaceNet AG
Joseph-Dollinger-Bogen 14
Munich 80807
Germany
Email: gert@space.net
Greg Hankins
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States of America
Email: greg.hankins@nokia.com
AI ELI5
On this page
§1The Problem: Too Many Blackholing Mechanisms
DDoS attacks flood networks with unwanted traffic. A common defense is to announce that traffic destined to a victim IP address should be discarded entirely, a technique called blackholing.
The problem: different networks have historically used different methods to signal this. Some use predefined next-hop IP addresses, others use BGP communities (special attributes that carry metadata about routes), and still others require out-of-band agreements or special BGP sessions. This patchwork makes network operations unnecessarily complex and error-prone, especially during an active attack when speed matters.
RFC 7999 solves this by defining a single well-known BGP community that all networks can recognize and understand. A network under attack can announce its victim IP prefix with this community attached, and neighboring networks that support it will automatically discard the traffic.
Before this, each network had its own way of saying 'drop this traffic.' Now there is one standard signal everyone understands.
§2The BLACKHOLE Community
The BLACKHOLE community is a well-known BGP transitive community. In BGP terminology, 'transitive' means the community travels along with the route announcement as it propagates from network to network.
The community value is 0xFFFF029A (expressed in hexadecimal). In decimal, the low-order two octets are 666—a number long associated with blackholing in network operator culture.
When a network announces an IP prefix and attaches the BLACKHOLE community to it, it is signaling: 'Traffic destined to this prefix should be discarded.' The semantics are advisory—neighboring networks can choose to honor the community or ignore it.
The community is most useful when combined with destination-based blackholing, where a network operator announces a highly specific IP prefix (often a single host, /32 for IPv4 or /128 for IPv6) covering only the victim, minimizing collateral damage to other traffic.
Think of it as a label on a route announcement that says 'discard this.' Networks that see the label can drop traffic to that destination.
§3How to Use BLACKHOLE Safely
RFC 7999 provides operational guidance for using the BLACKHOLE community responsibly.
First, bilateral peering relationships: if two networks peer directly, they should agree in advance that they will respect the BLACKHOLE community. It is not automatic; operators must explicitly configure their routers to honor it.
Second, multilateral peering (route servers): if a network participates in a shared route server (a central point where many networks exchange routes), operators should follow the route server's policy on BLACKHOLE communities.
Third, preventing leaks: when a network receives a BLACKHOLE-tagged prefix, it should add the NO_ADVERTISE or NO_EXPORT community to prevent the prefix from leaking outside the local AS. Accidentally announcing a more specific blackhole prefix to the public Internet can cause serious harm.
Fourth, prefix length limits: the RFC notes that routes longer than /24 (IPv4) or /48 (IPv6) are generally not accepted on the Internet. For blackholing to work, the blackhole prefix should be as specific as possible (/32 or /128), which requires special handling and explicit bilateral agreements.
Finally, authorization: receiving networks should verify that the announcing network is actually authorized to advertise the prefix before honoring the blackhole. For example, an attacker might try to announce example.com's IP space with BLACKHOLE attached to knock them offline.
Just like a stop sign only works if everyone agrees to follow it, BLACKHOLE only works when both networks have agreed to respect it. You must also be careful not to accidentally block someone else's legitimate traffic.
§4Security Implications and Validation
The BLACKHOLE community creates potential security risks if misused. An attacker could announce an IP prefix with BLACKHOLE attached to cause a network operator's legitimate traffic to be discarded across the Internet.
To mitigate this risk, RFC 7999 emphasizes strong validation: a receiving network must verify that the announcing network is actually authorized to advertise the prefix. This typically means using BGP route filtering and origin validation techniques such as BGP Prefix Origin Validation (ROV), which ensures that only the legitimate originating AS can announce a prefix.
Additionally, operators should only honor BLACKHOLE from trusted peers with bilateral agreements. Networks should implement strict prefix filtering, checking that announced prefixes match what the peer is authorized to advertise.
Note that once a BLACKHOLE community is attached to a prefix, that information should be protected from unauthorized modification during transit. Operators are encouraged to store all BGP updates carrying BLACKHOLE for long-term auditing and forensic analysis.
A bad actor could misuse this signal to knock a legitimate network offline, so you must verify that whoever is announcing the blackhole prefix actually owns it and is allowed to.