Opsec Working Group                                            W. Kumari
Internet Draft                                                    Google
<draft-ietf-opsec-blackhole-urpf-00>                        D. McPherson
Category: Informational                                   Arbor Networks
Expires: July 19, 2009
                                                        January 19, 2009

            Remote Triggered Black Hole filtering with uRPF

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on July 13, 2009.

Copyright Notice

   Copyright (c) 2009 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.

Kumari & McPherson        Expires July 19, 2009                 [Page 1]

Internet Draft            blackhole-urpf-00.txt         January 19 2009


   Remote Triggered Black Hole (RTBH) filtering is a popular and
   effective technique for the mitigation of denial-of-service attacks.
   This document expands upon destination-based RTBH filtering by
   outlining a method to enable filtering by source address as well. It
   also defines a standard BGP community for black hole prefixes to
   simplify associated semantics.

Kumari & McPherson        Expires July 19, 2009                 [Page 2]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

Table of Contents

   1. Introduction ....................................................2
   2. Background ......................................................2
   3. Destination address RTBH filtering ..............................3
      3.1. Overview ...................................................3
      3.2. Detail .....................................................3
   4. Source address RTBH filtering ...................................4
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................6
   7. Acknowledgments .................................................7
   8. References ......................................................7
   A. Cisco Router Configuration Sample................................8
   B. Juniper Configuration Sample....................................10

1. Introduction

   This document expands upon the technique outlined in "Configuring BGP
   to Block Denial-of-Service Attacks" [RFC3882] to present a method
   that allows filtering by source address(es). It also defines a
   standard BGP community to signal that Remote Triggered Black Hole
   (RTBH) filtering should occur for a network.

1.2 Terminology

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as described in BCP 14, RFC
   2119 [RFC2119].

2. Background

   Network operators have developed a variety of techniques for
   mitigating these types of attacks. While the different techniques
   have varying strengths and weaknesses from an implementation
   perspective, the selection of which method to use for each type of
   attack involves evaluating tradeoffs.

   A common DoS attack directed against a customer of a service provider
   involves generating more attack traffic destined for the target than
   will fit down the links from the service provider to the victim
   (customer). This traffic "starves out" legitimate traffic and often
   results in collateral damage or negative effects to other customers
   or the network infrastructure as well.  Rather than having all of
   their network affected the attack, the customer may ask their service
   provider to filter traffic destined to the target destination IP
   address(es), or the service provider may determine that this is
   necessary themselves, in order to preserve network availability.

Kumari & McPherson        Expires July 19, 2009                 [Page 3]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

   One method that the service provider can use to implement this
   filtering is to deploy access control lists on the edge of their
   network. While this technique provides a large amount of flexibility
   in the filtering, it runs into scalability issues, both in terms of
   the number of entries in the filter and the packet rate.

   Most routers are able to forward traffic at a much higher rate than
   they are able to filter, and are able to hold many more forwarding
   table entries and routes than filter entries. RTBH leverages the
   forwarding performance of modern routers to filter both more entries
   and at a higher rate than access control lists would allow.

   However, with destination-based RTBH filtering, the impact is that
   the attack is complete.  That is, with destination-based RTBH
   filtering you inject a discard route into the forwarding table for
   the prefix in question. All packets towards that destination, attack
   traffic AND legitimate traffic, are then dropped by the participating
   routers, thereby taking the target completely offline. The benefit is
   that collateral damage to other systems or network availability at
   the customer location or in the ISP network is limited, but the
   negative impact to the target itself is arguably increased.

   By coupling unicast reverse path forwarding (RPF) [RFC3704]
   techniques with RTBH, BGP can be used to distribute discard routes
   that are based not on destination or target addresses, but based on
   source addresses.

3. Destination address RTBH filtering

3.1. Overview

   A "discard" route is installed on each edge router in the network
   with the destination set to be the discard (or null) interface. In
   order to use RTBH filtering for an IP address (or network) a BGP
   route for the address to be filtered is announced, with the next-hop
   set as the "discard" route. This causes traffic to the announced
   network to be forwarded to the discard interface and so it does not
   transit the network and waste resources or trigger collateral damage
   or negative impact to other resources along the path towards the

   While this does "complete" the attack in that the attacked
   address(es) are made unreachable, it minimizes collateral damage. It
   may also be possible to move the host / service on the attacked IP
   address(es) to another address and keep the service up, for example
   by updating associated DNS resource records.

3.2. Detail

Kumari & McPherson        Expires July 19, 2009                 [Page 4]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

   Steps to configure destination based RTBH filtering:

     1: An address is chosen to become the "discard address". This is
        often chosen from (TEST-NET [RFC3330]), or from
        RFC 1918 [RFC1918] space.

     2: A route for the "discard address" is installed on the routers
        that form the edge/perimeter of the network, in all routers
        in the network, or some subset (e.g., peering, but not customer,
        etc.), with the destination of the route being the "discard" or
        "null" interface. This route is called the "discard route".

     3: A BGP policy is configured on all routers that have the discard
        route so that routes announced with the community
        [ TBD1 ] will have their next hop set to the discard address.
        The BGP policy should be made restrictive so that only BGP
        routes covering a defined number of hosts addresses will
        be accepted. That is, typically, only specific /32s are
        unless shorter prefix blocks are required. When filtering based
        shorter prefixes, extreme caution should be used as to avoid
        collateral damage to other hosts that reside within those
        address blocks.

     4: When RTBH filtering is desired for a specific address, that
        address is announced from a central router (or route server),
        tagged with the community [ TBD1 ]. The receiving routers check
        the BGP policy, setting the next-hop to be the discard route,
        which resolves to the discard interface.

     5: Traffic entering the network will now be forwarded to the
        discard interface on all edge routers and so will be dropped at
        the edge of the network, saving resources.

   This technique can be expanded by having multiple discard addresses,
   routes and communities to allow for monitoring of the discarded
   traffic volume on devices that support multiple discard interfaces.

   The technique can also be expanded by relaxing the AS path rule to
   allow customers of a service provider to enable RTBH filtering
   without interacting with the service provider. If this is configured,
   an operator MUST only accept announcements for prefixes from the
   customer that the customer is authorized to advertise, to prevent the
   customer accidentally (or intentionally) black-holing space that is
   not theirs.

   A common policy for this type of setup would be to accept from a

Kumari & McPherson        Expires July 19, 2009                 [Page 5]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

   customer their authorized aggregate block and then permit any more
   specific of the authorized prefix only if the blackhole communities
   are equal or similar to attached, append NO_EXPORT, NO_ADVERTISE.

   Extreme caution should be used in order to avoid leaking any more
   specifics beyond the local routing domain, unless policy explicitly
   aims at doing just that.

4. Source address RTBH filtering.

   In many instances the denial-of-service attacks are being sourced
   from botnets and are being configured to "follow DNS" (the attacking
   machines are instructed to attack www.example.com, and re-resolve
   this periodically. Historically the attacks were aimed simply at an
   IP address and so renumbering www.example.com to a new address was an
   effective mitigation). This makes a technique that allows black-
   holing based upon source address desirable.

   By combining traditional RTBH filtering with unicast Reverse Path
   Forwarding (uRPF) a network operator can filter based upon the source
   address. uRPF performs a route lookup of the source address of the
   packet and checks to see if the ingress interface of the packet is a
   valid egress interface for the packet source address (strict mode) or
   if any route to the source address of the packet exists (loose mode).
   If the check fails, the packet is typically dropped. In loose mode
   some vendors also verify that the destination route does not point to
   a discard interface - this allows source based RTBH filtering to be
   deployed in networks that cannot implement strict (or feasible path)
   mode uRPF.

   By enabling the uRPF feature on interfaces at pre-determined points
   of their network and announcing the source address(es) of attack
   traffic, a network operator can effectively drop the attack traffic
   at specified devices (ideally ingress edge) of their network based on
   source addresses.

   While administrators may choose to drop any prefixes they wish, it is
   recommended when employing source-based RTBH inter-domain that
   explicit policy be defined that enables peers to only announce
   source-based RTBH routes for prefixes which they originate.

4.1 Steps to deploy RTBH with uRPF for source filtering.

   The same steps that are required to implement destination address
   RTBH filtering are taken with the additional step of enabling unicast
   reverse path forwarding on predetermined interfaces. When a source
   address (or network) needs to be blocked, that address (or network)

Kumari & McPherson        Expires July 19, 2009                 [Page 6]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

   is announced using BGP tagged with a community. This will cause the
   route to be installed with a next hop of the discard interface,
   causing the uRPF check to fail. The destination based RTBH filtering
   community ([ TBD1 ]) should not be used for source based RTBH
   filtering, and the routes tagged with the selected community should
   be carefully filtered.

   The BGP policy will need to be relaxed to accept announcements tagged
   with this community to be accepted even though they contain addresses
   not controlled by the network announcing them. These announcements
   must NOT be propagated outside the local AS and should carry the
   NO_EXPORT community.

   As a matter of policy, operators SHOULD NOT accept source-based RTBH
   announcements from their peers or customers, they should only be
   installed by local or attack management systems within their
   administrative domain.

5.  Security Considerations

   The techniques presented here provide enough power to cause
   significant traffic forwarding loss if incorrectly deployed. It is
   imperative that the announcements that trigger the black-holing are
   carefully checked and that the BGP policy that accepts these
   announcements is implemented in such a manner that the announcements:

    - Are not propagated outside the AS (NO_EXPORT).
    - Are not accepted from outside the AS (except from customers).
    - Except where source based filtering is deployed, that the network
      contained in the announcement falls within the address ranges
      controlled by the announcing AS (i.e.: for customers that the
      address falls within their space).

6.  IANA Considerations

  This document requests registration of a regular Type, non-transitive
  BGP Extended Communities Attribute [RFC4360] from the First Come,
  First Served range to be named "Remote Triggered Black Hole Filter".

  This community will provide a standard method to signal a provider
  that RTBH filtering should occur for a destination and will eliminate
  the need for customers to track different communities for each

Kumari & McPherson        Expires July 19, 2009                 [Page 7]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

7. Acknowledgments

  I would like to thank Joe Abley, Rodnet Dunn, Alfred Hoenes, Donald
  Smith, Joel Jaeggli and Steve Williams for their assistance, feedback
  and not laughing *too* much at the quality of the initial drafts.

  I would also like to thank all of the regular contributors to the
  OpSec Working Group and Google for 20% time :-)

  The authors would also like to thank Barry Greene for his efforts in
  getting this implemented and Chris Morrow for publicizing the
  technique in multiple talks.

8.  References

8.1.  Normative References

   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

   [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September

   [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

   [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

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

8.2.  Informative References

   [2223BIS]  Reynolds, J. and R. Braden, "Instructions to Request for
              Comments (RFC) Authors", draft-rfc-editor-
              rfc2223bis-08.txt, August 2004.

Kumari & McPherson        Expires July 19, 2009                 [Page 8]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

Appendix A: Cisco Router Configuration Sample

   This section provides a partial configuration for configuring RTBH on
   a Cisco router. This is not a complete configuration and should be
   customized before being used.

Announcing router:
   ! The discard route
   ip route Null0
   ! Matches and empty AS-PATH only.
   ip as-path access-list 10 permit ^$
   ! This route-map matches routes with tag 666 and sets the next-hop
   ! to be the discard route.
   route-map remote-trigger-black-hole permit 10
    match tag 666
    set ip next-hop
    set local-preference 200
    set community no-export
    ! The community used internally to tag RTBH announcements.
    set community 65505:666
    set origin igp
   route-map remote-trigger-black-hole permit 20
   router bgp 65505
    no synchronization
    bgp log-neighbor-changes
    redistribute static route-map remote-trigger-black-hole
    no auto-summary
   ! An example IP that we are applying RTBH filtering to.
   ! All traffic destined to will now be dropped!
   ip route null0 tag 666

Filtering router:
   ! The discard route
   ip route Null0
   ! Matches and empty AS-PATH only.
   ip as-path access-list 10 permit ^$
   route-map black-hole-filter permit 10
    match ip address prefix-list only-host-routes
    match as-path 10

Kumari & McPherson        Expires July 19, 2009                 [Page 9]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

    match community 65505:666 no-export
   ! Don't accept any other announcements with the RTBH community.
   route-map black-hole-filter deny 20
    match community 65505:666
   route-map black-hole-filter permit 30
   ! An interface for source-based RTBH with uRPF loose mode.
   interface FastEthernet 0/0
   ip verify unicast source reachable-via any

Kumari & McPherson        Expires July 19, 2009                [Page 10]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

Appendix B: Juniper Configuration Sample

   This section provides a partial configuration for configuring RTBH on
   a Juniper router. This is not a complete configuration and should be
   customized for before being used.

Announcing router:

   routing-options {
      static {
          route {
              tag 666;
          route {
              tag 666;
      autonomous-system 100;

   protocols {
      bgp {
          group ibgp {
              type internal;
              export rtbh;

   policy-options {
      policy-statement rtbh {
          term black-hole-filter {
              from {
                  tag 666;
                  route-filter prefix-length-range /32-/32;
              then {
                  local-preference 200;
                  origin igp;
                  community set black-hole;
                  community add no-export;

Kumari & McPherson        Expires July 19, 2009                [Page 11]

Internet Draft            blackhole-urpf-00.txt         January 19 2009

      community black-hole members 100:666;
      community no-export members no-export;

Filtering router:

   policy-statement black-hole-filter {
      from {
          protocol bgp;
          as-path LocalOnly;
          community black-hole;
          route-filter prefix-length-range /32-/32;
      then {
          community set no-export;
   community black-hole members 100:666;
   community no-export members no-export;

   routing-options {
      forwarding-table {
          unicast-reverse-path feasible-paths;
      static {
          route discard;

   interfaces {
      xe-1/0/0 {
          mtu 9192;
          unit 201 {
              vlan-id 201;
              family inet {

Kumari & McPherson        Expires July 19, 2009                [Page 12]

Internet Draft            blackhole-urpf-00.txt         January 19 2009


Authors' Addresses

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   Email: warren@kumari.net

   Danny McPherson
   Arbor Networks, Inc.
   Email: danny@arbor.net

Kumari & McPherson        Expires July 19, 2009                [Page 13]