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Internet Draft                                                 W. Kumari
<draft-kumari-blackhole-urpf-02.txt>                              Google
Category: Informational                                     D. McPherson
Expires: April 27, 2009                                   Arbor Networks

                                                        October 27, 2008

            Remote Triggered Black Hole filtering with uRPF

Status of this Memo

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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. It also defines a standard
   BGP community for black hole prefixes to simplify associated semantics.

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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] and presents a method
   to allow filtering by source address. It also defines a standard BGP
   community to signal that Remote Triggered Black Hole (RTBH) filtering
   should occur for a network.

2. Background

   Denial of Service (DoS) attacks that starve out legitimate traffic by
   saturating circuits are becoming an increasingly common occurrence.
   Network operators have developed a selection of techniques for
   mitigating these attacks. Different techniques have different
   strengths and weaknesses -- the selection of which technique to use
   for each type of attack involves tradeoffs.

   A common attack directed against a customer of a service provider
   involves generating more traffic at the customer than will fit down
   the links from the service provider to the customer. This traffic
   "starves out" legitimate traffic. Rather than having all of their
   network affected the customer may ask their service provider to
   filter traffic destined to the attacked IP address(es).

   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

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   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
   you effectively complete the attack.  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

   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 cuts down on 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.

3.2. Detail

   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

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        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 necessary, unless
        shorter prefix blocks are required.  This might occur where
        larger numbers of attacking sources are located within a
        single prefix, or the employment of this mechanism is to drop
        traffic to 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.

     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 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 to 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

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   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 or the source address of the packet exists (loose mode).
   If the check fails, the packet is (generally) 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 on 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 the 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 w/ 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. 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.

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   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 current 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 severe
   unhappiness. 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
  - 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 within the announcement falls with the address ranges
  controlled by the announcing AS (i.e.: for customer 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

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7. Acknowledgments

  I would like to thank Alfred Hoenes, Danny McPherson, Donald Smith,
  Joe Abley and Joel Jaeggli 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 are not aware of who initially developed this technique,
  but would like to thank Chris Morrow for publicizing it.

8.  References

8.1.  Normative References

   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., and G. de
              Groot, "Address Allocation for Private Internets", RFC
              1597, March 1994.

   [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.  Requirement Levels",
              BCP 14, RFC 2119, March 1997.

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.

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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 the 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

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    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

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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 {
         # The discard route.
         route discard;
         # An example route to be RTBH filtered.
         route {
             tag 666;
     autonomous-system 65505;
   protocols {
     bgp {
       import remote-trigger-black-hole;
       local-as 65505;
   policy-options {
     policy-statement remote-trigger-black-hole {
       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 65505:666;
     community no-export members no-export;

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Filtering router:
   routing-options {
     static {
         # The discard route.
         route discard;
     autonomous-system 65505;
     # Enable feasible-paths mode uRPF.
     forwarding-table {
       unicast-reverse-path feasible-paths;
   protocols {
     bgp {
         group iBGP {
             import black-hole-filter;
   policy-options {
     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 65505:666;
     community no-export members no-export;
     as-path LocalOnly "^$";
   # Enable uRPF on an interface for source based uRPF.
   interfaces {
       so-0/0/0 {
           unit 0 {
               family inet {

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Authors' Addresses

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

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

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