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Versions: 01 02 03 04 05 06 rfc3882                                     
Network Working Group
Internet Draft                                                 D.  Turk
Document: draft-turk-bgp-dos-03.txt                         Bell Canada
Expires: June 2003                                        December 2002

           Configuring BGP to Block Denial-of-Service Attacks

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except that the right to
   produce derivative works is not granted.

   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

Conventions used in this document

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


   This document describes an operational technique that uses BGP
   communities to remotely trigger black-holing of a particular
   destination network to block denial-of-service attacks.   Black-
   holing can be applied on a selection of routers rather than all BGP-
   speaking routers in the network.   The document also describes a
   sinkhole tunnel technique using BGP communities and tunnels to pull
   traffic into a sinkhole router for analysis.

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Table of Contents

   1. Existing BGP-Triggered Black holing Techniques                 2
   2. Enhanced BGP-Triggered Black holing Technique                  3
   3. Sinkhole tunnels                                               4
   Security Considerations                                           7
   Disclaimer                                                        7
   References                                                        7
   Acknowledgments                                                   7
   Author's Addresses                                                7

1.   Existing BGP-Triggered Black-holing Techniques

   Current BGP-triggered black-holing techniques rely on altering the
   BGP next hop address of a network targeted by an attack throughout
   the iBGP network.  A customized iBGP advertisement is generated from
   a router participating in the destination/attacked AS where the next
   hop address for the targeted network or host is modified to point to
   an RFC 1918 (private internet) address.  Most routers on the
   Internet, especially edge routers, have static routes pointing RFC
   1918 addresses to the null interface. Those static routes drive all
   traffic destined to the network under attack to the null interface.

   When an iBGP-speaking router inside the destination AS receives the
   iBGP update, the advertised prefix will be added to the routing table
   with a next hop of one of the networks listed in RFC 1918.  The
   router will then attempt to resolve the RFC 1918 next-hop in order to
   qualify the route and derive a forwarding interface. This process
   will return a valid next hop as the null interface.  Assuming the
   router is properly configured to direct RFC 1918 destined traffic to
   a null interface, traffic destined to the attacked network gets
   dropped making the attacked network unreachable to the attacker and
   everyone else.

   While this technique shields the internal infrastructure from the
   attack, thereby protecting a large number of devices, it has the
   undesirable side effect of rendering the targeted/attacked network
   unreachable throughout the entire destination AS.  Even if a static
   route pointing an RFC 1918 address to a null interface is not
   configured on all routers within the destination AS, the modified
   next hop makes the traffic un-routable to its legitimate destination.

   Network operators usually use the BGP-triggered black holes for a
   short period of time. The technique causes traffic drops on all
   ingress points of the AS for traffic destined to the attacked
   network.  By default, routers dropping traffic into a null interface
   should send "ICMP unreachable" message to the source address
   belonging to the origin/attacking AS.

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   Once the procedure reaches this point, one of the source addresses of
   the attack traffic is hijacked by introducing a device with the same
   source IP address into the BGP domain of the destination/attacked AS.
   The device hijacking the source address collects the ICMP unreachable
   packets.  The source addresses of these ICMP unreachable packets
   reveal which edge routers within the destination/attacked AS the
   attack is coming from.  The network operator may then opt to manually
   stop the traffic on the routers from which attack traffic is

2.   Enhanced BGP-Triggered Black-holing Technique

   This paper describes a technique developed to instruct a selected set
   of routers to alter the next hop address of a particular prefix by
   use of BGP protocol. The next hop can either be a null interface or,
   as discussed later on in this paper, a sinkhole tunnel interface.
   This technique does not invoke an access list or rate limiting
   statement to treat attack traffic, nor does it involve a network wide
   change of the attacked prefix next hop address.  The next hop will
   only be changed on a selection of routers with the aid of BGP
   communities within the destination/attacked AS.

   To prepare the network for this technique, the network operator needs
   to define a unique community value for each destination AS border
   router that could potentially drive attack traffic to the victim.
   For example, a network with a BGP autonomous system number 65001 has
   two border routers (R1 and R2).  Community value 65001:1 is assigned
   to identify R1, community value 65001:2 is assigned to identify R2
   and community value 65001:666 is assigned to identify both R1 and R2.

   After the BGP community assignment, R1 and R2 must be configured with
   the following:

   1.   Static route pointing an RFC 1918 network to a null interface.
   2.   AS-Path access list that matches local BGP prefix advertisement.
   3.   BGP community access list to match the community value assigned by
        the network operator for the particular router (i.e. 65001:1
        for R1).
   4.   BGP community access list to match the community value assigned by
        the network operator for all router (i.e. 65001:666 for R1 and R2)
   5.   Under the BGP process, an iBGP import route policy should be
        applied on received iBGP advertisements to do the following logic.
        (Statements are in a logical AND order)
        a.   A policy statement to permit routes that match the following
             criteria and apply the following changes.

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              i.  Match for community specific to that router (i.e.
                  65001:1, for R1).
             ii.  Match AS-Path to locally generated BGP advertisements.
            iii.  Set BGP next hop to an RFC 1918 network.
             iv.  Overwrite BGP community with the well-known community

        b.   A policy statement to permit routes that match the following
             criteria and apply the following changes.
              i.  Match for community that covers all routers (i.e.
             ii.  Match AS-Path to locally generated BGP advertisements.
            iii.  Set BGP next hop to an RFC 1918 network.
             iv.  Overwrite BGP community with the well-known community

   After the policies have been configured on R1 and R2, the network
   operator can, in the case of an attack, advertise the targeted
   network that could be one or more /32 "host" routes into iBGP of the
   destination/attacked AS. The advertisement must contain the community
   value associated with the router(s) where the attack is arriving in
   addition to the well-known community (no-export). Using BGP
   communities preserves the original next hop address of the targeted
   network on all routers where the special route policy configuration
   is not present.  iBGP will then carry the prefix advertisement to all
   routers in the destination/attacked AS.  All routers within the
   destination AS, except the ones that match the community stamped on
   the prefix, will be oblivious to the community value and will install
   the network route with the legitimate next hop address.  Routers that
   match the community will also install the network route into their
   routing table but will alter the next hop address to an RFC 1918
   network and then to a null interface as per the route policies
   configuration and recursive route lookup.  The reason for matching
   locally announced networks is to make sure that no eBGP peer can
   misuse this community to drive any network to a null interface. It is
   recommended to blackhole the targeted/attached hosts and not the
   entire address block they belong to so that the blackhole effect has
   the minimum impact on the attacked network.

   This technique stops traffic from getting forwarded to the legitimate
   destination on routers identified as transit routers for attack
   traffic and that have route map matches for the community value
   associated with the network advertisement.  All other traffic on the
   network will still get forwarded to the legitimate destination thus
   minimizing the impact on the targeted network.

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3.   Sinkhole tunnels

   Following the "Enhanced BGP-Triggered Black-holing Technique", it may
   become a requirement to take a look at the attack traffic for further
   analysis.  This requirement adds to the complexity of the exercise.
   Usually with broadcast interfaces, network operators install network
   sniffers on a spanned port of a switch for analysis of traffic.
   Another method would be to announce a network prefix that covers the
   attack host address into iBGP, altering the next hop to a sinkhole
   device that can log traffic for analysis.  The latter technique
   results in taking down the services offered on the targeted/attacked
   IP addresses.  Inter-AS traffic will be sucked into the sinkhole
   along with Intra-AS traffic.  Packet level analysis involves
   redirecting traffic away from the destination host to a sniffer or a
   router.  As a result, if the traffic being examined includes
   legitimate traffic, that legitimate traffic will never make it to the
   destination host.  This will result in denial of service for the
   legitimate traffic.  A better alternative would be to use a sinkhole
   tunnel.  A sinkhole tunnel is implemented only at the entry point to
   the destination/attacked AS.   All traffic destined to the attack
   targeted host will be re-routed to a special path (tunnel) where a
   sniffer will be able to analyze the traffic.  After being analyzed,
   traffic will exit the tunnel and gets routed normally to the
   destination host.  In other words, the traffic will pass through the
   network to a sniffer without altering the next hop information of the
   destination network. All routers within the destination/attacked AS
   iBGP domain with have the proper next hop address.  Only the entry
   point router will have the altered next hop information.

   To detail the procedure, a sinkhole router with an optional sniffer
   attached to its interface is installed and configured to participate
   in IGP and iBGP of the attacked AS.  Next, a tunnel is created using
   for instance, MPLS Traffic Engineering, from all border routers
   attacks can potentially enter from (Inter-AS traffic) to the sinkhole
   router.  When a host or network is under attack, a customized iBGP
   advertisement is sent to announce the attacked host(s) network
   address with the proper next hop that insures traffic will reach
   those hosts or networks.  The customized advertisement will also have
   a community string value that matches the set of border routers the
   attack is entering from, as described in section 2.  The new next hop
   address configured within the route policy section of all border
   routers should be the sinkhole IP address.  This IP address belongs
   to the /30 subnet assigned to the tunnel connecting the border router
   to the sinkhole router.  Routers other than the border router(s)
   where the attack is entering from will install the customized
   advertisement with the proper next hop pointing to the host(s) under

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   Lack of community string match on other routers in the iBGP network
   will insure that the special route policy does not change next hop
   address.  Traffic entering from border routers that do not have
   matches for the special community will be passed through regular
   router interfaces to the legitimate destination.  The next hop will
   not be altered.  Attack traffic is then treated at the sinkhole
   device.  It might be required to analysis the traffic and send it
   back towards the host(s) it was originally destined to.  In this
   case, a default network route is configured to point to any interface
   attached and configured on the iBGP network.  That includes the same
   physical interface the tunnel is built on.  Traffic entering the
   sinkhole device from the tunnel will be sent back to the network due
   to the presence of the default route.  Routing protocols will then
   take care of properly routing the traffic to its original destination
   (attacked network).

   It becomes apparent that this technique can also be used for purposes
   other than analyzing attack traffic.  Legitimate traffic could also
   be pulled out of normal routing into a tunnel and then reinserted
   onto the backbone without altering the next hop addressing scheme
   throughout the iBGP network.

   MPLS Traffic Engineering with its many feature, is a good method of
   sliding traffic to the sinkhole device.  Features like QoS policies
   can be applied on the attack traffic, thus preventing it from
   competing with legitimate traffic.

   To be able to alter the next hop on the border router, a subnet of an
   RFC 1918 network is statically routed to the tunnel interface.  An
   example of the static route is:

   ip route Tunnel0

   Setting the next hop of the target IP address to will
   force the traffic to go through the tunnel.

   Traffic is received at the sinkhole interface via the TE tunnel.
   Subsequently, three methods could be installed, namely rate-limiting
   policies, QoS policies and access lists.  These policies could rate
   limit or drop traffic classified as attack traffic.  This process
   would be done on the interface of the sinkhole device.  Another
   useful application for a sinkhole router is to pull in traffic via
   tunnels to an inbound interface and have a default route statement
   forwarding the traffic out to an Ethernet interface.  The Ethernet
   interface is connected to the iBGP network and guarantees proper
   delivery of traffic but allows the use of a packet sniffer to further
   analyze the attack traffic.

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   This becomes very useful when it is not feasible to apply an Access
   list or a rate limiting statement on the BGP border router or last
   hop router before the attacked host or network because of hardware or
   software limitations.  Hence, instead of upgrading interfaces at the
   point of entry of attack traffic, the latter could be pulled into the
   sinkhole and treated on that device.  Operational costs can be
   rendered minimal if the sinkhole router is a powerful device.

Security Considerations

   It is very important to practice tight control over eBGP peering
   points before implementing the techniques described in this paper.
   eBGP customers might be able to blackhole a particular subnet using
   the Blackhole communities.  To eliminate the risk, the match for
   locally generated BGP advertisements in the special route policy
   should not be neglected.


  The views and specification here are those of the authors and are not
  necessarily those of their employers.  The authors and their employers
  specifically disclaim responsibility for any problems arising from
  correct or incorrect implementation or use of this specification.


   [1] B.  Greene "ISP Security Essentials - Best Practice Cisco IOS and
       other Techniques to Help an ISP Survive in today's Internet,
       version 2.0", May, 2001


   The author of this document would like to acknowledge the developers
   of the remotely triggered black-holing technique and the developers
   of the backscatter technique for collecting backscatter traffic.  The
   author would also like to thank all members of the IP Engineering
   department for their help in verifying the functionality of this

Author's Addresses

   Doughan Turk
   Bell Canada
   100 Wynford Drive
   Email: doughan.turk@bell.ca

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