Opsec Working Group                                            W. Kumari
Internet Draft                                                    Google
<draft-ietf-opsec-blackhole-urpf-04>                        D. McPherson
Category: Informational                                   Arbor Networks
Expires: September 30, December 4, 2009
                                                          March 30,
                                                            June 4, 2009

            Remote Triggered Black Hole filtering with uRPF

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   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.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3 .....................................................4
   3. Background ......................................................3
   4. Destination address RTBH filtering ..............................4
      3.1. Overview ...................................................4
      3.2. Detail .....................................................5
   4. Source address RTBH filtering ...................................7
      5.1. ...................................8
      4.1. Steps to deploy RTBH filtering with uRPF ...................8
   6. ...................9
   5. Security Considerations .........................................9
   6. IANA Considerations .............................................9
   8. ............................................10
   7. Acknowledgments .................................................9
   9. ................................................10
   8. References ......................................................9
      9.1. .....................................................10
      8.1. Normative References .......................................9
      9.2. ......................................10
      8.2. Informative References ....................................10
   A. Cisco Router Configuration Sample...............................11
   B. Juniper Configuration Sample....................................13
   C. A Brief History of RTBH.........................................15

1. Introduction

   This document expands upon the technique outlined in "Configuring BGP
   to Block Denial-of-Service Attacks" [RFC3882] to demonstrate a method
   that allows for filtering by source address(es).

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119. [RFC2119].

3. Background

   Network operators have developed a variety of techniques for
   mitigating denial of service attacks. While 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 the tradeoffs associated with each method.

   A common DoS attack directed against a customer of a service provider
   involves generating a greater volume of attack traffic destined for
   the target than will fit down the links from the service provider(s)
   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 destinations on their network be affected by 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.

   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 filtering
   leverages the forwarding performance of modern routers to filter both
   more entries and at a higher rate than access control lists would
   otherwise allow.

   However, with destination-based RTBH filtering, the impact of the
   attack on the target is complete. That is, destination-based RTBH
   filtering injects a discard route into the forwarding table for the
   target prefix.  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 filtering, BGP can be used to distribute discard
   routes that are based not on destination or target addresses, but
   based on source addresses of unwanted traffic.

4. Note that this will
   drop all traffic to / from the address, and not just the traffic to
   the victim.

   This document is broken up into three logical parts, the first
   outlines how to configure destination based RTBH, the second covers
   source based RTBH and the third part has examples and a history of
   the technique.

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119. [RFC2119].

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 the discard (or null) interface. In order
   to use RTBH filtering for a single IP address (or prefix), 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 prefix to be forwarded to the discard interface so that it
   does not transit the network wasting resources or triggering
   collateral damage to other resources along the path towards the

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


3.2. Detail

   Before deploying RTBH filtering, there is some preparation and
   planning that needs to occur and decisions that need to be made.
   These include:

       - what are the discard addresses?
       - what are the discard BGP communities?
       - what is the largest prefix that can be black-holed?
       - what is the smallest advertisement that your provider will

   Steps to configure destination based RTBH filtering:

   Step 1. Select your Discard Address schema. An address is chosen to
   become the "discard address". This is often chosen from (TEST-
   (TEST-NET [RFC3330]), or from RFC 1918 [RFC1918] space. Multiple
   addresses allow an operator to configure multiple static routes, one
   for each incident: = Incident #1 = Incident #2 = Incident #3 = Incident #4 = Incident #5

   Customer #1 who has a DDoS attack can be pointed to discard route Customer #2 can be pointed to discard route If
   capable, the router can then count the drops for each, providing some
   level of telemetry on the volume of drops as well as status of an
   ongoing attack. A consistent address schema facilitates operations.

   Step 2. Set Configure the Discard Route(s) on each router. router, 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.). The destination of
   the route is set to the "discard" or "null" interface. This route is
   called the "discard route". Implementation experience demonstrates
   the value of configuring each ingress router with a capability for
   dropping traffic via RTBH filtering.

   Step 3. Configure the RTBH BGP Policy on each router.  A BGP policy
   is configured on all routers that have the discard route so that
   routes announced with a chosen community 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
   necessary. Shorter  prefix blocks may also be required or desirable,
   for example if larger numbers of attack targets are located within a
   single  prefix, or the employment of this mechanism is to drop
   traffic bound for specific networks. When filtering based on shorter
   prefixes, extreme caution should be used as to avoid collateral
   damage to other hosts that reside within those address blocks. Full
   implementations will have  multiple communities, with each community
   used for different parts of a provider's network and for different
   security incidents.

   Step 4. Configure the Safety Egress Prefix Filters (Murphy Prefix
   Filters). Filters.  There is always
   a chance that the triggering BGP Update could leak from the network. This has happened [ Youtube/Pakistan
   incident ]. Egress network
   and so egress prefix filters are strongly recommended.  These egress
   prefix filter details may varying, vary, but experience has demonstrated that
   the following works:


     -  Deny all prefixes /30 through /32 from leaving your network. longer than the longest prefix that you expect
        announce. For example, if the longest prefix that you expect to
        announce is /24, deny all prefixes of length /25 though /32. If
        your triggering BGP update is only /32s, then this egress prefix
        filter will add a safe measure in case the NO_EXPORT community
        does not work.


     -  Deny all communities used for triggering RTBH filtering. This is
        also a "safety" measure in case the NO_EXPORT community does
        not work.

   Step 5: Configure the Trigger Router. Set  Configure the trigger router,
   workstation, or other device so that adding and removing the triggers
   can be done easily and quickly. The BGP Update should have the
   NO_EXPORT community as a mandatory attribute. An egress prefix filter
   or policy which prevents RTBHing RTBH filtering prefixes in the /8 to /24
   range is also recommended as a safety tool. The trigger router can be
   set up as a  iBGP route reflector client which does not receive any
   prefixes from its BGP peer. This allows a low cost router/workstation router /
   workstation to be used as the trigger router.


   Using the RTBH filtering:

   1. When RTBH filtering is desired for a specific address, that
   address is announced from a trigger router (or route server), tagged
   with the chosen "RTBH" community and with the NO_EXPORT community,
   and passed via iBGP. The receiving routers check the BGP policy, set
   the next-hop to be the discard route, resulting in a FIB entry
   pointing to a discard address.


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


   2.1 Multiple Discard Addresses for Incident Granularity. 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.
   As mentioned earlier, each router can have a discard address schema
   to allow the operator to distinguish multiple incidents from each
   other - making it easier to monitor the life-cycle of the incidents.


   2.2 Multiple "Trigger Communities" for Network Wide Granularity. The
   network can be sectioned into multiple communities, providing the
   operator with an ability to drop in discreete discrete parts of their network.
   For example, the network can be divided into the following

        XXX:600 RTBH filtering on all router
        XXX:601 RTBH filtering on only peering router
        XXX:602 RTBH filtering on only customers who peer BGP
        XXX:603 RTBH filtering on Datacenters (to see if the data center
      is the
                     source of attack)
        XXX:604 RTBH filtering on all customers (to see how many
      customers are
                being used by the attacker)

   Some diligent thinking is required to develop a community schema
   which provides flexibility while reflecting topological


   2.3 "Customer Triggered" RTBH filtering. 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's  trigger routers. If this is configured, an
   operator MUST only accept announcements for prefixes from the
   customer that the customer is authorized to advertise, in order to
   prevent the customer from accidentally (or intentionally) black-
   holing space that they are not allowed to advertise.

   A common policy for this type of setup would first permit any more
   specific of an authorized prefix only if the blackhole communities is
   attached, append NO_EXPORT, NO_ADVERTISE, or similar communities, and
   then also accept from the customer the original aggregate prefix that
   will be advertised as standard policy permits.

   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 denial-of-service attacks sourced from botnets 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 employing technique that allows
   black-holing to be based on 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
   an invalid next-hop - this allows source based RTBH filtering to be
   deployed in networks that cannot implement strict (or feasible path)
   mode uRPF. Before enabling uRPF (in any mode), it is vital that you
   fully understand the implications of doing so:

     - Strict mode will cause the router to drop all ingress traffic
       if the best path back to the source address of the traffic is
       not the interface from which the traffic was received.
       Asymetric routing will cause strict mode uRPF to drop
       legitimate traffic.

    - Loose mode causes the router to check if a route for the source
      address of the traffic exists. This may also cause legitimate
      traffic to be discarded.

   It is hoped that in the future, vendors will implement a "DoS-
   mitigation" mode in addition to the Loose and Strict modes -- in this
   mode, the uRPF check will only fail if the next-hop for the source of
   the packet is a discard interface.

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

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


4.1 Steps to deploy RTBH filtering 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)
   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. fail and the packets to be discarded. The
   destination based RTBH filtering community 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

  No action required.


7. Acknowledgments

  I would like to thank Joe Abley, Ron Bonica, Rodney Dunn, Alfred
  Hoenes, Donald Smith, Joel Jaeggli 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 Steven L Johnson and Barry Greene
  for 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

   [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.

9.2.  Informative References

   [Greene2001] Greene Barry Raveendran and Jarvis Neil "Unicast Reverse
              Path Forwarding (uRPF) Enhancements for the ISP-ISP Edge",

Appendix A: Cisco Router Configuration Sample

   This section provides a partial configuration for configuring RTBH
   filtering 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
    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 filtering with uRPF loose mode.
   interface FastEthernet 0/0
   ip verify unicast source reachable-via any

Appendix B: Juniper Configuration Sample

   This section provides a partial configuration for configuring RTBH
   filtering on a Juniper router. This is not a complete configuration
   and should be customized 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;
      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 {

Appendix C: A Brief History of RTBH filtering

   Understanding the history and motivation behind the development of a
   technique often helps with understanding how to best utilize the
   technique. In this spirit we present a history of Unicast RPF and
   RTBH filtering.

   This section provided by Barry Raveendran Greene:

   Unicast RPF Loose Check (uRPF Loose Check) was created by Neil Jarvis
   and Barry Greene to be used with dRTBH as a rapid reaction tool to
   DDoS Attacks. The requirements for this rapid reaction tool was based
   on post mortem conversation after the Feb 2000 attacks on several big
   content hosting companies. The summary of the requirement became the
   "Exodus Requirement" which stated:

   "We need a tool to drop packets based on source IP address that can
   be pushed out to over 60 routers within 60 seconds, be longer than a
   thousand lines, be modified on the fly, and work in all your
   platforms filtering at line rate."

   A variety of options were looked at to meet this requirement, from
   reviving COPS, to pushing out ACLs with BGP, creating a new protocol.
   In 2000, the quickest way to meet the "Exodus requirement" was to
   marry two functions. First, modify Unicast RPF so that the interface
   check was no longer required and to make sure that a "null" or
   discard route would drop the packet (i.e. loose check). Second, the
   technique where BGP is used to trigger a distributed drop is dusted
   off and documented.  Combining these two techniques was deemed a fast
   way to put a distributed capability to drop packets out into the

   To clarify and restate, uRPF Loose Check was created as one part of a
   rapid reaction tool to DDoS attacks that "drop packets based on
   source IP address that can be pushed out to over 60 routers with in
   60 seconds, be longer than a thousand lines, be modified on the fly,
   and work in all your platforms filtering at line rate." The secondary
   benefits of using uRPF Loose Check for other functions is a secondary
   benefit - not the primary goal for its creation.

   To facilitate the adoption to the industry, uRPF Loose Check was not
   patented. It was publicly published and disclosed in "Unicast Reverse
   PathForwarding (uRPF) Enhancements for the ISP-ISP Edge"

Authors' Addresses

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

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